Dual Deploy Ejection Setup

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

ckjohnson

Well-Known Member
Joined
Oct 19, 2013
Messages
307
Reaction score
0
Hey everyone,

My setup in my Norad is that my drogue is in the lower body tube, followed by the ebay, and the main in the upper tube. The ejection charge for the main is directly outside the ebay, and it pushes the main chute with all laundry out of the tube behind the nosecone. The ejection charge for my drogue is on a wire and runs alongside the laundry to behind the drogue. When it fires, it pushes the laundry out behind the ebay. Like so:

----------------------------------------------- ----------- -------------------------------- ----------------\
Motor Ejection Canister Drogue | E-bay | Ejection Canister Main | Nosecone >
----------------------------------------------- ----------- -------------------------------- ----------------/

I see a common practice is to mount the ejection charges to the e-bay, which would look like this:

----------------------------------------------- ----------- -------------------------------- ----------------\
Motor Drogue Ejection Canister | E-bay | Ejection Canister Main | Nosecone >
----------------------------------------------- ----------- -------------------------------- ----------------/

Wouldn't this require the body tube to separate below the drogue? The ejection charge would push the drogue down into the body tube while the e-bay ejects, wouldn't it?

I ask because I am always worried that the force of acceleration is going to push the laundry back on the ejection charge, tearing the wires out of the altimeter ports and was wondering if it would work to place the ejection canister directly behind the e-bay.
 
Last edited:
Okay, the formatting on this is making the "diagrams" confusing. I hope you guys can see what I am demonstrating there.
 
Both work. When I put the charge below the drogue I have it rest on the upper centering ring. It cannot shift down and tear any wires out. When the charge is at the top, it blows the sections apart (just like putting it at the bottom). No matter how long my shock cord is, I attach the chute about 3 to 4 feet from the ebay. It "will" pull the chute out if your charge is appropriate.
 
Thanks Titan! I got the same reply from a friend who also put a hardpoint (eye bolt) on the e-bay to wrap the wires around to prevent them from pulling on the altimeter.
 
First of all, forget the idea that the ejection charge pushes anything out of the BT. It doesn't.

The ejection charge pressurizes the BT and that pressure, pushing on the av-bay, or nose cone, causes those items to pop off of the BT like a champagne cork. If your parachute is very snug in the BT so it acts like a piston, you may get some ejection motion from the charge, but not much. How many G-Force rockets have you seen that fell flat with the nose cone off but the chute never deployed? The ones I've seen, the chutes fell out of the BT when the rocket was picked up by the fins. There was nothing holding the chute in, but the ejection charge never pushed it out either.

As long as you have a strong enough charge to get the two halves to stretch the shock cord to it's full length, and the parachute is attached at a position where it will come out when the cord is stretched, you will get deployment no matter where you place the charge. My warning is, don't overdo the ejections charge. Stretching the shock cord is good, slamming the two piece apart and shock loading the cord is not. Using an excessively long cord to avoid shock loading causes it's own problems, sometime leading to worse shock loading, especially in DD rockets.
 
You attenuate the shock of the ejection by z-folding the shock cord and taping 3-4 fold bundles with masking or electrical tape. Sorry I do not have a picture.

Here is an old thread that talks about it.

https://www.rocketryforumarchive.com/showthread.php?t=16779

You are right, and I use that method a lot. It only dissipates so much energy and probably less then you would expect. I've seen ejection charges slam the two halves the rocket on the end of the shock cords even when they were z folded. Blow it out or blow it up is a really stupid method in my opinion.
 
Found a picture. When I do ejection testing I make sure some bundles remain intact. There is a wide range of adjustment based on number of bundles, type of tape and how many wraps of tape.

Also, check out this thread and specifically post #6.
https://www.rocketryforumarchive.com/showthread.php?t=45956

I like the idea of using 3, then 2 then 1 tape on the sections of shock cord. I also like the idea of using the electrical tape. I've only used masking tape and I think that tears too easily. Also, when using 1/8" Kevlar, it seems like it will slip the loops out of the tape instead of tearing the tape. I've taken to only doing odd numbers of folds in the z-folds. I'll do 5 folds and then tape, then five more folds on top of that and tape again, then maybe another 5 folds so everything is in one bundle.

I'll have to try the 3 tape z-fold, 2 tape z-fold and 1 tape z-fold method. Then again, I have my charges set pretty well so it doesn't really need much z folds on the drogue side. With the drogue chute, I also don't have issues with main deploy since it's always above the fin can and everything stretches out before the main actually opens fully. I don't get a lot of shock loading on the system.
 
I've never seen that. I'm going to use the tape methods. Thanks for your help guys.
 
Back
Top