Calculating D motor ejection charge

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My son and I are building a 4 D cluster rocket. It's a BT80 based Patriot Missile using what I believe is a Apogee tube (we got it at a swap meet) that's fairly thin. I'm concerned about over pressurizing the tube if 4 ejection charges go off. Anyone know how I can calculate how much ejection (ie. how many motors with charge) I would need? The tube volume is about 127 cubic inches. Intuitively I think two would probably do it. Do you think I'd need to "plug" no-charge motors to prevent the pressure from escaping from the empty motor?

 

[brianc]

I'm concerned about over pressurizing the tube if 4 ejection charges go off.

I wouldn't worry about it. The possibility of all four ejections firing at
precisely the same instant is remote... Once the first charge releases the
nose and recovery, the remaining charges have a clear path to open air
and won't pressurize the tubing.

 

[Zeus-cat]

Another option is to use two pairs of mismatched motors. Select one pair with a delay that is the one you want and another pair that has a longer delay. For example, if you think you need D12-5's, use one pair of D12-5's and a pair of D12-7's.

 

[Handeman]

If the tube is big enough, and the fact they you will probably never get two or more motor ejections at the same time, you might need to add a little powder to the end of the motors to ensure a successful ejection.

I've never heard of anyone rupturing tubes because of ejection charges. That's not to say it hasn't happened, but I've never heard of it.

 

[powderburner]

As far as reasons not to worry about excessive ejection pressure, the posts above are all good ones.

Plus, if you did plug two motors (which would be tampering with motors, a whole different thread for discussion), and if they were somehow the only two motors that ignited out of the entire cluster, you would have a problem.

If you are afraid the tube volume is too much, how about breaking (cutting) the BT in the middle and installing a coupler with a solid bulkhead. That way, pressurization would push out the recovery system along with the fwd half of the rocket (the front end would act like one big NC).

 

[quickburst]

If clustering its best to stagger the delays between the 4 motors as suggested earlier or leave them all in tact. The reason is that its possible that if you remove the charges (which would not be easy, and may even be frowned on in some circles). There is a chance that the ones with the charges missing will be the only ones to light.

Can you spell L..A..W..N..D..A..R..T ?

 

[[email protected]]

Thanks for all the good answers. It seems like the consensus is that I don't have to worry about rupturing the tube because the charges are not likely to go off at the same time. The other issue is wear and tear on the internals from all the hot gas and soot from the charges. Since the rocket should need the full 7 sec. delay, I'm thinking of going with two D12-7s and two D12-0s. I'm also thinking of putting a disk of thin G10 on top of the D12-0 motors in the mount to prevent any material from burning forward (as happens in staging) and to provide a little extra seal for the pressurization. (This is what I meant by plugging the motor, not actually tampering with it.)

 

[TheAviator]

If you're going to plug a D motor, just go with an Estes D11-P or E9-P. No tampering whatsoever!

 

[Peartree]

I haven't done it myself, but according to some discussion here, you recovery system *may* deploy from the burnout of D12-0 motors. Some folk have flown that way on purpose because their designs required a less than 3 sec. delay. YMMV.

 

[Micromeister]

Thanks for all the good answers. It seems like the consensus is that I don't have to worry about rupturing the tube because the charges are not likely to go off at the same time. The other issue is wear and tear on the internals from all the hot gas and soot from the charges. Since the rocket should need the full 7 sec. delay, I'm thinking of going with two D12-7s and two D12-0s. I'm also thinking of putting a disk of thin G10 on top of the D12-0 motors in the mount to prevent any material from burning forward (as happens in staging) and to provide a little extra seal for the pressurization. (This is what I meant by plugging the motor, not actually tampering with it.)

you'll either blow the discs out or kick the D12-0 casings. You really don't have to worry about all the ejection charges. In fact your fear may end up causing the crash your attempting to avoid. What happens if one or more of you D12-7's fails to eject?
I used to fly a lot of 3, 4 and 5 motor D12 clusters. Clustering D12's is still my second favorite type of flying D12-0's have a good amount of burn thur gas "Kick" but it's really no were near the volumn of the ejection charge unless you try to cap or contain it. Then it's enough to straighten out standard engine hooks or pop out those G10 caps. C6-0's are the same.
My suggestion to help reduce the charring inside the model is to add a BT-80 coupler just above the motor mount. I've even done this as a retrofit on some of my Upscale as they aged. Most of the Upscale I still have in my fleet have more then 25 flight, some many more others like my 13.25Scale Bomarc right at 25. Shes in for a upgrade from 3 D12-3's to 4 D12's using the same airframe and stuffer tube.
In most of my Upscale i've never really worried about over pressure, I'm always more concerned with under pressure. usually using a BT-55 or BT-60 stuffer tube leaving about 12-14" of BT-80 as the chute compartment.
3-D12 Ramjet, Laser-X, Aerobee-Hi, 4 D12 Optima, Nova Payloader, Orbital Transport, and 5 D12 Crayon, Hobby Goblin. have never had an over pressure problem. but I did suffer an underpressure NO deploy on the OT causing major damage
Hope this helps a little.

 

[cls]

you won't have any problems with pressure.

but install a really sturdy baffle system to contain the hot particles from the ejection, because those will really cause heat damage on that airframe tubing.

you might consider:

- coating the ejection area with epoxy

- gluing in a piece of coupler tube, or other piece, as a liner

- adding some stainless steel mesh (like Aerotech baffle material)

- using an offset-holes-and-plate baffle system

- all of the above!