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Armadillo update: Pyrolusite, peroxide, Lox

Published by Elliot on Mon Mar 28, 2005 6:46 am
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chabot imagePyrolusite

Our last experiment for attempting to create a robust, long-lasting mixed-monoprop engine is over with. We built several engines using manganese dioxide (the mineral pyrolusite) instead of our expensive platinum group coated catalysts.

Pyrolusite is used in various water filtration and treatment processes http://www.awifilter.com/media/Specs/Pyroloxspec.htm . AWI was willing to sell us 50 pounds of the pure pyrolusite (without the quartz sand), and it was very cheap, only $3/lb, but it was finer grain than we would have preferred — about 20 mesh.

Initial drop tests with 50% peroxide on the material were surprisingly good, giving reactions visibly very similar to the expensive platinum catalysts, and vastly superior to silver catalysts. The fine granules filtered through even multiple pressed screens fairly readily, so we used a disk of nickel foam backed up by screens to support it. We eventually settled on welding a second support plate on top of the foam and screens to make a strong sandwich without any leak paths around the edges for granules to sneak around.

We assumed that this dense pack would need a large contraction ratio for efficient operation, so we used a small 1.25″ diameter nozzle throat with a 7″ ID catalyst pack.

Our initial test used 2kg of material, giving about a 3″ deep pack. There is a patent for a gas generator using mixed peroxide / fuel and pyrolusite catalyst with a single layer of catalyst that they claim reaches full temperature operation if it is preheated to 600C, so, even though we were dubious, we tried that first. We used a hydrogen / air mixture to preheat the pack, but when we put propellant through it, it did what all of our other single pack designs did — quenched out after briefly running hot.

We then put a spark plug underneath the pack to try it out as a flameholder-only (no hot pack) engine. This had some interesting results. If we started a low flow of peroxide and hit the spark, combustion would instantly start and the exhaust would go clear. However, it couldn’t do this much flow at all. A little more throttle and the exhaust would go straight back to cloudy peroxide. We had two theories for this: either that was all the flow the catalyst could support, or the flame was blowing out at the increased gas flow rate.

We tried adding a crossed-angle flameholder underneath the support plate, but this didn’t seem to help out at all.

We then build an engine with 5kg of catalyst, which did a bit better, but it stll couldn’t handle anywhere near full flow.

For completeness, we also built an engine using pyrolusite as a hot pack below the flameholder. This was able to reach full operating conditions like our other engines, but the hot pack was destroyed. Manganese dioxide melts at a rather low temperature. The dubious patent theorized that it was changing into manganese tetroxide when heated, but ours just melted into various funny shapes.

Peroxide Retrospective

We spent a year and a half working on the mixed monoprop engines. If we were aiming to do something as a stunt, we could get it done with our current engine designs. One bring up test to make sure the catalyst is settled properly, then you can count on several good runs out of an engine. Unfortunately, this doesn’t fit with our development goals, which call for vehicles that can make hundreds of flights, not just a few.

It is tempting to rebuild one more time with mixed monoprop and get some impressive flights off with the current 48” diameter vehicle design, because we have most of the parts, and we know how to make it work. Because I have no confidence that we will ever get the mixed monoprop engines working the way we really want, it would be basically a dead end development path.

It’s time to cut our losses on this path, even though I have over $50,000 worth of stuff at the shop that we probably won’t be able to use. Much of it was bought speculatively when we were still aiming for the X-Prize, but we had a good chance of eventually using most of it if we continued to build vehicles of that design. We are going to stack most of it up in a back room for now. If the 90% peroxide supply situation changes down the rode, some of it may be useful, if not to us, then to someone else.

The deep problem with the mixed monoprop is that we couldn’t find a catalyst that provided repeated long life after 1800 F firing cycles. Coatings just came off from the heat / flow / thermal cycling, and it was all downhill after the first run. I suspect rashig rings of solid platinum would work, but that would be heinously expensive. This experience also makes me somewhat dubious about catalyst pack based 98% peroxide motors for highly reusable vehicles, which have similar operating temperatures. For one-shot biprop vehicles it doesn’t matter, but I would assign some risk to an RLV expecting to make large numbers of flights using 98% peroxide catalyst packs.

90% peroxide with pure silver screen based catalyst packs remains long lasting (if you aren’t regen cooling an engine before passing it through the pack, in which case you should drop to 85%) and easy to use, but there is still a bit of voodoo with the pack assembly and break in. It was a damn shame that we lost our 90% peroxide supplier when X-L Space Systems went out of business. We would have made a lot more progress in the last year. Realistically, we still wouldn’t have won the X-Prize, but we would have been flight testing an X-Prize class vehicle at supersonic speeds.

The LOX plans

All of our propulsion efforts are now LOX centered. We are going to try and get a vehicle in the air as fast as possible, but there are enough new things to do that it will probably take us several months.

We plan on sticking with methanol as the fuel for the time being due to the easier cooling task, and the cleanliness factor. If we get to the point where we care about a few percent more performance, we might switch to something else. We want to keep things as simple as possible. If you spill LOX, you just let it evaporate. If you spill peroxide or methanol, you just hose it down with water. If you spill kerosene, you actually have to clean it up.

The basic vehicle plan is a single gimballed main engine with roll control thrusters.

We were really happy with our jet vanes with the peroxide motors, but the working environment would be much more difficult for a LOX motor. The V2 and Redstone used jet vanes under LOX/alcohol motors, but the vanes did erode, and we want a solution that can last for an arbitrary time.

