Rear Ejection questions...

[McKailas Dad]

Yes, I've searched. While I do understand the concept, in the construction I'm a bit confused.

I want to eject both the motor tube, and the 'chute.

I am using BT-80 body tube, 17 inches long, with a BT-50 motor tube, about 13 inches long. I have a nose cone mounted with screws, to be removable.

I have (temporarily?) made pink foam centering rings, and installed one near the motor end, and one an inch or so from the top of the motor tube.

If I glue a centering ring (thrust ring?) in the BT, to keep the motor tube from going up, how would I feed the shock cord as I am preparing the rocket?

Should the shock cord (Kevlar) be attached to the inside of the MMT? How else would the shock cord pass through the BT thrust ring?

I'm thinking of epoxying a small Kevlar loop inside the top of the MMT, then attaching a short (4 foot?) piece form there to the NC. I could bundle up the extra Kevlar and stuff it in the top of the tube as I am prepping for flight.

The other Kevlar, for the parachute, would be somehow attached to the outside of the motor tube, in the room (~10 inches) between the foam MMT CR's.

Man, this seems confusing to read. I'll see if I can take some pics to help describe what I am doing...

Can you show me pics on how you did yours?

Thanks!

[Fe Dude]

IIRC, on my estes sizzler (rear eject) the shock cord was mounted to the outside of the mmt, fed underneath one of the CR's.

Since you are using kevlar, I don't see an issue mounting the shock cord like you would normally do, mount a separate CR in the body tube to act as an engine block for your motor mount assembly.

You could always cut a small notch into your BT thrust ring for the SC to pass through.

Heck you could even use a solid balsa bulkhead with a screw eye, mounted further down the tube. That could act as both your thrust ring and make it so you aren't running kevlar up to the nose.

A little sanding work on the bulkhead and mmt tube should allow your kevlar to pass by without an issue.

The chute gets attached to the bottom end of the MMT, and gets wrapped around the MMT for its ride up. Use a short piece of kevlar with a fishing swivel tied on the end for a mounting point for the chute.

Just bouncing some ideas around. I hope you can decipher some of it. I've been pondering an upscale of the estes rocket, so I'm just sharing some of what I thought through.

Sam

[Micromeister]

Generally speaking:
Rear Ejection pods wrap the Chute or Streamer around the motor tube.

Are you familiar with the Swingwing Transport? or Skydart SST? Both have a BT-50 outer airframe with a BT-20 Motor/stuffer tube that slides out with both forward and rear centering rings attached permanently to the motor tube. Chute is attached with duct tape and wound around the motor tube just before it is inserted. It helps wonders to heavily Talc baby powder all over the chute or streamer.

You are correct about the KEVLAR Shockline being stuffed in ahead of the rear ejection pod. it's usually a good idea to add a single sheet of FPwadding just before inserting the pod to catch as much of the burning particles as you can before the shockline is dragged out.

AS for keeping the motor pod in place.
One of the tricks I use is to glue a 1/8" ring of the main Airframe to the edge of the rear centering ring as a motor stop. Works just fine of everything up to 4 D12's in a BT-80 body
Another way is to install a Tube coupling as a forward Pod stop inside the main Airframe but this seems to cause more trouble in my clustered models then help.

In all cases the 70-130lb kevlar shockline is alway knot & epoxy embedded in the Nosecone and attached around the forward pad centering ring through a hole no more the 1/8" for the outside edge of the ring.

Though I had a close-up photo of one of the Clustered Rear pods but can't put my finger on it'. Hope these to smaller pod pics will help a little.

[hornet driver]

This is what I use in my boxter BG as well as a similar design for all my BG's. It can be easily modified with a tether to the rocket body. I always use a piece of bt between the bulkheads--lessens the chance of binding on the way out--had that happen before-held in place with light friction but I've used spring clips with success in the past.

