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Trashed a vehicle, Self pressurized methane, Development work

February 11, 2009 notes:

February 11, 2009 notes:


Trashed a vehicle


It’s been a while, but we pretty much trashed a vehicle last month.  We were doing the first test of the “super mod” with completely full propellant tanks and an external high pressure helium tank with a computer controlled high pressure valve for tank pressure regulation.  The goal for this design is to get enough performance from one of the modules to do the level 2 lunar lander challenge without having to use Pixel, because we still worry a lot about slosh and propellant balance on the quad vehicles.  Moving to external pressurization is also one of the major performance growth paths for us in the future, so it is a useful development direction.


We didn’t really expect the first cut to be able to hover for 190 seconds, because the propellant capacity is still less than what we put in Pixel, and the vehicle is overbuilt in a couple ways – the legs weight as much as one of the propellant tanks since they were designed to be ok for a four module cluster, and the high pressure tank we are using is DOT rated for 3000 psi, even though we only load 2000 psi into it.


Cascade loading the high pressure bottle on the vehicle was a new operation for us, but it went very smoothly.  We loaded fuel first, then helium, then lox last.


While it would have been possible to use a big regulator for this amount of flow, using a computer controlled high pressure ball valve offers a lot of advantages.  It is easily scalable to an arbitrary size.  It allows us to tailor the tank pressure curve to minimize the range of throttle valve movement, so instead of holding, say 400 psi until the high pressure bottle is empty, then transitioning to blowdown, we can have the pressure start at 400 psi, and decay by 1 psi for each second of flight.  It also allows us to leave the propellant tanks unpressurized until just before firing.


We had tested the servo regulator over a good range of flow conditions, but we hadn’t tested it at high flow on a tank completely full of liquid with almost no ullage space.  When the engine throttled up for liftoff, the tank pressures overshot the target value due to a very noisy “pressure velocity” signal as the propellant valves and regulator valves were filling and sinking from the negligibly small ullage volume.  This was the same engine design used for all of Pixel’s 180+ second flights, which would have the chamber glowing bright red to orange early on, before dulling down as the chamber pressure decayed.  Pixel was initially pressurized to 425 psi in both tanks, but by liftoff time the tank pressures were usually down to around 400 psi in the lox and 410 in the fuel due to cooling of the ullage gas and the lox chilldown dump, and they dropped steadily from there.  This time, both tanks were at 435 psi when the engine went to full throttle.


The vehicle jumped in the air rapidly at this high thrust level, but almost immediately it started to burn through the side of the chamber.  It is possible that there was a manufacturing difference between this engine and the engine that Pixel did all the long flights with, but I suspect the issue is just that we were so close to the limits that the slightly leaner and slightly higher pressure mixture was just too much for it.  We were only aiming for 350 psi, which would have almost certainly worked out fine.


Normally, this wouldn’t have been a big deal.  I would have shut the engine off, and the vehicle would have bounced on the tethers.  However, this happened to burn through right next to one of the gimbal attach points, and a second after the flame started shooting out of the side of the engine, the gimbal let go, and the engine shot over to about a 45 degree side angle, sending the vehicle into a vicious cartwheel.  The valves started to close as soon as the vehicle hit the 20 degree tilt abort, but it still flipped completely over and came down hard on a single tether mount.


The tether mount broke.  If it had been a normal, half-full blowdown module load, it would have been fine.  If it had come straight down and loaded both tether mounts, it would have been fine.  Now here is the really painful part – what actually broke was the bolts holding the strap cylinders to the tank mount points.  They weren’t the right bolts.  The legs and tether mount points are all set up for a bit of a hammer fit for 7/16” bolts.  The other flight module has those everywhere, but this module had 3/8” all-thread holding the tethers on, because we didn’t have the right bolts when it was assembled, and we never went back to replace them.  That probably only had half the shear strength, and it very easily could have loaded and failed the loose-fitting bolts independently.


The second tether mount point then failed as well, and the vehicle crashed to the ground.  We have always used four tether straps, and the quad vehicles have four attach points directly on the frame, but the upper leg mounts on the modules made it convenient to only have two tether mounts and double up the straps.  A clear mistake in hindsight.


None of the tanks ruptured, but some of the plumbing around the engine broke, and pretty much everything at the base of the vehicle got burned before we could get the fire out.  We had to reposition our fire truck once while fighting the fire due to wind conditions, which was an unexpected complication.


