This is discussion archived from a time before the current discussion method was installed.
Heh. That reminds me of one of the Laws of Anime.
- 4 - Law of Constant Thrust, First Law of Anime Motion - In space, constant thrust equals constant velocity.
Semiapies: Now, there is one way in which fuel (and lack of it) is important, at least in interplanetary travel. You need the ability to change enough velocity (delta-vee) to not only reach your destination but match its velocity. Other arcanities of orbital dynamics can involve this, I'm sure. I seem to recall this coming up on Star Cops, where a (sabotaged?) ship en route to Mars has a failure that prevents it from making a necessary maneuvering burn, and the pilot says in horror, "Oh God, *we're dead*!" because they're simply out of range of any rescue and are just heading out into empty space, with insufficient fuel (by order of magnitudes) to simply "turn around".
Your Obedient Serpent: The point that Semiapies raises is, indeed, the crux of the classic short story, "The Cold Equations", which was made into an excellent episode of the 1980s revival of The Twilight Zone and later, into a terrible TV movie aired on The Sci Fi Channel. The main entry's dismissal of "oh, yeah, you also have to have enough fuel to stop" makes that sound like a trivial issue. However, this veers off from the main, ahem, thrust of the entry, which is the way Space Opera vessels tend to screech to a halt when they're "out of gas", much like the crashing airplane in the Bugs Bunny cartoon, "Falling Hare".
Red Shoe: Yeah, my main reason for dismissing it in the first place is that "We don't have enough fuel to get to saturn" is usually something you say *before* you set out. When you're stopped, if you have enough fuel to start moving, you also have enough to stop (by only going half as fast, ignoring overhead)
Seven Seals: "Space ships won't shake apart from going too fast." I'm no physicist, but it seems to me that strain is no less valid in space than back on Earth. A ship may not shake apart from moving at constant speed, no matter how great that speed is, but it might tear itself apart if accelerating too fast, no?
LTR - I've been wondering about that too, it won't shake apart from things like atmospheric turbulence, like an aircraft exceeding it's designed load capacities in an extreme dive or turn, but wouldn't it still be vulnerable to say, internal vibration from it's engines or other workings? And Newton's laws are still in effect even in an airless environment, trying to make a 90 degree turn in space will break your ship in half unless you're using somthing exotic for propulsion that lets you cheat inertia.
- For the first bit, a ship to have a hull harmonic frequency the same as that which is produced by some large piece of equipment (ie, enough to start breaking it) is very bad design, as should have been sorted out in a lunar test-run. For the second bit, if your ship can take maximum thrust without damage, there is no way you can damage it by reorienting, since inertia is measured only from the centre of inertia.
Red Shoe: As usual, "it's complicated". You won't shake apart from going too fast, but too much acceleration will do the same thing to the structure of your ship as it will do to all the squishy humans inside — but this only applies when you're speeding up, slowing down, or turning. The old standby, "We can't maintain this speed for long!" doesn't apply.
Kendra Kirai: Actually, technically it still could...there's the effect of particulate matter upon the vessel, which would naturally be increased...If the ship doesn't have a suitable Deflector Shield to handle the speed and the ammount of particulate, the impacts could well have the effect of shaking the ship as they impact the hull....And of course, that only applies to conventional drives. With Faster-Than-Light Travel , you have the problem of powering the drive, the drive possibly overloading, and the fact that with most all FTL drives I know of, the failure of the drive means you either die, just stop in whatever version of hyperspace you're in at the time, or you re-enter normal space, whereupon you're back under the influence of "normal" physics.
I honestly can't think of any examples where sublight travel will 'shake the ship apart' unless it's while the ship is actively accellerating, which is when the engines are going and can generate vibrations.
Also, you have to remember the most important fact of space travel. You not only have to have enough fuel to get there...you also have to have enough fuel to stop. That's incredibly important. If you're going at eight million miles per hour relative to your destination, all you're going to do is blow right past it into deep space...at best. If you're unlucky, you'll get pulled in by the gravity and either burn up in the atmosphere, or make a very nice crater. None of the likely scenarios are exactly favorable.
Gus: I guess the point is, if your are going to have your ship shake apart, you need some cause other than "we was a-going' too fast." It's not too hard to think of something plausible. Serenity's engine had a spinning part. That sort of thing could get out of alignment and start to shudder the ship pretty badly. Same thing applies to ships that rotate on axis for g-simulation. A bad ball-bearing or so for such ships could get a shimmy going. Jato units could all go haywire and start knocking things back and forth. Perhaps your deflector shields have forgotten how to do whatever magic it was that it used to do to absorb the shock of smashing into stuff.
Red Shoe: While the "needng enough fuel to stop" thing is true — and important, not having enough fuel to stop is a situation you should only get into if something happens to your fuel supply after you've reached top speed; from a standing start, any amount of fuel you've got on hand is enough to accelerate to some speed, then decelerate from it later. If fuel's low, you just go slower.
