Ejection charge only works when bay is empty

[Dufus Johnson]

Howdy, folks... new to the forum. I'm L1 and going for L2 and I'm trying dual deployment for the first time. Only ground testing so far and have mixed results. Basically if I try the calculated amount of BP on an empty bay, it shears the pins and ejects the nose cone quite forcefully. If I pack my parachute in there, the charge detonates but no shear, even at twice the amount. I've tried this several times now with consistent results. Now, my instinct suggests that partially filling the volume to be pressurized would result in higher pressure and stronger ejection, not weaker, so I'm wondering if someone can explain why this might be happening? It's not like I have the chute jammed in there or something. It's the same setup I've used successfully on engine-based ejections numerous times. Thanks.

 

[markjos]

Do you see any evidence of leaks? Any soot around bulkhead holes, fasteners, or loose airframe/coupler joints?

 

[Dufus Johnson]

Nope, I think everything is sealed up pretty good, and the ejection works consistently fine when the bay is empty, so other than the strange behavior when the parachute is in there, things seem good.

 

[OzHybrid]

The only thing I can think of is this....

How much does the chute weigh? F=m x a To shear a pin theoretically it's just the pressure force x area its applied / cross section of the pins it's applied to.
In the real world plastics behave differently depending on how fast things happen. So if you're having to accelerate a different mass, ie the chute is IN the nose cone, you will get a different result with the extra mass of the chute.
Ejection charges also vary with diameter in terms of the extra amount you might need. You haven't said what the diameter is. With chutes there are some packing methods that work better for some diameters. I've got a 48" chute packed into a 2 1/4 ID tube. Tried folding, deployment bag and other methods. Ended up stuff packing it like a sleeping bag. Find your method that works, then pack your chute close it up without any charge, then pull it all gently apart in a single action. Keep moving until all chute and lines are out. There should be no obvious snagging. Your chute should look like it will inflate if it hasn't already. Video it so you can see it.
If all that is ok, increase the charge till it works.( within the limits of common sense.) The usual common sense.

 

[Dufus Johnson]

Thanks, Norman and Mark. It's a 2.6" diameter, 11" bay depth, 3x #2 nylon screws. Supposedly I need ~65lbf to shear those pins, and two BP calculators suggest ~1/3g to get that done. I started at 1/2g, and when empty, that ejects the nose cone slightly more forcefully than I'd like. (Put some holes in my wife's blankets...) So far so good. But even 1g does not budge the nose cone when the bay is loaded with a parachute and rigging. My WAG right now is the charge is actually marginal and the empty bay is acting to focus the shock wave on the nose cone. When the bay is full of parachute, that can no longer happen. But that is pure speculation. I guess tomorrow I'll try increasing the charge again.

 

[Voyager1]

For a 2.6” body tube, I’ve never needed more than 2 x 2-56 shear pins. Typically, I would only use 1.

 

[OzHybrid]

3 pins on that dia is a lot. As said above, 2 max.

 

[tim cubbedge]

Are you using canisters or glove tips? I’ve seen similar problem on same size airframe when using glove tips. Changed it to canister and problem gone.

 

[OverTheTop]

My BP estimate would be to start at 0.45g (I would round up to 0.5g) then ground test, so your numbers sound about right. Three pins does sound excessive. Sure there are no leaks?

 

[Dufus Johnson]

Thanks, everybody! Fewer pins is something I did consider early on, but forgot about later with everything else going on. I'll give it a try, although I did have a worry about a single pin jamming up. Is that a thing people have seen? The glove tip "tip" suggests directing the charge does matter? Or is there another reason you think that fixed the problem? Regarding leaks, I'm guessing since it works empty that it's ok. I don't see any soot outside. No visible smoke. It feels like a fairly snug fit (adjusted with tape the same as my other launches), and the only holes either have screws, rivets, or are taped off (eg. wire passage through bulkhead to electronics). Thanks again.

