News Archive

Hot preheat mixture, E-Beam foam, self-preheat

Hot mixture concept

October 4, 2003 notes

Hot Preheat Mixture, E-Beam Foam, Self-Preheat

We performed a number of interrelated experiments in the last week. A lot of experience was gained, but we don’t have a clear win yet.

A general problem that we have had through all of our work with the mixed monoprop solutions has been ensuring a complete preheat of the catalyst. Passing a calibrated air / propane mixture through the catalyst will bring it up to a fixed temperature, but only if the reaction is started by heating part of the catalyst up to the temperature where propane begins to catalytically burn. We had been doing this by the unreliable method of sticking a torch up the nozzle until we got at least part of it red hot, then turning on the air / propane flow to let it bring the entire pack up to our target temperature of around 1600 – 1800 f.

This worked fine for catalysts that were a single monolithic block, but as we separated the blocks to prevent direct channeling, it got harder and harder to ensure that the gas flow would heat anything but the final block. Some of our recent tests with the three-pass catalyst and post catalyst flame holders are even harder to get started, and we have had a few test runs that behaved poorly due to, we think, incomplete preheats.

Hydrogen / air mixtures will start preheating on a room temperature catalyst, but it will sustain open air combustion over such a wide range that it actually flashes into flame even at the incredibly lean mixture ratio that we run at, which causes some problems.

I theorized that we could raise the temperature of the propane / air mixture we are feeding into the engine in a controlled manner, and that the hot mixture would react on the room temperature catalyst, starting at the top of the engine and working its way down, which would give the best characteristics.

I made a heat-exchanger by wrapping 25 or so feet of -6 aluminum tubing around a 2" pipe to make a fairly tight tubing coil. Sticking a heat gun full on in one end and roughly covering or plugging the other end to force most of the air out between the coils would get the end coil to about 250 degrees with 4 cfm of air flowing through it if you waited long enough.

I put some ½” ID plastic hose on the other end, but I was quite surprised at how much the air flow cooled back down flowing through it. I had expected the larger diameter to cause less heat transfer, and the plastic should have been a better insulator with slower heat transfer, but with 20 or so feet of tube, the 250 degree air at the end of the coil was back down to barely over room temperature at the outlet, even though the end right by the heat exchanger was very hot (and soft). I cut the hose down to just six feet, but there was still a 50 degree temperature drop along the hose. I finally just put the engine right at the end of the heat exchanger for initial testing.

I turned the propane up to our normal 5cfh and left it there for a while, but 250 degree preheated mixture didn't accomplish anything. With the restriction of the small tubing in the heat exchanger, I also found that the air compressor couldn't even hold 6 cfm of flow.

I did some experiments and found that sticking the big torch in the end of the coil at a very low flame (with the other end open) did not seem to hurt the aluminum tubing while gas was flowing. The end coil got up to about 350 degrees and seemed to stabilize. Turning on the propane for a while still did not get anything going in the pack.

I turned up the torch a little bit, and the temperature started climbing again. At about 380 degrees, the pack started making the "rocket engine sound" that we heard when the hydrogen mixtures autoignited in the catalyst pack, signifying actual free-air combustion in the pack instead of catalytic burning. Looking in the pack, there was light coming from deep inside (this was the four-deep catalyst pack), but it wasn't the red-orange we usually see in a preheat, it was white.

I watched it grow for a few seconds, then decided I should check to see if the propane mixture is way out of whack. It wasn't. I turned the propane completely off, but left the air flowing to cool it, and went back to look at it again. The white glow has continued to increase, and the entire bottom of the catalyst pack was slowly bowing out under the gentle air flow (the engine was laying on its side). I was wondering if it is was just clearing the line of the propane, but it keeps going. The pack had opened up enough to see that the inside is actually being consumed -- the air is burning the stainless steel catalyst! I shut the air completely off, and it very slowly starts to cool down. Four catalyst blocks were completely slagged.

Obviously the excess oxygen in the air was combusting with the iron in the stainless steel catalyst base, thermite style, but I puzzled over the exact behavior for a while. I was thinking that we were going to be sunk on using this method to preheat the engines, but after a while, a reasonable explanation occurred.

Our normal air / fuel mixture gives about 1800 - 1900 temperatures. However, that is with room temperature reactants. The preheat was at least 300 F hotter than room temperature (later evidence would point to more like 600 F over room temperature), so with the preheat, the final temperature could easily have been over 2200 F. 316 stainless melts at 2600 F, but the catalyst substrate is a "proprietary alloy", and some combination of events seems to have let it melt and start burning. I wondered if platinum may also be a combustion catalyst for metal burning.

On Tuesday, we set about to characterize things better. We potted a bare-wire thermocouple into a T fitting with ceramic sealant so we could measure the actual gas temperature right before it went into the engine, instead of measuring the surface temperature of the heat-exchanger coil at various places. Russ brought in a propane cooking burner that we could control better than the big torch, and we set the heat exchanger coil up on top of it. We used Cotronics tubular insulation to reduce the heat loss in the hose from the heat exchanger to the engine.

We slowly worked our way up the temperature range. I was surprised when we passed 400 F without anything happening, the external temperature reading I was taking in the initial test was clearly understating the gas temperature. Finally, just when we were about to quit, at 550 F, we got reliable, fast startup. This is the number, after repeated tests, a 500 F mixture won’t start, but a 550 F mixture will start every time, heating the entire catalyst brick to red hot in under two minutes. With this amount of additional heat, we had to lean the mixture out all the way to 3 cfh propane / 5 cfm air to keep the temperature under 1900 F. This seems to be more than just the initial temperature delta, so there is some additional process causing it to heat more efficiently than the cooler mix.

