October 3, 2006 notes:
Matt has done some work on the main Armadillo web page to
include our Nvidia sponsorship and X-Prize Cup
mission patch design, but he also gave the picture and video gallery pages an
update, so they aren’t four years out of date.
If you haven’t followed all the individual updates, they are worth a
look.
Development
I’m not going to review every single flight test, but some
of the things we have been changing are:
We have moved to using a crane truck instead of a big
palette lift for holding the tethers for vehicle tests. This gives us more height, lifts straight up
instead of tilting up, and now that Joseph has a CDL, we can load everything
onto the truck and drive it to our test sites, rather than coordinating a
rental delivery at the site and having us arrive separately with a trailer. It is a bit more expensive, but worth
it. We have rented it all the way
through the X-Prize Cup.
http://media.armadilloaerospace.com/2006_09_21/craneTruck.jpg
We fixed our shock absorber sticking problem by adding very
strong springs to force the piston out even if it is a little sticky, and added
round balls for rougher terrain landings.
The springs actually keep the vehicle slightly off the shocks when
empty.
http://media.armadilloaerospace.com/2006_09_23/newShocks.jpg
A full load of fuel is 110 gallons (not a firm limit for blowdown, but one 55 gallon drum per tank is convenient),
which we were getting very tired of moving around with our tiny little
peristaltic pump, so I finally bought a 25 gpm
rotating vane pump. We found that pumping
ethanol dries out the pump vanes, causing it to stall on startup sometimes. The solution is to give it a shot of WD-40 in
the inlet before and after use.
We moved to pressurizing all four tanks simultaneously,
rather than in pairs. It is possible
that we may yet trim the pressure balance a little for perfect fuel depletion,
but this is generally a good simplification.
I also realized that we could just use high pressure air hoses for this
instead of braided stainless hoses. The
new hoses are just as heavy, but they coil easier and don’t get any sharp wire
bits to poke holes in your hands.
We have gone through a couple setups for the on-board vehicle
video transmitters. We will have two
video streams, one looking down and one looking out.
We have tested our independent thrust termination
system. Russ’s company, Long Range
Systems, makes various pager systems, so he adapted one of their products to be
able to close our fuel isolation valve when commanded.
We shortened out lifting bars / tether attachment points so
we can leave them in for future over-the-road trips, and just carry a single
lever bar for tilting the vehicle up during balancing. We had originally thought it good to make the
vehicle moveable by hand with four people, but since we always have a lift
truck or crane for testing, it turns out to not be important.
On September 9th, we had a fairly successful flight
test in Oklahoma. We used the straight unlike-impinging (45 / 0
degree) stainless steel engine with a single row of lox cooling vents. We had intended to do a 90 second flight, but
we aborted the flight initially at 17 seconds to check the engine. It turned out to be ok, so we re-pressurized
and flew for another 65 seconds before hitting lox depletion. We always get some propellant imbalance after
we abort a flight and repressurize, so I had to trim
a fair amount right after liftoff, but it generally went well. If we hadn’t vented in between and let lox
boil off it would have cleared 90 seconds, but it wouldn’t have been close to
180.
http://media.armadilloaerospace.com/2006_09_21/OKSP65sec.wmv
The like-impinging stainless steel injector got quite hot,
but did not burn. The lox cooling vents may
be hurting, rather then helping. Analyzing
the telemetry showed that it went in and out of mildly rough burning with a
period of 15 seconds. I believe that was
lox boiling in the manifold when the injector got too hot, with resulting
cooler and rougher burning, which eventually let it cool down enough to get
liquid lox out. This injector is on the
shelf as a possible backup, but all future flights have been with “twisted
element” injectors.
http://media.armadilloaerospace.com/2006_09_21/postFlightInjector.jpg
We did some shop hops with the first aluminum twisted
injector, and the performance was better, but still not good enough for 180
seconds. We went ahead and made a
stainless version to do a qualification flight for the easier challenge, while
we continued work on other injectors.
http://media.armadilloaerospace.com/2006_09_23/stainlessTwisted1.jpg
http://media.armadilloaerospace.com/2006_09_23/stainlessTwisted2.jpg
http://media.armadilloaerospace.com/2006_09_23/stainlessTwisted3.jpg
90 second flight
Randall Clague from XCOR, is going to be our designated safety observer for the
X-Prize Cup, so he came to witness the test, along with a couple people from
FAA-AST.
This is a really big video that isn't all that exciting:
http://media.armadilloaerospace.com/2006_09_23/PixelQualification.wmv
http://media.armadilloaerospace.com/2006_09_23/PixelGroupShot.jpg
There were two issues we were concerned about right after we did this
flight: The lox pressure dropped a lot
more than the fuel pressure, causing a drastic mixture ratio change, and we
wound up burning some stainless from the injector at the very end, probably
because with the low lox pressure not bending the unlike impinging fuel
elements straight down, we got a recirculation region that we don't get at
normal pressures. There were also some
attitude oscillations towards the end of the flight that I didn’t like.
