Substantially Enhancing the Capability of the SLS Architecture by Utilizing EUS Derived Propellant Depots and Reusable Orbital Transfer Vehicles

Left: Orion space capsule with hypergolic fueled Service Module; right:  notional Orion spacecraft with a reusable EUS derived orbital transfer vehicle.  

It now appears that Congress will direct NASA to return Americans to the surface of the Moon in order to prepare for future crewed journeys to the surface of  Mars. But the Moon could prove to be much more than just a beyond LEO testing ground for astronauts and components.

At minimum, an interplanetary round trip to the Red Planet would require several hundred tons of  water for drinking, washing, food preparation, radiation shielding, the production of air, and for the production of liquid oxygen (LOX) and liquid (LH2)  for rocket propellant.

Supplying the water needed for an interplanetary spacecraft parked at LEO  would require a delta-v ranging from  9.3 km/s to 10 km/s from the Earth's surface. Minimum energy launch windows during the 2030s would require an additional delta-v ranging from 3.59 km/s to 4.81 km/s for Trans Mars Injections (TMI)  that could send humans to Mars in less than a year.

But at different locations within cis-lunar space, substantially lower levels of delta-v could be taken advantage of in order to launch cargo and crew into a Trans Mars Injection. And these interplanetary launch points within cis-lunar space are most easily accessible from the lunar surface rather than from within the Earth's deep gravity well. So utilizing lunar ice resources could greatly reduce the cost and the complexity of sending humans to Mars.


Delta-v to important destinations within cis-lunar space

Earth surface to LEO - 9.3 km/s to 10 km/s

LEO to EML1 - 3.77 km/s (~3 days)

LEO to EML1 - 4.5 km/s (~2 days)

LEO to EML2 - 3.43 km/s (~8 days)

LEO to EML2 - 3.95 km/s (~4 days)

Lunar surface to EML1 - 2.52 km/s (~3 days)

Lunar surface to EML2 - 2.53 km/s (~3 days)


Delta-v from cis-lunar space to Trans Mars Injection

LEO to TMI (2030s) - 3.59 km/s to 4.81 km/s

EML1 to TMI (2030s) - 1.04 km/s to 1.3 km/s

Because of its shorter travel times from Earth orbit,  EML1 would appear to be the optimal region for locating Deep Space Habitats (DSH), reusable lunar shuttles,  propellant manufacturing water depots that receive water from the lunar surface and for launching cargoes and crew on interplanetary journeys to the orbits of Mars and Venus.

A delta-v of   3.77 can transport crews to EML1 in approximately 4 days. It would require 3.95 km/s to travel to EML2 in 4 days but 8 days of travel and radiation exposure would be required to take advantage of a lower delta-v to EML2 at 3.43 km/s. A delta-v of 2.52 km/s would be required for a 3 day journey between EML1 to the lunar surface. 2.53 km/s would be required for a similar journey between EML-2 and the lunar surface. The significant presence of habitats and depots and reusable vehicles at EML-2 could also interfere with future radio astronomy on the back side of the Moon.

Taking full advantage of polar ice resources on the Moon would require a space architecture that utilizes orbiting depots to supply water and propellant to reusable interplanetary spacecraft and landing vehicles. Some space advocates have argued that propellant depots make heavy lift vehicle's an unnecessary expense. However, heavy lift vehicles would make it possible to easily deploy propellant manufacturing water depots to  EML1  for crewed interplanetary journeys to the orbits of Mars and Venus.

Similar depots located at LEO could also enable large payloads  originally deployed  to LEO by heavy lift vehicles to be later  deployed by reusable orbital transfer vehicles practically anywhere within cis-lunar space.

Expendable EUS for SLS launch vehicle (Credit: NASA)

In 2018, NASA will launch the first SLS heavy lift vehicle with an unmanned Orion space capsule and a hypergolic fueled Service Module and an interim upper stage. But NASA currently envisions crewed SLS launches in the 2020s to include an Orion space capsule with a hypergolic fueled Service Module and a large LOX/LH2 fueled Exploratory Upper Stage (EUS). 