We considered liquid or gas injection in the nozzle for attitude control, but I am still concerned about having enough control authority when we throttle down at the end of a boost, and LITV worries me at very low chamber pressures with minimal nozzle expansion. We have seen almost the limit case of LITV when we burned through the throat of a motor into the cooling channels, and it didn’t look like all that much side load. Gimballing should give maximum control authority at low engine thrusts. We will be testing an actuated gimbal rig soon.

Using two gimbaled engines to get roll control would require very low thrust engines for the initial test vehicle, which might cause cooling problems, and it would increase the total actuator count a fair amount. This still might be an option later, as I really prefer using main engine thrust for roll control, so it is impossible for thrust imbalances to overwhelm a separate roll control system.

We will initially be using cold gas for roll control to completely decouple it from engine performance, but the expectation is that we will be tapping off of our oxygen vaporizer for roll control gas. We will need to use larger valves for that, because at throttle down and on descent the vaporizer pressure may be as low as 50 psi.

The initial small test-bed vehicle will use off-the-shelf tanks. The target is 100 pounds of propellant and 200 pounds dry weight, which will give us plenty of propellant safety margin for hover tests and early boosted hops.

I bought two new 7 gallon stainless steel tanks from McMaster, part number 9934K33. ASME rated for 200 psi, it weighs 31 pounds, and holds 62 pounds of LOX if you fill it up all the way to the top. You aren’t getting to space on that mass-ratio, but it will be fine for our test vehicle. The tank was pretty dirty inside, so we spent a while cleaning it up for LOX service. Annoyingly, the fittings protrude internally a little bit, so you can’t drain cleaning fluid them completely. We tried getting everything out by drawing a vacuum, but blowing dry nitrogen through it worked a lot faster.

The follow on vehicle will be around 3’ diameter, with a single man cabin section and custom tanks. The first incarnation will probably be made out of 5083-O aluminum so we can drill, cut, and weld all over it with impunity. After the vehicle has been fully developed, we will fabricate another version using a high strength, heat-treated alloy that will cut the structural mass in half. The target is 400 pounds dry mass + 200 pounds payload + 1800 pounds propellant = 100 km ride with powered landing. This would be using a 5,000 lbf engine.

It is worth noting that Black Arrow made it to orbit on a cluster of 8 x 5,000 lbf engines in the first stage.

Lox engine 4

Our basic design is still to use a lox preburner to give gas / liquid mixing in the main chamber, which should allow deep throttling and give good performance and safe ignition characteristics. We can establish and vary the oxygen flow and temperature before starting the main fuel flow. With average oxygen temperatures of 250C or more, injected methanol auto-ignites in the chamber.

Right now we just have a single solenoid controlling the tiny amount of methanol going into the preburner. For deep throttling we are going to have to throttle that flow somewhat. Two different sized solenoids in parallel will give us three different throttle levels, which should be sufficient. After ignition, we can tolerate the gox coming out well below 0C if necessary, although combustion probably goes better at the hotter temperatures.

We cut the preburner off the composite wrapped engine to reuse it, rather than fabricating a new one. We fabricated flanges for the preburner and chamber so we can mix-and-match for testing, since the preburner seems to be working fine, while we are still experimenting with chambers.

The one thing we had to change in the preburner is the small methanol spray nozzle. We had been using these little nozzles http://www.bete.com/products/pages/pj.htm , which are tiny and make a very fine mist, but the little impingement post acts as a flameholder, and it burned right off. We have replaced it with http://www.bete.com/products/pages/um.htm , which uses internal swirl to produce the spray pattern. This is a much taller nozzle, so we had to build an offset chamber just for the spray nozzle on top of the preburner.

The spark plug isn’t burned at all, which is a good sign.

The thing we are testing in the new design is an alternate way of directing the fuel through the channels following the nozzle contour. We filled the channels with wax again, but instead of wrapping the engine with fiberglass and epoxy (which leaked in various places), we put the engine inside a matching tube and filled the entire throat saddle area with epoxy.

I wanted to use the Cotronics flexible epoxy, but my order still hasn’t shown up, so we just used our normal aircraft epoxy at a 4:1 epoxy to hardener instead of the normal 100 : 44 ratio to get a little more flexibility.

I had some concern about the epoxy saddle turning into a hybrid grain when we ran the oxygen preburner, but it doesn’t seem to have a problem. The initial gox that gets pushed back into the fuel channels is quite cold, but if I ran the gas generator for an extended period of time it might heat up enough to be a problem. We may arrange for our nitrogen purge on the fuel side to stay open during the preburner temperature dial-in.

We managed to screw up in two different ways, and melted through the chamber throat.

The first mistake we made was setting the fuel pressure too low compared to the lox pressure. This chamber only had a 1.25” throat, while our previous one had a 2” throat, so chamber pressure was a lot higher. High enough to almost cut out the fuel.

The second mistake was not analyzing data from a short run. When we fired it up it looked a little ratty, but I was just going to let it run until propellant depletion. After running much longer than I expected, the plume veered off to one side, and I shut everything down. If we had stopped and looked at the data after a short run, we would have seen the methanol flow rate plummet as the chamber pressure built, and seen that we were running at about 3x the O:F ratio we intended.

The lesson we are taking from this is that both tanks should be pressurized to the same pressure. If we want to tweak flow rates, we will do it with an orifice instead of tank pressure.

To look at the silver lining, we saw that a burn-through really isn’t that dramatic of a failure.

I took another big hunk of aluminum to machine a second identical chamber, but Russ misread one dimension when boring it out, and we are way too close to the cooling channels. It is probably going to rupture when we put pressure through it, but we are going to go ahead and fire it anyway. Russ and I usually split the machining work on the chambers – I do the outside and mill work, he does the inside and jacket. We can make a chamber a week until we get it right if necessary. My mill is chewing away on a third chamber right now.





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