Rocket design and ejection charge limits (low power)

[Gunstar]

When you are designing a large, low power rocket with a BT-80 body tube (or similar size) how do you determine if you can just use a stuffer tube inside to blow the nosecone off, or if you need to make it so the rocket separates into two halves?

 

[smstachwick]

Fly it and hope you get a core sample of the ground instead of somebody’s skull?

IDK, good question. Maybe somebody with more experience flying rockets of this size has an answer.

 

[MidOH]

Add a tissue baggy of black powder. It'll come off.

If my breath can pop the nose cone, any motor should.

 

[waltr]

I have used the on-line ejection change calculators. These are typically used for Dual Deploy but handy to get an estimate of required BP charge on larger BTs.

Just built a 4" Goblin and questioned if the 0.7gram that came with the motor was enough. Charge calc suggested 1.2gm so I setup a ground test. 1.2gram did the job of pushing off the nose. I doubt that 0.7g would have been enough to force the nose cone out fast enough to pull out the chute..

 

[Antares JS]

When you are designing a large, low power rocket with a BT-80 body tube (or similar size) how do you determine if you can just use a stuffer tube inside to blow the nosecone off, or if you need to make it so the rocket separates into two halves?

Both of those are solutions to the same problem and as far as I know, neither is better than the other. Either will work. I prefer stuffer tubes, personally.

 

[lakeroadster]

I have used the on-line ejection change calculators. These are typically used for Dual Deploy but handy to get an estimate of required BP charge on larger BTs.

Just built a 4" Goblin and questioned if the 0.7gram that came with the motor was enough. Charge calc suggested 1.2gm so I setup a ground test. 1.2gram did the job of pushing off the nose. I doubt that 0.7g would have been enough to force the nose cone out fast enough to pull out the chute..

Is there an on-line ejection charge calculator for low power rockets, like the OP is asking about? That would certainly be great.

 

[waltr]

This one works good.
https://rocketrycalculator.com/rocketry-calculator/bp-estimator/
another good one
https://www.insanerocketry.com/blackpowder.html
Both allow you to enter small tube diameters. The 'tricky' part of all charge calculators is 'guessing' on the force or 'psi' required to push off the nose cone.

Many people suggest doing the 'breath' into the motor tube to test nose cone tightness. If you can pop off the nose by blowing into the MT then a motor's stock amount of BP should be fine.
If nose does not come off or barely comes off and it is not too tight (lift test) then you might need to increase amount of BP in the charge. This is the case on my 4" Goblin. Nose will slowly slide off when lifting the Goblin by the nose so tightness is good. However, when doing the 'blow test' the nose just barely comes off. This is not enough to pull out the chute. Stock motor charge was 0.7grams and calculator (linked above) indicated 1.2 grams. Ground test with 1.2 grams ejected to nose with enough force to pull out the chute but not fully extend the nylon shock cord. Therefore, I increased the BP in the motor (AT 29/40-120 reload). The first flight deployed the chute perfectly.

On the other hand I build the BMS 3" school rocket with extended BT. On this one I added a Stuffer tube. The 'blow test' ejects the nose with enough force so use the stock amount of BP. Chute deployement has worked well with this rocket.