February 5, 2007 update
Pixel flies again
We made three good long flights with Pixel this month. There are three significant changes since the
X-Prize Cup: new landing gear, regeneratively cooled engine, and new gimbal
The landing gear is plenty strong now, but we have had them
fall off a couple times when we lift the vehicle up after a landing loads them
enough to upset the set screws holding them on.
We will drill explicit holes for the set screws so they act as pins even
if they loosen. The gear on the lox
tanks is also fairly sticky when cold, only very slowly extending after lifting
away from the ground. We had put some krytox lubricant on the sides, but we may be better off
leaving it just metal-to-metal. We are
considering adding some desiccant inside the gear to reduce the chance of ice
The first set of tests on the cooled engine resulted in the
graphite chamber cracking when it got hot.
We had carefully measured everything to give us some room for thermal
expansion, but the welding of the bottom closure to the tube had caused enough
warping that we were still torquing the injector down
directly onto the graphite chamber, instead of only to the outer aluminum
flange. We machined 0.030 off the end
of the next chamber for clearance, and it is still pristine after a 100 second
and a 70 second flight. We are strongly
considering going to a retaining ring based assembly to avoid the weld warping
and save the weight of the flanges.
The o-ring between the graphite chamber and the aluminum
injector is causing some trouble. We
have frozen it with lox pre-chill or purge, and we are getting four spots of
burning on it between the spokes in the injector pattern. We are going to try a helicoflex
metal seal from http://www.helicoflex.com/assets/pdfs/helicoflex.pdf.
I am still a bit concerned about heat soak after shutdown,
since we have 25 pounds of fairly hot graphite.
The o-rings would be the first thing to go, but it isnt out of the
question that some of the aluminum might get hot enough to lose temper. I am letting the entire fuel tank ullage bleed down to 50 psi
through the engine to provide some cooling after shutdown, but we have the
option of dumping some lox on it for additional cooling if necessary. I always had fears of the lox igniting the
graphite like a hybrid grain, but it has never been a problem.
We changed our purge plumbing so that it only blows the
injector manifold dry, not the feed pipe and cooling jacket. This avoids dumping a fairly significant
quantity of flaming alcohol out of the engine on shutdown, and has the nice
benefit of leaving the alcohol in the cooling jacket to absorb heat after
shutdown. You can hear it boiling after
the purge has shut off, which is a pretty good indicator that the temperature
isnt too terribly high, or it would have been gone a lot sooner. We probably have plenty of margin
with the current injector, but I fear that if I removed the film cooling to
increase Isp, the chamber would be a whole lot
hotter, and we might have a problem.
We have changed our gimbal linear
actuators to bugs from Ultramotion: http://www.ultramotion.com/products/bug.php.
The Electrak units that we had
been using were cheap, but we had to swap the motors for custom units to avoid
the thermal cutoffs, and we had to make internal modifications to reduce the
backlash. They were probably still fine for
our old, small engines and vehicles, but I have been pretty uncomfortable with
them on the big motors. One of our
flights had a data trace that showed a temporary drop in speed on one of the
actuators that looked like it was nearly stalled, and one of the actuators had
broken internally after the last fall-over-on-side flight.
I had originally selected the geared motor, but Ultramotion pointed out that the non-geared motor, combined
with a different lead screw pitch and pulley, provided the same speed at
greater force and efficiency, as well as weighing slightly less. The pulley drive arrangement is quite convenient, we swapped pulleys for a faster speed after we
did our initial testing.
The units cost quite a bit more than the Electraks,
at just under $1000 each, but we are now faster and stronger than before, and
we have some growth room if we need to add more speed or power in the
future. The hardware design is a lot
higher quality, and the extra mounting flexibility is nice. Ultramotion is more
than happy to work on rocket projects, and gives fast turnaround time.
