Ejection Charge Re-Visited

[MikeSModerator]

My 8″ rocket is going to be duel deploy.

The main is in a 48″ portion of the air frame. (48″ X 7.5″)

I mount My charge on the top of My electronics bay.

I have not figured My charge size yet, But I know it will
be large compaired to what I have used in the past.

Question:
Should I use two charge holders?
(Thinking…Two smaller charges would give the same pressure, but in a smaller ‘BANG’).. Or am I over thinking it?

Charge sizing: In the past I have used a much smaller charge
that math says to use. Most I have talked to and most on the
forum agree with that.

What is the discussion on a charge size for a 7.5″ frame?

Remember some of My charges sounded like small cannons
when the main came out. Most of the time I lean to the side
of (Make Damn Sure)

Thanks Mike

[Jeffrey Joe HintonModerator]

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.

[MikeSModerator]

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.

[new2hprParticipant]

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)

[Kenneth ReillyParticipant]

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.

[John A. WilkeParticipant]

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.

[MikeSModerator]

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.

[Bruce R. Schaefer]

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!

[greywolves]

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. 😀

[Warren B. MusselmanModerator]

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))

[Author]

Posts