Headlines > News > Armadillo Aerospace News: Tank insulation, Engine tests, Electronics, Flight tests

Armadillo Aerospace News: Tank insulation, Engine tests, Electronics, Flight tests

Published by Sigurd De Keyser on Sat Sep 9, 2006 10:38 pm
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Tank Insulation

We settled on using Pyrogel insulating blankets from Aspen Aerogels to insulate our tanks: http://www.aerogel.com/products/pdf/Pyrogel_6250_DS.pdf#search=%22pyrogel%20insulation%22

After Phil figured out the right pattern to make the gores for the spherical tanks, it went very well. We used a spray adhesive to attach it, and we tested all the combinations of dusty side / non-dusty side and painted / non-painted for best adhesion. Surprisingly, putting the adhesive on the dusty side worked best. The material still does shed some dust in the wind, but it is reasonably rugged, more so than the fastblock insulation we were previously using, and it is only about $5 / square foot, which is a tenth the cost of the fastblock. It also sheds water fairly well.

I was a bit surprised at how much of a difference insulating the tank made on our boiloff rates. I had been presuming that much of the boiloff was due to heat conduction from the rest of the 90 pound tanks that don’t get cooled that well during filling, but it turns out that the entire tanks get cooled a lot better with insulation. We used to have 10 psi in the tanks after filling, even with the vents open, but now it is just 3 psi and soon drops to 1 psi. We also insulated our test stand tank.


Engine Tests

We did some noise measurements to see just how disturbing we are to our neighbors. In our parking lot, noise is over 100 db, but it drops to 90 at the end of the building, and is only 80 at the main street. Traffic noise is 70, and this is an industrial neighborhood, so that really isn’t too bad. Testing below ground level in our receiving well with the water below it is certainly helping a lot, because our old vertical testing was louder than this with significantly smaller engines. We need to check the noise levels for the vehicle flight tests, which will be worse due to the exposed plume.

The element drilling of the stainless steel injector went better than I expected. I botched the very first element, which we welded over, but all the rest of the 38 unlike impinging elements were milled and reamed without a problem. I had problems with the 1/32” film cooling holes, which are about a 6:1 aspect ratio with this setup.

The elements are 45 degree fuel / 0 degree lox unlike impinging. 38 unlike impinging element pairs, 19 film cooling holes.


We had a rather odd set of tests with this engine. The first run had poor performance, slightly worse than the last aluminum injector. The second run had fantastic performance, 50% better than the first run, even though the only difference was more propellant loaded to make the run last longer. It wasn’t just a data collection anomaly, because the video clearly showed no visible film cooling in the plume because it was superheated much more. We did more runs at various pressure and tried to replicate the high performance run, but it never happened again.

The stainless injector suffered some burning right at the igniter exit hole, but it only slightly yellowed elsewhere. The burning at the igniter hole might have something to do with the high combustion efficiency, perhaps making a continuous ignition torch during the odd run.

I was very disappointed that we didn’t get a consistent high performance with this engine. We had always rapidly melted the aluminum unlike-impinging injectors, so I had hoped that the Isp would be much higher if the stainless kept it from melting.

One of my last theories about the injector performance was that the unlike fans were hitting each other very early because of the small distance between holes, and this essentially just turned the pattern into a single converging cone with less surface area than if the individual fans had more room. To test this, I made another aluminum injector with the exact same 38 unlike impinging element pairs / 19 film cooling holes as the last couple injectors, but with the following changes:

Tapered manifold inserts to keep the propellant velocity equal and higher around the manifold to increase cooling.
Elements twisted so the fans don’t intersect.
Flush drilled 50 degree impingement, 30 degrees from fuel, 20 from lox, so the resulting fans still won’t spread out towards the chamber walls, but also won’t converge towards the center as fast as the 45 / 0 angles we were using.


Note the deeper film cooling cutout to make the holes lower aspect and easier to drill.


In testing this, we saw a short burst of very high performance from the engine, then it got ratty. I believed this to be a sign that the lox was boiling in the manifold, due to the tapered manifolds drawing more heat out and / or the combustion being much better. I added a set of tiny showerhead holes at the inner border of the lox manifold to hopefully create a gas cooling layer.

We had several very high performance runs with this configuration.

A three-camera shot of a long, high performance engine burn:

We finally made the inside of the graphite chamber glow red hot, even with all the film cooling. The outside got hot enough that the radiative heating caused all the phenolic tubes insulating the tie rods to blacken and crack above the nozzle, making a rather disturbing snap-crackle-pop sound after the run.

