The metal washer that goes into DMS ejection charge well?

[amiliv]

There's this tiny metal washer that's supposed to go into the bottom of ejection charge well of AeroTech DMS motors. What's the purpose of that washer? To my untrained eye, it doesn't do much (or anything?). It's free floating, it doesn't provide any sealing. It has hole in it, and BP will go through it down to delay element one way or the other. Once the delay burns through, the BP will go poof instantly with or without it.

It just happened that it was missing from the packaging of one of the motors I bought at last launch. And I spent some amount of time procuring a replacement washer. Wondering what would have happened (if anything) if I simply flew it without that washer.

 

[tfish]

IIRC..the BP was getting sucked out or falling out when the delay element burned through/out. The washer keeps the BP in the well until it gets burned?

Tony

 

[TimothyG]

My understanding is it helps with preventing too much of the pressure generated during black powder burn from being ejected out through the rocket motor. I was told this could be more of a problem once you are using motor ejection on 3” or bigger rockets.

 

[crossfire]

There's this tiny metal washer that's supposed to go into the bottom of ejection charge well of AeroTech DMS motors. What's the purpose of that washer? To my untrained eye, it doesn't do much (or anything?). It's free floating, it doesn't provide any sealing. It has hole in it, and BP will go through it down to delay element one way or the other. Once the delay burns through, the BP will go poof instantly with or without it.

It just happened that it was missing from the packaging of one of the motors I bought at last launch. And I spent some amount of time procuring a replacement washer. Wondering what would have happened (if anything) if I simply flew it without that washer.

You need to use the washer if your needing motor ejection. I flew an early DMS K535 with no washer if not for using electronics. I would have had no ejection at all. BP gets sucked out the nozzle end.

 

[Steve Shannon]

Yes, without that washer the motor ejection charge doesn’t build up enough pressure to eject the chute or even the nosecone. I’ve watched it (not) happen.
As a rule, manufacturers don’t pack extra pieces just to confuse you.

 

[rcktnut]

The older DMS (first ones made) did not include the washer. After the problem was identified the washer was included to reduce the size of the "touch hole." The size of the hole needed to adjust the delay was just too big for proper ejection charge function. Washer solves that problem.

 

[Rocketjunkie]

I don't get why they don't use a normal size touch hole and use a smaller diameter drill on the delay adjusting tool. There is a separate one for the DMS any way. We always used a drill that would fit before reloads were introduced.

 

[TimothyG]

Cost of reconfiguring an entire product line I imagine. Mainly recertifications on everything due to a hole change.

 

[amiliv]

Thanks all! Makes sense.

 

[Steve Shannon]

Cost of reconfiguring an entire product line I imagine. Mainly recertifications on everything due to a hole change.

The rules don’t require recertifying everything based on a change like that. When a manufacturer makes a design change that exceeds manufacturing tolerances, the manufacturer is supposed to notify the organization that certified the motor originally. Typically the certifying organization would simply review the change and determine that the performance of the motor or the delay time would not be affected.
There could be changes that could affect performance or delays. That’s why this notification and review is required.

 

[beeblebrox]

I don't get why they don't use a normal size touch hole and use a smaller diameter drill on the delay adjusting tool. There is a separate one for the DMS any way. We always used a drill that would fit before reloads were introduced.

...It also costs a lot of money to have new molds made to change the aft bulkheads... Maybe the drill is bigger to make sure it won't work on model rocket motors so that noobs don't mess around, or an oversight when first designed, not realizing that the big hole would cause ejection gasses to vent outside via the nozzle. On motors where the nozzle throat is smaller than that hole it would not matter, but those washers are cheap, If you lose one you could use something else that fits with a small hole drilled in it the same size roughly. Since I have decided to always build an ejection baffle in my rockets, those washers and the rubber cap sealer is rattling around up in there after the flight. Shake it out and keep it just in case...

 

[Starfire73]

There's this tiny metal washer that's supposed to go into the bottom of ejection charge well of AeroTech DMS motors. What's the purpose of that washer? To my untrained eye, it doesn't do much (or anything?). It's free floating, it doesn't provide any sealing. It has hole in it, and BP will go through it down to delay element one way or the other. Once the delay burns through, the BP will go poof instantly with or without it.

It just happened that it was missing from the packaging of one of the motors I bought at last launch. And I spent some amount of time procuring a replacement washer. Wondering what would have happened (if anything) if I simply flew it without that washer.

My H135W didn’t include that baffle washer either. The on site vendor said I shouldn’t need it. I launched without it and it ejected just fine, but I knew I was taking a risk.

 

[AeroTech]

I don't get why they don't use a normal size touch hole and use a smaller diameter drill on the delay adjusting tool. There is a separate one for the DMS any way. We always used a drill that would fit before reloads were introduced.

There were several reasons but as Scott said, we didn’t want the drill tool used on standard SU motors because it won’t work properly for them, instead causing a safety hazard.

 

[jimzcatz]

I don't get why they don't use a normal size touch hole and use a smaller diameter drill on the delay adjusting tool. There is a separate one for the DMS any way. We always used a drill that would fit before reloads were introduced.

Tom, just continue using a 1/8th bit. Its worked for us for decades. I still use my Synerjet tool which has a 1/8th drill on every motor I adjust,reload or SU.