4
$\begingroup$

Looking to design a one-man fighter craft (using CGI) for space-based combat. To be at all useful in such a role the pilot must be able to turn the craft. There are three axes of rotation involved in the motion of any aerospace craft:

Pitch: This value changes when the craft rotates around its left-right axis.

Yaw: This value changes when the craft rotates around its up-down axis.

Roll: This value changes when the craft rotates around its forward-reverse axis.

Given that a craft operating in a vacuum cannot rely on aerodynamics (i.e. flaps, ailerons, etc.) to turn itself, rotation has to be accomplished by thrusters (of some kind) placed to provide torque.

Since more thrusters == more complexity in the design, and thus more things that can go wrong, I would like to eliminate any unnecessary thrusters.

Of the three axes of motion, which is dispensable in space-based operations?

$\endgroup$
11
7
$\begingroup$

All are equally dispensable, but I think the true answer to your question is "none are"

Theoretically, any rotation in 1 direction can be created from a series of rotations in the other 2.

For example:

  • to yaw 90 degrees left in a spacecraft with no yaw control, you can pitch up 90 degrees, roll 90 digress right, and then pitch down.
  • to pitch up in a spacecraft with no pitch control, you can yaw left, roll left, yaw right, and then roll right.
  • to roll, you can pitch, yaw, pitch, and yaw.

My Quaternion-fu is not perfect, but with the loss of any one of the primary 3 axes leaving only 4 functional thrusters, in 2 pairs, both at 90 degrees to each other, you should be able to perform any possible re-orientation in no more than 16 applications of thrust on the remaining thrusters. (8 rotations = 16 thrusts. As each rotation needs two thrusts, and all thrusts need to be in pairs as you need to start pitching up and stop pitching up in two separate applications of thrust.)

This limitation means you're unable to track a target along certain axes. If your enemies spacecraft is lacking yaw control, then you should manoeuvre to their left or right such that their main guns are unable to track you as effectively.

You're also going to be disorientated while yawing in a craft which lacks yaw control, as you'll be pitching and rolling all over the place to arrive at your target orientation. While your disorientated, you'll be vulnerable to attack.

This reorientation is difficult to perform manually without giving it your full attention, if you're in battle and need to track this complex manoeuvre is going to be easy to make a mistake and end up lost. If you need to change your target orientation mid manoeuvre (eg tracking a target heading in that unavailable rotation) you're unlikely to be able to do that without a computer.

All the thrusts in a manoeuvre need to be strongly ordered - you must wait for your pitch comes to a complete stop before applying roll, otherwise you'll end up with residual yaw at the end of your manoeuvre, this will further complicate tracking. If you had all 6 thrusters, then you could overlap thrust inputs if required for a much faster turn.

In open space, these limitations are equal for all 3 primary rotation directions.

There are multiple frame challenges to your assumptions here:

  • You can use a gyroscope to reorient your space craft, this requires no propellant and no external thrusters to reorient your craft. It's turning circles will be quite large though, so it'll be unable to achieve tight WW2-like dog-fights. But a craft with only 4 orientation thrusters will also make for some pretty boring dogfights too.

  • Thrusters need not be complex things that would need to be reduced to simplify design, they're amoung the simplest systems on modern spacecraft. You can make thrusters with only a single valve (I.e. a single solenoid) and no other moving parts, which requires only a single digital output from your flight control system (on / off), amplified through a single transistor. Hydrazine can be sprayed onto a strip of an Iridium catalyst which ignites the hydrazine and expands out a fixed nozzle, generating thrust.

$\endgroup$
4
  • 2
    $\begingroup$ To second the point about the simplicity of thrusters, those were literally the last system that the crew of the space shuttle worried about failures in during reentry. They are designed this way for a reason. $\endgroup$ – Adam Reynolds Mar 14 at 8:49
  • $\begingroup$ You probably don't want something quite as simple as a hydrazine monopropellant thruster, because you'll run into propellant capacity issues in extended flights (and potentially, the related ullage issues). It also bears remembering that when you've taken away aerodynamic requirements, it becomes a lot easier to fit turrets and things to point your weapons where you want, rather than having to steer the whole craft round. $\endgroup$ – Starfish Prime Mar 14 at 9:59
  • $\begingroup$ Are you sure you need 16 burns? Do you always have to stop the first rotation before starting another? $\endgroup$ – Loren Pechtel Mar 15 at 3:48
  • $\begingroup$ @LorenPechtel Like I said I'm not 100% sure, I think 16 is the minimum for an arbitrary reorientation but can't prove it. Theoretically you could go full async overlapping fire for thousands of burns and get in a complex mind blowing spin, but if you want zero rotational inertia at the end, your last dozen or so thrusts must be separate otherwise you'll end up with residual spin. $\endgroup$ – Ash Mar 15 at 3:59
1
$\begingroup$

One thruster. All possible directions.

enter image description here

https://en.wikipedia.org/wiki/Asteroids_(video_game)

Your game will be done Asteroids style. That will make coding easy and calculating speed and trajectory easy too. Your single thruster rotates nearly instantaneously to provide a vector force in the desired direction. The entire vessel need not rotate instantaneously - maybe the pilot is in a gimbal-mounted cockpit and the thruster rotates around her.

This gives you one thruster and one vector at a time. Piloting the ship for Asteroids was surprisingly tricky - the ship would continue on the prior trajectory until additional force was conferred using the thruster. The thruster was of course the brake as well.

Plus in your game you can have an asteroid come by from time to time as a tip of the hat.

$\endgroup$
1
  • $\begingroup$ This is not for a game, but for animation work, but thanks for the answer. $\endgroup$ – EvilSnack Mar 29 at 15:03
1
$\begingroup$

In space you have 6 degrees of freedom. 3 rotations (as you have said) and 3 translations (think forward/backward, left/right, up/down).

For movement you would need at least 1 translation (so thrust out the back) and 2 rotations (any 2 does not matter for movement but pilots may have a preference).

So to get somewhere you can point your nose at that location and thrust forward. Half way there you rotate to point away from it and thrust to slow down.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.