Ejection charges for large rockets

[DeeRoc29]

Folks with actual experience with large rockets: I'm preparing to ground test and am looking for suggestions on a starting point for grams of FFFFG to pressurize 90" of 10" ID tubing, pushing off a 40 pound upper section. This is a huge, 20 foot, 200+pound rocket that I'll have to lug over to the park to ground test, and I don't want to scare the locals more than I need to with multiple tests. Calculators are telling me 20-30grams but my gut says that is too much. I may start with half that. Thoughts from experienced folk appreciated.

 

[neil_w]

I don't want to scare the locals more than I need to with multiple tests.

I’m guessing once will be enough.

 

[kenstarr]

When I tested an 8" dia. by 30" long section, the noise was extremely minimal. I'd say start low. Maybe very low. Like 8g to start. Maybe less.

 

[Tractionengines]

How many and what type of shear pins? What's the tube made from? What are your charge holders? How tight are you packing BP?.... it all comes into play

 

[MClark]

I typically use 1 gram per 200 cubic inches.
35 grams

 

[fyrwrxz]

I tend to agree with Mark. Containment is primary. On my 7.5, I used some powdered graphite well burnished into the shoulders to reduce friction and used brass shim stock to back the shear pins to insure a clean cut. The reaction from the nayboors was interesting as I did it vertically and caught the nose coming down. I figured gravity woud be a factor in spite of no wind loading. The poor sod holding my wireless remote pyro initiator seemed to think it was gonna be a charge of dynamite. I used a 29/100 case open at the top with the match thru the nozzle and packed and taped dogbarf plug. Taped to the sidewall, pointed up. Worked pretty good for a brain damaged pyro. The bitch was repacking the main each time. I ended up just stuffing the mess back in as it wanted to bloom each time. I just wrapped the drogue in tape for testing after that. I'm a bear of little patience. I only used ~150g that day on the main as I stepped it up each time until happy. Not the brightest bulb inthe chandelier, but I didn't want it all banged up before the first flite. No pics, just a boring 3fnc. Maybe I'll bring it to ROCstock if I get my van fixed in time.

 

[fyrwrxz]

I’m guessing once will be enough.

Once is never enuff, isn't that the title of an old movie? Or maybe some old girlfriend said it. Never heard it after I got married, but that's just me.

 

[tjsnakez]

@UPscaler and I recently ground tested our 12” diameter 16’ tall rocket and ended up using 15g on the drouge and 18g for the main. 3’ long tube on the drouge side and 4’ long tube on the main side. Factoring in couplers into the volume, that’s 12”x18” on the drouge side and 12”x30” on the main side.

Is the 90” of tube open recovery area that is to be pressurized? If so, then 25g would be a good starting point.

 

[fyrechaser]

Just as a reference I use 6 grams of ffffg in 30" of 8" ID tube with a 8lb nose cone and 3 2-56 pins.

https://www.insanerocketry.com/blackpowder.html

 

[DeeRoc29]

I typically use 1 gram per 200 cubic inches.
35 grams

That's about what the calculators say, but I want to start smaller for the ground test.

 

[SMR]

Rules of thumb often give you a quantity of BP as a function of payload volume, but the effects of pressure vary greatly with the diameter of the rocket. What you want is the force that gives you on the nose cone or bulkhead If you figure the surface area of the bulkhead that moves (think of it like a piston), Pressure = Force / Area. So that area times the pressure gives you the force. You need to be able to sheer pins, fully eject the appropriate part with vigor, and push out any laundry. You'd be surprised how little BP you need with a large diameter rocket. For actual numbers, I only need 8 grams to blow off the 36 pound nose cone on my 10" Jayhawk.

 

[OverTheTop]

From Modern High Power Rocketry 2 the formula will calculate to 54g for that volume. That sounds like a lot! Given the large diameter the pressure is working on you could scale that back quite a bit. A few people upthread have mentioned 35g, and that is where I would probably start.

 

[kc9qzf]

F (lbf )·L(in) = m(g)·Rs·T (in · lbf /g) = m(g)·1937in · lbf /g.
If you use 250lbf, 90in you are lookin at 11.6g (oh my!)

 

[kc9qzf]

Here are the rest of the equations, I put them out a few years ago in a monograph, but it didn’t handle a monster airframe like this, but I think I will go try and dust off the old machine and see if I can make one!
Dan

BP4F Gas Properties: T = 3307◦R; R{specific} = Rs = R/M = 22.16ft · lbf /◦R; 454g/lbm Ideal Gas Law: P·V = m·Rs·T;A = π·D2/4; P = F/A; V = A·L; P·V = F·L
Rs·T = [22.16ft·lbf /lbm/◦R]·[3307◦R]· [1lbm/454g]·[12in/ft] = 1937in·lbf /g
P (lbf /in2)·V (in3) = F (lbf )·L(in) =
m(g)·Rs·T (in · lbf /g) = m(g)·1937in · lbf /g
P (lbf /in2) = F (lbf )/A(in2) =
F (lbf )·4/π/D2(in2)

 

[kc9qzf]

Here is a nomograph for this class of monster...
Dan