Re: Steering

Not just hover, but arriving at exactly a specified X ,Y ,Z  at a 
certain time and with a certain velocity and with a certain attitude 
(yaw pitch roll.)
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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>
>  
>


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>>>>>>>>>>>>>>>>----<<<<<<<<<<<<<<<<<
........ Alex Fraser  N3DER .........
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