January 8 and January 12, 2001 meeting notes
Silver screen engine testing
We test fired the little 0.65 by 2.55 cat pack engine
again this week, with two changes:
added a top injector plate with two 1/16 holes in it to provide a
pressure drop, and alternated stainless steel screens with the pure silver
Cutting the stainless steel screens (20 mesh) was much
harder than expected. We first tried
cutting them on the press with the little turned down socket that we cut the
silver screens with, but it just dug into the socket edge when pressed. We cut a few by chucking the now-notched
socket on the drill press, but it tended to grab and tear. I tried cutting discs with various tools
held in the boring head on my little mill, but it always grabbed before it cut.
What finally worked was making a punch and die set, where
you slide the screen in a slit across a thick walled cylinder bored to the correct
ID, place a tightly fitting punch into the cylinder on top of the screen, then
whack it with a hammer. I made the
cylinder out of stainless steel, and I intended to make the punch out of
stainless, but I mis-measured and botched the part. I made a quick one out of brass, which worked well for a while,
but slowly got chewed up as it cut screens.
Eventually it was wedging the screens down without cutting them cleanly
off, making it difficult to remove the punch.
The next time I do this, I will take the time to make the stainless
We packed the engine with 50 pure silver screens alternated
with 50 stainless screens, which required mild compression force. The stainless screens are drastically
stronger than the pure silver screens, so we expect them to make up the bulk of
the strength of the pack, with the silver screens just fitting between them.
Our first test was at 400 psi, and while it still took a
very long time to warm up with manual pulses, once it was active and I let it
run, it catalyzed cleanly and ran extremely evenly. It made 13 pounds of thrust, which was about what we expected
given the deep, restrictive cat pack.
In hindsight, I wish we had done a high pressure (800 psi)
run with the engine at that point, but we chose to do a long duration test
firing instead. The last long duration
test run we made was with one of the 75 pound thrust attitude engines with a
foam pack, and it started clouding up after 45 seconds of firing.
We ran this engine for 85 seconds at 400 psi, and the power
and catalyst stayed good the entire time.
There was a hint of uncatalyzed peroxide, with an occasional cloud a
couple feet from the nozzle, but there was no roughness to the run. It dropped about a pound and a half of
thrust 20 seconds into the run, which turned out to be when the copper gasket
After it cooled down, we examined the motor and found out
that we have finally reached the limits of alloy 360 brass for the engines. Two of the retaining threads had pulled
themselves almost free from the engine, unwinding out of the bar like a
helicoil when we took it apart. The 1
square bar stock had also bulged three hundredths of an inch on the hot end.
Our current vehicle motors have the closure at the cool end,
so they arent likely to have this exact problem, but they are also thinner
wall, and we run them at higher pressure, so it is something to keep in mind. Closure-at-the-bottom has the benefit that
if you have a leak, you just leak steam and oxygen instead of raw peroxide, and
it is easier to swap nozzles, but it does make the retention and sealing a bit
That evening, I tried to turn down the square bar down to a
cylinder, leaving just the bottom part to act as a flange that we could use a
nut behind instead of internal threads, but there wasnt enough wall thickness
left if I cut it deep enough to hold a socket head or nut instead of just the
The next screen test engine will have a 1 diameter by 2
long cat pack for a 0.25 nozzle throat, which should be a lot less
restrictive. It will use bolts and nuts
across a flange instead of the internal bolt threads. It looks like there is no getting around using stainless for production
rotor tip engines, because they will see extremely high pressures (4000+ psi)
and have run times of potentially a couple minutes.
New lander frame buildup
The legs arent done yet, but we got the main frame back to
start assembling things. The base tubes
are thicker wall, and there is a bit more triangulation in it, so it is
stronger (and heavier) than before.
There are extremely strong attachment points for tethers and a top
We cut a special bracket to make the top seat attach point
The bottom distribution manifold has been reconfigured so we
could remove a half inch cross fitting, which saves about three inches of
plumbing height under the seat, making it impossible to hit the seat bottom
even if the engine is pushed level with the frame bottom.
We built a new set of plumbing mounted to the back of the
vehicle that gets rid of the hanging valves off the tops of the tanks, and
allowed us to relocate the pressure transducer (and pressure gauge) to the gas
side instead of the liquid side, which should give us much more level readings
Russ finished the new power supply board.
The flight computer is all configured and tested with the
Esteem 1000 mW wireless units.
Phil did some maintenance on the vacuum pump, and replaced a
leaking ball valve on the fill cart.
Joseph sunk really good tether attach points into the
concrete. We had been removing the eye
bolts after each test session, but that was weakening the threads in the
ground, so they are now permanently epoxied in. The only way a tether is coming up is if it takes a good-sized
chunk of concrete with it.
Russ made brass anti-channel rings for the attitude engines.
We should fly again next week.
The rotary test stand has the tachometer configured for it.
The TZM bar stock arrived.
A $1100 hunk of metal. We are
discussing exactly how we want to configure the hybrid test motor.
I went to the Oklahoma Space Industry Development Authority
again, pushing for near term waivers from the FAA. I will be briefing the Oklahoma FSDO about our development
program next month. We still havent
gotten anywhere on our local waiver.