August 13, 20, and 24 2002 Meeting Notes

 

Last weekend was Quakecon, so we didn’t have an Armadillo meeting.

 

Lander Hops

 

We finally got the seated lander back in flying trim.  On Tuesday, we did all our plumbing and engine checks, then did a very conservative liftoff.  The computer box was not leveled properly, so it held an angle a few degrees off vertical, and skimmed along the ground.

 

Next Tuesday, we leveled the computer, and finally got the laser altimeter running off of the main vehicle power supply instead of a separate battery, which required adding a new 6v regulator to our power supply board.  We now have +5v for the computer and some sensors, +15v for the Crossbow iniertial unit, +6v for the laser altimeter, and unregulated +12v for some other sensors.

 

We loaded up for a short flight, and it lifted off and set down fine, then I lifted it up a bit higher, but when it came back down the second time, the computer reset, leaving one engine firing , causing it to flip over and land shark a bit.  We broke a gauge, a hose, and a valve, but nothing structural was hurt.  We had perfect telemetry until the second landing, with the laser altimeter data looking good enough to use for auto-throttle.

 

On investigating, one of the makeshift terminal lugs on the power supply board had vibrated loose enough to rattle, which was almost certainly the cause of the problem.  When the power supply board was built, we made do with soldering some crimp logs on the board to act as sideways screw terminals, but this shows that was a Bad Idea.  Russ made some somewhat better terminal connector posts for us out of standoffs, and we routed things to save a couple connectors on the critical path, but we really need to build a brand new power supply board with good screw terminals, and we should take the opportunity to power both of the available +5v / gnd connectors on the computer with separate connectors to give us a level of redundancy against connector problems.

 

We were ready to do another flight on Saturday, probably with the laser altimeter controlling the throttle, but it turned out that we were at the bottom of this drum of peroxide, so we didn’t get a chance.

 

http://media.armadilloaerospace.com/2002_08_24/landerHops.mpg

 

 

Radiative Modifications

 

On Tuesday, we tried our last bet for sealing the TZM chamber with materials on hand, a high temperature silca gasketing material.  It didn’t work much better, but when I re-faced the injector, I found that it was noticeably warped, so we may not have given the gasket material a fair shot.

 

The radiatively cooled engine running at temperature showed up several weaknesses that we are addressing:

 

The brass clamp ring was obviously deforming after hot runs.  We expected this, so we have moved to stainless steel clamp rings.  I started to make one on my Sherline mill, but I managed to strip something on the Z feed while plunging an end mill into a stainless plate, so I had Dyna Turn run off a few for us.  It is going to be very nice when we finally get our big mill installed…

 

The 316 socket head cap screws were suffering.  This wasn’t as expected, but they were definitely stretching, and getting generally screwed up to the point that a couple of them had to be cut off to separate the chamber.  System 22 www.system22.com was able to find me a small lot of A286 alloy fasteners in the size we need (10-32 2”), and get them to me on short notice.  These are rated for use to 1200 F, and retain strength quite a bit higher, so they should be fine until they start glowing bright red.  Interestingly, the matching flange nuts are silver plated to reduce galling.  To go beyond this level of fastener to inconel would involve long lead times and large lots, so we hope these do the job.  At about $8 each, they are already getting a bit expensive.

 

We have yet to get a good seal between the injector and the TZM chamber.  We have probably been facing three separate issues on this:  the silicide coating is not terribly smooth, the bolts and clamp ring were probably easing up as the runs got longer, and by the end of our experiments, the actual injector was warped, so sealing wasn’t possible.  We have gently sanded the top flange a little smoother, which should be ok for the silicide coating, which should have penetrated 0.003” into the metal, as well as built up 0.003” above it.  We may give it another try with the silica gasket material now that we have smoothed it, and have the high temp bolts and clamp ring, but I have also ordered the high tech solution – a few gas pressurized metal O-rings from www.helicoflex.com , which should be here any day now.  They are $50 each for 5, and they are not supposed to be reused.  Interestingly, for sealing onto rougher surfaces, they are silver plated (no peroxide leaking past that!), because silver is the most malleable coating they can put on it.  If higher temperature service is needed, they move to nickel plating, but it halves the roughness that can be tolerated on the surface.

