Re: Using both atmospheric oxygen and nitrogen for hypersonic propulsion.
- From: BradGuth <bradguth@xxxxxxxxx>
- Date: Sun, 24 Feb 2008 16:27:31 -0800 (PST)
On Feb 24, 10:14 am, Ian Gay <g...@xxxxxxx> wrote:
BradGuth wrote:
Very interesting, but where is all the usual Usenet of naysayism and/
or of topic/author bashings and if at all possible banishment?
. - Brad Guth
Well, if you'd like a modest nayism, look up the equilibrium constant
for formation of NH3 from N2 + H2 _at the temperature of a scramjet_.
Robert Clark wrote:
On Feb 22, 5:59 pm, Robert Clark <rgregorycl...@xxxxxxxxx> wrote:
This pages describes some of the problems with getting a scramjet
engine able to operate at the speeds necessary to reach orbit:
Scramjet.
"For a scramjet, the kinetic energy of the freestream air entering
the scramjet engine is large compared to the energy released by the
reaction of the oxygen content of the air with a fuel (say
hydrogen). Thus the heat released from combustion at Mach 25 is
around 10% of the total enthalpy of the working fluid. Depending on
the fuel, the kinetic energy of the air and the potential
combustion heat release will be equal at around Mach 8. Thus the
design of a scramjet engine is as much about minimizing drag as
maximizing thrust."http://en.wikipedia.org/wiki/Scramjet
The problem is even for the scramjet you have to slow the speed of
the incoming airstream down very much from the high hypersonic
speeds required for orbit. This kinetic energy of the air is turned
into heat. But then you can't recover this heat at 100% efficiency
to create thrust.
Then since burning hydrogen would only add 10% more of this energy
back in, any inefficiency in the system would make the combustion
process only barely able to make up for the drag caused in slowing
down the air, or not at all.
However, the 10% figure is only coming from combusting the
hydrogen
with the oxygen in the air. As usual in air breathing propulsion
the nitrogen in air is disregarded as far as chemical reactions to
create heat for thrust is concerned.
The problem is it is difficult to break the N2 bond in order for
nitrogen to chemically combine with other molecules. However, a key
aspect with hypersonic propulsion is that such high temperatures
are created that nitrogen already becomes dissociated. Then we can
get a higher percentage of heat generated if we combine the
hydrogen with both the oxygen and the nitrogen in the air.
At Mach 25, the air stream is moving past at about 8,000 m/s. This
has kinetic energy of 32 million joules of energy per kilogram of
air. Burning hydrogen with oxygen produces 142 million joules of
energy per kilo of hydrogen. The ratio of the mass of oxygen to
hydrogen in H2O is 8 to 1. So burning hydrogen generates 17.5
million joules per kilo of oxygen used. The amounts of oxygen and
nitrogen in air is about 21% and 78% by volume, or, equivalently by
molar amounts. This is about 25% and 75% by mass for oxygen and
nitrogen. So there is 17.5/4 = 4.375 million joules released when
burning the hydrogen per kilogram of air, actually a bit more than
10% of the 32 million joules per kilo kinetic energy of the Mach 25
airstream.
But suppose we can also react the hydrogen with the nitrogen in
the
air. The energy released by reacting nitrogen with hydrogen,
assuming the nitrogen has already been dissociated allowing it to
react, is 46.5 kJ/mol of NH3 (ammonia) generated. Given a molecular
weight of 14 of N, this is 46.5/14 = 3.3 kilojoules per gram, 3.3
million joules per kilo of nitrogen. So this amounts to .75*3.3 =
2.5 million joules per kilo of air. Therefore the total energy
released would be 4.375 + 2.5 = 6.875 million joules per kilo of
air, about 21% of the 32 million joules of kinetic energy of the
air per kilo.
This does result in a lower Isp since the hydrogen used to react
with
the nitrogen could have been used to react with other oxygen to
produce a more energetic reaction. However, the 21% of the air's
kinetic energy obtained gives us a much better chance of getting
more than break even when we slow down the hypersonic airstream.
Note also that since we don't have to carry the oxidizer along we
would still get a rather high Isp.
This also raises the question: could we do better by using a
different fuel than hydrogen? Hydrogen has been considered the best
fuel to use for hypersonic propulsion because of its high Isp.
However, this does not take into consideration the efficiencies of
reacting the fuel with both the oxygen and the nitrogen in the air.
