**5. The far future: human Saturn missions with nuclear pulse propulsion (NPP)**

Historical analyses of human missions to the outer planets have included many nuclear propulsion conceptual designs. Nuclear pulse propulsion was investigated and was considered a practical alternative to any chemical propulsion options. The round-trip impulsive delta-V for such missions was approximately 60 km/s [34, 38–41].

Human missions to Jupiter and Saturn were suggested in the 1960s. Large-scale exploration missions with many astronauts were planned. The primary propulsion system considered was nuclear pulse propulsion. Many small nuclear packages were exploded behind the vehicle, propelling it onto a high-thrust trajectory. For a human Jupiter or Saturn mission, the delta-V was approximately 60 km/s. While the round-trip Jupiter missions were designed to orbit Callisto, at Saturn, Titan was selected. Titan is the largest moon of Saturn, the delta-V to land there is high, 2.2 km/s, and the escape velocity is 3.17 km/s. These values represent an all propulsive landing on Titan and include a 20% delta-V penalty for gravity losses [23]. Such a propulsive delta-V was selected to make the comparisons with the other

**Figure 7.**

*Saturn NPP sizing and mission data, B = 0.01 Mp [23].*

moon landers as consistent as possible. If a lander were to be used for many flights, repacking parachutes or reoutfitting a robotic lander with additional parachutes on orbit may be cumbersome.

**Figure 7** shows the mass of the Saturn missions for a range of delta-V of 60–120 km/s. While the 60 km/s missions represent a fast mission to Saturn (approximately 250–500 days), a fast mission in both directions may require 120 km/s. The vehicle mass for the 120 km/s missions is over 47,000 MT, while the 60 km/s mission requires less than 6000 MT.

In Ref. [23], using ISRU for refueling a fast Saturn mission was analyzed. Saturn's atmosphere was considered a likely source of the nuclear fuels: helium 3 and deuterium. After analyses of the ISRU transportation systems, it was found that Uranus was a more likely atmospheric fuel source. The complexity of moving OTVs from low Saturn orbit to an assembly point (such a Titan) and the trip time for the low-thrust transfers (for a round-trip delta-V of over 94 km/s) were very prohibitive. With the Uranus option, the nuclear fuels were mining and sent to Titan. The round-trip OTV delta-V for lifting the mined fuels to a moon of Uranus was approximately 32 km/s. While the delta-V for mining transportation is lower, the need for an interplanetary transfer vehicle (ITV) is apparent. A relatively low-energy Uranus to Saturn transfer is possible with an NEP ITV. While the ISRU option for the fast Saturn mission is attractive, the time for gathering the nuclear fuels may require 10 years of mining operations.
