DMS Delays Too Short?

Do you have the bulkhead yet? If so slice it length wise, with hacksaw, dremel maybe you will find something out. If you don't want to, send it to me, I'll pay postage. I'd like to see how they look on the inside.

I have flew around 12 of the DMS motors with no ejection problems. H,I,J,s

I used a DMS I140 last week drilled to an 8-sec delay, but the delay actually fired at 2 sec after motor burnout. It caused a major zipper in my rocket but it is repairable. Aerotech will replacing the motor and said that mine had an "old style" delay system and their new delays are no longer prone to early ejection problems.

Most delay elements burn about 1/32" per second in atmospheric pressure. That is how deep the delay tool should be going. The rest burns faster during the burn.

If you are getting fast ignition, no chuffing, barely shortened the grain (or not at all), but still get an ejection while the motor is burning (blow by - defect in the forward closure sealing).

As a wild guess, I would say the real short delay after burnout would be similar to blow-by. Thinking the delay getting lit on the outside along the wall and burning inward to hit the powder. If an o-ring is on the delay face like with the reloads, it prevents immediate blow by, but the liner might not be thick enough to prevent the delay burning in a manner other than as an end-burning delay grain.

Kevin - Thanks for the reply. I don't know the details about the internal construction of these DMS motors. I did drill out 3/16", or 6 sec worth of the delay, to get an 8-sec delay. I followed the instructions carefully so the result was very unnerving and has me questioning whether to use DMS motors again. Whether this was blow-by or an uneven burn rate of the delay, I don't know.

clarinet said:

Kevin - Thanks for the reply. I don't know the details about the internal construction of these DMS motors. I did drill out 3/16", or 6 sec worth of the delay, to get an 8-sec delay. I followed the instructions carefully so the result was very unnerving and has me questioning whether to use DMS motors again. Whether this was blow-by or an uneven burn rate of the delay, I don't know.

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Yeah a bubble in a delay grain can make a big difference. Hard to say exactly what the problem is as I do not know the internals either. RMS reloads are more within the builders control than the DMS. Some masking tape on the delay or the delay insulator if it feels too loose and you are good.

Yes, I'm thinking about going back to RMS motors and no more DMS ones.

I have a lot of DMS (and RMS). Hoping to get to all DMS and have fewer cases to clean. Still have snap ring for research motors though.

I had the same problem last summer with a DMS I-140, ejection about 20' off the rail.

Mine got to about 700 ft before ejection, about half way to projected apogee. Fortunately the rocket wasn't zippered beyond repair and it came down on its main chute.

I thought they made a pill for premature ejection?

clarinet said:

Yes, I'm thinking about going back to RMS motors and no more DMS ones.

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Thrust rings that detach, cases that rupture, and delays that blow several seconds early. Can't wait to fly DMS!

I had 4 DMS 38mm motors I gave away and told him to fly them in a rocket he didn't care for. The problem I have is no matter how old the rocket is I hate to destroy it.

Buckeye said:

Thrust rings that detach, cases that rupture, and delays that blow several seconds early. Can't wait to fly DMS!

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Because the other brand is perfect too? Having sold both, I've had very few failures from dms. All of these issues have been resolved, though they were not super common anyway.

tbonerocketeer said:

Because the other brand is perfect too? Having sold both, I've had very few failures from dms. All of these issues have been resolved, though they were not super common anyway.

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+1, my AT DMS L1000-W didn't shed a thrust ring, CATO, or eject early. It was just like most of my AT SU flights fun, and painless. I did however just to be safe add some epoxy to the thrust ring and a couple of wraps of tape.

I know that most DMS's have performed well. I have only used one so far, but I do feel more confident about them after hearing Aerotech's comments about how they've improved the delay element reliability within the past year.

I have seen on numerous occasions, bp in the charge well....delay completely burnt...no bp charge fire.