After cleaning up, we stripped everything down and proof tested the tanks to 600 psi again, and they still turned out fine.  However, most of the gear on the vehicle will need to be replaced.  We are going ahead and building lightweight legs for it now, which will probably give it all of the performance margin we need   A new wiring harness has been built, and most of the other little parts are on their way, but this is a low priority project for us until the new Lunar Lander Challenge rules are announced.






Lessons learned:


Put hard limits on the servo regulator behavior, such that it will never throttle up when above the target pressure, no matter what the pressure velocity it.


Test the servo regulator with completely full tanks.


Use four independent tether attach points.


Use the right bolts.


Consider moving the gimbal mounting points to the top of the engine instead of down on the chamber, so they can’t get burned off.


Set up our big 1600 gallon fire tank with two fire hoses so we can cover both sides of a fire simultaneously.



Self Pressurized Methane


We have successfully flown a module on self pressurized lox / methane a few times now.  It looks like any other module flight, but the fact that it has worked fairly smoothly is exciting.


Vapor pressurized propellants, or “VaPak”, systems have some very tempting attractions.  You can fill your tanks completely full, yet still have 80% of your initial pressure when the liquid is fully expelled from the tanks, giving the mechanical simplicity of blowdown, but the mass ratio and thrust of externally pressurized systems.  Getting rid of helium can as much as halve flight costs in some situations, especially during testing with partial loads.  It may also be possible to simplify or do away with torch igniters and purge systems.


Air Launch LLC http://airlaunchllc.com/ was the most recent proponent of this with their QuickReach rocket, using lox / propane propellants.  They fired some large engines for significant durations before their development contract ran out.  Almost all nitrous oxide hybrid rockets, including Space Ship One and presumably Space Ship Two, also use vapor pressurization for the oxidizer.


There are a few downsides:


It doesn’t work very well for higher pressures, because density drops fairly precipitously as the saturation pressure increases.  I believe AirLaunch settled at 250 psi, which seems about right to me.  This is generally not a problem for an upper stage or an air launched vehicle, but it is a lower than ideal pressure for ground liftoff, where you would tend to choose a somewhat heavier tank for higher chamber pressures and Isp.  Nitrous oxide is often used self pressurized at higher pressures, but that is more due to the convenience of room temperature operation than any particular performance merit.


At liquid depletion, your tank is still full of a lot of cold, dense gas, which has a significant impact on mass ratio when compared to helium.  With upper stages this can potentially be turned into an advantage by allowing the gaseous propellants to burn in the engine at a reduced blowdown thrust (and presumably reduced efficiency), allowing your stage to “burn the tanks to vacuum”, which is even better than anything you could do with helium.  That isn’t so helpful for reserve landing propellant on a VTVL, where a major drop in thrust as you are coming in for a landing is a problem.


Propellant conditioning is an issue for repeatability.  Propellant can stratify into different temperature regions in the tanks, especially with slosh baffles.  We hoped that since the engine feed hoses would cause more boiling than the tanks, the convective cooling would stir things well enough, but it doesn’t work out that way.  We currently deal with this by “shaking the rocket” under the crane to stir the propellant as it warms up, but that isn’t a very scalable solution.


Our first test was using the exact configuration we flew with helium pressurized lox / methane, but allowing the tank pressures to come up by themselves with temperature, instead of adding helium.  The computer individually relieves pressure in the tanks as necessary to let them both arrive at the target pressure.  With the same injector that we used for the other flight tests, the engine made less than half the chamber pressure at full throttle, which was not enough to lift off.  The propellant density was less than 25% different at that pressure, so there was clearly two phase flow in the injector elements, reducing the total mass flow.


We made another injector with significantly bigger holes and got the vehicle up in the air for a few flights, but Isp was miserable.  We made another injector with more holes of the smaller size, and it improved somewhat, but it was still worse than the helium pressurized one.  I had been hoping that the self-atomizing nature of the propellants would make things better, but that seems to not be the case.  We are experimenting with other unlike-impinging designs now to try and get the performance back up.  The self-pressurized propellants will hopefully not have the same combustion stability problems we had with the helium pressurized



Once we knew that this was basically working, we stripped off all the insulation on the methane module so it would self pressurize faster.  The lox goes up in pressure faster than the methane, even though it has almost 3x the mass, since the difference in specific heat and boiling temperature more than make up for it.  We load the methane first, but the lox still winds up getting up to pressure and venting first.  It takes about 40 minutes for the preopellants to come up to 200 psi in our current configuration.  A lot of steps on the checklist went away without helium pressurization, but we now have a 40 minute hold between loading the propellant and firing.  The bulk propellant temperature does not rise evenly -- when the tanks first reach 200 psi we lift the vehicle up in the air with the crane and shake it around a bit to mix things up, which usually drops the pressure back down to 150 psi.  It takes another ten minutes to get back up to a more uniform 200 psi.  I want to try letting some dewars get up to 250 or 300 psi for a direct feed-in to a 200 psi controlled tank relief, which would be immediately usable and consistent, at the expense of wasting propellants in boiloff.


