On Tue, 4 May 2004, Donald Qualls wrote:
> > If you put TWO paddles on each side of a square (each paddle is 1/2 the
> > width of the side, 8 total), you now have roll control as well as
> > redundancy in paddles.
>
> True enough, Jerry, but there are very, very few rocket engines that are
> square...
The layout of paddles around the exhaust can be square even if the engine
(and the undisturbed exhaust stream) is round.
Think of it this way: take your orthodox pitch-and-yaw-only paddle
layout, and split each paddle in half, with separate actuators for each
half, but the two halves still pivot on the same axis. Move diagonally-
opposed half-paddles into the exhaust stream, and you've added a twist to
the flow, giving you a roll torque.
This doesn't work so well with fluid injection, alas, because it always
pushes the exhaust essentially straight outward from the wall. To do roll
control with fluid injection, you need either a square exhaust nozzle --
which is not a big deal, actually, it's been done -- or a fin sticking
inward from the nozzle wall with injection ports on both sides.
Mind you, roll torques are typically small enough, compared to pitch/yaw
torques, that you might not *want* to use the same system to handle them.
There are rockets which use a gimbaling nozzle for the turbine exhaust to
do roll control. And it's not unheard-of to just use little RCS thrusters
(even cold-gas thrusters!) for roll; both Ariane 5 and Atlas II do that.
The only issue there is that you may need to make some effort to trim out
roll torques from engine misalignment and the like -- Arianespace found
out about that the hard way on the second Ariane 5 flight, when the roll
thrusters ran out of fuel because the main engine produced more roll
torque than anyone expected.
Henry Spencer
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