But two of the principal companies (Boeing and Lockheed Martin) currently developing the SLS/Orion architecture are also--  privately developing-- an alternate space architecture through their joint company, the  ULA (the United Launch Alliance). In this alternate architecture, the ULA  will utilize their emerging  IVF (Integrated Vehicle Fluids ) technology to deploy a reusable ACES upper stage that could eventually utilize LOX/LH2  propellant depots for travel within cis-lunar space during the 2020s. IVF technology allows a spacecraft to utilize hydrogen and oxygen ullage gases for attitude control, power production, and for autogenously pressurizing propellant tanks, eliminating  the need for hydrazine and liquid helium. 

NASA could greatly enhance the capability, efficiency, and the safety of the SLS/Orion architecture by taking full  advantage of the ULA’s  emerging IVF  technology. While there's no reason to stop Boeing from developing the EUS as an expendable upper stage for the SLS, it would still be technologically advantageous for NASA to commission the ULA to use its IVF technology to convert some EUS vehicles into  reusable orbital transfer vehicles and others into propellant depots.

EUS derived propellant manufacturing water depot and reusable Orion orbital transfer vehicle.

 An SLS/Orion architecture utilizing EUS derived Orbital Transfer Vehicles (OTV-125) and EUS derived propellant producing water depots (WPD-OTV-125) would no longer require the expense and the enhanced risk of launching astronauts on top of a super heavy lift vehicle. So for beyond LEO missions in the 2020s, under this scenario, astronauts would simply be transported to LEO by Commercial Crew vehicles that would have already been in operational service since 2018.

Once in orbit, the commercial crew capsule would dock directly with the Orion-OTV-125 reusable spacecraft, transferring its crew aboard the Orion for its beyond LEO mission. Alternatively, a crew capsule could dock at the port of  a space station where the Orion-OTV-125 would also be docked at a different port. Both scenarios would mean that the ATV based hypergolic Service Module being manufactured by the Europeans would not be required for crewed beyond LEO missions, and would only be used once during the 2018 test mission. So only  the domestically manufactured Orion capsule being developed by Lockheed Martin would be preserved in this architecture.

A single SLS Block I cargo launch could be used to deploy two WPD-OTV-125 depots to LEO plus a two 1 MWe solar arrays. With at least 35 tonnes of propellant, one of the water/propellant depots could self deploy itself to EML1 along with its 1.4 to 2.8  MWe solar array. Another basic SLS Block I cargo launch could be used to deploy two reusable Orion-OTV-125 vehicles to LEO. But once this reusable depot based extraterrestrial architecture is deployed by the SLS,  private commercial launch vehicles will only be required to conduct crewed beyond LEO missions. This will allow the SLS to be used exclusively as a cargo launcher for the deployment of large and heavy spacecraft and habitats and other large and heavy structures.

Notional lunar water ice extraction, storage, and LOX/LH2 manufacturing facility.

Water for the propellant manufacturing orbital depots could be regularly supplied to LEO and to EML1 from commercial launch vehicles (Atlas V, Delta IV heavy, Falcon 9, and the future Falcon Heavy, Vulcan, and Vulcan Heavy vehicles).  However, once water is being manufactured on the lunar surface, water transported to EML1 would be supplied  exclusively from the lunar surface.


Since the  Orion-OTV-125 would derived from the EUS, it would be capable of accommodating up to 125 tonnes of propellant. But less than 35 tonnes of propellant would be required for an Orion-OTV-125 to transport its crew to EML1. And an equal amount of fuel would be needed to return astronauts to LEO. No aerobraking would be required.

A reusable Extraterrestrial Landing Vehicle (ETLV) would transport astronauts from EML1 to the surface of the Moon and back  to EML1 with a single fueling of LOX/LH2. A single SLS Block I cargo launch could deploy three ten tonne ETVL-2 vehicles to LEO, each with enough propellant to travel to EML1. The ETLV-2 could also serve as an back up OTV in case one or both of the Orion-OTV-125 vehicles becomes inoperable.

Until new RS-25 engines are in production, perhaps by 2021 to 2023, NASA will only be able to launch four SLS vehicles. And one of those launches will be an unmanned test launch in 2018. So after 2018, only three SLS launches will be possible before the RS-25 engines go into production.

Notional EUS derived propellant manufacturing water depot @ EML1 after refueling a reusable Extraterrestrial Landing Vehicle.

