April 2 and 6, 2002 Meeting Notes
Last weeks biprop testing had eaten a section of the inner
web out of the brass catalyst retaining pack, so I made a couple out of
stainless for testing. Cutting them
apart was a big hassle, so I added a dozen stainless spreading plates and
retaining plates to our CNC order with DynaTurn.
Russ and Phil had our current water cooled chamber nickel
plated over the brass, in preparation for running it in regenerative form with
peroxide instead of water providing the cooling.
The cat pack that was borderline on Saturday took a really
long time to completely clear today, so we decided to rebuild it with more
doubled up silver screens, which seems to be working well now. The total count is now 70 silver screens and
50 stainless screens, plus a few spacers.
We fired with kerosene and the new pack without any
problems, but we couldnt get the ethane to light again, for reasons we do not
We cut the new rotor blades.
We began using new test stand software with several new
features that make our lives easier: dynamic bias zero, remote pulsing,
constant display of pressure and thrust, and the ability to take multiple logs
before exiting. I need to buy a big A/D
system so we can start measuring a lot more things: peroxide flow rates,
kerosene flow rates, coolant temperature, wall temperature, etc.
We did all the bonding on new rotor blades, we will be
drilling the bolt holes next week. The
blades are set at 20 degrees of pitch this time, up from 10 degrees last
week. This will be higher than optimal
for static testing, but more appropriate if we do any vertical acceleration.
The fat fan blade extrusions from a different supplier arrived. They have over a 2 bar hole, which would
make the hub attachment trivial, but the extrusions arent completely
straight. We may ask about getting a
hand-selected set with better characteristics.
The big set of tests today was working up towards
regenerative cooling on the test engine.
We started out by calculating the peroxide flow rate that we
have for a long, clean biprop run (500 ml in 12 seconds), and turned the water
down until it matched. Our normal
coolant flow for long runs was higher than the equivalent peroxide flow. We dont have temperature instrumentation,
so we ran the water exit hose behind the trailer and let someone with a
carefully calibrated gloved finger tell us how hot it was getting. On a monoprop run, the water got about as
hot as a hot water faucet. With a
biprop run, the water did get to the point of having some bubbles in it,
showing some boiling.
We made the plumbing to run the peroxide through the cooling
jacket and back to the cat pack, and pressure tested it with water. The higher pressure coolant should suppress boiling
a fair amount over the dump-to-ambient water cooling.
We set up with everyone behind the concrete wall, watching
the engine on a closed circuit monitor, and very slowly worked our way up with
monoprop runs going through the cooling jacket, first with 200 ml of peroxide,
then 400, then 600, then 1000. Total
thrust is somewhat lower, due to the pressure drop in the cooled chamber, but
it does increase slightly as it pulls more heat out of the chamber. Based on the discoloration, the cat pack is
clearly getting hot earlier in the cat pack with the preheated peroxide. We could probably get by with a smaller cat
pack if we didnt mind a cloudy run until it warmed up.
The next run, we briefly pulsed the kerosene on a couple
times. Nothing exploded.
The next run, we held the kerosene on for a while. After about four seconds, the flame got rough, and we cut the
kerosene. It continued for the rest of
the run on monoprop without a problem.
We repeated this on the next run, and the behavior was the
same. After four seconds of hot fire,
things got rough. We re-pulsed the
kerosene later in the run, but it was still rough. We assume that we are seeing some boiling in the cooling channels.
We took the engine apart to look at the state of the
catalyst pack. Some literature has
claimed that 90% peroxide will melt silver packs if it has been preheated by
being used as a coolant, and that 85% should be the limit for that
application. There was some silver
plating under the cat pack, and a few of the bottom silver wires had a
semi-melted look, but none of the 32 mesh screen wires were melted through. This melting at the bottom was probably due
to the rough running pushing biprop combustion back into the pack. We have seen this when using a pressure
transducer in the chamber rough running pushes way more heat in odd
directions than smooth running does. It
is too early to say conclusively, but it doesnt look like the pure silver
screens are going to have a real problem.
Maybe the reported problem was dealing with plated screens that lost
their adhesion at temperatures below the actual melting point.
We arent absolutely positive that there are no stagnant
spots in the cooling jacket which might boil easier, but we do know for sure
that the coolant is getting plenty hot, so we should assume that it is a raw
heat transfer problem.