We somewhat inadvertently tested one of the major benefits of vapak propulsion -- when we were struggling to get enough thrust for liftoff we started short loading the lox, and we made one flight that went to liquid lox depletion.  The thrust dropped a lot, so the vehicle started to descend from its hover even after throttling up to max, but it continued burning smoothly as it transitioned to burning on the cold gox ullage.  It may be possible to set up a VTVL so that it is normally landing at a fairly deep throttle on liquid propellants, but can still maintain a constant descent for a little while by going to full throttle if it happens to transition to gas flow.  You want to make sure that your run out of liquid lox before liquid methane, both because it is a lot heavier, and because going to lox / gch4 would almost certainly fry a film cooled engine in short order.


We have been able to light the engines with just a spark plug instead of a torch igniter.  We had bangs doing it with the main propellant valves on the unlike impinging injectors, but using the manifold gas purges has been reliable in most cases.  We hope to be able to completely do away with solenoid valves after getting back to a well-mixed unlike impinging injector, but we have decided to stick with torch igniters for the time being.


Starting up and shutting down without manifold purges has worked fine.


Opening both propellant valves identically has worked fine.  With lox / alcohol, we always pre-chilled the lox manifold by briefly opening and closing the lox valve, which prevented us from physically linking the valves together (without adding a dedicated purge valve).  Especially after our recent experiences with valves getting out of sync, physical linkage is looking a lot more attractive.


Development Work


We are evaluating using larger 1” V-cut ball valves from AVCO instead of our current ¾” reduced port ball valves.  This should give us a little more flow at full throttle, but more importantly it should allow a lot more precision at very low throttles.  We have some hope that we will be able to run the engines at a deep enough throttle, almost a “pilot light”, that we could do full 100km suborbital flights without having to shut the engines off and relight them.  This would be a Very Good Thing, although it would make the vehicle unsuitable for true microgravity work.




We are building a linked set of main propellant valves, using one of the Ultramotion Bug linear actuators to drive them.  The KZCO rotary actuators we have been using for years don’t have the torque to turn two 1” valves at the speeds we want, but the Bugs have enough power to handle even larger valves if we ever need them.  Having independent valves gave us a few advantages in the past:  it was easier to plumb things up, we could open the main lox valve to chill the lox manifold without opening the fuel valve, and we could vary the mixture ratio dynamically, although those tables have been identity transforms for all of our actual tests.  The issues we had last year with one actuator moving and not the other, especially the resulting burn-through on Pixel’s engine, are making me think that the advantages aren’t worth it, even though we have since resolved those problems.  For lox-alcohol, we would need to add a dedicated chill valve, but for lox-methane, we could just use it as-is.




This first-cut test has some strength and play issues, but the second version has a longer isogrid plat that also serves as a mount for our spark box and pressure transducer.


Another possibility that linked valves enable is the ability to fly a differentially throttled vehicle with no required position sensing feedback on the actuators.  The control system currently uses vehicle position / orientation, rates, and actuator position to determine a desired actuator position that the motor is driven towards.  This works fine, but we have seen problems with the position feedback on both the KZCO and Ultramotion actuators.  It has never caused us an in-flight problem, but the danger is there.  If we could use angular acceleration as one of the inputs, the output value could just be a motor current, and you wouldn’t need to really care about the actual position.  You would certainly log it for analysis, but it would be like chamber pressure, not flight critical.  With independent valves, this would lead to drastic mixture ratio skews almost immediately, but linked together it would probably work fine.  You would still have to worry about the possibility of completely shutting off an engine, but there are strategies for that.


