October 18, 2003 Update
I have been getting a lot of people offering to volunteer to
help out with the project lately. I
apologize for not personally responding to each request, but we arent looking
for any more team members at this time.
We are rarely random-labor limited, so more people on site would be more
of a hindrance than help. However, I do
always appreciate suggestions sent in by people that have experience relevant
to what I am currently discussing in the updates, so feel free to send me an
email, or, better yet, comment publicly at http://www.space-frontier.org/FFO under
the current armadillo update.
After the rather surprising preheat behavior with the spray
nozzle injector head with the glow plug, we bought a bunch more glow plugs for
experimentation. They are a different
brand than the previous one, and we unexpectedly found that if they are left on
for too long without any extra resistance, they will burn themselves out. We added some extra wire to cut the current
a bit, but they still draw 15 amps each.
The easiest thing to try was to put the glow plug in a side
chamber above the engine inlet. We have
to avoid getting liquid splashed directly on it, which can cool it down too
fast, but having it branched off the main flow path seemed to shelter it enough. We had hoped that the decomposed peroxide
and vaporized methanol from the warmup pulses would find its way up through the
spreading plate holes and off to our little side chamber, but there doesnt
appear to be enough vapor circulation for this to work.
We welded a glow plug into the side of our long extension
chamber (with a lot of catalyst bale stuffed in the bottom) and welded a shield
tube around that, and tried using it under the 3-pass catalyst pack. This worked. A pulse of the main valve would let a slug of propellant into the
3-pass pack, which would decompose some of the peroxide and vaporize most of
the methanol. It is basically
impossible for a catalyst pack to completely decompose 50% peroxide, because
there isnt enough energy to vaporize the water, so you just get hot water of a
lower and lower peroxide concentration, plus gaseous oxygen. The 3-pass pack seems to do a decent enough
job, because the vapors ignited easily on the glow plug, and we were able to
get the extension catalyst up to full temperature without any propane or torch
heating at all.
We are currently manually pulsing the valve open and closed,
letting it cook off and blow steam for a while, then doing it again until we no
longer see any condensing water vapor coming out of the engine nozzle. If we go stick our mirror-stick under the
nozzle, the catalyst and spreading plate are red hot at this point. It may be possible to just use a continuous small
propellant flow instead of pulses, which is something for us to test next week.
This was a big success for us, completely eliminating the
need for our propane / air preheat system, and giving us the ability to do
in-flight restarts without any trouble.
The only downside is that the initial one or two preheating pulses do
spray a little bit of undecomposed propellant on the ground under the nozzle
until there is enough activated catalyst warmed up.
With this arrangement, the upper catalyst stays cool during
operation, not getting much over 200 F, while all the high temperature
combustion takes place in the extension chamber. This is probably good for the life of that catalyst.
Runs with this combination still quenched out after extended
runs, somewhat sooner than when we preheated all the catalyst (including the
3-pass) with the propane, which was basically expected.
We wanted to encourage re-circulation in the extension
chamber below the 3-pass pack, so we welded a plate offset from the 3-pass
retainer, which would force the flow out to the sides, which should give a turbulent
mixing zone underneath the plate. This
didnt seem to help much.
On the longer runs, the extension chamber would heat up to
red hot in places, but not uniformly.
We suspected the hand-packed (and re-packed after making changes)
catalyst bale was giving poor distribution, but we tried adding a second glow
plug to see if that effected anything.
One thing we didnt find out until the next testing day was
that the ¼ thick perforated metal retaining plate had gotten so soft and
warped that it had half-pulled past its seat, almost falling down into the
nozzle. That may have had some impact
on the mediocre test runs.
In an attempt to provide greater axial heat conduction from
the hot bottom of the engine to the cooler top to stop the progressive
quenching, we fabricated a couple catalysts out of rolled up copper screen, so
there would be straight copper wire runs to conduct a lot of heat axially.
The initial hope was that hot copper would be catalytic
enough, but it didnt work. We then
tried making a composite roll by taking one strip of the platinum coated metal
foil from one of our corrugated catalyst rolls, and rolling it along with 10x10
mesh copper screen. This roll had about
2.5 times the mass of a single corrugated roll, and all the copper should give
much better thermal conductivity, so I had high hopes for this arrangement.
