December 6, 2003 notes
All the new engine angle shims or tack welded in place so we
cant get any of them cocked at the wrong angle on subsequent assemblies.
We added a large battery mount and relay for powering four
glow plugs on the vehicle.
All of the motor valves are bolted to welded brackets, and
oriented horizontally. This requires a
90 before the engine, which hurts flow a bit, but makes it much more
robust. The KZCO valves have tended to
break the actuator away from the valve upon a hard landing when they are
We disassembled everything from the base of the vehicle and put
an insulation coat over the bottom of the tank to protect it from the engine
backwash during liftoff. We put two
gallons of fastblock-800 http://www.tamfg.com/pdf/FB800-Fastblock_800_Series.pdf
on the base of the tank in two coats.
One gallon weighs about four pounds, and we needed to mix in about 750
ml of water to get a good consistency for spraying.
We are looking into options for spraying RTV to replace the
fastblock, which is quite expensive and somewhat fragile. RTV wouldnt hold up on the actual engines,
but it would be more than sufficient for insulating the composite structures
from exhaust and suborbital reentry temperatures.
Our flight test pad is fairly thin, non reinforced concrete,
so an eye bolt sunk in it wouldnt do anything at all to stop a runaway
vehicle. We fabricated a rocket anchor
to guarantee that if we have any catastrophic failures in our early testing,
the vehicle wont get away. The base is
a 63 diameter steel tank end that we originally purchased when we were mocking
up our top cabin arrangements. We cut
in half a 10 section of thick C channel and welded it together to make a box
section, welded that to the tank end with extra support braces, and added a ½
thick plate on top with a 1 stock eye bolt.
Joseph buried this up to the eye bolt at our test site, so this should
easily handle 30,000+ pounds of lifting force.
Between the eye bolt and the vehicle base is a 20 nylon recovery strap
insulated inside tubular ceramic insulation from Cotronics. The recovery strap will stretch up to 4
before breaking , which greatly minimizes shock loads.
We expect to do two flights hanging under a crane, then two
flights ground launched without the crane, but still with the anchor, then fly
Joseph has also been doing a lot of site prep work, clearing
the one fence line that went down the middle of our property, and mowing down
the overgrowth. We also moved three
water barrels with stands, spigots, and buckets out to the shed for field use.
It looks like we are going to be able to use the Ashtech
G12-HDMA GPS http://products.thalesnavigation.com/en/products/product.asp?PRODID=74
for auto-hover and auto-land. It
updates at 10 hz, but the velocity data is cleaner than what I got from smoothing
and differentiating the laser altimeter data.
Operating the laser altimeter from the vehicle base would be tricky for
a couple reasons, and we need the GPS for range safety and altitude
verification anyway, so this saves a system for powered landing flights. The position data still has the normal GPS
discontinuities as conditions change, so for auto landing I plan on having the
vehicle aim for 2 m/s descent speed at 3 meters above the launch position, then
hold that velocity until the accelerometers register ground impact.
In the fast update mode that I use the board in, position and
velocities are output in ECEF (Earth Centered, Earth Fixed) coordinates instead
of normal gps lat / lon / alt coordinates.
This turns out to be really nice a proper orthogonal coordinate system
with the origin at the center of the earth.
I create a surface tangent coordinate system at launch time, and report
all values in meters from launch spot along the east, north, and up vectors.
The addition of GPS frames of reference and values added
several double precision floating point values to the telemetry stream, which
caused some problems due to different structure packing between GCC on the
flight computer and VC++ on the laptop, so I had to explicitly #pragma pack(4).
The GPS board is having some lock-up problems that are
bothering me, so I may need to talk with their technical support.
We slightly modified the 1000 lbf engine by adding 80 grams
of catalyst bale on top of the hot section monoliths to see if that would bring
the Isp back up to what we saw with the all-bale hot sections and still have
the high flow rates. We loaded up the
big tank with 15 gallons of propellant for a long run. It ran smoothly and started out at 485 lbf
from 250 psi tank pressure, which is still good, but it thrust slowly dropped
until, after 15 seconds or so, streaks of clouds started to appear in the
exhaust, which got steadily worse. I
stopped the propellant flow, but the catalyst still looked completely hot, so I
restarted it again. It ran fine for ten
seconds, then started clouding again.
This process was repeated with shorter and shorter good periods until
the propellant ran out. Thrust would
come back up on the restart after the catalyst had re-heat-soaked any cooled
areas, then decline slowly until clouds started appearing. It is a graceful failure, but still not
This is very disappointing, because it means we dont have a
formula that we can replicate for all the vehicle engines yet. The all-bale hot sections give great Isp and
run forever, but have too much back pressure.
If we cant figure out any way to use the high flowing monoliths in the
hot section, we may need to fabricate extra-wide chambers with only a thin
section of the catalyst bale.
I am worried about not having enough thrust out of the
engines for our early tests without putting the tank pressure higher than we
care to, so I bored out all four vehicle nozzles from 2.0 to 2.2 diameter. All of our small nozzles have ¼ thick
walls, so this still leaves plenty of strength.
Also in the bad-news department, both of our chamber
extension pieces for the 12 test motor arrived, but we found, to our horror,
that the bolt circles didnt match. I
immediately recalled a conversation with the engine fabricator where he
commented that my original drawing left the bolt circle very close to the
outside edge, and he recommended moving it in a ¼ inch. I agreed, but never updated my original
drawing. I looked at the original
drawing for dimensions for the extensions.
Damn. We should be able to
rotate the bolt circle and redrill.