[F-FT] Question/thought on Aerotech delays

This should be in the FAQ.

There are some clubs that will not allow you to install your igniter in the motor until after you are at the pad on motors "F" size or above, which would mean that you would have to take your unassembled motor with you out to the pad and assemble it there, or at least take the incompletely assembled motor, and hope you don't drop an O-ring on the way.

Al

I suspect the reason the igniter is placed all the way at the end of the propellent grains has more to do with making sure all the grains light simultaneously than igniting the delay with the igniter. The solid boosters for the Titan missile had a configuration not much different than an RMS reloadable, but no delay. They were ignited with what amounted to a small engine at the top of the stack and the reason given was that this would ensure that the whole stack lit at once. If ignition occurred at the bottom, it would not propagate rapidly to the top. If the igniter is all the way to the top of the grains, it really doesn't matter whether it lights the delay directly or lights the top grain which then lights off the delay along with all the other grains. If the igniter placement is lower in the engine, the delay should not ignite until the top grain does. I'd say the bigger problem might be that full thrust would not be developed quickly and the rocket might not be going fast enough to be stable when it leaves the rod.

Larry

David Erbas-White wrote:

Late ignition of the delay element would give you an undetermined amount of "bonus delay", regardless of thrust issues.

Motors with difficult to light propellant/grain configurations, where the delay element lights immediately, but the propellant does not, would "waste" delay time, prior to full motor ignition. Thus giving you a "short" delay in flight.

Any mechanical anomalies associated with the delay element, seen or unseen, can effect delay functionality. Small bubbles or "voids" will effectively shorten a delay, as will hot spots or "plumes" that might cause uneven erosion of the delay. "Burn through" of the delay liner will also shorten the effective delay.

With reloadables, You can add assembly anomalies to the equation, such as wrong delay element, improper installation of delay element, greasy delay element.

The ignitor installation is only a problem on some of the 18mm and perhaps 24mm RMS reloads. D and E stuff.

I've built tiny Magnelight ignitors small enough not only for any reload, but even for the old Apogee 13mm and 18mm competition motors.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

I'd have to say NO. The reason for the ignitor placement is to insure proper ignition of the propellant grain, not anything to do with the delay.

Take for example, the 29/40-120 E16 reload. There's almost 2 inches between the propellant grain and the delay. The ignitor is placed at the top of the propellant grain. The delay has no problem igniting anyway.

BTW, the original instructions for these motors had the propellant placed at the delay end of the motor instead of hte nozzle end. They work fine that way too, except a WL motor ends up producing black smoke instead of the characteristic WL smoke. Probably due to the spacer erosion betweent he propellant and nozzle. or maybe due to some bizare gas flow through the empty space.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

I don't think it's NARTS, it's R&D Judge Joyce Guzik who has been collecting R&D reports for many years. Of course, it's probably to facilitate putting together the NARTS summaries.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

The probelms would be in proper motor ignition, not from the delay.

Copperheads, for all their faults have a VERY energetic pyrogen. *IF* you can get them to fire, they do a pretty good job. If you wnat to try an interesting experiment, get a Quest Tigertail, and an Estes Solar ignitor. Fire each on your workbench on a flameproof surface, and compare the results. You'l be amazed.

That said, I use Firestar / Magnelite et al for igniting all my composite motors.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

As I just posted, I think this just happened to me. It was VERY different from a "chuff" and took maybe 3-4 seconds. A "chuff" is what you get from lighting the propellant somewhere other than the top, where it doesn't pressurize fully and the burn rate drops back down. The sound was VERY different.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

A good trick I too have used for years.

This can also happen in the MR C-slot grains, where the ignitor is fatter than the C-slot.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

This is what the Rocketflite folks told us to do igniting their big honken cored BP motors. It vastly reduced the cato rate on FSI F100s too. It causes the motor to come up to full thrust a bit slower, reducing the shock that causes the cato. But BP doesn't have the combustion characteristics of APCP, and igniting an APCP motor like this is likely to cause chuffing.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

correct.

I asked a motor manufacture after reading this report.

he told me the ignitor does not light his delays.

Would that be the one that has the 'bonus delays?'