January 3, 2004 Notes
We built a couple more test engines this week. We reconfigured the data acquisition to read
pressure between the cold pack and the hot pack, and final chamber pressure on
We built a welded engine with 600 grams of ring catalyst in
the hot section in a single layer, and a slightly drilled out spreading plate.
This started out with great thrust over 700 lbf, but it
tailed off after a couple seconds. This
turned out to be due to many of the rings on the bottom squashing flat under
the heat and pressure.
Initial: 730 lbf from 293 psi feed, 187 psi mid, 154 psi
Final: 575 lbf from 278 psi feed, 198 psi mid, 120 psi
We then ran it at a higher pressure:
808 lbf from 425 psi feed, 300 psi mid, 171 psi chamber
The self-preheating behavior of the higher flowing engines
has gotten so bad that we started sticking a pole up the nozzle throat during
preheat so it could build a little chamber pressure easier. Our old engines with compressed catalyst
bale in the hot section were so restrictive that a slug of propellant could
ignite and actually build up enough chamber pressure to make a loud bang when
it cooked off, which transferred a lot of heat to the catalyst. The free-flowing hot packs wouldnt even
build a couple PSI when propellant cooked off, making it very slow and messy to
bring them up to operating temperature.
The nozzle plug helped a lot, but isnt really an operational solution. We are probably going to look at going back
to preheating with a forced propane-air mixture.
We received yet another catalyst to try in the hot section,
a box full of little ceramic beads impregnated with catalyst. We didnt have good luck with the ceramic
monoliths that were directly exposed to liquid propellant, but we thought that
they might work our well in the hot catalyst section where they are only seeing
gas flow during operation. They should
avoid the crushing problem that the rings suffered from. When heated with a torch some of the little
balls crack and pop apart due to trapped moisture, but you can slowly bring
them up to red-hot without much damage.
We did several tests preheating these with propane flow in various ways,
and the tended to look (and occasionally sound) like nuclear Rice Crispies.
We cut open the ring engine and replaced the 600 grams of rings
with 520 grams of beads, which occupied a much shallower area. This turned out to be much more restrictive
than the rings, but showed no change in behavior over the length of the run.
380 lbf from 265 psi feed, 208 psi mid, 83 psi chamber
We removed 200 grams of beads, and the thrust went up:
515 lbf from 281 psi feed, 210 psi mid, 65 psi chamber (the
sensor tube must have been loose, this number cant be right)
We were afraid to pull any more of the beads out, because
the layer wasnt very thick, and any bowing of the supporting plate would let
them pull away from the edges and allow some gas to bypass the catalyst.
We received a new batch of spreading plates in different
sizes laser cut by Global Stencil. The
previous ones have all been 204 x 0.032.
We built another engine with 204 x 0.040 holes, which is a 60% increase
over the old ones, which should show a big improvement if the spreading plate
was at all a limiting factor. This
engine used two sections of 200 grams each of ring catalyst, supported by a
extra-thick (1/4) perforated plates with a 10 mesh 316 screen above it to keep
the rings from crushing through the larger holes.
Performance was steady, without signs of the squashed-ring
falloff from the previous motor, but total thrust was not any higher. This engine also behaved a little odd with
the warmup, never seeming to get completely hot during the warmup, but if you
just opened the valve it would run cloudy for about two seconds, then clear up
for a strong run. We suspect that added
a third layer of 200 grams of ring catalyst would make it start cleaner without
hurting the thrust much.
610 lbf from 270 psi feed, 180 psi mid, 127 psi chamber
I am now officially giving up on trying to increase the
chamber pressure for these engine dimensions.
It is probably just barely possible to get 700 lbf out of these engines
with supported arrangements of the ring catalyst, but it would be such a near
thing that any degradation of performance as they settled could make it
non-viable. A 2.2 throat from a 5.5
diameter catalyst chamber is just too large (6.25 : 1 area ratio). Our big motors are 9 : 1, which should be a
lot more forgiving, but we may want to go even more than that. The tiny nozzle we tested gave a spectacular
chamber pressure, but that was at almost 20 : 1 area ratio. We can probably get by with much shorter
engines with better area ratios, but it will probably still hurt our engine T/W
We are probably going to work out a way to mount one of the
big engines in the center, rather than building wider engines for the sides. We need to build another driver board and
rework all the base plumbing to do this.
A lot of little work has also happened on the big vehicle
we have made lifting harnesses for transport, our GPS unit is back and
installed on the electronics board, new wiring harnesses for the engine pods
have been made, the master cutoff watchdog computer box has been built, etc.