Ejection Charge Pressure

I was doing some reading on ejection charge pressures in the parachute chamber.
I'm not sure I fully understand things.
I have in mind a medium power (level 1) rocket design with a 3" body tube and 1.2g powder charge.

What I'm being told (I think) is that I should aim for an ejection pressure of around 15 to 20 psi. This equates (on my design) to somewhere around 150-200Ib force.

Where I am unclear is, does the cone really require that amount of pressure to leave the tube? Seems very high to me particularly as the rocket should be travelling at very low speed when the charge ignites.
The design I'm building shows a pressure of 18psi and an election force of 127Ib. I appear to be in the right range but it just seems a bit excessive to me.
I understand the maths and the physics but not the level of pressure required.
Clearly there must be a reason, but at the moment I can't see it. Can anyone help and explain to me why the recommended range is so high. I'm sure it will be totally obvious when explained.

I understand the

Thanks,

SO.

I will be interested in the answers to this as well. I have justified by thinking of the nose cone dragging the chute out of the tube, the additional pressure gives the nose cone enough kinetic energy too we come the friction of the tube in the chute.

I am no expert, but I can think of a few reasons. First, I do believe the idea is to impart enough kinetic energy to the cone so that it can pull the chute out even if the chute is snug. Second, as the cone is ejected the volume of the chamber increases and some pressure leaks, so the extra pressure gives a bit of extra cushion.

My question is what is the compartment size you're pressurizing--to be at 15-20 PSI off of 1.2g of 4f, you'd be looking between a 16 and 22 inch long parachute compartment. Does that seem to be about what your design is?

Anyways... the pressure you need to eject the nose cone is dependent on materials, configuration and retention method. If you have a good friction fit, where you can lift the rocket by the nose without it falling off, then ask: how much friction is holding my rocket together? 3 psi? 5? 7? The length of NC shoulder here will be key. Exceeding this will eject the nosecone. Will it be enough to push out the laundry? Hmm... how do you have it packed, and what force is required to push it out of the tube? A pull scale can give you the figure. Ground testing can confirm whether the charge is adequate, excessive, or not enough...

If using a shear pins, well... what size and how many do you need based on expected forces in flight, and what pressure is required to break that many of that diameter? If you've figured, for example, that 3 #2-56 pins are needed, then, you have to figure the force required to shear those three pins. #2-56 6/6 nylon screws shear around 35lb -3 of them, therefore, would shear at 105#. I'd size my charge to 50% greater than that --or around 150#-- to start with, and let ground testing dictate the final flight charge.

Either way, we wind up at ground testing. If 1.2g of 4f seems excessive to your thinking, and 0.7 (just to pick a figure off the top of my head) seems more reasonable... ground test at the 0.7 and observe the results.

**AND NOT FOR NOTHIN', but... realize: materials often act a bit differently when cold and when hot... if you ground test at 40 degrees ambient temperature, you may not get reproducible results when it's 77 and 94% humidity...**


Best of luck!


Later!

--Coop

You should not just consider the perfect case when your rocket is moving slowly at apogee - if it's arced over and moving at a fair clip you still want it to deploy properly and not go ballistic.

Also many folk use shear pins for keeping the NC attached through mach transition and avoid drag separation after motor burn out, so your charges should be strong enough to shear the pins
cleanly.

From my experience I use about 1.5g-1.75g on my 3" fiberglass rocket. Edit: with 3x 2-56 nylon shear pins

What is the airframe diameter and what is the length of the compartment it needs to pressurize?

Greg

Thanks all,

In answer to your questions, body tube - 3" diameter, parachute chamber length - 17.75" length, cone shoulder length - 2.5".
As far as the cone fit goes, if I tip the rocket it doesn't fall but I keep the fit as loose as possible, around a pound to pull it off. I have however read the note on Apogee rockets site regarding the pressure differential with altitude and the resultant force of 12Ib.
Food for thought.

SO.

Let's say you wanted a force of 1000 N (~225 lb-force) to take the cone off.

By my calcs 1.3 grams of BP would do it with an internal pressure of about 32 PSI. If you wanted half that force, you would use half that amount of BP.

A force of ~936 N would be about 1.2 grams of BP.

Greg