How do you make your ejection charges?

[jadebox]

I usually use a simple and effective method that someone showed me at a NEFAR launch years. I use plastic wrap, doubled onto itself and push a depression into it for the BP. I pour the power in the depression and insert the ematch. Form the plastic around the BP into a ball and twist it so the plastic wrap around the ematch lead. This ensures that the powder is compressed. I then wrap the whole thing in electrical tape so it stays that way.

 

[boatgeek]

I do yes to both glove tips and pipe caps. Before the launch, I'll measure out the appropriate amount of black powder into glove tips and tape them closed temporarily. At the launch, I pull them out, put in an ematch, and wrap the opening with electrical tape. The tape shoudl take up all the slack so the glove tip feels firm. That then is a tidy package that can be dropped into a pipe cap screwed to the bulkhead. I usually top up the pipe cap with dog barf and tape over the top. Yes it's more work, but the glove tip keeps everything nicely contained during assembly.

For small rockets with just an apogee charge, I'll just hang the glove tip down to near the motor mount and dispense with the glove tip.

 

[JohnCoker]

For those who don't want to watch the video, here's how I do it:

1. drill a 3/32" hole in the bottom of the centrifuge vial
2. pull the ematch wire almost all the way through the hole from the top
3. put a tiny bit of 5m epoxy around the ematch wire just below the head
4. pull the wire the rest of the way through (seating the ematch head near the bottom and sealing with the epoxy)
5. let cure (I typically make multiple and leave them in this state for later filling)
6. measure the BP and pour it into the vial
7. tap the vial repeatedly to settle the BP around the ematch head
8. fill the remainder with Estes wadding (overfilling slightly to make it a tight fit)
9. close the cap of the vial and make sure it compresses the wadding
10. wick a little CA around the rim of the cap so it can't open accidentally
11. write the charge amount on the vial with a Sharpie

 

[jahall4]

The expanding gasses from an ejection charge do very little if anything to "blow" the chute and harness from the airframe. It simply does not work that way. If it did, the typical dual deploy configuration would be prone to failure. Instead, the “laundry” (chute, harness, etc…) is pulled from the airframe by the inertia of the sections. Relying on the concept of blowing the chute out can result in deployment failure and hence becomes a safety issue.

 

[jahall4]

For those who don't want to watch the video, here's how I do it:

1. drill a 3/32" hole in the bottom of the centrifuge vial
2. pull the ematch wire almost all the way through the hole from the top
3. put a tiny bit of 5m epoxy around the ematch wire just below the head
4. pull the wire the rest of the way through (seating the ematch head near the bottom and sealing with the epoxy)
5. let cure (I typically make multiple and leave them in this state for later filling)
6. measure the BP and pour it into the vial
7. tap the vial repeatedly to settle the BP around the ematch head
8. fill the remainder with Estes wadding (overfilling slightly to make it a tight fit)
9. close the cap of the vial and make sure it compresses the wadding
10. wick a little CA around the rim of the cap so it can't open accidentally
11. write the charge amount on the vial with a Sharpie

Like the CA idea around the cap. Also you can trim the lip from cap.

 

[Scott_650]

This has been awesome thread! I have several dual-deploy rockets on deck for winter builds and a couple previous builds to mod for d-d - on top of everything I’ve picked up from the very experienced HPR flyers in our club this has been so informative. Thanks to all who’ve contributed.

 

[KenECoyote]

I came up with the glove tip method as well before finding out that others were already doing it, but I take it a step "easier" (?) and usually don't use masking tape...I twist the gove tip, bend the wire over itself, then wind it around until tight. I then write the charge size with marker on the charge (if using white or blue glove tips). Then it all goes into charge wells with dog barf (if needed) and tape it shut. Haven't had a failure yet and for the early years I only had Pyrodex.

Here's a pic of my most recent batch of charges...I only had clear gloves, so I had to use masking tape to mark the sizes, but it's the first time I've used masking tape.

 

[KilroySmith]

is pulled from the airframe by the inertia of the sections.

I don't fully follow your argument here. What do you mean by "pulled...by the inertia of the sections"? How does the charge create whatever effect you're discussing here?

 

[jahall4]

I don't fully follow your argument here. What do you mean by "pulled...by the inertia of the sections"? How does the charge create whatever effect you're discussing here?

Not an argument and not an effect beyond that of an expanding gas. Could rephrase as "pulled...by the momentum of the sections" if that helps.

 

[Rex R]

My fg bird weighs about 9 lb.s the ejection charge will get the front and rear sections moving around 10 mph relative to each other, the chutes don't really move until they get pulled from the airframe.

