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Armadillo Aerospace News: Lox engine

Published by Sigurd De Keyser on Mon Sep 20, 2004 4:46 pm
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chabot imageOur latest stab-in-the-dark on the 7” mixed monoprop engine was to try welding a 1.7” throat nozzle on instead of a 2.2” throat, but it was still rough at about the same throttle level. Our next stab-in-the-dark is trying a hot pack made out of the old “catalyst bale” that we used before we moved to the ring catalyst. We will be testing that on Tuesday. We aren’t giving up on the combination yet, because we have over $50,000 worth of mixed-monoprop vehicle stuff on hand in catalyst, nozzles, support plates, and tanks (the tanks and nose cones to replace the 48” diameter vehicle have arrived).

However, we have been working on the preburner lox / methanol engine, and it has been going easier than expected.

We implemented out new burner scheme with a long enough central pipe for the gox and ethanol to do most of the burning before it gets a chance to mix with the lox flowing around the tube, and it seems to work well. We estimated the tube length necessary by starting with a very long tube, firing the burner, then just cutting it off at the point where it seemed to reach a constant temperature. This should be conservative, because at operating pressure it should combust faster.

The vaporizer would work fine being just a long jump enclosing the burner, but mounting it would be a bit difficult, so we welded it to one of our monoprop tops, which allows us to feed fuel through a tube straight down the centerline. The vaporizer tube is off center, so the hot core doesn’t directly impinge on the fuel pipe. We have three sensor taps on the chamber top: vaporizer temperature, vaporizer pressure, and chamber pressure, which is a tube going down through the vaporizer top and closure plate to sense the chamber pressure. We need to sense like that, because the real combustion chamber will have a cooling jacket, making it difficult to add a pressure tap.

The burner jets are set at 0.100” for gox and 0.050” for ethane, and both regulators are set at 160 psi.

The vaporizer exit hole is 0.9” ID, with a 0.5” OD tube in the middle for the methanol. This is 0.44 square inches for the gox, or the equivalent to a 0.75” diameter hole. We initially had a 0.9” ID tube extending from the vaporizer for about an inch, which would serve as a tight mixing area before exhausting into the main chamber, but that seemed to burn off almost immediately when we added the chamber, and doesn’t seem to be necessary.


The attachment to the load cell and test stand is somewhat rickety, which gave us some vibration noise in the load cell measurements.

There is a lot to hook up for this engine: lox feed, methanol feed, gox feed, ethane feed, nitrogen purge, spark plug, ignition power, and three solenoids.


Our startup procedure is to begin the lox flow to let the feed line and burner get chilled down, then, when the vaporizer thermocouple reaches about 0C, start the burner, then rapidly crank up the lox flow. We can play with the lox flow as long as we want, increasing the flow decreases the vaporizer temperature and increases the vaproizer pressure. This does keep us from having precise run to run vaporizer pressures, because we can’t just open the lox valve the entire way without it dropping below freezing coming out of the vaporizer. Just the vaporizer made about 50 pounds of thrust at 94 psig pressure. We will be going to computer control of the burner oxygen / fuel pressure to allow us to heat more lox.

It does look like the gox coming out of the vaporizer still has a significant variation in temperature, because there was a visible flame in the center even when the thermocouple was only reading 200 C. The swirl around the chamber top probably pushes the cooler / denser gox to the outside. A turbulent static mixer would probably do a great job of evening it out, but it would have to be spaced pretty far away from the burner to keep it from being melted on startup.

We triggered the methanol injection without any chamber below it to watch the spray combustion pattern:


Autoignition was instant in the hot gox.

We then bolted on an uncooled chamber made of thick stainless steel with a 2” diameter throat.

We were running 100 psi methanol through a big solenoid, which flowed about 0.4 pounds per second to atmospheric pressure.

When just the vaporizer was running, there is only a couple psi of pressure in the chamber, but when the methanol was triggered, the chamber pressure jumped to 30 psig and it made about 125 pounds of thrust. The vaporizer pressure was 66 psig, and it was clearly sonic chocked, because that pressure didn’t budge when the methanol was pulsed on or off.