The bolts needed to be re-torqued after the run. We don’t know for sure if the tie-rods actually stretched due to the chamber thermally expanding (I really don’t think they got very hot with the phenolic tubes over them), or if the bottom clamp plate relaxed a bit more. I’m leaning towards the clamp plate being the problem. Cesaroni Aerospace is going to modify the design for some of our new chambers so we can grab the chamber from a flange at the top, leaving most of the engine hanging free below. This will save a decent amount of weight, and avoid most of the heat concerns.

When we increased the feed pressure to 400 psi, the injector burned through around the middle of the lox manifold ring. I made a new injector that had twice the film cooling holes to get the chamber temperature back down, and I staggered the lox cooling holes with half of them on the middle radius where it burned through previously. For some reason, I was having trouble drilling the holes, and I wound up adding a big countersink on them, thinking that might be a positive thing to cause the lox to spray out and vaporize faster.

The non-countersunk holes were machining errors, they don’t go all the way through.
Note the doubling of the film cooling holes.

After doing some flight tests with this engine, we saw that there was erosion on the face:

The countersunk holes are a mistake, they act as flameholders. We are trying an experiment by welding over all this mess, facing it off, and re-drilling just a single circle of straight holes at the outer band. I didn’t have any trouble drilling the straight holes this time, but we haven’t tested the injector yet.

I had already modified the stainless injector with this same pattern before we found the erosion, but it doesn’t seem to be burning it yet.

New Electronics

We have completed the new electronics box and transitioned the current vehicle over to it, and the second box is almost complete. The old box got stripped of its GPS and IMU and dedicated to the test stand. This is the first time we have ever had two complete sets of electronics at one time.

The final push to retire the old electronics was that we needed more driver channels to independently control each roll thruster to allow us to move propellant from side to side for dynamic balancing. The old box required us to tie together pairs of roll thrusters and purge solenoids, due to lack of channels (we burned one pair of drivers with a short). With the new box we also get current sensors on the drivers, more analog channels available, watchdog shutoff of up to four drivers (so we can do a four engine differentially throttled vehicle next), and what should be a higher reliability construction.

One of the three PC boards we had assembled at a board shop had a significant error – the identical looking 5v and 12v DC-DC converters were swapped. It is a very good thing that Russ checked that before we put an expensive computer or GPS on the system. The power and ground planes are so generous on this board that Russ wasn’t able to unsolder the parts with his equipment, so we have to send it back to the assembler to be fixed. They did turn it around in a single day, which was appreciated.

We planned on moving to the new Thales DG14 GPS boards, but despite what they claim, they don’t seem to be 100% software compatible with the G12 boards that we have been using, so we have an older G12 in there until I can figure out why my init sequence isn’t working.

I was just looking for some manuals online, and I found that Thales Navigation was just sold to Magellan a couple days ago. This particular board family started out with Ashtech, was sold to Thales, and has now been sold to Magellan, just over the years we have been using them. Changing to a different model wouldn’t be a large hardship, but I hope we don’t get orphaned.


Flight Tests

Disconnecting the paired tank ullages helped significantly with the vehicle balance problem, and if we carefully balance the weights before liftoff, we can have good flights. Now that we have sufficient driver channels, we can cause propellant to transfer from tank to tank in flight by firing only a single roll thruster instead of a pair, causing a drop in pressure in one tank, which allows propellant to flow over from the opposing one. We are probably going to do a manual trim for this, rather than a very low frequency automatic control loop.

With the tank ullages disconnected, the lox absolutely MUST stay below the saturation point, because at saturation, any change in the heat input to the tanks will result in different amounts of boil off, and a couple psi difference in the tanks leads to a large propellant transfer. Our insulation is pretty good, but we won’t want to sit on the pad for a half hour after pressurizing. Because our tank pressure drops during flight, there is the possibility that we could not be at saturation at liftoff, but reach saturation during flight. That would be bad.

Flight 5: August 12, 2006 hop1
Added tank insulation
Increasing oscillations, hit ABORT_TILT, then quickly hit SHUTDOWN_TILT

Flight 6: August 12, 2006 hop2
Cut all the gains in half to make it less twitchy
Much smoother
Ran to propellant depletion

Flight 7: August 15, 2006 hop1
Discovered that one of our bubble levels doesn’t read right upside down when centering the gimbal.
Increased gimbal range to 0.1 – 0.9 instead of 0.2 – 0.8 for a bit more control authority. The linear actuators don’t have limit switches, so I don’t want to stall them at their limits in flight.
Removed purge during igniter startup to prevent an initial transfer of propellant between tanks. We had added this when we thought we had fed some lox into the fuel manifold during a pre-chill while horizontal testing, but that turned out not to be the problem, so removing this doesn’t hurt.
Nice flight, drifted a bit, manually shutdown as it was starting to pull on the tethers.

