Too Much, Much Too Much.

What to do with all that power? Not motor power; it is after all only three 13mm A3s.

But the ejection charge from those three motors is channeled into a single BT20 tube and even with a baffle and six feet of shock cord the nose cone still hits the end with enough force to bounce back and prang itself on the fins. And this does nothing good for the streamers or parachute.

Anybody have any ideas as to how I could vent some of that pressure without increasing the risk of not actually deploying the recovery device?

It makes no difference if the shock cord is entirely Kevlar or a mix of Kevlar and elastic.
And no; there simply isn’t enough room to substantially lengthen the cord and still fit it and the streamer/chute into the available space.

Just a thought and I'm no expert, but maybe drilling two or three vent holes close to the forward end of the tube would help.

pepe said:

Just a thought and I'm no expert, but maybe drilling two or three vent holes close to the forward end of the tube would help.

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Possibly; but with the chute or streamer filling the lower part of the body tube just above the baffle, acting much like a piston, any vent holes at the top would be largely useless for venting all the pressure from below the chute/streamer. Now if they were placed down from the top a distance that equaled half the length of the rolled streamer or chute then perhaps the excess pressure would be vented yet the recovery device would retain enough momentum to clear the tubes end.

I considered vent hole/s just above the baffle of perhaps halfway between the baffle and the open end but I’m concerned about having things get halfway deployed and then. . .

Don't use the same delay on all three motors. That way only the shortest delay motor ejects the chute, and the other two blow into the wind.

I would have said use two motors with zero delay, but they don't make those in A3, do they?

In fact they only make a single delay time for each 13mm motor. A3-4 T is alls you got.

Either that or A10-3T

boomtube said:

In fact they only make a single delay time for each 13mm motor. A3-4 T is alls you got.

Either that or A10-3T

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Crap. There goes a brilliant answer down the toilet. :bang:

My only other thought is this, and it's not brilliant. Use a second, very tiny streamer for the nose cone, and separate the nose from the body (i.e., have each one come down on its own streamer). Easier to lose parts that way, but nothing else comes to mind.

You could try the vent hole idea, but it would take a lot of experimentation, and you may end up destroying the rocket before you find the right ventilation scheme.

I would try either venting two of the motors to the outside or drilling a couple small holes just above the baffle. Start small and work your way up. You could also just lengthen the shock cord.

There is no means by which I can vent just one motor, the motor mount is a 13mm FlisKit “Tress” style and it channels all motors into a single BT20 airframe tube. I really, really need to get a new camera so I can show this kind of stuff.

As for lengthening the cord; there is just not enough room available to add enough to make a difference.

I hadn’t considered separating the nose cone from the rest of the rocket. I’m not sure there is enough room in the main body for two separate recovery devices what with six feet of cord and all.

On another subject; does anybody make/sell chutes made from very thin fabric that packs down very small? Like a 9” or 12” chute that can pack easily into a BT20 tube and that actually opens after deployment.

Those plastic type chutes have a nasty tendency when packed that tight to just remain a blob on the end of the shroud lines after deployment.

CATO Chutes has an 8" ripstop. Never used it, so I don't know how well it packs. But you could call them.

Estes has the A10-PT which is a plugged motor. 2 of those and your A3-4 may be a possibility.

boomtube said:

I hadn’t considered separating the nose cone from the rest of the rocket. I’m not sure there is enough room in the main body for two separate recovery devices what with six feet of cord and all.

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If you have 2 separate components, you won't need any cord for the NC and a much shorter one for the booster's streamer. I also like DuctTapeAndRocketFuel's idea. But maybe to keep power balanced, go with ala A10s, with 2 plugged and 1 unplugged? The also make booster A10-0 engines that have no charge at all.

Try taping the cord in sections. Gather up a loop of cord, wrap with a little masking tape, go down the cord a little and do it again. As many times as you wish.

What happens is at ejection each loop will brake the tape and slow the cone down some.

I tried this on a rocket that was lost forever to the rocket gods on it's third flight. Saw it go up and it vanished. Shame too, I really liked it. Was a min Dia 24 flight on a D. Just long enough to be stable. No added weight. Designed from the start to go high. Guess it worked.

