Do Aerotech reload delays start at ignition or burnout?

I flew my first Aerotech reload last weekend, an E18W. Flight went very well except for a miscalculation on my part regarding the delay. Looking at the lengthwise slot in the grain, I thought the delay charge would start burning as soon as the motor ignited. Is that wrong? If so what keeps it from starting. The areotech propellant does't look like an end-burn grain to me...

The delay grains (on all core burning motors) light with motor ignition. However, the published delay value on the package tries to account for motor burn time.

However, I have had grains which were particularly hard to light, but the delay lit right away causing the delay charge to go off under thrust.

Kevin

The delay burns when the motor lights, but the delay time they quote is indeed from motor burnout. ie, if the motor burns for 2 seconds, and then has a 5 second published delay time, the delay grain actually burns for 7 seconds. It also burns much more quickly when the motor is firing under pressure, this is why long burning motors like the E11J or F12J only have relatively short delay times- the delay has to burn for a really long time!

Nate

kruland said:

The delay grains (on all core burning motors) light with motor ignition. However, the published delay value on the package tries to account for motor burn time.

However, I have had grains which were particularly hard to light, but the delay lit right away causing the delay charge to go off under thrust.

Kevin

Click to expand...

nute said:

The delay burns when the motor lights, but the delay time they quote is indeed from motor burnout. ie, if the motor burns for 2 seconds, and then has a 5 second published delay time, the delay grain actually burns for 7 seconds. It also burns much more quickly when the motor is firing under pressure, this is why long burning motors like the E11J or F12J only have relatively short delay times- the delay has to burn for a really long time!

Nate

Click to expand...

Thanks! That explains my miscalculation. I assumed the delay started burning at ignition, but didn't realize they were fudging the number to look more like an end burn motor. The extra time put me further past apogee than my chute could withstand, which taught me to stop using cheap plastic chutes...

You are correct about the AT motors not being end burners, they are core burners. The delays are designed to light at the same time as the propellant, which is why it is important to place the end of the igniter at the top of the propellant grains. On loads with spacers between the propellant, and delay, I place a piece of masking tape across the top of the upper grain to aid in igniter placement. The advertised delay time, is the expected burn time of the delay after propellant burn out. Hope this helps!

David

As stated above, the time delay is calibrated relative to burn-out, for compatibility with end-burning BP motors. For more detail, I go over that point early in the Electronic Recovery video.