Re: On Hydrogen Fuel


tomcat wrote:
I am beginning to wonder whether hydrogen is being used to full
advantage in our space program.

Here is my understanding of the aerospace use of LH2.


It is stored in a glass lined slosh baffled tank in it's liquid state
(typical rocket tank). These tanks are vented so that evaporation can
be expelled from the tank thus eliminating pressure build up.

Probably has to do with the glass lining, don't you think? If it wasn't
a vacuum seperated bottle in a bottle, then convesctive and conductive
heat gain would be high. The coldness of the LOX would promote thick
frost/ice accumulation on the tank outer surfaces leading to both
aerodynamic drag and ice chunk falling hazards.


It is extremely cold in the liquid state. Cryogenic and close to
absolute zero.

When chilled even further by inducing increased evaporation by vacuum
or injecting liquid helium which is a colder liquid it reaches what is
called a 'slush' state which is partly hydrogen 'ice' mixed with liquid
hydrogen.

Slush tanks make hydrogen 16% more dense than it's pure liquid state.
It also increases the fuel's ISP somewhat.

It increases it due to the fact that a leaner fuel mix is used. Perfect
combustion of H2+O2 to H2O cannot be counted on due to the short
latency time in the combustion chamber. LH2/LOX is burned at a 1:4
ratio instead of 1:8 ratio by mass/weight. With Slush-LH2/LOX the H2 is
birned at a 1:6 ratio, a 50% improved fuel efficiency. The Isp remains
constant as only the reaction of 2H2+O2=2H2O counts. None of the
unburned H2 adds thrust.


Now I have learned that liquid hydrogen is transported, both by rail
and truck, in carbon fiber composite tanks which hold the fuel in a
liquid state without evaporation at 250 atmospheres pressure. This
works out to be a little less than 4000 psi.

Composite tanks for compressed H2 have been approved in Europe for road
vehicle use to 10,000 psi pressure, with 2.35 safety margin, passing
tests to 24,000 psi.

Several ideas have come to mind. First, why not use these tanks in
place of the evaporation tanks on rockets? Second, why not build even
stronger tanks and super compress the hydrogen into slush, or even into
a pure solid state?

Stronger tanks up to 2,000,000 psi are theoretically possible but the
cost is out of this world. A new era of "conformable tanks" is just
dawning, and the testing process is bth time-consuming and costly.

DEsigning high pressure tanks for Hydrogen Economy fuel cell road
vehicles has uncovered a new failure mode never seen before and not
even predicted by prior experience. These tanks can be made to explode
where the tank turns to dust, and it happens so fast that high speed
cameras with 4000 frames per second barely capture the even on one
single frame, although sometimes it happens between frames.

Now you see it, now you don't, in 1/3000th of second.
http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/review04/st_p1_weisberg_04.pdf
http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/30535ar.pdf

They call it "benign" failure because it doesn't throw tank shrapnel,
but you are still left with a ball of Hydrogen rapidly expanding.



Titanium tanks can easily take 100,000 psi. Why not use titanium, add
a little helium, take hydrogen to a solid state and use that to fuel
SSMEs (Space shuttle Main Engines)? This would produce 'atomic'
hydrogen with an ISP around 750+, not the ~450 ISP of standard liquid
hydrogen.

For the same reason they don't use peanut butter: ductility. The reason
we can shape titanium is because of ductility. That's the thing you
want to avoid in high pressure tankage.



The real question is whether or not the SSME can stand up to 'atomic'
hydrogen with it's higher ISP. This, I believe, needs to be tested by
NASA.

Forget SSME. The experimentt is over -- it failed to produce what it
was being tesed for: cheap frequent reusable orbit insertion. Time to
move on. NASA has accepted that decision. Now it's your turn. It's
over.


A significantly denser hydrogen, perferably solid, combined with a
substantially higher ISP should result in spectacular performance.

There's no such thing as "spectacular performance" when it costs, $55m,
$500m or 1.2b per averaged flight. Get over it -- the entire concept
failed to deliver cheap regular frequent orbit. Move on to better
things. This is a cul de sac. Back out and start from where it went
wrong. You can't make progress in this dead end.


Atomic hydrogen is being experimented on by NASA. I wonder, however,
why it's application with real live equipment is taking so long to come
about. It is really just a solid form of the fuel that is currently
being used by hydrogen burning engines. Extra ISP is just extra ISP,
requiring at most some beefing up of the combustion chamber, throat,
bell and maybe the lines running from the tanks to the throat.


I welcome any comments on liquid hydrogen. It seems to me to be the
fuel of the future, not just the 'old' Shuttle.

NASA flew the Helios Prototype to 96,863 feet on 28 horsepower of Solar
Photovotaics-Powered plastic covered wings in 2001. Extend THIS, not
waste your time tweaking the Shuttle.

I predict that the first spaceplane to reach ISS will be a rocket
powered biplane with wings larger than 8,000 feet square. It will look
more like the B2-Spirit carrying the Concorde piggyback than it will
look like the Space Shuttle riding on the 747.

It will take on the bulk of it's oxidizer at 100,000 feet as it sprints
to orbit, and the wingloading will be less than 20 pounds per square
foot at takeoff. Like the Helios Prototype, it will have PV solar cells
on the upper wing surfaces for power during orbit.

It will carry both water and ammonia for return fuel, converted to N2,
O2 and H2 at station. The N2 and some of the O2 will be left at station
each trip for breahable atmosphere for an ever expanding LEO habitat
program, and the H2 and some of the O2 will be used for poiwered
decent. NH3 has 1.7 times the Hydrogen density of LH2. Nine liters H2O
has 1 kilogram of H2 and 8 kilograms of O2 contents, compared to 14
liters for one kilogram of LH2, 1.5 times the Hydrogen density.

It makes more sense to carry breathable air to station in forms
containing fuel values for refueling than to carry compressed air tanks
to station. Because of the long processing times, return fuel would
likely be made at station from previous deliveries, although solar
cells on wings permit staying as long as it takes for return flight
without depending on prepositioned return fuels.

There is a perpetual requirement for air and water deliveries to LEO,
so some launch budget must alway include these payload fractions
anyway. It's going to be a very long time before lunar deliveries of O2
will help supply.


Another option is CH4/LH2 Gelled Hydrogen with metal particles.
http://sbir.grc.nasa.gov/launch/GELLED.htm


The high solar power and long solar "days" of above atmosphere will be
used for making cryogenic gases in orbit for active cooling re-entry.
Return cargos may be returned seperately on their own unpiloted craft
assembled at LEO. This will bring the wingloading down below 5 pounds
per foot square. Oxidizer will be reloaded on return trip same way as
outbound for powered aeronavigation to any convenient chosen landing
port. Such a craft will do double duty as transcontinential air
carrier.

Now, as Jean Luc Picard would say: "Make it so".

.

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