Sci-Fi Civilian Vehicles, Equipment and Technology

I'm trying to decipher that diagram, but I don't have the background for it. Does it come in the form of oxygen or water?

It uses methane and oxygen as inputs. The power density is very low, according to the report, so I don't know how practical this design would be for spacecraft.

Shoot. For a minute there, I thought we had something really interesting.

I think the issues with methane are solvable where as the issues with Hydrogen are fundamental to chemistry.

I don't really see the supposed issue with hydrogen. Chemical powersources like this aren't really very interesting. It's only useful when you're already hauling chemical fuel around. The kind of missions where hydrogen's diffusion problem starts to matter are also the kinds of missions where you bring an RTG, a nuclear reactor or a solar panel.

Well, the diffusion problem also matters if you are building a rocket to get something out of a gravity well. Not everybody's lucky enough to live on a radioactive low-gravity world.

Not sure what you mean. The hydrogen is made and chilled not long before launch, it then spends a few hours at most in the tank while lifted off planet, and you may be using it for a few days or weeks in orbit. It realistically is never going to be a major issue for your fuel cells that hydrogen leaks out of your tanks.

Outside this specific frame, any other powersource is better. Chemical fuel for power is simply not power dense enough

Not to mention, the hydrogen diffusion issue is a major problem for your engines, much more so than for your fuel cells. Given the tyranny of the rocket equation, the fraction of hydrogen used by a fuel cell is going to be, no, HAS to be minimal compared to what your engine suse.

Edited by devak on May 13th 2022 at 7:44:02 PM

I'm thinking about all the methane on Titan. It's more or less useless as a power source because there's no free oxygen out there, and distilling it from water uses more power than it produces by burning the methane. I've long been looking for some sort of end run around that.

Hydrogen diffusion matters mainly for long-duration missions: weeks or months rather than days or hours. It is possible in principle to keep propellants like liquid oxygen and liquid methane stable and confined for long durations with insulation and active cooling. However, it is not possible for hydrogen, which will escape no matter how much effort you put into keeping it within tanks.

That said, it's about ratios. With sufficient measures you can probably manage storage of liquid hydrogen for several months.

As for methane, obviously it takes more energy to create than you get from it; that's simple thermodynamics. This is why you use a relatively low-power but readily available energy source to perform that conversion. On Mars, you'd rely mainly on solar panels, with support from portable, modular fission reactors. Farther out you'd have to rely more on nuclear power unless you can tap a local source, like Io's volcanoes or Jupiter's magnetic field.

These are core principles of ISRU.

Edited by Fighteer on May 13th 2022 at 2:18:14 PM

I was just looking for an efficient power source for smaller spacecraft. Shuttles, mechs, tugs, etc.

1. Solar panels and batteries
2. Fuel cells of various sorts
3. Radioisotope thermal generators
4. Nuclear power plants

It's easier and simpler for the fuel cells to siphon propellant from the main tanks, so designing them to operate on those propellants would be reasonable. If you want to postulate a breakthrough in methane fuel cells for your setting, that's perfectly fine.

If you're talking about Giant Mecha and Space Fighters, you're in a setting that already deviates enough from reality that nobody's going to get up in your face about that sort of thing.

Edit: There's a mechanism that can make fuel cells extremely viable and also help keep cryogenic propellants stored long-term. When a liquid expands to a gas, a lot of heat is carried away with that gas, cooling the liquid. This can be seen very easily with compressed air canisters that you can buy in stores.

You can bleed off that vaporized gas to stabilize the temperature of the propellant, but it has to go somewhere, right? If you send it into your fuel cells you can get useful power.

This is also a way to get fresh oxygen for long interplanetary trips: bleed it off of the propellant tanks as part of the mechanism for keeping them chilled. Since water is a byproduct of the fuel cells, you also get that for "free".

Edited by Fighteer on May 13th 2022 at 3:13:54 PM

Is deuterium any easier to contain then normal hydrogen? And if not what would be the best way to have long term stable fuel for a shipboard fusion reactor?

