**4. Mission planning and delta-V data**

**Table 3** provides the delta-V to reach low orbit about the moon and the moon's escape velocities. As Titan is the largest moon, the delta-V for escape is the largest: 3.17 km per second. These delta-V values will be important in selecting the most attractive moons for ISRU processing. The planetary gravitational constants and radii were found in Ref. [19].

## **4.1 Exploration vehicles**

As a prelude to human missions, a detailed survey of the major Saturnian moons is planned. The survey is driven by ISRU requirements: identification, prospecting, resource capturing, and utilization. The final steps will be power generation and manufacturing on the surface of the moons.

A first survey of the major moons will include orbiters and small landers. A suggested set of spacecraft would be nuclear electric propulsion transfer vehicle and a small chemical propulsion lander. The NEP transfer vehicle would use a complement

**39**

*Solar System Exploration Augmented by In Situ Resource Utilization: System Analyses, Vehicles…*

of science instruments to assess the ices and regolith on the moons' surface. A megawatt class radar system aboard the orbiter would provide data on ice thicknesses and regolith composition. Once an attractive site is identified, the chemical propulsion lander can descend to the surface and perform a series of in situ assessments. Chemical laboratories delivered to the surface can sample the ices and regolith. Samples may even be returned to orbit for caching and planned return to Earth.

The nuclear electric propulsion (NEP) vehicles or orbital transfer vehicles (OTVs) are described by the following mass scaling equation. The dry mass scaling

The low-thrust OTV delta-V values are noted in **Tables 4** and **5** for each round-

The chemically propelled oxygen/hydrogen propulsion landers were described with a mass scaling equation. In sizing the chemical propulsion landers, a vehicle

Mdry,stage (kg) = Mdry,coefficient • Mp (kg)

In almost every case, the chemical propulsion landers had a B coefficient of 0.4. With the very large delta-V missions at Titan, the B coefficient was 0.2. The lander

where Mdry,stage is the stage dry mass, including residual propellant (kg); Mdry,coefficient is the B mass coefficient (kg of tank mass/kg of usable propellant

The NEP vehicle has a specific impulse of 5000 seconds with a propulsion system efficiency of 50%. The power level of the reactor was 10 MWe, and the reactor specific mass was 10 kg/kWe. The propellant tankage dry mass fraction was 5%, and the fixed mass was 20 MT [31]. Additional mission assumptions are discussed

Mdry, stage (kg) = reactor specific mass (kg/kW) • P (kWe)

+ 0.05 • Mp (kg) + fixed mass (kg)

*Landing, launch, and escape of delta-V for the seven major moons of Saturn.*

*DOI: http://dx.doi.org/10.5772/intechopen.88067*

**4.2 Space vehicle sizing**

equation used was [31–35]

trip mission.

**Table 3.**

in the next sections.

mass scaling equation was used [31–35]:

mass); Mp is the usable propellant mass (kg).

specific impulse was 480 seconds [31, 36–38].

*Solar System Exploration Augmented by In Situ Resource Utilization: System Analyses, Vehicles… DOI: http://dx.doi.org/10.5772/intechopen.88067*


#### **Table 3.**

*Planetology - Future Explorations*

**3.3 Iapetus**

**Table 2.**

712 km. Its gravity level is 2.4 × 10<sup>−</sup><sup>2</sup>

*Gravity levels of Saturn and its major moons.*

have filled in numerous craters.

masses to and from the moon's surface.

**4. Mission planning and delta-V data**

manufacturing on the surface of the moons.

**3.4 Icy moon gravity levels**

radii were found in Ref. [19].

**4.1 Exploration vehicles**

Iapetus is one of the most distant large moons from Saturn, and its radius is

girdles a significant fraction of the equator. One hemisphere has very dark material, with an albedo is 2–6%, while the other hemisphere in comparison is very bright. The composition of the dark material is perhaps organic material and carbon. Simulations have suggested that the dark materials may have been deposited by numerous particle collisions. An alternative theory is that magma from the interior has risen to the surface and that magma may be visible in photos as materials that

**Table 2** provides the gravity levels of the major moons. While Titan has a surface gravity level of 14% of Earth, the remaining moons have gravity levels of 1 × 10<sup>−</sup><sup>2</sup> Earth gravity or less. Such low gravity levels may make transportation, operations, and industrial processing on the moon's surfaces very challenging. On the other hand, the low gravity is helpful is reducing the delta-V needed to transport large

**Table 3** provides the delta-V to reach low orbit about the moon and the moon's escape velocities. As Titan is the largest moon, the delta-V for escape is the largest: 3.17 km per second. These delta-V values will be important in selecting the most attractive moons for ISRU processing. The planetary gravitational constants and

