Help sizing BP charges for a BT-60 tube

[R3verb]

Hey Everyone,

I'm working on an Estes Star Orbiter that I've heavily modified to include DD. This is my first experience with DD and I'm having some issues with getting the charges to pop the drogue out. I did some pop tests today with mixed results. I'm using the Christmas lights ejection charge method (https://user.xmission.com/~jry/rocketry/mini-bulb-mortar/mini-bulb-pyros.html). The main worked perfectly with .5 grams of black powder. Nosecone popped off, drug the chute out with it, perfect test. I then tested the drogue, no dice. Removed the 1 sheer pin, still no dice. Doubled the black powder charge to 1 gram, still no dice. Looking at the 2 ends, it seems like the black powder on the drogue isn't burning all the way since there's a LOT of what looks like unburnt black powder. I'm also curious if not having a motor in the motor mount is causing an issue (basically a giant 29mm hole at the bottom of the tube). Any ideas? I don't have any e-matches so going with the glove tip method is out of the question for this weekend short of finding somebody locally with a few I can buy.

I'm happy to go bigger, I'm a little worried about blowing up the tube though. At what point do I need to be worried about trashing the tube?

For reference, the tube is a BT-60 (1.3") at 18" long. I've used 1" and 2" of tubing which translates roughly to .5 and 1 grams of BP.

Thanks!

 

[Handeman]

Yes, not having a motor in the MMT is huge issue. You can't pressurize the tube with that huge hole in the end. Also, it is not duplicating flight conditions. When you fly, there will be a motor in the rocket. You must put something in the MMT to seal that or your deployment tests are useless.

 

[R3verb]

Yes, not having a motor in the MMT is huge issue. You can't pressurize the tube with that huge hole in the end. Also, it is not duplicating flight conditions. When you fly, there will be a motor in the rocket. You must put something in the MMT to seal that or your deployment tests are useless.

I figured this was an issue. My question would be this, what should I put in there? The flight plan would be an estes F15-6. My sim puts the ejection charge happening JUST after apogee:



Optimum delay at 5.74s. So ideally, the ejection charge is still in the motor when the apogee charge fires. In that scenario, I'd probably just 3d print a 29mm plug and try that but, let's assume the motor ejection fires at 5.5s, then you're looking at spent motor in there. Would I then want to print a plug with a hole the size of the grain? A hole the size of the nozzle opening? Or is the assumption if the motor ejection charge fires before the DD ejection charge that the tube is going to pop open anyways?

Thanks for all the help on this!

 

[Handeman]

You are way over-thinking this. Just plug the motor hole and do your test and figure out how much you need. Don't go down rabbit holes!

 

[lakeroadster]

I figured this was an issue. My question would be this, what should I put in there?

A spent 29mm motor casing would do the trick... or a piece of wooden dowel.​

 

[SolarYellow]

29mm motors are 1-1/8 inch in diameter.

 

[R3verb]

Thanks for all the help everyone! I 3d printed a motor plug, tested with 1 gram and got a good separation. Time to fly it!

If you want to follow the build thread, it's here: https://www.rocketryforum.com/threa...-estes-9716-star-orbiter.181106/#post-2549289

Thanks everyone!