Aerotech DMS BP delay charge

[dhkaiser]

Per the 29-54mm DMS instructions...

"Use about ½ of the contents of the vial for 2.6” diameter
rockets and smaller, and the entire vial for 3” and
larger rockets. Note: additional ejection charge may
be required for rockets exceeding 4” in diameter."

These instructions seem a little vague since they do not consider the length of the rocket. For instance, I am considering flying my 2.6" DX3 SD with both BT's which will result in 40" of BT between the NC and forward motor mount centering ring. Do I use 1/2 vile of BP per instructions, or the entire vile considering the extra volume. Will the entire vile be enough?

How does one tell? I did fly my 3" Scorpion on a G80 29mm and it was fine. Doing the math V = pie r^2h and on the Scorpion V = 579.62. The DX3 with both BT's V = 849.49. Will the entire vile be enough?

 

[crossfire]

If it were me I would use it all. 1/2 for 2.6 and whole thing for 3" not a lot of difference in tube size. On my 4" rockets I add a little more. If you use long shock cords you will be fine.

 

[markkoelsch]

Use it all.

 

[fyrwrxz]

There's a formula for that. I use rule of thumb. Take your thumb-open vial-dump it all in. Easy-peasy!

 

[noffie79]

I think there's something to be said about three people saying to "use it all". Better to have too much than not enough. Use it all.

 

[dhkaiser]

Thanks guys. Yes I would use it all. The remaining question is, is it enough?

 

[kevinkal]

Per the 29-54mm DMS instructions...

"Use about ½ of the contents of the vial for 2.6” diameter
rockets and smaller, and the entire vial for 3” and
larger rockets. Note: additional ejection charge may
be required for rockets exceeding 4” in diameter."

These instructions seem a little vague since they do not consider the length of the rocket. For instance, I am considering flying my 2.6" DX3 SD with both BT's which will result in 40" of BT between the NC and forward motor mount centering ring. Do I use 1/2 vile of BP per instructions, or the entire vile considering the extra volume. Will the entire vile be enough?

How does one tell? I did fly my 3" Scorpion on a G80 29mm and it was fine. Doing the math V = pie r^2h and on the Scorpion V = 579.62. The DX3 with both BT's V = 849.49. Will the entire vile be enough?

Thanks guys. Yes I would use it all. The remaining question is, is it enough?

I suppose using the DMS motor makes it hard to do a ground test. Perhaps you could use a spent DMS motor, just load the charge well and run an igniter up the motor, fully retained in the motor tube.

It's good that you're considering the increased volume in your DX3. Roughly, for the same charge, the pressure will be proportionally less.
At the minimum, take a careful look at a Black Power charge calculator such as https://www.rimworld.com/nassarocketry/tools/chargecalc/index.html.

I just double checked your numbers, and I think you might be using the tube diameter, 2.6", in your calculations instead of the radius, 1.3". I got numbers that were 4 times smaller.

How is your rocket held together? Friction/tape? Shear pins?

I've noticed that the BP charge that has come with all my 29m, 38mm and 54mm RMS loads is all about the same charge.. 1.2 to 1.4g. I've had to add powder to these charges in my 4" rocket to have the required amount to consistently shear four 2-56 nylon screws/pins.

If you're using tape/friction beware of temperature and humidity related deformations that can make the rocket fit change drastically when in the hot sun on the launch pad. Two weeks ago, I had to remove a full layer of tape after the rocket heated up on the pad and failed my last second "friction" checks just before walking away from the pad. It was so tight, I don't know if the BP charge would have worked at all. It was good that I'd double checked.

 

[dhkaiser]

Thanks Kevin, yes I was using diameter rather than radius, Oops. And good point about fit, I am using friction with a little tape.

So my 2.6" DX3 would be V = 3.14*1.3^2*40 = 212.
My 3" Scorpion V = 3.14*2^2*24 = 301

These calculation suggest the full vile would be plenty for the DX3. However your suggested link states that smaller diameter airframes require higher pressures. For my 40" long BT and 11 psi DX3 their calculator gives a return of 1.2 g, that which is supplied with the DMS motors.

So now the question is, is 11psi enough? Your suggestion of a ground test with a spent DMS motor seems reasonable.

 

[noffie79]

I don't know how much powder is in a CTI 29mm F36, but we launched an Estes Partizon that has ~39" of BT to pressurize and everything ejected fine. I'm sure if you use all of it, you'll be just fine.

 

[jnelson]

Just for reference- AT's clear BP vials contain 1.4g of powder.