Hey all, (forgive the re-ordered snip-snip editing)
Top down design:
[EMAIL PROTECTED]:
> 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.
Absolutely, having a good understanding of the external forces
involved is key to smart control design. I've done a little research,
and not found much. The ground speed of winds at high altitude can be
100's of kph, but how gusty are they? I haven't found a good
reference. Just looking at wing camera footage from high altitude
aircraft, it looks like the weather up there is just as variable as
down here. Some shots show smooth as glass glides, while others show
the wing tips trying to clap together.
Another example is tank slosh. At certain resonances the liquid in the
tanks can impart a strong yaw force which can even lead to failure if
not controlled. Something as simple as a single baffle in the right
place may be all that is required.
Within the atmosphere keeping the vehicle pointed into the wind
significantly reduces drag. So to the extent that larger fins are both
more accurate in pointing, and more draggy, there is a fin size that
maximizes altitude for a given flight package. If active controls are
added to the mix then the optimal size for passive fins changes.
In flight the vehicle tends to arc. The separation between center of
pressure and mass create a moment arm that turns the nose toward the
ground. The ideal flight profile is a compromise between the gravity
loss and drag loss given the feasible motor thrust. This implies a
minimum control authority needed to counter the gravity turn. The best
profile may be mission dependent even if the vehicle doesn't change
much, due to payload or motor variation.
[EMAIL PROTECTED]:
> > [..] 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.
At some point i hope we'll be in a position to seriously brain storm
about this. There are a lot of approaches that haven't been really
documented (maybe even tried ;) (I'd mention linear aerospike arrays.)
There is an optimization to be considered here. Aero controls are only
effective for 60 seconds or so of flight, but that's the 60 seconds
when most of the loss and outside disturbances occur. In my opinion,
an optimized amateur orbital vehicle will have fins on the 1st
stage. If these will be passive, active, or both is somewhat murky.
Active fin control (AFC) vs thrust vector control (TVC) or reaction
control system (RCS), has the advantage of zero propellant usage. If
the weight of the AFC system is less than the others, it might be an
overall win.
Of all the systems i've heard of RCS is the single system that works
in all flight regimes. Thus potentially it requires less total design
time.
I'm skeptical of RCS-only however because of the need to resist the
gravity turn, which note is an aerodynamic issue. Blowing out fuel to
do something we could do with a fin seems considerably heavier to me.
On the other hand, suppose the nosecone was asymmetrical such that it
imparted a tangential force to the flight path. By controlling roll
with RCS a low fuel-cost solution might be possible. Same sort of thing
might be achieved with slight _negative_ stability held very close to
neutral by an attentive control system.
[EMAIL PROTECTED]:
> > > 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.
The road map we've been implicitly following is this:
. LV2 demonstrates telemetry and orienteering
. LV2+ demonstrates trajectory following
. LV3 design phase low cost amateur launch vehicle
. Motor shopping
. LV3 orbital operations
Crunched versions of LV2 have demonstrated telemetry and sensor
package. (Despite the transverse noise i'm not worried about the basic
IMU design.)
IMNHO we know how to solve the orienteering problem. If we had a
couple full time graduate slaves we could bang out a solution in a
couple months.
Fitting LV2+ with a control system(s) makes a lot of sense to me, and
i do agree that making the right choice of system is important, and
seems to require a peek ahead into the LV3 design phase.
Collectively we know what taking that peek involves: Some
aerodynamics, some due diligence on motors and controls, and a
credible mission model (simulator).
My personal problem has been (hu)manpower. I have only one unit at my
command. (Actually slightly less than one ;) So i've been
concentrating on finishing the orienteering phase, hoping to get to
the rest when that's done.
It would be great if there were someone willing to lead a peek-ahead
effort so everybody keep their eye out, or better yet, volunteer!
-------------------------------------------------------------
Young lady, in this house, we obey the laws of thermodynamics!
-- Homer J. Simpson
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