Charge calculations should be based on the amount of airframe volume you need to over pressurise to break the shear pins and push the laundry out. Ground testing is the best way to actually determine the size of charge you'll need to definately acheive deployment and I prefer one big charge over two smaller ones but that depends on the electronics being used and if using two, make the second one bigger because if the first one didn't pop the chute out, the backup one had better.

I know this may be kicking a dead horse..

I have always seemed to put large charges in.

I have alway used Verns Charge Sizing. He suggestes
on an 7.5" rocket using 1gram per 6" of airframe.
(airframe to be preasured)

Now. When caluculating I always measure my airframe
from the top of my EB to the bottom of my nose cone and
add.

Question. Should that measurement be from the top of my
EB to the bottom of my chute and shock cords? ....

This my be why mine always start out soooo large.

Last year my 7.5" rocket blew at apogee just fine but was too
strong and popped the nose cone off also. I will be dropping
the charges 1 to 1.5 grams. and have a 25' shock cords. last
years shock cords were 15'

I will be ground testing, but I have often wondered why
my starting point was so high.

No experience with anything that big, but I'd recommend ground testing starting with about half your calculated charge and work up. BP is cheap, but the rocket either not coming apart, or coming apart in too many pieces is very expensive. I'd have to think that large laundry should count against the total volume in the calculation though.

-Ken

p.s. my largest charge has only been 1.5g in my 4" rocket (main w/3 shear pins)

Mike,

The only thing I would have to add is that when you ground test, do it with the chutes and harnesses packed exactly as you would if you were flying it. Eliminate as many variables as possible.

Ken, you are correct, though I'd add one caveat - go a bit heavier on your charges than you might otherwise think on your ground tests, because if you DON'T separate, you might cook your chute 😯

Alternatively, I like to use a placebo for my ground test ejection charges - an old towel or something that occupies the same space as your chute.

Those of you whom I've had the honor to TAP know that I pretty much demand / expect / want ground testing. It is a simple thing that can save a lot of heartache. I got ahead of myself and wrecked a GORGEOUS rocket once... that bird was named "MayheM" (and it was flown long before the original MHM). It core sampled on a Kosdon K777 at Hartsel. I ground tested, but w/o the laundry. To this day I am convinced that my ground tests w/o the chute were not the equivalent of the flight WITH the chute.

Yes...Last years ground testing burned a hole in my chute. I was glad I did
not have the shock cord hooked to the rocket. It would have ran away.

Also this year I will have redundant charges. I built it for that but
last year did not use. Can not remember why. I will have a slightly
larger charge for #2. I am sure every one will hear it.

I have an old chute that I fried DOING a ground test. But that's good because I can simulate different chute sizes and weights with it now. Has worked successfully for numerous rockets, including the one in the picture to the left. I just add a towel or two, as JW said, to make it larger, as needed. It MUST be as exact as the real flight as possible. Look at the chute you're going to use: pack it, weigh it, measure it, and then prepare something out of the same, or similar, material to use instead of an expensive chute. After enough time, you'll know when the charge is right. I agree with JW, less is not more. As long as you're using fiberglass tubing (PR convoluted), you're pretty safe with larger charges. Just pack it with a Kevlar pad big enough so it won't get singed, or worse. BTW, calculations only give you a general idea. Ground testing is the only way to know for sure. Also, remember to make the secondary charge bigger than the primary. On my L3, even though Art and I ground tested it out at the North site—and all looked fine, the primary did NOT "chute" out the laundry. A larger secondary charge did, however, and saved my L3. I used Jim Amos’s WRC II as the secondary electronics. After the primary puff and no separation, then the secondary success, JW turned to Jim and said “Your WRC II just paid for itself.” A certification and a year’s worth of hard work literally hanging in the balance. BTW, I used a 60' and 40' set of 1/2" Kevlar cords on the rocket to the left. That takes up some of the extra charge, and remember to tape up the cords (reef) at intervals, that also absorbs some shock of a good and safe charge. Man, Mike, you make great rockets!

Photo by Ray LaPanse
I was looking at Rays great pictures, found this awesome looking rocket. Mike, in the picture, instead of behind the camera. 😀

I use the Art Hoag Method - I use the calculator to determine how much powder needed and then double it - at least.

((This used to be known as the Wilke method))

Concerning the charges do not seperate your charges to two canisters. Atleast dont split up the calculated charge in half. If you need 4 grams for your main, dont put two grams in two canisters. Put 4 grams in each. What I do, which is usaully using two different electronics is put two canisters one for each electronics at the calculated size. So if you need 4 grams I'd put two canisters of 4 grams. The chances they both go off at the same time are VERY unlikely. I think for the Uproar project we argued over too do two canisters or one, its personal preference.

With redundant charges and altimeters, you can be sure that they will NOT fire simultaneously - especially if you use different altimeter brand/models (smart idea by the way). As I said, I use a calculator based on internal volume and # and size of shear pins to determine my charge size and than I effectively double that amount (Note, I only use composite tubing, with paper or phenolic you might want to back that off substantially).

If I know that one of my altimeters will always fire later than the other (I do - and plan on it), the second charge I make even 20% larger just in case the first charge fails to push things all the way out. If you properly deploy on the first charge, it won't hurt because you'll have the open end of the airframe to dissipate the extra force and if you haven't fully deployed, it might be just what you need to fully kick things out - especially if your coupler/airframe joint is a bit loose and jams a bit.

Again, I don't build anything with paper of phenolic so be aware that you could just blow your rocket up or at least blow a nice hole in the side.

Most of all - ground test if you're nervous about it being a successful deployment.

By the way, my L3 was not ground tested. I was just extremely anal about getting a smooth coupler/airframe joint that was held together not by friction, but by shear pins. I calculated the correct amount of BP and essentially (on Art Hoag's advice) doubled the charge size. A perfect deployment.