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Rocket Racing League, Moving, Methane, LLC

Rocket Racing League

May 9, 2008 notes:


Rocket Racing League




We have been talking with the Rocket Racing League for years about the possibility of doing “vertical drag races” with a few VTVL rockets, but the topic recently came up of having us adapt our propulsion systems for rocket powered airplanes. I said “not interested” a couple times, because we don’t have an aircraft background, and it wasn’t really in line with our VTVL development work, but I eventually came around to the view that it would actually be a good deal for both companies.


The direction we were already heading with the film cooled engines makes all the right trades for RRL – simplicity and robustness at some loss of specific impulse. Making the engines as reliable as possible was our top priority, and knowing that a pilot is going to sit seven feet from the engine gives us extra incentive not to let anything slide.


Our film cooled alcohol engines have a lot more visible plume than XCOR’s highly efficient, regeneratively cooled engines due to the extra fuel on the periphery burning outside the engine, but they still don’t have the impact of a kerosene engine. We weren’t willing to switch to kerosene as a fuel for several reasons, so we did some quick testing of a “plume seeding system”, basically just spraying a liquid into the rocket exhaust to make it more “showy”. It worked spectacularly the very first try – when I triggered the seed solenoid, everyone though the engine had just burned a pound of stainless steel it was so bright. We have brilliant yellow, red, and green solutions already tested. Seeing a couple rockets / rocket planes flying at the same time with this on is going to be really damn cool.


http://media.armadilloaerospace.com/2008_05_12/greenPlume.jpg (during a low-throttle test)


It was awkward at Space Access last month talking with everyone else in the industry and not being able to discuss the RRL negotiations, but I’m glad everything is public now. The “21starts” video from last month was essentially running an RRL burn profile, and I was wondering if anyone was going to put two and two together before the press conference.


We have a couple engines on hand now that can do the job, but we are still making improvements: We are progressively leaning out the engine to bring the fuel consumption down. Equal propellant depletion is important for us, but heat load is going up as we move in on it.


We experimented with a couple coatings from www.jet-hot.com on the stainless steel. We had a chamber coated in their normal hot-rod “sterling extreme” coating, and it held up pretty well – the coating was burned off around the throat, but the entire barrel section stayed like-new shiny. We then tried one of their specialty aerospace coatings on both the injector and the chamber. It was supposed to be good for over twice the temperature of the previous coating, but it actually fared worse. It wasn’t exactly an apples-to-apples comparison, because the injector got leaner between the tests, but the higher temperature coating might be more susceptible to mechanical stripping in the sonic / supersonic flows. Jet-Hot has been a good company to work with, the prices were quite reasonable, and the service was very quick.





(I didn’t get a coating2_after picture)


We have moved from 304 to 310 stainless for the injectors, and our next batch of chambers will also be spun from 310 seamless pipe. The change in alloy makes a real difference – where we used to have a pitted surface on the injector, we now have a completely smooth (but still discolored) surface even after many minutes of burn time. Our current plan is to move forward with the 310 but not the coatings, but we may try adding the coatings back in later.


We have built a “test sled” that is representative of what we will be installing in the airplane, and we have moved to this instead of using Pixel for our test firings now. An added benefit is that it raises the engine another four feet off the ground, so we don’t cook the surface underneath the engine as badly, opening up a lot more options for test firing locations.




Our flight computer now has a display screen to show the current status to a pilot. My first inclination was just to mmap the framebuffer and pretend I was back in the days of DOS, but I decided to try and be a good linux programmer and use ncurses. It took me longer than I expected to get it working properly for displaying on the VGA for an application launched from a telnet session, and the performance was very bad. I wound up writing directly to the terminal device myself, spitting out all the escape sequences manually, but it was still quite appallingly slow. I have it working acceptably by only updating the various display items in a scanning fashion to avoid slowing it down on any individual frame, but I should have just followed my first thought and gone with a direct memory mapping. It turns out that having a little display screen on the flight computer is quite convenient for the pad crew, giving them a much more accurate pressure reading than the manual gauges, as well as all the valve throttle positions and other information.




We are just about ready to test a new igniter that has the solenoids welded directly on, eliminating a couple of the critical chamber-to-atmospheric potential leak points.


