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Armadillo update: The X-Prize Cup Event

Published by Robin on Tue Oct 11, 2005 10:52 pm
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The event was a mixed bag for us. We intended to do three flights at the 15 second burn time limit, with the possibility of the third one being a boosted hop, but we wound up only getting one flight off.

We weren’t allowed to do a test flight of the vehicle ahead of time because of the airport operating restrictions, but we did a full checkout and dry pressurization to see if we had any problems. Next year I hope this can be addressed, or we might bring a chain and run the vehicle tethered if necessary.

Our flight day preparations went well, and we were ahead of schedule on our checklist when it came time to fly. There were winds up to 35 mph on the day of the show, so we had to hold several times for the wind to drop below 20 mph, which was our listed operating limit. We honestly didn’t know how bad the wind would affect us. We had flown the open framework jet vane test vehicle in winds over 20 mph without incident, but with the conical shell, this vehicle had more side area.

It turned out to be a non-issue — when the winds dropped a bit below 20 mph, we lifted off smoothly, and the attitude and position hold worked just fine. Unfortunately, when the vehicle came down for landing, one leg landed on the steel plate we launched from, the other three legs landed off in the mud, and the vehicle tipped over. The computer was still operating fine, so I purged the pressure and we went to check it out. It looked basically fine, so we tipped it back up and checked out all the actuators.

The in-flight performance of the vehicle had been right on expectations, so we prepared to make the next flight a boosted-hop to 100′ instead of a hover at 20′. We went through all the checklists and got fuel and lox loaded, but when we went to load helium we found that the fall had punctured one of the pressure hoses, so we had to scrub.

This was very disappointing, but I’m glad we were able to at least fly something. We brought the vehicle back in to the crowd and pulled the shell off to let people see the insides of the rocket that had just flown, which turned out to be a pretty big hit. We also had one of our older vehicles (the “flying crayon”), two 5k engines, and a couple boxes of rocket parts at our booth, which people seemed to enjoy.

As for the rest of the event, XCOR fought a bunch of problems behind the scenes, but put on two perfect flights of the EZ-Rocket for the crowd.

Starchaser came all the way from England to have their rocket engine blow up in front of the crowd (at a very safe distance, of course). They had a nice test firing the day before, but some of us had commented that their ignition system was really dubious, and they were rolling dice every time they fired it. They had done a total of seven firings before the event, but it caught up with them in front of the public. I still consider them among the more credible of the ex-X-Prize teams, because they actually build things that fire and fly, as well as just doing the road-show bit, and their current plan-of-record for a big vehicle is fairly pragmatic.

All three of the Tripoli high power rocket launches were canceled due to wind and some technical problems with a launch rail, which was a shame.

All of the usual suspects were there with nice displays and mockups, which gave the crowd a lot to see.

While it wasn’t perfect, overall it seemed to be a success. The crowds were very good, bigger than I really expected (20,000 is being reported), and both the X-Prize Foundation and the local sponsors seemed very pleased with how it went. If it wasn’t an eleven hour drive, I would be very excited about doing launch operations in New Mexico. As it stands, we are still pursuing a direction with private Texas land, but SRS is clearly the leading commercial spaceport effort for projects like ours.

While we basically had our act together pretty well (which seemed to be appreciated by the event people), we had a few useful lessons learned from the effort:

We had set up a sun shield, but the flight control laptop screen was still difficult to see outside at 2:00 pm. I have been considering buying one of the ruggedized laptops with a transflective display screen to address this.

We had poor telemetry signal at only 250′ from the rocket, because the control site was in a slight depression. We addressed this by putting the Esteem unit on top of the tent to give it a little elevation. We should bring a pole mounted antenna in the future.

I was using an unplugged UPS to run the Esteem wireless unit and my laptop, but it died after only an hour and a half of operation, which didn’t seem right considering how heavy it was and how low the current draw was. We plugged it into the generator, but that made it hard to hear the radio communication. We use the generator to run the fuel pump during loading operations, so we either need to get two generators (with a long extension cord to reduce the noise at flight control), use a different fuel loading technique, or get a better battery solution. I am leaning towards just using a big car battery with a voltage gauge and an inverter.

We had another split in the tubing for our peristaltic pump while fueling. We have had this before, and it is easy to fix by just shortening the fueling tube, but we may want to think about changing our loading methods. Now that we are using ethanol instead of methanol we should be easier on the hose plastic, so just replacing it now might be good enough. XCOR uses pressure loading from a fuel drum, which we may do for the bigger vehicle. We tried that earlier with the big peroxide vehicles, but the plastic drums for 50% peroxide bulged too much, and we would up using vacuum loading instead. Ethanol comes in metal drums, so pressure may work out better. For the small vehicles, I still like measuring a precise amount into a carboy and pumping it in.

The metal road plate for launching isn’t turning out to be a great plan. It was a nice idea to consider just pulling it off the back of the trailer anywhere we want, setting the rocket up on it and flying, but asking the rocket to consistently land within 18 inches of its takeoff spot is a bit much, and landing half on / half off is worse than not having it there at all. We asked for a larger concrete pad, but it was too close to show time to get it done. We probably would have been better off just launching from the ground and letting it dig a little hole on liftoff. In any case, we are going to give the next vehicle a much wider base, even at the expense of adding a fair amount of frontal area for drag.

Related to landing, it is becoming clear that a gimballed engine is about the worst attitude control system for powered landing, because once one leg touches the ground, there is a sort of control inversion since the vehicle can’t push the grounded leg away, it can only tip over it. Jet vanes have this problem to a degree, but at least the drag on a tilted vane gives some restoring down force even though the side force isn’t rotating the vehicle. Differential throttling is the best, because it doesn’t generate any side forces at all. We fondly look back at our very first flying vehicle that could be set with one leg up at a 20 degree angle and still lift off straight by bringing the other legs up level first. At this point, gimballing still seems like the right choice for us, but I wouldn’t be shocked if we wound up making a future vehicle with differentially throttled biprop motors. The big peroxide motors didn’t have fast enough reaction times for good control authority, but I think that likely has much to do with the lag in changing the temperature profile of the catalyst packs, which wouldn’t be present on a lox motor.

We have talked about adding contact sensors to the legs to get the engine shut down ASAP to minimize the problems with the control inversion when a leg hits the ground, but I am still nervous about the issues with that.

One of the larger benefits of doing the show was that we worked through our flight systems with AST very carefully. The amount of oversight was completely absurd for 15 second flights in a vehicle that couldn’t possibly reach any uninvolved public, but we have essentially gone through all the issues necessary for getting a full experimental launch license for the bigger vehicle, so when we are ready to do that it should go smoothly (aside from the environmental issues, which are outside their domain). During the analysis, the AST folks made several good suggestions that resulted in genuine improvements to our flight control systems.

I think the X-Prize Foundation is on the right track with the rocket racing plan. I have never been a believer in the original X-Prize Cup concept of “racing” diverse X-Prize class vehicles, but the new rocket racing plan with spec racers of relatively modest performance sounds eminently doable to me, and will provide a good show for the crowds. To complement the rocket planes doing laps, I have been proposing “vertical drag racing” with VTVL vehicles. People sound excited about it, so we’ll see where it goes.

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