June 25, July 2 and 6, 2002 Meeting Notes

 

Tube Vehicle Work

 

We added metal reinforcement plates to the bottom bulkhead to keep the fin attachment tubes from pushing into the plywood as much.  This helps, but it still complains a bit when carried around by the fins, so we have been trying to move it other ways when possible.  Joseph can carry it from the parachute harness with his bobcat, and we can get a dolly under the engine bell and walk it around, but neither is really ideal.  We should probably design vehicles so they can be moved with a palette jack.

 

I haven’t been real fond of the fins in general.  When (not if) we break the fins off of this vehicle, we will probably rebuild with a flare bottom as the stabilization mechanism.  Flares give more drag for a given amount of stabilization, but it should be more robust.

 

Deakins Metal Spinning http://www.metalspinning.com/ is getting a quote for spinning the nose cone / upper fairing assembly for the vehicle out of 0.050” aluminum.

 

We have decided to use the big 30’ parachute for this vehicle, which will hopefully bring it down soft enough to not break anything.  It is designed for a 900 pound load, and we are going to be under 300 pounds, but we want a landing soft enough that the vehicle doesn’t sustain damage, not just soft enough that it is survivable for an light plane pilot with a crunched airplane.  We are going to use H motors to deploy the parachute.

 

For hover and auto-land testing, we can’t just have the altimeter strapped inside the tube, looking down through the bottom, because it doesn’t get a reliable signal until it is five feet away from the ground.  For testing, we strapped the altimeter to the top of the tube.  One incident of note was that when I noticed the altimeter signal acting much noisier than usual, it turned out to be due to a loose strap blowing under the optics.

 

In preparing for a hover test, we balanced the vehicle as well as we could by blocking it up under the engine bell and tipping it each way.  We would up adding about 10 pounds of lead ballast at the top to center the CG over the engine.

 

We were all set to do a captive hover test on the vehicle, when the computer flaked out.  This had happened a couple times recently, so we stopped work to see if we could get a definitive answer to the problem.  I had recently changed the electronics box back from the tall stack of PC104 modules in a VersaTainer to our older SBC with only two modules stacked on top of it.  I had several reasons for this:  the PC104 CPU had started losing its configuration data when left alone for a while, the SBC was slightly faster (133mhz vs 100mhz) which may keep it from being maxed out and dropping the occasional sensor sample, and the SBC has four serial ports on board, which we need to add the GPS along with the IMU and altimeter.  The drawback to this computer is that it twiddles the parallel port bits during the boot cycle, which causes solenoids to fire if attached to a vehicle.  We may want to add a separate switch for actuator power, which we only turn on after the computer has booted.

 

When it first falked during tests last Tuesday, it acted like the battery was too low, continuously resetting but not getting into the boot cycle.  Because of another issue with the configuration of the new A/D board on the system, I didn’t have the battery voltage logged to check.  That weekend, it happened again, and I knew the battery was almost fully charged, and it in fact came back to life after I opened it up and prodded it a little.  My theory at the time was that one of the not-very-well-tied-down serial cables might have shorted something out and put it into a reset condition.  I made a nice little bracket to mount all the DB9 and DB25 connectors on, which I was hoping would solve the problem.

 

When it happened again, we finally nailed down the real problem – this computer draws more power, and we are jumping over the two amps that our 5v power supply can provide.  It may boot, but once it heats up a bit, it drops the voltage and it resets.  When we used this computer in previous electronics boxes, we had a different power supply.  We considered cobbling something together so we could still do our hover test, but we decided to just let Russ make a new board with a much higher margin power supply.

 

Once we do some hover tests, we plan on flying this to parachute deployment altitudes.  The Washington AST office’s attempt to help with our local FAA waiver seems to have failed, so we are more or less resigned to doing our first mid altitude flight test at the Oklahoma Spaceport, even though it is a six hour drive.  We should be doing this next month.  The goals for the first test series will just be to fly light propellant loads to depletion, so we can see what the aero loads look like to the computer.  The only thing of note will be that it will be an actively guided launch, so it will fly without a launch rail of any kind.

