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Two 180 second flight tests, Module work

Three test sets to report on

August 13, 2007 notes:


Two 180 Second Flight Tests


We had three flight test outings this month, with somewhat mixed results.

We are in a little bit of an unfortunate situation. Cessaroni Aerospace, the company that has been machining our graphite chambers, informed us that their preferred grade of graphite for this application will not be available for four months due to the supplier being obligated to provide their complete production capacity to a major customer, leaving nothing available for the smaller customers. We are going to have to live with their second choice supplier for the rest of this year. They consider the exact supplier and grade of graphite a trade secret, so we don’t know the details, but the machining characteristics are definitely different on the new graphite, and we have suffered two failures with it.


We had one engine with the old graphite that had made ten consecutive long duration flights on Pixel without a problem.  We moved it to Texel, and cracked the graphite chamber on startup due to factors discussed and addressed last month. We replaced the chamber with an old, eroded one, but I didn’t mention in last month’s update that when we put the chamber into the engine, we found that it sat at a slight angle, and we had to wedge a small spacer on the side to get the injector in.  We couldn't recall if it had been like that originally, and the tilt was due to some weld distortion, or if it had been a result of either all the thermal cycling on the engine, or the last start that cracked the chamber. We went ahead and used it for the single module flight tests, and we didn't have a problem, but that was a light vehicle, each hop was less than 20 seconds, and it was the old style graphite.

We received a new chamber with the alternate grade of graphite, and installed that in the same engine. It still sat at a bit of an angle. Our next test was to attempt back-to-back 180+ second flights under tether with Pixel.  This was a full load, with a 2200 lb liftoff weight, so the engine operated at a high chamber pressure.  The startup was perfectly smooth, and the vehicle was performing flawlessly until 23 seconds into the flight, when the vehicle just sort of tilted over in place and triggered the automatic abort.

The chamber had cracked and ejected only half of the nozzle expansion cone, which resulted in a side force at the bottom of the engine in the couple hundred pound range.  The telemetry was interesting to look at.  When it happened, the gimbals started to get pushed over by the side force, but they ramped up to full force and stopped the gimbal motion, and even began to slowly push it into a correcting direction.  It wasn't enough to catch it, so the vehicle hit the tilt shutdown point.  As the valves closed and the chamber pressure dropped, the gimbal actuators were able to push the engine around again.

The sobering thing is that this is the first engine failure we have had this year that would have resulted in a loss-of-vehicle accident in a free flight.  All of the other times we have had a graphite chamber crack, the engine was perfectly happy to continue operating at a reduced efficiency with fuel leaking into the chamber, and we could safely land it.  This time, the abrupt failure would have happened at 60 meters altitude in a free flight, and it would have resulted in a catastrophic crash.

This could be taken as an argument for extremely strong gimbal actuators, but we already have 600 lbf actuators on there (they have a six inch lever arm from the engine midpoint, but the engine bottom is about 26 inches down from the pivot point), and I don't want to revisit that development decision.  If we had enough gimbal force we might, maybe, have been able to keep the vehicle stable, but I don't think it is a sure thing.

The obvious lesson is that if an engine doesn't seem to fit together right, it probably has something wrong with it.  We checked one of our other cooling jackets, and the engine wasn't tilted, so we do think the hard start probably bent this particular chamber.


As an extra corrective measure, instead of welding the fuel manifold at the bottom of the tube, we now use an o-ring and spiral retaining ring, just like we do for the injector side. Since we cut the ring grooves and trim the tube on the mill, this gives us an extremely accurate chamber length, and perfect flatness. The warping may have been due to the hard start instead of welding, but we had been considering going this route anyway, and it seemed like a good time to make the move.


We got a second chamber with the new graphite in, and fit it up with the new engine tube. It sat perfectly flat now, but it also revealed that the injector top and bottom faces weren’t exactly square. Again, we weren’t sure if it was due to original manufacturing, or part of the last hard start. We faced it off, and got the motor put back together with everything now nice and square.


There is one other problem that we have been trying to address: the igniter exit nozzles in the injector have been eroding since we moved to the four-way angled exits and the pre-drilled metering orifices in the igniter mount. I can see it happening in the telemetry as the igniter pressure drops significantly after the first second on the first fire, and somewhat less on each subsequent fire. It isn’t clear if it just chews it out until it reaches an igniter pressure that it can live at, or if it will keep going until the igniter pressure is too low for our ignition interlocks. We know we can make four runs in a row with it, so it isn’t absolutely critical, but we want to get it fixed. We have been welding it back up each time we take the engine apart, but we decided to try threading the injector for a stainless steel insert for this test.