 

[Scott_650]

I don't fully follow your argument here. What do you mean by "pulled...by the inertia of the sections"? How does the charge create whatever effect you're discussing here?

This is how it’s been explained to me: the charge creates a positive pressure that accelerates the ebay section, pulling the recovery gear out of the booster section like a locomotive pulling a train car rather than the charge “popping” the recovery gear out of the booster like shooting a paper wad out of a straw.

I always liked that paper wad vs locomotive analogy

 

[teepot]

OK, I have tried PVC caps, shell casings and the containers that come with the motors. My problem is I keep crushing the Av-Bay. It has happened 3 times now. The first 2 were shell casings holding the BP. This last one a day ago was an AT BP holder. I don't use more powder than what comes with the motor. I have switched from the Av-Bay bulkhead resting on a rim of coupler to a 1/2" plug sitting on the Av-bay edge. The first two times I blew up an Av-bay the 1/4 bulkhead cracked in half. This last time, with the AT container separated from the Av-Bay by several inches, still shoved the bulkhead into the Av-Bay coupler. The bulkhead was sitting on a rim of coupler stock. I have used a gram and a half in the first two explosions and the last time 1 gram. I figured I had packed the BP too much and used too much tape. This last time I pulled the e-match thru a hole drilled in the bottom and taped it. Put in the powder, added a little dog barf and closed the lid and put one piece of tape over it. The dog barf was not packed. I read you guys posts about packing the BP tight and taping them tightly and wonder why do I keep blowing up/in Av-Bays. All three collapsed Av-Bay used the piece of coupler as a stop for the Av-bay bulkhead. The first Av-Bays I bought used this method. So when I started making my own bays that is how I built them. Not any more. Oh, and the first dual deploy rocket I built, during ground testing I blew the BT apart. Any thoughts?

 

[cwbullet]

OK, I have tried PVC caps, shell casings and the containers that come with the motors. My problem is I keep crushing the Av-Bay. It has happened 3 times now. The first 2 were shell casings holding the BP. This last one a day ago was an AT BP holder. I don't use more powder than what comes with the motor. I have switched from the Av-Bay bulkhead resting on a rim of coupler to a 1/2" plug sitting on the Av-bay edge. The first two times I blew up an Av-bay the 1/4 bulkhead cracked in half. This last time, with the AT container separated from the Av-Bay by several inches, still shoved the bulkhead into the Av-Bay coupler. The bulkhead was sitting on a rim of coupler stock. I have used a gram and a half in the first two explosions and the last time 1 gram. I figured I had packed the BP too much and used too much tape. This last time I pulled the e-match thru a hole drilled in the bottom and taped it. Put in the powder, added a little dog barf and closed the lid and put one piece of tape over it. The dog barf was not packed. I read you guys posts about packing the BP tight and taping them tightly and wonder why do I keep blowing up/in Av-Bays. All three collapsed Av-Bay used the piece of coupler as a stop for the Av-bay bulkhead. The first Av-Bays I bought used this method. So when I started making my own bays that is how I built them. Not any more. Oh, and the first dual deploy rocket I built, during ground testing I blew the BT apart. Any thoughts?

In 40 years of flying rockets, this has never happened to me. In the 15 years since I return to rocketry, the only time I have seen this is when someone chooses too thin or too soft of material for AV lids. I guess it could happen if you use too much powder.

 

[Banzai88]

OK, I have tried PVC caps, shell casings and the containers that come with the motors. My problem is I keep crushing the Av-Bay. It has happened 3 times now. The first 2 were shell casings holding the BP. This last one a day ago was an AT BP holder. I don't use more powder than what comes with the motor. I have switched from the Av-Bay bulkhead resting on a rim of coupler to a 1/2" plug sitting on the Av-bay edge. The first two times I blew up an Av-bay the 1/4 bulkhead cracked in half. This last time, with the AT container separated from the Av-Bay by several inches, still shoved the bulkhead into the Av-Bay coupler. The bulkhead was sitting on a rim of coupler stock. I have used a gram and a half in the first two explosions and the last time 1 gram. I figured I had packed the BP too much and used too much tape. This last time I pulled the e-match thru a hole drilled in the bottom and taped it. Put in the powder, added a little dog barf and closed the lid and put one piece of tape over it. The dog barf was not packed. I read you guys posts about packing the BP tight and taping them tightly and wonder why do I keep blowing up/in Av-Bays. All three collapsed Av-Bay used the piece of coupler as a stop for the Av-bay bulkhead. The first Av-Bays I bought used this method. So when I started making my own bays that is how I built them. Not any more. Oh, and the first dual deploy rocket I built, during ground testing I blew the BT apart. Any thoughts?

