Ejection charge size

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pyrovette20

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I seem to remember there being a formula somewhere but cant find it now. The basics 5" dia. x 24'' approx. without recovery. Fiberglass airframe so it can take a little extra. Im guessing 2 1/2 - 3 grams of 4F
 

MarkM

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I seem to remember there being a formula somewhere but cant find it now. The basics 5" dia. x 24'' approx. without recovery. Fiberglass airframe so it can take a little extra. Im guessing 2 1/2 - 3 grams of 4F
I always ground test with recovery in place and connected as it will fly. It best simulates your flying conditions and provides an easy way to determine if that charge size will sufficiently pop out your chute.

The issue is also not what the airframe can take in terms of charge, but how that charge will "jar" the rocket and how forcefully it will deploy your recovery gear. You don't want it to be too forceful or it could jerk the rocket too much. And you want to be sure for the apogee the force isn't too much to shake the nose cone loose (even with sheer pinning); I've overloaded my apogee charge once or twice and even with a sheer pinned nose cone, the nose cone shook loose and the main came out far too early.

I use 4g 4F for my 6" x ~36" airframe.
 

blackjack2564

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I seem to remember there being a formula somewhere but cant find it now. The basics 5" dia. x 24'' approx. without recovery. Fiberglass airframe so it can take a little extra. Im guessing 2 1/2 - 3 grams of 4F

That's what I'd start with exactly. Just make sure you have nice long shock cords.
 

pyrovette20

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Thanks for the replys Ive had then on my computer. But my computer went down for about a month and lost a few things along the way. Im using 1'' nylon strap with nomex cord protecters and nomex heat shields. The rocket is a PR 5'' competitor w/18'' extension on the booster. Going up this weekend on a KBA M-3500.
 

stickershock23

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those are some really useful websites, one says webpage not found, one just sits and spins and never loads, and VCT I downloaded the file, and tried to extract it from the zip and it said "no file to extract" someone should check these site before they recommend them. :eek:
 

brianc

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those are some really useful websites, one says webpage not found, one just sits and spins and never loads, and VCT I downloaded the file, and tried to extract it from the zip and it said "no file to extract" someone should check these site before they recommend them. :eek:
Looks like Vern Knowles site recently moved to .NET instead of com...

https://www.vernk.net/EjectionChargeSizing.htm EDIT: the .COM address is working again...

I'll suggest PEBKAC on the VCT - works for me. ;)
 
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stickershock23

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thanks for the upadates guys. much better :D

that "Info" site is awesome.
 
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stickershock23

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Well, I just did a ground test. 2 #4 nylon schear pins, 3 grams ffffg it was perfect.

Blew the nose cone straight up just barely fully extended my shock cord at apogee of the cone!

I think its ready to fly, now I need the weather to hold!!
 

quickburst

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I seem to remember there being a formula somewhere but cant find it now. The basics 5" dia. x 24'' approx. without recovery. Fiberglass airframe so it can take a little extra. Im guessing 2 1/2 - 3 grams of 4F
My calculator says 2.43 grams. I usually start with what the calc says, ten go up from there.
 

cwbullet

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Guys,

I am looking for a reference for larger rockets and ejection charges. I have been reading a lot. From what I am reading, 15 psi is too large for a 10 inch rocket. If you are planning a 14 or 28 inch long compartment that is 10 inch in diameter, what do you use to calulate the powder size.

I used the infocentral calculator and it says 12 grams and 22 2-56 shear pins.

Are there any references you use for large rockets?
 

Pantherjon

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12grams??22 shear pins?? I would think that 10grams and 4 shear pins would do..Just a SWAG, but ground testing is your friend for sizes such as that...

Who really wants to put in 22 shear pins???!!!!
 

troj

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I am looking for a reference for larger rockets and ejection charges. I have been reading a lot. From what I am reading, 15 psi is too large for a 10 inch rocket. If you are planning a 14 or 28 inch long compartment that is 10 inch in diameter, what do you use to calulate the powder size.

I used the infocentral calculator and it says 12 grams and 22 2-56 shear pins.
Yikes! Both numbers seem big, to me.

I'd use three shear pins, and start ground testing with 4 or 5 grams.

-Kevin
 

jadebox

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I am looking for a reference for larger rockets and ejection charges.
Larger rockets don't need as great a difference in pressure because of the larger surface area on the bottom of the nosecone or bulkhead that is being pushed out. I think all the calculators take that into account.

https://archive.rocketreviews.com/tool_black_powder.shtml

The important thing is to ground test starting with the amount recommended by the calculator and increase it in the next test if necessary.

Most of my tests have worked fine with the size estimated by the calculator, so I don't think getting the amount of ejection charge exactly right is critical.

-- Roger
 
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