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Spaceflight is a relatively new phenomenon, dating back to the The '50s at the very earliest, yet it has captured the human imagination in a way that is unlike almost anything else. Dreams of going to space and visiting other planets have lived in our imaginations long before they became a reality, but now that we have visited many of the worlds in our solar system with robotic spacecraft and landed human beings on one (the Moon), the truth has begun to settle in a little bit. Spaceflight is hard, dangerous, and (ironically) very slow for human purposes.

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Science Fiction deals with this in a number of ways. The Mohs Scale of Science Fiction Hardness discusses the degrees to which writers fudge the details of space and space travel, from complete fantasy to as close to reality as possible. Along the way, a lot of authors make mistakes (knowingly or not) about the actual technology of rockets and their various propulsion methods. This article discusses the following topics:

  • Terminology associated with rockets
  • The realistic portrayal of distances in space
  • Propulsion methods: real, proposed or in development, and completely hypothetical
  • Historical, modern, and future rockets: their capabilities and missions

Completely fictional rockets and propulsion methods are matters best left for the many works that utilize them, but understanding what can (or could possibly) be done should help you come up with plausible alternatives.

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For rocket history, see The Space Race.

Note: Units in this article are in metric. If you need help converting them, let me Google that for you. To get you started, one mile (mi) is about 1.6 kilometers (km).

Rocket Terminology

Here we will discuss basic terms associated with rockets and rocket propulsion.

    Rocket Terminology 
  • Fuel:
    • For chemical rockets, the substance that burns along with an oxidizer
    • For other types of rockets, the substance that the power source consumes to produce energy that pushes the propellant
  • Oxidizer: Fuel needs oxygen (or an equivalent) to burn. This must be carried along with the fuel since there's no oxygen in space.
  • Propellant: The material that a rocket throws out of its engines to achieve thrust via Newton's third law, usually the byproduct of chemical combustion
  • Dry mass: How much a rocket weighs without any propellant
  • Wet mass: How much a rocket weighs with a full load of propellant.
  • Liftoff mass: How much a rocket weighs at liftoff, including propellant and payload.
  • Payload mass: How much payload a rocket can deliver into orbit (or farther), not counting the rocket itself. Sometimes referred to as "upmass"
  • Delta-V: This is a measurement of how much total change in velocity a rocket can achieve. A rocket with 8 km/s of delta-V can speed up (or slow down) by a total of eight kilometers per second with its available fuel. If that's not enough to get where you want to go, you're hosed.
  • Specific impulse (isp): Measured in seconds, this is a numerical value stating how efficiently a rocket engine converts its propellant into thrust. It's like the gas mileage of a rocket: a higher number means you get more total delta-V from a given amount of propellant.
  • Thrust-to-weight ratio (TWR): The ratio of a rocket's thrust to its weight, typically measured at liftoff. If this value is less than one, the rocket cannot get off the ground. Rockets with a liftoff TWR of less than one (and sometimes even more than one) will use detachable solid rocket boosters (SRBs) for an initial kick to the point where their main engines can take over. TWR increases as a rocket burns off its propellant supply.
  • Orbital velocity: How much total velocity you need to achieve and remain in orbit. For Earth, this is between 6.9 and 7.8 km/s depending on the shape of the orbit.
    • Note that orbital velocity is not necessarily how much delta-V you need to reach orbit from the ground. Gravity and atmospheric drag have to be accounted for as well, adding between 1 and 2 km/s to the requirement depending on the flight profile.
  • Escape velocity: How much total velocity you need to leave orbit and no longer be gravitationally bound to an object. For Earth, this is about 11.2 km/s.
  • Kerolox: Refers to a rocket engine burning kerosene (aka RP-1) and liquid oxygen.
  • Methalox: Refers to a rocket engine burning liquid methane and liquid oxygen.
  • Hydrolox: Refers to a rocket engine burning liquid hydrogen and liquid oxygen.
  • Hypergolic: Refers to a rocket engine burning a mixture of fuel and oxidizer that combusts spontaneously when they come into contact, or to said chemicals.
  • Stages: The various parts of a rocket that burn in sequence so that the parts that aren't needed can be thrown away
  • Booster: The stage of a rocket that burns first, used to get the vehicle out of the Earth's atmosphere
  • Second stage: The stage of a rocket that burns after the booster separates and accelerates it to orbital velocity. Some vehicles use third or even fourth stages.
  • Kick stage: A stage deployed once a rocket reaches orbit that sends the payload to another orbit.
  • Ignition: The moment when a rocket's engines ignite, usually a few seconds prior to liftoff
  • Liftoff: The moment when a rocket leaves its launch pad or platform
  • Max Q: Maximum aerodynamic pressure ('q' is the symbol for dynamic pressure) is the moment during ascent when the combination of air resistance and acceleration produces the greatest stress on a rocket's structure. Surviving max q is seen as an indicator that the mission will likely be successful.
  • BECO: Booster engine cutoff, used variously to describe the burnout of solid rocket boosters or the main stage, depending on the convention of the launch provider
  • MECO: Main engine cutoff, usually describing the shutdown of the rocket's first stage engine(s) in preparation for staging. Some launch providers describe the second stage as the main engine instead.
  • SECO: Second engine cutoff, typically describing the shutdown of the second or orbital stage's engine(s). Its use depends on launch provider.
  • Staging: Stage separation, the moment when a rocket's stages come apart. This is a dynamic phase as the risk of failures and collisions is relatively high.
  • Fairing: An oblate or ovoid shell used to encapsulate a rocket's payload during ascent. This protects it against air resistance, heating due to friction, and noise/vibrations while the rocket is moving through the atmosphere. After reaching space, it is no longer needed and is typically jettisoned.
  • Reentry: When an object in orbit (typically a spacecraft) descends to an altitude where atmospheric drag begins to slow it down significantly. Most heating during reentry is caused not by friction but by compression of air molecules while the object is moving at hypersonic velocity. To survive reentry, a spacecraft needs a heat shield.
  • Perigee: The lowest point of an orbit with respect to the Earth (the general term "periapsis" refers to orbits around any body)
  • Apogee: The highest point of an orbit with respect to the Earth (the general term "apoapsis" refers to orbits around any body)
  • LEO: Low Earth Orbit: up to 2,000 km
  • MEO: Medium Earth Orbit: between 2,000 km and 35,786 km
  • GEO: Geosynchronous Equatorial Orbit, aka Geostationary Orbit: a circular orbit exactly 35,786 km in altitude, in which a satellite remains over a fixed spot on Earth's equator
  • HEO: High Earth Orbit: above 35,786 km but still in Earth orbit
  • GTO: Geosynchronous Transfer Orbit: an elliptical orbit with LEO at its perigee and above GEO at its apogee, allows a GEO satellite to circularize its own orbit at the cost of some fuel
  • Polar orbit: An orbit that takes a satellite over the Earth's poles
  • Retrograde orbit: An orbit that goes against the Earth's rotation instead of with it
  • SSO: Sun-Synchronous Orbit: a special type of polar orbit that is always at the same time of day relative to the ground at each point in its orbit
  • LTO: Lunar Transfer Orbit: an orbit that intersects the Moon's orbit at apogee
  • NRHO: Near-Rectilinear Halo Orbit: an eccentric orbit planned for the Lunar Gateway to allow easy transfers between LTO and LLO
  • LLO: Low Lunar Orbit: A lunar orbit below 100 km in altitude, allowing for easy descent to the surface
  • Injection burn: Firing a rocket's engines to put it into a transfer orbit (GTO, LTO, etc.)
  • Circularization burn: Firing a rocket's engines to turn an elliptical orbit into a circular orbit
  • Deorbit burn: Firing a rocket's engines to bring its periapsis low enough to enter the atmosphere (if applicable) or hit the surface
  • Entry burn: Firing a rocket's engines to slow it down enough that it can enter atmosphere without being destroyed
  • Boostback burn: Firing a rocket booster's engines after ascent and staging to put it on a desired descent trajectory
  • Landing burn: For retropropulsive landings, firing a rocket's engines to bring it to a stop at the ground
  • Launch Abort: May be called during countdown at any time prior to liftoff if there is a problem preventing the rocket from safely flying
  • In-Flight Abort: An emergency situation in which a crewed vehicle must escape a rocket that is not operating properly
  • Launch Scrub: A total abort or cancellation of a launch, which can happen anytime between hours to mere seconds before liftoff due to a variety of factors (like weather or mechanical failures)


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Distances (How far away are things in space?)

Before we start, some notes on distances. Scifi Writers Have No Sense Of Scale, and this can sometimes even be considered an Enforced Trope when you need to write an exciting story and don't want to have to wait months or years for your characters to get anywhere interesting.

The point of all this isn't to intimidate you, by the way. It's to point out that if you're going to write a believable science fiction story, you need to accept these facts and work with them, intentionally ignore them (in which case you're probably writing Space Opera), or come up with some way to get places a lot faster than we currently can. A LOT lot faster.