One of the benefits of the LOC avionics bay design is the stiffy coupler which provides a thick ledge for the bay lid to seat on and the load of the ejection forces to bear against.

That said, I've never had any issues with the conventional (non-LOC stiffy equipped) avionics bay collapsing like that. I built a few av bays in my early early BAR days before I knew what I was doing.....and those have been fine too.

Maybe post a picture of your builds so we can see and maybe help?

 

[DragonRocketry]

OK, I have tried PVC caps, shell casings and the containers that come with the motors. My problem is I keep crushing the Av-Bay. It has happened 3 times now. The first 2 were shell casings holding the BP. This last one a day ago was an AT BP holder. I don't use more powder than what comes with the motor. I have switched from the Av-Bay bulkhead resting on a rim of coupler to a 1/2" plug sitting on the Av-bay edge. The first two times I blew up an Av-bay the 1/4 bulkhead cracked in half. This last time, with the AT container separated from the Av-Bay by several inches, still shoved the bulkhead into the Av-Bay coupler. The bulkhead was sitting on a rim of coupler stock. I have used a gram and a half in the first two explosions and the last time 1 gram. I figured I had packed the BP too much and used too much tape. This last time I pulled the e-match thru a hole drilled in the bottom and taped it. Put in the powder, added a little dog barf and closed the lid and put one piece of tape over it. The dog barf was not packed. I read you guys posts about packing the BP tight and taping them tightly and wonder why do I keep blowing up/in Av-Bays. All three collapsed Av-Bay used the piece of coupler as a stop for the Av-bay bulkhead. The first Av-Bays I bought used this method. So when I started making my own bays that is how I built them. Not any more. Oh, and the first dual deploy rocket I built, during ground testing I blew the BT apart. Any thoughts?

Is your sled long enough to fit tight between the caps when ebay is together. This will keep the lids from pushing on the coupler and smashing the ends.

 

[Antares JS]

OK, I have tried PVC caps, shell casings and the containers that come with the motors. My problem is I keep crushing the Av-Bay. It has happened 3 times now. The first 2 were shell casings holding the BP. This last one a day ago was an AT BP holder. I don't use more powder than what comes with the motor. I have switched from the Av-Bay bulkhead resting on a rim of coupler to a 1/2" plug sitting on the Av-bay edge. The first two times I blew up an Av-bay the 1/4 bulkhead cracked in half. This last time, with the AT container separated from the Av-Bay by several inches, still shoved the bulkhead into the Av-Bay coupler. The bulkhead was sitting on a rim of coupler stock. I have used a gram and a half in the first two explosions and the last time 1 gram. I figured I had packed the BP too much and used too much tape. This last time I pulled the e-match thru a hole drilled in the bottom and taped it. Put in the powder, added a little dog barf and closed the lid and put one piece of tape over it. The dog barf was not packed. I read you guys posts about packing the BP tight and taping them tightly and wonder why do I keep blowing up/in Av-Bays. All three collapsed Av-Bay used the piece of coupler as a stop for the Av-bay bulkhead. The first Av-Bays I bought used this method. So when I started making my own bays that is how I built them. Not any more. Oh, and the first dual deploy rocket I built, during ground testing I blew the BT apart. Any thoughts?

I have never seen this behavior before either and am puzzled. I have a few questions that may help narrow down what is happening.

Are your bulkheads plywood or something else?

Did you use shear pins? If so, What type, how many, and what size?

Are you plugging the hole that your ematch is coming through?

Are your charge canisters secured to the bulkhead with the side that opens facing away from the e-bay?

How is the ledge your bulkhead resting on secured to the e-bay body?

 

[Scott_650]

OK, I have tried PVC caps, shell casings and the containers that come with the motors. My problem is I keep crushing the Av-Bay. It has happened 3 times now. The first 2 were shell casings holding the BP. This last one a day ago was an AT BP holder. I don't use more powder than what comes with the motor. I have switched from the Av-Bay bulkhead resting on a rim of coupler to a 1/2" plug sitting on the Av-bay edge. The first two times I blew up an Av-bay the 1/4 bulkhead cracked in half. This last time, with the AT container separated from the Av-Bay by several inches, still shoved the bulkhead into the Av-Bay coupler. The bulkhead was sitting on a rim of coupler stock. I have used a gram and a half in the first two explosions and the last time 1 gram. I figured I had packed the BP too much and used too much tape. This last time I pulled the e-match thru a hole drilled in the bottom and taped it. Put in the powder, added a little dog barf and closed the lid and put one piece of tape over it. The dog barf was not packed. I read you guys posts about packing the BP tight and taping them tightly and wonder why do I keep blowing up/in Av-Bays. All three collapsed Av-Bay used the piece of coupler as a stop for the Av-bay bulkhead. The first Av-Bays I bought used this method. So when I started making my own bays that is how I built them. Not any more. Oh, and the first dual deploy rocket I built, during ground testing I blew the BT apart. Any thoughts?

