BP Calcs When Using a Piston

[jrains]

What are the guidelines for calculating BP when one's rocket uses an internal piston compared to the same rocket without a piston?

 

[OzHybrid]

Less than normal. Start at about half. Big "however" though.... Any sticking reduces your already reduced amount of BP charge effectiveness.

Test what you're going to do, do what you test, make sure you fully clean the inside sliding faces after and check the whole length of the sliding area and piston. And a piston length should be roughly 1.1x the diameter minimum.
YMMV

Norm

 

[Rocketclar]

Agree with above. I keep a spreadsheet of calculations AND tested results. That way when I prep a rocket, I can see what worked. I also video the ground tests. Several times, I needed to adjust and retest as the calcs didn't match what my rocket needed for an adequate ejection. I ground test with a spent motor and all components....parachutes, cords, d-links, electronics, etc to match as close as possible to what will happen. I have several piston rockets (Quantum tube and FG) and cleanliness is critical. Also, I talc my pistons especially in cold weather.

 

[jrains]

Agree with above. I keep a spreadsheet of calculations AND tested results. That way when I prep a rocket, I can see what worked. I also video the ground tests. Several times, I needed to adjust and retest as the calcs didn't match what my rocket needed for an adequate ejection. I ground test with a spent motor and all components....parachutes, cords, d-links, electronics, etc to match as close as possible to what will happen. I have several piston rockets (Quantum tube and FG) and cleanliness is critical. Also, I talc my pistons especially in cold weather.

Thank you for both of the responses to date. Will ground test - this is a must. The rocket is made of LOC 4" kraft body tubes, not Quantum or fiberglass. The nose is held in place by nylon shear pins. There is a lot going on here, hence the question.