There were a couple bright flashes as the mixing tube and part of the chamber pressure tap burned off and flew out the nozzle, but the pressures were dead smooth through the entire run. The heating behavior was interesting – it took longer than I expected for the chamber to start glowing, sitting there for a few seconds without any visible change, then very rapidly running up to orange hot while I shut off the methanol. The chamber appeared to be almost the exact same temperature from only an inch or two below the flange, all the way to the bottom. This implies that it was getting very rapid combustion, and the chamber didn’t need to be nearly that long.

When you have a sonic choke, you can determine the mass flow through it based only on the pressure, temperature, and gas composition. This page has a chart for various gasses: (note that pressures are absolute, not gauge)

Based on this, we were flowing about 45 pounds of oxygen a minute, or 0.75 pounds a second. With a 70 psi differential, the methanol was probably flowing around 0.33 pounds a second. Ideal lox / methanol ratio is 1.25 : 1, so we were quite lean.

On Saturday, we installed and calibrated a turbine flow meter for the methanol, so we could get better data. I used a 5 – 50 gpm model, so we needed to push the pressure up quite a bit to get it in range. For my future reference: low calibration 1.3v = 6.1 gpm, high calibration 3.6v = 26 gpm.

Saturday firing #1:

150 psi methanol

3.91 gpm methanol, which is outside the calibration range, and possibly not to be trusted.
88 psig / 102 psia vaporizer pressure
68 psig / 84 psia chamber pressure

Not quite critical flow from the vaporizer, the vaporizer pressure changed when the methanol started and stopped. However, it still had perfectly smooth combustion, so decoupling the vaporizer from the combustion chamber may not actually be necessary.

Saturday firing #2:

To increase the vaporizer to chamber pressure ratio, we welded a reducer onto the vaporizer exit, taking it from 0.9” to 0.79” diameter, reducing the gox flow area to 0.3 square inches, or an equivalent diameter of 0.62”. We welded an extension onto the methanol tube so it protruded well below the exit plane of the choke plate to guarantee that the methanol injection wouldn’t have any impact on the vaporizer pressure. We left it about an inch over, because we were curious how it would effect the combustion pattern.



445 psi methanol

8.12 gpm methanol
78 psig / 92 psia vaporizer pressure
66 psig / 80 psia chamber pressure

This was a little bit odd, because the flow was critical from the vaporizer, not changing at all when the chamber pressure pulsed from 2 psi to 66 psi on methanol injection, but the pressure ratio was even lower than the previous test due to the increased methanol going into the chamber. There are some details about thin plate sonic chokes and pressure recovery that may explain the discrepancy.

The heating pattern was much more splotchy than the previous runs, which stands to reason. We want to get the methanol injection point as soon after the sonic throat as possible to give it the highest speed gasses to tear the low velocity stream apart. The next time we open up the engine we will cut the methanol tube off flush with the bottom of the choke plate.

This run was too rich, so we dropped the methanol pressure, which should also drop the vaporizer to chamber pressure ratio.

Saturday firing #3:

300 psi methanol

6.43 gpm methanol @ 6.63 lb / gallon = 0.71 lb / sec
92 psig / 106 psia vaporizer pressure = 45 lb / min = 0.75 lb / sec
73 psig / 87 psia chamber pressure
250 lbf thrust

This also had critical flow from the vaporizer. This is slightly rich, so this area ratio of 0.3” effective diameter at the vaporizer to 2” diameter at the chamber throat does have a little bit of margin, but it would probably be good to give it a little more.

When we shut off the burner at the end of a run, the top chamber frosts over by the time we shut of the lox, but the chamber retains enough heat to still be glowing:


We had the inner section of our cooled chamber hardcoat anodized, so it is now ready to use. We expect that it will be able to handle low pressure runs like this, but probably not much more, because it has a full inch of aluminum between the cooling jacket and the throat. The throat will probably start to melt out when we push the pressures and heat transfers up. We need to make a saddle style milled slot cooling jacket, like the XCOR and SPL designs.


All in all, this seems to be going very quickly. All credit to Charles Pooley for evangelizing the lox preburner concept to me at Space Access this year. We are probably going to try doubling the chamber pressure and getting regenerative cooling working at this size, and possibly building a vehicle of the rough size of our 24” diameter test vehicle using a lox / methanol engine.

We might skip the 4” throat size and go straight to an 8” throat after that, which is a 15,000 – 20,000 lbf engine.

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