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Questions about Aerotech LMS motors - packaging problem?

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billdz

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I ordered two G79-7W/L and two G78-7G/L LMS motors. Both of the G79s seem to have an incorrect delay charge, and one of the G78s appears to have an incorrect grain.


1) Both G78 packages have the G78-7G/L box checked, but the instructions state: "For 7 second delay times, use RDK-03 Plus (.438" delay, .281" purple spacer). For 10 second delay times, use RDK-20 Plus (.531" delay, .188" green spacer)." Both of my motors came with the .531" delay and the green spacer. So I assume I was mistakenly sent G78-10G/Ls?


2) The instructions for the G79 state that the propellant grain with "two green stripes and one magenta stripe" should be placed in the aft position. However, in one of my packages, both grains have two green stripes and one magenta stripe. I assume this was an error, since the other package has one grain with these 3 stripes and the other has just one green stripe.

Thanks for any advice,
Bill
 

mikec

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I think you reversed G78 and G79 in your numbered list, but otherwise it sounds like you're right. Contact Aerotech. You could drill the delays to the right length but the grain thing won't work as is (I'm a little surprised the grains are different for the G78, are the core diameters different or the lengths?)
 

billdz

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Thanks for the reply. You're right, I did reverse them.

I have the drilling tool for DMS motors but it looks like the LMS requires a different tool, which I don't have. I'll probably just wait for replacements.

I don't have the motors with me now but my recollection is that all of the grains were the same size and length. The G78s had stripes while the G79s were unmarked and presumably interchangeable. The G78 produces green smoke while the G79 does white.
 

billdz

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Interesting that delay charges apparently have different delay times depending on the motor. For example, the G78 instructions say to use the RDK-02 Plus for 7 second delay, while the G79 says to use the RDK-02 Plus for 4 second delay.
 

eggplant

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Interesting that delay charges apparently have different delay times depending on the motor. For example, the G78 instructions say to use the RDK-02 Plus for 7 second delay, while the G79 says to use the RDK-02 Plus for 4 second delay.
The burn rate depends on the pressure, so longer burning/higher thrust motors will consume more of the delay element during the burn. These motors then need longer delays for the delays to last the same amount of time after burnout.
 

mikec

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Interesting that delay charges apparently have different delay times depending on the motor.
Yes, different propellants have different pressures and hence the burn rate of the delay is different while the motor is burning.

All you need to do to adjust the delay is drill a hole of depth 0.531-0.438" in the base of the delay grain, which you could do with a drill bit, a ruler and some masking tape, but if you're not comfortable doing this by all means wait for a replacement (or fly a rocket appropriate for a 10-second delay.)
 

Steve Shannon

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Interesting that delay charges apparently have different delay times depending on the motor. For example, the G78 instructions say to use the RDK-02 Plus for 7 second delay, while the G79 says to use the RDK-02 Plus for 4 second delay.
Not at all unusual. A portion of the delay burns while the motor burns at a rate that depends to some extent on the conditions within the motor, such as pressure.
The remaining portion of the delay is the part that must match pretty closely the delay you need before apogee.
If you use the same delay, and if a motor burns for two seconds the length of the remaining portion is shorter than if the motor burns for one second. Therefore a different delay length is needed if both delays must have a 7 second delay. There are complete threads dedicated to this one topic that explain it better than I just tried.


Steve Shannon
 

billdz

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Thanks for this info, makes sense that pressure and burn length would affect the delay rate.

I'm comfortable with drilling but get nervous when speaking about 3-digit fractions of an inch. So I'd need to drill off .093" (less than 1/8") to reduce the delay from 10 seconds to 7.
 

mikec

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So I'd need to drill off .093" (less than 1/8") to reduce the delay from 10 seconds to 7.
Yes. 3/32" roughly. Delays are hardly exact anyway, 20% is the allowed tolerance and they often vary even more than that, especially when Aerotech mispacks a motor kit :(
 

jnelson

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2) The instructions for the G79 state that the propellant grain with "two green stripes and one magenta stripe" should be placed in the aft position. However, in one of my packages, both grains have two green stripes and one magenta stripe. I assume this was an error, since the other package has one grain with these 3 stripes and the other has just one green stripe.
Definitely sounds like an error to me. If they're marked separately, and the instructions distinguish between them, there must be a reason for it. If both grains have the same dimensions, then the difference is probably with the propellant itself. I'm interested to hear what you find out from Aerotech.
 

billdz

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I'll post as soon as I hear something, thanks for all the replies.

I noticed that as delay time lengthens, the delay charges get bigger and the spacer gets smaller, so the combination is always the same length. So maybe we can't just drill off a piece of the delay, don't we also have to increase the size of the spacer somehow?
 

mikec

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tfish

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I'll post as soon as I hear something, thanks for all the replies.

I noticed that as delay time lengthens, the delay charges get bigger and the spacer gets smaller, so the combination is always the same length. So maybe we can't just drill off a piece of the delay, don't we also have to increase the size of the spacer somehow?
you are correct in that the each given delay and spacer combo will always be the same length (for a given closure). Drilling a hole in one end of the delay element does not shorten it's physical length...but it does shorten it's burn time.

Tony
 

billdz

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AT has advised they are sending me replacement parts. They say the motor would work with two 3-stripe grains, "but not optimally as the grain chemistry was
purposely developed for the motor to burn correctly.
"
 
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