Well, it’s the end of the month and its been a long time since I last wrote an update. When we started this blog, my goal was to have an update every 2-3 days on various projects, but this past month has been very busy. More importantly, I wanted to wait until after Pierce’s igniter had been succesfully fired before making my post. Now that Pierce has written up his most recent blog entry, I feel its a good time to catch everyone up on what’s been going on at MSS over the past month or so.

Igniter Work:

As of my last report, we were about to test out the new wire-mesh catalyst we had just gotten in, and proceed forward with a more sophisticated injector for the igniter. The test results for the new catalyst was as I had feared–they had insufficient reactivity. The flow rates you want for an igniter are just too high for any reasonably sized catalyst igniter. Now, for a large igniter (or a GOX/GH2 engine), the length required to get ignition might be feasibly, but it was way too long for what we are interested in. We also made a quick modification that day to the igniter to test if increased mixing would help the new catalyst to ignite better (on the theory that maybe the hydrogen rich zones were quenching the flame), but it still wasn’t igniting. It did warm up noticably, and we did have some signs that the catalyst was at least doing something, but it wasn’t good enough.

By the time I had run that test, I had already finished the detail design of the new igniter injector we were going to try out. It would have been a 3d metal printed part with integral injection elements of a unique hybrid sort. The whole thing was a little bit bigger than 1 cubic inch, and most of that was for the inlet ports for the GOX and GH2. We had spent the time working with the engineers at the printing company to iron out all the manufacturing difficulties, and had a design that we were pretty sure would do the job (at least all the analysis looked really good, and the manufacturability looked good too), however after further discussion, we decided to put the catalyst igniter onto the backburner. The problem is that even in the old NASA/Aerojet references where they were able to get reliable ignition from the bead catalyst based systems, they were still having substantial erosion of the beads that would increase the pressure drop through the system over time (or wear out components). This might be overcome by using a stronger, less thermal shock-sensitive subtrate like SiC, but that’s another research project, and we really need a good operational igniter soon for engine testing. The clincher in the decision was that it wasn’t looking like we could get the catalyst igniter debugged and working to the point we needed for engine testing by the time we would need it. There are also some real handling issues with high pressure hydrogen gas that we’re kind of happy to see go away.

Fortunately, about two months ago, when I started hitting some real snags with the 2nd Generation catalytic igniter, we decided to start development of an augmented spark igniter (similar in concept to what XCOR uses in their engines) system. Pierce took the lead on that, and we’ve actually had the igniter ready to go for nearly a month or two now, but we were waiting to test it until the the test trailer was sufficiently ready to go for us to run the tests. We’re still running into some minor snags with that, but are having a lot easier of a time with the spark igniter than I had with my catalytic ignter.

One thing we did discover was that testing our igniter using our rocket engine test stand was a little overkill. So last week I started the design of an igniter test cart. In hindsight, this is something that I wish we had decided to do back in December or January, but that’s life. We happened to pick up an extra data aquisition board last year, so we should be able to make the igniter test stand fully instrumented like the trailer without anywhere near as much hassle. This will be particularly nice when we start the 1000-firing endurance test on the igniter system. We have the cart in, but most of the other components won’t be here till early next week, so I’ll post some pictures at that point. One of the nice things about this test stand is that it will be capable of testing any sort of GOX/Hydrocarbon igniter system independently of the test trailer, which will allow me to do some dabbling on the side with alternative igniter designs without having to tie up our test trailer. The more I’ve looked at the trade-offs, the more I’ve become interested in resonance igniters. There are some groups that have had good success with GOX/Hydrocarbon resonance igniters, and they seem to offer most of the benefits we were hoping to get out of catalyst igniters without most of the worst drawbacks. Once I get some spare time, I’ve got an idea for a simple setup for testing resonance igniter hardware that can be easily used with the igniter cart once it’s finished.

