Gregory L. Hansen wrote:
In article .com,
Robert Clark wrote:
There was a study announced a few years ago on a more efficient
nuclear fuel, Am-242m, that would allow a trip to Mars in two weeks:

Extremely Efficient Nuclear Fuel Could Take Man To Mars In Just Two
Weeks.
Date: 2001-01-03
http://www.sciencedaily.com/releases...0103073253.htm

Taking the number 75,000,000 km for the distance at closest approach
for Earth and Mars, we can calculate the acceleration required to
reach
the half way point in 7 days, or 608,000 seconds (thereafter the
rocket
would turn around and use the engine to decelerate over 7 more
days.)
The formula for the distance travelled (s) at constant acceleration
(a)
over time (t) is:

s= 1/2 * a * t^2

So:

37.5E9 = 0.5 x a x (608000)^2 = 0.5 x a x 3.7E11 ,
so a = 0.2 m/s^2 .

Then the max velocity is: 0.2 x 608000 = 120960 m/s, about 121 km/s.

I found a report on line that derived some design elements for this
propulsion method:

FISSION FRAGMENTS DIRECT HEATING OF GAS PROPELLANT FOR SPACE ROCKET.
http://www.crs4.it/Areas/cfd/10-IWCP_article.pdf

It gives the Isp as 2500s, maximum.

Its great claim to efficiency seems to be directly exposing the
propellant
to fission fragments, while the more traditional nuclear rocket needs
to
let heat leak out from inside the fuel with the temperature limited
by how
well the heat is removed from the fuel and the temperatures that the
engine structure can handle.

For something not requiring americium, consider a more conventional
reactor with an array of cooling tubes running through it, made of
uranium
or plutonium or other fissile material of your choice. Forget the
thin
films of exotic materials, use bulk material with a switchable
neutron
source.

--
"Never argue with a fool. They will drag you down to their level and
win
by experience."

Extremely Efficient Nuclear Fuel Could Take Man To Mars In Just Two
Weeks.
"To meet the challenge of a light nuclear reactor, Ronen examined one
element of reactor design, the nuclear fuel itself. He found at the
time that of the known fission fuels, Am-242m is the front-runner,
requiring only 1 percent of the mass (or weight) of uranium or
plutonium to reach its critical state."
http://www.sciencedaily.com/releases...0103073253.htm

This seems to be a fundamental improvement in the efficiency over
uranium, plutonium fuels analogous to the efficiency of hydrogen over
kerosene as measured by Isp.


Bob Clark