We did have some issues with the pot feedback. The feedback is from a linear pot, which is
generally a good thing, especially if you have looked at the flimsy geared
arrangement on the Electraks, but the resistance went
slightly out of our range. We have 12V
excitation for all sensors, but we can only read 10V on the A/D channels. We have always been ok on our previous
sensors, but we had to add a resistor to get the Bugs in range. More seriously, one of the units we got had a
short-to-case at one point in the travel on the pot feedback. Ultramotion sent us
a new one next-day delivery, but we also found another issue: when the motor is
driving the screw into the bump-stop one direction, the pot seems to fall off
the end into undefined space. The range
is valid over the entire normal travel, but compressing the bump stop must push
it over the edge. The other one didnt
do that, so it is probably a result of having the stop set a few thousandths
An unexpected result of the high efficiency ball screws is
that the engine assembly wont hold its position without active control, it slowly slides back to a position slightly off
center due to the tension in the feed hoses.
We had to make a minor change in the way we calibrate the actuators due
In addition to the vehicle changes, we are continuing to
improve the ground support hardware and procedures. The pressurization hoses have been modified
with some different vent valves and check valves, allowing everything to be
done by one pad crew member, instead of requiring two people to open two tank
valves at the same time.
This ground liftoff flight was intentionally cut short to 60
seconds, because we still didnt know if the regen
engine was holding together properly.
Cracks in the chamber just result in higher fuel consumption as fuel
leaks into the chamber without going through the injector, so I would hate to
have the vehicle drop to the ground at 87 seconds because it ran out of
fuel. It turned out not to be a problem,
and the residuals were as good as ever.
I landed the vehicle off to the side intentionally, so it wouldnt be on
the concrete that had been cooking for so long.
I am investigating the oscillations that picked up later in the flight, it could be either the faster gimbals, the new roll
thruster locations, propellant slosh, or some pull on the tether bungees. It settled
down before landing, which is odd, and perhaps points to slosh.
Many of you are probably sick of quad flight videos, but
this one has the on-board camera pointing at the engine, which is an
interesting view. It also shows we can
land without breaking now
We have a number of things to be testing and improving with
tethered flights this month, but we hope to do untethered
flights in Oklahoma
next month. Im happy to hear that a
number of new teams are springing up looking to compete in the lunar lander challenge (I know of at least six prospects, outside
of the 2006 entrants), but I think it is going to be a long shot for anyone to
beat us to first place. The once-yearly
nature of the competition makes for a much livelier competition, but I will
bitch and moan about it when we do fully qualifying test flights without being
able to collect the prize. I do
encourage everyone involved to consider keeping a blog
of their progress. Even if you arent
comfortable doing it live where your competitors can see, it would be cool to
read a history start-to-finish at some later point.
We are probably going to wind up with a fuel feed pipe going
through the lox tank in the new design.
There are certainly concerns with that, but we can arrange to have a
completely seamless tube through the tank, with no weld joints separating the lox
and fuel. This arrangement gives us a
very convenient arrangement of all our gear under the tanks, and should allow
us to change to a different tank technology (filament wound or flowformed) with very little pain.
In theory, the strains on a through-pipe welded at both ends
would be the same as anywhere on the sphere, but we fabricated an aluminum
bellows to allow the tank to grow during pressurization without straining the
pipe. Bending aluminum like this is
certainly not good from a fatigue standpoint, but the total number of pressure
cycles isnt going to be all that high, and a failure
would only be about like a big burst disk letting go.
Our first design had an 8 diameter flat bulkhead on the
bottom, allowing us to mount the engine u-joint directly to the tank center,
but it bowed so much that the bellows bottomed out and the inner tube broke
during hydrotest at 790 psi. We have since redesigned to allow the
through-pipe to go down the exact center, and mount the engine below the
We tested several different pipe configurations for the
impact on ethanol in a cryo temperature tank. We filled a Styrofoam cooler with liquid
nitrogen, and made test tubes in different configurations filled with ethanol
(90%). The bare pipe obviously froze
first, followed by the 1/8 thick polyethylene liner, followed by the 1/8
thick phenolic liner, followed by the ¼ thick pneolic liner. By
far the best was a 1/8 air gap held by a couple o-rings, separating a
tube-within-a-tube. It took about 40
minutes to really freeze it, and at 20 minutes it was still completely liquid
(although < -100F !). We would be having worries about low lox
densities before the fuel froze.