We didn't realize it until I looked at the side view video and telemetry back
at the shop, but all the problem turned out to be was that the vehicle position
drifted enough that it was tugging on the tether bungee cords, which caused
lots of firing of the lox side roll thrusters, depleting a lot of lox ullage pressure, and also caused the oscillations. If
we had been flying without tethers, we wouldn't have seen either issue.
There was a slight drift in the integrated velocity position versus the true
position over 90 seconds of about two meters, which contributed to my not
realizing the exact position of the vehicle and the tethers. There were two separate problems with the
on-board video reception, we have it working better
now.
While the mixture ratio was pretty far off towards the end
of the flight, we could still tell based on the remaining fuel that the vehicle
probably wouldn’t have been able to make the 180 second flight, even if we
loaded 110 gallons of fuel.
I have been making several software improvements to the
flight control based on our testing. The
existing auto-hover logic just aimed for a zero vertical velocity, but it could
still slowly drift up or down, requiring continuous monitoring of
altitude. I changed this to an explicit
altitude hold when you release the hat control, which removes one of the things
I needed to worry about. I also added an
automatic descent from altitude based on the flight time and current altitude,
so even if I’m not paying attention, it should always get itself down before it
runs out of propellant (assuming nominal engine performance).
We finally made some significant strides in engine
performance last week. We took the basic
“twisted element” design, but broke it up into two rings of elements instead of
just one, and also thickened the injector deck so the holes have a greater L/D
ratio. Our first attempt at this, which
bridged from the outer fuel manifold to the new inner one, had bad startup
behavior because of the long trip that fuel had to make to get around the main
manifold, up the bridge tube, and back down into the inner fuel manifold. We fixed this in a second revision by
fabricating a (rather ugly) T-bridge directly from the valve inlet to both
manifolds.
http://media.armadilloaerospace.com/2006_09_30/FOF1.jpg (FOF = Fuel Ox Fuel, the manifold layout)
http://media.armadilloaerospace.com/2006_09_30/FOF2.jpg
http://media.armadilloaerospace.com/2006_09_30/FOF3.jpg
http://media.armadilloaerospace.com/2006_09_30/FOFspray.jpg
http://media.armadilloaerospace.com/2006_09_30/bridged.jpg
Our baseline shop hover test flight time rose from 35 to 45
seconds, so we now think we have sufficient Isp for the full 180 second flight. We also did a liftoff test with a full load
of propellant, which shows we have sufficient thrust, but the aluminum injector
started to melt. We are cutting a
stainless version now. If this doesn’t do
the job, we have a couple more things to try, but time is running out…
http://media.armadilloaerospace.com/2006_09_30/FOFflight.jpg
The melted aluminum injector was actually pretty favorable,
it held out for 17 seconds at full weight liftoff throttle before melting (not
burning), and the melting was very even all the way around. The stainless one will probably work.
http://media.armadilloaerospace.com/2006_09_30/melted1.jpg
http://media.armadilloaerospace.com/2006_09_30/melted2.jpg
X-Prize Cup
As a favor to the XPC organizers, we agreed to test a
proposed launch/landing pad concrete slab by flying a vehicle over it to see
how the surface held up. Local New Mexico civil
engineering students did the design and fabrication of the 4’ x 4’ test slab,
and one made a visit to the shop for the test.
It didn’t turn out so well. We
didn’t have a perfectly straight liftoff, and when I swung the vehicle back
onto the test pad, the exhaust plume actually kicked the 600 pound slab of
concrete around quite vigorously, causing it to break into pieces before we
could really get any thermal load data on it.
I wish it had been reinforced (the actual pads will be), but I wouldn’t
have predicted that behavior.
http://media.armadilloaerospace.com/2006_09_30/before.jpg
http://media.armadilloaerospace.com/2006_09_30/after.jpg
http://media.armadilloaerospace.com/2006_09_30/breakSlab.wmv
(this was the hardish start from the first FOF
injector design)
Texel,
the second Quad vehicle, is assembled and almost ready to fly. It still needs one lox tank insulated, and various
calibrations. We should be hopping it
any day now. I have configured
everything so both vehicles and two control laptops can all be operating simultaneously,
so we can actually fly them together if AST approves it. I still think Vertical Drag Racing would be a
great attraction for an event. We have
talked with the Rocket Racing League about it, but if anyone else is interested
in doing this, let us know.
http://media.armadilloaerospace.com/2006_09_30/pixelAndTexel.jpg
While it looks like we won’t have any actual competitors for
the vertical rocket challenge and the lunar lander
challenge this year, there is still a bit of drama with just our
performance. If we had a couple more
months it would look certain, but we are getting down to the wire now. It looks like the latest engine design has
sufficient Isp and thrust to
do the 180 second flight, but we haven’t demonstrated it yet. The 50 meter climb and 100 meter translation
for the prizes will also be new territory for the vehicles. We can’t test those before hand, because they
require a launch permit, and any failure is likely to be a loss-of-vehicle, so
we might as well do the spectacular crash for the paying audience. I’m fairly confident that it will work,
though.
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