Post Space Access Technical Update

The company had its annual migration to Phoenix two weeks ago for Space Access 2009, and we figured it was about time for a technical update.

Changing Directions on XA-0.1B
We have had some ups and downs since our last update, and a recent change in directions. This is a picture of XA-0.1b doing its first four engine test. The engine startup was perfect, but a previously damaged igniter solenoid left the igniter running until it burned through the side of the engine. The end result was one engine that had a finger-sized hole in its side, and another that had taken a decent flame toasting.

We were then faced with a decision. Do we repair the 500lbf engines or do we move on? The engines that were damaged were the original engines developed for XA-0.1 three years ago. These four flight engines had been fired over 500 times between the four of them, had been dropped, one had been landed on when XA-0.1 died, and they were generally mistreated more than a rocket engine ever should. More importantly we knew that the igniter design on the old engines was a weak link that was likely to fail again in the future. In the end, we decided that we had learned about all there was to know about them. And everything we had learned had gone into making the new 750lbf engines easier to deal with, more robust, and more efficient. And frankly, we were simply tired of dealing with an inferior design. So the decision was made to retire the 500lbf engines and to go with a vehicle using one centrally mounted 750lbf engine module instead.

B-750 Undergoes Reconstructive SurgeryOur original plan before the failure was that our next vehicle, 0.1D would be a single-engine vehicle with an all-aluminum frame. However, that new frame hadn’t been started yet, and 0.1B was now available, so we decided to modify it into a single-engine vehicle to get something flying quickly. While most of the group was off at Space Access, Orson (our intern for the spring) and I removed the old engines, cut off the old engine mounts and legs, removed the old T-bottle pressurant systems, and its mount points, and pulled all the old engine-specific cables and boxes off the vehicle. When Dave got back, he designed some new landing legs, a new engine mount interface, and some mounting interfaces for two, much lighter carbon fiber pressurant bottles. The vehicle dropped from a dry weight of around 850lb down to a prediction of around 350lb, giving a theoretical maximum flight duration in the 90-110s range (depending on how much performance we can squeeze out of the system).

We’re still planning on starting X0.1D (Xoid) soon, since the modified 0.1B (B-750) is likely too marginal to bank on for the LLC. But doing B-750 should allow us to start getting data on the engine module design and the single-engine flight controls a lot quicker, instead of waiting a month or two for the new vehicle to be built. In order to get this put together as quickly as possible, some of the tight integration that we were going to do for the engine module has been put off for a “Rev 0″ version that is simplified a bit. This will allow us to test out new features more iteratively, and hopefully avoid some of the issues we ran into in developing the old 500lbf modules.

B-750 and 750-2B Progress
B-750 Coming Back Together (picture by Ben Brockert)I’m strongly tempted to nickname this vehicle the Xom-B Phoenix, as it is quickly rising again from its ashes. After everyone else got back from Space Access, we had a welder friend, Harley Clairday (formerly with Protoflight), come out and do all the welding on the vehicle. It took two days over the past week, but the vehicle frame is now upgraded, and it’s standing on its own legs, with the new carbon fiber bottles mounted. The new engine module is on the vehicle now for hose and plumbing fitup, and the new cold-gas RCS thrusters have been designed, and mounting tabs for them have been fabricated. There’s still a fair deal of wiring and plumbing to do before the vehicle is ready to fly, but we’re still pushing at this date for a flight by the end of the month if possible.

While the vehicle has been coming back together, we’ve also been working on the engine in parallel. We had mocked up the engine structure a few months ago, using plastic, and had just gotten in the real aluminum parts and the gimbal actuators. Everything fits together tightly, and seems to be working the way intended. The new gimbal actuators (Ultramotion bugs like the ones we selected for XA-0.1B) and their mounting interfaces should work a lot better on this vehicle than they did on the old 500lbf engines.

The flight motor for this engine was originally going to be the 750-2 (the second Rev 750lbf design that had a slightly bigger throat and longer L* to correct for some issues we had caught before we fired the 750-1). During the ~80 test firings we did of the 750-1, we found a few more issues, which required doing some modifications to the 750-2 engines before they could be fired.

750-2 Oooh ShinyWe had found a cooling issue on the 750-1, that we had to make a workaround to fix. It worked, and we got off a 121 second firing last month, but we wanted to do things right for the new engine. In order to fix the cooling issue we ran into, we needed to shorten the nozzle a bit, and make some modifications to the coolant channels down near the nozzle end. We realized that these modifications were taking us most of the way to adding a saddle anyhow, so we also made that modification. We got the parts back from the machinist Tuesday afternoon, the parts fit together well, and were assembled and installed on the trailer by Wednesday night.

Yesterday, the guys did a dozen firings on the new engine. We did one longer run that may have reached thermal steady state, but we’ve got another round of tests for tomorrow, and possibly a few for early next week. We’re not out of the woods yet, but the early results look very encouraging. The new throat and nozzle gained us enough thrust to fly with, the longer L* gained us another 2% of c* efficiency, and the bell nozzle gained us at least a few percent of Cf efficiency. We’re not quite to where our goals for this engine are, but they’re definitely good enough to fly with, and there are still a few new injector elements we want to try when we have a chance. The injectors are pretty easy to interchange (it takes about an hour to swap them at the moment), so we can do those improvement tests in parallel with flight testing.

Part of the testing we did on the 750-1 included two days of testing out a dynamic throttle control algorithm. It seemed to work very well, but there are enough changes to the 750-2 that we’re going to run it again with the new engine to make sure our gains and relations are set right before we try to fly with it. We’ll also be testing the new gimbal controller as soon as we get the new gimbals and H-bridges wired up.

Anyhow, Michael asked me to keep this short, so we’ll post some more pictures of the engines and vehicle as they come together. If you’re out in the Mojave area, there’s a fly-in/space-in this Saturday where we’ll have the B-750 on display.