 

[tim cubbedge]

I had 2 rockets with identical size payloads I was ground testing. Both were 2.64" diameter airframe, both used three 2-56 nylon shear pins. Only difference was that one used charge wells and the other used nitrile glove finger tips. Initial test worked great on the payload with charge wells. Not so with the fingertips. I upped the charge a few times and could never get it to blow the NC off. I ended up switching to charge wells and has worked great ever since. Could I have used 2 shear pins, probably but was already set up for 3.

 

[Dufus Johnson]

Thanks, Tim. Well, naturally it is snowing today (which is actually great since we need the water), so further testing is on hold, but I will also look into adding canisters. I like the focus and repeatability it should provide.

 

[tfish]

I had 2 rockets with identical size payloads I was ground testing. Both were 2.64" diameter airframe, both used three 2-56 nylon shear pins. Only difference was that one used charge wells and the other used nitrile glove finger tips. Initial test worked great on the payload with charge wells. Not so with the fingertips. I upped the charge a few times and could never get it to blow the NC off. I ended up switching to charge wells and has worked great ever since. Could I have used 2 shear pins, probably but was already set up for 3.

I agree 100%.
Deployment charges are a system..of events. The 'container' is part. Glove fingers and balloons are the least efficent...lots of scattered unturned BP. Short fat ejection wells are also very wasteful..scattered BP. Don't try to make them better by sealing or tapeing them closed. Makes them even more..wasteful. Placing the ematch on top of the BP helps some..
The best..most efficient...reliable..repeatable..charge holders are long and skinny..5/16 ID x 3..or similar with the match placed on top if the BP.



Tony

 

[Dufus Johnson]

That is very interesting, Tony. How are you attaching those to the bulkheads? From the material and action shot, I'm assuming the tubing is a one time fire?

 

[Dan Griffing]

I had 2 rockets with identical size payloads I was ground testing. Both were 2.64" diameter airframe, both used three 2-56 nylon shear pins. Only difference was that one used charge wells and the other used nitrile glove finger tips. Initial test worked great on the payload with charge wells. Not so with the fingertips. I upped the charge a few times and could never get it to blow the NC off. I ended up switching to charge wells and has worked great ever since. Could I have used 2 shear pins, probably but was already set up for 3.

First, charge wells can be taped over with a pinch of dog barf and two layers of electrical tape to hold the BP ejection charge in until more pressure builds up for more complete BP combustion with a greater impulse of ejection gas.

Second, have you tried replacing the sheer pins with just two small pieces of masking tape? A 2.5 inch rocket might not need sheer pins on the main chute airframe compartment.

The problem might not be the sheer pins. It could be that your chute, flame protection and shock cords are packed so tightly that they’re preventing ejection from working correctly.

I had a similar problem even with motor ejection on a 4” rocket with single deployment and no sheer pins. Two earlier launches in the day worked perfectly with identical configurations, but the third turned into a lawn dart and destroyed the rocket that had achieved my L2 cert with DD a month earlier. Rocket recovery is binary and even when its working you don’t know how close your recovery setup is to having a failure. Lots of tests can help.

 

[tim cubbedge]

First, charge wells can be taped over with a pinch of dog barf and two layers of electrical tape to hold the BP ejection charge in until more pressure builds up for more complete BP combustion with a greater impulse of ejection gas.

Second, have you tried replacing the sheer pins with just two small pieces of masking tape? A 2.5 inch rocket might not need sheer pins on the main chute airframe compartment.

The problem might not be the sheer pins. It could be that your chute, flame protection and shock cords are packed so tightly that they’re preventing ejection from working correctly.

I had a similar problem even with motor ejection on a 4” rocket with single deployment and no sheer pins. Two earlier launches in the day worked perfectly with identical configurations, but the third turned into a lawn dart and destroyed the rocket that had achieved my L2 cert with DD a month earlier. Rocket recovery is binary and even when its working you don’t know how close your recovery setup is to having a failure. Lots of tests can help.

