Testing motor ejection

[smoon]

I have a RW AGM-58 where the aft end of the rocket weighs 8 lbs dry and the payload section forward weighs 4 lbs. I am concerned about drag separation after motor burnout, so I put 3 x 2-56 shear pins into the aft airframe/coupler connection point.

I am going to fly this with motor eject and chute releases, so I am not using electronics to control separation at this point. I am going to use motor eject. Because of the shear pins, I need to test ejection to make sure the ejection charge will shear the pins and separate the rocket properly. That is where I am not sure what to do. Testing ejection charges using canisters on an avionics bay is no problem. I am just not sure how to test motor eject.

Here is an idea I had, but I wanted to see what others that have done it before think.

My idea is to thread an igniter through the hole in the motor's forward closure, secure it in place, then fill the cavity with the calculated amount of BP, and seal it as if it were for a flight. Would lighting the igniter to ignite the BP simulate motor eject properly?

Thanks for any and all advice,

Steve

 

[qquake2k]

I don't see why that wouldn't work. Yes, an igniter will light BP. I've also heard of people putting the BP in the finger of a latex glove and wrapping it with tape. Or using a straw or small cardboard tube. I've used centrifuge tubes I got from Ebay, and threaded them through a spent single use motor case. I put a little dog barf in the tubes to keep the BP at the bottom.

 

[Buckeye]

It doesn't really matter what "end" of the tube you are pressurizing. The canister on the ebay side will give the same result.

A forum member will chime in and say that some of the BP on the motor side falls through the hole in charge well and burns in the motor compartment, thus reducing the effectiveness. So, maybe add a tad more BP on the motor vs. the ebay cannister.

This is a 4" rocket with 3 shear pins, so you will probably need around 1.5 grams as a starting point. 38mm AT reloads, for example, come with 1.4 grams that fill up the charge well. You will need to make provisions to hold more volume in the charge well due to added powder (and the igniter during your tests).

 

[ksaves2]

NO, NO, NO, NO!!!! DO NOT SIM MOTOR EJECTION WITH TUBE CANISTERS. ONE RUNS THE DANGER OF UNDER CHARGING THE POWDER!

The correct way was mentioned. Thread the ematch through the ejection bulkhead. Seal the bottom where the wire comes through on the "motor side" so little or no powder leaks in to the empty casing. Bear in mind, the powder one pours
into the well of a loaded motor, some of it leaks aft on top of the delay grain and though it is a small amount doesn't add to the forward push and is likely expended out the back of the burned out motor casing. Keep that in mind when doing
the sim. If you determine a powder charge that "just" lightly gets the NC off, you run the risk of undercharging if using the same amount of powder. You always want to add a bit more rather than less. Kurt

 

[smoon]

quake - your idea looks really good should I need to test motor eject on smaller rockets. I will keep it in mind.

Kurt - thanks for verifying my plan is the right way to go. I will make sure I seal the opening where the igniter wire feeds through the forward closure and keep in mind that a fudge factor will be added once I determine the right amount of BP.

Steve

 

[crossfire]

What motor are you going to use? A CTI Pro54 is very easy to add powder. I am planning on flying the same rocket in a couple of weeks using DD.

 

[ksaves2]

You're welcome Steve. I lost a rocket doing it the wrong way so I'm rather emphatic about it. Kurt

 

[smoon]

crossfire - I will be flying it on an AT J460T, which is a 54mm reload. Adding BP, if needed is very easy. As for DD, I am working up to that. With the chute release, I am in no hurry, though.

Kurt - ouch on the lost rocket, and this is exactly why I want to test the ejection. If the ejection charge failed to shear the shear pins, and the rocket came in hot, I would be extremely sad.

Steve

 

[ksaves2]

Yeah it was a Formula 54 I used a canister to test when I used motor eject for the flight. Black nosecone didn't come off and the rocket went it ballistic. A quick note is here: https://www.rocketryforum.com/showthread.php?129292-V2-for-my-L3&p=1680649#post1680649 Saved the rocket, put a reddish nosecone on it and had a forward closure failure the next flight. Fixed the burnout with a coupler repair and it flies fine now. You should have a nice flight
with the J460 Kurt

 

[smoon]

Wow, two major failures on consecutive flights. That would be tough to take, but I am glad that it flies great now.

My daughter has had similar luck with Nike Smokes. She really loves them, and had a PSII one that was her best flyer and had the most flights on it of any rocket we have, until a blow through of the motor just after liftoff cooked the inside of the rocket. I saved the nosecone and fins, and built her a replacement using Loc parts. The first flight on it was great. The second flight was on an AT G54 that must have been kind of old. It took forever to light, and when it did light only went about half as high as was simmed. It went in nose first before the ejection charge went off and broke the Estes nose cone and crushed the airframe. With a broken nose cone, I discarded the rocket and got her a Madcow 2.6" fiberglass Nike Smoke to match my 4". Due to a mistake by me where I placed the Nomex blanket, the chute got tangled with the shock cord and never deployed. It came in end first and broke two of the fins free inside the airframe. I am now in the midst of that repair, so it will be ready for our April flight.

With her luck, I have become almost paranoid about making sure everything is perfect before each flight.

Steve