July 12, 2003 Update
We tried an alternate catalyst design fro Catalytic Products
International this week. It is
basically a mass of thin stainless steel ribbons that have been crimped like
heating elements, then coated with a catalyst.
It is provided in a pack wired together like a hay bale. We were hoping that a platinum based
catalyst on a stainless steel base would be a metallic bond, but it turned out
to be more of a platinum-black powder coating, and the adhesion was
terrible. It was probably fine for gas
processing (the intended application), but you could rub the coating off with
gloved fingers, and rocket engine liquid / high speed gas flows took it right
off. Baking it on under flame helped a
bit, but not nearly enough. When we
fired an engine with this catalyst in it, we wound up with a big ring of black
goo on the ground under the nozzle where all the catalyst had been washed off.
We have one more form of metallic catalyst from CPI to test
next week, but if is the same type of adhesion, it isnt going to work for us
either. It is pretty clear that we need
a metallic bond, so we will probably have to get some material electroplated.
The plumbed-in preheating that we have used in our last
couple tests used a large, high-flow propane torch assembly with a tube on the
end to meter propane and air through the top of the engine. Because it relies on venturi suction for air
supply, we cant force the gas mixture through any pressure drop, like a check
valve, and the mixture ratio cant be adjusted independent of the total flow. We have done the tests with a manual ball valve
at the heater port on the engine, but if we get any of these preheated packs
working well enough to go in a vehicle with four engines, we want to be able to
make a connection in a single place, and have all the engines heated from there,
which will require check valves to isolate the engines (or four manual valves,
but we dont like that solution). We
need to move to a forced-air plumbing arrangement.
We got two new compressed gas cylinders in this week, a
hydrogen cylinder, and a breathing air cylinder. We dont want to use pure oxygen for the preheat, because the
flame temperatures would be far too high.
The extra nitrogen in compressed air is a good buffer.
The hydrogen was interesting if you just blow hydrogen gas
over a platinum catalyst in an open-air environment, it starts heating it up
even if it is at room temperature. If
the catalyst has been preheated at all with a torch, blowing hydrogen onto it
causes spontaneous ignition. This
differs from propane over the catalyst quite a bit propane wont do anything
on a cold catalyst, but if it is heated nearly red hot with a torch, propane
flow will keep it red hot, but wont actually start a flame.
There is a good NACA document on hydrogen burning properties
The fact that hydrogen flames start so spontaneously, and
over such a wide mixture ratio, caused us some problems. When we flowed propane / air mixtures in
from the top of the engine, it stayed a cool gas mixture in the plumbing, and
only burned on the active catalyst surfaces that had been heated. When we flowed hydrogen / air mixtures, it
would burst into flame inside our mixing system above the engine, which we
would prefer to keep cool. Only when
the mixture ratio was extremely lean would it only burn at the catalyst, and at
that point the resulting temperature was too low to heat the catalyst the way
we wanted. Hydrogen can actually make a
quite cool flame, down to 1200 K / 1700 F or so.
We werent getting the catalyst hot enough with a
non-burning flame, so we cranked up the hydrogen for a while, which made the
top of the engine very hot. When we saw
some little glowing bits fall out of the engine nozzle, we realized we had
probably gone too far. After a failed
engine test, we opened it up and found that our stainless steel spreading plate
had been completely slagged, along with a fair amount of catalyst beneath it.
We moved back to the more controllable propane mixtures for
the rest of our tests, and we also moved over to using an air compressor
instead of the bottled air, which was being consumed very rapidly. This arrangement works fine, but it does
require torch heating the bottom of the catalyst, and it takes a while for the
heat to propagate up the catalyst so that it starts catalytic burning through
the entire thing. I need to buy a
couple gas flow meters for this type of work.
Right now, we can tell if the gas mixture is fuel rich by lighting the
gas coming out of the nozzle, but that is about it.
Big Vehicle Work
We are building a conformal box for the final parachute
packing that fits up against the tank bottom.
The sides are cut from thin Nomex honeycomb boards, while the bottom
will be a hand lay-up of fiberglass cloth formed to the tank contour. We will be building a custom packing box
with a contour foamed bottom that the box can be placed in for vacuum / pressure
packing of the canopy.
We installed a second large chain hoist on the longer
ceiling girder at our shop to make moving the big vehicle around easier, and we
are putting big castors on the vehicle cradle that can properly handle the
fully loaded weight.
The big tank liner is cross-linked polyethylene, which is
compatible with peroxide, but the big manway flange closure is a fiber
reinforced vinyl-ester composite, which is not peroxide compatible. The most mass efficient solution would be to
replace the 2 thick manway closure with a dished aluminum flange, but we have
chosen to go a simpler route and just add an aluminum plate between the tank
and the closure, so the existing closure still takes all the pressure loads,
while the aluminum plate provides peroxide compatibility. We are keeping the central post as the main
parachute / lifting attachment point, and adding two 2 pipe ports through the
closure for plumbing to the engines and our fountain-fill port. Drilling through the thick composite plate
wasnt as bad as I feared, but we did use up a new bimetal hole saw in the