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The 90 day report
The 90 day study on the human exploration of the moon and mars
In 1989 the then president of the United States of America, George Bush, called for a Space Exploration Initiative. The response by NASA to this was the infamous 90-day report, which called for a fleet of massive ships to be built in orbit so that everything could be taken to mars prior to the start of a manned mission (The same methods that had been advocated ever since the 1950's).
The 90day report, or to give it it's full name the 90 day study of the human exploration of the moon and mars, starts from the faulty assumption that any trip to mars is going to be done in zero gravity.
Based upon this faulty assumption, they then went on to make a whole set of other equally faulty assumptions which all had knock-on effects for their mission plan.
For a start, they assumed that to minimise the exposure to zero gravity, you needed to take short duration, high energy orbits to get to mars. In turn, this would require you to use very big engines, taking all the fuel and supplies with you for the return trip.
Because of this, the ship would have to be large, and strong enough to cope with the accelerations needed for this plan. This would make it too heavy to launch from the ground, so it would have to be built and supplied in orbit. This in turn would require complex orbital assembly techniques to be developed, along with other (also high cost) technologies needing to be developed as well.
Because of the size of the ship, it would need a fleet of bigger shuttles continuously shipping components up, which again put up the price.
All of these things combined to give it a price tag of $450,000,000,000 which congress took one look at and said no. This basically meant that any similar future plan would be dead in the water.
After all of these complications, the mission plan was still only a "plant the flag" style mission like the Apollo moon program.
Robert Zubrin decided that a different approach to manned mars exploration was needed, so he came up with one. Mars direct began life as a reaction to this overly ambitious and unrealistic plan, and to others like it, and is explained in detail in his book "The case for Mars".