February 1, 2006 notes

 

Igniter / Engine work

 

We have built a new test stand arrangement that gives us two benefits – increased tank size (2 x 13 gallon spherical tanks – we will use the 2 x 100 gallon tanks on the vehicle for longer tests), and the ability to use the tanks either connected to our vertical stand, or a horizontal stand. We have also been experimenting with sound suppression methods, because we know we are sort of pushing our luck firing > 1000 lbf biprop engines behind our shop.

 

While testing the new arrangement, we had a little hard start on our test engine that blew the top closure off.

 

The ignition sequence on our engines has been:

 

Activating the vehicle (main trigger) opens the lox ball valve to 30% to allow lox to chill down the plumbing and start filling the engine with gox.

 

After a few seconds, I press the ignition button, which opens a solenoid that runs fuel to a spray nozzle at the top of the engine, and starts triggering the spark plug.

 

I have two options at that point – if the igniter is working properly, I release the ignition button and the main fuel valve also opens to 30%, putting the engine at “idle”. If the igniter isn’t working, I can release the main trigger and everything shuts down and auto-purges.

 

We have done over a hundred engine firings like this, but ignition of the igniter spray fuel is sensitive to exact chamber details, and we have had some experimental engines that wouldn’t light 100% reliably, and as the engines have gotten larger, it isn’t as clear when the igniter is working. Small throatless engines had an obvious jet of fire coming out the bottom, but on larger engines we only really got to see the lox clouds turn clear.

 

What specifically happened this time was that we changed valves for the new test stand, and at low throttles the flow through a ball valve is very sensitive to exact positioning, so the 30% lox throttle on the previous valves turned out to be slightly more open, and the little 0.030” jetted fuel spray nozzle was getting overwhelmed with oxidizer. The mixture by the spark plug didn’t become ignitable until the main fuel valve started to open, then it went bang. We got lucky with our first firing, then we had some problems, then we had it go bang.

 

We have been aware of the lox valve calibration issue for a while, and had been discussing adding a dedicated solenoid to meter in a specific amount of lox / gox, but I had been a bit reticent to add another actuator to the engine setup. We had also known that ignition sensing was getting to be an issue, and I was looking into some flame sensing possibilities, but I also didn’t really want to add another class of sensor. Nothing like a little bang to get me over my reservations.

 

We decided to build an independent torch igniter for the engines, in the style of XCOR’s, where it has separate fuel and oxidizer solenoids, a spark plug, and a throat constriction, so it builds igniter chamber pressure that can be reliably sensed. I realized that if I just put our existing chamber pressure sensor in the igniter chamber, the same sensor could be used for both ignition detection, and flight chamber pressure monitoring, because the igniter shuts down after one second of operation, allowing the main chamber pressure to fill back into the igniter chamber. We are currently using a 0.018” fuel jet, a 0.060” oxidizer jet (assuming it will be mostly gox, not lox), a 0.25” throat, and an L* of about 20 for the igniter.

 

I changed the control software to automate the ignition process, so as soon as I squeeze the trigger, the engine lights up to idle all by itself. There are two shutdown checks, the first to see if there is igniter pressure, then the second to see if there is chamber pressure after the igniter has shut down. We tested all the cases by alternately disconnecting igniter and main fuel feeds.

 

The engine had one cracked weld and had broken the graphite nozzle, but we fixed it back up and got it on the test stand with the new igniter. The zirconium oxide coatings we had put on the previous graphite nozzles didn’t seem to last long at all on the inside of the nozzle, so we just left the graphite bare on the new nozzle.

 

Our strategy for sound suppression on engine testing has been to move the vertical test stand down into our truck loading dock well so the plume is below ground level, and flood the well with several inches of water. This has worked very well. The only downside is that so much water and steam get kicked up during firing that camera positioning for viewing can be tricky.

 

http://media.armadilloaerospace.com/2006_02_01/pitTest.mpg

(this is with the old ignition style, not the torch igniter)

 

One issue with the new igniter is that there is a huge difference in behavior based on if the igniter is getting gox or lox. The test stand hoses leave a big warm gox bubble in the line when they are filled, so the igniter lights with a very low chamber pressure due to running extremely rich, and the main chamber also runs extremely rich for the first couple seconds. I started manually flowing some lox through the engine to get everything chilled, but I want to avoid this if possible, because we are considering linking our fuel and lox valves together with a single actuator, and I don’t want to add another valve just for chilldown.