It has always been one of these 2 issues...always. Grease on inside of delay well contaminating the BP OR bp clump blocking the hole. [ assuming bp is not damp]

Always pour small amount of BP in the well, use toothpick, altimeter screwdriver, wire from starter, anything to force the bp into hole & tap side of closure until BP disappears down the hole and fill gap between delay-o-ring & closure. When you can no longer get bp into closure, THEN fill the charge well & you will be ready to go.

Grease where it doesn't belong...your screwed with that one.
Always be careful not to over-grease the rubber washer & o-ring that sits on bottom of closure. Any excess grease in that area will "wick" into the bp, rendering it useless to transfer ignition into the actual charge well.

Doing the above will eliminate 99.9% of motor eject problems.

Altimeters & e-matches/ejection starters, well that's been covered at length is several recent threads.
I have hundreds & hundreds of flights under my belt & prepped hundreds of rockets for others over the years.
Pull test on all wire connections.
2 altimeters and 4 starters on large projects.
1 altimeter with 2 starters per charge [4 matches total] on small rockets.
The above eliminates 99.9% of any issues.

Drilling delays.
Something really wrong here. This many folks having issues warrants a double check.
Stick something in the drill hole and mark the depth. Remove & measure. It's simple, for every 1/32 in. should = 1 second time removed for delay. If hole is longer/shorter by 1/32 measure than the delay tool is telling you in seconds time to check the tool.
I have seen every tool sold, have some bits mounted incorrectly & need adjustment. Both for being too long & short.
When all else fails...do it like we have for years before these tools were sold. Use a drill bit with a depth collar and set it for time by the 1/32 rule!

Anyone having consistent problems, get a mentor to look over your shoulder and watch what you are doing. The odds of both electronics/match failure AND motor eject failure happening at the same time, on a regular basis are astronomical.

There's a ton of wisdom in that post, Jim!


[emoji1010] Steve Shannon, L3CC [emoji1010]

Thanks for all the advice, Jim. I have checked my drilling tool and it is calibrated correctly, so that doesn't seem to be the reason for the early ejection that occurred in my flight.

My AT DMS J270 flew perfectly this past weekend. I had drilled the delay to 10 sec and I got the expected 10 sec delay.

and my replacement I140 DMS from AT flew perfectly last week - I drilled it down to an 8 sec delay and that's what it did in the flight. The Madcow Torrent flies well on that motor.

I have flown three DMS I140 motors. Two delays occurred as were as drilled, but my last one flown back in Janurary, drilled down to 8 seconds and double checked, ejected at 2 sec after burnout. Rocket survived with a healthy zipper and chute survived with a little damage, so had a decent recovery. Model needed a facelift anyway, so no huge issue.

I have two more of these that were purchased in May. Going to fly one in two weeks at our launch. Hopefully the short delay issues have been worked out. Any more trouble and I will probably go back to RMS I161s and build them.

Tom,
I suggest you contact Aerotech (Karl) and give them the serial numbers of your two I140 DMS motors. They (he) will be able to tell you if they are the newer version in which they claim to have solved the delay element problem. If not, they may well offer to send you replacements. My old I140, which I bought in March, did exactly what happened to you (ejection 2 sec after burnout even though I drilled it to 8 sec). However, the replacement I140 that Aerotech sent me in May performed normally.

clarinet said:

Tom,
I suggest you contact Aerotech (Karl) and give them the serial numbers of your two I140 DMS motors. They (he) will be able to tell you if they are the newer version in which they claim to have solved the delay element problem. If not, they may well offer to send you replacements. My old I140, which I bought in March, did exactly what happened to you (ejection 2 sec after burnout even though I drilled it to 8 sec). However, the replacement I140 that Aerotech sent me in May performed normally.

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Excellent idea. Will try to do just that.

Anyone have any delay issues with the 38mm H100 DMS? I got one of those in May as well.

Flew another I140 DMS yesterday. Worked as desired and exactly as drilled.

Did not have a chance to get a check on the batch with AT, so used the one I had with the highest serial number.