October 23, 2001 Meeting Notes
We spent half of last week at the Space Frontier Conference
in LA. We had a good time, and picked
up quite a bit of useful information.
We will be getting the demo video we had running at the conference
converted to mpeg and put on the web site this week.
One of the high points was having Gunter Wendt, the NASA pad
leader for most of the Mercury, Gemini, and Apollo flights, climb up onto our
manned lander and say Ah, if I was forty years younger, I would want one for
Christmas! He also told us some
stories about banging on balky hydrogen peroxide thruster valves to stop them
from sputtering, and having a peroxide drum blow the lid through the ceiling
due to machining oil from a newly cut drum plug draining down into the drum.
Our FAA waiver application for our test range has now been
bounced between four different people the local FSDO, the regional manager,
the Fort Worth Center, and finally to the DFW tower. I hope to hear something from them tomorrow.
We assembled our new 12 ton floor standing press, which we
will be using to press the catalyst packs for the 600 pound thrust motor and
the upcoming larger motors. The first
pack we made for the 600 pound motor we had one of our neighbors compress in
their arbor press, but we need something we can use ourselves during our normal
The 1/4" perforated steel plate at the bottom of the
catalyst pack in the big motor is bowing pretty badly. We pressed it back into shape (had to squash
something in our new press
), and will be welding some supports underneath it
for a temporary fix. We will probably
convert from the mild steel, which is rusting badly, to a stainless steel that
will have better high temperature strength, but larger motors are almost
certainly going to need some extra bracing as well.
Our quick attempts at making anti-channel rings last week
didnt turn out so well. The two upper
ones that we slit before inserting just pulled away from the edge when they
heated up. The one that was folded a
bit to make fit seemed to hold its shape ok, but we are going to try to make
some this weekend out of thicker metal that needs to be compressed into the
bore like a piston ring.
Lithium Aluminum Hydride Hybrid Paper
Bruno Berger of the SPL was kind enough to dig up a copy of
an experimental report on a lithium aluminum hydride ( LiAlH4 ) / peroxide
hybrid that I had seen referenced, which is the closest I have found to the
interesting-sounding aluminum hydride ( AlH4 ).
This is not a stellar performer even in theory, but one of
the things they wanted to test was regression rates of metalized hybrid grains,
which were expected to increase with pressure, which allows throttling with
more constant O/F ratios.
They were using a reasonable sized cat bed peroxide hybrid
engine of around 230 lb thrust, with 200 - 300 psi chamber pressure and an
undisclosed nozzle ratio.
They pressed fuel grains with 5% polyethylene for increased
structural integrity at the expense of a few seconds of Isp, but it was only a
17% difference in grain strength vs pure LiAlH4 , so it's not clear it is
necessary to add. I believe the characteristics of aluminum hydride are
similar, so this is a noteworthy point. The pressed fuel grains gave
densities around 94% of theoretical.
Theoretical Isp for this mix was calculated as 297 at 1000
psi chamber pressure with 90% peroxide, expanding to 14.7 psi, and 319 vacuum
Isp (expansion ratio not listed).
They didn't fire at 1000 psi chamber pressure, but
calculated the appropriate corrections to conclude that the delivered Isp of
200 at around 250 psi chamber pressure was only 80% of the theoretical Isp, due
largely to poor measured C*.
They believe that the chamber temperature was not quite high
enough to properly burn the aluminum, and discussed possibly adding 10%
aluminum powder or 10% magnesium powder to the grain to increase the chamber
temperature in hopes of improving combustion efficiency. Another option
they didn't mention, that would probably be higher performing, would be to use
98% peroxide instead of 90% to increase the chamber temperature.
LiAlH4 has other disadvantages compared to AlH3, including a
low density of 0.917 g/cm3, a low O/F ratio of around 1.6 : 1, and a high
regression rate of 0.1 inch / sec at 200 - 268 psi (higher at higher pressures)
that would make a more awkward fuel grain size.
With 90% peroxide, LiAlH4 is not interesting as a hybrid
fuel, because it doesn't perform better than a straight polyethylene fuel
grain, which is much cheaper and has a completely gaseous exhaust that is
easier on nozzles and generally more benign.
If 98% peroxide got the combustion efficiency up, it would
offer a moderate Isp advantage over a hydrocarbon, but nothing to get excited
Aluminum hydride would probably have the same combustion
efficiency issues with 90% peroxide, so 98% may be required to get the
New vehicle design
We are considering building another manned vehicle before
moving on to the high altitude unmanned vehicle.
Moving to a dual tank design with a seated pilot in the
middle provides twice the propellant, and allows the pilot to be completely
enclosed and protected. We can build
some experience with strapping a person in before next years supersonic
We can build the new vehicle around the same 2 diameter
engine bulkhead that will be used on the ballistic vehicle. The main engine, manifold, and attitude
engines would stay connected together on the bulkhead, while the body tube and
tank(s) for either the manned vehicle or the ballistic vehicle are bolted to it. We were going to have to build this anyway,
so having it common between two vehicles would be nice.
We could use the existing outrigger legs for landing gear,
sliding them into mating tubes on the underside of the engine bulkhead. These same mount points could be use for
mounting fins for the ballistic vehicle.
The existing outriggers are about 14 pounds each, but we could save at
least five pounds with minor modifications since they wont need to support
engines or two foot long foam.
A seated person can just squeeze into the 2 filament wound
tubes we have. We could cut off a five
foot section, which weighs 50 pounds, make a top bulkhead for it, then cut out
most of the front third to let people get in.
Tall people would have their knees stick out of the front opening a
bit. We would hand-lay a fiberglass
seat base and foot well, so the pilot is completely separated from the
engines. Pad well with energy absorbing
rubber foam, mount a full safety harness, and the pilot should be well
protected from any orientation of crash.
Side or down firing attitude engines? Side firing engines would cut the hover time
by 25%, but down firing engines only 10 from the centerline may not have
enough control authority.
Where to mount the electronics box? Strapped to back? Mounted on top? Sideways on
bottom bulkhead, insulated from the engines?
Flat on top is easiest, but moves the CG up.
High or low side tanks?
If mounted high, the plumbing can just make a gentle 90 degree bend
directly into the joining T above the manifold. If mounted low, the CG will be better, which will help both
tilted landing and the attitude control, but the plumbing would need to turn
270 degrees, potentially leaving a pocket of liquid. There is two feet of choice in vertical mounting position.