Ejection charge placement

I'm just now getting into HPR electronics and dual deployment and have been testing ejection charges. It appears to me that the drogue chute ejection charge placed forward of the shock cord and parachute (ie at the aft end of electronics bay) requires a larger charge for a given ejection action than does what I've observed a motor ejection charge (based on published Cesaroni ejection charge amounts for various impulse levels). I speculate that this is because the motor ejection charge gases work on the chute, shock cord, and forward end of the rocket to push them all away from the fin can. It seems to me that with the ejection charge at the aft end of the electronics bay that works to push the forward end of the rocket away from the fin can but pushes the shock cord and parachute into the fin can, and the momentum of the forward end of the rocket is all that pulls the shock cord and parachute (now compressed into the fin can) out of the fin can. Therefore, I will need a drogue chute ejection charge larger than a typical motor ejection charge to deploy the drogue chute.

If the above is true, it seems to me that I should extend the wires of the ejection charge so that I can place the ejection charge aft of the shock cord and parachute, next to the motor mount. I speculate that the connection wires would add little friction during the ejection.

Comments please!

John

Keep in mind a problem with a free placement of a "hard" plastic canister can result in it blowing out the side of a thin walled rocket. Been there and done that. Recently had a bulkhead based apogee charge hang the shockcord a bit in a DD bird. Onboard upper bay video camera caught the sustainer whirling about closely to the upper bay. Fortunately didn't collide. Eventually the shockcord deployed the full length and then the main blew nominally. Staged deployment was less "stressful" on the bird anyways.

Easiest way to avoid blowout is a charge canister or simply put a long bolt through the bulkhead and zip tie the charge to the bolt. Plastic ultracentrifuge
tubes make nice disposable charge holders. Epoxy match in the bottom and good to go. Of course, if one uses fiberwound glass tubes, no trouble with blowout there. Kurt

I have done both methods, powder cup on the aft end of the av-bay, and a long wire with a charge on the top of the motor. I don't believe either method takes any more powder then the other, and both take less then what is typically in a motor ejection charge.

If you attach your drogue to the shock cord about 1 ft behind the av-bay and the drogue is only 2 ft long, you only need three feet of separation to get the drogue out of the lower BT. Most motor ejection charges will push the two parts apart to the end of the shock cord and induce some shock to the system as they pieces hit the end of the cord. BTW, that's also when the nose cone and main chute can come out of the upper section.

My recommendation is put the charge where ever you are comfortable with it, just don't make it too large that it causes the two halves to hit the ends of the cord with force. Even if all the shock cord doesn't come out of the lower tube, when the drogue opens and the fin can drops, the cord will come out.

Thanks for the input. Thinking about it now, your comment about placing the drogue chute near the av bay makes sense but I'm not sure why, on three of the four drogue tests, the forward end did not separate from the aft end. I had measured the force required to pull the forward end out of the aft end and the force was in the 10 to 20 pound range. Holding the forward end of the rocket up vertically, the friction of the av bay was just enough to hold the aft end in place. For the 2" rocket I used 1.2 g of BP, for the 2.6" rocket I used 1.2 g of BP, and for the 3" rocket I used 1.8 g of BP (the 4" rocket with 2.4 g of BP worked fine) and these failed to separate. The noses (main chute) separated well on all rockets fine with the same amount of BP and each had 2-56 nylon screws (2, 2, 2, and 3 respectively). The estimated force holding the noses in place was typically 80 to 120 pounds.

Any ideas on why I didn't get separation for the drogues?

Are my BP charges too small?

John

I put my charges on a lead so they are doing exactly what you say in blowing the laundry out rather than compressing it. Other ways to deal with it are to attach the chute some distance down the shock cord so while it's being pushed into the rocket the parts can separate using that shock cord length and then momentum pulls the laundry out.

How are you containing the BP while it burns? Also the charge required is not directly proportional to the tube diameter so you might need a larger charge than expected for the smaller tubes.

How are you containing the BP while it burns?

I use a kraft paper tube consisting of 3 glued wraps of 60# kraft with an insert of 3 glued wraps of 20# bond paper to reinforce the side wall of the tube. One end is sealed with a glued triangle fold. The ematch is placed into the tube and sealed with another glued triangle fold. The charge is finished with two wraps of masking tape, around the tube and pole to pole. I've been making fireworks (not professionally, but BATFE licenced) for over 20 years and I've made many of these types of burst charges. I could believe that one charge may have not been made well, but not three of four.

I did notice that when I removed the "laundry" from the "failed" drogue sections it was tightly packed and seemed to take much more force do remove it than when I had measured that force to pull out the coupling and chutes (ie the 10 to 20 pounds I had noted above).

