Space Forum / Space Flight / January 2006

Yes, and rockets are much, much less fuel efficient compared to
modern high bypass jet engines. (even compared to first generation ones).

Not to mention that though it's lighter, LH2 does not have anywhere
close to the energy per unit volume that aviation fuel does.
Oh, and the rocket exhaust would cause nitrous oxide pollution.

Beside the problem of handling cryogenic fuels, it doesn't make sense to
carry your oxidizer when it's all around, holding you up.

And sure, H2 is lighter than jet fuel, but that just means *really* big
fuel tanks, and they're going to be heavy, since they have to contain
seriously cold liquid without letting it boil away.

That may very very well be the case.

Maybe it has something to do with operating deep freezing equipment
near ordinary humans (passengers, various personell), and it has
something to do with the tendency of H2 to diffuse out of most
containers, over short time? And maybe it has something to do
with the fact that airlines tend to fly at constant speed over
their range, instead of explosively accellerating for the first
few minutes of flight?

best regards
 Patrick

In practice, the main reasons are probably historical (or "histerical"
as a friend of mine used to say).  But if you're thinking of "clean
sheet" designs...

On the pure-technical side:

H2 requires a *huge* volume of tankage, and pretty seriously insulated
too.  That's going to run up your frontal area and hence drag.  Not
nice for an aerodynamic vehicle.

Now to the reasons which are half-historical and half-technical:
Basically, nobody's done it before, so whoever tries to do it first
will have to spend all their own money to debug the technology.

For example, there's no existing airline-scale fueling infrastructure
for H2, so whoever tries to introduce this into service first will
have substantial up-front costs setting this up, and probably be
limited to a small number of airports at first.

Ditto there are no existing "off the shelf" H2/LOX jet engines and
other fuel-system components ready for Airbus or Boeing to incorporate.
Again, whoever goes first in designing for this will probably have to
spend a fair bit of money on R&D.

There's also the issue of persuading various governments' aircraft-
-licencing authorities that this is *really* safe:  The aviation
industry has *very* stringent safety standards.  In particular,
typical airline crashes-per-flight rates are below 1 per *million*
flights, and typical safety specs are at or below 1 safety-critical
failure per subsystem per *billion* flight hours.  Since these rates
are at least a factor of 10,000 better than the best achieved by
space-launch rockets, you can't just take "off the shelf" (space-
-launch) H2/LOX rocket hardware and drop it into an airliner, at
least not if you want to get it certified for revenue passenger
service.

None of these problems are impossible to solve, but all would take
nontrivial amounts of engineering effort, and hence money, to solve.
Even a "conventional" new airliner costs many billion dollars to
develop, and unlike the space-launch business, the government won't
bail you out if it's a flop.  So, you need a really solid cost
estimate before your bankers/stockbrokers will raise the money...
and it's hard to get a solid cost estimate in advance for "nontrivial
amounts of engineering effort".

ciao,

Storing liquid oxygen would be a waste of effort, when there is so much
free air to be had right outside.

Kerosene is low energy, but it has one quality that makes it far
superior to hydrogen for commercial aircraft: high density. Since the
mass of fuel tanks depends on the volume of the fuel to be stored far
more than its mass, and the mass of pumps depends more on the volume to
be pumped than the mass, high density wins. The drag on the plane from
bulky hydrogen tanks would also be a factor.

This advantage is so strong that it is seriously suggested that
commercial jet air fuel should be used for earth-to-orbit rockets
instead of hydrogen, despite hydrogen's higher energy. The numbers make
the two options very comparable to one another. And historically, for
multistage liquid rockets like the Saturn 5, kerosene has been the fuel
of choice for lower stages, with hydrogen only used for the upper
stages.

1) Petroleum is cheaper than LH2. Kerosene goes for about $0.50-$0.75
in bulk orders (or it did when I checked 10 years ago), while hydrogen
costs that much per liter, or more.

2) LOX is cheap stuff, about $0.10 per liter. But oxygen in the air is
even cheaper - it's free. And when you need about 2-3x as much oxygen
(by weight) as fuel, minimum, the LOX cost will add up.

3) No, LOX/LH2 is not lighter than kerosene and atmospheric oxygen. A
jet engine needs a minimum of about 3x the weight of its fuel in
oxidizer (or about 5-6x if the fuel is hydrogen). A jet aircraft
carrying 54000 gallons of jet fuel (like a long range version of the
777) has about 150 tons of fuel on board, or about half its weight. If
it had to carry liquid oxygen, it would need about 350 to 450 tons of
liquid oxygen. That's not very practical for a 300-ton aircraft. On the
other hand, a jet engine can get endless tons of oxygen from the air.
The aircraft doesn't have to carry that mass.

