On Sat, Dec 08, 2007 at 07:55:18PM -0800, Andrew Greenberg wrote:
> Hi mark!
> 
> 
> > I was just thinking about what controls for guiding the rocket that
> > would be available, and thinking about the single nozzle thing with O2
> > pumps pushing O2 into the combustion chamber as a way to provide some
> > vector's of control to the system.  (my first reaction is hmm, this
> > doesn't sound too plausible.)
> 
> Oxidizer injection (or, liquid injection) thrust vector control is
> possible, it's been done by other (even amateur) groups. But, it's not
> very proven, and it'd probably take a lot of development work by us.
> 
> > Anyway after seeing a picture of a 2 engine rocket on some magazine it
> > occurred to me that if we had 3 or 4 thrust controllable engines tied to
> > the airframe, then we'd have a good chance of having a controlled
> > flight.
> 
> Yes, this is clearly the easiest way to do thrust vector control since
> it requires only 3 or 4 valves. But, you now need three combustion
> chambers, three nozzles, etc.
> 
> > How realistic is designing an air frame with a 3 or 4 engine backside?
> > How realistic is controlling the thrust out put of the rocket engines?
> 
> The real question is, how realistic is it for us as a group to pursue
> thrust vector control? And the answer we've currently come up with is
> "sure, but it'll be hard and take a lot of time". So, maybe, the
> question is, is TVC the right control mechanism for this group? And
> although we don't know, it seems like we're leaning towards more simple
> "stone knives and bear skins" approach to control, which would be small
> control surfaces for lower stages and small cold-gas reaction control
> systems for upper stages.

I think its realistic. And if you don't figure out a non-aerodynamic
TVC design you will only be designing a missile not capable of orbital
insertion, (because control surfaces depend on aerodynamic forces
missing at high altitudes)

At least you need some sort of "plan for a plan" to get tot he point
that putting something into orbit is plausible.


> 
> > Defining how the vehicle thrust vector will be controlled is really a
> > key design choice.  Has this been made yet?
> 
> No, and luckily we don't have to right now. What's important is that we
> tackle the really hard preliminary issues first, which is the design of
> the avionics system and the state space observer we'll be using. Once
> we've gotten that going, launched it a few times, verified it's doing
> what we think, *then* we can start to think about closing the loop and
> doing control. And my guess is we'll want to start playing with control
> right away when we're ready, so that might mean doing something really
> simple like small control surfaces for a while. Then we can start to
> switch from large passive fins to small control surfaces, and then start
> to think about staging, etc etc. It'll be fun as heck to solve this
> problem when we get to it!

I don't know about this.  I think with the problems you are solving and
questions that are being asked, you only have 1 or 2 more launches before
you are waisting your time.

Also, without having some idea of how the control problem will be
addressed we won't know what experiments are critical we get data from
on the preliminary launches you are talking about.  Some design
attention up front is worth considering.

All the other systems and design issues are hard problems, but without
at least a plan for a plan you are designing a missile.  A missile with
some unneeded avionic components.

I know I'm likely giving my area of interest more attention than all the
other problems but these are fundamental questions.

Today we can start asking questions like:
If we can only have low pass filtered data for x/y acceleration using a
32 sample filter (i.e. a control loop cycle time of at least 0.0128 sec)
and we assume we have a infinitely fast control response, how much TVC
thrust will be needed to have controllability?  Where do we get into
instabilities and chaos, and why?  The answers to questions like this
will drive the avionics requirements that matter for orbit insertion.

Right now we can ask this question with an idealization of the TVC
however; we can't really start asking this for real without force
diagrams of some thrust vector control designs we think are plausible.


--mgross

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