March 1, 2006 notes

 

Attempted hover

 

We tried to hover the new vehicle, but it didn’t work out. We got the vehicle up on the stands, and everything went smoothly with the flight checklist, but when I throttled up the vehicle didn’t lift off. I eventually kicked the gimbal around with the joystick to knock it off the stands to see how it behaved swinging, but it never built up any more thrust. From all indications, it would have flown properly if it had more chamber pressure.

 

http://media.armadilloaerospace.com/2006_03_01/onStands.jpg

 

This engine had mode over 1100 lbf on the test stand previously, but when we tested it again it was down to 800 lbf. However, even at 800, it should have picked the vehicle up.

 

An additional loss of thrust is probably explained by higher lox temperature / lower density due to greater heat transfer into shallowly loaded large tanks versus highly filled small spheres on the test stand, and the longer time between closing off the tanks and firing the engine on the elevated vehicle versus the test stand. We are considering adding insulation to the lox tank to help this, but it shouldn’t be necessary if we aren’t chasing absolute maximum performance. We got some spray polyurethane foam to evaluate, but we might be better off with something like mineral wool that would be non-flamable.

 

For testing this vehicle I purchased a set of race car scales, which lets us tell exactly how much lox is loading and boiling off during the filling process. It also let us know that the previous weight we had for the vehicle was off a bit due to maxing out the old floor scales we had – the actual vehicle dry weight is 600 pounds even. At full load it will hold 1500 pounds of propellant, for a mass ratio of 3.5.

 

Using real bearings and shafts for the gimbal hinge points reduced the slop in the system considerably. Phil thinks that much of the remaining slop in the actuator can also be corrected by shimming some internal gears, but it is good enough for now.

 

New engine

 

The engine that was on the vehicle (4” ID regen chamber with radial injector, 2.75” throat graphite nozzle) is definitely damaged internally. We haven’t cut it apart yet, but it appears to have a failed weld. This engine had the top popped off with the old igniter and repaired, as well as a couple configuration changes, so we aren’t shocked that it isn’t healthy.

 

I had started milling a 6” ID regen chamber for a new motor, but I wound up messing up the first one, and due to some design considerations for what we thought the centennial challenge prize was going to require, we decided to change designs and go with an uncooled chamber again.

 

The graphite nozzles have been holding up impressively well. The zirconium coatings we tried did not last long, but a silicon carbide based coating seem to do much better. Our current testing is with a 12” long 6” ID / 8” OD phenolic tube over a 3” throat graphite nozzle, but we are having an experimental chamber fabricated that will be solid graphite with a carbon fiber reinforcement layer. We have reasonable expectations that such a chamber will be able to run for many minutes of continuous firing.

 

We wanted to experiment with axial propellant injection now that the head ends are getting to decent proportions. Radial injection was nice for the smaller engines, but the fabrication is less repeatable with the runout on tubing (and in my mill fourth axis chuck – one of these days I will fix that…), and we have a definite issue with stacking radial lox injection points over the radial fuel injection points as the engines scale up. Early French engines and the OTRAG engines use alternate posts of fuel and oxidizer injectors around the circumference of the engine, which apparently scales well to larger engines, but I can’t conveniently machine those with my current setup.

 

The new injector plates have a central igniter port, and a single ring manifold each of fuel and oxidizer with 60 degree unlike impinging injectors. The current one has 80 holes each, all 1/16” diameter. This flows slightly fuel rich with ethanol, but it should improve chamber life. This should be the rough size that we use for quite some time – somewhat over 3000 lbf. One engine on the current vehicle, and four engines on the upcoming 65” differentially throttled vehicle.

 

Working out the hole drilling process was important. I drill from the chamber side, first milling a land at the 30 degree half angle, then spot all the holes with a spot drill, then drill through in two pecks (about 0.25” total depth for a 4:1 L:D ratio), then go back and mill down another 0.06” to clean off the enlarged spot drill holes and leave a clean injector exit. The holes on the inlet side have the final chips cleaned off manually.

 

We made the torch igniter / engine mount so that it screws onto a -16 AN fitting, making that section portable between different injector plates.