As a prelude to human missions, a detailed survey of the major Saturnian moons is planned. The survey is driven by ISRU requirements: identification, prospecting, resource capturing, and utilization. The final steps will be power generation and

A first survey of the major moons will include orbiters and small landers. A suggested set of spacecraft would be nuclear electric propulsion transfer vehicle and a small chemical propulsion lander. The NEP transfer vehicle would use a complement

of Earth's gravity. This moon has a ridge that

**38**

*Landing, launch, and escape of delta-V for the seven major moons of Saturn.*

of science instruments to assess the ices and regolith on the moons' surface. A megawatt class radar system aboard the orbiter would provide data on ice thicknesses and regolith composition. Once an attractive site is identified, the chemical propulsion lander can descend to the surface and perform a series of in situ assessments. Chemical laboratories delivered to the surface can sample the ices and regolith. Samples may even be returned to orbit for caching and planned return to Earth.

#### **4.2 Space vehicle sizing**

The nuclear electric propulsion (NEP) vehicles or orbital transfer vehicles (OTVs) are described by the following mass scaling equation. The dry mass scaling equation used was [31–35]

 Mdry, stage (kg) = reactor specific mass (kg/kW) • P (kWe) + 0.05 • Mp (kg) + fixed mass (kg)

The low-thrust OTV delta-V values are noted in **Tables 4** and **5** for each roundtrip mission.

The NEP vehicle has a specific impulse of 5000 seconds with a propulsion system efficiency of 50%. The power level of the reactor was 10 MWe, and the reactor specific mass was 10 kg/kWe. The propellant tankage dry mass fraction was 5%, and the fixed mass was 20 MT [31]. Additional mission assumptions are discussed in the next sections.

The chemically propelled oxygen/hydrogen propulsion landers were described with a mass scaling equation. In sizing the chemical propulsion landers, a vehicle mass scaling equation was used [31–35]:

$$\mathbf{M}\_{\text{dry,stage}} \text{ (kg)} = \mathbf{M}\_{\text{dry,coefficients}} \bullet \mathbf{M}\_{\text{p}} \text{ (kg)}$$

where Mdry,stage is the stage dry mass, including residual propellant (kg); Mdry,coefficient is the B mass coefficient (kg of tank mass/kg of usable propellant mass); Mp is the usable propellant mass (kg).

In almost every case, the chemical propulsion landers had a B coefficient of 0.4. With the very large delta-V missions at Titan, the B coefficient was 0.2. The lander specific impulse was 480 seconds [31, 36–38].


#### **Table 4.**

*Round-trip delta-V map for Saturn's moons (seven moons).*


#### **Table 5.**

*Round-trip delta-V map for Saturn's moons (six moons, without Iapetus).*

## **4.3 Mission delta-V results**

Examples of the chemical propulsion lander masses are shown in **Figures 2** and **3**: Titan and Enceladus, respectively. At Titan and Enceladus, the smallest 1 MT payload landers would be for ISRU prospecting and exploration. The 50 MT payload landers would be for industrial scale ISRU propellant production plant delivery.

The NEP vehicle delta-V values are shown in **Tables 4** and **5**. The delta-V was computed with a low-thrust trajectory estimation algorithm. **Table 4** shows the round-trip delta-V values for trips between the seven major moons; the table shows that using Dione as a central exploration moon, the total delta-V for a fleet of OTVs is minimized. An option with only six moons was also investigated, noted in **Table 5**. As Iapetus is the most distant moon in the system of moons, a separate analysis was conducted excluding Iapetus. For both the seven moon and the six moon options of **Tables 4** and **5**, the moon Dione shows the minimal fleet delta-V. Though this is the moon showing the minimal delta-V for the entire fleet, the influence of the OTV and lander mass may change the optimal (or minimal mass) solution.

**41**

**Figure 3.**

**Figure 2.**

**4.4 Factory analysis**

*Solar System Exploration Augmented by In Situ Resource Utilization: System Analyses, Vehicles…*

*Saturn moon lander mass for Titan, round-trip flights, payload masses of 1–50 MT.*

The ISRU factory will allow the OTV to be refueled from water ices on the moons. Initially, the OTVs, their science landers, and factory lander are delivered to the centric moon. The OTVs are delivered with no NEP propellants and only

*Saturn moon lander mass for Enceladus, round-trip flights, payload masses of 1–50 MT.*

*DOI: http://dx.doi.org/10.5772/intechopen.88067*

*Solar System Exploration Augmented by In Situ Resource Utilization: System Analyses, Vehicles… DOI: http://dx.doi.org/10.5772/intechopen.88067*

