Ground test

Steven88 · Mar 19, 2019

Good evening fellows, tonight I did my first ground test with 100% success!! However, I have a couple questions. I used an ejection charge calculator to calculate how much black powder I would need and backed it off 15% for starting out. The main deploy seemed maybe a little weak? It has about a 15 ft shock cord and didn’t quite come to the end of it but ejected the chute and chute protector. The drogue section has about a 21ft shock cord and it almost used the full length of it up so I thought maybe that was good since it is longer. Both sections were pinned with three 2-56 nylon sheer pins. Is that enough black powder being used or should I be sure and use enough that my shock cord becomes taut upon ejection? Thanks in advice for the advice.

3stoogesrocketry · Mar 19, 2019

Steven88 said:
Good evening fellows, tonight I did my first ground test with 100% success!! However, I have a couple questions. I used an ejection charge calculator to calculate how much black powder I would need and backed it off 15% for starting out. The main deploy seemed maybe a little weak? It has about a 15 ft shock cord and didn’t quite come to the end of it but ejected the chute and chute protector. The drogue section has about a 21ft shock cord and it almost used the full length of it up so I thought maybe that was good since it is longer. Both sections were pinned with three 2-56 nylon sheer pins. Is that enough black powder being used or should I be sure and use enough that my shock cord becomes taut upon ejection? Thanks in advice for the advice.

What are your thoughts? Obviously you used a calc. to determine the right amount of BP. You pinned it together , you got basicly full extension on both sides. Good job

Nytrunner · Mar 20, 2019

Sounds like a success to me.

Any more ejection energy beyond ejecting the chute means more energy your harness has to absorb. Try another test using the full calculated recommended amount and see how that works out (it'll be more energetic!)

If you're happy with the chute ejection on the lower amount, go with it. If you're running redundant backup, make the full amount your backup charge.

Steven88 · Mar 20, 2019

I think the drogue section has sufficient black powder! For some reason the drogue section created a whole lot more fire and noise than the top section and launched the top of rocket several feet farther so I am thinking of added a little black powder to the top section

Steven88 · Mar 20, 2019

Good advice. Can’t wait until the week end to try it out

Bat-mite · Mar 20, 2019

My 2 cents. You'll get more separation in the air, and you don't want to stress your harness (the term "shock" cord is a misnomer because you don't really want to use it as a shock absorber), so IMO ground tests should not do more than get the chute out of the tube. If the chute comes out, gravity will take care of everything else.

cbrarick · Mar 20, 2019

I'd actually use more and go against what Bat-mite said. The rocket traveling thru the air has one force acting on it that we can't simulate on the ground - air pressure holding it together as it provides a downward pressure on the cone. I aim for separation "with authority." A little extra powder just wakes the sleeping people up, not enough ruins the rocket. You pick.

disclaimer" note I said a little extra, not a lot I'm guessing the bp calculator is smarter then me :>

T-Rex · Mar 21, 2019

A little more doesn't hurt. I accidentally added a lot more once, it tore the upper centering ring out of my Patriot..... I am of the crowd that believes that if the laundry comes out, gravity will do the rest, no sense adding unnecessary stress to the parts. If it barely comes out, well you will want to add a little more and try again.

Bat-mite · Mar 21, 2019

Whatever floats your boat. I always end up removing a little after my shakedown flight when I see the harness tighten.

grouch · Mar 21, 2019

A little too much is better than a little too little.

Handeman · Mar 28, 2019

I agree with Bat-mite. Especially for the apogee charge, you don't need full separation of the two halves, only enough to get the drogue in the air stream. If you use too much, then you get into the cycle of heavier cords, larger eye bolts, etc. which leads to heavier rockets and larger charges to move them apart which needs heavier cords, larger eye bolts, etc. I think the "blow it out or blow it up" is the worst method you can use. I look for the minimum charge to consistently separate the rocket.

Nathan · Mar 28, 2019

I like redundant deployment charges. For the primary charge I use the amount of powder determined from ground testing then for the backup charge I use about 20% more.

Banzai88 · Mar 29, 2019

Handeman said:
I agree with Bat-mite. Especially for the apogee charge, you don't need full separation of the two halves, only enough to get the drogue in the air stream. If you use too much, then you get into the cycle of heavier cords, larger eye bolts, etc. which leads to heavier rockets and larger charges to move them apart which needs heavier cords, larger eye bolts, etc. I think the "blow it out or blow it up" is the worst method you can use. I look for the minimum charge to consistently separate the rocket.

That's the advice that you gave me a while ago.....and since I started following it I've not had a single issue.

58pan · Mar 29, 2019

Can also add a little "shock adsorbing" to your shock cord.
i usually do a bunch of these little ties. If a ground test opens all but 1 or 2 I figure I'm good.
If you try this both free ends should exit in the same direction.

Bat-mite · Mar 29, 2019

58pan said:
Can also add a little "shock adsorbing" to your shock cord.
i usually do a bunch of these little ties. If a ground test opens all but 1 or 2 I figure I'm good.
If you try this both free ends should exit in the same direction.

Rubber hair bands work as well. I don't like gumming up my harnesses with tape.

YodaMcFly · Mar 31, 2019

Bat-mite said:
Rubber hair bands work as well. I don't like gumming up my harnesses with tape.

I use blue painter's tape, and either z-fold the cords, or roll into circles, depending on width.

Everything stays neat, but the painter's tape leaves less residue than the ejection charges, and I never have to worry about the tape failing to come off cleanly and foul the cord.

vcp · Mar 31, 2019

FWIW, I made an EC calculator that did a simple model of the separation velocity of the body/NC. One of the things it made clear was that a ground test only approximately modeled the free-body separation. Close enough though, and my model itself had lots of approximations. Everybody adds 'a little bit more' anyway.

vcp · Mar 31, 2019

Oh, the other interesting thing that calculator did was to show how dependent separation velocity is on shoulder length (i.e., a lot), but it's normally not noticed because the proportion of shoulder length/diameter/volume is usually about the same for most models.

That calculator was written in VB6 and doesn't run on modern windows. But I've been looking for a project to try in Python, so I might give that a try.

Ground test

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