Parasitic Gliders and Rocket Stability

Discussion in 'Rocket Boosted Gliders' started by JCL, Jul 7, 2010.

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  1. Jul 7, 2010 #1

    JCL

    JCL

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    I recently built and flew an Estes Shuttle XPress. One of the gliders flew well and the other still needs some tweaking, but we had some fun flights with it.

    Parasitic gliders seem so simple that I want to make more and adapt another rocket to launch them. I have a Chrome Dome which is very close in size to the Shuttle XPress booster.

    My major concern is stability. I haven't been able to find much online about this. It looks like a symmetric rocket like the Chrome Dome will do best with 2 gliders to keep it balanced. Or would it still work with 1?

    Is the change in CG the biggest influence on stability? Or is it the slower speed due to greater mass and drag?

    The Shuttle XPress gliders are only connected at the nose. Does this mean that these gliders are like pivoting fins, so this doesn't change the stability?

    I will be doing some experiments, but I fly at a local park with my sons so I would prefer to keep things somewhat sane.

    Joe
     
  2. Jul 7, 2010 #2

    Jerry Irvine

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    Static stabilty is simple CG-CP. Dynamic stability is different. If you have a parasite on one side like an Enerjet Aero-Dart with a Pterodactyl, with the glider just above the fins, it is stable but tends to rock a bit on angle of attack and can rotate or spin due to parts misalignments.

    But "stability" is stability. Any parasite has drag. Drag as a rule tries to damp perturbations.

    Tech Jerry

    http://v-serv.com/usr/kits/fdart.htm
    http://www.ninfinger.org/rockets/catalogs/enerjet72/72ejet12.html
     
  3. Jul 7, 2010 #3

    cjl

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    In general, adding gliders to the back will tend to increase stability. If you have an asymmetric arrangement, the drag of the glider on one side might cause the rocket to arc over, which could be bad (especially with longer burning motors). I would try to keep it symmetric if possible,

    Oh, and Jerry, drag does not tend to damp perturbations. Mass tends to damp perturbations.
     
  4. Jan 14, 2013 #4

    Ez2cDave

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    ke-6a.jpg ke-6c.jpg ke-6d.jpg ke-6b-2.jpg ke-2f.jpg
     
    Last edited: Jan 14, 2013
  5. Jan 14, 2013 #5

    Ez2cDave

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  6. Jan 14, 2013 #6

    rstaff3

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    I saw one rocket with a single glider arc over badly. The results were horrific. I don't know the stability calcs/thrust-weight ratio/etc of the rocket in question. But the event makes the question you asked worth asking.
     
  7. Jan 14, 2013 #7

    JumpJet

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    The new Manta Foam gliders that Estes sales seperately work great for this.


    John Boren
     
  8. Jan 14, 2013 #8

    luke strawwalker

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    I've flown single fairly large foam gliders (a shuttle) on a roughly 3 inch (Christmas wrapping paper tube) rocket powered by a D12 before... it works well but they do tend to arc a bit... main thing is don't launch in windy conditions!

    I think the off-center mass tends to make them arc more than the additional drag. I never experimented with counterweights to test this assumption, though. The larger the core rocket and smaller the parasite glider, the less pronounced the arcing effect, however... and of course a good high-thrust motor that gets things up and moving quickly certainly helps (from an arcing standpoint anyway).

    The main thing is to have enough motor power to actually get the thing up and moving in a stable manner-- IE without arcing too much at liftoff, and to keep the stack moving at a decent rate as drag increases (remember that drag quadruples as speed doubles (drag squares) so the faster the rocket goes, the faster drag builds up to retard it's gaining speed...) I tried this glider on a C6 powered rocket and it was arc-to-crash... bout demolished the rocket, though the glider was fine... stuck to D's after that... For this reason, I'd avoid D9's and stick to the higher average thrust motors, where you have a choice...

    As for the aerodynamics and stability, well, obviously you want to mount the glider pretty far aft-- usually just ahead of the rocket's fins. This usually puts the glider far enough back that it tends to shift the CG back SOME, but since gliders tend to be very light for their size, (mass/area ratio) the CP usually shifts back more to compensate, keeping the stability the same or slightly increasing the stability margin (CG/CP relationship in calibers). If the glider is free to rotate on a forward mounting pin, then it's contribution to stability will be slight-- (look up "pivoting forward fins" for more information). What's bad is, they tend to foul up the airflow over the existing rear fins of the rocket, which reduces the corrective force the existing rocket fins can provide, because of turbulence generated by the glider's wake during boost. Fortunately most rockets seem to be able to handle this "bad air" without too much tail-wagging or other bad flight effects... (at least in my experience). For this reason, I tended to prefer a secondary attach point near the rear of the glider-- I experimented with a tape loop (tape folded back on itself and attached end-to-end) to "pin" the tail of the glider to the rocket tube at the rear... it worked okay, but the connection isn't particularly rigid... didn't foul up seperation, but then this glider was pretty substantial mass-wise and the inertial and simple "peg" style hook into a hole in the side of the body tube came loose easy when the glider seperated-- which then caused the glider to pitch up sharply and rip it free from the tape (or the tape free of the tube). A tape loop on the back of the glider certainly doesn't help with a clean glide, though. Later I notched the back of the glider a little bit to slide over the leading edge of one fin a little bit, right at the root/tube joint, to give a little more "stable" connection with less "play" in it from the dinky tape loop. If the rear of the glider is pretty well fixed with respect to the rocket, then the gliders' wings/rudder will contribute to the rocket's stability far more than a simple "forward peg mount" will that is free to rotate and flop around some...

    In the end, it all comes down to experimentation to find out what works, more or less... :)

    Good luck with your projects! OL JR :)
     
  9. Jan 14, 2013 #9

    kjohnson

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    If you are going with a single parasitic glider, you need to make sure that you have a longer booster to account for the area of the glider.
    Take a look at the kits that have been produced with a single glider (Manta, F-22, Multi-roc, Discovery with HitchHiker, etc) and you'll start to see a pattern in the length of those rockets.

    If you booster is short, you need to have a smaller glider or add some nose weight, or think about using two gliders.

    kj
     
  10. Jan 14, 2013 #10

    aerostadt

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    My scratch-built 4x Orbital Transport flies very straight on ascent, but it's weight is in a much higher category and it has lots of aft fin area.
     
  11. Jan 14, 2013 #11

    kjohnson

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    Yeah, but the OT has a lot of stuff hanging out in the breeze on the non-glider side to counteract the offset drag..

    kj
     
  12. Mar 12, 2019 #12

    Ez2cDave

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