Amount of Blackpowder

Hello,

I'm almost finished building a rocket driven by one E and 3 D11-P engines.
Now I'm concerned about the amount of blackpowder that comes with the E reload.
I only use that ejection charge but I think it won't be enough.
The body tube is 43 inches in length and 3,5 inch in diameter.
Nosecone fits tight on the body tube.
Is the ejection charge that comes with the E engine enough?
And if it's not, how should I solve this?

A common way to reduce the airframe volume is with a stuffer tube. The attached picture shows the internal motor mount and connected stuffer tube next to the airframe.

You could add BP on top of the E motor but there is really no way to know how much and it's hard to test.

I guess that without the stuffer tube I will need more blackpowder than the little container for the e-engine can fit, right?
Don't know how much BP fits in that thing but my guess is that's not more than 1 gram.

Everything is already glued together so there's no way I can add a stuffer tube...

Aerotech ejection charges are quite strong. They assume that you will be using them in a large voluminous rocket.

Originally posted by BruuD
Hello,

I'm almost finished building a rocket driven by one E and 3 D11-P engines.
Now I'm concerned about the amount of blackpowder that comes with the E reload.
I only use that ejection charge but I think it won't be enough.
The body tube is 43 inches in length and 3,5 inch in diameter.
Nosecone fits tight on the body tube.
Is the ejection charge that comes with the E engine enough?
And if it's not, how should I solve this?

Click to expand...

My completely unfounded opinion is that you'll be OK.
The reason I'm thinking this is that the internal volume of the rocket is already pressurized. OK only to ambient pressures but it is not like the ejection charge first has to fill a vacuum the size of the body tube and then have enough left over to "pop the top", it just has to "pop the top".
Aslo, as the rocket gains altitude and the ambient pressure drops ever so slightly, if the rocket is anywhere near "sealed" the internal pressure will be just a bit above the surrounding pressure and thus be trying to "pop the top" on its own.
Short and simple, I think you'll be OK.
If there is any doubt however, I would suggest carefully drilling out some of the clay stopper above the ejection charge and adding a thin plastic baggy above the engine with, oh, I don't know, maybe 8 ounces of BP. THAT ought to be enough to deploy ANY recovery system short of the Space Shuttle.

Just my opinion, your milage may vary,

Just remember, when in doubt, add more explosives!



Greg

Originally posted by 11Bravo
oh, I don't know, maybe 8 ounces of BP.

Click to expand...

I sure ain't no high-power expert, but I have an extensive background with pyro mixes (demented childhood) and I can authoritatively tell you that eight ounces is waaaaaay too much.

Eight grams is probably too much.

The stuffer tube is one good approach. Try that----on a ground-test bench. Rig a special igniter directly to the ejection charge well, skip the propellant reload, and test the size and effectiveness of your ejection charge on the ground.

Touch up the shoulder of the NC and smooth the inside of the forward BT to make the fit a little more loose. It should **not** be tight. You should be able to pick up the rocket by the NC and not have it pull out, but you should be able to gently shake the rocket (while held by the NC) and have it slip off the shoulder.

If none of this works, you can also cut the BT in the middle of your rocket. Insert a bulkhead on the forward side and eject your recovery system from the middle.

Good luck!

Originally posted by BruuD
Hello,

I'm almost finished building a rocket driven by one E and 3 D11-P engines.
Now I'm concerned about the amount of blackpowder that comes with the E reload.
I only use that ejection charge but I think it won't be enough.
The body tube is 43 inches in length and 3,5 inch in diameter.
Nosecone fits tight on the body tube.
Is the ejection charge that comes with the E engine enough?
And if it's not, how should I solve this?

Click to expand...

IIRC the amount of BP in an E motor is 1 grain. (or what it gram? check the archives here, I'd found out from Estes at one time).

If you're using only one charge, a stuffer tube is almost necessary for that size rocket. The necessary centering ring would also serve to reinforce the body, near its weak (mid) point.

If it's not too late to modify, consider using 3 D11s with ejection charges and an E9-P (or 0). The D has 0.8 where they E has 1.0 of BP (whichever the unit). Three D11's popping will have 2.4 times the pop power of a single E9. Don't try to use ejections in all of them, because the burn times are different.

Once you get the grains vs grams right, you can go to an ejection charge calculator (EMRR, InfoCentral) and see what's needed. Remember you need the amount of open tube that needs to be pressurized, minus the motor mount, nosecone shoulder, etc.

I agree with the others that it sounds like too little. I've used Pratt cannisters at 1.5 g on my 4" rockets all the time.

A stuffer tube is the answer for a work in progress.

If you are using a reload and have a semi-accurate way to measure powder (and have extra powder of course), you could build-up the charge well with masking tape, insert the right amount, then cap it. This will result in a non-certified motor.

Yyyeeesss, the 8 ounces was a joke.
Unless of course he can get video!
HP48G says that 1 gram = 15.4323583529 grains
Or one grain = 6.4798100002E-2 grams, which is just a teeny tiny amount.

Greg

I've made some adjustments to the ejectioncharge holder wich comes with the reload.
It should be able to hold about 2 grams of black powder.
Hope it works

BruuD

Bruud,
This is one of those times when a ground test is really called for. You are making a fairly major change to the motor and rocket design and it would be worth taking the time to do a little testing. (And it might save you the agony of a crashed rocket.)
If you have an old motor case, or a reloadable case without the main propellant grain installed, you can install an igniter with the pyrogen head in the ejection charge compartment. Run the igniter leads out the nozzle and fill the rest of the motor case with wadding. (You are testing the ejection, not the motor thrust.) Install your ejection test charge, and insert the recovery system as you would for a normal launch.
When you strap this thing down to a ground test stand and trigger the igniter, you will find out if the ejection charge works OK. You can run a series of tests and safely determine if less BP will still get the recovery gear deployed.
This is much more safe than risking a failure-to-deploy, or risking a way-too-much-BP-look-at-all-the-pieces-coming-down failure.

That should do it methinks and I'd prolly just go for it. A ground test couldn't hurt tho. I'd put some soft buffer behind the rocket to absorb the shock. One of my ground tetst stripped a rail button