 

The warping of the injector may require us to make it out of stainless, but our next injector is going to have a slightly different design that may let it stay closer to the peroxide temperature instead of the chamber temperature.

 

I also broke down and finally bought a pair of cavitating venturis from Fox Valve www.foxvalve.com so we can see if that brings our runs back to the extreme smoothness that we have seen on some other engines, as opposed to the 7% or se roughness level that we are currently seeing.  Their prices are pretty expensive, around $600 for a fancy tube fitting, but we will finally get to see if they are worthwhile over the simple tuning jets we are currently using.

 

30 Second Regen Run

 

We tested a 2” diameter regeneratively cooled biprop today made out of aluminum, with very good results.  The inside of the chamber was hardcoat anodized to act as a layer of insulation, but it was mostly just plain old aluminum.

 

We started off with full blast water running from the hose through the cooling jacket.  This worked perfectly as expected, because it should certainly cool better than the water flood cooling that we had used on the other aluminum chamber.

 

0.080 peroxide jet

0.040 kerosene jet

250 psi regulated pressure

steady 47 lbf

14 second run.

 

We then cut down the water flow rate until it was measured to be what the peroxide flow would be during operation.  This was doubly conservative, because we chose a slightly lower flow rate, and the water was operating at atmospheric pressure, while the peroxide would be at at least 200 psi, which would hold off any boiling.

 

After eight seconds of burning, the water exhaust turned to steam, and I stopped the kerosene flow.  It quickly cooled back down under the remainder of the monoprop run.

 

We richened the mixture by moving to 0.050 kerosene.  This started boiling after 9 seconds.

 

We richened the mixture again by moving to 0.060 kerosene.  This did not boil the water at all.

 

This is VERY rich, but nice and safe for the time being.

 

We then plumbed it up in actual regenerative cooling mode.  For our first run, we started at a lower 200 psi, which dropped the thrust to 35lbf, but it ran fine the entire run.  In hindsight, lowering the pressure on a regen engine actually increases the heat load into the coolant, because heat transfer scales with the 0.8 power of chamber pressure, so higher pressure causes steeper thermal gradients, but less total heat transfer per unit of peroxide.

 

We then increased the pressure to 250 psi, and made another perfect run at 48 lbf.  Right after the run, we checked the inlet tube with an IR thermometer, and found it to be 223 F.

 

We then loaded two liters of peroxide for a long run, which also went flawlessly for 30 seconds.  We measured our kerosene flow on this run, and found out just how rich we were actually running – we burned 1.7 liters of kerosene with the 2.0 liters of peroxide, for an O:F of 2.2:1, about three times as rich as optimal.  Our true Isp was also quite low, only 151 s.  Going to a leaner jet would get us another 10% or so, but we still need to improve our injector to get the rest of the performance we want.  We will be happy when we have a measured 200 Isp.

 

With regenerative cooling, we are not limited to the lower combustion chamber pressures that we would be with radiative cooling, so we decided to switch out our fiberglass kerosene tank for a tank that could handle higher pressures.  Back when we first started the biprop testing, we were using a nitrous tank for the fuel, but the single ended tank was a big hassle to fill and vent.  We had tapped another port into the bottom of the tank, but we changed over to the fiberglass tank before we ever used it.  Today, we swapped back to this tank, but we didn’t bother to plumb the sightglass in yet.

 

The regulator we had running the constant pressure feed would only get us to 380 psi, and it dropped off during the run, but it worked fine, making 67 lbf for 10 seconds.  However, on termination we had a serious kerosene leak past the fuel solenoid.  We shut off the bottle valve, but it was still leaking, so we depressurized and pulled everything apart.  We drained everything out, and found out that we had never washed this tank out after it had been tapped, so there were lots of aluminum chips throughout the system, which wedged the solenoid open and scratched the bottle valve seat.  We flushed everything out, dumped out the engine, and set things back up for another run.