    Distances 
  • Earth's radius is 6,371.1 km.
    • The tallest artificial structure ever built by humans is the Burj Khalifa in Dubai, which stands at a total height of 829 meters (just 171 meters shy of 1 km in the sky).
    • The highest point on Earth's surface is Mount Everest, about 8.8 km above mean sea level.
    • Most commercial aircraft fly at a maximum altitude of about 11 km.
    • The edge of the Earth's atmosphere is not a definite barrier, since it decreases in density gradually as altitude increases and there are detectable atmospheric particles thousands of km above the surface. However, "space" is commonly defined as beginning at around 100 km in altitude, known as the Kármán line. A human being reaching this altitude is considered an astronaut.
    • Technically, you could orbit the Earth at any altitude, at least as long as you don't run into something like a building or a mountain. But below the Kármán line, atmospheric drag will slow you down rapidly. Even above it, satellites have to expend propellant occasionally to maintain altitude against the constant braking due to drag.
  • To reach orbit, it's not enough to go up; you have to go sideways very fast. This is why rockets arc over as they ascend until they are almost entirely horizontal by the time they are out of the atmosphere. Objects in orbit are falling, still pulled by the Earth's gravity, but they are moving so rapidly that they miss the ground. (The Hitchhiker's Guide to the Galaxy is right!)
    • Most satellites hang out in what is called Low Earth Orbit (LEO). This is arbitrarily defined as anywhere from the boundary of space (100 km) to 2,000 km. LEO satellites typically remain above 250 km to prevent atmospheric drag from pulling them down. The International Space Station orbits at 400 km.
    • In LEO, each orbit of Earth takes about 90 minutes. It takes most rockets between 4 and 10 minutes to reach this orbit from the ground, depending on their design. Getting back to Earth from LEO takes about the same amount of time depending on trajectory and how much acceleration your spacecraft and its passengers can tolerate.
    • Geostationary orbit, also called Geosynchronous Equatorial Orbit (GEO), is exactly 35,786 km above the surface. A satellite in GEO will remain more or less above the same point on Earth's surface because its orbital period is exactly the same as the Earth's rotation.
  • Our Moon's orbit is slightly eccentric but averages 384,399 km from the Earth. That is ten times farther away than a geostationary satellite and sixty times the Earth's radius. It took the Apollo 11 astronauts 76 hours to go from Earth orbit to lunar orbit.
  • The next nearest major body to Earth is the planet Venus (not Mars). It can be as close as 40 million km and as far away as 261 million km, depending on the relative positions of the planets in their orbits.
  • Mars can get as close as 57 million km and as far away as 401 million km. At its closest approach, it is 150 times farther away than the Moon. At the same relative speed, the Apollo astronauts would have taken 470 days to get there. In fact, you'd go much faster on a Mars transfer orbit, but the minimum transit time with current propulsion methods varies from six to nine months.
  • One astronomical unit (AU) is the average distance from the Earth to the Sun, defined as approximately 150 million km, or 1.495978707×1011 m.
  • The farthest body visited by any human spacecraft is 486958 Arrokoth, a Kuiper belt body beyond the orbit of Pluto. It was imaged by the New Horizons probe on January 1, 2019. Its average distance from the Sun is 44.6 AU, or 6.6 billion km. New Horizons launched on January 19, 2006, so it took just under 13 years to get there.
  • One light-year (ly) is the distance light travels in one year. This is approximately nine trillion (9.46×1012) km.
    • You may hear the term "parsec" (pc) mentioned. This is defined as 3.26 ly and is a unit used by astronomers in parallax calculations. It is not a unit of time!
  • The distance to the nearest known star, Proxima Centauri, is 4.244 ly (40 trillion km). At the speed New Horizons is traveling relative to the Sun, it would take almost 100,000 years to get there if it were aimed in the right direction (it is not).
  • The distance to the center of the Milky Way galaxy is approximately 25,000 ly. If it were capable of getting there at all (it isn't), New Horizons would take about 577 million years to do so. The distance to the edge of the galaxy is about the same.
  • The observable universe is estimated to be approximately 93 billion ly in diameter. This is larger than the age of the universe because it is expanding, so something that emitted light 13 billion years ago would be ~45 billion ly away at the moment it reaches Earth.


Propulsion Methods (How fast can we go, really?)

So you want to get somewhere interesting in space. How quickly you get there, and whether you can get there at all, depends a lot on your propulsion method. First, the basics. Then we'll cover real propulsion methods, starting with the simplest and going on to more complicated and theoretical methods.

On Earth, you can push off of things to generate acceleration. These things include the ground, the water, and the air. Technically, you are taking advantage of static and fluid friction. In space, there is nothing to push off of or grab onto (with some extremely speculative exceptions at the edge of known physics). All objects must obey conservation laws; the most important being the law of conservation of momentum. The only way to make yourself move in one direction is to take some part of yourself and throw it in the other direction. This is, at a fundamental level, how all rockets work.

Momentum equals mass times velocity. In general, the faster you can throw something away, the more momentum is transferred and the more acceleration you get from the deal. It is generally easier to make lighter materials go faster. Thus, a rocket engine is literally throwing the lightest stuff possible (gas particles) away from the rocket as fast as possible. What you choose to throw and how you make it go fast defines the type of engine you are using.

We will start with the simplest engines: compressed gas, then work our way through chemical-thermal, nuclear-thermal, electric-thermal, ion, and finally exotic drives. Note that all but the "exotic" drives are either in use or are proposed for use, with substantial engineering work already done. First, however...

The Rocket Equation

All rockets are limited by the amount of propellant they can carry. Propellant is heavy: a typical orbital rocket is over 90 percent propellant by mass. The more propellant you have, the more powerful your engines have to be to lift it and the larger the rocket will be. Make a bigger rocket and you need more propellant to get the same delta-V. This fundamental principle, enshrined in the Tsiolkovsky rocket equation, tells you the maximum delta-V you can get out of any particular rocket. The choice of engine (and thus propellant) is extremely important because it gives you the parameters for the equation.

To get off of a planet, you need enormously powerful engines, since you need to overcome gravity (and atmosphere, in many cases) simply to get high enough to reach orbit. However, once you are in orbit, you can use less powerful but more efficient engines to get to your ultimate destination.

Compressed Gas Engines (aka Inert Gas Thrusters)

If you've ever filled a balloon with air and released it without tying the end, you've seen a compressed gas rocket engine. Of course, a balloon can only hold so much air before it bursts. The compressed gas engines (more commonly termed "thrusters") found on rockets and spacecraft use tanks ("pressure vessels") that can withstand hundreds of times Earth's atmosphere.

Since pressure wants to move from high to low, the compressed gas (typically nitrogen) will rapidly escape through the nozzle once the valve is opened. Because of the extreme simplicity and fast response time of such rockets, they are often used for maneuvering systems, which need to be as light as possible. However, it is impossible to reach orbit on compressed gas because the efficiency is too low.

Another type of inert gas engine is the steam rocket. These are not talked about often for a reason: steam is a very poor propellant for launching from Earth. That said, steam has been proposed as a propellant for interplanetary rockets, using water ice found on asteroids and moons to refuel.

  • The first and second stages of the Falcon 9 rocket use nitrogen cold gas thrusters for maneuvering once they are out of the atmosphere and their engines are shut down.
  • Perhaps the most well-known steam rocket of the late 2010s is the one flown by "Mad" Mike Hughes in his efforts to prove the Earth to be flat (possibly for publicity). His attempt to launch such a rocket in February 2020 resulted in his death.

Chemical Thermal Rocket Engines (aka "Conventional" Engines)

Compressed gas is cheap and simple, but not very efficient, and difficult to store at extremely high pressure. What if, instead, we took a liquid or solid with high energy potential and ignited it? The resulting gas would expand extremely rapidly, and we can use that to get thrust. This is basically how chemical rocket engines work. Of course, there's no oxygen in space to sustain combustion, so it is also necessary to bring along an oxidizer.

There's still the problem of how you get the propellant, under pressure but not extremely so, to go through a nozzle at very high velocity. Pressure goes from high to low, so if we just burn it, part of it will go back into the fuel tanks, right? So let's discuss the three main ways to accomplish this.

Solid Fuel Rocket Motors

Solid-fuel rockets are typically referred to as "motors" rather than "engines", since there are virtually no mechanical parts and there's no combustion chamber. A solid-fuel rocket packs the fuel and oxidizer into a mixture that is stable until ignited under particular conditions. Think of it as a relatively slow-burning explosive... in fact, some missiles literally use properly prepared mixtures of explosives as propellants. The most popular solid fuels in modern space rockets involve a mixture of synthetic rubber with ammonium perchlorate and powdered aluminum. The binder provides structure and fuel, perchlorate an oxidizer, and aluminum is a dense, high energy fuel.

The solid motors we see on full-scale rockets typically involve a casing with the propellant mixture packed into a specially-shaped mold with a channel down the center. They ignite from the top down and the channel forces the hot gas out a rocket nozzle. This nozzle may be steerable, as on the Space Shuttle.

There are examples of solid fuel rockets everywhere, from the AJ-60A motors on the Atlas V rocket to the solid rocket boosters (SRBs) on the Space Shuttle to the Minotaur IV, which is a rocket with four solid fuel stages. Solid fuel rockets have extremely high thrust-to-weight ratios and are thus ideal for accelerating extremely rapidly. However, they don't generate chemical energy as efficiently as liquid fuels, meaning they can get to orbit but not much more. Solid fuels have the advantage of being easy to store; there are usable solid rocket boosters dating back decades in the U.S. inventory.

Pressure-Fed Engines

A pressure fed rocket engine uses inert, pressurized gas to force the fuel and the oxidizer down toward the combustion chamber where they can be mixed and ignited. Pressure fed engines are not as common as the other kinds of conventional rockets, but one example was the Quad Rocket developed by Armadillo Aerospace.

