October 25, 2003 notes
On Tuesday, we did several things to test improvements to
our highly successful combination of last week.
We used the short (2) chamber extension, and packed it with
one 1 thick foil monolith catalyst and covered that with 110 grams of catalyst
bale. This extension is 6 ID, and the
monoliths are only 5.5 OD, so we packed the bale catalyst around the sides as
much as possible. It turns out that the
Champion diesel glow plugs we are using have the same thread diameter and pitch
as 1/8 NPT, but with a straight thread.
This allows us to weld a 1/8 F-F pipe union on the outside of the
chamber, and thread the glow plug into it.
With some anti-seize on it and a bit of torque, it seems to seal even
though the taper is only on the female threads. The glow plug tip does protrude slightly past the end of the
union into the chamber, but we didnt expect quenching to be a problem.
I made new three-pass catalyst plates. Because the upper catalyst doesnt get much
over 250F, the monolith has plenty of structural strength to support itself
without the cross bars, so I removed those for better flow. The upper plate is also 0.050 thicker to
give a bit more turn-around volume.
Russ welded on some spacers at the top of this plate to guarantee that
it wouldnt bow up and close off any of the top flow. Instead of standing the perforated flame holder away from the
bottom of the lower plate, we welded it directly to the plate, which gives a
0.25 gap between the bottom of the catalyst and the perforated plate.
Somewhat to our surprise, the short chamber did hold plenty
of catalyst to function properly. We
made several good runs, proving the following:
We dont need all the volume and catalyst we had in the long
The glow plug doesnt need to protrude into the chamber to
light the preheat vapors.
A single glow plug works fine.
The glow plug is only necessary for the preheating, it doesnt
play a part in the long run functioning.
We made some fairly long runs without the glow plug powered after
opening the throttle.
The new three-pass-pack-plates with the forced standoff at
the top did provide even flow all around the engine.
The perforated plate worked fine welded directly to the
plate, the standoff wasnt important.
It looks like the perforated plate, in combination with the 3-pass pack,
was the magic ingredient, inducing lots of little turbulent zones instead of
letting the output of the top pack rush straight down.
Now, a couple things that were only partially successful:
We added a small solenoid in parallel with the ball valve so
we could do preheats with more control.
I had hoped that it would be possible to just run a small amount of
propellant through until it completely preheated, but it definitely needs pauses
to get the bottom pack glowing red hot.
A constant low flow will get the pack hotter than simple peroxide
decomposition levels, but not to the point where the exiting water vapor is
superheated to the point of clarity, which usually coincides with a red-hot
pack. My theory is that the output of
the 3-pass pack has free oxygen, methanol vapor, and quite a bit of liquid,
moderate concentration peroxide.
Because a lot of the oxidizer is still in liquid form, this gives a very
rich, relatively cool burning, that isnt hot enough to force the rest of the
peroxide to decompose. If you let a
slug of propellant into the engine and close the valve, it will sit in the
first two channels of the 3-pass pack, giving lots of time for the peroxide to
more fully decompose. This fills the
engine with pure oxygen, so when the next slug of propellant comes in, it has a
lot more oxygen to burn with.
We tried a couple runs with food grade peroxide. The first run took much longer to preheat,
and started out cloudy, but did clear up and run clean. The second try ran cloudy the entire time,
probably because we werent patient enough with the preheat. After these runs, we did another run with unstabilized
peroxide, which behaved normally, so the catalyst didnt seem to be
poisoned. We can probably make food
grade work, but we are going to try unstabilized semiconductor grade next, if
we cant get FMC to sell us any more of the 50% propulsion grade.
On Saturday, we replaced the foil monolith catalyst in the
extension chamber with more of the catalyst bale. We like the monoliths a lot more for repeatability, but we are
somewhat motivated by the fact that we have exactly enough monoliths to
possibly build a ship-set of four engines if we use bale catalyst on the
bottom. We have more catalyst on order
(we are going to 600 pores-per-square-inch, from 400, for all of them), but it
is 4-5 weeks out.
Phil built the catalyst with 680 grams of bale, and pressed
it to 1000 psi indicated on our press gauge.