But  just three basic  SLS Block I cargo launches would be required to deploy a reusable EUS derived cis-lunar transport architecture (WPD-OTV-125, Orion-OTV-125, and the ETLV-2) capable of transporting humans to EML1 and to the lunar surface and back.

However, the deployment of the lunar landing vehicles could be delayed until the new RS-25 engines are in production. This would allow NASA to use their last four Space Shuttle Main Engines to be used to launch a DHS (Deep Space Hab) to EML1. An SLS propellant tank derived habitat with over 510 cubic meters of pressurized habitable volume would have a dry weight of 22.4 tonnes. So in theory,  a single SLS Block I cargo launch could be used to deploy two or three pressurized habitats to LEO. A partially fueled Orion-OTV-125 could then be used to transport one of the pressurized habitats to EML1.

SLS propellant tank derived habitat (Credit: NASA).

This would leave one or two pressurized habitats at LEO with a combined pressurized habitable volume exceeding that of the ISS! All of that would be achieved with  a single SLS launch and a reusable propellant producing water depot architecture routinely and sustainably  supplied with water from commercial launch vehicles.

Links and References

Moon first, then Mars? Congress moves to shift space priorities

The return to the moon, Lori Garver, and the price of ambition

Lori Garver Questioned Astronauts about NASA's Next Destination?

Seeing the end of Obama’s space doctrine, a bipartisan Congress moves in

What about Mr. Oberth

Human Lunar exploration architectures

Earth Departure Options for Human Missions to Mars

A Study of CPS Stages for Missions beyond LEO

Deep Space Habitats

First Human Voyages to the Martian Moons Using SLS and IVF Derived Technologies

Congress Requires NASA to Develop a Deep Space Habitat

UltraFlex Solar Array Systems

Utilizing Lunar Water Resources for Human Voyages to Mars

At EML4, an OTV-400 interplanetary booster  undocks with a fuel depot (WPD-OTV-400) before proceeding to dock with the  Odyssey interplanetary spacecraft. 

by Marcel F. Williams

Long interplanetary journeys to  Mars, or to the orbit of Venus, or even to some of  the NEO asteroids could take several months or even years before their human occupants finally return to the relative safety of the Earth's surface. Such interplanetary voyages will require a substantial tonnage of water  for drinking, washing, the preparation of food,  the manufacturing of oxygen for air,  the production of  liquid oxygen and hydrogen for propellant, and for appropriately shielding humans inside of  habitat modules from the dangers of cosmic radiation and major solar events.

NASA delta-v estimates for  achieving Mars Transfer Orbits from LEO during the 2030s range above 3.8 km/s to just below 5 km/s-- just to reach the vicinity of Mars. Additionally,  a LEO departure for Mars would require launching the huge amounts of water and propellant for the voyage out of the Earth's enormous gravity well. This would require a delta-v ranging from 9.3 to 10 km/s.

However, if  crewed interplanetary vehicles are launched from an Earth-Moon Lagrange point, the delta-v requirements to achieve a Mars Transfer Orbit would be substantially lower. The delta-v requirements for departing cis-lunar space from one of the Earth-Moon Lagrange points could be less than 2 km/s-- especially if an Earth flyby or an Earth and Moon flyby (even better) are taken advantage of during the initial trajectory burns.

Providing water and propellant for  crewed interplanetary mission from the lunar surface to an Earth-Moon Lagrange point would also have a substantially lower delta-v requirement.  A  delta-v of less than  2.6 km/s would only be required to supply water and propellant to an interplanetary gateway at an  Earth-Moon Lagrange point. Contrast that with the  enormous  9.3  to 10 km/s delta-v that is need to  supply propellant and water to an interplanetary vehicle located at LEO.  The lunar supply of water and propellant to an interplanetary spacecraft at an  Earth-Moon Lagrange point would also have the additional economic advantage of being able to use single stage reusable vehicles. 

Earth-Moon Lagrange Points, the optimal gateways to interplanetary space within cis-lunar space.

So  launching  crewed interplanetary space craft to Mars from one of the Earth-Moon Lagrange points (L1, L2, L4, or L5) has a substantial delta-v advantage over launching crewed interplanetary spacecraft from LEO--  if such spacecraft are supplied with fuel and water from the surface of the Moon. 

