Re: Steering

Replies interspaced

Bill Clawson wrote:

>First off, you can always linearize the turning of the
>valve to engine thrust via a look up table.  We do
>this in disk drives because the position error signal
>from the disk drives is often-times non-linear in
>nature.  Ideally it would be linear, such that the
>amount off track is directly proportional to some
>position error measurement, but this is rarely the
>case these days due to the nature of disk drive
>read/write heads, etc.
>
And if it gets lost you just reset the heads to zero and retry, all you 
lose is access speed as you have to retry the data access. At under 10 
feet of altitude this would mean a bad landing for a rocket. One non 
linear element can be compensated  for with a closed loop system, when 
you get to two non linear elements the best reaction is luck, next best 
is some hunting time for the system which if it gets to long in duration 
goes beyond the systems ability to recover. It would appear as wobbly 
movements of the machine with occasional loss of control.

>  
>
>Also, you can take into account any time delays in
>building up thrust for a given change of valve opening
>via the overall servo loop.  This is pretty common
>stuff in the servo world, so I wouldn't be too worried
>about that.  We get this all the time when dealing
>with electronics and physical systems that don't
>always respond right away, due to large amounts of
>inductance or what have you.
>  
>
But you can't have acks and nacks or have the machine force a line low 
to stop the system.  Also the issue of scaling comes in where the more 
mass  you try to move the slower your controls work (with a reasonable 
energy budget of course). 

>Lastly, slop in the valve mechanics themselves can be
>compensated for, if not taken out immediately, by the
>overall effects of the servo loop.
>
>I would think that in a vectored thrust stearing
>configuration you would also want to make sure that
>the thrust vector for each engine DOES NOT pass
>through the CG of the vehicle, but is off of the CG
>such that the change in thrust really does cause the
>rocket to change where it is pointing.  Otherwise,
>there is no point to vectoring your thrust.  
>  
>
Yes and how far from the center of gravity is the moment arm, the more 
moment arm the less throttling to effect the same RATE of turn.

>Lastly, you are going to have to solve for the effects
>of gravity by increasing the overall thrust in
>proportion to the cosine or sine of the angle you have
>now oriented your rocket (I leave it up to the readers
>to do the trig) and for the change in the total thrust
>you have caused across all of the engines due to
>reducing the thrust on one rocket.  NOTE: The easiest
>way to compensate for changing the thrust on one
>rocket is to increase the thrust of the opposite
>engine by the same amount.  If this does not cover
>your rocket configuration, then the overall math is
>only slightly more difficult.
>  
>
I will do more  CAR  (Coffee Aided Research) in the morning.  I think 
that the complexity of solving these problems is greater than the 
complexity of adding attitude thruster rockets.
 
My order of suitability for manned flight;

1 Gimbaled main engine  with  attitude thrusters
2 Vanes on main engine with attitude thrusters
3  multiple independently gimbaled thrust engines
4 multiple independently vaned thrust engines
5 multiple variable thrust

    Of course 1 and 2 are multiple rocket machine with the 90 degree 
placement of the attitude rockets allowing for their size to be greatly 
reduced. if rockets were shaped like pancakes this of course wouldn't be 
true as you could not get a good moment arm. Rockets are shaped more 
like pencils for aerodynamic reasons so you can get further away from 
the center of mass going fore and aft, hence the smaller thrusters needed.

>
>
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>


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........ Alex Fraser  N3DER .........
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