4oz Motor Bulkhead Blowout

[BlueComet24]

I made a tooling set for 4oz (1/2", 12.7mm) semi-endburner motors. I use 1/2x3" tubes (12.7x76.2mm). The nozzle is 1/4" (6.4mm) into the tube and 1/2" tall before tapering at 45° to the wall of the tube. The nozzle diameter is 1/8" (3.2mm). The short core into the fuel grain is 1/2" tall and also 1/8" diameter. The fuel grain, from the inner nozzle opening to the bulkhead, is 1.75" (44.5mm), and the bulkhead is 1/4" thick.

http://i.imgur.com/Ve72L6B.jpg

 

I've made 3 motors to these specs so far, and all have CATO'd about 5-10m up. I've made all 3 with my hot ball milled BP and rammed by hand. This last one I rammed with 1/8" fuel increments and I drilled an ~1/8" hole through the bulkhead for an ejection charge to deploy a chute. It took off and CATO'd ~5m up as usual. I was able to recover it for examination. The tube and nozzle remained intact, but the bulkhead blew out. This surprised me, as there should be about 1.25" of solid BP before the bulkhead upon ignition, so it shouldn't just blow out like that. I wonder if the grain could have cracked, even thought I only stored the motor for 2 days, and sealed in plastic. My nozzle/bulkhead material is ground up kitty litter with 10% paraffin wax melted in. It works just fine for 1/4" motors, but I'm wondering if it doesn't have enough grip against the tube for these larger motors. I'm thinking I'll try using screen-mixed 75/15/10 BP for my next motor, and if that doesn't work, then 60/30/10. Do you think this is the right thing to change, and do you have any other advice?

[Mumbles]

In principle it should be fine. The first place I'd look is your compaction. As you progress into larger motors a solid grain becomes more important. Since you're ramming these, I'd ensure you have a stout hammer and a solid base to ram against. A tree stump or wooden post anchored in the ground both work well. A table or workbench may bounce a little too much. Something like a solid rubber mallet will also have these sort of bouncing type issues. You might even be able to press them with an arbor press if you have access to one.

 

What you describe sounds like a fuel grain problem. There could be cracks, air pockets, or perhaps a small gap between the grain and the tube. Waxing may help with the final issue. Taming the fuel slightly as you mentioned may also help to stop the issue while you make any necessary adjustments.

[FlaMtnBkr]

Is your BP granulated with dextrin in it or pressed and corned?

 

If so the grains may be too hard to reliably compact into a solid grain.

[BlueComet24]

I used mill dust for the fuel. The granulated stuff was the ejection charge.

[Sparx88]

I'm no rocket expert but I had those same issues starting out. You do need a very solid base and a heavy mallet or hammer to really get the fuel and delay above the core compacted well. All of it needs to be for that matter but especially the clay bulkhead if using one. 3/8" or better. Now once you have that issue worked out you will then find out if your tubes hold up. 3/16 wall thickness minimum for 4 Oz 1/2" id tubes.

[lloyd]

Generalizations abound, but I think you have a specific problem.

 

It appears that your motor tube is 'inflating' with the pressure, and fire is creeping between the grain and the tube walls.

 

There are several ways to combat that:

 

The first is to increase the nozzle size with respect to the tube i.d. Never start with smaller than a 33% nozzle. Then, if you can go smaller, fine -- go smaller. 33% is _about_ as small as you can get away with using good, hot powder.

 

The second is to eliminate the core entirely at first, then lengthen it as you progress. Eh... that involves new spindles. The easiest way is just to eliminate it altogether... you can experiment later. If you have just a 'plain' nozzle former, use that, then 'drill back' as you wish to have longer cores.

 

Third would be to press your powder MUCH harder (I know... you're ramming). I don't know if you're using a tube-support, but that will help.

 

Fourth would be to significantly reduce the height of the powder increments.

 

Fifth would be to try tube waxing. I know for a certainty that it works, and works well... but you really need to resolve 1-through-4 before trying the tube waxing.

 

LLoyd

Edited April 29, 2016 by lloyd

[calebkessinger]

Probably just granulating the fuel with 2% wax and making any sort of good tube support would at least get you a functioning rocket.

 

Nozzle size isn't near the issue once you get tube support and real pressure, then you can press whistle in them.

[Asaster420]

"My nozzle/bulkhead material is ground up kitty litter with 10% paraffin wax melted in. "

 

Could the wax be an issue? I understood that no oil or wax should be added to the litter. I have even used it without any milling down ahead of time and still had no issues

[CrossOut]

As a general rule, i always use a bulk head at least as thick as the ID of my tube. since you are using standard BP, your rocket can burn through that layer pretty quick. especially for core burners.. w/ my 4oz tooling my rockets impulse for just under a second (then again my spindle is a lot longer....) since you have a long fuel grain at the top, it sounds like you need to ram / press your fuel grain a little tighter as everyone has mentioned. Also try using a delay comp before your drilled bulk head. 1/4" isnt much especially when it has a hole in it.

Edited June 17, 2016 by CrossOut

[Oinikis]

I'd suggest making the bulkhead longer. As a rule of thumb, I make my bulkhead at least as long as it is wide(ID). the clay is a brittle material, and I don't expect it holding at this ratio. The only time I reliably used shorter bulkheads, they were cast out of concrete into an aluminium tube with it's end rolled inwards, and even then it was thicker than yours. So, my advice is making the bulkhead the same lenght as it is wide. However, do not fill the space above the bulkhead, either lenghten the tube or shorten the grain.

Edited June 23, 2016 by Oinikis

[BlueComet24]

I'd like to thank all of you for your help in getting my motors to work. After I made several of the changes suggested here, they fly, and fly high! What I changed was ramming the motors on a solid post for better compaction and making the bulkhead as long as the tube ID (12.7mm or .5"). I already was using very small increments of fuel (about .125" or 3.175mm after ramming; about 1g of mill dust), which I'm sure helped to prevent grain cracking.

I'm very satisfied with the performance of these motors. They are able to lift a header of at least 32g nice and high, even when made with BP made with commercial airfloat charcoal. This means they should be able to lift 1.75" ball shells, which was one of their intended purposes.