Charge wells were not the problem as that is exactly how I do it. It was the rubber glove tips that failed and I believe that is what the OP is using.

 

[Dufus Johnson]

Interesting to see everyone's perspectives. I'm actually using short pieces of tubing with the ends folded over and taped with electrical tape. I have yet to see any unburned powder residue, even on the ones that don't open up where it simply cannot escape detection. However, I do like the idea of ejection canisters and think it will improve the system. Chute is not packed tightly. It is the same setup as I regularly use successfully on engine ejection flights, so it's a bit strange. Hopefully fewer pins and/or more BP and/or canisters to direct the energy will make this issue go away.

 

[spigalau]

Just checking - you do have something blocking the motor mount ?

 

[Dufus Johnson]

Just checking - you do have something blocking the motor mount ?

not for the very first test!

 

[OverTheTop]

Regarding the high number of shear pins, you can keep using three and drill out the core of the pins to reduce the cross sectional area. I do that on my 54mm Tomach. That way you keep the shear pins but they are not excessively strong.

 

[DragonRocketry]

I use 1/4" launch lug material to make my charges. I do not like the finger tip as I seen to much variance in the charge pressure.
Here is video of how I do my charges in small rocket that I cannot put a charge well in.

Ejection charge

 

[teepot]

What size motor are you going to use? Ejection charge size varies with motor size.

 

[Dufus Johnson]

What size motor are you going to use? Ejection charge size varies with motor size.

I'm using RMS 38mm H through J (360 and 720), but I'm plugging the casing.

 

[teepot]

The H's and I's I've used have 1.5 grams of BP. The J I have is the same iirc. So that's the amount I use in my charges. And sometimes a little more.

 

[Voyager1]

Interesting to see everyone's perspectives. I'm actually using short pieces of tubing with the ends folded over and taped with electrical tape. I have yet to see any unburned powder residue, even on the ones that don't open up where it simply cannot escape detection. However, I do like the idea of ejection canisters and think it will improve the system. Chute is not packed tightly. It is the same setup as I regularly use successfully on engine ejection flights, so it's a bit strange. Hopefully fewer pins and/or more BP and/or canisters to direct the energy will make this issue go away.

Have you considered using centrifuge vials as your canisters? Many people do, as I do, and find them very effective?

 

[MProcket]

I would concur with using centrifuge vials (Eppendorf tubes) or other similar ‘hard’ sided containment holder. I was ground testing my L1 DD using heat shrink tubing and BP and could just barely get the nose cone off using 2x the calculated value of BP. When I switched to the Eppendorf tubes, the same amount of BP caused the body tube to crack. Hindsight, I should have gone back down to the calculated value and tested up from there using the new containment method. So switch to charge wells or hard sided tubing or Eppendorf tubes and start at low BP amounts first.

 

[Dufus Johnson]

Have you considered using centrifuge vials as your canisters? Many people do, as I do, and find them very effective?

I would concur with using centrifuge vials (Eppendorf tubes) or other similar ‘hard’ sided containment holder.

I have come across these before in various places, but didn't know their name, nor had I considered using them for this application. And they seem cheap and available. So thank you very much. I'll definitely look into it more.

 

[teepot]

I used some florist single flower water tubes. Hard plastic, rubber top with a hole in it. Didn't seem very energetic when at the av bay. So I added more BP and moved the charge down by the motor and blew the rocket up. Now I just use charge wells. So start small with your charges in the tubes.

 

[Dufus Johnson]

So I added more BP and moved the charge down by the motor and blew the rocket up. Now I just use charge wells. So start small with your charges in the tubes.

Yep, that definitely sounds like good advice. Thanks.

 

[Dufus Johnson]

Update: I added charge wells, reduced the amount of BP way down, packed the charges a bit better, and went with only one pin. I had no problems with separation on several ground tests after that. Just did a test launch this morning and it went pretty much perfectly other than dinging a fin on a rock. Thanks to everyone for the help. Much appreciated!!