During the 2030s, entering High Mars Orbit during a Conjunction Class Mission (330 to 560 day stays) would require an additional delta-v of 0.9 to 1.9 km/s, while entering High Mars Orbit during  Opposition Class Missions (60 day stay)  would require an additional delta-v trajectory burn ranging from  0.9 to 3.8 km/s. The additional delta-v requirements for reaching High Mars Orbit adds further support for minimizing the initial delta-v requirements when departing from cis-lunar space. So, again, supply fuel and water from the Moon from an Earth-Moon Lagrange point gateway would appear to be the optimal way to begin crewed interplanetary journeys. 

But how much water is there on the lunar surface? And how technologically difficult would it be  to extract large quantities of water from the lunar regolith?

A spectral analysis of the ejecta plume from the impact of  a Centaur upper stage  into the  Cabeus crater at the lunar south pole was conducted in 2009.  The analysis indicated that the lunar regolith in the shadowed crater contained water ice with concentrations ranging from 2.7% to 8.5% by mass. This suggest that  in some of the permanently shadowed craters at the southern pole, the lunar regolith there may contain as much as  27 to 85 kilograms of ice per tonne. 

The dark purple and blue areas represent neutron emissions from the Moon's polar regions that indicate  hydrogen-rich regions on the lunar surface covered by desiccated regolith (Credit: NASA) .  

In the Moon's northern polar region, the Mini-RF on board the Chandrayaan-1 orbiting probe strongly suggest that 40  craters in the northern polar region my contain as much as 600 million tonnes of water-ice. So there is clearly no shortage of  water resources on the lunar surface.

In order to provide enough water for human activities at a lunar outpost, a cis-lunar transportation system, and to supply propellant and mass shielding for five Conjunction or Opposition Class missions to Mars during the 2030s: 2030, 2033, 2035, 2037, and 2039, at least 500 to 1000 tonnes of water is going to have to be annually manufactured on the lunar surface.

A lunar water and fuel manufacturing depot along side of mobile LOX and LH2 storage tanks and a mobile microwave water extraction robot (Water Bug).

NASA researchers have demonstrated that a simple one kilowatt microwave oven could extract as much as a tonne of water from the regolith at the lunar poles over a one year period. A single mobile robot with a 100 kw powered microwave oven, therefore, should be  able to annually extract 100 tonnes of water from the regolith from the shadowed areas at the lunar poles. Ten such mobile microwave units might be able to extract 1000 tonnes of water per year from lunar polar regolith resources.

Solar extraction of water from lunar regolith brought from permanently shaded lunar craters at the lunar poles. Transparent domes allows sunlight to heat a layer of lunar regolith from the top while solar heated metal tubes below filled with methanol heat the regolith from below. Water vapor is deposited within regolith insulated cold trap canisters.

However, a simpler method may only require sunlight to passively extract water from the lunar regolith. If one tone of regolith from the shadowed areas of the  lunar poles is composed of  approximately 5% water ice then mobile lunar excavators capable of digging up and depositing at least one tonne of regolith into a solar heater could produce at least 50 kg of water per day (18 tonnes of water per year). Just a few electric powered or fuel cell powered excavation robots on the lunar surface could, in theory,  deposit at least one ton of icy regolith to a solar heater per hour (more than 400 tonnes of water per year).

A reusable water tanker shuttle on a microwave sintered launch pad after being loaded with lunar water and lunar fuel for its flight to a water and propellant depot at one of the Earth-Moon Lagrange points.

The Odyssey interplanetary vehicles would be crewed spacecraft capable of transporting 12 astronauts to High Mars Orbit and back to cis-lunar space using solar photovoltaic powered propellant producing water depots supplied with water from the lunar surface. The Orbital Transfer Vehicle (OTV-400) and the IAGH (Interplanetary Artificial Gravity Habitats) would both be derived from the SLS fuel tank technology.

 The OTV-400 would use  a common bulk-head LOX/LH2 fuel tank derived from the SLS fuel tank technology. An IVL(Integrated Vehicle Fluids) type of technology would be used to utilize ullage gases for attitude control. Space Works has proposed a similar type of OTV using IVL technology that would store more than 450 tonnes of LOX/LH2 fuel. However, the OTV-400 would also use photovoltaic powered cryocoolers to  virtually eliminate any hydrogen and oxygen boil-off.
 