The problem is that when I tried differentiating the rate gyro signals on a test flight, the resulting rate acceleration signals just looked like noise.  I decided to try some direct-reading angular accelerometers form http://www.cfxtech.com/ to see if the signal looked clean enough to use. It was interesting playing around with these.  They are rated for 25 rads / sec^2, or 1400 degrees / sec^2, and if you roll one of them between your fingers, they are constantly going full scale as you move them around, because they are so easy to accelerate at very high rates.  However, when bolted onto a heavy box, the values you get when the box is swung around stay a lot lower.  On the vehicle in flight, they were even lower.  One ton vehicles don’t change rotation rates all that quickly (our cartwheeling module notwithstanding).




What we discovered was that the wire rope isolators that we mount the electronics box with shake around at something like 50hz.  We got a much cleaner signal when we hard mounted the angular accelerometers directly to the vehicle frame, with no isolation.  When I went back and looked carefully at some of our previous flight data, I can nicely see that the gyros are tracking the box shaking, which was why the derived angular rates were so wacky.  Unfortunately, hard mounting the IMU or the entire electronics box isn’t an option, because the accelerometers pick up too much vibration, and that kills our hover control.


We did a flight test with the hard-mounted angular accelerometers, and the signal does look good enough to consider for control use.  Interestingly, the roll axis was quite a bit noisier than the other two axis, presumably because the micromachined sensing element was in line with the engine thrust.  Probably the really right thing to do would be to hard mount a set of fiber optic gyros directly to the vehicle for accurate body rate and derived angular acceleration, and have a separate comfortably isolated set of accelerometers.  I have been meaning to try  http://www.fizoptika.ru/ gyros, which is what Crossbow uses internally, but they didn’t return my last inquiry, and dealing with a Russian company directly may have some difficulties.


We are building up five new electronics box lids.  At this point, we don’t have a single “virgin” electronics box – they have all been through a vehicle crash at some point.  They still work, but we don’t want to rely on them much longer.  We are only making modest changes in the systems, like bringing out three more analog inputs, adding three dedicated digital inputs for panel switches, and using different relays for the watchdog cutoffs.  We are going to build two complete brand new boxes, but we will hold off on assembling the other three until we need them, since there is almost $20k of parts in each box (dominated by the $11.5k Crossbow FOG IMU).


For years, I have had some enthusiasts extolling the virtues of rapid prototyping to me.  It sounds great – build anything you want directly from CAD drawings in only a few days!  However, the reality isn’t quite up to the promise yet.  A few years ago I tried getting a regen cooled chamber fabricated with SLS, but all the vendors returned a “will not quote” on it.  Recently, we did get pretty good results on some cast chambers from http://www.proivc.com/ , but the surface finish is still rather poor, and the thin wall structure we had made showed a bit of ovaling.  In addition, two other parts I tried to have fabricated there weren’t feasible for the process.


In the last month, we have been working with http://dyna-tool.com on some pintle designs fabricated with a new direct-laser metal system from http://www.3dsystems.com/products/sls/sinterstation_pro_slm/index.asp.  Until recently, most metal rapid prototyping work was “selective laser sintering”, which didn’t produce a completely solid part.  You could bronze infiltrate it as a secondary operation, but that is a poor substitute for stainless steel in most cases.  The latest machines can directly produce non-porous parts in stainless steel, aluminum, and other materials.


Dyna Tool is still learning all the tricks of the process, which is complicated by the fact that the original developers are in Germany, and the initial parts have been somewhat flawed.  There is still a lot of promise here, and we are going to stick with it for a while.  I do think that we will eventually get to the point where we send a CAD file out Monday morning, and we get the part that we want back on Friday for testing n Saturday.  Build time and expense is fairly proportional to part mass in a given material, so it probably isn’t going to be making major structural pieces any time soon, but I can see a lot of detail pieces that would be convenient to make small runs of.  Several of us are practicing up on solid modeling tools now.





We will be taking one of the methane engines to White Sands Test Facility to test nozzle extensions in a vacuum chamber soon.  We have both a traditional bell extension, and a dual bell to test.






Richard Garriott http://www.richardinspace.com/ stopped by for a visit last weekend, which was fun.  The early Ultima games were significant inspirations for my game development career, but despite being in the same industry for over fifteen years, we rarely bump into each other.  His first hand experience with both the Russian and American space programs in valuable, and I learned a few new things just from chatting with him.  I hope we can pick his brain more in the future, and he likes the idea of skydiving off one of our vehicles in the future.  He already has his own space suit…


Now that the new FAA amateur rocket regulations are in effect, we expect to do some higher speed, untethered free flights before the next update.  We should be able to work our way up to nearly 8000’ essentially in our back yard, which is going to be very, very convenient.




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