Ejection Charge Problems

[Gus]

Can anyone point me to information on ejection charges for Estes C, D, and E engines?

What I'm looking for is something similar to the thrust-time curves produced for the engines themselves.

I've been working on a scratchbuilt and have had two ejection charge failures. The first used a long pistion-type ejector which resulted in the piston moving hardly at all and the body tube blowing itself in half due to overpressurization.

The second, lost today, used a much shorter pistion, which the ejection charge (on a D) blew right through the bulkhead holding it in place.

It seems the ejection charge is much more rapid and violent than I thought.

Any info would be appreciated.

Thanks.

By the way, the only parts of this rocket which have come through both failures unscathed were the nosecone and transition made for me by Sandman!

 

[Missileman]

Yes it is a rapid and violent expansion (hey its gunpowder in there)
If I am not mistaken the Estes D engine has .86 grams of BP in its ejection charge.
Pistons are funny animals.
Any change in temperature can affect its tightness in the body tube.
Put talc. powder of some kind on your piston to help it move.
This may sound funny but here is how I test my piston eject sytem.
With the piston, chute, and cone in place I blow through the motor tube. If I can blow everything out with shear lung power I am good to go.

 

[Gus]

Jim,

I used the "blow technique" on the second version and it worked fine, top popped right off, no problems. But it never occured to me the actual ejection charge would be so fast and powerful that it would blow the piston right through the bulkhead.

The only gratifying aspect of this failure is that I've now absolutely proved that I'm not nearly the "blowhard" some people claim.

 

[Micromeister]

Gus:
Folks have been looking for that data a long time, as far as I know it does not exist.

Jim is right on with the Blow techinque, but internal pistons are very finicky, Good luck

 

[Elapid]

graphite powder would be a great lubricant for your piston ejection system... if a bit messy.

 

[DynaSoar]

Originally posted by Gus
I've been working on a scratchbuilt and have had two ejection charge failures. The first used a long pistion-type ejector which resulted in the piston moving hardly at all and the body tube blowing itself in half due to overpressurization.

Are you using vents? I built a rear/piston ejection Bertha clone (search for thread "Ahtreb") and Len rightly noted that if there weren't some pressure relief it'd likely create its own. You might consider it if you haven't.

I was concerned it'd cause too much drag and that some siphon action might do bad things, so I put a piece of striping tape above it so it hangs over the hole, and folded it over so there's no adhesive there. The flow on the up phase should keep it in place and covering the hole, but there's nothing to prevent it from flapping out due to internal pressure.

And now that you've rasied the question again, I'm starting to wonder if one 3/16" hole, near the full extension of the piston, is enough. Perhaps I'm better safe that sorry, yes?