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Ejection charge pressure?
#1
Hi All:

   Finally getting a few minutes to get back to rocketry and building my LOC Bruiser.

   I'm going to try it as a single-deploy with a cable cutter. I'm in the midst of spec'ing out the ejection charge, and since it will be a 7.51x33" compartment, it will take a fair amount. The calculators I'm seeing online for smaller diameter rockets generally suggest around 15PSI.

   However, there's also some commentary about how large diameter rockets don't need that much pressure due to the much larger surface area, with suggestions ranging from 5PSI upwards.

   I'll obviously be ground testing, but since the 5-15PSI range for the amount of body tube to be pressurized includes the possibility of doubling the charge from 6 to nearly 12 grams of BP, I'm curious about what other people have used for charge sizes in large diameter rockets.

- Rich
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#2
This is a good website with an explanation and calculator:
  http://hararocketry.org/hara/resources/h...on-charge/

  5psi is ok without sheer pins, and with a fairly full compartment. But, I'd recommend two #4 sheer pins, which are about 40# each. Go for about 250 pounds on the bulkhead, which would be about 4.5 grams.  I have used 5 to 6 grams on an 8" rocket with four #4 sheer pins.  Be careful not to have the shoulder too tight, but it should not be loose.  Good luck!

  -John DeMar, L3
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#3
(05-02-2016, 09:12 PM)jdemar Wrote: This is a good website with an explanation and calculator:
  http://hararocketry.org/hara/resources/h...on-charge/

  5psi is ok without sheer pins, and with a fairly full compartment. But, I'd recommend two #4 sheer pins, which are about 40# each. Go for about 250 pounds on the bulkhead, which would be about 4.5 grams.  I have used 5 to 6 grams on an 8" rocket with four #4 sheer pins.  Be careful not to have the shoulder too tight, but it should not be loose.  Good luck!

  -John DeMar, L3

Thanks! Yep, from what I saw the 5PSI range should be suitable. Now I just need to get the final epoxying done so I can do a ground test.
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#4
When i ground test I usually start off with what the calculations say, then back it down a bit t o see what I can get away with on the ground.
Then I increase it, knowing there will be aerodynamic forces being applied during descent, to make sure I get that chute out.
Usually I wind up close to the initially calculated amount.
Greg
Greg Young - L3
NAR #42065
TRA #00234
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#5
A point of reference, my HoJo 7.51" x 16" section uses 4g of BP to give a healthy separation.  3g is marginal.

So 8 grams would seem to be a good estimate for your 33" section.
John Derimiggio
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