March 28, 2004 notes
We tried building another 7 engine with only 600 grams of
ring catalyst in the hot pack, since the 700 gram engine from last week seemed
to still be running at full temperature after the screen burned away and let
the rings jump around. The engine came
up to temperature fine, but on throttle up it lost a lot of temperature and
never gained it back. We decided to
stick with 700 grams of rings in the hot packs.
We built up a complete ship set of engines like this, and
tested them individually. They all make
a bit over 800 lbf at 250 psi feed pressure, generally losing about 18 psi
through the hot pack and 36 psi through the plumbing / spreading plate / cold
pack. The temperatures still arent
quite as high as the engine that burned the retaining screen (the stainless
burning may well have added the extra heat), but they are plenty good
enough. Two of the engines are running
a bit rough, probably because the rings werent able to be packed as solidly in
the recycled engine chambers with stubs of old plates still on the sides. The next time we build fresh engines, we are
probably going to try vibration settling, then weld the retaining plate on top
under a fair amount of hydraulic pressure.
I did a significant amount of rewiring on the vehicle to set
it up for spark ignition instead of glow plugs. The MSD Small Engine Controllers only draw about 1.25 amps each,
so there isnt any need for the dedicated battery and relays we used on the
glow plugs. The vehicle is also all
wired up for a thermocouple for each engine now, but the amplifiers havent
arrived yet. To avoid having to make
new electrical connectors for each engine, I started sharing power and ground
for the three sensors at each engine.
It turned out that it probably would have been easier to just make new
connectors, but there was a benefit I hadnt considered: driving the valve
potentiometers with 12V (regulated) instead of 5V more than halved the sensor
noise. We are completely maxed out on
the current A/D board now: Each engine
has a pot feedback, a pressure transducer, and a thermocouple. We also have a pot feedback on the master
cutoff valve, tank pressure, battery voltage, and recovery tank pressure for a
drogue cannon if we decide to add one.
We spent quite some time chasing two sensor problems: The tank pressure signal had been coming and
going for a while, which was finally tracked to a problem we have seen before
someone had twisted the AMP CPC off by the strain relief instead of the locking
ring, which twists the wires internally.
When I pulled out the contacts, the signal wire had a section visibly abraded
through. Cutting it off, stripping it
back, and replacing the connector fixed it.
The other problem took longer to track down, but Russ finally figured
out that we needed a capacitor across the driver board power terminals to avoid
some noise issues.
On Jamess suggestion, we bought a bunch of steel to make
some serious work tables. The cheap
banquet tables we have all over the shop have been severely protesting their
use as welding and engine assembly tables.
James welded up two tables while we were chasing electrical issues.
We finally got everything out for the warmup test in the
parking lot, and loaded 20 gallons of propellant. Since we dont have the vehicle thermocouple amplifiers yet, we
used one of the test stand sensors to display the temperature of one of the
engines. All the engines lit right up
and ran clear, but I had selected too high of a throttle setting for the
warmup, and we didnt reach the desired temperature for full flow. After the run, we found some heat damage
under the vehicle: the pressure gauge face had bubbled up, and the master
cutoff valve indicator and wiring harness was cooked. We added some insulation around the cutoff valve, but we are
probably going to look into getting a stem extension for it so we can get the
actuator out from the middle of the base.
We will locate a new tank pressure gauge remotely.
We added another thermocouple so I could watch two engines temperatures,
and loaded up another 20 gallons. The
engines again all lit right up, and at a lower throttle setting one of the two
temperatures I was watching rapidly climbed to proper startup temperature
(800C), but the other one was having trouble getting over 600C. I throttled up anyway, and all four engines
came evenly to pressure, bucking the vehicle up a bit even at the 125 psi we
had the tank at. Engine zero was still
running rough, but as far as we can tell, the vehicle should be flyable right
We arent going to try and hover it until we get the
thermocouple amplifiers for the vehicle, because that data is too critical for
us to guess at during warmup. We also
want to replace some of the heat damaged components and insulate things better,
but a hover test next weekend is a possibility, weather permitting.
Our drum pressurization propellant loading has been working
well, but when the drums bulge up we often wonder how much safety margin we
actually have, so we filled a drum up with water and started pressurizing it
(on the other side of a wall) to see what would happen. It was very anti-climactic: At 40 psi, the
drum deformed enough that the bung seal no longer worked, and pressure leaked
out. When it dropped down to 35 psi, it
sealed itself back up. We had only been
pressurizing the drums to 7 psi, so we dont have anything to worry about.
We have made a lot of interesting melted parts over the last
several months while developing the mixed-monoprop engines. Get your very own Armadillo Droppings: