September 28, 2005 notes:

 

Ready For X-Prize Cup

 

Most of our effort has been focused on getting the vehicle all dialed in for the demo flights at the X-Prize cup in a couple weeks.

 

This particular vehicle is not very well balanced because of the odd tank shapes and relative sizes of the various components. We have used a combination of a few plates of ballast weight and gimbal center offsets to get it to stay centered during the entire flight. The way the position hold code works, any CG offset or gimbal misalignment will translate into a fixed position offset that the vehicle will wind up at when everything settles down. I can always manually steer the vehicle around, but I don’t want to rely on that. With the initial positions and weights, that center point was a few meters away.

 

There are a couple little control system wrinkles that are obvious in retrospect, but took a couple flight tests to realize. When the flying vehicle is offset from the start position, the control logic will cause the vehicle to point back towards where it wants to be. If the vehicle is to the right of the desired spot, the engine plume will be directed to the left to make the nose rotate back to the left to head the vehicle back that way. However, pointing the engine to the left will accelerate the vehicle to the right as it is rotating the vehicle, so it actually does the opposite of what you want in the short term. This has to be accounted for by making sure that the gain from the horizontal velocity is enough smaller than the gains from attitude and rate that the temporary increase doesn’t make it oscillate badly. This behavior is much less noticeable on differentially throttled vehicles or attitude-jet vehicles.

 

Related to that, dialing in the gimbal angle so that it is pointing exactly through the cg on the pad will cause the vehicle to accelerate that direction immediately after liftoff if the CG isn’t straight above the engine, forcing it to swing back to correct. We tried to remove all the ballast weights and use just gimbal offsets, but that caused enough of an initial horizontal shove that we wound up leaving 12 pounds of ballast on one side of the vehicle to let the gimbal stay a bit straighter.

A testing lesson we have learned is that when we support the vehicle on stands that are intended to blow out when the vehicle lifts off, you want the metal “sails” that catch the wind to be at the bottom of the stands, rather than the top. Intuitively, you would think it would be easier to get them to tip over with the sail at the top, but when the rocket plume hits the ground, all of the gas is directed horizontally very close to the ground. We had some tests where the stands basically just scooted sideways when they had a sail on the top, because the wind on the bottom from the deflected plume was enough to equal the extra leverage that the top sail got. Cutting off the top sails and putting them on the bottom lets the stands reliably blow out cleanly from underneath elevated tests.

 

We knew we were begging for it with this vehicle in “open form”, so we weren’t all that surprised when we finally did catch the tether ropes on something sticking out of the vehicle. We tipped the vehicle over and bent a couple legs.

 

We wound up killing two birds with one stone – it had been decided that it would be more crowd pleasing to fly the vehicle with the conical aeroshell, so we changed our mounting for the cone to be robust enough to handle a tether catch on a dropping vehicle, and tethered it from the top of the cone, as we had done in the big peroxide vehicles.

 

Putting the cone on had two unexpected effects. On the positive side, some of the oscillations that I couldn’t seem to make go away just disappeared when the cone was put on. Russ mentioned that the oscillations looked like a parachute without a vent hole, so we are theorizing that the rocket engine plume entrained enough air (you can see the cool mist around the lox tank being sucked down in some videos) that the flow over the messy vehicle and flat blast plate was destabilizing. Adding the smooth aeroshell apparently cleaned that up.

 

On the downside, we were then getting noisy signals on the accelerometers when we tried to take off from the ground, evidently from acoustics inside the cone. We added acoustic foam around the electronics box and changed the liftoff software a bit to get around the problem. We know that bad vibration on the accelerometers can cause problems with the automatic throttle control, so I have also added some emergency override controls to let me nudge the throttling down on top of the automatic control if necessary.

 

A totally unexpected thing we saw was the big effect that the hot plume had on the steel plate we liftoff from. It was an www.xprizenews.org forum member that pointed out that heating the center of a plate causes it to crown up in the middle, which caused our tie-down test chains to go taught last month, and we saw this clearly on the bigger steel plate with some of our liftoff tests. The surprising one was when the vehicle flew a couple feet off to the side and strongly heated the edge of the plate, causing it to bow corners-up into a half-dish shape. This is still a little worrying, and could result in a tip-over. We are pretty confident now that the vehicle is going to stay within a foot of its liftoff point and land with all four legs on the plate, but if we go off course for some reason, it might be an issue.

