Charges — What’s Everyone’s Experience

[Steve JensenParticipant]

Languishing from Oktoberfest withdrawal (rainless…), I’ve been ground testing my new 4 inch diameter rocket.

It’s a 4 inch diameter fiberglass rocket.

Particulars are

Aft drogue charge — 4 inch tube with 24 inches of space and 3 x 4-40 Nylon shear pins. No Drogue. I’ve found 2.0 grams BP in the surgical tubing format holder seem to work.

In the fore section with a rather large chute I have about 12 inches stuffed with the chute in front of a 5 inch piston (fiberglass too) that has some space under it. It is set up with two 4-40 Nylon Shear Pins. I used 1.1 grams successfully.

I keep hearing of altitude vs. ground testing variances, so any feed back would be appreciated. Of note this is a large heavy rocket 100 inches height and 192 ounces with only a 54mm MT. With an L motor, about 10K or so. With a mid size K, just under 6000. Should be fun. Just low and slow.

[new2hprParticipant]

I’ve got a similar size (though much lower and slower than yours). 4″ LOC paper tube (based on Fantom EXL).

Drogue:
24″ drogue
about 12″ space (very loosely packed)
no shear pins
1.1g charge

Main:
78″ main
about 10″ space
3x 2-56 nylon shear pins
1.5g charge

Both charges extend the 15-20′ kevlar to full length with authority. Both charge holders are 1/2″ pvc caps attached to the bulkheads of the ebay (yes, that means the drogue charge is above the chute, but it has worked 100% with the loose packing).

With 4 38mm motors, I haven’t even broken 3k yet!

-Ken

[Warren B. MusselmanModerator]

I tend towards beefy charges… in my similar 4″ bird I use 1.5g for the drogue (4″x12″) and 2 or 2.5g for the main (4″ x 24″). With composite birds, I rarely worry about overpowering things so long as the bearing surfaces are clean and smooth.

Of all people, Art Hoag told me that he never ground tests, he overbuilds – a philosophy I’ve long followed. Just compute the amount of BP your volume requires and make your charges at least 1.5 times larger. I personally go double. My successful L3 cert was done that way.

W

[Chris LaPanse]

With that particular rocket, I would personally use 1.5-2g, and I would tend to ground test. It will almost definitely separate with less, but I like to be sure. On a paper or phenolic rocket, I would be a little more conservative, since an excessive charge is more likely to damage the tube, but with glass, I go fairly heavy.

[Bruce R. Schaefer]

The only tube I ever blew up in ground testing was a 3″ phenolic tube using a little over 2 grams, which is why I converted to convolute fiberglass soley. I’ve yet to blow that up. With the PR fiberglass, you’re pretty safe regardles of charge. There is a fine line between not having enough charge to separate and having too much. Pack the chute well and protected and blow that sucker apart. If your chute shreds, at least you’ve broken the stability of the rocket, and it will land much safer than if it DOESN’T shoot the laundry out. Make sense?

[Steve JensenParticipant]

Flat spins w/o chute are repairable. I’ve even had one rather large rocket remain undamaged from this.

Ballistic recoveries usually involve a trash bag…

[Warren B. MusselmanModerator]

My 4″ bird with carbon/glass on a phenolic tube and 3 layers of tip to tip has impacted from over 8K via a flat spin and flown multiple times since then.

[Ed Dawson]

I know there is lots of response that seems to favor “blow it up, or blow it apart”, but I tend to be a bit cautious of over pressurising when using pistons. I have had a couple failures that I attribute to beefing up a charge too much when using a piston. I think if the charge is too strong then the piston just expands and binds in the BT. This seems to be particularly prone to phenolic.

[Jeffrey Joe HintonModerator]

Most powder calculators are not calibrated for our launch platform altitudes so an extra little helps assure deployment. In my humble opinion, it is still best or wisest to GROUND TEST flight configurations as much as possible for deployment with authority. When the rocket is still vertically travelling upward, in coast or at motor burn out, there are extra forces at work keeping the nose on. Too big of a charge for drogue deployment may also cause main deployment as the shock or force of the charge may not be properly directed internally. Bulk heads might take on the burnt of the charge force but that push travels the length of the bird.

[Bruce R. Schaefer]

And, I agree with both of the previous posts. I AM NOT a piston guy, Warren is and JW is; we have all had different experiences. There are SO MANY VARIABLES at work. Do, as JW says, “Dance with the girl who brought ya.” In other words, whatever has worked for you in the past, will for you in the future. I made all my mistakes, embarrassingly, achieving my L1. My successes are based on the minds and experiences and friendships of this club. Trust us, but go on calculations and instincts, my friend.

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