[Micromeister]

Hornet Dirvers pod is very similar to those used in quite a few Estes ejectable pod gliders. The Scissor Wing Transport, Crusader Swing Wing and Skydart SST. only reall difference I see is the estes extended pods all have a forward end of the tube centering ring to help protect the chute or streamer from the hot ejection gas and ensure the pod completely clears the model body.

have all three models out in the shed but can't get too them right now as the entrance is blocked with basement junk while we are doing some Home Improvements.
Best I can do is a T3 downscale of the Scissor wing Transport with Streamer rather then chute but they work exactly the same way.

[BABAR]

From your post it sounds like you are NOT building a boost glider, where the glider portion of the rocket completely separates from the engine pod. Instead I am assuming you want rear ejection but BOTH the body and the engine pod will descend together.

1.You can attach your rocket body shock cord to the nose cone and to the FRONT of your engine pod. For strength you can loop it around the ejection pod just BEHIND the front ring and feed it through the ring between the ring and the motpop ejection pod tube so it comes out in FRONT of the forward centering ring. This shock cord is going to be blasted by the ejection charge, so needs to be tough enough to handle that but pliable enough to coil up nicely between the front of your pod and the nose cone.

You will feed the cord in before you load the pod, then stuff the pod up the tail of the rocket. You will have a separate shock cord connecting the chute to the ejection pod, this can be attached anywhere between the forward and rear centering rings. Concur with der Meister that a few sheets of wadding either in the body just after you tuck in the shock cord before you stuff the pod in, OR you can put it in the pop pod before you start. This may save your shock cord a bit.

Your chute and pod shock cord will go around the pop pod tube between the centering rings. Resist the temptation to use the pod as a spindle and just wind them around. Fold your chute so it is just a bit shorter than the space between front and back centering rings, fold your cord lengthwise just a bit shorter. Sandwich the cord between the chute and the motor mount, and wrap the chute short width around the mount. It is kind of a PITB to get in but has a much higher probability of deploying than if you just wind everything around the tube.

2. One other option is to attach your rocket body shock cord OUTSIDE the rocket body, with the other end attached to the front of the motor mount BEHIND the forward ring, the outer attachment point is at or just forward of the CG of the rocket body alone (without the pod). You can use a short piece of elastic at the motor mount attachment with Kevlar on the side running along the outside of the rocket body. Can also use a small inconspicuous Kevlar attachment point on the body and a small snap swivel on the shock cord so you can dis-attach it for display purposes.

Advantage of this is that in this case your rocket body will fall and therefore impact the ground either tail first or sideways. If you use standard rear ejection your shock cord will come out the tail of the rocket and your rocket body will pretty much always impact pointy side first.

3 final suggestions.

Make sure your nose cone is screwed in or strongly glued to the body so it doesn't blow out.

Make SURE the pod slides out very easily. This is especially true because you are going to be using a lot more chute to recover your pod and rocket together than just the pod alone, which is the case for rear eject boost gliders.

You said you are using foam centering rings. The forward ring will get some blow back of hot gas, add some paper or Mylar or something in front of it to keep it from melting

[go for burn]

Apogee has a video.I built a few models using this system they were flawless. Type in Apogee rear ejection video. For me it was the top one.I am new to computers so input is always excepted.

[plastic_gorilla]

So I am working on my first rear ejection as well. It is also my first B.Bertha. I am considering putting a bulkhead in as a tie off and blast plate for the ejection charge. I brought this up in another thread but is seems best bringing it here. Rex R had suggested 1/8" ply (going to the store tomorrow!) Pretty excited to try it out!

to make sure i get it right - are you all hooking the kevlar directly to the interior anchor and then the mount? or are you putting in any sort of shock to buffer the pull of the line against the body. i was planning on using the supplied 18" chute attached to the engine mount and then kevlar then shock cord to the bulkhead. but it sounds like that may be inadvisable.

any thoughts appreciated!

pg

additional edit below - forgot to mention

also i was thinking of the rear ejection for the possibility of switching out engine mounts - jump up to a D (or higher if i feel brave - or visit a place with fewer trees)