I’m a moron when it comes to physics, so please forgive me if I sound stupid.
If I understand things correctly, when we send a probe to Mars (for example) we basically point it where we want it to go and send it off. Then the probe essentially “coasts” to it’s destination.
But if a space ship was to maneuver in space, ie change directions and destinations, how would that work? How could a rocket engine in outspace exert force to turn a vessel if space is a vacume?
Something that’s been bugging me since I was a kid.
A rocket’s exhaust does not push against air or anything of the sort.
This may seem counterintuitive, since when we want to move we need to push off something (a wall, the floor, whatever) but a rocket works bt throwing its exhaust out really, really fast from one end, and as Newton demonstrated if you throw things off one end of an item, the rest will move in the opposite direction.
That is why you need to brace yourself against the recoil of a rifle. The round is going out one end reall, really fast, which kicks the rifle body back in the other direction in proportion to how much more massive the rifle is than the round. Think of the rocket’s exhaust as the round, and the rocket as the rifle, and you can see that you move by recoil.
Get some skates and go stand on some ice. Throw a large mass away from you. To conserve momentum you will begin moving in the opposite direction.
p[sub]initial[/sub] = 0 = p[sub]exhaust[/sub] + p[sub]rocket[/sub]
p[sub]exhaust[/sub] = -p[sub]rocket[/sub]
Since mass is always positive, you can see that velocities must be in opposite directions.
As far as I know, this is not really always correct: trajectories in space, especially long distance ones, are often calculated to take advantage of the gravitational effects of other large objects, so it’s not just a question of pointing a spacecraft in the direction of its destination. See gravity assist.
Yet it can still be called “coasting”. It’s certainly true that if you wish to actually arrive at your destination you must indeed take its movement and anomalies along the path into account.
With care, you can also bend the path significantly without the use of fuel by passing close to a massive object.
I think the point was that by passing near a massive object (e.g Jupiter) and then timing a specific duration of burn you can pick up some speed. I dont think you could call that coasting.
That said there are things called gravitational corridors that minimize fuel consumption. One paper –> Optimal capture trajectories using multiple gravity assists; Stefan Jerga, Oliver Jungea, Shane D. Rossb; Communications in Nonlinear Science and Numerical Simulation, Volume 14, Issue 12, December 2009, Pages 4168-4175; Redirecting
Not much to slingshot off of between here and Mars, though. I found the idea of a transfer orbit to be curiously elegant when I first heard of it. It’s just half of an elipse; the lowest point coincides with the orbit you’re leaving, and the highest point matches the orbit you’re trying to reach.[sup]*[/sup] And of course you have to time it so that you and Mars get to the same point at the same time.
Reverse that description if you’re going to Venus.
Specifically, it’s pushing against the exhaust while the exhaust is still inside the rocket (in the engine).
Imagine the engine is a hollow sphere with hot compressed gas inside trying to escape anywhere it can. Obviously, it’s pushing equally hard against every point on the sphere, and all these pushes cancel out, as they’re in every direction. But make a hole in one place, and now the push has no effect on the sphere at that point. This means that the push at the exact opposite point has nothing to balance it, so stuff happens. A hole in the back means the engine moves forwards.
I think “in the engine” would have to include the often quite big metal cone that opens rearward. I think this contains some of the combustion and helps focus the expanding exhaust rearward, and the exhaust exerts a big share of thrust on the inner surface of this cone. Perhaps pressure in the ball of fire just aft of this cone contributes to the force on the cone - there is still combusion happening there, and at least in some rockets like the Saturn V there is still fuel being distributed, wet, on the inner surface of the cone. So, “in the engine” could extend many feet backwards from the trailing edge of the rocket. Anybody know more specifically?
Also there is aerobraking, where a spacecraft is intentionally ‘dipped’ into the atmosphere of the planet/moon? (technically possible, as some moons have a atmosphere those I don’t know if it has been done) to slow it down.
One of the easiest ways to visualize it was something I learned as a kid. Set off an explosion inside a cylinder with an open nozzle at one end. The explosion pushes outward in all directions, as explosions are wont to do. Visualize this cylinder sitting upright on a launch pad. The force northward is counteracted by a force southward, and vice versa; similarly for eastward and westward. But the force upward is not counteracted by a force downward, because that’s escaping through the nozzle. Net result: the cylinder moves upward – not because it’s ‘pushing against’ anything, but because it’s propelled by the internal force of the explosion. Causing an ongoing explosion by feeding in fuel and oxidant keeps the force going.
Also explains why you’ll never see interstellar ships being like they’re often designed in Scifi, such as the cut-away drawings of the Enterprise, which contain ludicrously small amounts of fuel for their size.
Because if you eject 1% of the mass of the ship at speed N, the whole “equal and opposite reaction” thing means you’re only increasing the velocity of the ship by 1/100th of speed N. So in order to reach .25c, you’d either need to eject that 1% at 25 times the speed of light, or you’d need to eject 25% of the ship’s mass at the speed of light. (This doesn’t take into account the energy curve that requires more energy as you approach the speed of light, so it’s not factually true, only an illustration of the problem.)
