Re: On Hydrogen Fuel


H2-PV NOW wrote:
Look at that big external tank that the shuttle and two solid rocket
boosters are strapped on. Now WHERE INSIDE do you fit that?

Just kind of 'squish' it a little like a tube of toothpaste, shape it
into a nice triangle and use this tank, properly reinforced with
composite and titanium, as the vehicle itself. Put the cockpit and
crew quarters somewhere near the middle so that the fuel will stop
interstellar radiation.


You are misunderstanding what is meant by efficiency. H2 can only burn
when it finds O2. In the tiurbulent gas streams and combustion
environment it gets a very short amount of time to do that.

Theoretical efficiency is 100% if every molecule of H2 finds an O or O2
the mate up with. Because of the short latency time in the combustion
chamber that 100% fuel efficiency is not possible. "Rich" mixutes of
fuel and oxidizer means that there is extra fuel in the mix looking for
O2 partners and the chances of finding them are increased. LH2/LOX
rockets typically have double the H2 component, 4O per 1H, wasting 50%
of the fuel as unburned expelled molecules in the rexhaust stream.
Slush/LH2 only wastes half of that, for a ratio of 6O per 1H ratio.

The goal is not to conserve fuel, but to get sufficient power, in the
short time that it will do you any good. Efficiency, then is getting
all the power you possibly can in the short latency time that fuel and
oxidizer are lingering in the combustion chamber. If they burn
downstream you get no lift from that.

A completely redesigned engine is indicated here. The extra power of
virtually complete 'burn' should more than make up for increased
weight. And, it should be capable of dealing with 'atomic hydrogen'
pressures and difficulties.


I think you are over-estimating the virues of "atomic" hydrogen. Fuel
cells reduce hydrogen to atomic form through catalysis, and they do it
quietly, efficiently, and non-spectaularly. Fuel cells are not rocket
engines because of some magic property of atomic (non-molecularly
bonded) hydrogen.

Instead of trying to squeeze more power out of hydrogen as a fuel, stop
making it work so hard and accomplish more. The problem is NOT that
hydrogen is not giving lots of power already -- the problem is it is
being asked to lift 8 to 16 times its own weight in oxidizer from a
dead stop under the thickest part of the atmosphere.

That's the problem to solve: how do you avoid the weight and volume
penalty of carrying 8 to 16 times the fuel mass (of oxidizer) until you
are clear of 90% of the atmosphere? SKYLON and SABRE claim they are
solving that problem by air-breathing up until they are at Mach 5.5,
and filling their LOX tanks at high altitude from the thin air at high
speeds.

It's a very intelligent approach. Unfortunately the math is not that
clearly on their side. In order to chill air to LOX temperatures they
need additional LH2, which is colder, but not that much colder than
LOX. Air is 90% N2, which will drain heat from LH2 without contributing
any lift, so the N2 has to be jettisoned with it's chill subtracted
from the LH2. The process of seperating O2 and N2 has to occur very
fast or else too much LH2 is being lost, and the take-off LH2 load
begins to get too big and too heavy for the mission objective, which is
to reach LEO as a Single-Stage-To-Orbit vehicle.

The margins are slim. The allowable time window is narrow. The
technology must work perfectly and it must work extremely well.

For these reasons, plus the small cargo payload, SKYLON does not look
like a useful answer even if it does work as advertised.

The atmosphere seems such a long way up. At 300,000 feet the
atmosphere has all but disappeared. A rocket engine waverider will
reach 300,000 feet in about 2 1/2 minutes. Putting a lot of air
compression and molecular separation equipment on board for that 2 1/2
minutes is, I believe, contraindicated. Ditto for using an
airbreathing rocket engine -- which does not yet exist -- and suffering
a tradeoff decrease in power for just the first 2 1/2 minutes of
flight.

Nicer, easier to just put a good rocket on a triangle waverider. Time
to high orbit should not exceed 5 1/2 to 6 minutes. Attach a couple of
SRBs to the bottom of the waverider and additional speed should turn
'high orbit' into 'escape velocity'.


There's no such thing as "Leprechan Hydrogen", some magical hydrogen
that is different from the ordinary kind. Hydrogen as H2 is more
compact than hydrogen as H+H atomic non-molecular form. I don't care if
you freeze it into bricks, the molecular form has reduced in size from
atomic solitaires . . .


