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Engines, Gimbals, Vehicle

Engines

May 12, 2005 notes

 

Engines

 

We made a new aluminum biprop chamber with reduced and tapered cooling channels for higher velocity. The chamber has 20 passages of 0.094” (3/32”) width and 0.060” depth along the chamber, tapering to 0.030” depth at the throat. At 6 gpm (23.1 cubic inches / second) alcohol flow, this gives flow rates between 17 and 34 fps.  Slop in the chamber jacket will make the actual flow rates a bit lower than this.

The chamber thickness was also tapered slightly to make it thinner at the throat, giving us a 1.40” throat diameter instead of 1.25”. This also makes our L* even lower, which hurts Isp, but also lowers chamber temperature.

 

We have previously has aluminum hardcoated at Dallas Precious Metals, but it usually takes a few days. Russ was researching the hardcoating process and found that one of the methods basically involves immersing the part in a sodium hydroxide / water solution and hitting it with a high voltage / current AC power supply. He experimented a bit with using a welder as the power supply, and we seem to be getting pretty good results from the “homebrew hardcoating”. We coated the new engine.

 

We started off with 25% (by mass) water added to the methanol for extra cooling.

 

DOT-5 brake fluid has been reported to work as an alcohol additive to continuously form a silicon layer in the combustion chamber, but we found that it didn’t dissolve at all in methanol, and while it did seem to dissolve in ethanol, a slightly visible puddle still separated out at the bottom after a little while, even though the color from the brake fluid was evenly dispersed. Ethyl Silicate does seem to dissolve in methanol, but we haven’t tried firing it in an engine yet.

 

We made several long firings on the new engine, and it has held up fine, even when we removed the water from the methanol. Isp has dropped a bit, down to 178 from 188, almost certainly due to the reduced L* of the new engine. We tried a new injector plate that had the fuel jets firing directly into the gox jets instead of at 90 degrees, but it actually performed worse. The injector also had the ability to puddle a bit of fuel, and we managed to get a preburner hard-start, which popped the solenoid off and bent a flange. This injector design also had a recirculation area at the top of the chamber that eroded some aluminum away.

 

Because of the eroded top and bent flange, we went ahead and made another chamber. The latest engine has a 1” longer chamber, which will bring our L* back up and a little higher, so we expect Isp to exceed 190.

 

Theoretical calculations for various lox / alcohol / water mixtures at 150 psi chamber pressure pressure:

                volume O:F isp     temp K
ethanol  1:1      225     3105    tiny bit better at slightly leaner ratio
75% ethanol  1:1     219     3058    peak O:F
methanol        1:1     219     3096
methanol        5:6     222     3054    peak O:F
75% methanol 1:1     208     2923
75% methanol 4.5:6   214     2903    peak O:F

The watered mixtures don’t drop the combustion temperature all that much, but they can absorb a good deal more total heat as a coolant.

 

Our long test runs have all been at full throttle, because the jetted solenoid as a preburner fuel control doesn’t work well for throttling. When the lox is throttled down, preburner pressure drops, giving more delta-P across the jet and spray nozzle, resulting in more fuel flow and excessive temperatures. We have modified a small ball valve to accept one of our KZCO actuators to finely control the small amount of fuel going to the preburner.

 

Our blast deflector problems are pretty much gone. After the extra-high temperature bricks (McMaster 9355K1) failed miserably, we poured a tray of “ultra-high temperature cement” (McMaster 9876K1). This was sort of “puffy” and we didn’t expect it to do well, but it lasted a lot better than the bricks. Still, it dug at least a half inch pocket each run. Finally, James talked to a local refractory supplier and they suggested “Louisville” fire bricks. These have lasted very well indeed, eroding less than a quarter inch each run, with no flying sparky bits. We can also get them in 12” x 12” x 3” bricks, so we are just going to make a central slab that we replace every ten runs or so.

http://media.armadilloaerospace.com/2005_04_23/engineTest.mpg

 

Gimbals

 

When we initially set up the engine gimbal we had ball joints at each pivot – one at the top of the engine, and one at each end of the linear actuators. This was under-constrained, and the setup could flop around. We replaced the engine pivot ball joint with a U-joint, which held things together properly, but we also realized that the actuators only need a U-joint on the base plate and a hinge at the engine side, which will tighten things up even more. Even after all the linkages are tightened, there is still a little bit of play in the linear actuators themselves, but I don’t think it will hurt us. If we are forced to make the assembly completely rigid we will need to go to a ball screw based actuator.

 

The closed loop control on our valves and jet vanes was always simply full speed one way or the other, which gave them a “jittery” feel that can be seen in the close-up videos and on the telemetry graphs. This was acceptable for those cases, but the long arm on the gimbal magnifies it a lot. I finally got around to adding software pulse width modulation to all the motor drives and adjusting the control loops so the speed ramps down as it approaches the desired position. It is very coarse, running off the 186 hz IMU tick with only eight levels, but it still dramatically smoothes out the motion, and I doubt the slower response will effect anything. It is likely that we wouldn’t trip the thermal cutoffs on the motors now that they don’t slam back and forth continuously at full power, but we will continue to remove the cutoffs, because we have found them to be extremely conservative, and tripping one in flight would be disastrous.

 

Vehicle

 

Delivery times for aluminum tanks was excessively long, so we are just going with a steel tank for the alcohol and a stainless steel tank for the lox. For 10 gallon tanks, mass ratios are a little better than the 7 gallon ones, at 37 pounds, but it is still pretty heavy. We had to lay the tanks sideways for everything to fit, but what first looked like a problem turned out to be a benefit. We have a manifold under each tank that drains the tank from two points, which should make us basically immune to the slosh and swirl issues that can crop up when draining a tank from a polar boss.

 

We are leaning towards using two gimbaled engines instead of one engine plus roll control thrusters, because this is the likely configuration of the next larger vehicle, and the current 350 pounds thrust engines that we are firing would be the right size for the vehicle. However, I wouldn’t be shocked if we flip flopped again on this issue as the vehicle comes together. We are also still discussing the merits of a single preburner with gox hoses to two engine chambers versus each chamber having its own preburner. We are almost certainly going to use just a single set of throttle valves and fork the flow to the two engines, because there is no situation where we would want to control them independently.

 

We are going to need to build new electronics again, because a twin biprop engine needs more actuators and sensors than we currently have available. Our Diamond A/D + DIO board has plenty of bits – 24 digital outputs and 32 analog inputs, but we don’t have connectors for all of them, and we need more H-bridges for the motor drives. A single engine with roll thrusters can just barely fit the existing boards if Russ makes a few trace changes, which may yet be the strongest argument for it. On the other hand, we will certainly need more capability for the next vehicle, so we might as well get on it now…

A single engine needs the following bi-directional motor drive / feedback lines:

LOX throttle

Fuel throttle

Burner micro-throttle

X gimbal

Y gimbal

 

The following actuators without feedback:

Purge solenoid

Spark box points signal

 

If we use roll control thrusters, we need two more piloted solenoid actuators.

 

The following sensors:

Preburner pressure

Preburner temperature

Chamber pressure

LOX tank pressure

Fuel tank pressure

Pressurant tank pressure

 

We also track five internal values: three isolated battery voltages, regulated 12v and regulated 5v.

 





 






 
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