November 30, 2005 notes:
The 2.75 ID throatless engine
with twice-around spiral cooling channels worked fine without melting, but Isp dropped from the 2 ID engines
even though the cooled section of the chamber had been lengthened from 9 to 12. The fact that it didnt melt even without
film cooling was a good sign that the spiral cooling passages were a good
improvement, but the drop in Isp
here, especially after seeing the huge drop in Isp on
the 6 ID engine, shows that this injector design just isnt scalable with
increases in diameter.
We are pretty sure that what happens is that all the lox
vaporizes to gas in the one inch long section of tube between the lox injector ring
and the fuel injector ring, making a uniform fast moving gas stream for the fuel
to inject into. The fuel stream can only
penetrate a short distance, probably under an inch, before it is strongly swept
downstream. In a 2 diameter motor this
gives decent mixing, but as we get larger and larger, it leaves a huge hole in
the center that is basically just pure oxygen with no fuel mixed with it, and
an excessively rich mixture near the walls.
In the video last month looking down the throat of the 6 ID engine you
can actually see a ring of combustion fire around the outside edge and a dark
To avoid this problem, we need to get the propellants both
injected very close to each other in axial distance. We will probably be going to top mounted
injectors for the 6 and larger engines, but at the smaller sizes our igniter,
pressure transducers, and engine mount assembly takes up too much room on top. I only just recently realized that I can
crank the entire spindle on my mill off at an angle, which allows me to
automate drilling of unlike impinging injectors around the tube. To get them to impinge very close to the
wall, this does require a single welded plate between the lox and fuel
injectors, which I worry a bit about.
Russ makes a good, deep weld from both sides, then
I mill the welds down flat and drill the injector holes. I will feel better when we go to bigger
engines and an injector fabrication strategy that allows an open air gap between
propellant welds or seals.
Going to an unlike impinging injector did not have the
results I expected.
I was concerned that it might run rougher, since the conventional wisdom is
that combustion stability is worse on unlike impinging versus like impinging,
but the run was positively eerie smooth -- only a single pixel jitter on the
thrust curve, under 1% total roughness.
However, the Isp was
worse than the unlike impinging injector.
The chamber did burn through after about 15 seconds, when the fuel started
boiling in the cooling channels. Watching the fuel flow meters tells you
what happened in a burn through -- if the fuel flow notches up when the first
signs of trouble show up in the plume, then the wall burned through because
enough heat wasn't being removed from it and liquid sprayed into the chamber,
so you need to increase the coolant velocity. If the fuel flow starts
going down somewhat smoothly before things go to hell, the fuel is starting to
boil in the cooling channels and you need to reduce the coolant velocity. We probably lost the intrinsically fuel rich
layer near the wall with the better mixed propellant.
Interestingly, in the two seconds that the fuel was starting to boil before the
chamber melted through, Isp
climbed steadily. I have an instantaneous Isp graph, which is often interesting to look at in
cases like this. Even though the mixture ratio was getting extremely lean
of peak, the fact that the fuel was coming in as a gas was letting a lot more
of it combust in the short chamber. At
this very lean ratio, Isp
came fairly close to the theoretical value.
We have reached the conclusion that an L* of 12 just doesnt
give the propellant enough time to burn and give a decent Isp,
and making throatless engines with much longer tubes
becomes a worse boring job and more difficult to cool. The next engine we are testing has a 2:1 area
contraction rato, and will have an L* of about 20. I am also going with slightly deeper cooling
channels and only a single wrap spiral to reduce the heat transfer to the
This was kind of interesting: Matt took two photos of our injector water
flow test on the new engine. The one
without a flash shows how it looks to the naked eye, with smooth fans, but with
a flash, it shows a much more turbulent instantaneous view.
Matt has been busy with work at his day job lately, no we
dont have any nice engine test videos
We went through several designs of ground contact sensors
before settling on a spring contact threaded over our shock absorbers, but we
had a little surprise when we welded studs onto the springs for electrical
contact every one of the springs broke at the weld point when we just pressed
it together with our bare hands. I would
have thought welding would have just annealed the spring at that point, but it
made it extremely brittle instead. We
rebuilt them with soldered on brass studs instead of welding, and that seems to
have worked fine.
(old version, new version has the
plate on the end of the shock, which comes down and touches the spring, instead
of the plat on the spring which comes down to touch the shock tip)
In retrospect, we should have put a fountain tube on the
bottom vehicle tank as well, so we could pull ullage
gas out on the bottom without running another tube down the side of the
While we will continue to have our independent watchdog
computer that can override the main computers fuel valve commands to shut
things down if the main computer dies, we are adding another completely
independent valve that can be triggered to close by a radio signal independent
of our normal telemetry stream. This
will be the big red button that a range safety officer can control without
having to go through the remote pilot.
This makes us triply redundant for flight termination, and is key to our plan for hopefully being able to demonstrate some
high flying and powerful rockets in front of spectators next year.
As seen in this picture, we are using a lot more sanitary clamps
for our larger plumbing sizes, which have been quite nice to work with. The only downside is that we had to make
aluminum versions ourselves for welding onto our tanks and pipes, because they
only come in stainless. This vehicle is
plumbed to flow for a > 5000 lbf engine, but we
expect to be doing most of our flying next year on a smaller 2000 lbf engine.
Eight helium bottles are mounted directly to the intertank wall with custom milled brackets:
The computer plate (with wire rope vibration isolators)
bolts to mounts welded directly to the lox tank: (having extra hemispheres
around for fit up is a big help!)
We still have to bend all the hard line for the high
pressure bottles, but everything is pretty much in place:
The vehicle has a six foot base, and the CG is about seven
or eight feet off the ground:
We are still waiting on a couple parts, but it should be
functional relatively soon.
I get asked fairly often what people can do to support
Armadillo. Unfortunately, there isnt a
really great answer. The best way to
support a company is to buy their products, but we dont have any consumer
products at the moment. We shut down the
web store a little while ago because it wasnt worth the hassle for a hundred
dollars a month or so. It will probably come
back when we do some more publicity oriented things next year, but for now there
is only an indirect way.
Roughly 1% of the retail price of Id Software products
provides the funding for Armadillo Aerospace.
Retail stores, publishers, partners, and family are first in line, Armadillo gets what is left of my share. Buy Doom 3 or Quake 4, or Doom RPG on your
cell phone, and a bolt goes on a vehicle.
The X-Prize Cup was our first serious paying customer, and
we are working together to do much more impressive things next year, so I would
also encourage people to attend the X-Prize Cup events in the future. They should get pretty exciting fairly soon.
We are in this for the long haul. Armadillo is operating at a sustainable level
for me, and the team members (all still unpaid at the moment) are just as driven
as they ever were. We continue to make
steady progress, learning lessons and expanding our skill sets with each new
vehicle generation. We feel that
something fairly close to the current designs will soon give us capabilities
that are marketable.
Im not really interested in attracting investors, but if
someone wanted to put several million dollars behind us, we could certainly
talk about stepping up the pace of things.