March 26 and 30, 2002 Meeting Notes
We are getting a batch of 11 1 cat pack diameter engines
CNC machined in 316 stainless at DynaTurn of Elk City, OK. The price was very reasonable, so we are
going to have two complete sets of attitude engines sitting on the shelf, as
well as cat packs for the rotor and a test stand engine. If these come out right, I will probably get
a batch of 1.5 diameter engines as well.
We are probably never again going to make four copies of something by
hand, it is worthwhile to just get ten copies made by CNC. Even making two copies of something by hand
is sounding a little dubious in some cases.
The supplies for our Really Big Engine are starting to
arrive. We have a set of 1 pipe
fittings, a four foot length of 1 braided line, a 1 servo ball valve, and our
custom order of 12 diameter stainless steel screens just arrived. We will need to order a bit more silver for
it, and our anti-channel rings are a custom run from Smalley that we are still
waiting for. We are probably going to
have the engine rolled and welded out of stainless, because it will be too big
to reasonably machine. We will probably
make a couple different nozzles for it, but in high pressure form, it should be
good for 5,000 pounds of thrust or so.
On Tuesday, we did nothing but work in the shop. We are getting a lot of things permanently
mounted on the big trailer, including a slick new filling arrangement using big
spring-return ball valves. We have
everything pulled off of our old fill cart, so when we do VTVL tests, we will
have the trailer at the edge of the pad, and use a 30 long hose to run out to
the vehicle in the center. We finally
got around to permanently mounting our vacuum pump and adding a nice switch to
it. We also have a couple AC outlets
permanently mounted on the trailer, which we either connect to a wall outlet
when testing locally, or to Josephs generator when running in the field. Our field work should be very similar to our
shop work, with minimal setup time.
Russ and I finished the new hub for our next rocket rotor
test. The 7068 aluminum we used is
really nice it has 2.5x the tensile strength of 6061, and machines much
nicer. It is about 3x as expensive, and
only comes in round stock, but we will probably use this or 7075 for all our
future aluminum work, because raw material cost is a very small fraction of
what we spend.
I made the new tip engine mounts out of 7068 aluminum
instead of brass, and I am drastically milling down our right angle nozzle
blocks, which will save a lot of stress on the blades.
Neil has located another supplier of extruded aluminum
airfoils that offer a 2 bar hole, which we are going to move to for high
stress cases, but we are probably going to do our computer RPM control at
relatively low speeds with our current blades first, and possibly get them on a
low performance flying vehicle.
On Saturday, we did some more shop work, including mounting
a sight glass on our kerosene tank, calibrating a new small load cell, and
fixing the wiring on our control box so we can run two arbitrary solenoids at a
After finishing everything up, we set out to get the biprop
working right. The switchbox we had
been using to control the kerosene was very messed up, so we hoped that after
fixing that, things would work right again.
Our first run, with an 0.070 peroxide jet and an 0.018
kerosene jet, didnt light, and gave the characteristic rich cloud
exhaust. At least the solenoid was
working consistently. This was a
combination that did light with older cat packs, but this was the first 36 mesh
silver pack we had made, and it had a lot more than our usual amount of
compression, and had 80 discs in it from when we were trying random things last
We tried going to an 0.090 peroxide jet, but it was still
rich. We pulled first 10 discs, then 20
discs (pairs of discs, actually) out of the pack, but it was still too rich.
We ran the other cat pack from the rotor engines on the
biprop, and got it to light. That pack
was put together hastily for the rotor test, and wasnt ever smooth, but it did
work. We could then compare the
monoprop numbers and see that the new pack was twice as restrictive. As we have always seen before, a rough
monoprop engine will always make a rough biprop engine.
We started a new pack from scratch, with a new
technique: instead of making the first
block of screens just 10 silver alternated with 10 stainless, we doubled up the
silver, so it went 2 silver, then one stainless. This approximates the 80 mesh screens we had at the top of the
other packs. We did minimal compression
on the pack, so it filled the entire chamber with just 60 silvers and 50
interspersed stainless, plus a few extra stainless spacers. We werent at all sure that the pack would
work well, but it ran perfectly smooth.