Trailer Work:

The commissioning of the trailer ended up being a lot more time-consuming than we had originally expected. We had a lot of problems with leaks at high pressure, and have had a lovely time fixing them, but we’re done with the IPA side, and most of the way done with the LOX side now. We’ve learned some very valuable lessons about rocket plumbing (mostly learned the hard way) which should pay off when we go to actually lay out the plumbing on our flight vehicles. We made several modifications to the blast frame system so that we could quickly mount/dismount the blast frame between tests (for a reason that will be explained a little bit later). Last week we did a cryogenic test of the trailer with Liquid Nitrogen that went very well. We still want to wire the trailer with video cameras, but that’s the only major item left to be done on the trailer.

Test Facilities:

We’ve only made it up to our remote facility once or twice since our last post. We finished the plumbing for the two new 5,000 gallon water tanks, as well as redoing a lot of the plumbing that connects our water supply (a spring) to the tanks, and the tanks to the water supply down below at the test stand. One of these days I’ll post pictures of the site and the water tanks, to give a better idea of what we’re talking about. The last main item of work we need to finish before we can start hot-fire testing is to finish hooking up the power lines from the new generator to the rest of the equipment we’re running.

“Heat Sink” Engines:

The main thing I have been working on over the past two months has been the design of our first hot-fire engines. Shortly after my last post, while we were discussing the engine design, we realized that it might make sense to test out more than one propellant injector style for our vernier engine. Our verniers need to be able to throttle over a decent range–more than normal for a rocket engine, but a lot less than what is usually considered deep throttling. Pierce had been designing a small swirl injector, but we weren’t sure how well it would do with throttling, so we decided to also develop a pintle injector at the same time. I had done a lot of previous research on pintle injectors while I was still at BYU, so I decided to take a swing at designing one a prototype.

One of the problems with pintle injectors is that there really isn’t a lot of openly available data on how to design one. There are several references which describe the key design parameters (skip distance, pintle diameter to chamber diameter ratio, relative injection velocities, etc) however only one or two of the references even gave general ranges of values typically used. I decided that the best way to approach the problem was using a statistical screening experiment to find out which factors were most important, and which general area of the design space looks most promising. We ran into some manufacturability issues with the pintle injector due to the very small annular gaps required for an injector of the size we’re using. The tolerances required would have been very expensive, however we were able to come up with a clever way of avoiding the problem that should allow for a more stable injector that is much easier to manufacture. We have our first three pintle injector sets in now, and we will be doing some initial cold-flow testing of them this next week (we’re waiting on an o-ring that I had accidentally forgotten to order, as well as some dyes we’ll be using to more easily visualize the flow). We plan on doing some later, more thorough cold-flow testing before we start firing the engines, but for now we just want to make sure the flow is fairly symmetrical and that the flowrates at given pressure drop are close to what we designed for.

Somewhere in the middle of my two or three iterations on the pintle injector design, I discovered that the heat sink engine Pierce had designed had the igniters at the wrong location for a pintle injector (particularly one with as radiused of a head-end dome as mine), so I ended up also taking on the design of the overall engine. After some analysis of the actual workload required to switch injector sets (which would be done fairly frequently in the testing process originally planned), we decided to go with a partially water-cooled chamber with a movable nozzle piece. The water-cooling of the actual chamber walls helps shorten the amount of wasted time between testing, and the movable nozzle piece allows us to figure out the optimal chamber length for our flightweight vernier. The chamber ends up being a bit more expensive and complicated, but the savings in testing time should hopefully make up for it. All of the parts are now either manufactured, being manufactured, or out for quote.

Pierce finished the design on his swirl injector, and we found a clever way to integrate his injector into one of my manifolds so the amount of work to switch in his injector should be just about the same as switching in one of mine. His parts should be back from the machinist by sometime next week.

Anyhow, those are the main highlights of what we’ve been up to over the past two months. We’re getting a lot closer to our first hot-fire of the engines, even though things have taken a lot more time then we expected. However, by the time we actually do get to testing, the system we test should be a lot higher quality, and more
likely to succeed than we had originally planned. Stay tuned for more details.