Laser Ignition and APCP ignition temperature.

Hello everyone,

I am currently designing a two stage 29mm minimum diameter two stage rocket for a competition. Looking into alternative methods of igniting the second stage, lasers came into mind. My question is, how hot does APCP need to get in order to ignite?

Thanks in advance

Why not use the tried and true method of an igniter? You might also look into Head End ignition.

https://www.rocketryforum.com/showthread.php?17252-Igniting-engines-with-lasers

I haven't been on the forum in a while, totally forgot about that helpful search function. Thanks for the link!

After reading that thread, lasers are probably not the best. On to the next method!

While lasers are sometimes used for ignition, it is always used to initiate some sensitive primary material that then ignites a secondary pyrogen that has the job of igniting the propellant.

This isn't that far out of an idea. I can think of several ways to make this safe and practical. Probably cheaper (really?) and easier to use electrical methods. This is an interesting thought problem, though.

We did some tests with modified NASA standard initiators with a glass (Quartz?) window. There were no issues igniting anything, APCP, AN props, BKNO3. I do not know the wattage of the laser, it was IR and gave a single pluse with enough energy to damage the fiber optic cable so it could not be reused.
I was not the laser guy, I was the propellant guy.

M

Out of my comfort zone again, but...I'll ask anyway. The one question I have, and just for conversation sake, how would you aim a laser to the -top- of the grain, where composites are typically lit?

Is that why it was found impractical?

McKailas Dad said:

Out of my comfort zone again, but...I'll ask anyway. The one question I have, and just for conversation sake, how would you aim a laser to the -top- of the grain, where composites are typically lit?

Is that why it was found impractical?

Click to expand...

Fiber optic or rigid light pipe. That's the cheap part (but also a consumable one).

From https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19890005798.pdf the bridgewire in the standard NASA initiator has a 300K resistance of 1 ohm and will ignite its pyrotechnic charge in 1.5 ms at a 5 amp constant current with is 0.05 Joules. In a laser ignition system, an optical fiber replaces the bridgewire. In a laser ignition system, a 33 watt IR laser could be focused into a fiberoptic light pipe to deposit 0.05 joules in 1.5 ms into a pyrogen charge, and be equivalent to the bridgewire.

While you could build a laser ignition system, it is likely to cost several hundred dollars at the cheapest for a 50 watt fiber coupled diode laser and a 100 amp constant current power supply. At a minimum, you will need a new fiber every time you ignite a motor, and to fit in a small rocket the laser may be a 1-shot device as the weight and bulk of a heat sink may not fit. When you consider that an e-match cost $1, and can be fired using a cheap $20 timer circuit supplying 2 to 3 amps, it doesn't make much sense to use laser ignition for this application.

Bob

Bob - please, no timers - today's four channel altimeters are MUCH safer for lighting motors....

That's just an example Fred, meant to show that a conventional igniter makes more sense.