Another possibility for the problem may be that I used cloth to plug the motor mount at the forward end and this cloth did not effectively seal the motor mount. What do others do to seal the motor mount for ejection charge testing?

John

I would say you have to loosen up the laundry. 10-20 lbs.+ to remove the laundry is high. It should be packed that you can remove it with very little effort. You want to get the laundry packed as close as to sliding out on its own as you can. How big are your drogues? I never have a problem with space or packing to tight with them, it is the mains that can be a problem at times.

A few layers of masking tape to be sure of the seal on the motor mount would be good insurance for the next test too - I made that mistake once

I would say you have to loosen up the laundry. 10-20 lbs.+ to remove the laundry is high. It should be packed that you can remove it with very little effort. You want to get the laundry packed as close as to sliding out on its own as you can. How big are your drogues?

The 10 - 20 lbs was mostly the friction fit of the coupler. I measured the actual force to push out the laundry only at less that 0.5 lbs. The drogues are 18". I am using about 15 feet of half inch tubular nylon for the shock cord and this is taking up a lot of space along with the kevlar chute and cord protectors. It seems a bit tight. I could reduce this somewhat by replacing the shock cord with quarter inch round kevlar.

I also use an additional cord protector on the motor end of the shock cord to protect from the motor ejection charge. I've assumed that I should leave the motor ejection charge in place with a long delay as back up to the altimeter ejection. Is this reasonable?

I had not considered I was 'blowing the laundry in' when my dual deploy was firing.

I have only had one drogue 'hang' on one flight. I have never really did the math on the charges..had flown and deployed on 38mm so started with 1.3g of bp and my larger bird had flown with a 54mm so measured out 1.7g for it.

Initially had used 'latex fingers' with a nylon zip tie to hold the finger and charge to the eMatch. All the cool guys at the trf had the blast caps type wells mounted to there lids, so broke down and bought some doghouse charge wells. They seem to work well and not so fiddly in the field to prep.

I am happy with this style of avbay lids and wells and terminal blocks as I can pretty much preprep everything , leaving the avbay buttoned up on site and add the charge.

My dilemma is this..after growing up on lpr and mpr and motor deploy..when I pack lower laundry to protect the chute from av bay charge instead of the motor charge..it feels wrong, backwards ...even the shock cord / harness attachment of the nomex protector and the chute placement on it seem wrong. When I asked my gurus locally.. got answers like 'do it the way you like' or my favorite 'if you packed it and it worked..do it the same way'

Kenny

I've been using 1.5 g of BP for the drogue side of my rocket for 5 years now. The tube volume is 4" x 26". I z-fold the shock cord three times and tape with electrical tape, then z-fold two times and then once. The last flight had the two layer z-fold still taped together when it landed. It didn't tear open when the main opened either. I've never had a drogue fail to deploy.

As for blowing laundry in.... I don't believe that really happens. It sound good and logical, but the actual way it works, not so much.

How many times have you gotten a rocket back with motor ejection where all the dog barf was still in the rocket even though it popped the chute perfectly? It happens a lot to me. What about large rocket with relatively small charges like the G-Force with a Hobbyline motor. Several times I've seen the nose cone pop off, the rocket fall flat and when it was picked up, the chute fell out of the tube. The ejection charge pressurizes the tube and forces the joint apart, it doesn't apply much force at all the the items in the tube like dog barf or laundry.

Although the ejection charge can have some effect on the laundry, the tighter the joint, either because of tape or shear pins, the less effect the ejection charge has on the laundry inside the tube. Unless the laundry is wrapped tight and is snug in the tube so it works like a piston, it won't move much unless you put a really large charge in there. Then the two part flying apart are going to rip the laundry out of the tube before they put large shock loads on the recovery system. All you need for a drogue charge is enough to separate the two pieces about the length of the shock cord, the air will catch the drogue and do the rest.

When testing a drogue charge I always recommend putting a motor case in the hole. Don't try to stuff rags or anything porous in there and expect to get accurate results.

I usually use a charge calculator and then test - test - test !!!

It actually takes less BP than I normally use.

Here is the View attachment Ejection Charge Calculator.xls I use.

Cant remember who I got it from or I would give them credit

rdbones said:

I usually use a charge calculator and then test - test - test !!!

It actually takes less BP than I normally use.

Here is the View attachment 247162 I use.

Cant remember who I got it from or I would give them credit

Click to expand...

Thanks. These calculations are close to what I had been using. I think my problem was the way I was stuffing everything into the tube. I had to use a lot more force to pull the chute and cord out than I expected. The tube length is a bit too short and I had to cram them in. I've changed out the half inch nylon tubular shock cord for quarter inch round kevlar and things aren't so tight.

I also don't think the position matters that much, other than in terms of protecting the parachute and bridle from the hot ejection charge gasses.