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Armadillo Aerospace: Quad tests, module flights

Published by Sigurd De Keyser on Sun Jul 15, 2007 4:15 pm
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By John Carmack, Armadillo Aerospace; I’ve been crazy busy at Id with WWDC and E3 last month, and as a forewarning, the next six weeks looks equally busy, so updates might be late…

Quad Tests

We got in a bit of trouble last month. After we delivered all of our telemetry data from the permitted free flight in Oklahoma to AST for analysis, they pointed out a couple anomalies: our initial battery voltage was at a level that should have triggered a no-go, and the vehicle had rolled to a point that should have triggered an abort. These were due to the fact that we had replaced the lead-acid batteries with lithium-ion batteries to save weight, and relaxed the roll control dead band to reduce ullage gas depletion. I had written about these in the last couple updates, and we had tested them with tethered tests, but we had not updated our permit with the changes. AST agreed that we had not endangered anyone, but we were in violation of our permit rules. We have agreed on adopting new procedures to ensure that engineering changes get propagated to the AST paperwork, but we have dropped a few rungs in AST’s eyes.

We set out to test a modified injector with reduced film cooling to give more equal propellant utilization and higher Isp. This should have given us 220+ second flight capability while carrying the 25 kg X-Prize Cup payload, but immediately after takeoff it was clear from looking at the exhaust plume that something wasn’t right. After a teardown, we found that the metal Helicoflex seal had a raised, pinched area in it, and that had caused a crack in the graphite chamber when it was pressed into place. This was almost certainly caused by careless handling of the seal on a work table while we were swapping injectors. In the future, we are probably going to just use new seals on each engine. They are fairly expensive at about $130 each, but we shouldn’t be taking the engines apart much more.

We made the decision to not bother testing the hotter engine before XPC, and just go with the flight-tested pattern. We have dropped 20 pounds off the vehicle since we did the 192 second flight, and that wasn’t to complete depletion, so we have 200+ second capability as-is, which should be plenty for the 180 second level two flights. My wife scolds me sometimes about not just sticking with a working solution, and she is usually right.

I finally worked out everything necessary to use the Septentrio attitude sensing GPS system with our electronics box. The antenna / receiver combination is much more sensitive to interference than our previous GPS system, and we weren’t able to hold a fixed integer ambiguity lock for the attitude solution until I completely removed power from the old GPS board that was still inside the box. I was a little surprised to find that level of interference between two receivers. We later found that our wireless video systems also cause interference with the GPS. We are going to have to make some corner reflectors for the video transmitters.

It took me an appallingly long time to get the 3-axis attitude values from the GPS into the correct reference frame for the existing flight control software. It was frustrating having to leave the box and antennas propped up at different angles out in the middle of the parking lot while working on the software inside, but the worst part was that I felt that I had gone through every possible orientations of the quaternion output before I finally just switched over to the Euler angle output and eventually got it working. I’m supposed to be good at all that 3D math. Sigh.

We brought out Texel, our second Quad vehicle, and propagated all the updates we have made to Pixel over. We are in the process of making sure that our team skill sets are fully redundant, so I started training Russ on running the laptop flight control software to fly the vehicles (Joseph is also training Phil on driving and operating the crane truck). We went out for a tethered test, but we managed to get a bit of a bang on startup, which cracked the graphite chamber.

There were three factors that were different from our last dozen or so successful flights: We had reduced the starting pressure from 300 psi to 275 psi, because during the free-flight in Oklahoma we noticed that the engine throttled almost all the way down to the minimum throttle level when transitioning from a hover to a decent at higher altitude. This caused the engine to seem like it almost shut down, so we were trying to bring the throttle position up a bit by operating at a lower pressure. This was also the first time Texel had alcohol in it since last October, so the line to the igniter was completely dry. Finally, the lox pre-chill before engine start has been done manually, and Russ did it a little shorter than I usually do.

This was the last of our pristine graphite chambers, and unfortunately we had a miscommunication with Cessaroni Aerospace about pre-ordering more blanks to have on hand, so we are stuck with a bit of a lead time. We are probably still ten days out from getting a good sized shipment in.

To address each of the issues we believe led to the startup bang (which wasn’t much as hard starts go, not damaging any metal, but the graphite is fragile), we are now doing the following:

We are back to 300 psi minimum pressure. The engine cough doesn’t seem to hurt anything, so we are going to stick with what has been demonstrated to work. We will eventually have to revisit this for in-flight restarts, which will always be at a lower pressure. We may need to have a different idle throttle position based on current tank pressure, but we will work that out post-XPC. We are going to bleed fuel into the igniter early in our flight prep process to make sure it is going to instantly get fuel during the start sequence. I have made the lox pre-chill part of the automated start sequence, so it will be exactly the same duration on every start.

Module Flights

While we don’t have any perfect chambers, we do still have one with an heavily eroded throat that resulted from a pinched o-ring on an earlier engine. We certainly weren’t going to try and do back-to-back 180+ second flights with it, so we decided to finish up the first modular rocket and try and get it in the air.

