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Vehicle work, Rocket Anchor, GPS, Engines

Preparing For Flight

December 6, 2003 notes


Vehicle Work


All the new engine angle shims or tack welded in place so we can’t 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 oriented vertically.


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 wouldn’t hold up on the actual engines, but it would be more than sufficient for insulating the composite structures from exhaust and suborbital reentry temperatures.



Rocket Anchor


Our flight test pad is fairly thin, non reinforced concrete, so an eye bolt sunk in it wouldn’t 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 won’t 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 free.


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.


GPS Velocity


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.


Engine Work


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 good.


This is very disappointing, because it means we don’t 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 can’t 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 didn’t 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.






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