We had the glow plug in a small chamber welded to the side of
the engine top, with a hole through the side to an air gap between the
spreading plate and the top of the catalyst roll. The idea was that this would
keep the glow plug out of the initial liquid flow, but not occlude any of the
pack with a plug shield. This was able
to light the vapors from a warmup pulse, but we werent able to get the entire
pack preheated. It seems like it is
important to actually have two separate catalyst zones, a 50% decomposer that
isnt expected to get very hot, then an open area with a glow plug, then a
catalyst / flameholder that gets preheated to high temperature, and sustains
the real combustion during burns.
We wanted to re-test the 3-pass pack over the long chamber
with the dual glow plugs, because we wondered if the near-fall-through of the retaining
plate may have been a reason for poor runs.
Russ pounded the plate flat and directly welded it to the bottom of the
extension, so it wont have the chance to fall through again.
We also replaced the solid diverter plate under the 3-pass
catalyst with a plate of perforated metal.
The idea was that it would be better to have dozens of small
recirculation zones instead of a single bug one.
We self-preheated the engine and made a good run. We doubled the propellant load and had
another perfectly clean run. The
heating was extremely uneven in the chamber, with one side getting bright red
hot, while the other side stayed dark.
The exhaust didnt have any hint of unburned propellant, but it clearly
wasnt burning evenly in the chamber.
We suspected it was the unevenness of the hand-packed catalyst bale in
the extension, but just to make sure, we rotated the chamber 180 degrees under
the 3-pass pack and made another long run.
This was a Very Good Idea, because the red side did not rotate with the
chamber, it stayed in line with the 3-pass pack. We had suspected that warping during assembly had cut off flow on
part of the top plate, and this looked conclusive. I am going to have to make another set of 3-pass plates to fix
this, but this run also stayed perfectly clear, without any quenching, so we
We double propellant again, and made our longest mixed-monoprop
run ever, at 23 seconds, still without any hint of quenching.
This was the limit for loading in our normal test stand
tank, so we pulled out the big tank and plumbed it up.
We doubled the propellant load again, and made another
perfect run of 47 seconds.
This was looking extremely good, so we decided to double the
propellant again, aiming for a 95 second run, which is longer than our X-Prize
burn trajectory. While we were mixing all
the propellant, we swapped out the pressure gauge we were using on the big
tank, because it was looking a little dubious at the low pressure we were using
(250 psi). It turns out that the gauge
was indeed reading incorrectly (low), so, in conjunction with having a noticeable
blowdown pressure drop with this much propellant (76.8 liters / 20.3 gallons) even
in the big tank, our last run wound up going for 125 seconds at somewhat over 200
Still no sign of quenching.
This is Very Good.
We have spent months working on the mixed-monoprop scheme, but until
now, we were never 100% sure that is was actually going to work out for the
X-Prize vehicle. Now that we have
unlimited burn times and self-preheating, it looks like we have made the right
It looks like the perforated turbulator / flame holder plate
under the 3-pass pack is key, but we need to do more tests to isolate the
effects. When we took the engine apart,
we found that some of the welds holding the bottom retaining plate on had
cracked. Russ is going to actually fill
all the edge perforations to give us a much stronger weld for the next test
sessions. The uneven heating in the
chamber is putting a lot of thermal stresses on various parts, which should go
away when we get the next 3-pass cat pack built. The perforated turbulator plate clearly shows the dead side of
the 3-pass pack as a segment of missing combustion along the outside.
One annoyance from an otherwise triumphant day -- I had WinDaq
crash on me (taking my entire laptop with it) twice on the long runs, which
lost the test data. Anyone else
experience problems with their Ethernet acquisition systems on WinXP?
Run with a single glow plug, to see if the dual glow plug
arrangement was important. We dont
Cut power to the glow plug after the run starts. We held power through the entire run, but we
dont think it is necessary.
Run with a short extension chamber instead of the long
one. We know it can be less than half
the length, but we dont know how short we can go.
Try with one of the rolled foil catalysts as a secondary
catalyst instead of the catalyst bale.
This would help engine-to-engine consistency.
Try with food-grade 50% peroxide instead of the unstabilized
FMC 50% we have been using. FMC only
sold us this peroxide under the condition that it not be used in flight
Try a new 3-pass catalyst with good spacing at the top and less
Put the big nozzle on and see how much thrust we can make
with this combination. All we need for low-altitude
test flights of the big vehicle is 500 lbf / engine, which we have almost made even
with the small nozzles. We should be
able to get twice that if we use large diameter plumbing all the way from the
tank to the engine.
Make sure throttling is stable and predictable.
Once we have everything dialed in, we will have to order
some more catalyst to make a full ship-set of four engines for low altitude
testing of the big vehicle, and whatever we need for building up the full size 12