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Things are going well

August 21, 2008 notes:

August 21, 2008 notes:


Things are going well


… I just can’t talk about much that we are working on. Armadillo has been running at a slight operating profit so far this year, three team members are full time, and everyone is getting paid. However, most of our effort is going to the Rocket Racing League project, and all information about the project is going through them. With the Lunar Lander Challenge coming up, there should be more publicly available information bits soon.




We are prepping the same three vehicles for the Lunar Lander Challenge that we took last year. The only real changes that have been made are to move to the film cooled engines and new startup / purging sequences. The electronics boxes have had pull down resistors added to all the A/D channels for some extra ability to detect wiring faults, but we are pretty happy with how the rest of the systems performed last year.






We had a weld crack on an injector, and we have made a couple changes based on that: We now just use 310 alloy for the injector face that is exposed to combustion, and 304 for everything else. 310 has better high temperature properties, but is more susceptible to weld cracking. This saves money as well, since 310 is many times more expensive. Welding our internal cross drilled rings used to be just a fusion weld, because it was a fairly tricky position, but James now uses a smaller torch head, fine welding rod, and an automated rotary table to allow the parts to use a good bevel and filler material. We saved some money by just cobbling a motorized rotary table together out of some old valve actuators and a manual rotary table:


http://media.armadilloaerospace.com/2008_08_21/rotaryTable.jpg (the piece clamped down is just an aluminum test scrap)


We have poured a pad for tethered hover testing at our new location, but there was a recent FAA re-interpretation of the law that absurdly states that testing under a tether, as we have been doing for over eight years, is now considered a suborbital launch, and requires a permit or waiver just as a free flight would. This is retarded and counterproductive in so many ways, and the entire industry is lashing back over it, but it is an issue we have to deal with in the next couple months.




We are still struggling with the methane engine for NASA. We have fired five different injector combinations, but we still don’t have a combination that can run stably at a high performance level without burning either the chamber or injector face. We are making progress, with the last engine running at only a couple percent roughness and lasting 15 seconds before eroding some of the face, but it definitely isn’t working as well as the alcohol engines, which have less than one percent roughness and run indefinitely.





We have been fairly unhappy for some time with the Parker cryogenic hose couplings that we use for lox due to significant leakage during filling operations, but when we tried using them for LNG (differently keyed, of course), they were completely unacceptable. The LNG isn’t quite as cold, but it leaks a lot easier. We are now using couplings from Carter Cryogenics, which work perfectly so far. We are going the other direction now, using a Carter coupling for lox, and it also seems to work well. The high flow couplings are very expensive, so we probably aren’t going to retrofit all of our vehicles, but future connections will be Carter.




In general, cryogenic fuels seem to be much more troublesome than cryogenic oxidizers. A little leakage around a valve stem from lox isn’t an issue at all, but with methane in can easily catch fire during an engine burn and give you an almost invisible fire to deal with. We lost some sensor wiring after one test due to this. Conditioning the plumbing and injector for startup is similarly more troublesome – if you get hot lox or a gox bubble, the engine generally doesn’t mind running very rich for a couple seconds, but getting a fuel gas bubble can result in damaging lean conditions. You can’t expect both propellants to go through gas / liquid transitions at the same time even if they are both cryo, due to differences in the plumbing and manifolding. In a vertical configuration, flowing some propellant through each side in sequence is probably a good idea, but horizontally that would be unacceptable due to mixed propellant pooling.


The large difference in element sizes seems to make it more difficult to get a good high performance injector pattern. For our split-triplet alcohol injectors, the fuel and oxidizer holes are within 20% of the diameter of each other, but a similar methane injector requires oxidizer holes 50% larger. We are currently working with an O-F-O true triplet design to keep the holes closer to the same size, but that has forced changes to the manifolding and deck thickness to get the exit points close enough together.


Somewhat to our surprise, a methane engine is also more difficult to ignite. We had a couple frightening tests where the igniter was operating with a strong pressure signal, but the engine was just streaming out propellant for the 400 milliseconds between igniter check and first combustion chamber pressure check. We have doubled the mass flow through the igniter to account for this.


The promise of methane is still as a self-pressurized upper stage propellant that burns all the way to vacuum in the tanks. I can’t recommend it for booster applications at the moment, although letting it self pressurize to 200 – 300 psi and avoiding helium in the logistics chain may still be relevant.


We have modified our horizontal test sled to be bolted down to one of our anchor chain boxes to allow us to conduct test firings while the crane truck is off at a remote airport for rocket racing league work.






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