Figure B.24. Spacecraft velocity increment, acceleration time, and propellant consumed as a function of Isp for a 1,0001 spacecraft.
From this crude example it seems that a reasonable preliminary design of a fusion rocket for a fast Mars mission involves thrust power of the order of 10 GW with an Isp of the order of 106 m/s and a spacecraft mass of the order of 100 tons. Since the trip would last no longer than 20 days, this mass may be adequate, if the reactor can be made sufficiently compact and light. If that was not feasible, and mass had to be of the order of 1031, a practical "fast" Mars mission is possible only with a modest Isp = 105 m/s rocket using 1 GW power.
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