 

The engine continues to work well, reliably making 1050 lbf at 350 psi tank pressrure. After the first fifteen second run, we didn’t measure any appreciable erosion of the graphite throat, but when we took it apart after the second run and measured it more carefully, we saw 0.05” wall erosion on the throat, and one area of slight pitting. It is possible that we didn’t get an accurate measurement after the first run, since sticking calipers up a hot nozzle on the test stand isn’t super-precise, but it might also have suffered some when I flowed some lox over it to chill the plumbing while it was still fairly hot. We are trying the silicon carbide based coating on the nozzle for the next firing. It appears much harder and better adhered that the zirconium oxide coating after it cured, we’ll see how it does with the firing. We also want to investigate metallic plating to protect graphite.

 

Interestingly, we also saw some apparent reaction of the graphite and the woven silica gasket that we used to seal the graphite nozzle to the chamber. The frilled edges of the gasket that were exposed to the combustion gasses did slightly melt, and the graphite had shallow indentations under the gasket. We are going to try using a graphite sheet gasket on the next test. It will increase the thermal conductivity from the nozzle to the chamber, but we don’t think that will be an issue.

 

While repeatedly testing just the igniter, we did manage to freeze the lox solenoid once. This just results in an engine that fails to start in a completely benign way, and it shouldn’t be an issue with realistic vehicle operations, but we may try and do something about it.

 

The other issue we had was that the retaining plate that clamped the graphite nozzle to the chamber warped after the run was over and all the heat soaked out of the graphite. This was expected. A thick stainless steel plate would probably work, but we are going to try a ceramic insulating spacer on top of an aluminum clamp plate.

 

I was originally hoping that we could just make the clamp ring out of machinable ceramic (McMaster 8479K4 Alumina-silicate), but it turned out to be far too brittle to put any bending load at all on. In fact, the test flange with all the bolt holes literally came apart in Russ’s hands when he was looking at it. There is a fairly involved firing process you can go through that is supposed to increase the strength by 40%, but it involves a programmable temperature controlled furnace that can reach 2000 F, and I don't think I would be all that impressed with even a 40% toughness increase. As a spacer in compression, it seems to work fine, and it really is easy to machine.

 

Vehicle complete

 

We received our new motors for the gimbal linear actuators, which was the last remaining piece for the new vehicle. The stock motors were 4 amp stall current and, more critically, had very conservative internal thermal cutouts that would stop the motor from moving if it was jiggled back and forth rapidly by the computer for ten seconds. We replaced them with 12 amp stall current motors to give us a little more speed, a lot more force, and run-until-they-die capability. They have the same mounting pattern, but we did need to grind a flat on the new motor shaft.

 

We moved the engine over to the vehicle, got everything hooked up, and went through a full system test on the vehicle. We found a few issues, but nothing big:

 

Something was wrong with the fuel tank pressure gauge, causing it to respond extremely slowly, as if it had a clogged snubber in front of it. We pulled it off, but the aluminum attach point was galled, so we are going to have to weld a new one on.

 

The gox igniter solenoid isn’t working now, something must have happened to the wiring.

 

When we loaded lox into the tank, we realized that we should stand off and insulate a lot of our wiring, because all the aluminum around the base of the vehicle gets real cold, and I worry about some of the wire insulation and tie wraps.

 

We still have a hot gox bubble in the lox feed line to the engine, we will try and rotate around some of the sanitary clamps so it flows continuously downhill.

 

We need to use real shafts and bearings on the gimbal to engine hinge mount, because the current hole-and-bolt arrangement contributes half the slop in the assembly. The other half is from the lash in the linear actuator acme screw, which we can’t do anything about. We have some ball screw actuators, but they are much larger than what we want to mount here.

 

Vehicle dry weight is 540 pounds. With the current engine, we would only be able to lift off with 400 pounds of propellant, but we will eventually upsize to a bigger engine that would allow a flight with the full 1500 pound propellant load.

 

Flying soon!

 

http://media.armadilloaerospace.com/2006_02_01/fullVehicle.jpg

http://media.armadilloaerospace.com/2006_02_01/chillDown.jpg