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: > > [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. 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. > > [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.) > 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. > > > [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 > 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! > --mgross > ------------------------------------------------------------- > Young lady, in this house, we obey the laws of thermodynamics! > -- Homer J. Simpson > > _______________________________________________ > psas-airframe mailing list > [EMAIL PROTECTED] > http://lists.psas.pdx.edu/mailman/listinfo/psas-airframe
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