alternate working fluids for nuclear thermal rockets?

Is there some nice concise source for the ISPs of nuclear thermal
rockets using various working fluids, like ammonia or methane? I thought
I had a method of guessing what the ISP might be but it seems to be giving
me values that are wildly wrong.

In particular, why is it CO2 doesn't seem to be discussed
as a working fluid? It's cheap. I suspect low ISP is the answer but
as I say my method turns out to be utter crap.

James Nicoll

--
It's amazing how the waterdrops form: a ball of water with an air bubble
inside it and inside of that one more bubble of water. It looks so beautiful
[...]. I realized something: the world is interesting for the man who can
be surprised. -Valentin Lebedev-

James Nicoll wrote:

Is there some nice concise source for the ISPs of nuclear thermal
rockets using various working fluids, like ammonia or methane? I thought
I had a method of guessing what the ISP might be but it seems to be giving
me values that are wildly wrong.

In particular, why is it CO2 doesn't seem to be discussed
as a working fluid? It's cheap. I suspect low ISP is the answer but
as I say my method turns out to be utter crap.

CO2 is bad for two reasons. First, it has a high molecular weight,
and does not easily break down into components of lower molecular
weight. Second, it is oxidizing at high temperature, so it is not
compatible with ultra high temperature refractory materials like
carbon and carbides.

Realize that ammonia will decompose into nitrogen + hydrogen
at high temperature and low pressure, so the molecular weight
of the exhaust of a nuclear thermal ammonia rocket is lower
than you might expect.

Paul

Mercury would be a nice candidate, but the folks down range wouldn't
appreciate it much...

James

On 9 Nov 2003 12:49:13 -0500, (James Nicoll) wrote:

Is there some nice concise source for the ISPs of nuclear thermal
rockets using various working fluids, like ammonia or methane? I thought
I had a method of guessing what the ISP might be but it seems to be giving
me values that are wildly wrong.

In particular, why is it CO2 doesn't seem to be discussed
as a working fluid? It's cheap. I suspect low ISP is the answer but
as I say my method turns out to be utter crap.

James Nicoll

(James Nicoll) wrote in message ...
Is there some nice concise source for the ISPs of nuclear thermal
rockets using various working fluids, like ammonia or methane? I thought
I had a method of guessing what the ISP might be but it seems to be giving
me values that are wildly wrong.

In particular, why is it CO2 doesn't seem to be discussed
as a working fluid? It's cheap. I suspect low ISP is the answer but
as I say my method turns out to be utter crap.

Isp is essentially the momentum of the exhaust, while rockets are
limited by operating temperatures - in other words the energy of the
exhaust. Since momentum = mv, and kinetic energy = mv^2, to maximise
v you minimize m. CO2 has a molecular weight of 44, while hydrogen
has a molecular weight of 2. CO2 also has three molecules, so some of
the energy in the exhaust gets lost wiggling the atoms around, but
this is a relatively minor effect.

Ammonia turns out to be the second best NTR propellant, since it
decomposes in the nozzle to nitrogen and hydrogen, and is a lot denser
than hydrogen so your tanks aren't obnoxiously huge.

There's a lot more information he

http://yarchive.net/space/rocket/nuclear_thermal.html

-jake

In article ,
James Nicoll wrote:
Is there some nice concise source for the ISPs of nuclear thermal
rockets using various working fluids, like ammonia or methane? I thought
I had a method of guessing what the ISP might be but it seems to be giving
me values that are wildly wrong.

To a sloppy first approximation, it scales inversely with the square root
of the molecular weight. However, that's not as simple a calculation as
you might think, because some of the molecules in question break down --
in particular, an ammonia NTR will have a lot of nitrogen and hydrogen in
its exhaust.

With current technology, solid-core NTR is no better than chemical fuels
unless it is running on LH2 or just possibly something (e.g. ammonia) that
breaks down easily to a mix containing a lot of hydrogen. That's one
reason why you don't see a lot of interest in alternate propellants.

In particular, why is it CO2 doesn't seem to be discussed
as a working fluid? It's cheap. I suspect low ISP is the answer but
as I say my method turns out to be utter crap.

Propellant cost is utterly insignificant for near-future NTR systems.