Flight 8: August 26, 2006 hop8
When we moved to the new electronics box, our hacked-up tether shield didn’t fit anymore, so we tried a new idea to keep the tethers from catching on anything – we put a five foot piece of PVC tubing over the end of the tether, so the vehicle would have to go at least five feet higher than the bungee stretch before a tether could loop over one of the protruding parts of the vehicle. This seems to work well so far.
Used staggered cooling hole twisted injector.
We added our “payload” weights for the Lunar Lander Challenge to the vehicle as barbell weights right at the vehicle feet. We might as well move our CG a little farther away from the gimbal point…
Flew to propellant depletion.
Suffered injector erosion.
No video.

Flight 9: August 29, 2006 hop1
Stainless steel injector.
Added automatic propellant pumping code.
It drifted a lot on liftoff, and I didn’t like the way the plume looked, so I manually aborted it. Inspection didn’t reveal any problems.
The hard drop on the tether caused the engine to slip a bit in the gimbal mount, resulting in an off-center engine that needed to be fixed. We need to tighten the set screws harder.
(all combined two-camera videos are wmv files instead of mpeg due to aspect issues)

Flight 10: August 29, 2006 hop2
Drifted to tether limit, simultaneous ABORT_ROLL_CONTROL and ABORT_TILT
A rolling liftoff pipe knocked over the second camera at the end of this video, so we welded little bits of angle to the pipes so they don’t roll as far.

Flight 11: August 31, 2006 hop1
Disabled automatic pumping code. It may have been doing the wrong thing on the last two flights, causing increased drift.
Doubled position hold gains relative to orientation hold gains.

Perfect 35 second flight!
A view from just one camera for folks without windows media player:
There is still some stable tilt in the system, so we are going to try and trim that with manually commanded propellant pumping, because I didn’t like what the automatic system was doing.

Flight 12: September 2, 2006 hop1
Packed up and headed for the Oklahoma Spaceport, hoping to do a > 90 second flight.

Joseph now has a CDL with hazmat certification, so we can transport large quantities of all our consumables wherever we need. We outfitted our current trailer with all the placards, logs, and extinguishers required, but we are probably going to move to using a rented crane truck in the future, which will allow us to move enough consumables for multiple full duration flights. There is definitely something to be said for having everything you need on one truck, instead of relying on things to get delivered to your test site.

Unfortunately it rained the entire day. We tried to fly the vehicle a couple times when it cut down to just sprinkling, but the GPS PDOP values never cooperated. The flight computer currently will not initiate a flight if the PDOP is over 275, and it will abort and auto-land a flight if it jumps over 400 during the flight. We occasionally got a PDOP under 275, but it never stayed there long. We may be able to relax these numbers, but this is the first time we have ever seen it this bad, so I am probably going to stay conservative. We probably shouldn’t be flying in the rain anyway. We got the vehicle off the stands twice, but it did a GPS abort within a couple seconds. We are probably also suffering a bit from vibration at the higher thrust levels required for flying with a full propellant load, so we are also going to add foam insulation to the new electronics box. The upgraded GPS boards are supposed to be more vibration resistant, so it will be nice to move to them after I figure out what changed in the initialization sequence.

500 pounds of ethanol / 700 pounds of lox loaded
ABORT_GPS due to PDOP exceeding 400, steady auto-land sequence.
Repeated twice before the lox reached saturation and the liftoff attempts became immediate tip-overs.



We are sometimes getting some raw ethanol pushed out by the purge at the end of the flight when the combination of gimbal angle and tether swing is just right. If the film cooling holes were at the extreme outside edge of the fuel manifold it would always get pushed out very quickly, but they are inset enough that if everything is hanging at an angle there can be some fuel retention on one side, which gets blown out on the next swing from the tether.

Our shock absorbers are frozen up again. We have had corrosion problems with them from sitting in puddles of water after the lox frost melts. We tried greasing them up, but it didn’t seem to do the trick. We may need to fabricate the side load assembly ourselves.

We have found that the Jefferson piloted solenoid valves we use for roll thrusters and engine purges will actually open briefly if you hit them, which is extremely disconcerting. Our pressurization hose tends to hang close to the roll thrusters, and if it swings against them we will usually get a burp from the thruster, which can disturb our balance. Again, I am looking forward to moving back to a differentially throttled vehicle next.

Pretty much everything is ready to go on assembling the next vehicle. James has finished welding all four spheres, and all the parts are here. We have often said that it would be cool to do a time lapse video of assembling the entire thing, but in reality we wind up moving the vehicle around to different parts of the shop during assembly too much.

In the NVIDIA-theme, our two vehicles for the X-Prize Cup are named “Pixel” and “Texel” (Pixel get an added bonus by being a Heinlein character).


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