Case 1: H2O with NTR out of HEEO

We can use the propellant from space in steam rockets to perform the Mars mission. How much propellant would
this require? The answer depends on the trip time and the mass of the vehicles delivered to Mars.

The mission scenario starts with an LH2 NTR taking the Mars payload from Low Earth Orbit (LEO) to HEEO.
FIGURE 12 shows this. The Mars payload is unfueled; its propellant tanks are empty. Then at HEEO, the propellant
tank is filled with water from space. A steam NTR takes the payload the rest of the way. FIGURE 13 shows this.
Recall that the analysis suggested that the exploration and tanker missions could provide between 3500 and 5500 Mg
of water at HEEO.

An LH2 NTR needed to lift a given payload from LEO to HEEO weighs about 40% of the payload. For example, a
Mars lander weighing about 80 Mg would require an additional launch of about 55 Mg of fueled booster. The
booster would consist of about 48 Mg LH2, 6 Mg tank and 2.6 Mg nuclear rocket.

Table 4 shows the amount of water needed for various combinations of trip time, crew cab and ECCV, and lander
combinations. The water propellant requirement at HEEO is estimated both for a smaller, 1000 MW engine at a
colder, 800 C mixed mean outlet temperature,and for a larger, 3000 MW engine operated at a warmer, 1200 C
temperature. Note that less than about 1500 Mg of water propels the heaviest Mars payload option when the trip time
is the longest, 210 days. Long trip times can be considered with this option because the several hundred Mg of water
propellant can be used to shield the crew from dangerous doses of galactic cosmic radiation.

Note also that a warmer engine permits 150 day trips of smaller, 30 Mgcrew cabs and would consume about 2900
Mg propellant. A smaller crew cab would not be dangerous because of the availability of the 2900 Mg of shielding
water.

Case 2: LOX/LH2 out of SSF orbit (LEO)

An alternative mission scenario would usepropellant from space to take the vehicles from the Space Station
Freedom (SSF) orbit to low Mars orbit. Water at the SSF orbit would be split into hydrogen and oxygen and then
liquefied. Then cryogenic propulsion would be used to go to Mars. As an incidental byproduct, the SSF would have
access to 100's or 1000's of Mg of cryofuel.

This mission scenario would require that the propellant be brought from the comet to LEO, not just to HEEO. The
options to lower the orbit of a captured water payload from HEEO to LEO are either to use orbit decay or to use
propulsive maneuvers. Orbit decay would place a major fraction of the 3500 to 5500 Mg of water at LEO. But it
would entail skimming the Earth with2000 Mgpackages of water ice.

The orbit decay option would have ice in orbits that skim the atmosphere. The penalty associated with orbit decay
is time delay. The orbit decay must be controlled so that the payload does not melt or disintegrate when it skims the
atmosphere. An analysis indicates that such an orbit decay of 2000 Mg ice packets is feasible(see the Appendix).
The ice would never reach the Earth surface if an accident occurred. The compressive strength of the ice packets is
low enough that they would disintegrate should an accident occur that results in a deep penetration into the
atmosphere.

The propulsive option to bring thepropellants from HEEO to LEO would necessarily use steam propulsion. The
lower specific impulse of steam NTRs results in the consumption of about 75% of the water, leaving only about 25%
for the missions. This would reduce the 3500 Mg at HEEO to about 880 Mg at SSF orbit, and would reduce] 5500
Mg of water at HEEO to about 1390 Mg s.

TABLE 5 shows the cryofuel requirements starting from either the SSF orbit or HEEO for various Earth-Mars
transfer times and payload masses. Note that all the 150 day missions from SSF can be achieved with less than about
3980 Mg of propellant. This suggests that the orbit decay option would provide substantial payoff because it would
provide the required fuel.

A water splitting facility (WSF) provides fuel for the Mars mission. Whether in SSF orbit or at HEEO, the WSF
requires about 1 Megawatt of electricity. A study(ISU 1990) showed that a 1 Megawatt WSF would produce about
0.6 Kilo-Mg of cryofuel per year.

Case 3: LOX/H2 out of HEEO

From a nuclear safety and risk perspective, it would be very desirable to have a mission scenario that keeps the
nuclear activities in guaranteed safe orbits and guaranteed safe distances from either SSF or any of the near Earth
orbits. We would station a water splitting facility at HEEO. We would thereby keep its 1 megawatt nuclear electric