Ready to fly, the module weighs 455 pounds, over 100 pounds of which is in the legs. If you also take out the electronics box, then the raw tanks, plumbing and engines that you would replicate in a larger configuration are about 320 pounds. I am pretty happy with how the engineering turned out with the modules, essentially everything is right at the base of the vehicle, and you could basically move everything to a different tank set with almost no changes. Each module can fly as an independent rocket right now, but if pure differential throttling with no gimbal movement works out on the four module system, our next generation of modules will have much tighter and lighter packaging. I am also seriously considering a module arrangement with a fixed engine beside the tanks, so we could let the bottom of the spheres just bump into the ground with rubber pads. Landing gear weighs more than you think it should, and keeping the height low helps landing stability and transportation. With a fixed engine design and chem.-etched tanks, we should be able to get to a mass ratio of 5 with the modules.

We headed out to our test site, once again getting a bit lucky with the weather here. I noticed I had one of my three year old son’s toy robots in the back of my SUV, so I decided to tie-wrap it on for the rocket’s maiden voyage. Russ can ride a little bit later. J

Without having to worry about the propellant balance problems that the Quads have, we were able to do four short flights with a single propellant load, just re-pressurizing in between to make up for the long engine purges after shutdowns.

On the first hop, it sort of fell off the stands and went into a severe roll. It seemed to catch it and roll back, but drastically overshot. I suspected that my roll gain calculation might be wrong for very high rates, allowing the overshoot.

On the second hop, we moved to a smoother patch of concrete that wasn’t chewed up from previous flights, and stuck a shim under the short leg to make the vehicle more stable on the stands. It took off straighter, but immediately went into a strong roll. Clearly we had the roll thrusters backwards, even though we checked that at least three times, comparing against the quad.

We switched the roll thrusters for the third flight, and it took off fine. The reason the roll seemed to correct and overshoot on the previous flights was that when it passed 180 degrees of roll, it was interpreted as -180, which caused it to change firing directions for another 180 degrees. It still flew off to the side a bit on liftoff, but that is certainly due to the heavily and asymmetrically eroded chamber nozzle we are using. Stability was great, and I drove the vehicle around under the tether a bit.

We did a ground liftoff and landing for the fourth flight. Everything went fine. We need to get a softer grade of rubber for the landing pads, because even with a lot of holes drilled in them, the total surface area per pound is higher than on the Quads, and it hits fairly jarringly. I have been logging peak accelerometer values for our ground landing flights for a while now, and I think I am finally almost comfortable with using that as a ground-contact signal for automated engine shutoff to reduce the landing bounce and worries about the control inversion when a leg is on the ground.

If we had brought more helium, we would have gone ahead and flown a 90+ second flight today. We have a few things to improve before the next module flight, and I want to move future work with the 3-axis GPS over to the module so the two Quads stay identical with the flight proven configuration, but I was thrilled with how well everything came together. The new start sequence gave ultra-smooth pressure rises on all four flights. I tend to think that the longer, automated lox pre-chill was the main improvement. The higher thrust to weight ratio of the vehicle is very apparent – it really leaps into the air on throttle up. I still think vertical drag racing with two of these will be really damn cool.

Now that we have proven that the module works as designed, we are going to build up the remaining four modules that we have parts on hand for. James is going to work almost full time on the fabrication work, so I wouldn’t be surprised if we have the tank sets bolted together next month. All the plumbing, wiring, and testing will take more time, but I’m pretty sure we will have a four module system done for display at XPC. A tethered test at XPC might be possible, but winning the Lunar Lander Challenges is the main priority. Again, it is a little unfortunate that our development scheduling gets hampered by the fixed scheduling of the LLC at the XPC.

The four module vehicle will probably be flying the Space Diving mission next year. If we don’t wreck it in testing high altitude flight, we are considering doing a staging demo, flying the fifth module off the four module cluster, which would be a solidly 100km+ vehicle. The commercial manned 100km vehicle should be a cluster of six to nine improved modules (higher mass ratio and aspect ratio) in a single stage for full redundancy, with a nice cabin on top.


Matt is finally out from under his own crushing work load, so we have some pictures this month:

We are consolidating all the propellant loading controls in one place. We will be making a metal frame soon.

Here is the current engine design. The tapped holes on the bottom allow a pressure test plate to be bolted on to check for post-valve leaks. It might also be useful for a nozzle extension at some point.

I’m sure most people are appalled at us using rubber bumpers for “landing gear”, but I rather like them.

Both of our electronics boxes are converted over to the lithium batteries and honeycomb box walls. Everything on the bottom is just stuck down with RTV instead of strap clamps. 30 pounds lighter that the originals.




The roll thruster mounts were just the cut off pieces from the quads, after we moved them inboard. We are going to rebuild these to hang the valves down more vertically and use 90 degree fittings for the nozzles.


Feel free to discuss this in the Official Armadillo Aerospace forum.

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