This was a very tight press. The
engine worked fine, preheating seemingly better than before, but we werent
logging data at the time.
We then swapped the small nozzle for the 2 throat nozzles,
and made a couple test runs. The thrust
was surprisingly about the same as the much smaller nozzle. It turns out that the catalyst was packed
tightly enough to create a huge pressure drop.
When we pulled the engine apart, we had one surprise the perforated
plate on the bottom of the new 3-pass pack had burned through in a spot. We believe this was due to it being pressed
directly onto the lower catalyst without an air-gap.
We rebuilt the catalyst with 330 grams of fresh catalyst
bale, and only pressed it enough to get it seated with a half inch or so gap
under the top pack, which barely registered on the press gauge.
Unfortunately, we decided to change two things on the next
test, the catalyst density and the mixture ratio. We went from our normal 5:1 by volume O:F ratio, which is
lean-burn, to 2.5:1 by volume, which is rich burn. They both, in theory, give about the same Isp, but the rich burn
is 170 F cooler in operation. We were
not able to get this self-preheated very well, and it didnt burn clean. There was lots of pops and flame coming out
of the nozzle during the preheat, but the insides only got partially red
hot. We tried again with the mixture
ratio at 3:1 by volume, but we couldnt get that to work either. At this point, we wanted to go back and try
our normal mixture ratio, but we were completely out of methanol, because my
last order didnt arrive on time.
Just for something to do, we tried an ethanol mixture at 8:1
by volume, which is lean burn at about the ratio we normally run the methanol,
but it couldnt be preheated either.
Assuming that our normal mixtures functions properly on Tuesday, this
does show that using methanol for its catalytic properties is actually
I have started machining the 3-pass plates for the 12
diameter engine, and we are having an extension chamber for that
fabricated. Firing is probably two or
three weeks away. If we hit our
expected numbers with that, we will start the fabrication work for the complete
X-Prize set of four.
Here are the results for various 50% peroxide / alcohol
mixtures using the Isp code available at http://www.dunnspace.com/. Surprisingly, I had to add data for the
various alcohols to the propellant library.
Mass and volume are O:F ratios. Isp
is with a 300 psia chamber pressure with a nozzle sized for expanding to 14
psia at sea level, shifting equilibrium calculation.
Good online chemical data book: http://webbook.nist.gov
Heats of formation:
C1H4O Methanol 57.1 kC/mole 239 kJ/mole
C2H6O Ethanol -66.4
C3H8O Isopropanol -76.0
methanol ethanol isopropanol
Mass Volume Density Isp Temp (K) Isp Temp (K) Isp Temp (K)
---- ---- ---- ---- ---- ---- ---- ---- ----
12 9.6 1.138 133 923 151 1174 160 1299
10 6.7 1.130 144 1060 163 1341 170 1454
8.6 5.7 1.123 153 1186 170 1448 167 1384
7.5 5.0 1.116 161 1304 168 1388 165 1317
6.7 4.5 1.109 168 1414 165 1330 162 1252
6 4.0 1.103 170 1434 163 1275 160 1190
5.5 3.6 1.097 168 1393 161 1222 158 1131
5 3.3 1.091 166 1354 159 1170 156 1077
4.6 3.1 1.085 165 1315 157 1122 155 1036
4.3 2.9 1.080 163 1278 156 1077 154 1009
4 2.7 1.075 162 1243
3.8 2.5 1.070 161 1208
3.5 2.4 1.065 160 1175
For comparison, 90% peroxide monoprop gives density 1.381 / Isp
134 / temp 1030 K, and 98% peroxide monoprop gives density 1.430 / Isp 146 /
temp 1226 K.
Obviously, there isnt a heck of a lot of difference in the
performance of the various alcohols. In
theory, isopropanol offers a couple percent better bulk density for a given
Isp, but that is way down in the just-doesnt-matter range. Methanol reacts better on platinum
catalysts, which is a far more important factor for us. We are 15% or so off from the theoretical
Isp values, so there may be some room for improvement, but we arent going to
do any engine revs specifically targeting performance, because what we are
getting is good enough.