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Valve movement, Methane flights, Super module, Tesla

December 12, 2008 notes:

December 12, 2008 notes:

Valve movement

We are pretty sure we know what caused the valve movement problem that downed Pixel at the LLC.  Our systems run the propellant valves through a set of relays controlled by a watchdog microcontroller, which drives the valves closed if the main computer doesn't continuously toggle a keep-alive signal bit.  We found that with the valves on the bench, we could get interruptions to the current when we tapped on the relays in the electronics box.  We believe that acoustic vibrations from the operating rocket engine could cause the contacts on the relay to intermittently lose contact.

This seems plausible for a number of reasons:

We see little spikes in the current trace during the problem times, as if the relay contacts were only occasionally closing.

The acoustic energy gets much worse for ground liftoffs than when elevated four feet above the ground on tether stands.  We have seen similar problems with GPS units in the vehicles.

It is worse for Pixel than the modules, because the heavier GLOW needs more thrust, and the electronics box is closer to the engine.

It wasn't a problem for the Rocket Racer, because the electronics box was isolated farther away in the cockpit.

We tore one of the relays apart to look at the construction, and there wasn't even a real spring in them, it just relied on the springiness of the metal conductor.  Our usage of the relay is not energized during normal operation so that a failure of the watchdog's battery won't bring the vehicle down, which means there was nothing but the spring tension keeping the current path open.  These electronics boxes are also now three years old, so they could be getting a bit less springy with fatigue.

We discussed various options for changing the cutoff system, but we still came down on the side of wanting real, physical relays instead of a purely electronic solution.  Russ found some relays with much stronger springs for us to replace the current ones with.  This is a good example of how safety systems can have negative consequences.  We have seen several other examples of this over the years, and most people don't really appreciate it.

We still have some concerns about the limit switches inside the valve actuator.  The switch is actually wired up as a DPDT with two diodes, so there is a tiny spot where the motor is actually disconnected as the switch changes position, which we can see as a 5 msec divot in the current traces.  We have made a modified actuator circuit with four diodes and a DPST switch that avoids the issues, but it allows an unloaded actuator to spin past the stop point on momentum.  With a valve on, it seems to work fine, and we have flown the methane module with it running the fuel valve with no problems.  We are probably going to replace all the actuator electronics with custom boards in the future.

From a reliability standpoint, it is tempting to remove the valve limit switches altogether, and just have the computer always keep the valve in the right range.  That would remove a possible engine failure case, where the limit switch somehow interrupts your control of the valve, as well as also getting the diodes out of the control path, but it adds problems to our termination sequences -- we currently just drive the valves closed on a hard shutdown command, since it is possible to have burned through the feedback wires while still having control of the valves (speaking from experience), and we still want to be able to close them off in that condition.  We might consider having a physical stop and just driving the motor for a fixed amount of time that would be safe to stall for, but that would still mean that we need to be very careful about not leaving a motor drive enabled when we exit the flight control program.

Methane Flights

We have had a fairly miserable time with the methane engine work we are doing for NASA.  The basic idea was to adapt our alcohol engine for methane, adjusting the element sizes to get the mixture ratio correct.  Bottom line -- it didn't work.

We made over a half dozen different engines with unlike impinging elements for methane.  If we just used an alcohol engine as-is, it could be made to burn once we sorted out igniter values and startup sequences (need to chill the methane manifold as well as the lox manifold, making horizontal testing rather undesirable), but the mixture ratio was grotesquely rich.  Whenever we shrank the fuel elements down to bring the mixture ratio up, we ran into combustion stability problems.

We have never had high frequency stability problems with alcohol, even when deep throttled down to a negligible injector pressure drop, but we saw a lot of "interesting" things with methane.  Our film cooling was getting badly disrupted, such that even a poorly performing engine would make the entire chamber rapidly heat red hot.  We killed many pressure transducers, caused several spark plugs to disintegrate, and constantly vibrated swagelock fittings off.  We made one run with a vise grip clamped over the swagelock nut.  In the video playback, you could see the vise grip fall off the engine, which could be correlated with the pressure signal disappearing on the telemetry trace.  We then put a hose clamp over the nut and torqued it down until the clamp was solidly bent over the nut.  It still came off.