Liquid hydrogen would be lighter than an energy-equivalent amount of
kerosene, but you'd need 5 or 6 times as much oxygen as hydrogen,
minimum. 50 tons of hydrogen would call for 300 tons of LOX. Since
airliners are typically under 50% fuel by weight, you're eating up all
of the aircraft's weight budget on fuel and oxidizer.

4) Carrying the oxidizer is an unnecessary extra effort. Jet engines
can not only gulp down endless tons of oxygen from the air (for free!),
they can use air for excess reaction mass. You can run the engines
fuel-lean and just keep pumping excess air through the engine so you
throw extra mass out the back. When you're carrying all your oxidizer
on board (and it's the heavy part of the propellants), you can't waste
propellant like that.

5) There are several practical issues. Liquid hydrogen takes huge,
bulky tankage (because it needs massive insulation and because liquid
hydrogen is only about 1/10th as dense as kerosene). You won't be able
to pour LOX or LH2 into the bare metal hollows in the wings, unlike
kerosene. That means you're wasting weight and volume on LOX/LH2
tankage that wouldn't be wasted on kerosene.

6) Jet engines that ran on LH2 might be nice, but they have the
practical issues (see point 5) of trying to store LH2 on a plane.

The short summary is: kerosene jet engines are more fuel efficient than
LOH/LH2 engines. Storing cryogenic fuels would eat up a lot of room and
weight on an aircraft, particularly the bulky LH2 tanks. And finally
LOX/LH2 is more expensive.

Mike Miller

First of all, why bother with LOX, when you're flying through air, which is
20% O2?

Second of all, have you ever looked at the density difference between LH2
and kerosene?  The LH2 tanks on a jet aircraft would be huge.  Compressed
H2's tanks would be extremely heavy and impractical for an aircraft (or just
about any other transport) to use.

Lastly, commercial LH2 is typically made from petroleum.  As such, it's more
expensive to manufacture, store, and handle than petroleum products, partly
because it's cryogenic.  LH2 made from electrolysis of water is even more
expensive, partly because most electricity comes from burning petroleum
products.

Jeff

You are wrong on all counts, except that petroleum is cheaper.

LH2 is so light that the amount of space and weight
needed for fuel for a flight is greater than that of
the same amount of hydrogen in petroleum, and that
on the conditions for burning used by jets, the
hydrocarbon gives off more energy than the hydrogen
in it provides, due to burning the carbon and
releasing the bonding energy.

Also, at the altitudes being flown, there is enough
air that the oxygen is free.  Since the ratio of the
mass of oxygen to hydrogen in maximal energy burning
is 8 to 1, why carry the oxygen at all?

bob_jenkins@burtleburtle.net wrote in news:1134509904.059395.104300
@z14g2000cwz.googlegroups.com:

And vastly more practical.  It would be absolutely pointless
to carry an oxygen supply and the liquid hydrogen would be
extremely bulky.  I think a hydrogen-fueled aircraft has been
demonstrated, using air-breathing engines; the oversized
fuel tanks were obvious.

--Damon

I've no idea as to the relative weights of LOX/H2 and jet fuel (since
the plane is atmospheric why would it carry LOX?) but suspect that the
pressurized tanks to hold the LOX or H2 would weight rather a lot more
than the tanks holding unpressurized jet fuel.

The ubiquity of the internal combustion gasoline engine means that one
can find petroleum products just about anywhere on the globe,
including jet fuel or aviation gasoline (ok, perhaps not as prevalent
as regular unleaded but stilll :).  At the present time, it is a bit
more difficult to find LOX or H2.  The ever fun "chicken and egg"
bit...

rick jones

Am 13 Dec 2005 13:38:24 -0800 schrieb "bob_jenkins@burtleburtle.net":

ad 1: Forget LOX - it is easier and cheaper to use the oxygen provided
by the air.
ad 2: LH2 is a nasty stuff, that has two major disadvantages: a) its
very low density that necessitates large tanks and gives so much air
resistance and tankage weight, and b) the necessary low temperatures
needed to prevent the LH2 from boiling off (around -420 deg. F, if I
remember right). So you need very good insulation, that gives you
another weight penalty.