Let's consider using just carbon (C) for the fuel. This page shows
the bond energies for some simple molecules:
Bond Lengths and Energies.
http://www.science.uwaterloo.ca/~cchieh/cact/c120/bondel.html
Reacting carbon with oxygen to produce CO produces 360 kJ per mol
of
CO produced. This is 360/16 = 22.5 kJ per gram of oxygen used, or
22.5 million joules per kilo of oxygen, and 22.5*.25 = 5.63
million joules per kilo of air.
We need also to see how much energy we could get by reacting the
carbon with the nitrogen, again assuming the N2 in the air has
already been dissociated.
For CN, the bond energy is given as 308 kJ/mol. Assuming this is
the
same as the energy released when C and N are combined this amounts
to 308/14 = 22 kJ per gram of nitrogen used or 22 million joules
per kilo of nitrogen; and 22*.75 = 16.5 million joules per kilo of
air. Then the total is 5.63 + 16.5 = 22.1 million joules per kilo
of air. This is quite a significant proportion of the 32 million
joules per kilo of kinetic energy of the air at Mach 25.
However, the situation is complicated here by the fact that the
energy released per kilo of carbon is so much less than for
hydrogen. Still the final Isp and exhaust speed depends on the
specific heat of the reaction products and their molecular weights.
The specific heat of CO for example is smaller than for water,
which means it can be raised to a higher temperature for the same
input of heat.
Bob Clark
The page "Bond Lengths and Energies",
http://www.science.uwaterloo.ca/~cchieh/cact/c120/bondel.html
, shows the bonds energies for HN and HO are quite high at 391 and
366 kJ/mol, respectively. The problem is hydrogen would also have to
be dissociated to form these compounds. It might be possible to use
some of the heat produced from slowing the air stream to dissociate
the hydrogen. However, it's not clear if this would result in more
energy being delivered to the air stream than just reacting the
hydrogen as is since you would have to subtract off the amount of
energy it took to dissociate the hydrogen.
Perhaps if practical means of storing dissociated hydrogen, i.e.,
atomic hydrogen, were found it could work. Then for HN we would get
391/14 or 28 kJ per gram of nitrogen, 28 million joules per kilo, and
28*.75 = 21 million joules per kilo of air. And for HO, 361/16 = 22.6
kJ per gram of oxygen, 22.6 million joules per kilo, and 22.6*.25 =
5.7 million joules per kilo of air. This gives a total of 26.7
million joules per kilo of air. The energy per kilo of hydrogen used
you would get for both of these reactions is also quite high at 386
million joules per kilo.
Other possible choices for the fuel might be lightweight elements
such as lithium. I found some reaction energies with lithium here:
LITHIUM CHEMISTRY IN NSTX.
http://nstx.pppl.gov/DragNDrop/Operations/LLD_design_meetings/24-APR-...
Lithium reacts with oxygen (O2) to form lithium oxide, Li2O,
producing 143 kcal/mol = 600 kJ/mol of energy, or 600/16 = 37.5 kJ
per gram of oxygen, 37.5 million joules per kilo of oxygen, and
37.5*.25 = 9.375 million joules per kilo of air.
Lithium reacts with nitrogen to form lithium nitride, Li3N,
producing
47.5 kcal/mol = 200 kJ/mol of energy, or 200/14 = 14.3 kJ per gram of
nitrogen, 14.3 million joules per kilo of nitrogen, and 14.3*.75 =
10.7 million joules per kilo of air. The total would then be 20
million joules per kilo of air.
As with carbon, the energy per kilo of the fuel though is low.
However, lithium has the advantage that that it reacts both with
atmospheric oxygen and nitrogen at low temperature, i.e.,
dissociation would not be required. This means it could work at much
lower speeds.
Nitrogen.
Reactions.
http://en.wikipedia.org/wiki/Nitrogen#Reactions
Bob Clark
--
*********** To reply by e-mail, make w single in address **************
Your nayism is noted, and perhaps even sufficient, however it seems
3He is a somewhat useless element unless you can deductively think a
little outside the box.
At some point most everything known to physics burns and/or is
subsequently transformed into other matter, with the original energy
never exactly going away, just somewhat phase shifted or transfered
into a new form.
Why not use LRn222, as a rather nifty use-it or lose-it kind of energy
cache of extremely fast moving ions?
.. - Brad Guth
.
- References:
- Using both atmospheric oxygen and nitrogen for hypersonic propulsion.
- From: Robert Clark
- Using both atmospheric oxygen and nitrogen for hypersonic propulsion.
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