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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:<br /><br />Extremely Efficient Nuclear Fuel Could Take Man To Mars In Just Two Weeks.<br />Date: 2001-01-03 <br />http://www.sciencedaily.com/releases/2001/01/010103073253.htm<br /><br /> 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. The formula for the distance travelled (s) at constant acceleration (a) over time (t) is:<br /><br />s= 1/2 * a * t^2<br /><br /> So: <br /><br />37.5E9 = 0.5 x a x (608000)^2 = 0.5 x a x 3.7E11 , <br />so a = 0.2 m/s^2 . <br /><br /> Then the max velocity is: 0.2 x 608000 = 120960 m/s, about 121 km/s. <br /><br />I found a report on line that derived some design elements for this propulsion method: <br /><br />FISSION FRAGMENTS DIRECT HEATING OF GAS PROPELLANT FOR SPACE ROCKET. <br />http://www.crs4.it/Areas/cfd/10-IWCP_article.pdf <br /><br />It gives the Isp as 2500s, maximum. <br /><br />This page gives the formula for the mass ratio in terms of the velocity change over the trip, or delta V, "DESIGN-CENTERED INTRODUCTION TO AEROSPACE ENGINEERING", http://www.adl.gatech.edu/classes/dci/space/dci10.html , as:<br /><br />M1/M2 = e^(deltaV/g*Isp) <br /><br /> So in this case: M1/M2 = e^(4.937) = 139. If the ship itself weighed 100 tons the ship plus propellant would weigh 14,000 tons.<br /><br />This is large, but the DESIGN-CENTERED INTRODUCTION TO AEROSPACE ENGINEERING web page gives the mass ratio to reach Earth escape velocity for standard chemical rockets as with the Apollo missions as 18.7. So this nuclear Mars mission would be less than <div class="Discussion_UserSignature"> </div>