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Flameholders

Flameholders

February 22, 2004 notes

 

Flameholders

 

We had a couple more things to cut-and-try on the vehicle engines to see if we could get them to behave better, but I decided to step back and think about the problem a little more.

 

Our working theory about the operation of our mixed-monoprop engines is that the “cold pack” decomposes some of the 50% peroxide and vaporizes the methonal, which gives a somewhat combustible atmosphere that can be ignited by the glow plug and stabilized behind the perforated metal plate.  With a stable flame in the open space between the cold pack and the hot pack, the remaining liquid (water and undecomposed hydrogen peroxide) will be vaporized and heated enough that when it enters the “hot pack” it decomposed and further combusts without progressively quenching the pack as all of our early attempts would do, even if we had preheated the pack to very high temperatures.  When the hot pack has been preheated enough, it radiates quite a bit of heat back up to the perforated metal flameholder, helping to stabilize the flame.  Once it gets fully established, the engine runs very stably, but the problem for us has been getting the engine up to its operating steady state.

 

If our theory is correct, it should be possible to operate an engine without a hot pack at all and still get something vastly different than just a foam of decomposing 50% peroxide.  It probably isn’t possible to thermally decompose all the peroxide with just flameholding, because the temperatures really aren’t that high, but it should still be cooking pretty good.  Our combinations of perforated metal flameholders, glow plug, and the various spacing gaps have changed from engine to engine, and we are probably only hitting on good combinations occasionally out of luck.

 

We decided to do some more observable tests by running a cold catalyst pack with various experimental flameholders under it, exiting directly to the air without a nozzle.  Our first flameholder was made by cutting a length of stainless pipe lengthwise and welding it together in a cross of U shapes, with four 1/8” holes drilled in the top to let some mix directly in.  It was expected that the flow past the tubes would also re-circulate somewhat back into the tubes.  The glow plug was extended well into the tube, shielded from the main flow.  Several tests with this showed nothing but the expected cloud of partially decomposed propellant, but we noticed that the glow plug was getting cooled off when the propellant flowed, most likely from the 1/8” hole near it, so Russ welded that one closed.  This was a great thing to see with the camera zoomed way in.  The next test showed an extremely spectacular plume of flame coming out of the engine, clearly held on the cross.  This was the night and day difference that we expected to see with an operating flameholder.

 

We repeated the test, but had trouble getting it started again.  We were finally able to, but it certainly wasn’t a sure bet.  We then added another engine section so the flameholder was buried up away from any possible atmospheric oxygen, and we couldn’t get it to light at all.  We tried firing horizontally and with a nozzle attached, but it was still very spotty.  We theorized that it might need some small flow from the back side of the flameholder, so we drilled a 1/16” hole somewhat past the tip of the glow plug.  This seemed to light reliably, but when it had a nozzle on, it chuffed pretty badly.

 

We went ahead and tried putting a hot pack and nozzle underneath the flameholder.  The one we had on hand was the partially melted bead catalyst, which turned out to be a mistake.  With the glow plug hot, I just turned on the propellant flow without any warming pulses.  The engine chuffed and misbehaved fairly badly, but it did heat up to red hot operating temperature all by itself, which was an absolute first for us!  When we took it apart, we found the beads had melted the rest of the top retaining plate and generally made a mess of things.  We are swearing off experimenting with the ceramic beads, because they have such a tendency to cause the metal to start burning.

 

On Saturday we first tried to just repeat the open pack tests, but we again had difficulties.  We had accidentally bent the flameholder tubes a bit when we were working on it, and it might have disturbed a sensitive balance.  We decided that the glow plug just wasn’t doing a reliable job of ignition, so we switched over to a spark based system.  We had some spark gear that we got with the XCOR igniter we had been experimenting with (yes, we could stick the torch igniter on the side to damn sure start a flame front, but it would triples the engine complexity and fluid count, so we aren’t about to use it on the vehicle), and on a tip from aRocket I had picked up a few additional types of spark plugs from www.sparkplugs.com .  We worked up a good driver system and mounted a spark plug in another leg of the flameholder.  On multiple open tests, this seemed to light reliably.

 

We made a new hot pack with the last of the ring catalyst we had, and test fired the engine.  It didn’t seem to want to catch for several seconds, then it abruptly lit and started running clean, but still chugging badly.  The flameholder seems to be very pressure sensitive.

 

We tried an alternate flameholder design with a circular milled channel and single crossbar.  I had milled this out of brass because I loathe milling thick stainless steel.  It might melt, but it might be cooled sufficiently by the propellant flowing past it.  If a brass design works out, we can have DynaTurn or EnTek mill a bunch in stainless for us.  We had great difficulty getting it to light at all, although we did get it to catch once.  It might be plug / vent positioning, or two other possible theories:  This ring is on the outside edge of the chamber, so propellant would only be flowing past one side, halving the recirculation.  The brazed connection of the ring was probably letting some fluid flow down the outside, possibly drowning the spark plug.

 

We obviously have some development to do here, but it looks like it is going to be possible to get our engines to just start almost immediately, with none of the warmup hassles we have been dealing with.  This is a really big deal for us, removing the only disadvantage of our propulsion system.

 

In other work, the new motor drive board has been potted and installed in the electronics board, which is now as complete as we need to make it.  We could run 4, 5, 8, or 9 engine configurations, giving us a lot of flexibility if we miss performance targets or want to add redundancy.  The new bulkhead is firmly bonded into the cabin-at-the-bottom cylinder, and it should be our strongest work by far.

 

http://media.armadilloaerospace.com/2004_02_22/flameholderTests.mpg

 

http://media.armadilloaerospace.com/2004_02_22/cross.jpg

http://media.armadilloaerospace.com/2004_02_22/brassRing.jpg

http://media.armadilloaerospace.com/2004_02_22/brazedRing.jpg

http://media.armadilloaerospace.com/2004_02_22/sparkPlugs.jpg

http://media.armadilloaerospace.com/2004_02_22/pottedBoard.jpg

http://media.armadilloaerospace.com/2004_02_22/installedBulkhead.jpg

 

 

 

 

 

 

 

 

 





 






 
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