 

We have all sorts of ways of determining altitude for parachute deployment – GPS, laser altimeter, integrated accelerometers, and barometric (I have a sensor, but I have never integrated it).  However, the current plan is to use a manual telemetry triggered deployment command, and only have a backup timer deployment that fires if telemetry is lost for some reason.

 

Later flight tests will involve leaving a propellant reserve, and reengaging the engine for stabilization and impact attenuation before landing.  We will need to cut away the parachute to keep it from tipping the vehicle over after landing, and if there is any significant drift, it may need to cut away even before landing to allow it to kill horizontal velocity.  We don’t have any form of ground speed sensor that will allow us to do that automatically yet, because the GPS update rate is too low.  It may require a blended GPS/inertial system, although some form of radar may work better.

 

There are some appropriate looking cable cutters and pyro releases at: http://www.sweeney-special-effects.com/index.html

 

 

Two Inch Biprop Work

 

I built a new fuel injector ring that has a complete annular manifold around the chamber, and a small 0.005” gap between the fuel ring and the combustion chamber that feeds from the manifold.  This should give an almost perfect distribution of gaseous ethane, but will probably be too low of a velocity for kerosene atomization.  It will also cool the injector ring better.

 

We were planning on testing an immersion cooled chamber, where we just have the entire thing submerged in a bucket of water.  This definitely doesn’t work for high heat load engines, but we are curious if it may work with low pressure peroxide biprops, which run cooler than most rocket engines.  We made the chamber out of aluminum for good thermal spreading, and we want to try hard-anodizing the inside to reduce heat transfer into the metal, as the Agena upper stage engine did.  We tried a new aluminum alloy for this: 2011.  Russ reported that he could easily take 0.125” cuts on this with excellent chip control, and he turned out the complete chamber in an hour and a half.  In general, we use 7068, the strongest aluminum, when possible, but it is only available in bar stock.  7075 is the next strongest, and can be had in more forms, including plate.  If we don’t need maximum strength, and we have to cut a lot of metal, 2011 looks good.  If we try a regeneratively cooled 2” motor, it will probably be made from 2011.  We only use 6061 when we need a specific form, like the thick wall structural tubes that our fins mount on.

 

Before trying the immersion cooling, we wanted to get parameters for a clean run on the horizontal test stand with water jet cooling, because we had both a new injector, a longer combustion chamber, and I had slightly jetted down the peroxide from 0.120” to 0.110”.  I had realized that the 0.120” jet that we had been using on the test stand with the 2” cat packs was only appropriate when we were making 100 lbf of monoprop thrust, which is a lot more than we are flowing during the biprop tests.

 

The biprop runs that we were calculating Isp from were extremely smooth, but when the ethane mixture was richened, it would go into extreme roughness.  At the time, I thought it was due to either the ethane evaporation process or poor combustion, but in retrospect, it was probably due to the peroxide flow getting feedback when the flow rate dropped on biprop ignition.  Peroxide flow decreases at least 30% when it goes into biprop mode.  The indicated (oxidizer only) Isp was increasing up to the point where it went rough, so we likely did not find the peak in our tests, and we were probably running lean and hot.

 

The initial drop to 0.110” was way too tentative, and we did still experience rough biprop burning.  I dropped it to 0.080”, and the times we did get ignition were all smooth.  Now that I have gone back and referred to my earlier calculations at the April 20th 2002 update, it looks like the peroxde jet should be 0.060” to get a 22 psi pressure drop across the jet.  When we have pressure drops like this, we reliably get smooth runs.  In many cases, we still get smooth runs even without any effective jetting (like the smooth biprop runs with a 0.120” jet) due to the catalyst pack providing damping, but we can’t count on it.