This was a very interesting test, because alcohol doesnt
freeze like water does. It just gets
more and more gelatinous, rather than clearly crystallizing. When it got very cold it took on a honey like
consistency, and just got thicker and thicker.
When we removed the tubes from the liquid nitrogen and let their outside
warm slightly, we could dump an alcohol slug out of the tube. Also of interest, the solid slug was pretty
much invisible in a bucket with alcohol in it, not showing the clear refraction
difference that water does. This test
leads me to doubt the viability of subcooled propane
/ lox, a combination sometimes considered for launch vehicles because of the
better density. While it may technically
still be a liquid at lox temperatures, it probably isnt very fluid, and an uninsulated wall common with a lox tank would likely have
quite a bit of residual propellant clinging to it, especially if it was a
completely submerged tank.
We are going into production on five individual modules (two
tanks + one engine) for our first run.
We intend to fly each module as an independent unit, and we will
probably fly a two module system in Titan II differentially gimbaled form, and
a four module system in differentially throttled form with the gimbal actuators replaced with tie rods. We may also fly a four module system with
four gimbaled engines, controlled by two independent electronics boxes for full
system redundancy. We want one module as
a spare, in case we mess one up.
This means that we are producing parts in lots of five
through forty parts each, depending on how many go on each module. This is a great design exercise for us, and motivates
us to do everything CNC, instead of just cobbling things together with a band
saw / drill press / welder. Russ is
working on the computer mounts, and James is starting to learn CNC programming
so all the load isnt on me for part programming.
We went through several different designs before settling on
a simple bracket arrangement at the tank equators for holding the modules
together and acting as leg attach points.
our first 5383 (Sealium) tank to failure, with good
results. The tank was 36 ID, ¼ prespun thickness, like most of our previous tanks. The source blank to spin these hemispheres is
just under 48 diameter, so there is a fair amount of
thinning in some places as the 1800 square inch blank deforms into a 2034
square inch hemisphere. The spheres are
81 pounds dry and 950 pounds full of water, for a mass ratio of 11.7.
The burst was at 860 psi,
significantly better than our previous results.
There is likely still some room for improvement, as the initial burst
point was at one of the spots where James had a hole on the root pass, which
meant that it had a broader area with nothing but filler material when it was
closed back up. We are going to consider
moving to either 5083 or 5556 welding wire (from the current 5356) to increase
the strength of the raw weld that doesnt alloy any of the base metal in.
I have been working on a document with all the various
trades in aluminum tankage, but the bottom line is
that for tanks used in the as-welded condition, 5059 (Alustar)
is the clear winner, and we will probably be moving to that in the future,
although there is a fairly large minimum order.
7039 or 7005 can deliver similar strength, and possibly a bit more with
post-weld aging (somewhat countered by the higher density), but the welding is
more challenging, and the alloys need to be painted / coated for corrosion
resistance. After that, 5383/5083/5086,
in that order, are the best bets. 2219 looks best if you can solution heat
treat the entire assembly after welding, or if you can provide thickened weld lands. If you can friction stir weld, 2195
lithium-aluminum wins big. I do not
think there are any prospects of us FSW our tanks.
Several other teams are also working with hemispheres from www.amsind.com. They are great to work with, and the prices
are good, but the spinning is not CNC, so there is a fair amount of variability
to deal with.
Paul Breed reports at http://unreasonablerocket.blogspot.com/2007/01/normal-metal-tanks.html
on a smaller and thinner stainless steel sphere. I suspect that the welding was the weak point
of the test, and with practice they could do better, but with just that data
point, the aluminum tanks look significantly better.
Kevin Sagis reported in private
tested two 20" 5086 tanks today. Material was 3/16" prespun thickness.
