قسمت1
Trashed a vehicle, Self pressurized methane, Development work
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.
http://media.armadilloaerospace.com/2009_02_12/before.jpg
http://media.armadilloaerospace.com/2009_02_12/burnThrough.jpg
http://media.armadilloaerospace.com/2009_02_12/after.jpg
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
propellants.
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.
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