Pressure-fed engines are also used in the emergency abort systems of some crewed rockets, such as the SpaceX Dragon 2. This is due in part to their near-instantaneous response, with no need for turbopumps to spin up. They also use hypergolic propellant to avoid the need for separate ignition systems. For low-thrust engines, the pressure of the propellant tanks alone may be enough to supply their needs. This type of engine, again burning hypergolics, is used in the maneuvering systems of almost all spacecraft.

Pressure-fed engines are limited by the pressure that can be stored in either the main tanks or the pressurizer tanks, and are typically not efficient enough to be used as the main engines on an orbital rocket.

Pump-Fed Engines

Rocket engines are all about managing pressures. Pressure flows from high to low, and so the maximum power you can get out of an engine is based on the difference between your tank pressure and the exterior pressure. Unless, that is, you use a pump, which can force propellants into the engine at extremely high pressures — in some cases, hundreds or even thousands times that of Earth's atmosphere.

However, you need something to turn the pumps, and that something has to be powerful. There are two primary kinds of pumps: turbopumps, which work by burning some of the fuel and oxidizer to make what is in effect a miniature rocket, the force from which turns the turbines that power the pumps; and electric pumps, which are driven by motors that are in turn powered by batteries. Batteries are heavy, and unlike propellants don't lose mass as they discharge, note  so they are only suitable for smaller rockets.

There are many, many types of turbopumps. When you hear references to "expander cycle," "staged combustion cycle," "tap-off cycle," "gas generator cycle," and similar things, they are all talking about different ways to run a turbopump while obtaining the maximum efficiency from the combustion of the fuel and oxidizer, all while minimizing mass and cost. A complete discussion of these is beyond the scope of this article.

  • Turbopump-fed engines go back a long time: the infamous V-2 Rocket of World War II used them in its design.
  • Electric pump-fed engines are less common: the best-known modern example is the Rutherford engines powering the Electron rocket.

Liquid Rocket Propellants

This is a brief digression to discuss the various liquid propellants that are or have been used in rockets. Most of these need an additional ignition source, which could be an entire article on their own. Hypergolics are a subcategory consisting of two chemicals that spontaneously ignite when they come into contact with each other. This is a valuable advantage, especially for rockets that must be restarted often, but they are often highly toxic and thus very difficult to work with.

Many, many chemicals have been tested for Liquid rockets, the book Ignition describes them in detail. The propellant chosen is a tradeoff between exhaust velocity, density (a denser propellant means smaller tanks, pipes, etc. are needed for the same thrust and delta V), handling characteristics (storage, toxicity, ease of loading, among others), and random other engineering characteristics.

  • Ethanol. The first suborbital liquid-fueled rockets used this. It is relatively inefficient and was eventually discarded in favor of other alternatives, it did burn cooler and provide superior regenerative cooling, easing the engineering of early rockets. It also had the unique qualification of being consumable by humans.
    • The German V-2 rocket used ethanol as its propellant.
  • Hydrazine family (hypergolic). There are several variations including monomethyl hydrazine, symmetric dimethyl hydrazine, and unsymmetric dimethyl hydrazine (UDMH). Highly toxic, but easy to store, so it can be loaded into a spacecraft and stays stable for long time periods.
    • Hydrazine is extremely common as a fuel, but one example of its use includes the Apollo Lunar Lander.
  • Hydrazine monopropellant. Hydrazine by itself can decompose to produce lots of hot gas for a rocket. The exhaust velocity is lower than a bipropellant, and requires an ignition source, but only half the number of tanks, pipes, and such are needed. Used in some reaction control systems, but not powerful enough for main propulsion.
  • RP-1 or rocket-grade kerosene. This is essentially a highly refined jet fuel and is extremely common and cheap. It is liquid at room temperature, requiring no special storage, and so a fueled rocket can remain on the pad for days if necessary. It is more efficient than hypergolics but less than methane or hydrogen. The main problem with kerosene is its tendency to "coke", or generate long polymer chains that stick to the insides of engines and gum them up. This makes reuse of kerosene engines difficult. Rocket grade gasoline or diesel fuel could in theory be used, but would perform about the same as kerosene so there is no point in developing them.
  • Hydrogen. This is the most efficient chemical fuel that we can reasonably use (higher theoretic efficiency can be gained from other chemicals, but they are so volatile and or toxic that they're not worthwhile). It is the lightest and must be chilled to 20 Kelvin to become liquid, so storing it is extremely difficult. It also has a tendency to escape from any tanks it is held in, so is not suitable for long-duration missions. It is also very low density, so reasonably sized engines have a hard time generating enough thrust to take off: hydrogen engines are either restricted to second or above stages, or are combined with solid or liquid boosters for the first part of a launch.
    • The second and third stages of the Saturn V Rocket used hydrogen propellant, as did the Space Shuttle Main Engines.
  • Methane (refined natural gas). Occupying a happy medium between kerosene and hydrogen, methane is lighter than kerosene, does not coke at typical rocket temperatures, burns cooler than hydrogen, is easier to store than hydrogen, and has an efficiency somewhere higher than kerosene. In return, it has a somewhat lower density than kerosene, and is cryogenic. For various reasons, including supply and the widespread adoption of kerosene, methane has not been used as a fuel until very recently. The lack of coking is a big draw for reusable rockets.
    • The best-known use of methane is the Raptor engine which is intended for use on SpaceX's Starship rocket. A major factor in this design decision is that liquid methane and oxygen can be produced on Mars relatively easily using the Sabatier reaction.

All bipropellant fuels must be burned with an oxidizer. Two are in common use:

  • Liquid Oxygen: The most powerful oxidizer available that isn't toxic, explosive, or too difficult to handle in some other way. Easily available from the air around us. However, being cryogenic, it is difficult to store for long periods. It is used in launch vehicles with all the fuels listed above except hydrazine.
  • Dinitrogen Tetroxide: The most powerful hypergolic oxidizer, apart from some fluorine based ones that would produce toxic combustion products. Like its partner hydrazine, it is highly toxic but easy to store; it can be loaded into a tank using proper procedures and remain stable. Decomposes to red nitrogen dioxide, producing red clouds when rockets that use it start.

As mentioned above, other chemicals have been used or tested in the past. Some worked, but are less powerful than the above combinations, such as nitric acid and hydrogen peroxide as oxidizers. Many were more powerful than the listed combinations, but other difficulties mean they weren't used in production rockets: the fluorine based oxidizers mentioned above that produce toxic exhaust are one example. The book ignition linked above describes the difficulties in detail.

Metallic hydrogen has been proposed as a rocket fuel with many times the energy density of liquid hydrogen, but actually creating it and storing it stably is the stuff of fiction at the moment.

References

  • Most of the engines in Kerbal Space Program use chemical propellants and are modeled after real-life equivalents from across spaceflight history. For gameplay abstraction purposes, the game only has one type of generic liquid fuel and oxidizer each (though there are mods that remedy this); what they are is not explicitly stated anywhere in-game. Although liquid fuel is likely to be kerosene due to the fact that it's also what the game's jet engines run on, it's also something that can be synthesized off-world and doubles as fuel for a nuclear thermal rocket (see below).
    • The Realism Overhaul mod does represent actual real-life propellants and their respective densities, and even simulates the boil-off effect for cryogenic fuels like liquid hydrogen.
    • One interesting engine concept arising from rocket and jet engines using the same fuel in the game is the CR-7 RAPIER (Reactive Alternate-Propellent Intelligent Engine for Rockets) engine, inspired by the real-life SABRE concept, which is capable of using both atmospheric air and oxidizer for operation. It is less efficient than the dedicated jet engines and is a Jack-of-All-Stats among bipropellant engines as well, but being able to operate as both makes it an all-in-one engine for spaceplanes.

Electric propulsion (Ion engines)

Ion engines, or Ion thrusters, are in a completely different class from chemical rocket engines. The basic principle of an ion engine is that an inert gas such as xenon or krypton is excited by an electric field and loses some of its electrons. The ionized gas can then be accelerated using a magnetic field to extremely high velocities. In contrast to chemical rockets, here a heavier molecule is more efficient since it carries more momentum at any given velocity. However, xenon (the heaviest noble gas) is extremely rare and thus very expensive.

While ion engines can have fuel efficiency many times that of chemical rocket engines, they have very low thrust, making them suitable only for maneuvering once a spacecraft has reached orbit. For this reason, they are also unsuitable for crewed spacecraft as their thrust is so low that it would take a long time to get anywhere. It is possible that future technology will improve ion engines to the point where they are practical for human interplanetary travel, but they will probably never be used to get off a planet into space.

Ion engines are used by many satellites today, as their exceptional fuel efficiency allows satellites to remain in orbit for many years on a relatively small fuel budget. One example is SpaceX's Starlink satellite constellation, which uses krypton ion thrusters.

References

  • In certain parts of the Star Wars franchise, it is claimed that spacecraft use ion engines as their main propulsion source. This implies an astonishing breakthrough in ion drive technology, assuming the writers aren't just picking cool words out of a dictionary.
  • Kerbal Space Program allows the player to unlock and use the IX-6315 Dawn ion engine near the end of the tech tree. Although it still has very low thrust compared to bipropellant engines, it is orders of magnitude more powerful than real-life ion engines due to practical reasons: the game does not allow time acceleration while any engine is burning, so a realistic ion engine would require the player to sit through hours-long maneuver burns in real time. This way, ion engine burn times are measured "merely" in tens of minutes.

