How would you steer a spacecraft?

[Germanunkol]

Hi,
First of all I'm sorry for choosing the "general physics" forum, but I didn't find anything better suited.

I have been watching Star Trek and other Science-Fiction series and I am interested in how you would, in theory, make spacecraft s navigate in space. Most SciFi Spacecraft designs tend to have one or more engines pointing backwards, which propell the craft forward. Rockets that we fire into space also mostly need to focus on the forward movement. What I haven't found so far though is how the "enterprise" could move to the right or left, or turn. Aircraft-Flaps aren't the answer, so what is?

A spacecraft , once launched, would (theoretically) not slow down in space. So even slowing down would need an engine in the front, pointing in the opposite direction to the engine at the back of the craft.

Then why do almost all spacecraft s that artists design for movies and games only have forward engines? Is it simply style? Or is there other ways of turning an object in space?

 

[russ_watters]

Spacecraft use both steerable engines (they pivot) and thrusters pointing in different directions to steer.

To slow down, they turn around and fire their engines "forward".

 

[Janus]

Small adjustments can be made by the thrusters already mentioned. large changes would be made by rotating the whole ship with the thrusters so that the main engine was pointed in a new direction, and then firing it. For instance, for braking you would rotate the ship 180° and then fire the main engine. It wouldn't make sense to carry an extra engine just for braking.

 

[Filip Larsen]

A spacecraft , once launched, would (theoretically) not slow down in space. So even slowing down would need an engine in the front, pointing in the opposite direction to the engine at the back of the craft.

You are correct in noticing that spacecraft s usually aren't exposed to major drag forces and therefore maintain their mechanical energy associated with their motion. Note, that the velocity (as for instance measured relative to Earth) may still change (both magnitude and direction) without maneuvering due to gravitational forces.

Spacecrafts that employ rockets usually have many small thrusters point in different directions to control their attitude (the direction they point in space), but for controlling the velocity they, as Russ said, employ one or more big thruster pointing in single direction which is opposite the direction which the spacecraft accelerates when maneuvering.

Then why do almost all spacecraft s that artists design for movies and games only have forward engines?

While it may not be historical correct, I blame George Lucas for introducing spacecraft s that most unrealistically moves like airplanes and ships. The only post Star Wars movie with realistic spacecraft physics I can remember right now is Apollo 13. It would be interesting to know if there are others (well, there is of course always the classic "A Space Odyssé").

 

[jmatejka]

I believe the International Space Station uses torque from gyroscopes for "attitude adjustment"

 

[Germanunkol]

Wow, thank you for all those quick replies!
Hm. It is very interesting how almost no movies take the time to make their spaceships maneuver in any of the mentioned ways.

jmatejka, I found this, it's an interesting read though probably not suited for the PC-game we're planning on making... http://en.wikipedia.org/wiki/Control_moment_gyroscope

Is there any theoretical spacecraft -engines that have never been developed but could also steer the ship?

 

[DaveC426913]

It is very interesting how almost no movies take the time to make their spaceships maneuver in any of the mentioned ways.

It has nothing to do with not "taking the time"; they are written this way deliberately.

1] A movie is, first and foremost, a story. One does not put into a story that which will detract from the story, and that includes good science. Painful to say, but true.

2] You must recognize that most films are not science fiction films; they are action films. Hard science is awkward to portray while keeping action going. You can count the number of recent science fiction films on your hands.

 

[Filip Larsen]

It has nothing to do with not "taking the time"; they are written this way deliberately.

And if I may add ...

3] Movies more often than not depicts complicated stuff in a much simpler way so the target audience can understand what is going on, even if it grossly unrealistic. Making a movie with, say, physically realistic space combat is no doubt possible, but I bet its really hard to do without loosing (or boring) the audience in the process.

4] Once certain "models" gets popularized by movies they tend to stick around even if almost everyone know they are wrong. For instance, even though you would think almost all in the audience now would be familiar with how a computer operates, some movies still features movie computers from a decade or two ago complete with weird beeping noises and snappy looking graphics.

 

[QuantumPion]

You are correct in noticing that spacecraft s usually aren't exposed to major drag forces and therefore maintain their mechanical energy associated with their motion. Note, that the velocity (as for instance measured relative to Earth) may still change (both magnitude and direction) without maneuvering due to gravitational forces.

Spacecrafts that employ rockets usually have many small thrusters point in different directions to control their attitude (the direction they point in space), but for controlling the velocity they, as Russ said, employ one or more big thruster pointing in single direction which is opposite the direction which the spacecraft accelerates when maneuvering.



While it may not be historical correct, I blame George Lucas for introducing spacecraft s that most unrealistically moves like airplanes and ships. The only post Star Wars movie with realistic spacecraft physics I can remember right now is Apollo 13. It would be interesting to know if there are others (well, there is of course always the classic "A Space Odyssé").

The TV series Babylon 5 had somewhat more realistic space combat. The ships had thrusters pointing in different directions for rotating, and they accelerated in short bursts for strafing. Sort of like that old arcade game "Asteroids".

