Tuesday, October 20, 2009

2.375" Motor Update

I've been talking with Magnus from AIR recently about this motor and he has convinced me to use erythitol based propellant. I first heard about erythitol on Scott Fintel's site and the biggest attraction seemed to be ease of casting as well as a decent Isp even at relatively low chamber pressures. One of the other attractive things that Magnus has pointed out to me recently is that it functions well in a motor with a high L/D. Click here to see Magnus' latest motor that utilizes nine segments and has an L/D of around 15. One slight drawback is that it's hard to ignite, but from a safety stand point this is actually a good thing.
I'll keep the length of the motor the same for now, but in the future I could conceivably lengthen the casing and add two propellant segments without any additional changes and increase the total impulse by about 36%. The resulting increase in Kn would increase the chamber pressure, but it should still be well within the design limitations of the casing and the closures.
Rather than using a hotter igniter, I have decided to add a simple pyrogen cannister to aid in ignition. The design is a small wooden cup that will have a KN/SU/RIO propellant cast into it which will be ignited by a "standard" BP igniter. For added safety, the igniter will be inserted through a NPT plug fitting that can be screwed into the bulkhead just prior to testing. The cup will have a burst diaphragm attached to help ensure a quick ignition of the pyrogen grain.
Wood may not seem like the best choice, but I used a wooden disk in the original design to insulate the bulkhead and it was reusable a surprising number of times with just a bit of refurbishing each time.


Anonymous said...

Are you going to put a pyrogen on the burning surfaces? I highly recommend it, the only way that your going to achieve instant ignition without any slow startups or progressive burns due to the ends of the grains not igniting


in that you can see the ends of the grains have been painted and the upper 5 grains painted on all surfaces and lower 7 ends only

just somthing to think about

Randy Dormans said...

I've thought about it(:
I'm aware that KNER is more difficult to ignite than other sugar propellants but there are a number of factors that lead to quick ignition and I'm not convinced that "painting" the ends with a pyrogen is the only way to guarantee quick ignition. I'll start with the pyrogen canister using an easy to ignite and fast burning mix of KN/SU/RIO. While not a particularly hot burning mix, it should provide plenty of heat, pressure, and gas flow. I may include a burst diaphragm on the nozzle as well for this first test. If it doesn't work I may end up painting on a pyrogen. Thanks for the info.