Yes, deuterium is easier to store than protium hydrogen as it diffuses more slowly, but it's not a super strong effect. One problem though is that deuterium-deuterium fusion produces more neutrons and more radioactive tritium and less energy per unit mass than proton-proton fusion.

Refining deuterium for a fuel cell seems a bit excessive.

Yeah, that's ridiculous. The only reason to use deuterium is if you want lower-temperature fusion.

Isnt it also a key element in most of the easier to achieve fusion processes though? IE: Your going to have to be storing it any way unless your fusion tech is very developed?

I was asking about starship fusion reactors not fuel cells, just brought up by the topic of storing hydrogen for them

Edited by Imca on May 13th 2022 at 3:27:28 AM

Fair, but with nuclear reactors you can combine the deuterium with something else. Methane might work. The carbon might soak up a neutron or two but the bond won't affect the reaction.

It will however affect the radioactivity of the exhaust when there is carbon-13 in the methane which becomes the long-lived, biologically active carbon-14.

Probably best not to use near habitable planets, then.

You would never want to fire any sort of nuclear engine in a planetary ecosphere. In orbit is probably fine, since any dangerous particles would diffuse rapidly into the vacuum. And there are certain designs that don't have radioactive exhaust: a nuclear-thermal engine using plain hydrogen as propellant is completely safe to operate. The problem there is that it doesn't have enough thrust to lift off: the thrust-to-weight ratio on such a design is substantially less than one.

You may also run into problems with the nozzle design and the exhaust velocity. Firing it in an atmosphere could generate some very hazardous effects due to the sheer energy of the exhaust, even if the exhaust is not itself radioactive.

For example, the hydrogen exhaust would probably combust with atmospheric oxygen after leaving the nozzle. You'd be riding a giant afterburning fireball. I don't know how you could make that safe.

Edited by Fighteer on May 14th 2022 at 9:38:51 AM

To be fair the system I am using is a particle accelerator as the drive engine, with fusion providing the power.... though to be honest I cant imagine that works too well in atmosphere either.

A particle accelerator would only irradiate you if it was pointed at you...probably. Depending on how much mass you're working with it might make the air explode.

By definition, all Newtonian rocket engines are "particle accelerators". Particles are accelerated and they provide thrust. The difference is in how that acceleration occurs.

Of course, what you're probably talking about in context is magnetic acceleration. Electric ion drives already do this; while they are extremely efficient, their thrust is very weak. You could in theory power one with a nuclear reactor, in which case they'd have much higher thrust, but even then you couldn't lift off of an Earth-sized planet using them.

Nor would you want to fire them in the atmosphere as the exhaust velocity is many times higher than chemical rockets. They would cause serious problems for anything behind them.

If you mean something like a cyclotron that accelerates subatomic particles to relativistic velocities, the thrust provided by that would be absolutely miniscule, mainly due to the immense mass of the hardware, and if you tried to use it in an atmosphere you would severely irradiate anything in the exhaust plume.

Edited by Fighteer on May 14th 2022 at 10:11:04 AM

So technically with the Gundam Megaparticle accelerators they use for beam rifles and beam sabers... does this mean those could be used for recoil boost since those particle accelerators are far smaller? :P

A gun is technically a "particle accelerator". You are a particle accelerator when you throw a ball, or cough.

I have no idea what a "Gundam Megaparticle accelerator" is. If they have tiny cyclotrons in their guns that accelerate beams of charged particles, then yes, there would be recoil from that, because of Newton's Third Law. This applies to any such situation.

The recoil is determined by momentum, and subatomic particles don't have a lot of individual momentum even if they are moving very fast... hence why the thrust of a rocket using that kind of technology would be very low. Even a laser has recoil; it's just very, very, very, very, very tiny.

Edited by Fighteer on May 15th 2022 at 5:43:31 AM