Aerotech DMS motor confusion

[Dave S.]

Sometimes the picture shown on web sites like Apogee show the motor and parts that come with it like this no metal washer, but with an extra red cap, rubber bands, and a ring (thrust ring?):



other times the photo looks like this with one red cap and the metal washer:




I have bought motors that have the 1st picture on the site - but when I get them they look like the 2nd photo with the thrust ring already part of the case.

Not complaining, just asking. I'd ask Apogee, but I figured if I am wondering, other newbies must be too

 

[n3tjm]

They just havent updated the photos to match the current version of the motor.

Don't forget to use the metal washer if you are using motor eject.

 

[Dave S.]

They just havent updated the photos to match the current version of the motor.

Don't forget to use the metal washer if you are using motor eject.

Thanks for the reply. All I have ever had came with the metal washer, which I always use.

 

[Steve Shannon]

The washer is important. I have seen rockets fail to eject and had people ask if they should have used the metal washer.

 

[AeroTech]

The original DMS motors had a different name, glue-on thrust rings and other minor differences. The internal ballistics haven’t changed.

 

[Rocketjunkie]

Look in the ejection well and see if the metal washer is there. If not, save the motor until you've used another and dig out the washer from the spent motor and use it again.

 

[Sooner Boomer]

Look in the ejection well and see if the metal washer is there. If not, save the motor until you've used another and dig out the washer from the spent motor and use it again.

How do you get the washer from a motor that's had the ejection charge fire?

 

[Kallahan11]

How do you get the washer from a motor that's had the ejection charge fire?

You just pick it up off the field.

 

[Rocketjunkie]

Dig it out with a small screw driver.

 

[n3tjm]

Last time I flew an I65-PW, the motor was not plugged. It didn't come with the washer because it was a -P motor, but I wanted to use motor eject. So I 3D printed a "washer" that matched the angle of the well. Super glued it in. Worked like a charm.

 

[Art Upton]

Ok , I've flown many DMS motors, albeit about 4 years and more ago. They never had a washer in them.

What is it for now and why is it needed?

 

[Art Upton]

The washer is important. I have seen rockets fail to eject and had people ask if they should have used the metal washer.

What is it for? Never seen them in the past. Is it to "tamp down" the BP in the well to make sure it is against the delay grain? We used to use some wadding for that before under the red cap.

 

[DigBaddy]

What is it for? Never seen them in the past. Is it to "tamp down" the BP in the well to make sure it is against the delay grain? We used to use some wadding for that before under the red cap.

Ensures ejection charge pressure doesn't get sent backward. Thread on it here.

https://www.rocketryforum.com/threads/aerotech-washer-purpose.173025/

 

[Art Upton]

Ensures ejection charge pressure doesn't get sent backward. Thread on it here.

https://www.rocketryforum.com/threads/aerotech-washer-purpose.173025/

Thank you. They worked for me before they came with them OK. Now it seems you need to have them...

So for the DMS motors I still own that don't come with washers do I need to order some?

Edit: seems I can get them at the hardware store.

 

[enkindler]

While you may get away with it from time to time, my failure to produce a check list or at least mark the instruction sheet with a pen resulted in me failing a level 2 cert flight yesterday almost certainly due to the lack of the washer.


I Was flying a Wildman Journey 75 with a AT DMS 38 J270W-14A. My calculated (optimistic) altitude with just under 5000' with an optimal delay of 13.1 Seconds. My primary method was an Eggtimer apogee with the motor intended to be secondary. But with the tolerance of delay grains I fully knew that the motor charge may fire first.


I was using the included instructions but not marking them. When I went to pull out the washer I removed the igniter from the bag first and the covering tube fell off and into the snow. As I wanted to avoid contamination I looked around and decided to just to empty my pre-weighed charge which was in a centrifuge tube like they supply and put the igniter tip in the tube to protect it.

Well because I wasn't marking off items on the instruction sheet, the distraction resulted in me missing adding the washer first.

My flight was perfect on the powered section but the rocket did go ballistic and became a lawn dart.
After reviewing the video I took you can see that the motor charge went off at the early side of tolerances and the puff is consistent with the charge energy exiting through the back of the rocket.
While hard to see the arrow is pointing at the in tolerance but early cloud from the event. What is hard to tell from that image is that it is 1000's of feet above the exhaust plume just below it.


