Bonus Delay

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Feb 4, 2004
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I have read I think here and elsewhere that the AT bonus delay may be caused by short grains in a longer casing, say the E-16 in the 29-40/120. My thought is why not put the grain in with the spacer towards the nozzle instead of towards the delay? Would the burn still be the same? Wouldn't this make the delay ignite at the same time as the grain? If I get around to doing some field tests I'll post the results, but I'd like to here some theroys or solid facts before I start burning up reloads.
That doesn't explain the "bonus" delays on the SU motors.

My Commando Cody rocket at NARAM flew on a D21-4 and I got an 8 sec delay. The chute opened 5 ft from the ground...but at teminal velocity!

The 6 oz of nose wt. kinda did a number on the parachute at that speed! real damage except the chute and a little mud on the nose cone tip.

I'd think I remember that somewhere I read sometyhing about AT, in the first version of RMS, had the motor assembled with the propellant grain right next to the delay grain, essentially the opposite of what it is now with the grain at the bottom.

You are correct. The first release of the AeroTech RMS-18, 24 & 29 reload instructions stated that the propellant grain in 'short' loads was to be place forward against the delay grain.

This was changed to moving the propellant grain back against the nozzle after AeroTech received reports that the White Lighting smoke and flame were dimished in the original configuration. This happened because the paper spacer tube would burn and produce dark smoke.

Testing demonstrated that the position of the propellant grain in the casing did not affect the delay.
Is there any air gap between the delay and the grain in SU's? I've never had a bonus delay with one but I've only used G40's & 80's..... and F23 BJ EconoJets.
There is definitely something odd that goes on. I had a big lot of SU E15's and F32's I got when Rocket Vision went out of business that have been problematic. These motors are several years old.

I've noticed with these problematic motors that if the motor sits on the pad for several seconds chuffing before the motor comes up to pressure, you will get the looooong delay.

I don't know if the delay isn't getting lit right away due to oxidation on the surface or what, but it gets your blood pressure up. I've also thought it was possible that the copperhead ignitors that came with them slid downward a little and actually light the propellant somewhere in the middle, causing slow pressurization and late lighting of the delay grain. Just a theory.

I only have a few left and have been using super hot home-made ignitors with them and having better luck. I might try opening one up just to see if the propellant or delay look funny.

The delay grain is extremely hard to ignite, and usually does not start until the propellant ignites... so if you get a chuffer, the motor is consuming propellant trying to ignite. Once the motor ignites... the motor does not have as much fuel... so after burnout, the delay grain remaining is longer... so the delay time is longer....
If the delay is so hard to ignite why not dip an end of it like an igniter and get some extra fire in there?
You want the delay grain to start at the same time the propellant does.... I have seen cases where HOT igniters have started the delay grain, and the propellant never ignites... or ignites a few seconds after the delay starts burning...
Originally posted by dasbeermeister
Is there any air gap between the delay and the grain in SU's? I've never had a bonus delay with one but I've only used G40's & 80's..... and F23 BJ EconoJets.

In AeroTech SU motors (E15, F25, G40, etc.) the propellant grain is loaded into the casing first. The delay grain is then pressed into the casing until it stops when touching the propellant grain.