Your Obedient Serpent: Red Shoe, the reason "needing enough fuel to stop" isn't trivial is because, when you're traveling in space, you're not aiming between stationary points: your point of departure is moving, and your point of arrival is moving — and they're moving at very different speeds. It's not a matter of just "decelerating", it's a matter of being able to match velocities with your destination.
On top of that, most of the "out of gas" stories have something happen to the ship's fuel supply en route. In good, hard SF, this means that you don't have enough "delta vee" to match velocities with your target or any reasonable abort destinations. In bad SF, this means that you come screeching to a halt, or your orbit decays.
Timing is also important, as in the Space Cops episode mentioned above. A real-world example — if Apollo 13 hadn't made a significant course correction, they would have swung out into deep space like a David Bowie song. They didn't know if the main engine was damaged, so they had to use the LEM's engine to make the burn.
A good primer for this stuff is Winchell Chung's ATOMIC ROCKETS site, specifically http://www.projectrho.com/rocket/rocket3b.html
Dark Sasami: "If fuel's low, you just go slower" is a bit indicative that Wiki Editors Have No Sense Of Scale. The fact that you can still go the right direction doesn't mean it's not a crisis, if it's your great-grandkids who will be making the landing. Or did you forget to pack more than a hundred years' worth of oxygen, water, and Astronaut Ice Cream? Of course, it helps if you don't need oxygen...
Looney Toons: The bit about crashing airplanes was removed, and I'd just like to note that there is an analogous situation to that as well, although it doesn't fit under this topic — that of the ship that spirals directly into a sun or augers into a planet because its engines go out while it's in orbit. Completely ignores the basics of orbital mechanics. We ought to have a trope for this, but I can't think of a good, punchy, pithy name for it.
Ununnilium: Orbit Like An Anvil?
Looney Toons: Hmmm. Flies Like A Brick evokes the same imagery, and has the added benefit of referencing both NASA (it was an unofficial description of how the Space Shuttle handles) and The Hitchhikers Guide To The Galaxy...
Dark Sasami: If you've got a realistic spaceship that can casually park in the orbit of a planet and take off when you feel like it, why wouldn't you plummet if there were a major failure? The way I figure it, there are a few factors involved: one, you're going to have other traffic to account for, and two, course correction is very simple for the ship's autopilot, so you probably won't waste cycles settling into the precise orbit that will keep you there for ten years at a time. You're going to aim for "good enough for government work." And then, when there's a massive malfunction, you'll destabilize relatively quickly — perhaps not minutes or hours, but probably within a few days, and then it's all matter of how far out you parked and how thick the atmosphere is.
If propulsion is really cheap you might actually find a lower or slower "orbit" that's aided by outward thrust is more convenient than a gravitational orbit — you're doing the flying-saucer bit, just somewhat further out. Somehow I've never seen that addressed by science fiction; centripetal force is not the only one available when you're near a planet. And if the engines go out when you're doing that, you're toast.
YYZ: I hate to nitpick, but the Hitchhiker's quote is "Looks like a fish, moves like a fish, steers like a cow."
Dark Sasami: There may have been some conflation there with "The ships hung in the air in much the same way that bricks don't."
Your Obedient Serpent: I dunno — the "Instant Orbit Decay" thing really does boil down to Space Friction, ultimately.
Dwayne Hicks: And, as we know from dealing with black holes, Gravity Sucks.
Red Shoe: The point about not being able to stop at the end, while well taken, is not really so important that it needs to be mentioned twice in the space of three paragraphs. I'm not sure which place it fits best, so I'll leave it to the Wiki Magic.
Zaenos: I propose an alternate title for this trope: Science Friction
Allonymous: I don't think the part about the top speed of the ship being only limited by the speed of light is correct. The maximum speed of a rocket is determined by the exhaust velocity and the amount of fuel expended. In other words, the maximum velocity of the ship can't exceed the velocity of the exhaust particles unless the total mass of exhaust emitted (probably the mass of fuel expended) is greater than the total mass of the ship, including remaining fuel.
Maat Mons: I would like to address the beginning of this article: “Isaac Newton turned the world of physics upside down when he observed his first law of motion: 'A body in motion will tend to stay in motion at a constant velocity unless acted upon by an outside force.' This was earth-shattering stuff when he introduced the notion - in 1687!”
In “A Guide to Newton's Principia” I. Bernard Cohen wrote “The first law states the law or principle of inertia which Newton had learned from Descartes's Principia, where it was also the first law.” World Book Encyclopedia also credits Descartes with this law of motion. It says “In the mid-1600's, Rene Descartes, a French philosopher and mathematician, challenged the long-standing assumption that an absence of motion was the natural state of all objects. Instead, he proposed that all objects have inertia-that is, they maintain whatever their state of motion unless otherwise disturbed.”
In “Philosophiæ Naturalis Principia Mathematica” (Mathematical Principles of Natural Philosophy), published in 1687, Newton wrote “The principles I have set forth are accepted by mathematicians and confirmed by experiments of many kinds.” This was in reference to the three laws of motion as well as six corollaries. So, Newton himself attested that inertia was not earth-shattering in 1687.