Light sails, solar sails, and laser sails

Photons have no mass, but carry momentum. The quadrillions of particles blasted off the Sun every second that form the solar wind also carry momentum (and mass, but we digress). These can be harnessed by building a giant sail, literally. Demonstrations of these technologies have been made several times as of 2020, but as yet no operational spacecraft has used them.

A solar sail is designed to capture the flow of high-energy particles from the solar wind. A light sail is designed to capture the photons emitted by the Sun. A laser sail takes a light sail a step further by using a directed laser beam from a planet or satellite to generate thrust.

While solar and light sails have extremely low thrust and are impractical for human spaceflight, laser sails (powered by gigawatt laser arrays) have the potential to send our first relativistic spacecraft to other stars, being able to achieve velocities up to 10 percent of the speed of light, all without carrying or expending a drop of fuel, since their propulsion comes from an external source.

The Breakthrough Starshot program, still in its earliest stages, is a proposal to build thousands of laser sail probes, each no larger than a postage stamp, and boost them off to our neighboring stars to send back information about what's there. Even the simplest of these probes would require the construction (and powering) of lasers thousands of times stronger than anything we can currently build.

Another issue with sail-powered spacecraft is that it's not as easy to slow down once you get where you are going, requiring either an alternative propulsion system or creative use of "tacking". Robert Forward proposed a means for decelerating an interstellar light sail in the destination star system without requiring a laser array to be present in that system. In this scheme, a smaller secondary sail is deployed to the rear of the spacecraft, whereas the large primary sail is detached from the craft to keep moving forward on its own. Light is reflected from the large primary sail to the secondary sail, which is used to decelerate the secondary sail and the spacecraft payload.

The best-known solar sail project to date is the Planetary Society's LightSail spacecraft. LightSail-2 was launched on a Falcon Heavy rocket in June 2019 and successfully demonstrated photonic propulsion in low Earth orbit.

Nuclear pulse propulsion (Orion drives)

This propulsion system was proposed in the earliest days of the space program and even tested (at smaller scales and without using nuclear bombs). Put very simply, an explosion generates force, so you can propel a spacecraft by detonating a series of bombs beneath a large shield. Some of the kinetic energy of the detonations will push against the shield, creating thrust.

Nuclear pulse propulsion has the theoretical capability to achieve significant fractions of the speed of light and is one of the few systems discussed here, achievable with current technology, that could provide "constant" thrust for long durations. Modern designs would not use large nuclear bombs, but rather a number of small pellets, each of which creates a small thermonuclear pulse.

To date, no nuclear pulse spacecraft has flown and international treaties against the deployment of nuclear arms in space may restrict the development of such a spacecraft for a long time to come. There's also the understandable problem that you wouldn't want to use it to get to space, or even in low Earth orbit, given the "minor" problems associated with setting off nuclear explosions in or near the atmosphere. Such a drive system would also require the construction of hundreds or thousands of times more nuclear bombs than currently exist today, with understandable political ramifications.

The Orion drive is the only method of getting to another solar system within a human lifetime that we could potentially achieve with current technology.

Nuclear thermal propulsion (fission and fusion drives)

This is the last category of propulsion system in this list that we have the current technology to attempt (in principle, and with fission). In essence, the spacecraft carries a nuclear reactor that runs on some fuel source, like uranium/plutonium (for fission) or deuterium/tritium (for fusion). This reactor produces an extremely high energy flux, which can be used to accelerate an inert propellant to far greater velocities than chemical rockets. Typical designs use water for its ease of storage, relative chemical inertness, secondary use as coolant and for drinking, and relative lightness. The most efficient possible designs would use hydrogen.

Nuclear thermal propulsion achieves extremely high theoretical efficiency, with fission being up to three times better than chemical propellant and fusion much more. Such drive systems would allow casual interplanetary travel, but would still need far too much fuel to be able to manage interstellar travel in a reasonable time frame. Fusion drives could operate under constant thrust for a significant portion of a trip, giving human passengers a semblance of gravity.

The major obstacles to such drives are:

  • As with nuclear pulse drives, the idea of putting a full-scale nuclear reactor in space is a little intimidating to many nations.
  • They would probably not have enough thrust to get to space, never mind the problem of spewing highly radioactive exhaust into the atmosphere.
  • The amount of fuel needed could be seen as an escalation of nuclear proliferation and would be very expensive regardless.
  • While we can build fission reactors, making them small enough and foolproof enough to put on a rocket is another matter. We don't yet have working fusion reactors and probably won't be able to build anything remotely small enough to put on a rocket for at least fifty years.

References

  • 2001: A Space Odyssey and its sequels feature spacecraft with nuclear drives. The Discovery in 2001 uses nuclear thermal propulsion of a unspecified type, requiring vast tanks of hydrogen to sustain. The Leonov in 2010 uses muon-catalyzed cold fusion, a real idea dating back decades, but one that cannot be run at a net energy gain with our current understanding of physics. The Universe in 2061 uses an even more advanced version that can sustain fusion with almost any fuel, including water, and indeed a plot point involves siphoning water from Halley's Comet to enable a direct flight to Jupiter.
  • In The Expanse, the invention of the "Epstein drive", a variation of nuclear thermal propulsion, enables rapid interplanetary travel. The liberty taken here is that Epstein drives have a mass-energy efficiency far beyond that of any known fusion process, letting ships travel without the gigantic fuel tanks that would otherwise be necessary.
  • Kerbal Space Program has the LV-N Nerv Atomic Rocket Motor, which is a fission version of this. Large, heavy and too weak to lift even half its own weight under Earth's gravity, but its fuel efficiency in vacuum is over double of even the best bipropellant liquid fuel engines, making it a very popular choice for interplanetary flight among players. For gameplay abstraction purposes, it runs on the same liquid fuel as bipropellant engines, it just doesn't require oxidizer, thus saving mass for carrying more fuel.

Magnetic scoop fusion drive (Bussard ramjet)

This hypothetical propulsion system is based on the fact that the vacuum of space is not actually empty. It contains extremely diffuse particulate matter, mostly individual hydrogen atoms, with an average density of about one atom per cubic centimeter, called the "interstellar medium". This can potentially be used as fuel.

A "Bussard ramjet" spacecraft would be equipped with a magnetic "net" hundreds or thousands of kilometers in diameter, scooping the interstellar hydrogen into fuel tanks which would then power a fusion reactor. By definition, this would be a constant thrust craft and thus suitable for interstellar exploration.

Such a ship would need to travel relatively fast to be able to gather enough hydrogen to power itself, needing a boost or kick to get started, but it would also encounter significant drag from all of those particles hitting the net. From the scientific literature on the topic it is not clear if the drag would be greater than the actual thrust that could be obtained from the collected fuel.

References

  • Most spacecraft in the Wing Commander universe use Bussard ramjets to gather enough fuel to sustain their high acceleration and one of the EU novels has a subplot in which a ship "stalls" by being forced to go too slowly, taking several weeks to crawl back up to a useful velocity. The canon video games make no mention of this, however.

RF resonant cavity thruster (aka the EM drive)

This hypothetical propulsion system supposedly uses aspects of the quantum vacuum to provide a small thrust without an external source of propulsion and without expending any propellant. While prototype drives have been tested on several occasions in ground laboratories, and even supposedly in space by the Chinese, they violate the physical law of conservation of momentum and have thus far failed to demonstrate any real thrust that is not explained by other phenomena.

In short, the EM drive doesn't work, although people continue to hope that it might.

Antimatter drives

This proposed propulsion system uses the most efficient possible fuel: antimatter. While nuclear fission converts up to 0.1 percent of the rest mass of its fuel into energy, and nuclear fusion converts up to 0.7 percent into energy, antimatter-matter collisions convert 100 percent of their combined mass into energy. In principle, a rocket powered by antimatter (probably using some sort of thermal acceleration system similar to that of a fission or fusion engine) would need only kilograms of fuel to achieve the same impulse as tons of fusion or thousands of tons of chemical propellants and could easily get to other stars while providing constant thrust for a comfortable 1 G living environment.

In practice, however, getting the antimatter is a bit tricky. We currently produce and store positrons and anti-protons in large particle colliders like CERN, and a recent breakthrough (as of 2020) involved the creation of a complete antimatter atom. Yes, one atom. To produce the quantities of fuel required to even get to orbit from the ground would require trillions of dollars and entire national energy budgets worth of power.

To achieve antimatter propulsion, we need a currently unimaginable breakthrough in particle physics that would allow us to manufacture and store it at scale. It is the stuff of science fiction dreams, no less than a century away even in the most optimistic future.

References

Kugelblitz drives (black hole power!)

The most exotic propulsion system ever conceived by physics (so far), a black hole engine (aka Kugelblitz drive) would literally be powered by a tiny black hole. Moreover, we believe that we know how to build one. Light does not have mass, but it does have energy. Using the mass-equivalence principle in Einstein's special relativity, if you concentrate enough light energy (via lasers) into a small enough space, it would warp spacetime in exactly the same manner as a high concentration of matter, potentially collapsing into a black hole.