 

It takes about six hours of milling to make one of these injectors, so we can test a lot of variables quickly. Version one had 90 degree injection angles and didn’t have the spot holes cleaned off, which resulted in an obviously messy injection pattern with water. We fired it and it worked, but performance wasn’t great. Version two has the 60 degree angle elements with the ends closer together and other changes necessary to allow this, as well as improved pockets for assembly on the top side and chamber centering, and a smaller flange bolt circle with more holes. This injector burned through in operation, but it logged by far the best Isp we have seen (details after we have a perfectly clean run of data to back this up). Version three will add tapered manifold inserts to keep the propellant velocity uniformly higher in the rings for better face cooling, and make more changes to ease assembly of the main propellant inlets.

 

Cold flow tests on version two looked a lot better, but there were still a couple small stray sprays that weren’t axial with the rest of them. I may try reaming the holes to final diameter on a later version, or drilling with tiny end mills, but for now we just want to replicate version two with better cooling.

 

We blew out the graphite gasket between the nozzle and the phenolic chamber while testing version one. For version two, we took a belt-and-suspenders approach – version two upped the bolt count on the flange and brought the holes closer in so it could take more torque without bending, and we machined a pocket in the phenolic tube to capture the gasket and center the nozzle.

 

The phenolic tube is holding up better than we expected. The 2:1 diameter contraction ratio makes a huge difference in ablation rate compared to the chambers on the X-Prize Cup vehicle, which only had a 1.25 or so diameter contraction, and ablated rapidly.

 

Between the graphite nozzle and the aluminum retaining flange we have been using a half inch thick ring of machinable ceramic, but I am sick of it, because the rings are so brittle that they break every time we torque the bolts down. It is just a compression spacer, so it doesn’t really hurt anything, but I want a better solution. We added a pocket to retain the chunks and sprayed some adhesive on the back so it all stays together, but it still sucks. We will probably try a phenolic spacer or a solid stainless steel flange. Some brake pad like material would probably be a good insulating spacer. Anyone have suggestions?

 

Clamping graphite nozzles under phenolic tubes probably isn’t going to work for long runs, because the phenolic is ablating at the contact point of the gasket, leaving a surface that will probably leak if we run it long enough. We hope the solid graphite engines work out, but if we wind up using an ablative chamber with a graphite throat we will probably have to make a tapered joint between them and slide the nozzle in from the top, so it continuously pushes down into the charring ablative.

 

As the engines have been getting bigger, we have been chewing through our firebrick blast deflectors faster. On the last refurbish we added a graphite plate on top of the bricks, and it doesn’t seem to have suffered at all after a ten second burn.

 

1500 lbf (running at reduced feed pressure)

After the injector base burned through, it rapidly burned through the top closures and sprayed lox above the engine, charring a lot of wiring and setting the computer vibration insulation foam on fire. Putting that out with the fire hose resulted in us getting some water inside the electronics box, which required us to actually replace a PC104 connector due to corrosion. At one point the box was sealed reasonably well, but the gasket needs to be replaced, and we should conformal coat everything anyway.

 

http://media.armadilloaerospace.com/2006_03_01/burnThrough.mpg

 

http://media.armadilloaerospace.com/2006_03_01/threadedIgniter.jpg

http://media.armadilloaerospace.com/2006_03_01/injectorTop.jpg

http://media.armadilloaerospace.com/2006_03_01/injectorBottom.jpg

http://media.armadilloaerospace.com/2006_03_01/version2.jpg

http://media.armadilloaerospace.com/2006_03_01/burnedInjector.jpg

 

 

Centennial Challenge

 

The lunar lander centennial challenge is our top priority this year unless something else pops up. We had a commercial opportunity that was exciting, but it seems to have fallen through.

 

I’m not thrilled about landing on inclined, boulder strewn fields, but the payload and delta-V requirement are easier than we expected. Having two levels and consolation prizes is a good thing.

 

As soon as we can show that the new engines can make two 90 second burns, the current vehicle should have level one in the bag. We will need software changes and a remote video system, but no other significant modifications. To take the big level two prize we will need a completely different landing gear arrangement, and the total performance may be pushing it a bit. If our new engine Isp is as good as it briefly looked, we may be able to modify this vehicle for level two, but we are expecting to have to use the upcoming 65” diameter vehicle, which will have a better mass ratio.

 

It is unfortunate that the prizes can only be claimed at the X-Prize Cup, because that will encourage us to sit on the vehicles after they have been proven out, rather than flying them hard and potentially crashing them.