AT Universal Delay Drilling Tool - small hole, big difference?

I've used the delay drilling tool on several motors and it seems to work fine, but I'm trying to figure out the science. Relative to the size of the delay element, the tool makes only a small hole. If the undrilled element has a 14 second delay, shouldn't a hole that reduces the size of the element by almost half be necessary to reduce the delay to 8 seconds? But the tool only makes a hole about 3/16" wide and less then 1/4" deep. How does this small hole cause a 6 second reduction in the delay time?

billdz said:

I've used the delay drilling tool on several motors and it seems to work fine, but I'm trying to figure out the science. Relative to the size of the delay element, the tool makes only a small hole. If the undrilled element has a 14 second delay, shouldn't a hole that reduces the size of the element by almost half be necessary to reduce the delay to 8 seconds? But the tool only makes a hole about 3/16" wide and less then 1/4" deep. How does this small hole cause a 6 second reduction in the delay time?

Click to expand...

It makes the web thinner, the whole delay doesn't need to burn away, just the thinner section.

You only need a tiny well for the fire to fit through

Interesting, thanks for the info. What about the residue? When I drill into a reload, the hole is almost filled with powder, which I clean out. But on a DMS motor, the hole can't be cleaned. Does that make a difference?

Not especially. It's all to do with inhibited burn surfaces vs. not. Loose APCP is a nitroiously bad insulator :-D

The reason you don't have to drill half way through a delay to cut the time in half is because the delay element burns much faster when under pressure during the motor burn. When the pressure goes away at the end of the burn, the rate at which the delay element burns is greatly reduced.

Looking at an AT Hobbyline delay that is 0.75" long and gives a 10 sec delay in a motor with a 2 second burn time. At 1/32" per second for the 10 second delay, that has to burn through 10/32" to take the 10 second delay from the end of the burn. That means that the rest of the 0.75" or 24/32" delay has to burn 14/32" during the 2 seconds of propellant burn. So it is burning almost 7 times as fast under pressure during motor burn then after when there is no pressure.

If you drill the delay 2/32", it still burns the 14/32" from the bottom of the drilled hole during pressure, leaving only 8/32" of delay left when the pressure drops off. If you did drill half way through the delay grain trying for a 5 sec. delay, you would only have 12/32" of delay left and since it burns 14/32" during the motor burn, your ejection charge would fire before the motor was done burning. Not a good thing.

Got it, thanks.