Ejection charge

I'm building a rear ejection rocket, 24mm MMT in a BT-80 airframe. The motor tube is 300mm long and the airframe tube is 310mm long. The front of the motor tube butts against a bulkhead. The front centering ring is 30mm from the front of the motor tube.
I need to find out if the ejection charges on LPR BP motors are all the same. During ejection testing do I need to use 24mm motors or can I use far cheaper A8-3s?
Also posted in YORF.

The volume that counts is the volume that needs to be pressurized. If the volume being pressurized is primarily the 24mm MMT, then you will be fine.

The amount of BP in Estes BP motors varies with the diameter of the motor, i.e., the larger the diameter the greater the volume of BP.

Greg

To expand on Greg, calculate your volume. Find a reference on what the ejection charge size is for Estes motors,and for whatever other motor you have in mind. Google for a charge calculator, and see if what you get with either type of motor is sufficient.

mikemech said:

I'm building a rear ejection rocket, 24mm MMT in a BT-80 airframe. The motor tube is 300mm long and the airframe tube is 310mm long. The front of the motor tube butts against a bulkhead. The front centering ring is 30mm from the front of the motor tube.
I need to find out if the ejection charges on LPR BP motors are all the same. During ejection testing do I need to use 24mm motors or can I use far cheaper A8-3s?
Also posted in YORF.

Click to expand...

Hi Mike,

Based on your post, I gather you intend to "ground-test" the system by strapping down the rocket and testing to see how well the ejection works. You would like to know if a less expensive "A" motor would have the same or similar ejection as the more expensive "D" or "E" motor.

The A motor saves you $1.50 - $2.60 over the D motor. My personal opinion is the small savings is not worth the additional variable. I would test with the same engine size you intend to fly.

Doing an advanced search on this forum revealed that 18mm motors are approximately .5 grams BP, and D12 are about .85 grams BP, Dont know how accurate this information is. I know that in my SA-3 GOA scratch built, I use two C6 motors with different delays because both going off at once ruptured the main 24mm motor tube, the Booster Section on the SA-3 is 3" diameter by about 8" long, its a cluster model with a central 24mm D12-0 and four 18mm outboard tubes of which 2 are parachute tube and two are 18mm motors. The motor selection was based on the fact that the C6s burned the same length of time as the D12.

https://www.rocketryforum.com/showthread.php?70827-SA-3-Goa-Semi-Scale-2-Stage-with-Cluster-Booster

Thanks, guys. That's pretty much what I anticipated. I'll find and read that Sport Rocketry article and do the calculations. Then I'll just stick an A8-3 in and light it. If it ejects, good. If not, try a C-11. If that ejects, good. If not, redesign, rebuild, or use more talc or dry graphite. Yeah, I'll do that.

Follow up
A8-3 worked fine. Shot the MMT ~2m in the air.