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Hydrogen preheat, Pancake preburner

November 14, 2004 notes

November 14, 2004 notes

 

Sorry about the missed update last week, my time has gotten a whole lot tighter recently with new engine work at Id going into production use, but Armadillo work continues at the same pace.  No, we aren’t quitting because the X-Prize has been won.  We have a bunch of pictures of the recent work, but due to a network change I can’t get at them right now, and if I don’t do the update today, I would be in danger of missing another week.  I’ll try and just dump them up here soon.

 

The custom electronics boards still aren’t here.  Not pursuing the electronics as top priority immediately after the last vehicle crash is shaping up to be a rather large judgment error.  Without a live flight control system, the work on the vehicle doesn’t get the priority it should, and we are reduced to “tinkering” in many cases, which is basically what all the LOX engine work is at this point.

 

Hydrogen Preheat

 

All of the work with the LOX engines does drive back home a lot of the advantages of the mixed-monoprop, so we ran a set of experiments to try to address the warmup issue.  We set up an instrumented hydrogen / air burner to measure the exact flame temperatures produced.  There seemed to be fairly large variables based on exactly how we positioned and oriented the thermocouple at the end of the burner tube, but with it clamped in one position, the numbers were pretty sensible:

 

(hydrogen flow numbers with an air flowmeter, uncorrected)

 

CFM air           CFH hydrogen             temp C

10                    10                                370

10                    12.5                             465

10                    15                                561

10                    20                                800

 

The lower flows couldn’t be ignited directly, but if you ignited it at the richer (still extremely lean) levels, you could dial the hydrogen flow back down and the flame would be sustained.

 

We had sworn off the ceramic beads before after they caused a lot of internal burning of stainless engines, but I wanted to take one more shot with them in a very high contraction ratio form to see if the reduced pressure drop could make it viable.  We built an engine with a 7” catalyst ID contracting to a 1.25” throat diameter, holding 1222 grams of beads in a 2.5” depth.  We choked the maximum propellant flow down to around 5 gpm, which should prevent an initial inrush flood from quenching the entire engine.

 

Just flowing the hydrogen / air mixture over the catalyst would cause the temperature to slowly rise due to catalytic surface burning, then, at a sufficiently high temperature, it would ignite into a real flame.  At flows of 10 CFM air / 20 CFM hydrogen, it took about five minutes to get the exhaust flow under the engine catalyst up to 100 C, but then only five minutes more to get it to 600 C.  It doesn’t take much hydrogen to get the engine heated up, but it does take a lot of compressed air.  A single high pressure cylinder of air is only able to heat a small engine like this, it wouldn’t be reasonable to heat a big engine with bottled air.  A portable compressor or blower would be reasonable ground support equipment if it guaranteed us a smooth, predictable startup.

 

The engine still ran awful like the earlier bead engines – lots of chugging and shaking and messy exhaust, but it did climb up to full operating temperature of over 900 C with only a single pack level.  Afterwards, you could tell by shaking the engine that lots of the beads were pulverized inside.  The prospect of a simple engine that is just a single poured in layer of catalyst is very enticing, but we have yet to make it work like our dual level metal catalyst engines.  The metal ring catalysts are still the only thing we have tried that works reliably at our full combustion temperatures, but there is probably some hard ceramic berl saddle random packing that can hold up to the thermal shocks if we were willing to try more experiments.

 

Pancake Preburner

 

We built and tested a couple new LOX engines in the last two weeks.  Our preburners stuck off the side seem to work pretty well, but the packaging is very awkward.  We made two iterations of preburners that flow radialy in and out between plates, resulting in a preburner the same diameter as the main engine combustion chamber.  The first one we made had the spark plug positioned too far away from the fuel spray, so it wouldn’t light.  We stuck a spark plug in the side, which did get it to light, but it also served as a local flameholder and let the combustion burn through the aluminum wall.

 

The next one moved the spray nozzle up, and changed a couple other configurations things, and it ignited and burned fine.  There was a slight anomaly in the combustion temperatures – we are using Bete PJ-24 spray nozzles, and the first burner showed that to be still a bit too high of fuel flow for the amount of lox we are vaporizing, but the second one had the outlet temperatures much lower, implying that the movement of some of the passage holes wasn’t giving it the same combustion time before choking it off with all the cold lox.

 

We welded together our second regeneratively cooled chamber.  There are 20 cooling channels of 0.125” width by 0.065” depth running the length of the chamber.  Each cooling channel ends with a 0.0325” injection hole directly into the chamber, so there is no top manifold.  The total engine length was 9.8”, the throat was 1.9” diameter, the straight internal chamber was about 7” long by 3.6” ID, for an L* of about 30.  The preburner exit plate has 20 holes towards the outside, one for each fuel injection point, so there are 20 separate gas / liquid impingement points.

 

We have made a new test stand base out of a 3’ by 3’ piece of ¾” thick steel, because we have been digging a pit in the concrete with all the lox engine firings.

 

The engine made a very good looking plume, but almost immediately it started melting out the aluminum nozzle, spraying molten aluminum all over the blast deflector.  We didn’t hardcoat the chamber this time, but we are pretty sure the larger issue was that the new injection arrangement actually got our combustion efficiency out of the toilet, and the heat was just too much.  We measured a solid 186 Isp at only 125 psi chamber pressure (after the throat had been eroded quite a bit), which is about 90% of theoretical maximum at that (rich) O:F ratio.  Our previous runs were grimly low performance – under 130 Isp, which was worse than the mixed monoprop.  This new arrangement should give over 200 Isp with 300 psi tank pressure once we get the oxidizer flow up a bit more and keep the throat from melting out.

 

At the low efficiencies we previously had, the heat could be conducted through the 1” of solid aluminum from the throat to the cooling channel, but when the combustion got going a lot better, it just chewed the throat out until it reached the point that it could cool it.  We ran it again, and it didn’t get noticeably worse.

 

Our next chamber will have an outside contour to follow the nozzle, and I will probably cut down the channel depth near the throat.  We are going to try a fiberglass wrap closeout first, but if that doesn’t work we can either build a saddle, or try a gun-drilled nozzle.

 

 





 






 
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