I can only express myself with cases
case 1 multiple engine vehicle with variable thrust steering making landing. object 0 velocity at point X, Y with Z =0 (ground)
cross wind drifts rocket at 5 mph to the right (from observer), right engine increases thrust tilting rocket to left canceling drift. Thrust line moves off center. If the increase in the right engine thrust does not equal the increase in acceleration do to gravity and a tilted thrust line then you must adjust the throttles to keep velocity where you want it. The balance of altered thrust for steering and control of velocity would depend on the moment arm of the engines to the CG which in most rockets (all rockets!) is fixed. Intuitively I feel the balance of these forces, acceleration due to altered thrust line and increased acceleration due to added thrust on right engine would only balance at certain attitudes and at other attitudes you would have to increase or decrease total thrust to maintain goal velocity of vehical. Now if the response of the engines thrust to valve opening is not linear then you have a control problem which is unworkable. You can either have precise control of velocity or of exact location, but not both (at any one time). Doesn't this put you in the same position as a pendulum attached to a pendulum and predicting the position at the fifth swing? I'm not saying the rocket acts as a pendulum, but rather that you have the onset of chaos. No matter how fine you try to make your control system for the rocket you will be fighting a losing situation with two non linear forces acting on the same body?
I love coffee.
Henry Spencer wrote:
On Tue, 4 May 2004, Alex Fraser wrote:
...it occurred to me that you surly could steer the rocket with the various methods, but you could not accurately position the vehicle. If you are trying to land on the X mark (or fly through a small fixed donut) and it is important to really hit that mark you will find you move the machine sideways by altering the thrust line (vanes, gimbels or what ever)...
Actually, most of the methods mentioned can be used to position a vehicle
in hover, which seems to be what you're really thinking of. RCS, fluid
injection, CG shift, all work for that. About the only ones which don't
are the aerodynamic ones, which require some amount of forward motion.
And you can do a precision landing even with aerodynamic controls, in
principle, by simply not slowing below minimum controllable speed until
the last instant. (Admittedly this may make for a hard landing.) Real
vertical landings -- as opposed to what you might see in an airshow --
are *not* done by slowing to a hover well off the ground and then slowly
descending. They're done as a rapid, decelerating descent, with descent
rate reaching zero roughly at touchdown.
Henry Spencer [EMAIL PROTECTED]
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