June 8, 2003 Update
Digital IO board
We were doing a dry test run on the small vehicle when we
noticed that the drogue ejection actuator was behaving very erratically. It turned out to be a solder blob that had
landed on the PC104 board during the re-soldering of our secondary power
input. Our attempt to repair it wound
up killing the entire board, so I replaced it with a newer IO board (a
WinSystems UIO-48). This board didn’t
seem to work at all, but after some investigation, we found that it was
functioning correctly, but that it just couldn’t drive the 15 mA needed to
trigger the opto-isolators on our driver board. Looking at the specs, it only
claims to source 2.5 mA, so it isn’t surprising. What was surprising was that the first board we were using also
only listed 2.5 mA source, so we don’t see how that worked at all. The board (a WinSystems IO-48) must have
additional buffering, but we decided to change vendors and get a board that was
actually rated for what we were using.
The 104-DIO-48S from Access I/O products sources 32mA, and
seems to work fine for us: http://www.accesio.com/go.cgi?p=../104/104-dio-48s.html
Big Vehicle Work
We built the launch stand for the big vehicle, which is a necessary
step before mounting any engines:
http://media.armadilloaerospace.com/2003_06_08/launchStand.jpg
We did a full fit-up of the cabin and crush cone onto the
bonded tank flange, and built the engine wiring harness:
http://media.armadilloaerospace.com/2003_06_08/bigVehicle.jpg
The sea-catch toggle release (http://ourworld.compuserve.com/homepages/seacatch/
) arrived, and will be perfect for our helicopter drop tests. We will have to engineer a little motor
drive actuator to use them for the main parachute releases on the big vehicle,
but they are going to work great. You
can barely see this dangling under the tank in the launch stand photo.
Russ and Neil traveled to White Sands Missile Range to meet
with them and AST representatives to discuss the timeline for Armadillo doing
space launches from there. There is a
daunting amount of paperwork to be done, but Neil has been diligently pounding
away at it for a while.
50%
H2O2 success
We are hopefully very close to getting more 90% peroxide,
but, after our many failures to make anything happen with 50% peroxide
mixtures, we finally got something going.
We are using a platinum impregnated ceramic monolith
catalyst from Catalytic Products International http://www.cpilink.com/
:
http://media.armadilloaerospace.com/2003_06_08/catalyst.jpg
We had been expecting platinum based catalysts to be notably
poorer than silver based catalysts at room temperature, probably requiring
preheating to do much of anything.
Surprisingly, eye-dropper squirts of 50% peroxide react faster on this
than on our silver screens. There is a
good chance we will move to this style of catalyst even for 90% peroxide,
because it is much lighter, and has much less pressure drop. Engine thrust to weight ratio will actually
be important to us for the first time on the X-Prize vehicle.
The ceramic monolith has 400 fairly large pores per square
inch, so it doesn’t offer enough resistance itself to spread out liquid flow
from the engine inlet. We welded in a
sheet of stainless that had many small chem.-etched holes at the top of the
catalyst chamber, stood off with a few spacers to give the liquid room to
spread out. We tested with water until
it was sealed well and distributing evenly.
These holes are still too large to provide an injector pressure drop for
stability, but they get the flow reasonably evenly spread out.
The catalyst monolith is structurally strong enough that it
doesn’t need any kind of supporting plate at all, which is nice.
We are testing a 5.5” diameter by 2” thick catalyst pack, with
a 1.25” diameter nozzle throat. There
is no post catalyst combustion volume besides the converging section. Our weld-on flanges are due in soon, so we
will be testing various extension lengths to see if it makes a difference. The idea is that methanol will catalytically
burn on platinum, and the mixture is intimate, so it may not need any separate
combustion volume at all.
We tried flowing some raw 50% peroxide through the engine
first. It was only partially
catalyzing, leaving a lot of peroxide foaming on the ground, but this was still
a lot more active than when we flowed 50% through our screen based packs.
For the first methanol burning test, we used a propane torch
stuck in the nozzle to pre-heat the ceramic catalyst pack. Propane also catalytically burns on
platinum, which allows you to do an interesting demonstration: you can light a propane torch and heat a section
of the catalyst until it is hot, then kill the torch and just play the gas onto
the catalyst, and you can move a red hot glowing spot over the catalyst without
any visible flame. It is still
difficult to heat the catalyst inside the engine, because oxygen is still
necessary for combustion, so you can’t just stick the torch way up inside the
closed engine.
We were using a solenoid for the propellant, so we knew we
weren’t going to flow too terribly much through the engine. The flow started about the same as the raw
peroxide, with some foaming peroxide coming out the nozzle, but 1.5 seconds
later, the engine roared to life with flame jetting out the bottom. We had only loaded a small amount, so it ran
out in a couple seconds, but we were elated at the first truly positive signs
we have seen from a 50% peroxide combination.
We repeated that test successfully, then loaded up more
propellant for a longer run. The long
run test displayed a truly severe chuffing at around 1 hz intervals, almost as
if the engine was being shut down and restarted repeatedly. This wasn’t too surprising, given that we
didn’t have any explicit injection pressure drop anywhere, and the chamber
pressure was probably barely 50 psi with the little solenoid feeding that big
throat. We made additional runs with jetted
orifices, changed to a small ball valve, and eventually used the little
cavitating venturi we had Fox Valve make for our small biprop tests, but the engine
hasn’t ran stably yet. The CV changed
the nature of the instability some, but it was still very unsteady.
We are assuming that the system has some issues like we have
seen in our other catalyst designs, where a very active surface at the top can
cause unsteady disruptions of flow before everything settles down to a steady
push through the active part of the pack.
Our screen based engines need a number of inert screens at the top to
run smoothly, so we probably need something similar for these. One thing we realized after most of our runs
was that the we preheated the pack much more thoroughly on the later runs, such
that there wasn’t any wet startup period.
This probably got the heat all the way to the very top of the catalyst,
which triggers the flow disruptions.
We tried packing a couple discs of inert foam above the
catalyst, which increased the frequency of the chug, but didn’t diminish
it. We can’t easily add more at the
moment, but when our weld-on flanges arrive, we will have more fabrication
options.
We may try flowing a significant amount more propellant into
the engine and see if it smoothes out.
We can experiment with less aggressive pre-heats. I have ordered an adjustable heat gun that
we will use to blow hot air through the center of the pack, and let it
circulate back down around the outsides.
This should give a good, controllable heating.
We are going to see if it is possible to use various acids
to strip catalyst from the top 1/8” of the monolith, which would give us a flow
straightener before it hits active material.
Overall, we are very excited about this. Throttling may not be as good as the screen
pack monoprops, but the plumbing is exactly the same, the thrust to weight is
much better, the Isp should be much better, and the propellant cost is about
1/10th that of 90% peroxide, with no availability problems. It is also more gratifying to see flames
come out of your rocket engines. J
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