Shorting BP in Aerotech DMS for Piston

[DrewW]

Attempting my L1 cert in a couple weekends and I'm trying to square up the last few details.

I'll be launching a PML AMRAAM3 which uses a piston ejection system. In an effort to minimize opportunities for screwups I've also opted for an Aerotech DMS motor to fly on. The instructions that come with the piston system explain that significantly less BP is needed for a piston system to effect the needed separation so I'm curious if anyone has experience on shorting the BP for the Aerotech DMS motors, or any relevant advice on PML piston systems, or commentary on whether or not I should just use a full charge for the L1 cert flight.

 

[heada]

I did my L1 on the 4inch version of the PML AMRAAM. I used 1 gram of BP and it worked great. For your 3", between 0.75gram and 1 gram should work.

 

[jmmome]

To be safe, ground test a smaller BP amount if you have another method of igniting the BP. "Old School" is using a single Christmas mini light with the glass broken to expose the filament, and a shrink tube heated and shrunk around the light holder. BP inside the opening of the tubing, with the end sealed up with wadding and tape. The link below describes it in detail:

https://www.perfectflite.com/Downloads/Ejection.pdf
I always use pistons on L1 through L3 rockets, and always use the maximum recommended BP for the tube diameter and the length of tube. As long as your shock cord is strong and securely fastened to the piston and the rocket, you'll get a big BOOM, but 100% deployment.

My two cents.

 

[HyperSonic]

I use a piston in my 4 inch airframe about 12 inches long for both drogue and main. I use .8 grams for primary and 1 gram for backup.

 

[UhClem]

I haven't used any DMS motors but I always decreased the BP charge provided with the Aerotech reloads when using a PML piston. Including my L1 certification flight on a Quaser. (H180). I never measured but just eyeballed the charge.

These modern quantum tubes could be different since the fit of the piston will vary with temperature.

 

[afadeev]

I'll be launching a PML AMRAAM3 which uses a piston ejection system. In an effort to minimize opportunities for screwups I've also opted for an Aerotech DMS motor to fly on. The instructions that come with the piston system explain that significantly less BP is needed for a piston system to effect the needed separation so I'm curious if anyone has experience on shorting the BP for the Aerotech DMS motors, or any relevant advice on PML piston systems, or commentary on whether or not I should just use a full charge for the L1 cert flight.

Unless you are prepared to empirically test and determine the smallest amount of BP that is sufficient to eject the nose cone and the chute, I would not mess with the stock DMS ejection charge.

I flew my L1 cert bird (PML Bumble Bee) with stock DMS ejection charge (forgot what H-DMS motor I used), and it worked just fine.

HTH,
a

P.S.: one thing to 3x check with piston is that they still slide smoothly when you get to the launch site on the launch day. Temperature and humidity variation scan have unpredictable effects on the piston system tolerances, so validate that everything is moving smoothly before the flight!

 

[DrewW]

Unless you are prepared to empirically test and determine the smallest amount of BP that is sufficient to eject the nose cone and the chute, I would not mess with the stock DMS ejection charge.

I flew my L1 cert bird (PML Bumble Bee) with stock DMS ejection charge (forgot what H-DMS motor I used), and it worked just fine.

HTH,
a

P.S.: one thing to 3x check with piston is that they still slide smoothly when you get to the launch site on the launch day. Temperature and humidity variation scan have unpredictable effects on the piston system tolerances, so validate that everything is moving smoothly before the flight!

@afadeev thanks for the comments especially about checking that the piston moves freely on cert day. I'll add that to my checklist. I have an Aerotech H283 I'll be using for my attempt, the instructions for the piston system call out grams of BP given the tube size, empirical calculations having been performed. I think I'm settling on using the full BP load delivered with the motor just so I don't have to worry about measuring and prepping at the site.

 

[afadeev]

I have an Aerotech H283 I'll be using for my attempt, the instructions for the piston system call out grams of BP given the tube size, empirical calculations having been performed.

Those calculations are nice, but unless they were performed for exactly your location's altitude and your choice of BP (3F? 4F? etc), they should only be taken as guidance, or a starting point for own first-hand experiments.
That's why you always test and validate ejection charge amounts for HP rockets on your own rockets, in your own back-yard!

I think I'm settling on using the full BP load delivered with the motor just so I don't have to worry about measuring and prepping at the site.

That's a reasonable approach.
Weighting small amounts of BP at a windy launch site is probably not going to work out too well.
Test in advance. Prep and measure the desired amounts of BP at home. Package and seal them, build the motor, seal it in a zip lock bag.

Then arrive at the launch site, put the motor into the rocket, rocket onto the rail, insert the ignitor into the motor last, and off you go!

a

 

[HyperSonic]

P.S.: one thing to 3x check with piston is that they still slide smoothly when you get to the launch site on the launch day. Temperature and humidity variation scan have unpredictable effects on the piston system tolerances, so validate that everything is moving smoothly before the flight!

What I did to try and avoid humidity and temperature issues was to seal the inside and outside of my phenolic piston with thin epoxy, then sand smooth for a good fit in your airframe. The thin layer of epoxy prevents humidity from entering and causing problems. The inside of your airframe is not sealed so you still need to pay attention to the fit. I use baby powder for a lubricant.