We again set out to try to do back-to-back 180 second flights with Pixel. Everything started out fine, but at about 30 seconds something was ejected from the engine. The plume was a bit more orange than usual, so we were quite surprised to see that it still made a complete 182 second flight, and had pretty good propellant reserves remaining. Initial inspection showed that we had ejected about a quarter of the nozzle expansion cone. Looking at the telemetry, we could clearly see that this time the gimbal actuators were able to overcome the side force and hold it as necessary to compensate. The stainless steel igniter nozzle insert was completely gone, leaving the injector face a bit chewed up. We were able to find the slagged piece of igniter on the ground, rather to my surprise.


It is possible that the remains of the stainless igniter plug might have caused a crack in the chamber that led to the nozzle fracture, but we currently believe that this grade of graphite just isn’t as strong. To compensate, we are going back to using a solid block of graphite without a step to submerge the fuel manifold. This is stronger for two reasons, it keeps all of the graphite under compression from the cooling jacket, and it avoids the stress riser of the step. We were considering doing this anyway, but this was the clincher. To keep our same engine length, this does cut about 7” off of our L*, but we are still quite generous at around 64”. I am a little worried about overheating the fuel manifold at high altitudes where the plume will expand to directly contact it, but we will worry about that later.

We decided to go ahead and build up a new injector, rather than repairing the existing one with the failed stainless igniter plug experiment. To help the igniter problem, we reduced the lox jetting significantly, which should give both a lower chamber pressure and a cooler flame. We were looking a lot closer at flatness this time, and we found that very few of our machined engine components were completely flat, even before welding. We trued things up as necessary, but I am probably going to start using ground blanks exclusively for engine components that need to be completely flat. Just machining both sides doesn’t necessarily solve it, because clamping down a warped plate lets it bend back to its original shape after you machine it and relieve the clamping pressure. We are going to go back and investigate some other injector geometries post XPC that may also reduce the number of welds in our engines.


For the third time, we loaded up for a back-to-back 180 second test. The first 180+ second flight went fine, and the chamber did not crack, but the igniter was burned out worse than before. Looking at the telemetry, it was surprising to see that the chamber pressure was nearly the same, even though we had cut the lox (actually gox during startup) orifice size in half. It clearly started cutting through the igniter throats almost immediately, so it seems we went from very lean to close to stoichemetric in the igniter. Going back and looking at our earlier tests before we changed to the mill-machined igniter orifices, I saw that our igniter pressures were less than half of what they are now, which is a significant factor for the melting. We are adjusting the igniter throats to bring the igniter pressure back down, and changing the machining to reduce the length of the throats.


We are going to repair this igniter, and move this entire engine over to Texel to use for the 90 second level 1 runs, then build up another one for Pixel and the 180 second runs. With ten more weekends between now and the X-Prize Cup, I am pretty confident we will be in good shape.



Module Work


James has been working hard on fabricating the rest of the modules, so we can hopefully have a full four module vehicle at least for display at XPC. All of the fuel tanks were hydrotested this week, but we got a surprise when one of them failed at 590 psi. A good tank bursts at over 800 psi, and we hydro each of them to 600 psi. James marks all the imperfections he finds in the hemispheres as he is going through the cleaning / fitup / welding procedures, and this one let go right where he had marked a flat spot on the tank and subsequent but weld misalignment. This was the worst of the issues on the tanks, with about 0.080” mismatch over five inches or so, and it took over 30% of the strength out of the tank. We have stepped up our investigation into options to reduce the critical nature of the tank welding. The tanks aren’t the dominant cost in a module, so it is worth it for us to explore some options to get better tolerances and a thickened weld land, even if it costs a decent chunk more.


We worked out all the nosecone attachment hardware for the module, and managed to get it fabricated and installed in time to take the module and one of the quads down to http://www.quakecon.org/ this year for display. It was fun to have both of my (John Carmack) endeavors present in one place, and a lot of the quakecon crowd was pretty well informed about Armadillo. James, Joseph, and Phil spent a lot of time talking with the gaming crowd about the rockets. The younger kids especially seemed to get a kick out of it.


We fabricated a new, more robust roll thruster mount that hangs them completely inside the legs for easier handling.


We have purchased our own crane truck, which we will be picking up in two weeks. This is going to make our flight testing go a lot smoother, because we can make semi-permanent mounts on the bed for all the dewars, tanks, and vehicles, and we can make dedicated stowage areas for all our hoses and toolboxes. All signs point to us continuing to make two to three flight test outings a month, and more will start happening at the Oklahoma Spaceport and eventually Spaceport America as time goes on.
























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