Pyramid semi-success

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rocketmanMV

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Loaded an I-211 short into my test mule pyramid this afternoon. I used equalateral triangles for the sides and this made for a squattier rocket than most pyramids that I've been able track down. I set up a trap door on hinges for the 48" chute to come out, all very near the forward tip. The boost was spectacular; all I could have hoped for; straight up, a huge square of rising flame and smoke, amplified by the recessed rear bulkhead. Alas, the chute never deployed; the charge pushed the upper interior bulkhead back instead of booting out the chute. That blue tape holding the trapdoor shut worked just a little too well... I'll have to cut a substantial hole in the side to repair all the interior damage. Still, the thing that I was most worried about, stability, was fine.
 
I have a couple of pics of my pyramids, e-mail me an I will send them to you
 
Heres a pic of my small pyramid on a H180. The large pyramid pic is of a I211.
 
I mounted a 75mm motor tube in the pyramid, then glued centering rings to the motor mount. The parachute is put inbetween the centering rings. On the big pyramid I used a Perfectflite mini timer and a flashbulb to eject the motor tube, the chute unrolled and deployed from there. On the small one it is the same design but motor ejection. The parachutes are custom, do you have a way of making your own chute?
 
Is the parachute ejected from the base of the pyramid? Where is it anchored? BTW, my chutes are just leftover PML or Rocketman, leftover or pirated from other projects.
 
I have a weird question: What would be the problem with using a truncated interior pyramid to house the motor mount with a chute on top of it and a full pyramid that covers the truncated one entirely for the outer body? The full pyramid would function essentially as a nosecone. If my experience with dealing with the rear window of a Dodge Caravan without a wiper is any indication, the aerodynamics of the airflow pushing back up onto the aft end of the truncated pyramid could be enough to prevent drag separation and could simplify the design by just allowing the motor ejection to blow the full pyramid off the top to deploy the chute.

Just a thought, and, as always, there could be a serious flaw in my thought process.
 
Kermie, that's not such a wierd solution; I've thought of that one too. The whole parachute cannon at the base of the pyramid thing is a bit kludgie as it shifts the CG way aft, so your solution keeps all that recovery system weight far forward.
 
The only reliable parachute ejection I have had so far in my various tetrahedra is sideways... what I have named "Lateral Thinking Technology (patent pending)" in a vehicle called Braques Big Black Breast.

You can see it at https://www.palimpsest.co.uk/tet/tet.html

I had two singularly unsuccessful early flights with the big black one with rear exit... mostly owing to the chute being too tight which was due to the tube being rather too short for the chute ;-)

https://www.palimpsest.co.uk/KLOB/KLOB.html

I recently tried another rear exit design called Nothing2Hide last weekend but the base drag literally sucked the chute out which was poetic as I thought the only gremlin in those earlier flights was packing it too tight ;-)

Good thing it was undersized. N2H went on to a abbreviated apogee trailing its chute and returned to earth scarcely marred. A wet wipe and 3 inches of tape fixed it to fly again (once I have redone the chute deployment).

I have had two spacks using integral ejection charges (can't get the delay short enough with either Pro38 or Aerotech) but three perfectly timed ejections using a magnetic apogee detector and Pratt ejection canisters have made me change my ways.

I would have great reservations about the truncated tet within the big tet nosecone tho the idea has some appeal esp. since it would seem to allow a generous compartment for the chute.

An early ejection would be fighting against the huge base suction generated by the geometry's aerodynamics. A late ejection would require a charge big enough to eject the truncated inner tet >upwards< away from the plummeting outer tet.

I think that forward ejection is the way to go - I will be able to give you some empirical results as soon as I have refitted N2H.

I think you had a problem b/c of the tape. Tape, even lo tack masking tape, in tension is surprisingly strong (I learned that the hard way).

I think that a little attention to the chute exit door shape will solve your problem. It needs to be shaped so that it is held in position by the airstream on the way up yet free to fly off with some BP encouragement at apogee or afterwards.

The nice thing about tets (or pyramids if you insist) is that they have the terminal velocity of a hay bale. My first tet flight on a G spacked but took only 15 minutes with some 5 min epoxy and a file to prep for a second try with an H... which also spacked b/c the delay was too long and the chute was still too tight.

Short delays (and/or MAD triggers), loosely packed chutes, and any egress except rearwards are the ingredients for success IMHO

YMMV

Paul

Captain Tetrahedron

Team Uranus

UKRA 1276 Level 2
 
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