With a failure rate that high my only suggestion is to seek out an experienced HPR flyer for some advice and oversight - I’ve watched dozens of dual-deploy flights at launches with multiple clubs and never once saw anything like what’s described above happen. Seen CATOs, chute failures, wonky delays, etc but no blown-out, collapsed or crushed ebays.

 

[jahall4]

OK, I have tried PVC caps, shell casings and the containers that come with the motors. My problem is I keep crushing the Av-Bay. It has happened 3 times now. The first 2 were shell casings holding the BP. This last one a day ago was an AT BP holder. I don't use more powder than what comes with the motor. I have switched from the Av-Bay bulkhead resting on a rim of coupler to a 1/2" plug sitting on the Av-bay edge. The first two times I blew up an Av-bay the 1/4 bulkhead cracked in half. This last time, with the AT container separated from the Av-Bay by several inches, still shoved the bulkhead into the Av-Bay coupler. The bulkhead was sitting on a rim of coupler stock. I have used a gram and a half in the first two explosions and the last time 1 gram. I figured I had packed the BP too much and used too much tape. This last time I pulled the e-match thru a hole drilled in the bottom and taped it. Put in the powder, added a little dog barf and closed the lid and put one piece of tape over it. The dog barf was not packed. I read you guys posts about packing the BP tight and taping them tightly and wonder why do I keep blowing up/in Av-Bays. All three collapsed Av-Bay used the piece of coupler as a stop for the Av-bay bulkhead. The first Av-Bays I bought used this method. So when I started making my own bays that is how I built them. Not any more. Oh, and the first dual deploy rocket I built, during ground testing I blew the BT apart. Any thoughts?

Like @Banzai88 said a pic would be a big help, as there is something fundamentally wrong if your Avbay collapse as you seem to be describing. Here is some video of a avbay the is practically indestructible. You are welcome to PM me if you would like.

 

[rfjustin]

do you have photos?

https://www.rocketryforum.com/threa...-needs-built-first.167169/page-4#post-2189200

 

[boatgeek]

OK, I have tried PVC caps, shell casings and the containers that come with the motors. My problem is I keep crushing the Av-Bay. It has happened 3 times now. The first 2 were shell casings holding the BP. This last one a day ago was an AT BP holder. I don't use more powder than what comes with the motor. I have switched from the Av-Bay bulkhead resting on a rim of coupler to a 1/2" plug sitting on the Av-bay edge. The first two times I blew up an Av-bay the 1/4 bulkhead cracked in half. This last time, with the AT container separated from the Av-Bay by several inches, still shoved the bulkhead into the Av-Bay coupler. The bulkhead was sitting on a rim of coupler stock. I have used a gram and a half in the first two explosions and the last time 1 gram. I figured I had packed the BP too much and used too much tape. This last time I pulled the e-match thru a hole drilled in the bottom and taped it. Put in the powder, added a little dog barf and closed the lid and put one piece of tape over it. The dog barf was not packed. I read you guys posts about packing the BP tight and taping them tightly and wonder why do I keep blowing up/in Av-Bays. All three collapsed Av-Bay used the piece of coupler as a stop for the Av-bay bulkhead. The first Av-Bays I bought used this method. So when I started making my own bays that is how I built them. Not any more. Oh, and the first dual deploy rocket I built, during ground testing I blew the BT apart. Any thoughts?

How are you sizing your ejection charges? I found that the "X times volume" formula in MHPR2 gave grossly oversized charges in a couple of my projects. The calcs that estimated a desired pressure/separation force and went from there were much better for me. What sizes are your rockets and ejection charges?

If you're using very long shell casings, you might be getting more force than expected due to really good combustion. Jim Jarvis talked about the high altitude system he uses with a very long barrel is very energetic.

 

[jahall4]

If you're using very long shell casings, you might be getting more force than expected due to really good combustion. Jim Jarvis talked about the high altitude system he uses with a very long barrel is very energetic.

