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200 Isp, 45 second radiative burn, Altimeter Auto Throttle

August 27 and 31 meeting notes

August 27 and 31 meeting notes


Armadillo Labor Day cookout:  http://media.armadilloaerospace.com/2002_08_31/hotdog.jpg


200 Isp, 45 second radiative burn


We tested our fourth generation kerosene injector on Tuesday.  Basically, it has an even tighter venturi, down to 0.6" (with a 0.5" throat), and instead of the fuel coming in as a high velocity jet from the side of the venturi, a stainless pipe comes in from the outside on the catalyst pack side of the venturi, and it has a pretty sizable hole in the bottom, before a plugged end.  The idea is to have very low velocity fuel stream straight down the middle of the very high velocity (nearly sonic) peroxide decomposition, much like an SSME lox post injector in reverse.  The way we bring the fuel pipe in is pretty neat – we drilled out a swagelok tube-to-pipe fitting so the tube can slide completely through the fitting, making the ferrules into a bulkhead seal.  This technique opens up a few other interesting design possibilities for future work.


We put the radiative chamber together with the high temperature bolts, the stainless clamp ring, and the silica gasket material (not the gas-filled O-rings, which hadn’t arrived yet). We also wet sanded the silicide coated flange, and used a torque wrench to evenly tighten the flange to 6 ft-lb on each of the #10 socket head cap screws.  All of the runs sealed absolutely perfectly.  When we finally got around to checking, 6 ft-lb is a lot higher than cap screws that size should be torqued, even in a high end alloy, but it did the job.  For 18-8 stainless #10, 2 ft-lb is recommended, and about 3 ft-lb for 316 stainless.


We started off very rich with 0.080” peroxide / 0.050” kerosene (but not as rich as we were running the regen engine), and immediately on lighting, it was obvious things were working a lot better, because the engine started glowing much more rapidly, and much higher up.  There was still a visible bias opposite the fuel inlet, probably because the flow had enough momentum that it was not "turned" completely axially when it came out of the drilled hole.


Our oxidizer-only Isp was 246, up from a previous best of 192!  We shrank the fuel jet a good chunk to 0.045”, and the oxidizer-Isp increased a bit more.  We shrank it some more, and it finally nosed over a bit, which would still be at increasing "true Isp".


Thrust was 45 lbf +/- 5% at 250 psi feed pressure.


We will have to get our sight glass hooked back up on the new tank so we can measure the kerosene flow, but this was with an 0.040" kerosene jet, while a 0.060" jet resulted in a measured O:F of 2.2:1 on Saturday, so that would put us around 5:1 O:F.  On Saturday we double checked the load cell calibration and found it to be a couple percent optimistic, but that still means we have a true Isp right at 200 seconds, which I am quite happy with.


Long video of the entire burn: http://media.armadilloaerospace.com/2002_08_31/burnedTZM.mpg


We loaded up four liters of peroxide and let it run.  The burn was nice and monotonous, but after 45 seconds of hot fire, the plume started deflecting slightly, and some sparks started coming off the nozzle.  I killed the kerosene, and let the rest run out as monoprop.


We burned the silicide coating off at the nozzle, which allowed the TZM to vaporize a hole.  We have another TZM chamber already machined, but I’m not sure that I want to spend another $3000 for the silicide coating.  We obviously have to tone down the heat transfer with some combination of richer fuel mixture, lower chamber pressure, better film cooling, and thermal barrier coatings if we want to run radiatively cooled.  At the moment, our regenerative designs are looking good (and much cheaper!), so we are leaning that direction.  Being able to see the combustion distribution by the glow of the chamber has been quite useful, so we might make up a batch of disposable stainless steel chambers that we can just let glow red for a few seconds.


We may be near the theoretical max Isp for this pressure (220 at a leaner mixture), but we wanted to get rid of the uneven combustion, so we modified the fuel injector tube to actually take a 90 degree turn (cut, bend, weld), instead of having a hole drilled in the side.  This gives straighter, and even lower velocity fuel injection, which should improve the shear mixing.


On Saturday, we installed the modified injector on the regen engine for tests.  We used water cooling instead of peroxide cooling, because we aren’t expecting this small of an engine to be able to run steady state with the improved combustion temperatures against the reduced peroxide flow.


We had several frustrating runs with leaks around the injector, leading us to believe that the injector was left somewhat warped by the long radiative runs.  This has been a chronic problem for us, so we are going to go ahead and make an injector out of stainless that should hold up better.


We were able to get good enough data to learn something interesting – the new fuel pipe with the 90 degree bend was performing worse than the drilled pipe, only slightly better than the side injector.  We believe this is due to it sticking down into the venturi, so there is less opportunity for high shear mixing before everything expends out into the main chamber.  We are going to test this theory by making the next injector tall enough that the bent pipe exits well above the plane of the venturi.


We have been buying parts for a 1000lbf regen biprop, which we should be testing within a month.


Altimeter Auto Throttle


We did a normal five gallon test hop of the lander to make sure everything was still working properly after the skid on the ground after losing the computer last time.  We had replaced some components, and changed from AN jets between two aluminum fittings to custom drilled jets in a single pipe fitting to make the engine mounts more rigid.  Everything worked fine.


We prepared to try the auto-throttle again.  We loaded a somewhat lighter load of peroxide, because the last time we tried this, June 8 Notes, we had a pretty bad crash due to improperly simulating the valve behavior.  I was prepared to kill the auto throttle immediately on this test.


After warming the engines, I briefly clicked the altitude hat up, which would cause it to set a target altitude half a meter above the current altitude.  The vehicle leapt off the ground, and I rapidly killed the auto throttle, but on reviewing the video and telemetry, it had behaved exactly the way it was supposed to, bobbing a couple times in hover before I had killed the throttle.


I lifted it up again with the auto-throttle, and it did indeed hover just where it was supposed to.  However, it started rolling fairly rapidly, so I told the auto throttle to land, which it did properly.  After looking at the data, it appears that the roll fiber optic gyro was going completely nuts.  We power cycled the entire system, which seemed to clear up the problem.


We loaded up for another run, which started out perfectly, with it sitting at a nice hover, and me flying it side to side a bit, but the roll gyro went away again mid-flight, starting the fast roll again, and causing it to tip over as I set it down.  We broke another hose, but everything else was fine.




On investigating the next day, it appears we may have a thermal issue.  The logged IMU temperatures climbed from 88 F to 112 F over the course of the tests.  This is still well within the rated limits, but the roll axis would be the one mounted on the bottom, which is bolted to the plastic case, which may well be significantly hotter than the location of the sensor.  Crossbow recommends mounting to an aluminum plate.


With the auto-throttle working well, we should be able to do a perfect DC-X style flight path out at the 100 acre site now.  We will also be able to develop high speed auto-landing, which will allow the computer to save the vehicle automatically in the event of telemetry failure, or any failure on the remote pilot laptop system (like the joystick failure that caused the “splat” crash Dec 22, 2001 Notes  ).  It will also enable soft, upright landing after a parachute cutaway.


The laser altimeter we are using, a Laser Atlanta Advantage, is unfortunately a bit of a pain for our application.  We converted it from a battery pack to using our main system power, but it still requires us to manually press a membrane switch to turn it on, and the continuous reading mode is a commanded toggle that requires us to start the altimeter and the flight control program in a specific order.  The housing shape, like a police radar gun, is also annoying from a vehicle integration standpoint.  I also get a few bogus values from it occasionally.  I may wind up buying a more expensive one.




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