During the Apollo program, NASA investigated many options for up rating the Saturn V, from stretching the 1st stage to adding enormous solid rocket boosters, and even a nuclear powered upper stage - NERVA - that could be used for missions to Mars.
Two nuclear powered NERVA spacecraft being assembled in orbit, with the crew being ferried with an early design space shuttle, for the journey to Mars.
The Saturn V could also have carried an Orion spacecraft to orbit. Powered by nuclear explosions, the Orion spacecraft leaves orbit for Mars.
Orion Mars Spacecraft:
The Orion Mars spacecraft shown here has a crew accommodation section, 2 Mars Landers in the form of lifting bodies, and enough small nuclear devices to propel the ship to and from Mars.
Given the colossal power of nuclear explosions, enormous Orion ships were envisaged, to be launched from remote desert areas such as Jackass Flats in the Nevada Test Range.
The major problem with trying to devise a ship to travel between the stars is that of fuel. Robert Bussard devised a ship that would use a huge magnetic funnel to collect hydrogen from space to use as fuel in an interstellar ramjet.
Bussard Funnel Thrust:
While Bussard's spacecraft concept looks appealing, calculations have shown that the thrust of the engine may not be enough to overcome drag caused by the magnetic funnel.
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