Stewart Cobb wrote:
>
> Doug Jones wrote:
>
> > Yep. I put a fair amount of work back at RotRock into means for
> > torquing that huge momentum wheel over during the ascent burn.
>
> Differential throttling (simulating the cyclic-pitch function of
> a helicopter rotor) seems like the best bet. There are a lot of
> ways to do that, but most of them require actuators with response
> times shorter than the rotation rate. Others require knowing a
> lot more about the construction of the bearing hub and its fluid
> seals than I do right now.
Actually, if you have an even number of engines in contrarotating pairs,
you only need to worry about the precession forces at the engine
bearings; all gyroscopic effects will cancel on a macroscopic scale (not
to say that the precession torques on the bearings will be trivial at
225k rpm). Then you use differential throttling *between engines* in
one or more pairs, like the rudder control in a Chinook helicopter
(which actually applies differential collective to the front and rear
rotors to generate a net torque) for roll and one axis of pitch/yaw.
With four engines, that would put the natural (i.e. easy to control)
axes between the engine pairs, but that's just a matter of mounting the
hardware.
> And then, of course, the engine wheel has to be rigid enough to
> withstand these oscillatory torques. That's not a problem for
> helicopter rotors, because they're not rigid; the blades flap all
> over the place. But I don't think the engine wheel can be allowed
> to do that.
With multi-engine setups, there wouldn't need to be oscillatory torques;
no angular accelerations on the engines that rise or fall faster than at
startup (in fact, you'd want them a great deal gentler than that).
Note, too, that we're only talking about torques due to bearing
friction, not the torque you'd need to spin up an engine like that; that
means, among other things, that the engines don't have to be matched
down to the fifth decimal to give a flyable vehicle; you just need a
trim system, as on any aircraft.
--
Love wealth above life itself, and starve in splendor.
-- Elvish proverb
Donald Qualls, aka The Silent Observer NAR # 70141-SR Insured
Rocket Pages http://silent1.home.netcom.com/launches.htm
Telescope Pages http://silent1.home.netcom.com/astronomy.htm
Lathe Pages http://silent1.home.netcom.com/HomebuiltLathe.htm
Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.
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