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.


2 comments:

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

http://www.thefintels.com/aer/st131g.jpg

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.