Ground Test

[Jowayen]

This pic is from a ground test with a nose insert for my Big Daddy 38mm MMT. The nose insert has piston ejection using a 38mm piston.
This pic was 36 frames after ignition (0.6 seconds). The parachute yanked the insert forward about 6 feet!
I only used 0.33 grams (5 grains) of FFFg

Looks like too much black powder but I'm hesitant to use any less.

What's the minimum ejection charge you've ever used successfully?


Orange is the powder still burning in the insert (lower left). The 'chute is in a kevlar sock still in the piston (upper right)

 

[Banzai88]

I've used some pretty small ejection charges. The trouble with 'small' is that it can be small enough to not reliably ignite and EVERYTHING depends on how you pack and contain the charge. Only ground testing will reveal if you are at that point.....but 4 GRAINS is smaller than the head of a typical ematch! (I've been reloading metallic cartridges for over 40 years, so YES, I know what grains are).

5 grains is the smallest I've ever gotten to ignite consistently and I used it in a 38mm min diameter rocket, with the charge contained in a centrifuge vial and tightly packed. That's just enough to cover the ematch head in the bottom conical part of a 5ml vial.

The charge is generally used to pressurize the body tube and the energy imparted to the nose cone is what is used to drag the parachute/recovery harness out of the body tube. The picture only hints at your design, but it looks like you're shooting for a parachute cannon to get the laundry out?

In any case, the 'size' of the charge isn't nearly as important at the 'effect'. I always size my charges to get the parachute out and recovery harness about 2/3rds to 3/4ths the way fully extended during ground testing. In the air, that translates to full extension without any shock loading.

ETA: Something is off with your math. 4 GRAINS = .26 grams. .33 grams = 5.1 GRAINS. https://www.google.com/search?q=4+grains+to+grams

 

[waltr]

I do use charges as small as 0.4 grams in smaller DD rocket. For these I roll a 2x2" piece of paper around the red ematch cover to form a tube, taped on. Load the BP into the tube which fills the ematch cover. Squish and bend over the end of the paper tube and wrap tape around it.
These always ignite the BP.

 

[Jowayen]

Should've read 5 grains. My fat finger mistake. Thanks. (updated)

https://www.rocketryforum.com/threads/what-did-you-do-rocket-wise-today.48649/page-1382#post-2399963 (#41459)
This is the layout of the nose insert.

All my other rockets use the 5ml centrifuge tubes with a foam ear plug mashing everything together, then taped shut.
If I used one of the 5ml centrifuge tubes in this rocket, it would lay sideways. Thay may shoot a hole thru the side of the "cannon" since the 5ml tube is about the same length. I don't even want to try that. Instead, I take a 2x2 square of aluminum foil, pour in the charge, stuff the e-match into the powder, fold it up as flat as possible, then tape it shut. This takes up very little space between the bulkhead and the piston.

I have since ran some numbers using the slow-motion video I took of this ground test. I come up with 0.5 N of force.
The shock cord I have is sized to handle about 60x this load so I think I'm worried over nothing.

 

[Jowayen]

I do use charges as small as 0.4 grams in smaller DD rocket. For these I roll a 2x2" piece of paper around the red ematch cover to form a tube, taped on. Load the BP into the tube which fills the ematch cover. Squish and bend over the end of the paper tube and wrap tape around it.
These always ignite the BP.

This is real close to what I'm doing. Thanks