Can You CUT 24mm Delays to Shorten Times?

As opposed to drilling AT delays...if I want to convert a med. 24mm RMS delay to say, 3 sec. (for a boost glider), can/should I just slice the delay and then add a bit of spacer to fill the gap (or glue the existing one or sum'pin), or should I actually drill it?

In my search, I found one AT fact sheet on how to drill, but it talks about "I" loads...big stuff, using a 3/16" drill. In the same thread was a chart on delay size (length) for all the different loads & delay times....18mm and up.

For small motors, slicing to the proper length (while ensuring the parts don't shift) SEEMS to make the most sense.

Am I right?

Deandome said:

As opposed to drilling AT delays...if I want to convert a med. 24mm RMS delay to say, 3 sec. (for a boost glider), can/should I just slice the delay and then add a bit of spacer to fill the gap (or glue the existing one or sum'pin), or should I actually drill it?

In my search, I found one AT fact sheet on how to drill, but it talks about "I" loads...big stuff, using a 3/16" drill. In the same thread was a chart on delay size (length) for all the different loads & delay times....18mm and up.

For small motors, slicing to the proper length SEEMS to make the most sense.

Am I right?

Click to expand...

load one and test one. I would do a static test.

you can buy new hobyline cases at www.whatsuphobbies.com

Ben

Easier to drill out the delay...much easier!

Deandome said:

For small motors, slicing to the proper length (while ensuring the parts don't shift) SEEMS to make the most sense.

Am I right?

Click to expand...

No. Slicing the delay would shave off CONSIDERABLY more time than drilling it to the proper depth. Remember, the delay burns during thrust, too. By slicing off a piece of the delay, much more of the delay will burn during thrust, leaving less left at the end of the burn for the "proper" timing of ejection.

Slicing off pieces of the delay is NOT recommended for the hobby line delays, IIRC. So, I'd drill the delay to adjust the timing as you would any other AT delay element.

MarkM said:

No. Slicing the delay would shave off CONSIDERABLY more time than drilling it to the proper depth. Remember, the delay burns during thrust, too. By slicing off a piece of the delay, much more of the delay will burn during thrust, leaving less left at the end of the burn for the "proper" timing of ejection.

Slicing off pieces of the delay is NOT recommended for the hobby line delays, IIRC. So, I'd drill the delay to adjust the timing as you would any other AT delay element.

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But delays for a given load are the same material (there are a couple of diff. formulations in total, though)....and clearly the only difference between a 3, 6 or 9 sec. delay is the overall length of the delay element. AT even provides you w/those specific dimensions: https://www.aerotech-rocketry.com/c...Flyers_Data_Sheets/aeroTech_delay_lengths.pdf

I thought the only reason drilling would be preferable would be so the element cannot move...it's locked into place in the vertical axis...or maybe that's some kinda "sanctioned" alteration of a motor whereas slicing wouldn't be.

Can you tell me where to find drilling instructions for 24mm motors?

Whacking pieces off the delay element ain't going to do nothing but either give you a burn through or a VERY short, probably 0 or instant delay.

A burn through will ruin your forward closure and probably your rocket as well.

IIRC, drilling 3/16 of an inch reduces the delay time by 1 second. Someone please correct me if I'm wrong.

You gotta keep in mind that yes, the delay element ignites at the same time the propellant does and yes, the delay element burns faster during the burn than it does at motor burnout.

If a delay is marked at say 10 seconds for an example, that means that at the end of the burn, there should be enough of the delay element left to burn for another 10 seconds.

I have heard that motor ejection delays have a tolerance of plus or minus 1 second or 10% whichever is greater.

BTW, AT now has RDK's available for the 18-29mm hobbyline RMS motors. I would recommend going that route.

Donaldsrockets said:

IIRC, drilling 3/16 of an inch reduces the delay time by 1 second. Someone please correct me if I'm wrong.

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With the exception of some of the delays used with Black Jack loads (slightly slower), once the propellant burns out, the delay burns 1/32" per second. Drill this amount to shorten the delay. (I.e. drill 1/8" deep to shorten a delay by 4 seconds.) The diameter of the drill isn't critical and when drilling a RMS delay, assemble with the hole facing the propellant. Slicing off the same amount works also but you will need a spacer to make up the missing length.

Deandome said:

As opposed to drilling AT delays...if I want to convert a med. 24mm RMS delay to say, 3 sec. (for a boost glider), can/should I just slice the delay and then add a bit of spacer to fill the gap (or glue the existing one or sum'pin), or should I actually drill it?

In my search, I found one AT fact sheet on how to drill, but it talks about "I" loads...big stuff, using a 3/16" drill. In the same thread was a chart on delay size (length) for all the different loads & delay times....18mm and up.

For small motors, slicing to the proper length (while ensuring the parts don't shift) SEEMS to make the most sense.

Am I right?

Click to expand...

Sure! Make it harder to do, increase the possible failure modes, while adding no positive benefit whatsoever!

Sounds great!

Rocketjunkie said:

Slicing off the same amount works also but you will need a spacer to make up the missing length.

Click to expand...

I've never done it myself but one TARC team I observed last spring cut their delays that way, used no spacer and had no problems.

I've never sliced delays, always drilled them back. I have "dialed back" many delays, on everything from 24mm up through 54mm with no problems experienced. For the most part, my delay times have been pretty darned close! I may have just been extremely lucky up until now, but I use a great deal of caution and take my time.

Its not 3/16" = 1 sec. It's more like 1/32nd = 1 sec.

n3tjm said:

Its not 3/16" = 1 sec. It's more like 1/16th = 1 sec.

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Bugger! Where can we get the right poop here? What a mess.

Chuck

Drilling the delay effectively does the same, but with less area for error, as "cutting" the delay. The reason they are the same is that the burn "front" is still burning from the deepest part of the drilled hole, up at the same rate it would if you had cut it to that depth/length. The left over "lip" that burns out from the drilled hole is of little consequence. I have had good luck drilling delays, and the only time it was not the correct time length, was when I did not drill the correct depth. Also, from Aerotech's web site, as a general rule, all the delays burn at 1/32nd inch per second, once the core propellant has burned so that there is no induced internal pressure adding to the burn rate.
Here is an article from Aerotech discussing drilling.
https://www.aerotechrocketry.com/cu...tions/RDK_Instructions/rms_delay_mod_inst.pdf
Here is an article that lists the length of their delay elements.
https://www.aerotechrocketry.com/cu...Flyers_Data_Sheets/aeroTech_delay_lengths.pdf

mikeyd said:

Also, from Aerotech's web site, as a general rule, all the delays burn at 1/32nd inch per second, once the core propellant has burned so that there is no induced internal pressure adding to the burn rate.

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One demerit each for them other guys with the bad juju. And this WILL go on your permanent record!

Chuck