Dual Deployment with Aerotech DMS motors

[HenryC]

I have several Aerotech DMS motors (H, I and J) that I would like to use on my dual deployment rockets. For the rockets where the optimum delay is less than or equal to the maximum 14 second delay in those motors, I was going to drill them to be a little longer than the optimum delay so they would act as a back up for the drogue chute. I have one rocket that has an optimum delay of 18 seconds for the J270. Would it be OK to just not put any BP in the ejection charge area of the motor, thus solely relying on the drogue chute charge from the altimeter? I have never flown a DMS motor without using the ejection charge so I want to be sure there are no unintended actions.

 

[tim cubbedge]

If the delay is too short, I do just as you suggest, do not put any BP in forward closure. Just make sure you put the red plug in or some dog barf & masking tape. I do this on RMS or DMS motors.

 

[cerving]

If the delay is too short, I do just as you suggest, do not put any BP in forward closure. Just make sure you put the red plug in or some dog barf & masking tape. I do this on RMS or DMS motors.

^^ This ^^
I rarely use motor ejection... not surprising, right?

 

[HenryC]

If the delay is too short, I do just as you suggest, do not put any BP in forward closure. Just make sure you put the red plug in or some dog barf & masking tape. I do this on RMS or DMS motors.

Thanks Tim. Hope to see you at the SRA launch in a couple of weeks. I had to skip the July launch due to a high fever (did not want to risk spreading COVID if that's what is was, test result came back negative).

 

[Handeman]

If the delay is too short, I do just as you suggest, do not put any BP in forward closure. Just make sure you put the red plug in or some dog barf & masking tape. I do this on RMS or DMS motors.

I'm curious, why the red cap and/or dog barf and masking tape?

I never put the the powder in the ejection well of the RMS motors I use when I fly DD and I never put anything over the wells. I've been doing this since 2007 and I've never noticed anything bad about just leaving the well open.

My biggest issue is remembering to drill delays and use powder when assembling the motors the few times I do fly motor eject.

 

[tim cubbedge]

I'm curious, why the red cap and/or dog barf and masking tape?

Mostly a precaution in the rare case that the smoke/delay grain burns through touche hole. Was told by both CTI & AT dealers. I’ve seen people go as far as put epoxy in CTI motors when pulling B.P.

 

[Handeman]

Mostly a precaution in the rare case that the smoke/delay grain burns through touche hole. Was told by both CTI & AT dealers. I’ve seen people go as far as put epoxy in CTI motors when pulling B.P.

Looking at the delay/smoke grain when I clean my cases, I know the smoke/delay grain burns through the touche hole on everyone one of my DD flights. That's never been an issue.

To me, plugging that hole sounds like a solution to a problem that doesn't exist. It's like some of the other "best practices" that fall into that category, like putting swivels on parachutes. That started with the LPR guys that used fishing swivels on chutes, not because they needed the swivel, but because the snap hook made it easy to swap chutes. It just evolved into a "best practice" to put swivels on parachutes.

 

[Dan Griffing]

I have several Aerotech DMS motors (H, I and J) that I would like to use on my dual deployment rockets. For the rockets where the optimum delay is less than or equal to the maximum 14 second delay in those motors, I was going to drill them to be a little longer than the optimum delay so they would act as a back up for the drogue chute. I have one rocket that has an optimum delay of 18 seconds for the J270. Would it be OK to just not put any BP in the ejection charge area of the motor, thus solely relying on the drogue chute charge from the altimeter? I have never flown a DMS motor without using the ejection charge so I want to be sure there are no unintended actions.

I used a J270W with redundant dual deployment on my Zephyr using the 6–channel Eggfinder Proton. The motor ejection was a third level of redundancy, and it went off three seconds after apogee.

There’s nothing wrong with added levels of redundancy, especially when too little redundancy means having a destroyed rocket and risk to anything on the ground that it hits.

I loved the J270W because its 2.6 second burn gives an impressive launch to watch, an its moderate acceleration doesn’t stress the airframe much or compact the chute, shock-cords, or other packing as much as the higher thrust motors.

 

[Dan Griffing]

^^ This ^^
I rarely use motor ejection... not surprising, right?

I used a DMS J270W with a dual deployment-configured Apogee Zephyr for my L2 in June with an Eggtimer Proton with redundant drogue and main charges. The loaded weight was 8.25lbs. The Jolly Logic Altimeter3 max altitude was 3032 feet. drogue descent was 47fps, the main deployed and 800 feet for a gentle landing at 13fps under a 48” Fruity Chutes toroidal Iris less than 200 yards from the launch as determined by the Eggtimer GPS. It was a real Full Monty.

 

[Michael Wilson]

Might have been said before, probably a good idea to have redundancy especially with out a back up from the motor.