I am not sure what the failure rate is without the washer, but as they sold the DMS motors for a while before adding it, you may get away with it....until you don't.


Due to some experimentation I had well over a dozen test charges with the load I used and once I developed the initial load and added 20% I never had an in flight or ground testing failure and I was using to sheer pins with a fairly loose fit that makes other explanations for the failure to deploy unlikely.
Still 100% my own fault for not taking steps to ensure distractions didn't cause me to miss some steps.
But as there is fairly ambiguous replies to this question on here I wanted to share what is a pretty reliable example of failure due to not using the washer on a DMS motor.

 

[Kallahan11]

If the ejection charge went off and the puff came out the back of the rocket did you eject the motor? What did the wreckage look like, was the motor in the rocket? was the motor retention system still on the rocket? What happened with the eggtimer Apogee, should that have not gone off at apogee regardless of the motor ejection charge?

 

[enkindler]

If the ejection charge went off and the puff came out the back of the rocket did you eject the motor? What did the wreckage look like, was the motor in the rocket? was the motor retention system still on the rocket? What happened with the eggtimer Apogee, should that have not gone off at apogee regardless of the motor ejection charge?

No evidence of any separation at all.
The ejection charges are typically kept below 15psi and in this case were 11.5psi or about 80 foot pounds of force on the nosecone which would be more than enough to shear 2 #2 shear pins.
While I don't have the specs for all motors, on a random spent H100W I have, the hole in the charge bay is larger than the nozzle as seen in the picture below. All that has to happen is enough of the charge to pressurize the spent motor casing to cause the PSI in the rocket body to be too low to eject the laundry.


In this rocket it seems that dropping from the planned 11 PSI to 6 PSI would have been more than enough to result in a recovery failure. Even if the entire charge were to be directed to the spent motor casing, those pressures are well below the pressures it just experienced during powered flight. It could easily hand the pressure from the entire charge with no damage, but doesn't need to take the full pressure to result in a failure to deploy.
IIRC in model rockets, when you didn't want an ejection charge to do anything the rule of thumb was to size a vent hole at least as large as the diameter of the ejection charge. But that was to fully disable the effects of the ejection charge for clustering etc...

 

[green dragon]

I Was flying a Wildman Journey 75 with a AT DMS 38 J270W-14A. My calculated (optimistic) altitude with just under 5000' with an optimal delay of 13.1 Seconds. My primary method was an Eggtimer apogee with the motor intended to be secondary.

Were you using a seperate charge for the eggtimer apogee ?

Hard to believe a secondary charge, outside the motor. would go out the back through the casing.

 

[enkindler]

Were you using a seperate charge for the eggtimer apogee ?

Hard to believe a secondary charge, outside the motor. would go out the back through the casing.

I don't have enough information as to what failed there.
But the Eggtimer Apogee was housed in the nose cone with the charge being below the chute in one of the wildman ejection lighters with the cardboard tube.

Either the motor charge disabled the eggtimer charge or the ~17mm hole, if the motors charge blew that much out seems close to large enough to still drop the pressure low enough that sheering two pins would be questionable.

The math with that gets a bit complex and I will revisit in the future but it is non-trivial.

Most likely the charge was disabled but that is a guess.

The motor charge fired just under a second before apogee and there was no separation is the only real reliable data I have from the video.
If I can figure out how to post the video in a way where the hosting provider compression doesn't remove the rocket completely I will post it here.

But yes the question on that cascading failure is an unknown and always will be for this launch but the charge was large enough for 3 pins according to the rules of thumb and I only had two.

As this event really convinced me to move to electronic ejection completely more data will probably have to come from another source.

But it is quite clear that Aerotech thinks the washer is important and I fully believe their assessment.

 

[kramer714]

Were you using a seperate charge for the eggtimer apogee ?

Hard to believe a secondary charge, outside the motor. would go out the back through the casing.

Years ago I asked about the washer, I was told they added the washer after they introduced the delay change tool. It can make the hole in the bottom of the charge well too big, the washer has a smaller hole keeping the pressure up in the charge well.

 

[86mustang408w]

After my high power flight I got home, noticed something was rattling around. It was the washer and plug from the ejection charge, it hit my chute stop and was inside the airframe still, lol.

But like said the washer is to help to the charge, and to make sure it doesn't go back down through the motor out the nozzle. It can cause serious issues if not put in.