 

All of the tests have show that bang-bang roll control thrusters really don’t consume much pressurant, even with helium. We were worried about this, but it turned out to not be a big deal.

 

The thermal load on the vehicle base and legs also wasn’t as bad as we expected. We were worried after melting so many things on the test stand, but 30+ second burns at close proximity are much worse than the few seconds near the ground at liftoff and landing that the vehicle sees.

 

So, we should be good to go. The plan is to try to make three flights in an hour at the X-Prize Cup event. The first two will be similar to the hover tests we have been doing, just going somewhat higher, but, pending a final approval, it looks like we are going to try and make the third flight a boosted hop to 100’ or so. This will involve a code path and flight conditions that we haven’t tested on the vehicle yet, because we didn’t want to risk wrecking it before the event. It would be the first thing we would try when we got back, so if everything gets approved, we might as well try it there. The crowd may get to witness a rocket crash…

 

We have done thirteen liftoffs, but these are representative samples:

 

Four early open / elevated tests: (13 megs)

First test had a leaking lox fitting
Second test was a 26 second burn-to-depletion with five gallons of ethanol (half full)
Third test was a short test of adjusted gain to reduce oscillations
Fourth test was a ground liftoff

http://media.armadilloaerospace.com/2005_09_10/allHovers.mpg

 

Just the last ground liftoff without the cone: (3 megs)

http://media.armadilloaerospace.com/2005_09_10/groundLiftoff.mpg

 

A later, not quite dialed in ground liftoff with the cone: (3 megs)

http://media.armadilloaerospace.com/2005_09_24/2005_09_24_hoverTest.mpg

 

Perfect flight: (3 megs)

http://media.armadilloaerospace.com/2005_09_24/2005_09_27_hoverTest.mpg

 

 

Pictures:

http://media.armadilloaerospace.com/2005_09_24/2005_09_24_a.jpg

http://media.armadilloaerospace.com/2005_09_24/2005_09_24_b.jpg

http://media.armadilloaerospace.com/2005_09_24/2005_09_24_c.jpg

http://media.armadilloaerospace.com/2005_09_24/2005_09_24_d.jpg

http://media.armadilloaerospace.com/2005_09_24/2005_09_24_e.jpg

http://media.armadilloaerospace.com/2005_09_24/2005_09_24_h.jpg

http://media.armadilloaerospace.com/2005_09_24/2005_09_24_i.jpg

 

 

We have also done a bit of engine development. The regeneratively cooled throatless engine with film cooling holes ran for 30 seconds without burning through, but there was some aluminum starting to come off just above the film cooling holes, so it probably wouldn’t have run for too much longer. We have made a second one that moves the film cooling holes up another inch, reduces the lox injector count from 20 to 16 to keep the mixture ratio richer, and I carefully reduced the runout on the cooling channels a bit. With 0.030” deep cooling channels, the runout on my mill’s rotary chuck makes a significant difference. We will test this when we get back from the XPC, and I expect it to work. Unfortunately, it isn’t quite big enough to fly the vehicle, so we will have to hog out bar stock instead of just using the conveniently sized pipe and tube that we experiment with.

 

I am drilling the injector pipe for the 5,000 lbf test engine right now, which is the last part to fab for it. This will be an ablative test engine to just check out the injector behavior, but I am getting a gun-drilled tube quoted to make a regen version. We will probably have this engine with us at the XPC, but we won’t fire it until we get back.

 

We decided to just use a manifold of eight small carbon fiber pressure bottles for the big vehicle. They are only $475 each, and the packaging will work out great, stacking them around the edge of the intertank between the two spheres. We will be initially splitting them evenly between the lox and ethanol pressurization systems (completely separated this time to avoid the need for check valves), but they may wind need to add a fifth one to the lox side because of the extra cooling the pressurant will receive.

 

We haven’t quite decided if we are going to put a seat in the next vehicle or not. We don’t expect anyone to actually ride in it, but there would be some testing benefit to having it in a theoretically commercial configuration. The downside would be an extra 4’ of length to the vehicle.

 

http://media.armadilloaerospace.com/2005_09_24/2005_09_24_f.jpg

http://media.armadilloaerospace.com/2005_09_24/2005_09_24_g.jpg