Most of these engine had a perfectly flat reading on the chamber pressure sensor (in the five or ten seconds before the sensor would die), but the sensor was only sampled at 200 hz.  However, you could hear the screech audibly in video playbacks.

We tried increasing the pressure drop across the injector by using smaller and smaller element holes until we couldn't lift off at full throttle and max tank pressure.  It still had problems.  We tried varying the feed pressures between the lox and methane to change the relative injection velocities.  Still had problems.  We had made a couple engines that could run on the test stand at full throttle, but none of them could dynamically throttle on a hovering vehicle for more than ten seconds or so without something coming apart.

Finally, we gave up and made an injector with like-impinging elements instead of unlike-impinging.  We immediately had great vehicle flights.  The Isp was lousy, but it wasn't much of a surprise that we would need a longer chamber to give the separated propellants time to mix.  The chamber was actually frosted over in the upper part, so we added a few inches to the chamber, and performance went up by 20%.  The nozzle still wasn't very hot, so we added another three inches, but the Isp only improved by 5%.  With some changes to the injector elements, we have finally brought the performance up to the level of our lower performing alcohol engine on module flights, but that still isn't saying much.  We have another injector going in for test this weekend.  The injector face also wasn't getting the heating that the unlike-impinging designs got, so we moved back to 304 from 310 for future like-impinging designs.

It is interesting to note that because a vehicle loaded with methane has a lower mass ratio than the same vehicle loaded with alcohol due to the different fuel densities, the blowdown pressurization curve that is almost perfect on the alcohol module winds up needing a higher initial pressure and a steadily rising average throttle position when on methane.  We wound up taking off the 80 pounds of LLC payload ballast that we had on the module to bring the mass ratio up a bit.  We may yet wind up investigating V-cut ball valves for more precise deep throttling.  Related to that, we also have more mixture ratio skew during throttling and more pressure skew across the blowdown range.

We are also trying a new fabrication technology for making chambers – rapid prototyping investment casting from http://www.proivc.com/ .  This is more expensive than the spun-pipe based chambers we have been using, but it frees us from the limits of standard pipe sizes, and makes changing nozzle dimensions easier.  The surface finish is rougher than we had hoped, and there was some slight ovaling in the chamber, but it is still promising.  We haven’t fired them yet.




We have made five good 60 second flights with different injector / chamber combinations.  They are all pretty boring, but this is representative:


We also tried flying the same vehicle with self-pressurized lox / methane instead of helium pressurized.  It couldn’t generate enough thrust to liftoff, but ignition, shutdown, and throttle up were all smooth, so this is quite encouraging.  It is clear that the flow is two-phase in the injector, because the reduction in liquid density alone would not explain the level of reduced thrust we had.  Interestingly, it also didn’t make much more thrust as we took the self-pressurizing level from 150 to 200 to 250 psi.  We have a new injector with 2.5x the element area to try next.  If we have good success with this, it opens up several interesting options for us.

We are planning on starting higher altitude flights at our home base in February, as soon as the brand new amateur rocket regulations go into effect.  We should be able to fly to 8000’ altitude right behind our shop, which is going to be a HUGE benefit for us.  We can learn a lot, and hopefully get the “inevitable tragic loss of vehicle” as we go into a new flight regime out of the way without having to make long road trips.  Coupled with Spaceport America just getting their spaceport license, 2009 is finally the year where everything is going to come together.

Super module

Even though Pixel was not hurt very badly in the LLC tip-over, we are going to try to adapt a module for next years level 2 lunar lander challenge.  The module can fly for about two minutes in the current configuration, so we are adding an external pressure bottle to allow us to fill the tanks completely full of propellant.  We may also need to fabricate some lighter weight legs, which we would pull in a bit to give it a smaller footprint for landing in the clear area of the "lunar surface"

We could probably get by with just using a big Tescom regulator for this purpose, but it was a good opportunity to go ahead and develop the servo regulator technology that we will need for the much higher flow rates on passenger carrying vehicles.  We tried making a ball valve controlled pressure regulator several years ago, but backlash on the ball valve stem gave us problems with the control algorithms at the time.  With the current error-diffusion pulse width modulation that we use, it worked out much better this time, and we can nicely control tank pressure to an arbitrary curve over a flight.  It is useful to sort of mimic a blowdown pressure curve to avoid requiring the propellant ball valves to operate down in the very non-linear cracking range.