**Figure 2.**

*Planetology - Future Explorations*

**4.3 Mission delta-V results**

*Round-trip delta-V map for Saturn's moons (six moons, without Iapetus).*

*Round-trip delta-V map for Saturn's moons (seven moons).*

Examples of the chemical propulsion lander masses are shown in **Figures 2** and **3**: Titan and Enceladus, respectively. At Titan and Enceladus, the smallest 1 MT payload landers would be for ISRU prospecting and exploration. The 50 MT payload landers would be for industrial scale ISRU propellant production plant

The NEP vehicle delta-V values are shown in **Tables 4** and **5**. The delta-V was computed with a low-thrust trajectory estimation algorithm. **Table 4** shows the round-trip delta-V values for trips between the seven major moons; the table shows that using Dione as a central exploration moon, the total delta-V for a fleet of OTVs is minimized. An option with only six moons was also investigated, noted in **Table 5**. As Iapetus is the most distant moon in the system of moons, a separate analysis was conducted excluding Iapetus. For both the seven moon and the six moon options of **Tables 4** and **5**, the moon Dione shows the minimal fleet delta-V. Though this is the moon showing the minimal delta-V for the entire fleet, the influence of the OTV and lander mass may change the optimal (or minimal

**40**

mass) solution.

delivery.

**Table 5.**

**Table 4.**

*Saturn moon lander mass for Titan, round-trip flights, payload masses of 1–50 MT.*

#### **Figure 3.**

*Saturn moon lander mass for Enceladus, round-trip flights, payload masses of 1–50 MT.*

#### **4.4 Factory analysis**

The ISRU factory will allow the OTV to be refueled from water ices on the moons. Initially, the OTVs, their science landers, and factory lander are delivered to the centric moon. The OTVs are delivered with no NEP propellants and only

**Figure 4.**

*Moon base factory masses, for 50 MT payload landers.*

oxygen and hydrogen propellants for the science landers (with a 1 MT payloads) and 50 MT payload factory lander. Once in orbit about the centric moon, the factory lander with the 50 MT ISRU factory lands. The mining and conversion of water ice to oxygen and hydrogen for the factory lander's ascent to orbit are conducted. Additional hydrogen is created on the centric moon and that hydrogen is delivered to the orbiting OTVs. The OTVs can then be dispatched on their moon exploration flights to the other remaining moons. The factory lander will have to perform several flights to deliver the full propellant loads for the orbiting OTVs. **Figure 4** presents the estimated masses of a series of propellant factories. The factory masses are based on the lander Isp and the factory design, which is a function of the level of integration with the lander.

For example, a light propellant factory has separate tanks for lander and factory. A heavy propellant factory has separate tanks for lander and factory but has higher masses for enclosures that protect against the elements, winds, micrometeoroids, etc. Also, higher masses are included for foundations for cryogenic surfaces (creating a stable structure for the base). For a super lightweight propellant factory, propellants and all fluids are fed to and stored in lander tanks. Appendix A delineates the masses in the heavy configuration.

In **Figure 5**, the masses of the ISRU factory and the OTV propellant masses needed for the Saturn moon survey are compared. With the cases without Iapetus, six moons are surveyed. The mass of the ISRU factory is 50 MT. In all cases, the total OTV propellant load is significantly higher that the factory mass. Thus, the use of the factory can enable not only the first survey of the moons but many more. Typically, nuclear reactors have been designed for a 7-year life at full power and a 10-year overall life (operating for the last 3 years at a reduced power level). Given the typical 7-year lifetime of a space nuclear reactor [13], the number of OTV flights can be 6–7.

**43**

propellants.

**Figure 5.**

factory.

*Solar System Exploration Augmented by In Situ Resource Utilization: System Analyses, Vehicles…*

To fully explore the moons, a fleet of OTVs and landers were conceived. One OTV and one lander would visit each moon. At the central or centric moon, a 50 MT ISRU factory is landed. The 50 MT payload includes an ISRU factory and a set of empty propellant tanks. The factory creates the propellant for all of the NEP OTVs and landers. The propellant tanks will be filled with the moon-derived ISRU

The lander was designed to bring that propellant into orbit to refuel the OTVs. The lander propellant tanks are fueled with oxygen and hydrogen. The lander then ascends to the escape conditions of the moon's orbit. Then the lander performs a rendezvous with the OTV(s) and refuels OTV(s); it may also refuel the smaller exploration landers aboard the OTV(s). The newly fueled OTV with its exploration lander completes the orbit transfer to another moon. After completing its exploration, the OTV returns to the moon with ISRU propellant

An additional case for Titan with a 100 MT lander payload was included. This case was added as the lander delta-V for Titan is the highest of all of the moons; therefore, a larger propellant mass is needed for each flight. A larger lander payload case might be attractive in reducing the number of flights needed for refueling the OTVs.