This tiny black hole would, in turn, emit Hawking radiation, a form of quantum leakage from its event horizon that occurs because of the black hole cutting off certain frequency modes of the quantum vacuum. note  The smaller the black hole, the faster it emits such radiation, so this suggests an ideal size that would potentially power a spacecraft for years before evaporating.

To make a Kugelblitz, you would need hundreds of gigawatts of laser energy all focused into a space smaller than the width of a proton, firing at exactly the same moment. If you miss, you get, well, a lot of laser energy flying around. If you succeed, you get a tiny black hole that could be captured (somehow) and used to provide virtually unlimited power over its lifespan. Its radiation would be used to accelerate a propellant (probably hydrogen) in the same way as a nuclear thermal engine, or its radiant energy could be captured with a reflector and turned into a beam for photonic propulsion.

Alcubierre drives (warp speed!)

Perhaps the ultimate expression of theoretical physics, the Alcubierre drive does not describe a power source but rather a propulsion method that is based on mathematics arising from the theory of general relativity. GR establishes that nothing can move faster than light through spacetime, but it does not establish that spacetime itself cannot move faster than light. Indeed, right now there are regions of space that are receding from us faster than light, such that no information emitted from them will ever reach us.

The Alcubierre warp metric, as it is technically known, involves creating a field of warped spacetime (hence the name) in which space ahead of a ship is compressed while the space behind it is expanded. An object within this "warp bubble" would be carried along much like a surfer on a wave, experiencing no acceleration. As there is no limit as to how fast any patch of space can move relative to any other patch, this velocity could exceed the speed of light, allowing FTL Travel.

As you may imagine, however, the idea has a few problems. First, we don't have anything remotely like the technology to compress spacetime on the scale needed. Imagine a Kugelblitz style drive but with the lasers mounted on the craft itself, or a gravity drive (see below). Second, we don't even know if it's possible to expand spacetime behind the craft. In most formulations, this would require some kind of "exotic matter" with negative mass, which is not believed to exist. (If it did, you could build perpetual motion machines and other lunacy, not just warp drives.)

Still, the Alcubierre drive remains the proposal for high-speed or FTL travel that has the best correlation to known physics, and there is some hope that it may one day become technologically possible. NASA takes it seriously enough that its EagleWorks laboratory is experimenting with the idea on very small scales.

References

  • Star Trek explicitly uses the Alcubierre concept in its warp drive technology. Of course, it adds a helping of technobabble to turn it into a coherent plot device. Warp engines in Star Trek are powered by antimatter (see above).

Wormholes (jump drives)

These are less a propulsion system than a proposed means of achieving FTL Travel. Based on another mathematical artifact in the theory of general relativity, a wormhole is a hypothetical bridge (technically, an "Einstein-Rosen bridge") between two distant points in spacetime, like folding a piece of paper over on itself and punching a hole through both sheets. Indeed, this is the analogy used in literally every work that features it.

If a spacecraft could somehow create wormholes on demand (or access natural or artificial wormholes created or maintained by some technology), it could "jump" across spacetime without traversing the distance between the two points. Of course, the physics of general relativity also say that wormholes are unstable, lasting only fractions of a second and creating event horizons that "spaghettify" anything crossing them. It doesn't do you much good if your spaceship is swallowed, stretched out into a thin stream of atoms and disgorged as a cloud of undifferentiated subatomic particles.

There are ideas to deal with these problems, but all concepts of traversable wormholes require "exotic matter", much like Alcubierre drives: something with negative mass-energy that could be used to stabilize or prop them open, and there are no reasonable proposals for creating them to begin with. However, the idea of using wormholes for travel remains a staple of Space Opera and even some science fiction.

References

  • The sci-fi horror film Event Horizon is set on a ship that attempted to use a prototype jump drive to travel out of the solar system and back. It turns out that the alternate dimension that it crosses through is analogous to the Biblical concept of Hell and opens a doorway into our universe for pure evil.

Gravity drives

Now we are moving from "maybe physically possible" into "there is no current theory that would allow this, but it would be really cool". If we can manipulate the force of gravity itself, we would achieve godlike power, one of the simplest manifestations of which would be to propel spacecraft to any speed we could desire (up to the speed of light, of course).

In its most basic form, a gravity drive involves creating a strong artificial gravity well near a spacecraft. This gravity well would attract the ship to it, whereupon the ship moves the field farther ahead, and so on, like a carrot in front of a mule. This would violate conservation of momentum, but is already so far beyond anything in current physics that you might as well not worry about it.

References

  • The Humanx Commonwealth novel series by Alan Dean Foster envisions the "posigravity drive" which uses exactly this principle to achieve FTL Travel. Co-discovered by the humans and Thranx, the two species working together come up with an improved version called the KK drive. The hypergenius Ulru-Ujurrians later modify the KK drive to be able to land on and take off from a planet without tearing its surface apart.


Real-world rockets and rocket engines

It's time for an inventory of the rockets and rocket engines that are in use or have been used in the past. We won't cover everything here, just the more famous ones with some notes about their design and significance.

Specific types of rocket engines

Bell nozzle engines

The most common type of rocket engine involves a combustion chamber (with a bunch of stuff behind it) forcing high-pressure gas through a nozzle shaped roughly like a bell. There is a common belief that the nozzle itself is the engine; this is untrue. The nozzle, however, is a critical part of the overall design, because it acts to increase the velocity of the exhaust by reducing its pressure. It also balances the pressure of the exhaust with the pressure outside the rocket.

If the escaping gas is at much higher pressure than the surrounding air (or vacuum), it expands rapidly to the sides once it exits the nozzle, costing a lot of power — you want as much exhaust as possible going straight backwards. If it is at a much lower pressure, the surrounding air pushes inwards and back up the nozzle, causing "flow separation" instability that can destroy the engine. This is why most vacuum-optimized engines can't be fired at sea level.

Since rocket boosters in particular have to operate in these two very different regimes: a gradient from sea-level pressure all the way up to vacuum, you will often see two different kinds of engines in use: sea-level-optimized and vacuum-optimized. The latter have much larger nozzles meant to reduce the exit pressure as near to zero as possible, while the former attempt to achieve a happy medium between pressure at the ground and pressure in space.

Rocket nozzles also have to be cooled because of the extreme heat of the exhaust gases they have to contain, or they could melt or crack under the stress. There are several types of cooling in use:

  • Regenerative, where some cold fuel is circulated through the walls of the nozzle and combustion chamber before being pumped into the engine proper. This has the side benefit of heating the fuel up to improve combustion.
    • The Merlin 1D engine on the Falcon 9 uses regenerative cooling for the sea-level portion of its nozzle.
  • Ablative, where the nozzle contains or is made of a substance that breaks away as it heats up. Such nozzles are inherently unable to be reused.
    • The RS-68 engine on the Delta IV uses ablative cooling, making its exhaust slightly orange in color when it would otherwise be mostly blue, as it it is composed almost entirely of water vapor.
  • Film, where some amount of unburned or partially burned fuel is allowed to run along the walls of the rocket nozzle to absorb excess heat.
    • The F-1 engines on the Saturn V used film cooling, easily visible in close-up images as a darker part of the exhaust.
  • Radiative, where the nozzle is designed from a high-temperature alloy that radiates heat as rapidly as possible.
    • The Merlin 1D engine on the Falcon 9 second stage uses a combination of film and radiative cooling on its niobium nozzle extension. It can be seen glowing bright orange in video.

Aerospike engines

The aerospike is a type of rocket engine designed to be used at both sea level and in vacuum without losing efficiency. Such an engine, if it were to be built and operated, would make single stage to orbit (SSTO) rocket designs feasible. Unfortunately, no aerospike engine has ever successfully propelled a production rocket, as all such attempts have failed, been abandoned, or are still in development.

Simply put, an aerospike works by removing most of the rocket nozzle and aiming the exhaust flow towards a spike or linear surface that forces it to go in the proper direction. The other side of the exhaust flow is contained by air pressure, initially squeezed into the surface of the "spike" and later expanding outward, but always ending up in a linear flow.

The challenges involved in building an aerospike include the increased complexity and mass of the engine, the difficulty of steering the engine, and most importantly, the extreme difficulty of cooling the nozzle.

Multiple rockets using aerospike engines have been proposed, developed, and or tested to varying degrees. Among these are the Firefly Alpha, ARCA Space's Haas 2CA, and the VentureStar SSTO. An aerospike design was considered for the Space Shuttle Main Engine before the traditional bell design was adopted.

Others (scramjet, VASIMIR, etc.)

Rocket stages and SSTOs

The rocket equation

Properly named the Tsiolkovsky rocket equation, and colloquially referred to as "the tyranny of the rocket equation", this is perhaps the most well-known equation in rocketry, although it doesn't exactly flow off the tongue when written out. It tells you how much delta-V you can achieve with a given mass of fuel and a given mass of rocket.

Put simply, to get a rocket to go farther (add delta-V), you need more fuel (propellant). Adding fuel adds mass, both in the fuel itself and the tanks to hold it. If you add mass, you need more (or more powerful) engines to push it all. Adding engines also adds mass, meaning you need more fuel to get the same delta-V. For any given fuel source and engine, this equation reaches a maximum whereby you can add infinitely more fuel and engines without getting any more performance.