I believe that H2 'ice' is denser than LH2 and sinks in the H2
'liquid'.


Atomic H is very unstable and seeks the first form
of stability as H2. This is an exothermic chemical reaction that
liberates heat energy which causes a chain reaction that causes all the
H to form H2, but now highly excited by the thermal energy released.
Heat turns to kinetic energy which becomes expansion and pressure
energy and the end of that is ruptured tanks, death and destruction.


The same is said of LH2. But those problems were solved. I really
don't believe that 'atomic hydrogen' is all that much different.


For WHAT? So you can avoid looking at the real problem of trying to
lift too much oxidiser from a dead stop under the thickest atmosphere?

SKYLON is not the best answer, but it is far better than trying to lift
too much oxidizer in the first place.

The first place SKYLON can be improved is with much bigger wings. NASA
Helios prototype flew to 100,000 feet in 2001, the same altitude that
SKYLON picks up its oxygen supply, but the Helios got there on big
wings with 28 horsepower of electric motors. Those electric motors were
powered by solar cells,

There are several proposals to launch from 100,000 feet from platforms,
including an unmanned kite, and one proposal to launch from balloon.


Instead of launching from a balloon use helium and vacuum to greatly
reduce vehicle weight within the atmosphere. This means that gravity
will not only be reversing itself by helping with the airfoil but will
also be 'pushing' the vehicle up. This will show up in the
calculations as a greatly reduced dry weight yielding an extremely high
Mass Ratio.


Stop asking Hydrogen to lift backbreaking loads all on it's own. It
can't do more than physics allows and Leprechan Hydrogen found in a
box of Trix is not any kind of answer.

Shed the weight. Use the air for lift. [ Thumbs up! ] Take on the oxidiser at the last
altitude there still is plenty. Be already going fast when you load the
oxidizer. Begin going fast after 90% of the air is below you. Hydrogen
can do this much and no more. Settle for what the laws of physics says
is the limits.

The Shuttle costs $10,000 per pound of payload delivered. Accept it
that is too high a price to pay. "Atomic" hydrogen is not cheaper, but
it's more deadly, for no important gain in trading dihyfrogen for
monohydrogen.


According to NASA's Glenn Research Center there is a good deal of
difference. ISPs of 750+ are being touted about. In short, the
expected ISP is roughly double that of LH2.



So what? You pay $15,000 per pound to orbit instead of $10,000 and get
lots more crashes on TV to talk about.

The problem is not that Hydrogen is not powerful enough already. The
problem is we are loading it down with too many burdens to carry. If
you want $10 per pound payload to orbit you have to shed the weight,
you have to let the Earth's low altitude air carry some of the weight,
and you need to load the heaviest part of the fuel supply after the air
is very thin, and do this when you are already moving faster than
non-astronauts move through the air.

You are redesigned a fine fuel instead of redesigning a fat pig of a
ship. Even NASA has given up on the Shuttle. It never did what it was
created to do: frequent, regular, low-cost, routine flights to LEO with
a cargo equivilent to a highway truck load. Even if the external tank
was half as big, filled with twice as powerful Leprechan monohydrogen,
there would still be two solid rocket boosters strapped on. The LH2/LOX
external tank is not recovered, and it's cost is not the main barrier
from meeting the objectives: frequent, regular, low-cost, routine
flights to LEO with a cargo equivilent to a highway truck load.

The thing that is strapped to the tank is the problem, and nothing you
can do to that tank and its contents will change the thing strapped on
it.


The Shuttle was fine for it's time, but that was then and this is now.
The waverider triangle with enormous fuel capacity, vacuum panels,
composite wrapped tanks, should be capable of putting 250,000 pounds in
orbit or, with the use of SRBs, take the same weight to the Moon or
beyond.

New fuels, hydrogen based, may greatly increase 'thrust to weight'
and/or 'burn time'. These new fuels, therefore, may make inexpensive
interplanetary missions doable with 8 years. And, I believe it to be
possible new fuels or not. It is possible to achieve 7 1/2 minutes of
burn time with conventional hydrogen tanks and rockets. Use a couple
of SRBs as RATO units and Outer Space is possible.


tomcat

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