 

We had a lot of solenoid problems that basically invalidated almost all the testing we accomplished.  There were three distinct issues conspiring against us:

 

Some times, there wasn’t enough current to open both peroxide and fuel solenoids together.  This was likely more the fault of the wiring than the battery, but we hooked two batteries together to get enough push to reliably open both.  We have had enough battery issues that I am going to put a high current 12v power supply on the trailer for running the solenoids, instead of relying on batteries.  A 50 amp auto battery charger will probably work well.

 

The pushbuttons started to fail on us.  They were from Radio Shack, and really weren’t up to the 8.8 amp draw of the solenoids.  We finally tossed out that box and moved to one that we use for manually firing vehicle engines, but I am ordering a big button box designed for heavy machinery that will positively make this problem go away.

 

We had bizarre flow issues, where even monoprop runs would give different (but still smooth) thrust levels on different button pushes.  We changed out the solenoid, and when it was taken apart, we found there was a small piece of hard white plastic stuck at the valve seat.  We had seen this exact problem a long time ago, and we had thought it was a fragment from one of the manual ball valves that had started leaking.  We have a completely different plumbing setup now, but one of the spring-return ball valves on the fill manifold has been leaking a bit, so I wouldn’t be surprised if that is the problem.  The problem is likely due to some peroxide wicking into a tiny area in the valve seal, then decomposing and cracking it, but it might also be due to excessive vacuum during loading, because the valves are only rated to 29” Hg, which is a 97% vacuum.  Does anyone have any experience with vacuum failure modes of ball valves?

 

So, with all these issues, almost none of the data is likely worth much, but here are the runs we did on Saturday, all at 250 psi regulated pressure:

 

0.110” peroxide, 0.055” ethane (no regulator, high 80s temperature): did not light

0.110” peroxide, 0.044” ethane: lit briefly right at peroxide depletion, so we assume too lean

0.110” peroxide, 0.070” ethane: lit briefly after several tries

0.110” peroxide, 0.080” ethane: lit, but ran very rough.  I let it go, and it melted a hole in the aluminum chamber where it wasn’t getting much cooling water.

Russ made another chamber quickly, and I cut the peroxide jet down more to avoid roughness.

0.080” peroxide, 0.080” ethane: lit briefly after several tries

0.080” peroxide, 0.070” ethane: lit after several tries and burned smoothly.

0.080” peroxide, 0.060” ethane: did not light

0.080” peroxide, 0.060” ethane, mess with electronics: good smooth burn after a couple tries, but an odd discrete step in the biprop performance.

0.080” peroxide, 0.060” ethane: did not light

0.080” peroxide, 0.070” ethane: did not light

 

We were at the last of our peroxide drum, so for our final run we tried a 0.030” kerosene jet.  The solenoid was obviously plugged up again, but it still lit.  Kerosene lights far, far easier than Ethane, but we have yet to demonstrate a high Isp run with it.

 

I will have a high amp power supply and good pushbuttons for our next test, but we need to find a good inline filter for our loading system.  We need an all-stainless (ideally 316, 304 acceptable) liquid filter that won’t trap liquid, and can stand up to at least 600 psi (ideally 1000) during loading.  McMaster doesn’t seem to have what I want, so I am open to suggestions from anyone.  I may buy something low pressure that we can use for our biprop tests, but we will need a high pressure one for our vehicle tests.

 

The machining of our TZM radiatively cooled chambers will be finished next week, which we will send them off for two different oxidation protection coatings.  We are going to have Dallas Precious Metals platinum plate one of the chambers, and http://www.hitemco.com/ put a silicide coating on the other one.  The goal is to be able to do 80 second biprop burns multiple times.

 

We are considering making a heat soak chamber for the 5.5” engines, which will allow us to get some experience with the vertical test stand, and give an intermediate step before the 12” engine.  We do hope to have a fully functional 3000-5000lbf reusable biprop by the end of the year.  If it is regeneratively cooled, we will be able to run higher pressures, but a radiatively cooled engine will probably be limited to around 150 psi Pc to keep the heat loads acceptable.