A major factor in the equation is the efficiency of the engine. This is the maximum energy you can get out of the propellant, taking into account its chemical properties and also the performance of the engines used to accelerate it. The technical term for this is "specific impulse", written Isp or just "isp", and is typically notated in seconds (defined as the theoretical time that a rocket engine can operate, expending one kilogram of fuel to maintain 1 Newton of acceleration).

  • The Space Shuttle Main Engines (SSMEs), burning hydrolox, have a specific impulse in vacuum of 453 seconds.
  • The Merlin 1D engine used by the Falcon 9 and Falcon Heavy rockets, burning kerolox, has a specific impulse in vacuum of 348 seconds when using a vacuum optimized nozzle extension.

Multi-stage rockets

Almost all orbital rockets use multiple stages. The idea behind this is that, once a portion of the rocket's fuel is expended, the heavy tanks and engines can be discarded and a smaller, lighter rocket will do the rest of the work. There is a trade-off between efficiency and complexity - in theory, you could have as many stages on a rocket as you could physically build, but since each stage needs engines and other hardware that take up space that could be used by propellant, you reach a point of diminishing returns.

Another advantage of staging is that rocket engines perform differently in the vacuum of space than they do in Earth's atmosphere. Near the ground, you need powerful, high-thrust rockets optimized for use in atmosphere to push through it and into space. Once in space, however, you can use more efficient vacuum-optimized engines to push the spacecraft into orbit without worrying about atmospheric drag (not as much anyway).

Reusing the parts of a multi-stage rocket is challenging because each stage must contain its own systems for reentering and landing.

Single-stage rockets

A single-stage-to-orbit, or SSTO, is considered by some to be a Holy Grail of rocket design. You only need one vehicle to get to orbit, deploy a payload, and return to land.

As yet, no single stage rocket has been able to reach orbit, and this is due to a variety of factors including the lack of suitable engines (see above for the difference between bell nozzle and aerospike engines) and the substantial differences between aerodynamic and vacuum regimes. A vehicle that is designed to move through atmosphere has very different technical requirements than a vehicle designed to move through space, and accommodating both in the same vehicle adds mass and complexity that reduce performance.

Part of the drive for SSTOs is the cost savings from not having to throw away parts of the rocket on every flight. However, continual reductions in the cost of delivering payload on multi-stage rockets have all but eliminated the economic argument for them.

Proposed and attempted SSTO vehicles include the Skylon, the DC-X, the Lockheed Martin X-33, and the Roton SSTO.

Reusable rockets and spacecraft

One of the things that makes getting to space hard is the cost. Part of this is just the sheer effort needed to put stuff in orbit, but another part is the fact that most rockets are expendable. It has been noted that rockets are the only transportation method that we throw away after each use. If you drove a car from one city to another and it was immediately thrown in a crusher, forcing you to buy a new one, nobody would drive anywhere. If a new passenger aircraft had to be built for every flight, it would be too expensive to fly.

Obviously, one way to cut down the cost of spaceflight is to reuse rockets. This is harder than it may seem at first glance, though. An orbital rocket needs as much fuel as possible to do its job, leaving little left for a landing. Also, it's designed to go to space, not to fall back down to the ground. You can over-design your rocket to leave enough margin to recover it, but then you lose some potential payload.

There are also political considerations around reusable rockets. State-sponsored space programs insulate themselves from politics to some extent by spreading their supply chains over many districts to engage as many constituents as possible and guarantee high-quality jobs. Every rocket reused is a rocket not built and potential money not being earned by voters.

Nevertheless, if we want to get to space cheaply, we must find ways to reuse rockets. There are several methods that have been proposed and attempted.

Suborbital rockets

One way to keep your margins high enough for landing is not to go to orbit at all. Such is the case with suborbital spacecraft, which don't need the high performance of orbital rockets because they use much less energy. Suborbital rockets (also known as sounding rockets) have their uses: monitoring weather, delivering experiments that need brief periods of free-fall, point to point transportation, and and space tourism.

Suborbital rockets can land in a number of ways, including aerodynamically (wings and runways), via parachutes, or propulsively by firing their engines to come to a stop as they touch down.

Some examples of reusable suborbital rockets include the Blue Origin New Shepard (propulsive) and Virgin Galactic's SpaceShipTwo (aerodynamic). The SpaceX Starship is designed for orbital operations but a suborbital version is also planned for point-to-point transport. It will land propulsively.

Spaceplanes

A spaceplane is a vehicle shaped like an aircraft that is designed to reach orbit, then descend, reenter the atmosphere, and use aerodynamic surfaces to control their descent for touchdown on a runway or landing strip. Lift surfaces are pointless in space, of course, but they can increase surface area to reduce the stress on a vehicle from reentry heating.

Spaceplanes are launched on traditional rockets, which may or may not be expendable. Generally, the spaceplane will act as its own propulsive stage, either taking the place of a second stage or deploying as payload and then firing its own engines to modify its orbit.

Notable spaceplanes include NASA's Space Shuttle, the Soviet Buran (which never flew crew), the Boeing X-37b (an uncrewed orbital test bed for the U.S. Department of Defense), and the Sierra Nevada Dream Chaser (which has yet to fly).

Parachutes

If you look at the history of spaceflight, most spacecraft intended to return to Earth have used parachutes to accomplish that task. This includes all space capsules used for human flight with the exception of the spaceplanes mentioned above. After the vehicle reenters Earth's atmosphere, it uses air resistance to brake, then it deploys one or more leader parachutes (known as "drogues") to provide initial deceleration, followed by one or more primary parachutes (known as "mains") to slow it down for a safe landing. Parachute landings may occur on land or on water.

Parachute designs are thus common, but it is worth noting those that accompany vehicles intended for refurbishment and reuse. These include Boeing's CST-100 Starliner, the SpaceX Dragon, NASA's Orion, and in the future the first stage of the Rocket Lab Electron rocket and the engine block of the ULA Vulcan rocket. The Space Shuttle's solid rocket boosters also descended via parachute and were recovered from the ocean, but it was acknowledged some time into the program that it would have been cheaper to build new ones than to refurbish them.

Retropropulsion

All of the above are interesting and effective ways to reuse rockets, but the golden age of science fiction told us that rockets of the future would fall from the sky and land on their tails by firing their engines. Yet it took nearly seventy years from the first orbital rocket to the first propulsive landing of an orbital rocket. This feat was achieved by the SpaceX Falcon 9 on December 22, 2015.

To land an orbital rocket using its engines requires a number of design considerations that reduce its effective payload to orbit. In an optimal scenario, such a rocket must reserve about 10 percent of its first-stage propellant, which is fuel it can't spend on a payload. Falcon 9 and Falcon Heavy have several modes depending on the mass and destination. A return to launch site (RTLS) landing requires about 30 percent of the booster's fuel, while an ocean landing on an automated platform can cost as little as the 10 percent mentioned above.

China's Long March 8 and Blue Origin's New Glenn are rockets currently in development with planned retropropulsive booster stages.

SpaceX's Starship, currently in development, will attempt to take this a step further by landing both the first and second stages of the rocket. If successful, it will be the first orbital rocket to be 100 percent reusable.

Historically significant rockets

Saturn V (USA)

The Saturn V rocket to this day retains the title of the largest and most powerful orbital rocket ever built and operated successfully by humans. Standing at 110.6 meters tall and capable of delivering close to 7.9 million pounds of thrust, it was flown 13 times between 1967 and 1973 with a perfect operational record. (An engine shut down prematurely on one flight, but did not affect its performance.) Saturn V is of course best known for its role in the Apollo program, delivering U.S. astronauts to the Moon: the only humans to leave Earth orbit as of 2020.

The Saturn V's first stage was powered by five F-1 engines, themselves the most powerful single combustion chamber liquid fuel rocket engines ever built. These ran on kerolox for its high energy density. Its second and third stages were equipped with J-2 engines, burning hydrolox for maximum efficiency.

N1 (USSR)

The N1 rocket was the Soviet Union's attempt to build a Moon rocket to match the Saturn V. It was a monster: three stages, 17 meters in diameter at the base and equipped with thirty NK-15 kerolox engines. The second stage used 8 NK-15 vacuum optimized engines and the third stage used 4 NK-15 kerolox engines. While a bit shorter than the Saturn V at only 105.3 meters tall, it was capable of 10.2 million pounds of thrust.

The N1 would have been impressive had it worked. It attempted launch four times, all of which ended in failure. The second attempt crashed down on the launch pad at Baikonur Cosmodrome in Kazakhstan, utterly destroying the complex in one of the largest non-nuclear explosions ever produced by mankind. After two more failures and without any funding to continue the program, it was finally cancelled in 1976. A combination of rushed deadlines, engineering oversights, poor testing procedures and infighting between design bureaus spelled the downfall of this rocket, and it wouldn't be until the collapse of the Soviet Union in 1991 that information on the N1 was finally revealed to the world.

Sea Dragon (USA)

The most famous of the conceptual rockets. The Sea Dragon was a monster of a rocket first conceived by Robert Truax in 1962, and had it been built, would've stood at 150 meters tall and been capable of delivering 80 million pounds of thrust, dwarfing even the Saturn V. It would've been a two-stage rocket powered by massive single engines on each stage. It was planned to be built as cheaply as possible to reduce costs of getting payloads into orbit. Instead of using complex turbopumps like most rockets do, the Sea Dragon instead opted to use pressurized nitrogen tanks to feed the RP-1 fuel and LOX into its engines, as it makes it much easier for the rocket to be refurbished and reused.

