Rear Ejection

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Murdnunoc

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Hey folks,
Looking for a few pointers on setting up rear ejection for my 2.6" SpaceX Starship.
Mainly, how to protect the shock cord from the ejection blast.
If I hook it to the center of the nose cone, that will put it right in front of the motor tube and in the path of the E-charge. Anchoring it to the side wall with a flat mount just in front of the internal thrust ring seems like a better bet to keep the cord aft, but I'm still concerned that when packing it into the body, it will unfurl all over the place.

Planning to use a 100# Kevlar cord about 8' long. The elastic shock cord and little chute in the pics are stand-ins.

Can wadding be used somewhere in the mix to shield the shock cord? How and where would wadding get packed in the loading process?

And should the chute have its own shock cord, or have the shroud lines hooked directly to the motor tube?

Thanks for thoughts,
Darren

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You will probably get a multitude of diverse and conflicting opinions on this. Here's what I would do:
1. If your nose cone is not glued in, consider attaching the shock cord to it. Then it will be easily replaceable.
2.Pack the upper shock cord next to the nose cone, then pack the wadding. The ejection charge will hit the wadding first, then as the pop pod slides out it will pull the upper shock cord out. By then (a split second later) the heat of the ejection charge will be dissipated by the release of pressure.
3. Consider using a longer piece of Kevlar instead of elastic for the shock cord. The shock cord doesn't need to be stretchy if it's long enough. And Kevlar is fire resistant.
4. As for using a shock cord for the pop pod, if you have enough room for it then that's fine. If space is tight then it might interfere with the operation of the pod. Your call.
 
So, are you doing rear ejection to have the rocket come down nose first and minimize landing damage to the fins? How about some pics of the full rocket?
 
Thanks for the thoughts.
The elastic is just a stand-in for the Kevlar shock cord. Looking at 8' long.
The nose cone will be permanently attached, but I may be able to still replace the shock cord now and then by tying a sliding knot.
I like the idea of pinning the bulk of the shock cord against the nose cone with wadding shielding it. I'll just have to pack it a few times to see how it goes.
 
So, are you doing rear ejection to have the rocket come down nose first and minimize landing damage to the fins? How about some pics of the full rocket?

Here's what I've got so far!
I want the clean body to nose transition that will only be achievable with a permanent joint that can be filled and finished smooth. I built up a little 1.5" Starship with traditional nose cone, and it just doesn't look quite right with the seam there.
As I think more, the nose-down landing is the opposite of a scale flight, since the full scale will be landing on its fins. But I guess I have to choose whether to have it look pretty sitting on the pad, or fly and land like the real thing [emoji848][emoji849]

Maybe I can build another version with a standard recovery.

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Hey! I guess I can jam a little wadding into the length of motor tube that's forward of the front centering ring. I've left that extended so there's something to wrap the shock cord around.
Probably want to trim it shorter than I have in the pictures, but there should be some room for a little wadding directly above the engine. That should shield at least a portion of the heat, eh?
 
The method I've used after some troubles is attach the shock cord external to the rocket, such as tied to the back of the fin or epoxied there with a nonfraying guide. It exits the back ring through a notch. Pure kevlar might work internal with little protection, but I've always used elastic to reduce stress. My second rear ejection rocket, first that worked, had a ridiculously small chamber in front of the mount and internal elastic in there too. After a failure on the first launch, I used an elaborate wadding scheme but at least the small space meant nothing was going to move until full ejection. Packing that rocket was an exacting ritual. Rear eject often is, tricky and space is limited. Can handle fast descent and hard landings with a sturdy nose cone though.
 
Got one of these baffles ordered. By the design, it looks like the wide part of the cone goes on the motor end. Figured this would be the best design for being very near the ejection blast.
I don't know how well this baffle will hold up to the blast, but I got a spare that I can try laying some .001" stainless steel on the rear end to shield it if the first one turns out to get roasted.

This baffle may not get rid of all the fire in these tight quarters, so I'll be packing wadding into the shock cord area to further protect it, and try to keep the shock cord as far to the side as possible.

Thanks for the input! It's starting to come together the longer I turn things around in my head.View attachment 375721
 
It works! Ran a test with just some BP balled up in an old motor casing, and the whole mess seems to deploy very nicely.
Good feeling of peace of mind that makes me feel good about moving ahead with the build and dressing it up as a nice and shiny Starship. IMG_1262.jpgIMG_1263.jpgIMG_1264.jpgIMG_1265.jpgIMG_1266.jpg
 
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