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Jan 19, 2009
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How about ditching the hardware altogether and using a kevlar harness? I've seen some smaller (38mm) av-bays using this method.

I'm considering doing exactly this with my 2.6" av-bay. I built it out of 2.6" tube coupler with a cut down tube coupler like a stiffy tube. I'm looking at using small screws with blind nut to hold the endcaps on and using a loop in the kevlar to anchor it internally in the av-bay. Loops on either end would hook to the rest of the recover harness.

Your thoughts?



Adrian A

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Jan 22, 2009
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I'm looking for lightweight E-bay alternatives for dual deploy on a 2.6 dia. rocket. I am thinking of building a rocket to launch on the new AT I49, or I59 and need to keep the weight down. I'd prefer to build something a little bigger than minimum dia. and think a 2.6 inch rocket is doable while still staying around 2 lbs loaded weight. See Impending doom thread about an I49build/ launch. http://www.rocketryforum.com/showthread.php?t=4146

Aside from the motors which weigh a lb. or so the biggest mass contributor will be the e-bay. The LOC E-bays are built like a tank but weigh a ton. I have a LOC standard length e-bay for a 3.0 BT rocket and the thing must weigh 10 ounces, maybe closer to a pound ( I need to weigh it). I'm thinking I could lose the stiffy coupler and use aluminum threads and nuts on a 2.6 LOC type e-bay and it will still probably be too heavy. Any thoughts?
Ah, now this is a fun question. Here's how I would look at it:

An av-bay needs to do 2 things: (1) Hold the altimeter in a pressure-protected enclosure that has electrical outputs and can be disassembled for altimeter access (2) couple the 2 sections of the rocket together and provide attachments for all the recovery harness. Since the diameter is much larger than required for the altimeter, you can do these functions separately and save a lot of weight.

Start with the structural coupler function. You just need a coupler, a single bulkhead, and a section of kevlar harness that passes through the bulkhead and is knotted/potted in place. I'm not sure how much the coupler would weigh, but let's say it's 20 grams. The thin bulkhead and kevlar should only add another 10, so you've got 30 grams so far. The kevlar would have loops at both ends to tie the main and drogue harness to. No metal required. This bulkhead can be much thinner than usual for 2 reasons: (1) It's glued into the coupler all around the edges, so its edges are captured all around and it works together with the coupler to form a strong member (2) the kevlar that passes through to it takes all the recovery shock load, so the bulkhead takes none, except for just its own weight and the altimeter holder.

Now for the altimeter holder function. It only needs to be strong enough to hold the altimeter and resist the pressure of the ejection charges. A 24 or 29mm cardboard tube, a 0.03" thick G10 sled, and 1/16" plywood for bulkheads at each end would do the trick. It should weigh about 15 grams without the altimeter or batteries. I would drill a 29mm or so hole in the coupler bulkhead, glue one end of the 29mm tube to it, and close off the other end of the tube with one 1/16" x 29mm bulkhead. Now you have an open-ended can for your altimeter sled to go, that's permanently attached to your coupler. Cut a simple rectangle of G10 for the sled, and glue an oversized bulkhead to one end. The altimeter sled goes into the open can and is attached to your coupler bulkhead with screws around the overlap. Still no all-thread required. Now your altimeter is sealed within its own container in the middle of the rocket, your single coupler bulkhead is sealing off the drogue side from the main chute side, and your main chute airframe is connected to your fincan with pure Kevlar harness, end-to-end. To connect the altimeter pressure container to the outside world, either attach a pressure tube along the coupler bulkhead to go between the altimeter tube and the outside, or just make the altimeter tube offset enough so that it's flush with the edge of the coupler. The latter may cause too much mass offset, though. Later today if I get a chance I'll draw a picture.

So I think the av-bay minus the electrical components should weigh about 50 grams or so. Add a Parrot altimeter, some Featherweight screw switches, wire, and a li-po deployment battery for about another 15 grams. Then the grand total is about 65 grams, or 2.2 oz, or 1 1/2 9V batteries, or .14 lbs, or about 1/5 the mass of the LOC ebay you described.