Indeed, the Sea Dragon was ahead of its time when it came to the concept of reusable rockets, predating SpaceX's Starship by decades. The two stages would've been equipped with inflatable air bags to slow their descent through the atmosphere and into the ocean, where they would've been recovered. The rocket's large size would've outputted so much thrust on liftoff that it would've destroyed its engine along with the launch pad had it been launched on land. To counter this, the Sea Dragon would've been built in a shipyard and towed at sea (hence the name), where the ocean water provides a good buffer to dampen the destructive shockwaves. Ballast tanks attached to the engine nozzle would've sunken the rocket vertically to make it ready for launch.

As ambitious as the Sea Dragon was, its large size made it Awesome, yet Impractical for NASA to justify the costs of building it. Combined with NASA's budget being slashed as a result of The Vietnam War (which saw the cancellation of many projects), the Sea Dragon was ultimately canned and shelved.

STS/Space Shuttle (USA)

The Space Shuttle (officially known as the Space Transportation System) is the most successful crewed spaceflight program in human history. Five Shuttles were built and these flew a total of 135 missions between 1981 and 2011, launching such important payloads as the Hubble Space Telescope and most of the International Space Station. Significant parts of the Space Shuttle were reusable, including the SRBs and the orbiter itself.

The Shuttle lifted off with the help of two solid rocket boosters, the largest solid-fuel rockets operated in history (prior to the SLS, below). Its three RS-25 main engines burned hydrolox and carried the vehicle from ground to orbit, requiring significant advances in engine design to be operable in both regimes. It had an orbital maneuvering system burning hydrazine. The Shuttle used its heat shield tiles to survive orbital reentry, then its wings allowed it to aerodynamically glide to a runway landing.

The Shuttle failed in operation twice, killing the crew both times. In 1986, Challenger's main fuel tank exploded shortly after liftoff due to the failure of a solid rocket booster. In 2003, Columbia disintegrated during reentry as a result of damage to its heat shield on liftoff. Both incidents were traced to inadequate safety procedures; the Challenger disaster was particularly egregious as the decision to launch was political, despite unanimous warnings from engineers that conditions were unsafe.

Its safety record notwithstanding, the biggest problem with the Shuttle was its cost. Nominally around 500 million USD per launch, the total price tag over its lifetime averaged out to nearly 1.6 billion per launch, and it never met its goal of rapid reuse, flying no more than nine missions per year even with four operational orbiters. Simply put, it cost far more to refurbish each Shuttle than was originally promised, and safety issues plagued the program throughout its lifetime. Every incident led to more time and money spent on refurbishment and inspection, and as the existing orbiters reached the end of their lifespans, no new ones were built to replace them.

In 2004, the Bush administration announced the termination of the STS program once the International Space Station was complete. U.S. crewed launch capability was supposed be taken over by the Constellation program, which was itself shelved in 2010 due to cost overruns, a year before the final Shuttle mission. For nearly nine years afterwards, the United States relied on Russian Soyuz rockets to transport crew to and from the International Space Station, until the SpaceX Crew Dragon flew in 2020.

Important active rockets by nation

China

China's rockets are built and operated by the China Aerospace Corporation (CASC). The majority of them are part of the Long March (Chang Zheng in Chinese) family, which are identified primarily by number and subtype. Beyond that, these rockets are very different in design, with some intended for small lift duty, some for medium lift, and some for heavy lift.

Active Long March variants and their payloads include the 2C (2,400 kg to LEO), 2D (3,100 kg to LEO), 2F (8,400 kg to LEO), 3A (8,500 kg to LEO), 3B (several variants, estimated payload 13,000 kg to LEO), 4B and 4C (4,200 kg to LEO), 5 and 5B (25,000 kg to LEO), 6 (500 kg to SSO), 7 and 7A (13,500 kg to LEO), and 11 (700 kg to LEO). The 2F, 3A/B/C, 5, and 7/7A variants are also capable of reaching GEO orbit.

Chinese media does not widely advertise the capabilities and design features of the country's rockets, so confirmed details are scarce. Most of its launches occur from land bases, with the result that the spent booster stages often fall on civilian populations.

Europe

The European Space Agency (ESA) operates two major rocket systems: Ariane 5 and Vega.

Ariane 5 is a heavy-lift vehicle designed primarily for geostationary launches. Using two solid rocket boosters and a hydrolox main stage equipped with a Vulcain 2 engine, it can lift over 20,000 kg to LEO and 10,865 kg to GTO. The second stage can be either a hydrazine (hypergolic) or hydrolox version. The first-ever Ariane 5 launch failed mid-flight due to a software bug, becoming one of the most costly programming errors in history. It is expected to be retired in favor of the upcoming Ariane 6.

Vega is a medium-lift vehicle designed for small satellites and rideshares, with a maximum payload of just under 2,000 kg. It is a three-stage rocket, all of which use solid motors. Its Z23 second stage failed on July 11 2019, causing a long delay in flight operations, which resumed September 3, 2020. Its AVUM upper stage failed on November 17 2020 due to an assembly error.

India

The India Space Research Organization (ISRO) operates two families of orbital rocket: the PSLV and GSLV.

The Polar Satellite Launch Vehicle (PSLV) is a medium-lift rocket originally meant to launch into sun-synchronous orbits but also capable of small geostationary and even interplanetary launches. The first stage is a solid rocket booster, the second stage uses a Vikas engine burning hydrazine, the third stage is also solid, and the fourth stage uses hydrazine. Its maximum payload capability is 3,800 kg to LEO.

The Geosynchronous Satellite Launch Vehicle (GSLV) is a medium-lift rocket capable of carrying 5,000 kg to LEO. Uniquely, it uses liquid-fueled boosters to assist a solid-fuel first stage. The second stage uses hydrazine and the third hydrolox.

The GSLV Mark III is unrelated to the GSLV, despite sharing the name. It is designed for geosynchronous missions but also for human spaceflight, and is capable of lifting 10,000 kg to LEO. Its first stage consists of two solid rocket boosters, its second burns hydrazine, and the third stage burns hydrolox.

The most notable launch of PSLV is the Chandrayaan-1 lunar vehicle, which reached the Moon on November 8, 2008. The GSLV Mark III in turn launched Chandrayaan-2, which successfully entered lunar orbit on August 20, 2019. The lander, however, went off course and was destroyed on impact with the Moon's surface.

Japan

The Japanese Aerospace Exploration Agency (JAXA) operates the H-II and H3 rocket families, built by Mitsubish Heavy Industries. The H-IIA is currently in service, the H-IIB made its last flight in 2020, and the H3 is expected to go into service later in the same year.

Both variants of the H-II are two-stage, medium-lift rockets, using LE-7A hydrolox engines on the first stage and LE-5B hydrolox engines on the second stage, assisted by solid rocket boosters in various configurations depending on mission requirements. Payload to LEO is up to 15,000 kg for the H-IIA and 19,000 kg for the H-IIB. The A variant famously carried the Emirates "Hope" Mars mission in 2020 and the B variant was the standard platform for the HTV resupply vehicle. It will be replaced by the H3 rocket and the upgraded HTV-X resupply vehicle.

Russia

The Soyuz rocket family is, all together, the most frequently used launch vehicle in the world, with over 1,700 flights since 1966. The modern version is the Soyuz-2, which in its most powerful variant can lift up to 8,200 kg to LEO. The Soyuz-2 first stage is powered by four liquid-fueled (kerolox) boosers using RD-107A engines, which separate in a dramatic maneuver known as the Korolev Cross, and a core with an RD-108A kerolox engine. The second stage uses either an RD-0110 or RD-0124 kerolox engine and the optional third stage uses either S5.92 or 17D64 hydrazine engines.

Soyuz is the only rocket besides the SpaceX Falcon 9 that is currently certified to carry humans to orbit and it was the only means of reaching the International Space Station for the nearly nine-year period between the retirement of the Space Shuttle in 2011 and the launch of Crew Dragon aboard Falcon 9 in 2020.

Soyuz is infamous for its longevity and has had high overall reliability, but there have been some notable failures. Most recently, the MS-10 mission to the ISS failed when a booster didn't detach from the core properly. The crew survived thanks to the Soyuz capsule's launch escape system.

The Proton-M is Russia's main heavy-lift launch vehicle, with a payload to LEO of 23,000 kg. It is unique in many ways, among them that all three of its stages use hypergolic hydrazine as propellant. The first stage uses six RD-275M engines, the second stage uses 3 RD-0210 and 1 RD-0211 engines, and the third stage uses one RD-0212 engine. There are three optional fourth stage variants.

The Proton-M rocket has had some notable failures, the most well-known of which occurred July 2013 and was caused by the incorrect installation of angular velocity sensors. The rocket veered off course almost immediately after launch and crashed very close to the launch site, causing the largest known spill of hypergolic rocket propellant. Video of the crash went viral and it is among the most viewed rocket failures in history. Russia intends to replace the Proton-M with the Angara-5, but that has run into numerous delays.

United States

Antares, built by Northrop Grumman, is a medium-lift rocket mainly used for Cygnus resupply launches to the International Space Station. Its maximum payload to LEO is 8,000 kg. The first stage is powered by two RD-181 kerolox engines and the second stage uses a Castor 30B solid fuel motor. It has optional third stages as well. Antares previously used AJ26 engines adapted from Russian NK-33s, but when one of these failed catastrophically in-flight in 2016, they were replaced with the RD-181.