 

We were just about out of our first five gallons of kerosene, which showed us two more things.  The five gallon pail of kerosene from Home Depot actually had some dirt or other crud at the bottom of it.  We have been working under the notion that we don’t need to worry about contamination on the kerosene side, but we should probably start filtering to make sure there isn’t anything that can plug a solenoid or jet getting through.  The other thing we noticed was that the new kerosene we bought, a different brand, had a noticeably different odor.  Kerosene is such a loosely defined term that we should probably buy a drum of Jet-A or some other more specific spec for our work.

 

We reduced the fuel jet size to 0.050 for the next run to see if that would still run stably with regenerative cooling.

 

We started it up, and…

 

KABOOM!

 

Our very first engine explosion, just as the catalyst pack reached operating temperature.  Everyone was safe on the other side of the concrete wall, watching it on the video monitor.  We recovered all of the pieces, some of which were pretty impressive looking.

 

Obviously the kerosene leak had allowed kerosene to saturate part of the catalyst pack.  We had dumped the engine out, but it had been sitting there on its side, with the venturi injector funneling the kerosene down to the screens for quite a while, and surface tension probably held quite a bit in there.  Phil had even said that we should do a water run to clean out the engine, which might have saved it.  Or it might not have, with water not being a very good kerosene cleaner.  What we need to do if we ever suspect kerosene in the catalyst pack is to take it apart and put it in the ultrasonic cleaner with alcohol, then let it completely dry off.

 

The explosion happened in the middle of the catalyst pack, and rather surprisingly, it didn’t even damage the load cell or regen chamber, because almost all of the force was expended radially.  The test stand was rather mangled, with the center bar pushed down to the trailer, and we got peroxide all over everything, but we cleaned it all up and built a new vertical test stand by the end of the night.  We had been planning on doing this before testing a hypergolic engine, but if we had done it earlier, we probably wouldn’t have had this problem today.

 

Test biprop engines vertically!

 

All things considered, we are quite happy with the results today.  We will continue testing the regen chamber with better injectors, leaner mixtures, and possibly higher pressures, hoping to push the Isp up to 200 or so, and we are laying out what we need to build a regen chamber for the six inch cat packs, which should be good for up to 1000 lbf, which is a valid engine for a single-man space shot vehicle in a differentially throttled cluster of four.

 

Regen engines have a couple minor drawbacks: a pressure drop through the cooling jacket, and potentially trapped peroxide in the jacket when you close the throttle.  We may add a nitrogen purge to the cooling jacket for that reason.  The upsides are that you won’t radiatively cook your vehicle, you can run high pressures, and, probably most significantly, it was dirt cheap to make.  The TZM chamber started with a $1200 piece of bar stock (to make two chambers), required $1200 worth of specialist machining (again, for two chambers), then required a $3000 (for a single engine!) silicide coating, and looks like it will require $100 worth of high end components every time you take it apart and reassemble it.  Each step also took a couple weeks to get done.  The regen chamber required under $100 worth of aluminum and parts from McMaster, which arrived in two days, Russ did the machining in a couple days, and the hard coating was $60, the minimum order, done in two days.

 

We may have some difficulties melting out the throat on the big engine with a simple cooling jacket, which may require going to a slightly more complicated chamber design, but we are going to try the simplest approach first.

 

We need to get some high flow nitrogen regulators before testing a bigger biprop(at the 100 acre site!).  At a minimum, we need to be able to expel about one gallon a second at 250 psi, but being able to expel two gallons a second at 500 psi would also be nice.  That is at least 8cfm of pressurized volume, or 133 cfm of standard pressure volume.  We could guarantee a source pressure of 1500 psi, but we would probably have to swap bottles each run at that pressure.  Anyone have any suggestions?  Anyone know what the max flow rate out of a standard nitrogen bottle valve is? 

 

http://media.armadilloaerospace.com/2002_08_24/regenRuns.mpg

(water cooled, 30 second regen, high pressure, kaboom)