On Sun, Dec 09, 2007 at 05:30:30PM -0800, rq17zt wrote:

I'm sorry I'm new here, who is rq17zt?

> Hey all, (forgive the re-ordered snip-snip editing)
> Top down design:
>  > 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.

I know some folks will put the fuel in a badder to reduce slosh.

> 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.

yes, and this is cool stuff.

>  > > [..] 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.)

wow, those are cool... http://en.wikipedia.org/wiki/Aerospike_engine
> 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.

2 stages?  I hadn't really grocked this point for some reason.  You
will need multiple stages wont you?  Which launch will test a multi
stage rocket with semi-controlled flight for both stages?

> 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.
>  > > > 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

any plan for multi-stage testing?

> 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.

how do will the orienteering problem be solved?

> 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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