On Wed, 22 Oct 2003, John Carmack wrote:
> ...I still leave my engines in line with
> my gyro axis for setup convenience, but if you make your vehicle axis
> between the engines like this, you can get pure torques.
Interestingly enough, for the Apollo LM, they used a similar trick but
much more extreme. The LM had extensive inertia coupling between the
control axes, because the thruster axes were at 45deg to the principal
axes of the moment of inertia. During the testing on Apollo 9, the
angular acceleration vector was sometimes 15deg away from the torque
vector!
The fix was a new set of non-orthogonal (!) axes, based on where the
angular accelerations actually went. The revised software transformed
attitude and attitude-rate errors into components around those axes, and
then used those components to compute jet firings on the real axes.
Problem solved.
The standard accounts of Apollo 9 make the LM testing sound like a smooth
success, but in fact the guys in the back rooms had a lot of work to do
afterward, and there's a reason why Apollo 10 repeated some of the tests.
Henry Spencer
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