An overall fleet mass of the OTVs and the landers was then estimated. The fleet consisted of one NEP OTV and lander designed to visit a specific moon. Each OTV carried a lander with a 1 MT payload. For the seven moon options, seven OTVs and seven exploration landers were sized. Each OTV was sized to operate from a central moon. As noted above, an additional lander operating from the centric moon was

Using the mission delta-V values and vehicle sizing data, the following optima for moon exploration were found. In **Table 6**, the total mass of the OTV

added to provide the ISRU refueling capability for the OTV fleet.

*DOI: http://dx.doi.org/10.5772/intechopen.88067*

**4.5 Mission scenario results and interpretations**

*Moon factory mass comparison with OTV propellants needed.*

*Solar System Exploration Augmented by In Situ Resource Utilization: System Analyses, Vehicles… DOI: http://dx.doi.org/10.5772/intechopen.88067*

**Figure 5.**

*Planetology - Future Explorations*

**42**

flights can be 6–7.

**Figure 4.**

*Moon base factory masses, for 50 MT payload landers.*

integration with the lander.

eates the masses in the heavy configuration.

oxygen and hydrogen propellants for the science landers (with a 1 MT payloads) and 50 MT payload factory lander. Once in orbit about the centric moon, the factory lander with the 50 MT ISRU factory lands. The mining and conversion of water ice to oxygen and hydrogen for the factory lander's ascent to orbit are conducted. Additional hydrogen is created on the centric moon and that hydrogen is delivered to the orbiting OTVs. The OTVs can then be dispatched on their moon exploration flights to the other remaining moons. The factory lander will have to perform several flights to deliver the full propellant loads for the orbiting OTVs. **Figure 4** presents the estimated masses of a series of propellant factories. The factory masses are based on the lander Isp and the factory design, which is a function of the level of

For example, a light propellant factory has separate tanks for lander and factory. A heavy propellant factory has separate tanks for lander and factory but has higher masses for enclosures that protect against the elements, winds, micrometeoroids, etc. Also, higher masses are included for foundations for cryogenic surfaces (creating a stable structure for the base). For a super lightweight propellant factory, propellants and all fluids are fed to and stored in lander tanks. Appendix A delin-

In **Figure 5**, the masses of the ISRU factory and the OTV propellant masses needed for the Saturn moon survey are compared. With the cases without Iapetus, six moons are surveyed. The mass of the ISRU factory is 50 MT. In all cases, the total OTV propellant load is significantly higher that the factory mass. Thus, the use of the factory can enable not only the first survey of the moons but many more. Typically, nuclear reactors have been designed for a 7-year life at full power and a 10-year overall life (operating for the last 3 years at a reduced power level). Given the typical 7-year lifetime of a space nuclear reactor [13], the number of OTV

*Moon factory mass comparison with OTV propellants needed.*

#### **4.5 Mission scenario results and interpretations**

To fully explore the moons, a fleet of OTVs and landers were conceived. One OTV and one lander would visit each moon. At the central or centric moon, a 50 MT ISRU factory is landed. The 50 MT payload includes an ISRU factory and a set of empty propellant tanks. The factory creates the propellant for all of the NEP OTVs and landers. The propellant tanks will be filled with the moon-derived ISRU propellants.

The lander was designed to bring that propellant into orbit to refuel the OTVs. The lander propellant tanks are fueled with oxygen and hydrogen. The lander then ascends to the escape conditions of the moon's orbit. Then the lander performs a rendezvous with the OTV(s) and refuels OTV(s); it may also refuel the smaller exploration landers aboard the OTV(s). The newly fueled OTV with its exploration lander completes the orbit transfer to another moon. After completing its exploration, the OTV returns to the moon with ISRU propellant factory.

An additional case for Titan with a 100 MT lander payload was included. This case was added as the lander delta-V for Titan is the highest of all of the moons; therefore, a larger propellant mass is needed for each flight. A larger lander payload case might be attractive in reducing the number of flights needed for refueling the OTVs.

An overall fleet mass of the OTVs and the landers was then estimated. The fleet consisted of one NEP OTV and lander designed to visit a specific moon. Each OTV carried a lander with a 1 MT payload. For the seven moon options, seven OTVs and seven exploration landers were sized. Each OTV was sized to operate from a central moon. As noted above, an additional lander operating from the centric moon was added to provide the ISRU refueling capability for the OTV fleet.