It looked a little borderline in monoprop mode, with a fairly cloudy
start and wisps of cloud while running, but that may also have been due to the
fact that it was raining during some of our testing.
The important point was that the biprop run was
PERFECT. Instant light, dead
smooth. This was with the 0.070
peroxide, 0.018 kerosene jet combination, at 250 psi regulated pressure.
We then made another, longer run, and everything continued to
work great. A lot of the subtleties
dont come out well on video, but it is still very nice looking. There was a brief green flare midway through
the burn, which turned out to be a small chunk of our brass retaining plate
being consumed by the engine. Im going
to make one in stainless tomorrow.
The thrust graph is picture-perfect, with only a couple
percent thrust variation:
Thrust went from 12 pounds in monoprop mode to 18 pounds in
biprop mode, which highlights how much pressure drop we have across the
catalyst pack in monoprop mode at these low pressures. The chamber pressure jumps from around 100
psi to around 150 psi when the kerosene is burning.
With this success, we set up to try and light the ethane,
which we had yet to get to burn. We put
an 0.090 jet on the fuel port for the gaseous ethane, and started with 200 psi
ethane pressure. There was a faint
flame and some burning going on, but nothing dramatic. We increased the ethane pressure to 250 psi,
and the ethane lit right up, making a great looking plume of blue mach
diamonds. It started and stopped
instantly on multiple pushes.
We ran a couple more tests, and observed that the ethane
combustion seems to be a lot hotter, because we could watch our cooling water
starting to steam and bubble a bit very rapidly, while the kerosene ran for 20
seconds without a problem. The ethane
was making a little more thrust, so the chamber pressure was a bit higher,
which contributes some, and the combustion was likely a lot better with the
gas/gas mixing, but we also thought we might be running on the lean side, since
we had just come off the too lean to burn right level. We had never been able to make the kerosene
too lean, because we are at our smallest jet size, and we are likely still rich
We sort of botched our last ethane test, because we didnt
wait for our nitrogen pressure to get to the full 250 psi on the peroxide side
before starting the test, which resulted in extreme richness on the ethane
side. Still, we got to see what an
over-rich ethane condition is like.
When it was extremely rich, it would light briefly, then blow
out. When the ratio got down to just
very rich, it gave a puffy, blurry blue exhaust plume.
We also noticed while running the ethane at higher pressures
that the regulated ethane pressure dropped during operation. The ethane was only self-pressurizing to 395
psi this cool evening, so when we were drawing gas off, it could drop all the
way to 320 when it is self pressurizing.
Instead of raising the ethane regulator pressure, we should increase the
ethane orifice size and stay with 250 psi regulator pressure. The mixture ratio will still be changing
somewhat, because the cooler ethane later in the run will be more dense. Not sure if we should worry about that.
We will do more rigorous testing of the ethane next time.
We are having the current cooled chamber nickel plated, in
preparation for possibly running it in a regeneratively cooled manner. We will repeat some of our tests from today,
making sure to exactly measure the cooling water flow rate, before trying to
cool with peroxide, and that will be a test we run at the 100 acres, from
behind our earth berms. There have been
reported problems that 90% peroxide, when used as a coolant, picks up enough
extra heat that the decomposition temperature melts silver catalyst. Our pure silver / stainless packs may or may
not suffer from this. We will see. We have noticed a bit of silver plating on
the stainless screens at the bottom of the packs, which is probably a good
thing for activity, but it does mean that there is some erosion from the silver
screens. We still havent seen any hint
of wearing out the pure silver screens in any of our packs.
Right now, our engine is in three segments: catalyst pack,
fuel injector ring, and cooled chamber/nozzle.
The fuel injector ring is getting pretty hot, warping a bit after we run
it hard. The next cooled chamber we
make will have the fuel injector directly in the chamber section. While it wont have any coolant circulating
around it, it will at least have a better conduction path to the cooled
areas. We may try making the next
cooled chamber out of aluminum. That
will provide much better heat transfer than the brass, but still seems rather
more likely to melt if anything goes at all wrong. Something else to test at the 100 acres.