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geof
18th May 2009, 10:51 PM
I forget. Is the rule of thumb to remove 1/16in per 1sec with 1/8in drill bit???

[Also, just out of curiosity, why isn't it L/n inches per sec reduction, where L is the physical length of the delay component and n is the default seconds of delay?]

Geof

shreadvector
18th May 2009, 10:56 PM
I forget. Is the rule of thumb to remove 1/16in per 1sec with 1/8in drill bit???

[Also, just out of curiosity, why isn't it L/n inches per sec reduction, where L is the physical length of the delay component and n is the default seconds of delay?]

Geof


Correct information is provided on the Aerotech website. Anything posted here could be wrong and lead to an expensive failure of your motor/rocket.

As a favor, there is a sticky thread in the "Beginners and Education" forum with the direct linsk to the Aerotech information:
http://www.rocketryforum.com/showthread.php?t=539

Your desired links are in the first message and easy to find.

NAR29996
18th May 2009, 11:01 PM
It's 1/32" per second of delay. Not too sure if the size of the bit matters (within reason), as I use a slightly larger size. No problems using it with my 18, 24, & 29 mm sets.

jimzcatz
20th May 2009, 03:00 PM
You are correct,its 1/32 per sec

bobkrech
20th May 2009, 07:26 PM
http://www.aerotech-rocketry.com/customersite/resource_library/Instructions/RDK_Instructions/rms_delay_mod_inst.pdf is the official AT delay shortening procedure.

All other methods are not allowed.

Bob

bobkrech
20th May 2009, 07:34 PM
You are correct,its 1/32 per sec

It's not quite that simple.

The delay column burn rate depends on the delay composition, the delay grain temperature and the barometeric pressure.

1.) If the delay composition batch is not throughly mixed, the burn rate can be variable.

2.) The burn rate is proportional to the grain temperature, and increases with temperature.

3.) The burn rate depends on the background pressure. The burn rate is faster at sea level than at altitude.

Bob

rstaff3
20th May 2009, 07:57 PM
Still, the 1/32" is the rule of thumb and is reflected in the procedures. I challenge anyone to gen any more resolution. The 1/32" works for all AT motors because the difference in burn rate with pressure, etc, is accounted for by the delay design.

terryg
21st May 2009, 03:12 AM
I have found that 1/32" never shortens the delay enough in the reloads I have used it on. Use this a starting point and then drill a bit more (TLAR) (That looks about Right). With a normal + or - 20% on the delays anyway, there is a great deal of judgement in this anyway.

cjl
21st May 2009, 09:16 AM
Oh, and the reason it isn't a simple L/N formula is fairly simple. The delay ignites with the rest of the motor, not at the end of the burn. This means that when the delay "starts" at burnout, much of the delay element is already burned. Second, the delay burn rate is pressure dependent, so during the motor burn, it is burning the delay at a much higher rate than the rate the delay burns after burnout. Because of these factors, delays have to be tailored to the motor that they are included in, and if you drilled them using that basic proportionality rule you mentioned, you would remove far too much material.

MarkH
21st May 2009, 02:21 PM
Oh, and the reason it isn't a simple L/N formula is fairly simple. The delay ignites with the rest of the motor, not at the end of the burn. This means that when the delay "starts" at burnout, much of the delay element is already burned. Second, the delay burn rate is pressure dependent, so during the motor burn, it is burning the delay at a much higher rate than the rate the delay burns after burnout. Because of these factors, delays have to be tailored to the motor that they are included in, and if you drilled them using that basic proportionality rule you mentioned, you would remove far too much material.

True, but if you look at the AT delay chart by propellant type, the delay lengths increase in increments of 1/32" / sec starting from the shortest delay. E.g a G71R-4 is 0.500 inches, a G71R-7 is 0.593, and a G71R-10 is 0.657. Well maybe not, the delay length only increases by 2/32" from a 7 sec delay to 10 sec delay... maybe it should be a G71R-9, or maybe there is some rounding in there on some of the delay times.

rstaff3
21st May 2009, 06:20 PM
My meager observations are:

I've use the 1/32 rule on various hobbyline and 29/120 loads including the 24mm ones with good success. I haven't had any more overly long or short events than on the stock delays (no stats, just rough memory). I seldom need more accuracy since I managed to get by for years with the stock delays. My sims don't have enough fidelity to need more accruracy. Weather conditions often change the optimal delay anyway.