And there are a couple of other disadvantages of LOX/LH2 around. So
the use of fuels makes sense, that can be stored easily without
cooling and having a high (energy) density. Somewhen mankind will
switch over to regenerative fuels as alternative, just because it has
to out of necessity - due to lack of petrol oil at all...

cu, ZiLi aka HKZL  (Heinrich Zinndorf-Linker)

Petroleum-derived kerosene certainly is likely to be
cheaper than petroleum-derived hydrogen.

With nuclear-generated fuel it may be the other way
(iff extra steps to turn nuclear hydrogen into nuclear kerosene
are needed).

Existing jumbo aircraft would lose about half their interior
volume if they had to haul liquid hydrogen with the same
energy as their present maximum loads of kerosene.
However, because it would be lighter, some of that volume
could be taken back for the same range; maybe they'd lose only
30 percent.

If LOX were carried, rather than the motors just breathing
the high-up air as airliner motors now do,
it and hydrogen together would mass, if I've figured correctly
at http://www.eagle.ca/~gcowan/boron_blast.html#BtMT ,
283.7 kg/MWh. A typical C8H18 masses 78.8 kg/MWh
(http://www.eagle.ca/~gcowan/boron_blast.html#BtE )
so kerosene is unlikely to exceed 80 kg/MWh.

Carrying oxygen is nuts if you don't have to.

--- Graham Cowan, former hydrogen fan
http://www.eagle.ca/~gcowan/Paper_for_11th_CHC.html
boron as energy carrier: real-car range, nuclear cachet

Commercial airliners are air breathing and get their oxygen from the
air "for free" so they do not need the LOX component. It would be
possible to fuel air breathing jet engine with hydrogen and this has
been extensively studied in the past. There have even been a few
experimental types that used hydrogen as a fuel. However, although
hydrogen produces more energy per unit mass than kerosene (jet fuel),
it is much less dense even in its liquid form. This means that in terms
of energy per unit volume it is much worse than kerosene, so that the
fuel tanks on a hydrogen fuelled aircraft need to be much bigger and
therefore produce much more drag. The other problem with liquid
hydrogen is that it needs to be kept at very low temperatures in order
to remain liquid, which requires large ammounts of insulation,
especially on long flights. These are also problems for LOX/H2 rockets,
which is one reason space launchers often use kerosene or some other
non-cryogenic fuel, especially for the first stage. The other great
problem with hydrogen is that currently there is no real infrastructure
for supplying it in the quantities required by the airline industries
and substantial investment would be required to provide this
infrastructure in the form of manufacturing plants, storage tanks and
pipielines etc. I hope this helps.

Room temperature hydrocarbons are very easy to store in any old wing or
body tank, out of sight and out of mind of the passengers, who don't
want to be reminded that they are sitting on a flying bomb.

Cryogenic liquid hydrogen, on the other hand, requires a large somewhat
symmetrical tank, where it is much more obvious to the passengers that
they are mere specks in the greater physical order of things.

What we can convert to LOX/H2 more urgently and easily is trucking.

As well as LOX/H2 jumbos, and giant LOX/H2 SSTO and RLV rockets, I
would also like to see hydrogen dirigibles and giant sailing cargo
vessels. It only makes sense, especially if we can use large wind farms
to make the H2 and liquid air - O2, CO2 and N2, Ar, etc.

http://cosmic.lifeform.org

There's little point carrying LOX  if you're flying through a supply of
oxygen.  And keeping H2 liquified would add far more weight than you
save by using a lighter fuel, plus it's very expensive to do.

A good case can be made for using jet fuel for rockets as well as jets,
also because it doesn't have to be kept so cold.  That's what the first
stage of the Saturn V used (kerosene, anyway, IIRC).

There have  been experiments with hydrogen fuel in aircraft.

Zeppelins had bags of hydrogen to keep them up, and diesel fuel to run the
engines. They actually had to release hydrogen to trim the ship as the
fuel was burned off, so some trials were made on burning the hydrogen. It
turned out that hydrogen worked so poorly as a fuel that a 300 HP engine
would only put out 50 HP on hydrogen, so the idea was abandoned as
impractical.

The eventual answer to the trim problem was to condense the water out of
the engine exhaust. The water produced from burning fuel oil weighs more
than the original fuel, so they always had ballast to match the weight
that was burned off.

--

  John Halpenny

bob_jenkins@burtleburtle.net Wrote:

Gas instead of kerosene was studied in the 1990s. In the future we'll
see very large planes (kerosene will desapear 100 years before gas).
Imagine a A380 with the first level full with gas. To build A380 was an
intelligent move.
Rémy