Aft-ish Ejection/magnetic shear bolts/rear recovery canister/rocket modularity

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Aug 25, 2016
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I have been toying with some ideas over the winter, and I have built a kluge test bed rocket. I apologize if these ideas have been discussed before, but a rudimentary search did not reveal such. Also wasn't sure if this should go in LPR or recovery or here. Sorry if I erred.

I have always been intrigued by aft ejection methods. I had been told that the big problem with separating the rocket in some place other than the nosecone, you ran the risk of drag separation during the glide phase before apogee. After watching some sims in Rocksim, I noticed on a multi stage rocket that the difference between the drag on the first stage and the second was only around 4-5 newtons. So if I could resist a little bit more than that, it would prevent drag separation. This seems to be done in high power rockets with shear pins, but with paper rockets that would seem problematic. I have some experience with neodymium magnets from modeling Warhammer 40k models and thought it was easy to tailer the forces with different magnets, why not use this force in place of shear pins?

Well if this solved the drag separation problem, it opened up another one if not two. I needed to protect the parachute from the separation charge nastiness and possibly need to deal with increased pressure in a smaller area. I am not sure if I need to reinforce the walls of the rocket as the ejection charges are sized for having the entire volume of the rocket to expand in vs maybe 25=33% of the volume in this design. Once I decided to use a baffle, this with the needed coupler allowed for almost the entire section from the end of the engine mount to the coupler to be double walled. If I added a little more coupler material I could double wall this whole section to the magnetic mount. I decided that it was worth the weight for now until I see what I am dealing with from the increased pressures.

Lastly how to eject and protect the parachute? Since I needed a bulkhead to resist the ejection blast, I decided to use this bulkhead to protect the parachute. The test rocket is a BT-60 tube, so I installed a 29mm tube inside it with centering rings and an eye hook at the forward end for an anchor for the parachute cord. On the aft centering ring of this inner tube I installed the magnets (inset from the edge to allow the coupler to pass to the centering ring)


From here I made a basswood door that was slightly smaller than the diameter of the BT-60 coupler as to allow the coupler to pass by. The magnets are installed a 1/8 inch past the centering ring to act as posts to help keep the door retained. The door is 1/8 thick so the door and the posts are flush at the top.


I inserted two tiny roofing style nails in 1/8" square balsa to act as the magnetic opposite. These mail heads would provide ~8-9 newtons of attractive force to keep the coupler system tight, and the oversized balsa would keep the door from moving of the 'pegs'. Also in this image is the top of the baffle i installed with the brass hook (non ferris) to mount the short cable.


I used a wire fishing leader to connect the aft section to the blast door as it should resist the unkind environment coming from the baffle. Also it has swivels and operable hooks to keep it from getting bound up (hopefully). This wire is connected to the blast door brass eye hook, to pull the door off and pull out the shock cord and parachute. Hopefully the blast door should be kept in place from separation until the pulling of this wire by the ejection/separation blast.


This is untested as of yet as I have either been under fire restriction, a bunch of snow or wifey edict. I hope to test it in the next couple of weeks. I am also building another aft section with a E/D mount to mate up with the forward section. I think this design could add to some degree of rocket modularity as long as it stays stable.


Baby Birtha Mod.jpg

I hope to hear your thoughts. I hope to launch it soon on a Aerotech d10-5 as its too heavy for a C6-6. The addition aft section will allow for stronger engines.



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The magnets are a nifty idea. Looks like you've taken and Baby Bertha and turned it into a regular Big Bertha. The baffle is nice too.

Word on the D10's make sure you have a spare igniter, and that you insert it ALL the way to the top (friggin lumpy firstfire Jr caught in the slot)

I flew a couple D10's this weekend. One sent my Patriot to 920'. The other chuffed and spit the igniter. With a fresh igniter jammed all the way in, it still chuffed about 5 times, the rocket hopped 8" up the rod and sat back down, paused, THEN took off (just as I stopped recording figuring it was another fail).
Finally got to test the rocket this weekend with mixed results. I can happily report that the aft facing ejection worked flawlessly. Unfortunately it worked 20 feet off the ground and ripped the shroud lines through the plastic and reinforcements on the parachute, but this used so much energy, the remains fall from 10 feet went without damage. The rocket was too heavy for a C6, I knew it, Rocksim, knew it, but I let the range officer talk me out of the D10-5 and here we are.....

The damage to the rocket (Lower Section) came from the motor mount exploding. When I picked up the rocket, all the was left from the motor mount was the forward centering ring hanging by friction alone. The motor mount tube, and aft ring were gone. Never seen the center tube blow out like that. It was like 8-9 year old glue as it was an old rocket I modified for this test. Either that or too much back pressure, but it was a standard Apogee BT-60 baffle with the 1.5 tube length distance from motor to baffle. I am finishing the other rear end for this rocket that has the D/E mount, but now I am nervous about back pressure......
I have been using a similar ejection design on my G80 Mach busters for a few years now. I had some of the same problems early on. using a tight friction fit to prevent drag separation a couple rockets burst far before ejection. The culprit was the heat of the motor coupled with the sudden drop in outside pressure caused the air inside to expand. Now I reinforce the entire area with a coupler and have added a vent. The vent helps with thermal expansion and purge some of the expanding ejection gas. The plywood piston is epoxy coated and is attached to a very long harness. With all that gas expanding in such a small space it is like a gunshot. A simple knot on the harness pulls the piston and recovery out. I hope this helps with your design.