BP ejection force

[geof]

How much length of BT70 can a single Estes motor reliably and strongly pressurize for ejection? And, would it change for a cluster, or would it not change because the ejection pressure for the 2nd motor would just leak out of the casing from the 1st?

Geof

 

[shreadvector]

How much length of BT70 can a single Estes motor reliably and strongly pressurize for ejection? And, would it change for a cluster, or would it not change because the ejection pressure for the 2nd motor would just leak out of the casing from the 1st?

Geof

Bigger motors have stronger ejection charges because they assume that they will be flown in larger rockets. Clustered motors will create a larger pressure (if they all go off at the same time). The effect of leakage out the nozzle is almost nil.

Calculate the internal volume of the Mean Machine and convert that into an equivalent length of BT-70. That is a good volume for a D12 or E9 motor.

 

[geof]

Well, after much googling and forum searching, I think I have some answers, which I'll post here for posterity.

I used the information given here:
https://www.info-central.org/recovery_powder.shtml
and the *claim* from an old TRF thread that the grams of BP ejection charge are 0.6g for ABC, 0.8g for D, and 1.0g for E.

For my BT70, then, I calculated that I need 0.92g BP for 24in of BT70, and 1.4g of BP for 36in of BT70. This used the "150 pounds" threshold recommended at the website. (Before you panic, read the webpage very carefully.)

Now my rocket is a 7-motor cluster 3E+4C, but I can't be assured that they will all go off perfectly simultaneously. Therefore, I'm going to choose 24in of BT70 tube (actually more like 21in) in the design and place a bulkhead there, leaving the remainder of the tube as a payload section.

G

 

[loopy]

That was gonna be my suggestion...stuffer tube, baby!

 

[geof]

That was gonna be my suggestion...stuffer tube, baby!

Hmmm, a stuffer tube would work, too, but add weight and might burn out the lower stuffer tube CR. I think my solution of simply breaking the rocket at a point considerably below the NC is probably better.

G