[F-FT] Question/thought on Aerotech delays

Matt Steele's team ( who name escapes me now) did a R&D report on just this topic at last years NARAM. I know NARTS was collection all the R&D reports. I don't know if they are published yet. You might try contacting NARTS. There conclusion of the report show a defiant correlation on igniter placement and time of delay.

possibility?

Yes.. The delay is timed so that it starts burning at the same time as the motor, so the ignitor should be at the top. Will it have much difference if it's down a bit (a small bit)(in practical use)? Probably not. Halfway down, you might run into some weird problems. The only reason that you'd do "halfway" is that the leads on the igniter are too short, in which case, you got the wrong igniter...

One big technical issue, is CrapperHeads. If you use them, be sure to have a "fallback igniter" (non Crapperhead)!

possibility?

Why is the correlation defiant???

David Erbas-White

I wonder how big a motor they included. For a 6" long motor, it might not have the same impact if the igniter is 1/3 of the way down as on a 36" long motor.

Why would anyone not want to put the igniter all the way in anyway? The only thing that comes to mind is that you don't have the proper igniter.

The team name was "Pod Bay Doors". There was a recent article in Extreme Rocketry (#44) on this topic and I assume it was based on the R&D report.

However, the thesis of that report is that the _radial_ position of the igniter effected the delay time. I have a few problems with that thesis.

1) It only applies to delays that have been drilled. 2) It assumes that the igniter is lighting the delay. 3) There was a lot of hand waving involved in the "proof". Matt no longer had access to the data which he claims supports this thesis. Not that I don't trust him but I really dislike that sort of argument.

If the igniter were in fact lighting the delay, then we would see many more instances of misfires that light the delay element but not the motor. Since I have only seen misfires where both failed to ignite, I can only conclude that the igniter does not light the delay. Not very often anyway.

If the igniter doesn't light the delay, then the delay must be lit by the main propellant charge. If the propellant grains are ignited some distance from the delay, then because the combustion products are trying to exit through the nozzle, they may stagnate at the forward end and prevent ignition of the delay element for some period of time.

An extreme case of this was recently documented by Cesaroni in one of their Pro38 motors. A non electric match type igniter was positioned part way up the motor. The motor lit in a non normal fashion and ended up producing a pool of molten aluminum from the case. Read the FAQ item for "Can I use a different igniter?" on their web site.

Bruce Can> Matt Steele's team ( who name escapes me now) did a R&D report on just this

In such a case, I think you'll find that the igniter would, in turn, light the propellant. After all, the flame and exhaust gases from the burning delay only have one way to go -- out the nozzle.

-Kevin

I maybe luckily have not had any problems with copperhead ignitors. Now I am going to qualify this with the fact I have not been at this as long as some have but still the way some talk they should never in a million years light a motor. I have only had 3 that have not worked properly (as it first appeared) but by looking over things it was my mistake that was the cause. Maybe I have been lucky but I have a bundle of them and plan on using them till I have a reliable cheaper alternative.

If the kit uses a spacer tube (an E or F in a 29/40-120) then the igniter is at the end of the fuel grain. Masking tape is used to prevent the igniter from going any further. Works fine.

Sound to me like a presentation was made only so Matt could stand up and present something (ego trip thing). "I'm Matt Steel and will tell you something you don't know" "I have no proof, but I'm Matt Steel".

It seems to me, that in general, the delay is harder to ignite than the propellant, so the fact that you don't see many flights where the delay ignites but not the propellant, it doesn't surprise me... Once the delay ignites, it will get the main fuel burning.. Trust me.. Often called a "chuff". If the delay is burning, the only way the propellant wont catch is if the propellant was made of a non-flammable material! Propellant torches the delay, things usually work.. Delay torches the propellant, things usually work. The ideal is that both light at the same time, and the motor should perform as expected.

Yes, but...

In these motors, the delay is of a different size, and with different insulators, than the HPR motors (don't know if this makes any difference, but who knows?). Also, for these motors, the delay tends to be shorter, anyway (in comparison to the S/M/L delays of the HPR motors).

David Erbas-White

There are times I've put an igniter in, and it has 'stopped'. Only because I knew it wasn't in far enough did I 'twirl' it and get it to slide in farther. The end of the igniter sometimes catches on the gap between grains.

David Erbas-White

I had two igniters fail on me at NSL (these were not Copperheads). In both cases, the igniter burned slightly were the 'junction wire' was, which was about 3/4 inch from the end of the igniter -- but none of the rest of the pyrogen burned. Thus, in both of these igniters, there was a 'burned spot' about 3/4 inch from the end, and no other visible damage. Now, if this is typical of a failed igniter, that means it would be typical for a misfire to NOT light the delay, because the delay will only be lit by the igniter if the whole pyrogen portion burns. Therefore, I would assert that your hypothesis is false (based on the only iginiter failures that I've PERSONALLY seen).

David Erbas-White

Then you try again. I've had small RMS motors where I couldn't get the igniter thru the nozzle due to the pyrogen. Disassemble the motor and rebuild with the igniter in the proper place...

I myself have never seen an igniter stuck >

When building a motor, I'll often take the igniter and bend it at the length it should be when inserted in the motor. That way, if I'm installing it at the pad, I'll know if it makes it "home"

I suspect that the problem is mainly due to the igniter getting stuck before it reaches the top of the core, and the user doesn't realize it's not all the way in.

BTW, the Ellis 29mm I69 instructions currently say to put the igniter in about 4-5 inches (it's a 17" motor). Putting it all the way in tends to cause a cato. The first one I tried was before they started telling people not to put it in all the way, and sure enough it catoed. The second time, I put in according to the new instructions and it flew fine -- but the delay was over-long, which I suspect was due to the placement of the igniter.

David,

I think I saw a case of the delay igniting first, then the motor igniting last month at a launch. It was Bob Kaplow's rocket, so he should be able to chime in with the motor type and what igniter he used. We could see smoke from the igniter, then a little flame, then nothing, then a little more flame, and then it came to life. It wasn't a normal chuff like I've seen before, this was different. I can't tell you how many seconds it was between first smoke and liftoff, but I'd guess 4-5 at a minimum. The ejection charge fired while the thing was under thrust

- or dang close to the end of it, and the rocket hadn't gotten more than a few hundred feet up. Bob did a good job building that rocket, because it didn't rip itself apart.

Bob Wiersbe

David Schultz wrote:

I distrust him. I distrust his arguement.

Jerry

You know Matt too!?!

Okay.

Jerry