Using the mission delta-V values and vehicle sizing data, the following optima for moon exploration were found. In **Table 6**, the total mass of the OTV


#### **Table 6.**

*Total masses of the lander and OTV fleets, for moon-centric options, including seven moons.*

#### **Figure 6.**

*Total masses of the lander and OTV fleets, for moon-centric options, including seven moons.*

and lander fleets using a central moon are shown; the minimal mass is 958 MT for the moon Dione. **Figure 6** also presents the fleet mass data. While Dione represents the minimal fleet mass, the optimum is fairly broad over a set of four moons: Enceladus, Tethys, Dione, and Rhea. After further exploration and consideration of currently imprecisely known factors (ice composition, regolith strength, ease of access to any subsurface moon oceans, etc.), an optimal moon can be selected.

**Table 7** presents the same analyses, excluding Iapetus. In **Table 6**, the minimal fleet mass is found for the moon Dione. In **Table 7**, the minimal fleet mass is for the moon Tethys: the fleet mass is 785 MT. While Tethys represents the moon

**45**

*Solar System Exploration Augmented by In Situ Resource Utilization: System Analyses, Vehicles…*

with the minimal fleet mass, the optimum is fairly broad over a set of four moons:

*Total masses of the lander and OTV fleets, for moon-centric options, including six moons (without Iapetus).*

By excluding Iapetus, the total fleet mass is significantly reduced; for Dione, the fleet mass is reduced from 958 to 800 MT. Further restrictions of the moon exploration fleet (e.g., from six to four moons) may be needed to fit within the payload capacity of future interplanetary transfer vehicle (ITVs). As Iapetus is more remote than the other moons, it is possible that a separate ISRU space base may be more

Due to the low gravity level of the moons, a moon-orbiting space base may be attractive. Processing control of ices in a low-gravity environment may be very difficult. It is suggested that an artificial gravity space base in orbit about the moon may be the best location of resource processing. After a series of small missions have been conducted, a large ISRU space base may be established. The base might be a refueling point for extensive exploration. The water ices from the moons would be brought to the space base for processing, to allow the refueling of the landers and

**5. The far future: human Saturn missions with nuclear pulse** 

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 approxi-

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

*DOI: http://dx.doi.org/10.5772/intechopen.88067*

Enceladus, Tethys, Dione, and Rhea.

the NEP OTVs.

**Table 7.**

**propulsion (NPP)**

mately 60 km/s [34, 38–41].

attractive than a centralized ISRU space base.

*Solar System Exploration Augmented by In Situ Resource Utilization: System Analyses, Vehicles… DOI: http://dx.doi.org/10.5772/intechopen.88067*


**Table 7.**

*Planetology - Future Explorations*

**44**

**Figure 6.**

**Table 6.**

can be selected.

*Total masses of the lander and OTV fleets, for moon-centric options, including seven moons.*

*Total masses of the lander and OTV fleets, for moon-centric options, including seven moons.*

and lander fleets using a central moon are shown; the minimal mass is 958 MT for the moon Dione. **Figure 6** also presents the fleet mass data. While Dione represents the minimal fleet mass, the optimum is fairly broad over a set of four moons: Enceladus, Tethys, Dione, and Rhea. After further exploration and consideration of currently imprecisely known factors (ice composition, regolith strength, ease of access to any subsurface moon oceans, etc.), an optimal moon

**Table 7** presents the same analyses, excluding Iapetus. In **Table 6**, the minimal fleet mass is found for the moon Dione. In **Table 7**, the minimal fleet mass is for the moon Tethys: the fleet mass is 785 MT. While Tethys represents the moon

*Total masses of the lander and OTV fleets, for moon-centric options, including six moons (without Iapetus).*

with the minimal fleet mass, the optimum is fairly broad over a set of four moons: Enceladus, Tethys, Dione, and Rhea.

By excluding Iapetus, the total fleet mass is significantly reduced; for Dione, the fleet mass is reduced from 958 to 800 MT. Further restrictions of the moon exploration fleet (e.g., from six to four moons) may be needed to fit within the payload capacity of future interplanetary transfer vehicle (ITVs). As Iapetus is more remote than the other moons, it is possible that a separate ISRU space base may be more attractive than a centralized ISRU space base.

Due to the low gravity level of the moons, a moon-orbiting space base may be attractive. Processing control of ices in a low-gravity environment may be very difficult. It is suggested that an artificial gravity space base in orbit about the moon may be the best location of resource processing. After a series of small missions have been conducted, a large ISRU space base may be established. The base might be a refueling point for extensive exploration. The water ices from the moons would be brought to the space base for processing, to allow the refueling of the landers and the NEP OTVs.
