Almost any conceivable human mission to Mars involves transfer of some assets to Mars orbit, and some assets to the Mars surface. A widely accepted surrogate for estimating mission cost for human missions to Mars is the required initial mass in low Earth orbit (IMLEO). This, in turn can be calculated by estimating how much mass must be delivered to Mars orbit (MM0) and how much mass must be delivered to the Mars surface (MMS), and multiplying each figure by its appropriate "gear ratio": mass required in LEO to deliver one mass unit to Mars orbit or the Mars surface.

The overall gear ratio for transfer from LEO to Mars orbit or surface can be subdivided into a product of subordinate gear ratio factors for each step along the way. Gear ratios for propulsive steps can be estimated from the rocket equation. Gear ratios will be estimated for various mission steps using several chemical propulsion systems. For aero-assisted orbit insertion and entry, descent, and landing, the recent models developed by B. Braun and the Georgia Tech Team were used to estimate entry system masses (see Section 4.6).

The gear ratio for propulsive transfer from LEO to Mars orbit depends on whether the orbit is elliptical or circular. The gear ratio is greater for a circular orbit. However, when ascending from the Mars surface to rendevous in Mars orbit, the gear ratio is considerably higher for the elliptical orbit.

When nuclear thermal propulsion (NTP) is utilized for Earth departure, two important factors are the minimum altitude allowed for start-up and the propulsion system dry mass fraction. We have parameterized these and estimated gear ratios for a range of values. If NTP can be fired up in LEO, and if the dry mass fraction is as low as 0.2 to 0.3, the use of NTP in place of LOX/LH2 for Earth departure can reduce IMLEO by —40%. However, if the NTP must be raised to an altitude of over 1,000 km prior to start-up, and if the dry mass fraction is —0.5, the benefit of using NTP diminishes to almost nothing (see Table 3.9).

In situ resource utilization (ISRU) on Mars for propellant production can reduce the landed mass (MMS) and, if an elliptical orbit is used, it can also reduce the propellant mass required for orbit insertion and Earth return. Mass savings from use of ISRU are estimated to be significant.

Finally, a recent (2006) NASA Mars mission plan was analyzed in terms of gear ratios and it was found that the estimated value of IMLEO was —1,600 metric tons (mT), whereas the NASA claim is that IMLEO = 446 mT. NASA mass estimates for human missions to Mars appear to be overly optimistic.

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