What's generally expected by calculation is to burn all the BP. Obviously that never happens, but you want to burn nearly all of it so results are reliable, consistent, otherwise you may have a surprise failure. IOW never rely on burning some portion of the charge to obtain the desired separation. Start Ground testing with what the calculation says is required and increase the amount of BP as desired. If you have to increase lots, something is wrong that should be addressed.

 

[Donnager]

I use either the pipe cap on the bulkhead or surgical tubing. I may try the BT tube types in the future.

I've never liked how thin and fragile a glove finger was. The ones with surgical tubing are just a short length, zip tie an e-match at one end, pour in your powder, shake it down and pinch the other end with another zip tie. Those seemed to have less burning particulates than the cup on a bulkhead. You also get more 'Bang' than 'Whoomp'---if you get my meaning.

 

[teepot]

How are you sizing your ejection charges? I found that the "X times volume" formula in MHPR2 gave grossly oversized charges in a couple of my projects. The calcs that estimated a desired pressure/separation force and went from there were much better for me. What sizes are your rockets and ejection charges?

If you're using very long shell casings, you might be getting more force than expected due to really good combustion. Jim Jarvis talked about the high altitude system he uses with a very long barrel is very energetic.

The rockets are mostly 3". Between 4' and 6'. The shell casings I figured were to long and acting like cannons. This last time I used the container from an AT DMS. I used 1 gram. So less than a full charge from a AT DMS motor. The charge wasn't in contact with the bulkhead. I think the main problem is the bulkhead sitting on a stop made from a piece if coupler. I'll see what happens with a double thickness of bulkhead material with the bulkhead resting on the coupler end.

 

[teepot]

Like @Banzai88 said a pic would be a big help, as there is something fundamentally wrong if your Avbay collapse as you seem to be describing. Here is some video of a avbay the is practically indestructible. You are welcome to PM me if you would like.

I would think fiberglass is way stronger than cardboard. The new way I'm going to build my Av-Bays is as described.

 

[teepot]

With a failure rate that high my only suggestion is to seek out an experienced HPR flyer for some advice and oversight - I’ve watched dozens of dual-deploy flights at launches with multiple clubs and never once saw anything like what’s described above happen. Seen CATOs, chute failures, wonky delays, etc but no blown-out, collapsed or crushed ebays.

I have.

 

[boatgeek]

The rockets are mostly 3". Between 4' and 6'. The shell casings I figured were to long and acting like cannons. This last time I used the container from an AT DMS. I used 1 gram. So less than a full charge from a AT DMS motor. The charge wasn't in contact with the bulkhead. I think the main problem is the bulkhead sitting on a stop made from a piece if coupler. I'll see what happens with a double thickness of bulkhead material with the bulkhead resting on the coupler end.

1 gram charges in a centrifuge vial definitely shouldn't be pushing the bulkhead into the AV bay. A double thickness of coupler would definitely help, as might a coupler bulkhead which keeps the bulkhead centered over the coupler ring. On a 5.5" rocket, I epoxied some stops on the end of the coupler to try to keep everything from pushing through. It wasn't a totally unqualified success, but it worked.

 

[teepot]

I have never seen this behavior before either and am puzzled. I have a few questions that may help narrow down what is happening.

Are your bulkheads plywood or something else?

Did you use shear pins? If so, What type, how many, and what size?

Are you plugging the hole that your ematch is coming through?

Are your charge canisters secured to the bulkhead with the side that opens facing away from the e-bay?

How is the ledge your bulkhead resting on secured to the e-bay body?

So were the other club members. The bulkheads are 1/4" 5 ply plywood. 2x56 shear pins x 2. Yes , I seal the hole. The first two were attached to the bulkhead. The last one was not. All the failed av-bays were the drouge side. Using 1 to 1.5 grams of BP. No more than what comes with a motor. And I think the biggest problem is that they all rested on a ring of coupler material. So I'll change all the av-bays to an 1/2" plg style resting on the coupler end.

 

[Banzai88]

Perhaps the issue isn't the coupler itself, but perhaps the bulkhead is just ever so slightly too small in diameter? I've only ever used commercially available av bay caps from the various vendors/manufacturers, and all of them were a little too large to slide perfectly and took a little sanding to get the 'perfect' fit. If the fit were too small, it might cause what you're seeing.

 

[teepot]

Is your sled long enough to fit tight between the caps when ebay is together. This will keep the lids from pushing on the coupler and smashing the ends.

yes, they butt up against the nuts for the eye nut.

 

[Antares JS]

(...) I think the biggest problem is that they all rested on a ring of coupler material. So I'll change all the av-bays to an 1/2" plg style resting on the coupler end.

I have several e-bays built like that and have not had an issue. Had the ring come off when you had the collapsed e-bays?