Atlas V, built by United Launch Alliance (ULA), is the final iteration of the Atlas family of rockets, which dates all the way back to the 1950s. The current version is powered by a single RD-180 kerolox engine, with a Centaur upper stage powered by an RL-10 hydrolox engine and optional solid rocket boosters for additional thrust at liftoff. It has a wide variety of configurations supporting payloads of up to 20,520 kg to LEO and is capable of interplanetary missions. Atlas V has a perfect operational record over more than 80 missions. It is the second most used active commercial rocket in the U.S.. Atlas V will be replaced by Vulcan-Centaur (see below).

Delta IV Heavy, also built by United Launch Alliance (ULA), is the second most powerful operational rocket today, used mainly for high-energy GEO transfers and interplanetary missions. It consists of three Delta IV cores powered by a single RS-68 hydrolox engine each and a second stage powered by an RL-10B2 hydrolox engine. It is capable of lifting 28,790 kg to LEO. It is extremely expensive and thus doesn't fly that often. Delta IV is notable for having bright orange insulation on the hydrogen tanks and for lighting itself on fire just prior to ignition (to burn off excess hydrogen near the engines).

Electron, built by Rocket Lab, is the first rocket by a startup company to achieve operational profitability while not part of any government contracts. It is a small-lift vehicle (capable of carrying 300 kg to LEO) that provides dedicated launch services for commercial smallsats and cubesats, and has recently secured government contracts. The first stage is powered by nine Rutherford kerolox engines and the second by a single Rutherford kerolox engine. It is the only operational orbital rocket to use electric pumps, powered by large lithium-ion batteries that are ejected during flight. Rocket Lab has begun experimenting with soft-landing the first stage boosters using parachutes with the ultimate goal of reusing them.

SpaceX (United States)

Falcon 9 is a partially reusable, medium-lift rocket built by SpaceX. Its payload capacity is 15,600 kg to LEO (reusable) or 22,800 kg (expendable). Its first stage is powered by nine Merlin 1D kerolox engines and the second stage by a single Merlin 1D vacuum optimized kerolox engine. In 2020, it overtook Atlas V as the most flown commercial rocket in active service. It is also the only reusable orbital rocket in active service as of the end of 2020.

In 2016, SpaceX achieved the first ever propulsive landing of an orbital rocket booster, and now routinely lands and reuses them, with the goal of 10 flights per booster before extensive refurbishment. It also achieved the first recovery and reuse of a rocket fairing. The second stage is not recoverable. Falcon 9 became the first commercial rocket to lift humans to orbit on May 30, 2020, on the Demo-2 mission.

Falcon 9 has failed twice, but only once in flight. In June 2015, a structural failure destroyed the rocket during ascent on a cargo mission to the International Space Station. In September 2016, a fueling anomaly caused the total loss of the rocket and payload on the launch pad. Several Merlin engines have failed in flight but none of those have caused a loss of mission.

Falcon Heavy is a variant of Falcon 9 intended for high orbit and interplanetary missions, consisting of three Falcon 9 first-stage cores strapped together. Falcon Heavy is the most powerful operational rocket today (second only to the Saturn V on the all-time scoreboard), able to carry about 30,000 kg to LEO in fully recoverable mode, and up to 63,800 kg in fully expendable mode. It has flown three times as of 2020, all successfully. The side boosters detach first and perform a return to launch site maneuver, with a perfect landing record (so far). The center core flies farther and has landed in only one of three attempts.

After the one successful drone ship landing of the center core on Falcon Heavy's second mission, the booster later toppled in high seas and was destroyed. This has led to a popular meme among SpaceX fans: "the curse of the center core".

Falcon Heavy is expected to be extensively involved in supporting NASA's Artemis program, including launching supply craft to the Lunar Gateway as well as components of the Gateway itself.

Others

Future rockets

Long March Family (CASC, China)

Long March 8 is intended to be a partially reusable launch vehicle whose first stage booster will land propulsively like a Falcon 9. The first test flight is planned for 2020, but there is scarce official data about its design and capabilities. It is expected to have a payload capacity of 7,600 kg to LEO.

Long March 9 is a super-heavy-lift launch vehicle that is currently in conceptual study, with a first flight potentially occurring in 2030. In terms of capability, it is expected to offer payload capacity of 100,000 kg to LEO, but its primary purpose will be to take Chinese astronauts to the Moon.

New Glenn and New Armstrong (Blue Origin, USA)

Blue Origin, owned and operated by Jeff Bezos, is developing two orbital rockets. The first of these is named New Glenn, a heavy-lift vehicle planned to enter service in 2022. New Glenn is expected to have a payload of 45,000 kg to LEO and will be partially reusable. Its first stage booster will land propulsively on an ocean platform in much the same way as Falcon 9. The second stage is expendable. New Glenn's first stage will be powered by seven BE-4 engines using methalox, The second stage will be powered by two BE-3U engines burning hydrolox.

New Armstrong, also from Blue Origin, is a proposed super-heavy-lift launch vehicle with very few public details. Based on the company's naming conventions, it is believed that New Armstrong is meant to send astronauts to the Moon.

Space Launch System (SLS) (Boeing/NASA, USA)

The Space Launch System, or SLS, is a super-heavy-lift launch vehicle that is currently under development by Boeing at the request of NASA. As of early 2021, the first core stage is in final trials with the goal of launching in an uncrewed test flight in 2022. SLS has several variants, the first of which has a payload to LEO of 95,000 kg, and a planned upgraded version which can carry 130,000 kg. When operational, SLS will briefly claim the title of most powerful rocket in the world, although it will still be less powerful than the Saturn V.

SLS will use a core stage powered by four RS-25 engines burning hydrolox and assisted by two solid rocket boosters. The initial version of the second stage will use a single RL10B-2 engine burning hydrolox. The proposed Exploration Upper Stage (EUS) will use four RL10 engines, also burning hydrolox. The primary use of SLS will be for the NASA Artemis program, intended to return humans to the Moon. It may also be used for interplanetary missions depending on availability.

The SLS program has faced criticism over its handling and practicality. The rocket alone costs 1.5 billion USD and more than 20 billion will have been spent on its development by the time it launches. In combination with all other components of the Artemis program (the Orion capsule and the Human Landing System), each Moon mission will cost close to 8 billion USD when the total price is averaged out. This is less than Apollo, which cost close to 10 billion USD when adjusted for 2020, but even so is seen by many as unsustainable.

Additional criticism targets its cadence, as it will be capable of no more than one flight per year. SLS is also not very ambitious, reusing significant amounts of technology from the Space Shuttle program. It is a "safe choice", but faced with upcoming competition from commercial rockets such as SpaceX's Starship, it may be obsolete before it ever gets to the Moon. Time will tell which approach will be successful.

Starship(SpaceX)

Starship, under development by SpaceX, is a super-heavy-lift launch vehicle that is intended for full reuse. Its first stage is the Superheavy booster, powered by up to 28 Raptor engines burning methalox, and its second stage is the Starship vehicle, powered by six Raptor engines burning methalox. As of 2021, prototype Starships are undergoing flight tests in the shipyard in Boca Chica, TX while large-scale manufacturing facilities are being built.

Starship's designed payload capacity is at least 100,000 kg and potentially up to 158,000 kg to LEO, and if its orbital refueling program succeeds, it will be able to transport that same payload to the Moon or Mars. When operational, the full Starship/Superheavy stack will be the largest and most powerful rocket ever built, with nearly twice the thrust at liftoff of the Saturn V. It is so powerful that it will primarily operate from platforms at sea; otherwise it would cause unacceptable noise pollution.

In contrast to SLS, which is expendable, meant for dedicated NASA missions, and will fly no more than once per year, Starship is designed for mass production and full and rapid reuse. Each booster will carry its Starship to space, return to land at its launch site, be refueled, and launch again, up to eight times per day. Each Starship will ascend to orbit and await one or more tanker missions that will dock, refuel it, then return to land. The Starship will then go to the Moon, Mars, or other destinations.

There will also be a version of Starship designed for suborbital, point-to-point flight on Earth, completing intercontinental trips in under an hour that could take 12 to 24 hours by commercial jet.

This rapid reuse is expected to reduce the cost of access to space by at least an order of magnitude, if it works, which is not guaranteed at this point. It would, if successful, be a revolutionary vehicle.

Early orbital flights of Starship could take place as early as 2021, but it is not expected to enter full service until 2022.

Vulcan Centaur (ULA, USA)

Vulcan Centaur is a heavy-lift rocket under development by United Launch Alliance (ULA). It is expected to enter service in 2021, and will carry payloads of up to 27,200 kg to LEO. The Vulcan first stage will be powered by two BE-4 engines (developed by Blue Origin) burning methalox, and assisted by up to six GEM-63XL solid rocket boosters. The Centaur second stage will use two RL-10 hydrolox engines.

Vulcan Centaur will replace Atlas V in ULA's lineup, and will have a Heavy variant that is expected to replace Delta IV Heavy, used for high orbit and interplanetary missions.

Vulcan Centaur is provisionally designed for partial reuse. Its first flights will be fully expendable, but there are plans to detach the engine section of the Vulcan first stage (the heaviest and most expensive part of the vehicle) and allow it to reenter safely using a heat shield. After this, it will deploy parachutes for an aerial helicopter catch attempt.


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