We saw a couple component failures this month that are worth mentioning:

Our fuel loading manifold is made of PVC valves and fittings, since it only operates at 70 psi.  We saw a crack on one of the elbows a couple weeks ago, but it had gotten a lot worse, and needed to be replaced. While it is low pressure, there are hoses hanging off of it on a moving vehicle, and it can kick around quite a bit when liquid loading / unloading is completed, so fatigue was probably an issue.  There was about eight months of service on it, so we could probably just replace it with identical components, but it was worth the extra couple hundred dollars to build the entire thing out of brass valves and fittings.

The second failure we saw was a cracked flare on a high pressure braided line.  We were just going about our business when we heard a loud hissing sound from the back of the truck.  All by itself, with no provocation, the flare on one of the lines coming from a helium six pack just decided to crack and start leaking.  This was a -6 braided stainless over Teflon line with brass fittings from McMaster, operating at close to the rated 2500 psi pressure.  This particular hose was several years old.  Over tightening may have been an issue, so I am going to look into getting a convenient open-end torque wrench.  It was also a brass hose fitting going to a stainless fitting, which might conceivably have something to do with it, but that would be unfortunate, because we do that often.  If that is a known problem, we can go back to using stainless hose fittings, which isn't a big expense at the -4 and -6 sizes that we now use (it is a big difference on -16 hardware).



It has been a year and a half since we were able to perform engine and vehicle tests at our shop due to the increased size of our vehicles and some (not unjustified) complaints that arose from our activities. A long drive to a remote test site, without all our fabrication equipment present, has certainly reduced our efficiency.


We were negotiating a hangar lease at Hensley Field / Dallas Naval Air Station for a while, but the city changed their plans for the facility, and sort of pulled the rug out from underneath us. Fortunately, a large hangar at the Cado Mills airport outside Dallas just opened up as a sailplane company closed down, and we took it. It is much more space than we actually need right now, but it should serve us well for the foreseeable future. You could fit a Falcon 9 in the big hangar with no problems. This big hangar is only part of the facility (the owner still needs to move the last few planes out):



Right now things kind of suck, with most of our stuff in boxes at the airfield, but our machine tools and welders still at the old location, but by next month we should be more productive than ever. We have conducted our first couple engine firings at the new location, but we are going to have to pour some concrete pads for VTVL testing. We are actively working to build ties in the local community to try to fend off the eventual noise concerns. The first time we fired an engine at dusk, the local police said the 911 switchboard “lit up like a Christmas tree”, with people concerned that something had exploded at the airfield. We participated in a local community festival the following week to meet a lot of the neighbors and get the word out about our activities. Our crane truck loaded up with Pixel and a rocket engine on the back won third prize in the parade. J



Methane Work


We have a big truck full of LNG at our new site, ready for us to start burning. We adapted up our big run tanks for some long methane burns with a modified injector, but we ran into a few issues.


As XCOR warned us, methane is a lot more “leaky” than alcohol, and we burned some wiring when a small gas leak from a fitting ignited. Welding more things together is the final solution for this, but we are going to have to be careful with our connections during development.


The second run was shut off automatically when the chamber pressure transducer reported that the chamber had dropped to atmospheric pressure. It turns out that the stainless steel tube to the transducer had actually broken. We have known for a while that we need a better transducer mounting solution, but it isn’t clear if the methane engine was suffering from more vibration than the alcohol engines. This was the final impetus needed to get me to mill a nice mount for all of our pressure transducers (lox tank, lox manifold, fuel tank, fuel manifold, igniter, chamber):



Another worrisome issue was that after a burn of only a few seconds before the abort, the entire stainless chamber blued, while the barrel section usually remains mostly uncolored with alcohol. I suspect the methane plumbing needs to be chilled more thoroughly, and we are getting a very lean initial mixture. I am still leery about dumping large quantities of methane to the atmosphere around or testing sites, we probably need to set up some long remote vent lines.


We will get more tests done next month.



Lunar Lander Challenge


There was some confusion about our participation in the 2008 LLC now that we are committed to the RRL work, but we are absolutely planning on being there again this year.


The only thing we are going to change from our 2007 effort is to move the engine to our new film cooled ones (that don’t hard start), and move the module payload weights to the top of the vehicle to increase control authority with nearly empty tanks. We will do a bunch of hover tests, and a practice run in Oklahoma before the event, but that will be about it. This would have been a big issue holding us back from more aggressive tests, but the RRL and NASA work is going to keep us plenty busy in the meantime, so it looks like it will work out fine. Of course, it would be our preference to just go do the LLC flights next month, rather than waiting for October…




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