CO2, with a molecular weight of 44, gives performance grossly inferior to
a LOX/kerosene chemical rocket. Moreover, at high temperatures it shows
some tendency to break down to CO and O2. This improves performance a
little, but that free oxygen is very hard on an orthodox NTR core and
chamber. (There are materials that can stand it, but they're very
different from the ones in LH2 or ammonia NTRs, and the technology for
oxidizing-propellant NTRs is poorly developed.)
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |

(James Nicoll) wrote in message ...
Is there some nice concise source for the ISPs of nuclear thermal
rockets using various working fluids, like ammonia or methane? I thought
I had a method of guessing what the ISP might be but it seems to be giving
me values that are wildly wrong.

www.astronautix.com has a lot of engines, and includes the ability to
look at them by propellant.

http://www.astronautix.com/props/nucmonia.htm
http://www.astronautix.com/props/nuccohol.htm
http://www.astronautix.com/props/nucarlh2.htm

But there's no systematic method there for calculating Isp, just
examples of nuclear thermal with various reaction mass types.

Mike Miller, Materials Engineer

(James Nicoll) wrote in message ...
Is there some nice concise source for the ISPs of nuclear thermal
rockets using various working fluids, like ammonia or methane? I thought
I had a method of guessing what the ISP might be but it seems to be giving
me values that are wildly wrong.

In particular, why is it CO2 doesn't seem to be discussed
as a working fluid? It's cheap. I suspect low ISP is the answer but
as I say my method turns out to be utter crap.


To the first approximation, the main requirement for the working fluid
of a nuclear thermal rocket is: low molecular weight.

The Isp of a NRT using a fully expanded exhaust flow (i.e., infinite
area ratio nozzle) is:

Isp = Sqrt[2/(g-1)] * ao
(units: m/s. Divide by 9.8 m/s/s if you want Isp in seconds.)

...where ao is the speed of sound of the gas in the heated propellant
before expansion, and "g" is the ratio of specific heats: g = cp/cv.
Since sound speed is given by:

ao = Sqrt[g R To / MW]

...where "To" is the initial temperature, R is universal gas constant
(8314 J/kg-K), and MW is the molecular weight of the gas. "To" is
usually set by operating conditions of reactor. So, if you want a
high Isp, then you want high sound speed, and therefore low molecular
weight, e.g., hydrogen, ammonia, methane. I do not see any
conceivable advantage of CO2 over ammonia or methane.

The dependence on the specific heat ratio "g" would suggest you want a
small "g", but "g" is inversely related to the degrees of freedom a
molecule has (think of it as the "squishiness" of the gas: for a
monatomic line helium: g = 1.666, for diatomic like oxygen or
nitrogen: g = 1.4, for CO2: g = 1.3, etc.), so you can only lower "g"
by raising the molecular complexity of gas, which raises the MW
unacceptably.

Thus, to the first approximation: you want a propellant with low MW
and you take the "g" that gives you. Then you starting thinking about
storing the working fluid and the logistics of actually using it...
--
Andrew J. Higgins Mechanical Engineering Dept.
Assistant Professor McGill University
Shock Wave Physics Group Montreal, Quebec CANADA
http://www.mcgill.ca/mecheng/staff/academic/higgins/

James Nicoll wrote:
Is there some nice concise source for the ISPs of nuclear thermal
rockets using various working fluids, like ammonia or methane? I thought
I had a method of guessing what the ISP might be but it seems to be giving
me values that are wildly wrong.

In particular, why is it CO2 doesn't seem to be discussed
as a working fluid? It's cheap. I suspect low ISP is the answer but
as I say my method turns out to be utter crap.

CO2 is not as inert at high temperatures as it seems at low ones.
Ignoring all else, the important thing is the weight of the molecules/atoms
coming out of the exhaust.
Both NH2 and CH4 are good as they have lots of hydrogen.
CO2 by comparison is relatively heavy.

From (James Nicoll) :

In particular, why is it CO2 doesn't seem to be discussed
as a working fluid? It's cheap. I suspect low ISP is the answer but
as I say my method turns out to be utter crap.

James Nicoll


The problem is about molecular mass. It is 2 for hydrogen and 44 for CO2.
Assume you have a core reactor able to undergo X degrees, that is a molécular
energy E. Because E=1/2 MV², for a given E, the velocity V is larger if M goes
down. With tha same energy E, CO2 with a mass 22 times larger than H2 will give
a V near 4.5 times smaller (exactly square root of 22).

Now, if you have a gaseous core with the fission material in the plasma state,
your problem is not the energy, it is the radiation given off. You could then
use lead as fuel because it could be used too as radiation shield.

Yvan Bozzonetti.

The original context for this was a thread on "ultimate rockets",
rockets where the dominent cost was fuel rather than labour. Obviously
that doesn't describe the current state of affairs.
--
It's amazing how the waterdrops form: a ball of water with an air bubble
inside it and inside of that one more bubble of water. It looks so beautiful
[...]. I realized something: the world is interesting for the man who can
be surprised. -Valentin Lebedev-