 

[QuantumPion]

Wow, thank you for all those quick replies!
Hm. It is very interesting how almost no movies take the time to make their spaceships maneuver in any of the mentioned ways.

What about the very old arcade game, Asteroids!

 

[binbots]

How does thrust work in a vacuum? Doesnt it need a somthing to push through?

 

[DaveC426913]

How does thrust work in a vacuum? Doesnt it need a somthing to push through?

No. Propulsion works better in a vacuum because there's nothing impeding the exhaust.

The expanding gases do all their work inside the combustion chamber. Once the exhaust passes beyond the rim of the nozzle, it has done all its useful work and is nothing but waste. The waste can be better dispersed if there's no air pressure to slow it down.

 

[Germanunkol]

The way the ships in Babylon 5 steer would actually be interesting for a pc-game, there's still enough action and they at least attempt for realistic movement.

You must recognize that most films are not science fiction films; they are action films. Hard science is awkward to portray while keeping action going. You can count the number of recent science fiction films on your hands.

I'm still torn between making the game realisitc and filling it with action... For example, realistic would mean that you don't hear any sounds while flying around space. That wouldn't be very good for a game though...

 

[K^2]

The way the ships in Babylon 5 steer would actually be interesting for a pc-game, there's still enough action and they at least attempt for realistic movement.

http:\\ifhgame.ru

You're not alone in that line of thinking. The game is free, fairly realistic in terms of its physics, and the space combat is very cinematic. Give it a try.

 

[DaveC426913]

Agreed. Bab5 has done an admirable job of trying to stick to realistic physics while keeping the action going. A rare and noteworthy achievement.

 

[Percussim]

Generally the thrusters are called "retro rockets", don't need anything to push against because they work within the realms of Newtons Law of equal and opposite reaction. The gyros have a property called rigidity in space and once set up remain in a fixed position irrespective of what the spacecraft does - thus if the craft drifts from its position or attitude an error is detected and rockets fire to readjust the attitude. This is the basics of inertai navigation (IN), aircraft stabilisation, autopilot assistance used in earlier long haul planes and still used for attitude indicators.

 

[Skaperen]

I believe the International Space Station uses torque from gyroscopes for "attitude adjustment"

And that can be a separate (set of 3) gyros intended only for the adjustment. Of course, they will have to rotate rx degrees to effect x degrees of the station that has a mass of r times the flywheel mass. Very fine adjustments should be easy that way. Large coarse adjustments could take a long time and might be better achieved by other means.

 

[Percussim]

I think the response of gyros is as fast as the electronics and other response times can make it, practically, but, within limits of friction the gyros respond instantly. The gyros do no work - simply provide data of error for course and attitude adjustments - Sudden, step change errors should not occur in space, but may be corrected by manual control of retros if necessary - torque from gyros is not the issue - position detection is

 

[rcgldr]

Note that the space station and Hubble telescope don't have on board thrusters to prevent their low orbit from decaying due to the slight amount of atmospheric drag. Both rely on shuttle visits to get occasional orbital boosts. In the case of the Hubble, an unmanned mission will attached a thruster for "de-orbiting" the Hubble so it's large pieces that wouldn't burn up on re-entry end up crashing into the ocean (unlike Skylab where all they could do is change the orientation, and due to calculation errors, some parts ended up crashing into Australia).

Many rockets or spacecraft have side thrusters away from the center of mass to control attitude (pitch, roll, yaw). The space shuttle does a back flip in order to use it's main engine to slow it down for controlled de-orbit to set up the re-entry path, then front flips back up right so it''s oriented at the proper attitude for re-entry into the atmosphere.

 

[eah2119]

What's the fuel used for the short spurts of propulsion?

Whenever I see videos of the shuttle making its was to the ISS, I always see short and small bursts coming from the craft in different directions. I realize that they are controlling the exact position of the craft, but what is the fuel used? I know they use liquid hydrogen and oxygen to lift off, but what do they use the same fuel for maneuvering the orbit? It must not be that strong, right? It's also white in color and that strikes odd to me because I would think fuel would combust.

 

[rcgldr]

What's the fuel used for the short spurts of propulsion?

Monomethyl hydrazine (MMH) with an oxidizer, nitrogen tetroxide (N2O4).

 

[Filip Larsen]

What's the fuel used for the short spurts of propulsion?

For controlling attitude spacecraft s use what is called a Reaction Control System (RCS). For the Space Shuttle, the RCS uses two propellants (hydrazine as fuel and nitrogen tetroxide as oxidizer) which by the way are hypergolic meaning they combust spontaneously when mixed in the thruster. The fuel system is then pressurized using helium. You can read more details about the Shuttle RCS on [1].

[1] http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-rcs.html

 

[Gestahl]

For example, realistic would mean that you don't hear any sounds while flying around space. That wouldn't be very good for a game though...

Well you wouldn't hear any sound in the vacuum of space itself, however inside the ship one would still hear all kinds of familiar combat sounds as you're hit, shooting, or even the change in velocity.