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."