For the LLC, we are just hanging the pressure bottle opposite the ballast weight in the middle of the vehicle, but we plan to mount the pressure bottle up inside the nosecone for high altitude flights.  Launched from a stand without legs, that should be a 100km capable vehicle.  Paying customer work for the Rocket Racing League and NASA take priority over this work right now, but we should be ready for a three minute hover test fairly soon.




This is only tangentially related to Armadillo, but many of you will still be interested.  My Tesla roadster, number 30 in the production run, arrived this month.  I used to be known for my crazy turbocharged Ferraris, and I have a lot of great car stories to tell from those days, but Armadillo wound up getting me out of the supercar world by taking over all of my discretionary spending and eventually making me fairly frugal and money conscious.

When I first heard about the Tesla, it piqued my interest because I had occasionally looked in on the high performance / homebrew electric car world, and I personally knew Elon Musk through SpaceXElon was daily driving a McLaren F1 when I first met him, and he has been one of the most open and honest people I have met in the aerospace world, so I asked him for a straight up opinion about the prototype vehicle.  "It is blow-your-mind awesome." he replied.

I talked about the car a fair amount, but I hadn't made any six-figure personal purchases since founding Armadillo, and Armadillo was still bleeding lots of money at the time, so it probably wasn't going to amount to anything.  Out of the blue, Anna decided to get one for my birthday (my wife is awesome!).  That was a few years ago.  Being a software developer, I am sympathetic to schedule slips, so I didn't fret much about it.  Every month or so, someone would ask me when the Tesla would be getting here, and I would just say "sometime soon".  Finally, "soon" came.

At the peak of its modifications, my twin-turbo Testarossa had pulled 1010 hp at the rear wheels on a dyno.  This is a level of performance that has basically ruined the experience of most high performance cars for me.  When I later bought a stock F50, my driving impression was basically "Meh.  Pleasantly quick, I suppose." (I committed a sacrilege and put a mild turbo system on it to make it live up to its looks).  The Tesla wasn't going to knock me out with neck snapping acceleration, but that wasn't what I was looking for.

For years, my drive to the Id Software office afforded me an almost daily opportunity to run out fourth gear on the service road, which let the crazy cars earn their keep.  After we moved to a new location, the route just wasn't there anymore, and I would find that weeks had passed without the throttle ever going to the floor.  For just tooling around town, high winding, high boost engines just don't come into their own -- it takes a degree of planning before aiming for 9000 rpm and 25 pounds of boost.  They were also, well, "loud" is understating it a bit.  Everyone at the office always knew when I was arriving, and in a lot of situations, exercising the cars would just be anti-social.

Responsiveness is what I wanted from the Tesla, and it delivers.  There is only a single forward gear, so there is no shifting or being in the wrong gear.  You have the full torque of the motor from a dead stop, with no power band and clutch to deal with.  Your right foot really is an "accelerator" rather than a "throttle", and it honestly does transform the driving experience.  Coupled with excellent traction control, there is very little reason not to floor it every time you come away from a stop, getting smoothly pulled forward like you are on an enormous rubber band.  As Robert Duffy put it, "You are driving a railgun."  The car is obviously very quiet, but the engine has a nice whine as you accelerate.  It doesn't have the wide-eyed, slack-jawed, religious-experience type of performance that my Testarossa used to have, but there is a lot to be said for racking up a dozen happy-car-grins a day with your normal driving.

Internal combustion drive trains, with all the gears, clutch, oil, and exhaust start seeming remarkably primitive in very short order.  I have hopped back and forth between BMWs and the Tesla for a couple weeks now, and while there are plenty of creature comforts that are much better in the BMWs, every time I pull away, I wish I was in the Tesla.  I am using it as my daily driver now, whenever I don't have to haul any big packages for Armadillo.  As I am driving it, the range is only about 150 miles on a charge, but that is still plenty for what I need, and I just plug it in every night.

I do also confess to enjoying the irony of my driving an electric car.  I am fairly hostile to most of the environmental movement, finding it generally a modern tribal religion that justifies condemnation and control of others in the name of protecting the environment.  I care nothing at all for the environment in isolation, only for how it positively impacts human life -- civilization is all about beating the environment into forms that suit us better.  An "electric car" used to be a conspicuous sign of righteous sacrifice, but you won't get any self-flagellation points for driving a Tesla.  Too much fun.




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