Heat transfer is basically proportional to chamber pressure,
times chamber temperature minus wall temperature, times chamber surface area.
The cooling problem appears to be independent of pressure in
a given engine, because chamber pressure, and therefore heat transfer, is
directly proportional to peroxide flow, which is proportional to heat absorption. That means that high pressure engines and
throttled engines shouldnt put more heat into the coolant, although higher
pressure would demand more conductivity out of the wall material. I had been carrying around assumptions to
the contrary for a while.
Flame temperature is determined by peroxide concentration,
fuel ratio, and to a minor extent by peroxide inlet temperature. We are already running very rich on the
kerosene, so we cant reduce heating much more that way. We had been considering options with lower
peroxide concentration, and we had identified some fuel mixtures (containing
ether and some other low flash point additives) that would probably still auto ignite
at the lower cat pack exit temperatures, but considering all the problems we
have had lighting ethane even with 90%, I am getting dubious about autoignition
at any lower temperatures. We might try
diluting to 85% and seeing if the kerosene still lights. We might be able to ignite with the peroxide
circulating through the cooling channel, even if it wouldnt when water cooled,
because there will be a little temperature boost.
Putting 5% water in the combustion chamber can drop the
flame temperatures a fair amount.
The required chamber volume is the throat area times L*, the
characteristic length, which is an empirically derived value for each basic
type of engine. Our first biprop engine
only had an L* of 17, and it clearly was not enough. The current engine has an L* of 30, which is at the low end of
what is recommended. For a given shape
combustion chamber, surface area will be proportional to the volume to 2/3
power. When you double the throat area
(which doubles the thrust), the chamber volume must also double, but the
surface area only increases by 2 to the 2/3 power, or 1.59. Because the total coolant flow is
proportional to throat size, big engines will have a lower total amount of heat
absorbed per unit of coolant.
Surface area to volume for a cylinder is lowest when the
height is equal to the diameter, which would be about 1.6 for this engine to
keep the same L*. That would only be an
improvement of about 10% over what we have now. A true spherical chamber of 1.84 diameter would be an additional
13% less surface area for the same volume.
We could shrink our chamber volume a little, and make it the
optimal dimensions to cut maybe 20% of the heat transfer. Or, we could make a much bigger engine and
get really significant reductions in cooling load.
We could attempt to use some of the fuel as an additional
coolant. At best, this would be a 10%
benefit, given the much larger flow of peroxide, and the worse thermal
characteristics of kerosene. We could
circulate the kerosene once around the chamber, but the conventional wisdom is
that with normal kerosene (as opposed to RP-1), you would get coking. Almost certainly not worth it. Right now, the fuel injector is a separate
ring between the cat pack and the cooled chamber. When we make a new chamber with the fuel injector integral, there
will be better heat transfer to it, so there will probably be some small
benefit as the kerosene impinges on the opposite wall and draws heat from it.
We certainly dont have enough fuel flow at this engine size
to think about film cooling, and even in much larger engines, it doesnt make
as much sense for peroxide.
A high thermal conductivity material, like copper or
aluminum, will transfer heat a farther distance, so there is less chance of a localized
hot spot causing a burn through. The
maximum allowable distance from flame to coolant probably doesnt change with
scale, which is a benefit for small engines, allowing straight cooling
cylinders, instead of having to closely follow the nozzle profile. The allowable distance would also decrease
with increased chamber pressure.
However, a high thermal conductivity material will put much
more chamber heat into the coolant.
Our current chamber is nickel plated brass, which is sort of
middle of the road. Going to aluminum
would make for a very lightweight engine, but we would transfer lots more heat
out, which we clearly cant afford at this size engine. Going to stainless steel might significantly
reduce our heat load, but we might wind up melting the throat out of the engine
unless we changed how we route the coolant.
, 316 SS has a thermal conductivity of 9.4 BTU/hr/ft^2/degF
360 brass has 67 BTU/hr/ft^2/ft/degF
Most aluminums have between 80 and 125 BTU/hr/ft^2/ft/degF.
Something like jet-hot coating on the inside of the chamber would
prevent a large amount of the heat transfer, but the conventional wisdom is
that ceramic based coatings dont last inside combustion chambers. We could try it out on the current engine
Summary For Future Work
We could try burning 85% peroxide.
We could try getting the inside of the current chamber
We could build a larger biprop engine to take advantage of
the scaling law.
We could build a cooled chamber out of stainless steel.