Trying to stabilize the gas temperature at a given level is challenging, because there is a very significant (over a minute) delay between the time an adjusting to the burner flame is made and the time the gas temperature stabilizes at a new level, due to the heat sink nature of all the plumbing. On Saturday, we constructed a heated bath for the heat exchanger coil that used a thermostatically controlled 2000 watt cartridge heater. We picked a silicone oil rated for use at 500 F, but it turned out that we had significant boiling problems even aiming for 400 F temperatures, because the cartridge heater surface was significantly hotter then the fluid temperature. It also took a very long time to heat the entire bath up to temperature, and it didn’t look like the 2kW heating element would be able to hold a sufficiently high temperature once the heat losses through the container were accounted for as well as the gas being heated. We abandoned this development line, and went back to the direct flame heating for the remaining tests of the day. We may look into putting elements directly inside the gas flow in the future, possibly by cannibalizing a really big heat gun to let it take pressurized air instead of drawing it in with a fan.

We also got our nickel foam discs back from Galco with their e-beam deposited platinum coating. The very thin coating hardly changed the appearance of the nickel foam at all, but it was still quite reactive with peroxide. However, it had a couple bad characteristics for us related to the extremely low thermal mass of the foam. If you heated it up with a torch, it cooled down Really Fast after you removed the heat. Propane also did not seem as easy to catalytically burn on it, as it wouldn’t hold red heat with cold propane flowing through it, even though it started a flame on the other side. Heating it with a torch could also actually melt the foam if you held it in place long enough. This was a little surprising, as nickel has a higher melting point than stainless steel, but the tiny foam cells can’t conduct any significant heat away, so they can heat up to almost the full temperature of the torch flame fairly quickly.

The first test we did with the foam was to put 30 foam discs in one of the extensions underneath the chamber with the impingement spray nozzles. The hope was that the fine mist of propellant hitting the high surface area catalyst might start combustion in the top space, rather than tunneling down and progressively quenching the pack. The heated preheater was undergoing the transition back from the silicone oil bath to open air heating, so we tried to preheat it the classic way, torching the bottom before starting cold flow of air / propane. This was difficult to gauge, because it never held enough heat to glow for any length of time, and the propane flowing through it also didn’t bring it to red heat. We waited for it to get fairly hot, then tried various solenoid pulses of propellant, but we didn’t get anything useful out of it.

The other thing we were going to test was sticking a glow plug at the top of the injection engine. We never got a glow plug to do anything useful in a catalyst-free engine, but there was hope that it could be used to ignite the oxygen and methanol vapor that is given off when the propellant hits even a cold catalyst. We tried this first with a double block of the corrugated monolith catalyst under the impingement injectors. The first pulse of the solenoid gave a noticeable bark, even with a cold catalyst, but continuous spray from the injector resulted in a good deal of liquid coming out of the nozzle. Thankfully, Russ checked under the nozzle, and found that the catalyst was actually red hot! This was very exciting. We tried a couple more combinations of foam and monolith catalysts, and we were able to get them apparently (from the bottom, at least) nicely preheated by just pulsing a bit of propellant in, letting it decompose and ignite on the glow plug. We were not able to get sustainable thrust from them, but the spray injector arrangement has never actually worked out for us.

When we got the hot preheat arrangement working again, we set up to do some more tests with the three-pass catalyst. We did one test with the long extended chamber that preheated nicely, but still fell off after 15 seconds of operation. The next test we took the small extension and completely packed it with the platinum coated foam discs to fill the post-three-pass-catalyst. We started the hot preheat, but after a minute or so, Matt noticed a couple hot sparks dropping out of the engine, which is usually a Bad Sign. Looking up with the mirror, the pack had white heat deep inside it, so I shut off the air and propane. Interestingly, it wasn’t making the “rocket engine sound” we usually associate with a burning monolith. That sound must come from gas reaching sonic velocity in the individual monolith pores, which doesn’t happen in foam. When we took it apart, somewhat to my surprise, we found that the three-pass catalyst seemed unharmed, but the foam discs seemed a little beat up. When we pulled the block of pressed-together foam out, it felt strange when you shook it. We peeled it apart, and found that the center had been completely melted out.

This was interesting, because we had solid conditions for this test – the inlet gas temperature was just over 550 F, and the gas composition was 3 cfh propane / 5 cfm air. This combination should not have been able to get even close to the 2651 F melting point of nickel, but the remaining hot oxygen, even diluted by normal atmospheric nitrogen, plus the CO2 resulting from the combustion of the propane, was able to burn the nickel. It is obvious that the platinum coating was far from pinhole-free, because otherwise the platinum would have served as an oxidation barrier, but it seems like platinum with holes in it may actually be a burning catalyst for underlying metals.

We are really hoping that self-preheating with pulses and a glow plug can be made to work, so we don’t need any of the propane ground support equipment at all. That would also give us the ability to do in-flight restarts if necessary, a big benefit for powered landing.

Minor notes:

We have hopes (again) that we will be getting our initial deliver of 90% peroxide soon, so we may finally get back to flight testing vehicles again.

To make some of our testing more accurate, I bought a medium weight scale suitable for measuring our typical propellant runs. Our standard mixture of 8 liters of peroxide and 1.6 liters of methanol came to 23.25 pounds (26.5 – 3.25 lb for the Teflon bottle). I was probably slightly underestimating our Isp by using theoretical density values, which are at a temperature somewhat cooler than our shop temperature.

Unsorted pictures from this week:

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