November 22, 2003 notes
Vehicle modifications and tests
Because the new engines have the mounting flange on top, and
hang down a lot lower, we had to raise the vehicle up a fair amount and
relocate the engines. We moved the
engine mount points inboard, and made an extension to mount the wire rope
isolators directly under the support struts.
We chopped off the old extension brackets we had previously used for the
isolators (hurray for the plasma cutter
), so this was actually a small net
savings in weight. If we started over,
we would just have longer side panels and angle braces and avoid the extension.
Useful tip: we were using a carbide hole saw to cut the
opening for the new engine plumbing into the bulkhead plates, and it was very
problematic after the first ¼ or so of aluminum depth. On the second hole, we discovered by
accident that if there was already a hole drilled through the plate somewhere
on the perimeter of the circle being cut, it goes dramatically easier because
it can clear chips and get a good bite.
The remaining holes went much faster.
For the new engine mountings, I made full milled rings for
the 3 degree roll cant for the engine mounts.
Previously we have used separate little angled blocks, but with the flat
engine top it is nice to make a single piece with the exact angle. We still use separate blocks on the opposite
side of the bulkhead plate, but if I make new bulkhead plates I will mill the
angles directly into pockets around the bolt holes.
We filled the 850 gallon tank all the way up with water,
which took nearly three hours, and started pressurizing it from our fill
trailer. These style tanks always make
some distressing snap and pop sounds the first time they are pressurized, but
this one is ten times the volume of the smaller ones we use, so everyone was a
bit on edge. We took it to 450 psi without
incident. We were a bit concerned about
the support leg bonds and the cabin bond possibly having problems as the tank
stretched, but none of them seem any worse for the experience.
We did six landing drop tests on the wire rope isolator
landing gear. We did 6, 12, and 18,
then added 50 gallons of water to the tank and did 6, 12 and 18 again. There is a bit of a bounce, but everything
held up fine. We were concerned about
the aluminum channels we had used as spacers, but there wasnt any sign of
bending. The isolators do take a set
after they have been hit a few times, especially in the shearing-over-the-coils
direction, and they were rather resistant to just prying back into shape, so we
may need to make some jigs to allow us to push on them with a hydraulic press
to square them back up.
We took delivery of a liquid nitrogen dewar, which we plan to
use for pressurizing the big vehicle tank for flight operations. Filling the entire tank to 300 psi takes
almost six big nitrogen bottles, but a single dewar holds enough nitrogen to
allow three flight tests. It can only
put out 250 psi nitrogen, so we may still need a couple bottles when we want to
go to higher pressures.
The last aluminum distribution manifold we made was leak
free at the welds, but we were having some problems sealing at a nicked AN
cone, so we tried to just weld up a stainless steel manifold without any custom
milled parts. This turned out to go
surprisingly well we chopped off the threads of some 16 AN fittings on the
band saw, ground them by hand to a somewhat circular face profile, cut a
section of 2 pipe, plasma cut a circle to weld on the end, used the carbide
hole saws to cut side holes in the pipe, and welded it all to one of the
flanges that came with the 2 KZCO valve.
It probably took less time to do this than it took to do the precision
helical boring for the aluminum ones on the mill, and the extra length on the
pipe gave us a lot more room for things.
New engine test
We got our new catalyst order in, so we welded up what we
hoped would be a production engine. The
600 cells-per-square-inch catalyst was only slightly heavier than the 400 cpsi
324 grams vs 275 grams.
We built the engine with three separated catalyst blocks in
the hot section:
Note that the stack is upside down compared to the engine,
the screens go at the inlet side. The
blue caps are just indicators of open space where the glow plug ignition
When we put it on the test stand, we had high hopes. The engine preheated much faster than
previous engines, likely due to the denser cold catalyst doing a better job at
decomposing the 50% peroxide before the glow plug chamber. The catalyst all got evenly orange hot from
the preheat, and the run was perfectly smooth and clear. However, it still didnt make the nozzle
glow with the first run, and there were cooler spots in the catalyst at the end
of the run.
We doubled the amount of propellant and let it run for 12
seconds, which it did perfectly, but only a couple patches on the chamber
heated to red hot, and there were some dark areas in the catalyst at the end of
the run. Measured Isp was still low, as
we saw on the previous monlith-only engines.
We are going to make a few changes in the next engine we build,
because we arent confident that this wont quench on longer runs, and we dont
want to risk the vehicle.