I had a long kevlar thread cord in it and on the first flight had a small cord ding in the tube, someone suggested this to me and it worked for the second flight.

See ya,
Rod

boomtube said:

In fact they only make a single delay time for each 13mm motor. A3-4 T is alls you got.

Either that or A10-3T

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1/2A3's come in both 2 and 4 second delays but don't know if you're committed to using A's.

I will pile onto the suggestion to use either 1 A10-3T plus 2 A10-PT or 2 A10-3T and 1 A10-PT.

boomtube said:

What to do with all that power? Not motor power; it is after all only three 13mm A3s.

But the ejection charge from those three motors is channeled into a single BT20 tube and even with a baffle and six feet of shock cord the nose cone still hits the end with enough force to bounce back and prang itself on the fins. And this does nothing good for the streamers or parachute.

Anybody have any ideas as to how I could vent some of that pressure without increasing the risk of not actually deploying the recovery device?

It makes no difference if the shock cord is entirely Kevlar or a mix of Kevlar and elastic.
And no; there simply isn’t enough room to substantially lengthen the cord and still fit it and the streamer/chute into the available space.

Click to expand...

Because the ejection happens far away, it's hard to see exactly what is happening. Are you sure that the nose cone is shooting out so fast that it is rebounding back and hitting the rocket? Or could it be that the delay is not good for this motor-rocket combo, and the chute is opening when the rocket is traveling fast? In which case, maybe the momentum of the rocket stretches the shock cord, not the force of the ejection, and it is the rocket rebounding toward the chute and NC, not the NC rebounding toward the rocket.

In either case, it seems like elasticity is what causes a rebound. Have you tried all kevlar? (I wasn't sure if you meant you've tried all kevlar, all elastic, and a mix, or if you meant you were open to all these suggestions). Kevlar is not elastic, so I would think there would be little chance of rebound.

I think with kevlar, there is a way to kill momentum by making a small loop near the BT, then feeding another loop through that one, and another loop through the second, a fourth loop through the third, etc. all the way up to the chute. It needs to be loose enough, so when you pull one end of the cord, all of the loops pull out of each other in order, and the friction of unraveling the chain kills some of the energy. It's sort of like what Rob was suggesting above. I've never done it or seen it done, but I've heard of it.

Depending on the size of the NC, you could get a tiny bit more space by cutting off the bottom of the NC and gluing an attachment point up inside the very tip. Then you'd have the volume of the NC to pack with more cord or other recovery gear.

Top Flight Recovery sells thin mill nylon chutes. They are more compact than regular nylon, but I'm not sure they are any more compact than plastic.

Kevlar can be very small for its strength, so might be able to add a foot for every inch or 2 of elastic you lose.

I have become more aware, through various lessons, of the importance of allowing plenty of space for the recovery gear, wadding, and pressure control. There's usually more talk about large rockets having trouble getting enough pressure, but the opposite is more my experience.

I am not so sure that I would recommend using more than one plugged motor (with no ejection charge) in a three-motor cluster. If you only use one motor with ejection, you are betting everything on always getting successful ignition on at least that one motor. I don't know about you, but my record for achieving successful/complete cluster ignition is nowhere near 100%. With single-motor-ejection, you would also be betting on that motor having completely successful operation, with no "dead" delay or ejection charges.

Using two motors with ejection would greatly improve your odds of success. And if those two motors somehow fail to ignite, your rocket might not be leaving the pad anyway.

Using multiple motors where each has an ejection charge may not cause a severe environment. Remember that all three are not going to ignite at the same instant and they are not going to burn for the exact same perfect duration. Odds are that the resulting ejection gas will almost certainly be delivered at slightly staggered time intervals. They may only be a couple hundredths of a second apart, but once the first ejection gas hits the NC and removes it, the other ejection charges are simply venting through the empty body tube. Odds are infinitesimally small that you are actually getting all three ejection charges at the same instant.

If you want to let the NC separate completely, I suggest you seriously consider a streamer. Often takes less volume inside the body tube, and can work very well with a component as small as a NC.