For what it’s worth, the Enterprise (Star Trek ship) was supposed to have been built and serviced in orbit, not launched from Earth, and her impulse power (used for maneuvering and short-distance travel) was based on matter-antimatter annihilation (the antimatter being contained in force fields). “Warp drive” was of course a classic “doubletalk drive” invented for the purpose of making travel arcoss interstellar distances possible. So while I agree that the show’s vessels would not function in real life, the assumptions are not as far out of place as Chimera suggests.
“In the engine” would include the engine bell, yes. Relatively little combustion (i.e. active chemical reaction) takes place in or downstream of the engine bell; there are clearly luminous emissions from very hot exhaust gases, but the bulk of the actual chemical reaction (and associated release of chemical energy) has, for the most part, taken place inside the combustion chamber. Any liquid fuel drizzling down the interior surface of the engine bell isn’t contributing its chemical energy to thrust; it’s done to keep the bell cool.
The performance of a rocket is rather complex and requires some understanding in the field of supersonic gas flows. Gases traveling at supersonic speeds behave rather differently than those traveling at subsonic speeds, and this is important for the performance of a rocket engine:
-a subsonic flow will be at its fastest when traveling through a restriction (a “throat” in the pipe), and then will slow down when the flow expands into a larger area.
-a supersonic flow, upon exiting that same restriction, will accelerate to higher speeds as it expands in the rocket bell.
This is of course ideal, since we want to chuck that exhaust out the back with as much speed as possible, converting as much thermal energy into kinetic energy as we possibly can. Hot rocket exhaust is a waste; we want fast rocket exhaust. And that’s why the bell is there; to provide a smooth expansion duct for the supersonic flow. Ideally the flow expands to the point that it achieves ambient pressure by the time it reaches the exit plane of the bell. For a rocket climbing through the atmosphere this is not possible at all altitudes, which is why we see some interesting effects during NASA’s rocket launches. Click here for an explanation of “overexpanded” and “underexpanded” supersonic flows. Note that with a lot of rocket engines you can observe the effects explained there, particularly underexpansion seen at high altitudes via NASA’s long-range tracking cameras.
This also accounts for some other interesting luminous effects seen behind some rocket engines and afterburning jet engines. Shock diamonds may be visible in the exhaust plume as shock waves bounce back and forth through the plume gases. As this page suggests, there may be a tiny bit of combustion taking place in the shock diamonds, but this combustion does not contribute to the thrust of the engine in any way.
If you were to build the best space fighter (TIE, X-Wing a la Star Wars), would you build it for speed, or manueverability-as in which of the two would you emphasize?
In Earth’s atmosphere, you very well, at the speeds at which fighter planes typically travel, can get quite a bit of manueverability without sacrificing much top speed, which of course is there on demand via afterburners (at a steep cost in fuel). The optimal velocity at which your instantaneous turn rate maxes out is your corner speed: in an F-15 that might be 400 knots. An F-15’s top speed is c. 1700 knots, full burners, so that’s about a 4:1 ratio.
But, if you try to translate that model to space, you run into some rather severe constraints. First of all, to even get anywhere, you have to be going much faster than 1700 knots-at that speed, you’ll reach the moon in oh about 9 days (forget Mars or other more distant objects). But as speed increases, the amount of force needed to manuever (i.e. change your course and/or velocity) increases exponentially (delta V, as they call it).
In reality, a space battle between opposing fleets of fighters would involve them zipping past each other at e.g. 20,000 knots velocity at 5,000 km distances, having a few scant seconds to aim their energy beam weapons, take 2-3 hours to reverse course and get within weapons range of the enemy again, only to once again swoop out of sight in different directions. If you try to stay slow to maintain your “manueverability,” then you’ll find that the enemy would be in complete control of the engagement, able to harass you (or bugger off) at will. In WWII this is why 400 knot fighters became state of the art and 250 knot biplanes obsolete, even tho the velocity difference there was much smaller than in my hypothetical space encounter.
Picard’s first Enterprise was built at Utopia Planitia in orbit around Mars, but Kirk’s first Enterprise was built at the San Francisco Yards, and though Memory Alpha says it’s in Earth orbit, I know of nothing in any movie or episode that actually coroborrates this. It’s always been my understanding that there’s a debate about whether Kirk, Pike, and April’s Enterprise was built in orbit or on the ground, and the recent reboot actually had it built at Riverside Shipyards in Iowa.
Also, impulse in Star Trek is achieved by fusion reactors. M/AM reactors are for warp speed only.
Other than the Moon, of course, and Phobos and Deimos at the other end of the trip. Once you get to the Moon, if you plan your orbit right, the rest of the trip is essentially free.
I don’t think it’s really that much of a design question because of how different flight in space is.
Planes get maneuverability from things like wings, but a big wing (best for tight maneuvering) is lousy for high speed because of drag. A delta-shaped wing is great for high speeds, but not so hot for tight maneuvering.
In space, all of your maneuvering and speed comes from one source - the rocket. You rely on small attitude jets to help you spin on your axis, but any time you want to change velocity (that is, speed and/or direction), you turn the ship and fire your main rockets. So “corner speed” has no relevance to space fighters. They always have a 1:1 ratio, being able to corner as quickly as they can accelerate in any other direction. (Though the ratio might not make much sense for space fighters in the first place, since they have no top speed as it’s applied to planes).