One failed at 925 psi, other about
850 psi. The 850 failure appeared to fail
in weld, the 925 failure definitely failed in HAZ.
The one that failed at 925 had an equatorial thickness of only 0.125" in about
6" of circumference (the other tank had a minimum thickness of 0.150")
so we clearly did a better job on it.
Our tanks have done somewhat better than that by direct
pressure/volume comparison, so he is corresponding with James on exact welding
XCOR has also bought some smaller hemispheres from them for
tank ends, but I dont know how they turned out.
Here is James welding document with lots of details:
We have been operating with a fairly standard business
liability insurance policy for our shop (which was still quite a challenge to
get), but for us to perform flights under an experimental permit or launch
license, we need to have explicit insurance for third party damages, and one of
the sites that we are considering for larger static / tethered tests is also
requiring us to have additional insurance.
The X-Prize Cup covered the insurance at the event last year, but they
werent required to, so we may need to cover our own operations there this year
Phil did all the work on this, here is his update:
Procuring insurance for Armadillo operations has not been as
horrible as it could have been. A little research led us to Ralph Harp of
Falcon Insurance in Houston.
Ralph has been doing "Odd-ball" insurance as he puts it, for 20
years. His specialty is Aviation, balloons, Smoke jumpers, and other various flying
things or activities. He likes Aerospace and Rockets, and has insured
several sounding rocket launches.
Ralph and his underwriter Chris Barnett with Houston Casualty Company are
excited to support the budding industry of commercial space access. They
understand the industry, speak the language, and are willing to accept a
certain amount of risk while providing a reduced cost premium for the AST
required coverage for 3rd party liability as well as operational liability for
every day testing.
Through the process of negotiating the rate and assessing the risk, the more
information we provided the more they reduced the premium. The video and
description of past failures helped them quantify their actual risk based on
history. Something that is seriously lacking for them use as
justification for reducing premiums.
We have finally ironed out a deal to cover an unlimited number of free flight
tests under AST issued Launch Permit or Launch License, as well as coverage for
static testing and tethered flights for a 12 month period regardless of the
location. We are not able to disclose the exact premium amount, but I can
say it is under $50K and over $40K. Your mileage may vary depending on test
locations, total liability, and how much information you are willing to share
with the underwriter.
The coverage amount is $1M for static and tethered vehicle testing and $3M for
free-flight testing under Permit or License. This represented the minimum
amount of coverage AST would accept for the Permitted flights at the OKSP
If we are to apply this to another location we will have to mitigate risk to
the same level, which just so happens to be a single 3rd party casualty.
This is the minimum level of coverage AST will allow at this point, and they
appear to be unwilling to move on this for now.
The insurance agent and the underwriter request that we keep them in the loop
and treat them like a member of the team. The better we build this relationship
moving forward, the easier it will be to increase the limits of liability for
much larger endeavors such as orbital flights.
Ralph said he would be happy to speak with anyone regarding coverage of their
test programs. They can provide single event coverage as well as yearly
Ralph B. Harp
Falcon Insurance Agency of Houston, Inc.
Ph: (281)540-8822 or 1-800-880-8822
If you are developing a product that is intended to fly, you are in the aviation
industry. That makes your insurance more expensive!
If you want to go over seas to Lloyds, they won't even talk to you unless the
premium amount is over $50K. They wanted upwards to as much
as $100K or more for what we needed.
If there are only 1 or 2 people willing to accept the risk, the policy will be
Don't forget that the guy doing the underwriting has someone looking over his
shoulder asking him to justify the numbers with data, and we don't have much by
comparison to other aviation endeavors.
Sharing your failures and lessons learned is a good thing! They figure if
you don't share those things you must have something to hide, and the premium
goes up very fast.
AST's Graphical Risk Matrix is a helpful way to quantify risk level of certain
elements of your flight systems. It's easy for the underwriter
to interpret that kind of data.
Again, they like information overload. It helps them place odds on weather
you "can" fail in a fashion that causes 3rd party damage.