The twin habitat modules of the IAGH would rotate to produce a 0.5g simulated gravity for the six humans inside each module. The IAGH would also  be  appropriately water shielded to protect  its most radiation vulnerable occupants (25 year old female astronauts) from excessive exposure to  cosmic radiation and its  heavy nuclei component in a addition to major solar events-- for up to four years-- during solar minimum conditions. 50 cm of water shielding would be required to appropriately shield the light weight twin SLS fuel tank derived living areas: 118 tonnes of water shielding for each habitat module, 236 tonnes of shielding in total. 

A reusable Orbital Transfer Vehicle (OTV-400) for transporting crewed artificial gravity habitats to the orbits of Mars, Venus, or to the NEO asteroids. While some of the ullage gases are utilized for attitude control, most of the ullage gases are reliquified by photovoltaic powered cryocoolers to prevent fuel loss, a technology already developed by NASA.

Conjunction Class Missions would only require one reusable OTV-400  storing close to 400 tonnes of LOX/LH2 propellant. The higher delta-v Opposition Class Missions will require two reusable OTV-400 boosters. Entering orbit around Mars and reentering cis-lunar space will also require the IAGH  to dump the water shielding from its habitat modules in order to substantially reduce the Odyssey's mass just before the final trajectory burns to enter Mars orbit or to enter cis-lunar space.

Coupled with a large water storage tank and a photovoltaic powered electrolysis plant  and cryocoolers, the OTV-400 would function as a water storage and hydrogen and oxygen producing propeelant depot (WPD-OTV-400). Still equipped with its own rocket engines, it could self deploy itself practically anywhere within the inner part of the solar system  while still being able to manufacture LOX and LH2 anywhere where there is a source of water.

The WPD-OTV-400 water and propellant depot would have the ability to transport itself to Mars orbit from the Earth-Moon Lagrange points after producing enough fuel for its on flight and filling up with enough stored water originating from the Moon. However,  once water is being manufactured on Deimos and Phobos, using the same technologies employed on the lunar surface, propellant from the lunar surface will no longer be required to replenish water and propellant supplies in orbit around Mars. 

Reusable Odyssey I interplanetary space craft with a crew of 12 at EML4 in a trajectory burn configuration for a Conjunction Class mission to High Mars Orbit  in the year 2033. 

After its initial trajectory burns on its way to Mars, the  OTV-400 boosters and the ETLV-2 vehicles (Extraterrestrial Landing Vehicles) would separate from the IAGH  and re-dock at its central axis.The Odyssey would, therefore, be reconfigured  to produce artificial gravity for its 12 person crew for their multi-month journey to Mars.   Liquid carbon dioxide rockets housed in each habitat module, would be used to rotate or to stop the rotation of the Odyssey. Cables will extend from the IAGH core more than 100 meters from the central axis of the vehicle. A series of light weight cylindrical metal or ceramic shells woulds also expand outwards creating rotational arms that would act as levers to increase, decrease, or stop the rotation of the Odyssey. 

Reusable Odyssey II interplanetary space craft with a crew of 12 at EML4 in a trajectory burn configuration for a 1000 day Opposition Class mission to High Mars Orbit (60 day stay) to explore the martian moons Deimos and Phobos while also deploying  satellite constellations at Sun-Mars L1 and Sun-Mars L2 to provide global communications for future human missions to the surface of Mars.

After several months of travel through interplanetary space,  the Odyssey would reconfigure itself again to prepare for an Orbital Insertion trajectory burn and the water shielding within IAGH modules would be dumped into space.  But after a day or two in Mars orbit, the Odyssey will once again reconfigure itself to produce artificial gravity for its crew. The  water shielding for the habitats could be fully  restored within a few hours or a few days from a pre-deployed  WPD-OTV-400 already in high Mars orbit. Returning to Earth will also require the Odyssey to be refueled by the orbiting water and propellant depot in Mars orbit.

Once in high Mars orbit, each fully fueled  crewed ETLV-2 vehicle would have enough fuel to travel to one of the Martian moons for a few days of exploration and sample retrieval  and back to the Odyssey. And if they wanted to travel to the martian moons a second time then they could refuel at the orbiting Mars depot (WPD-OTV-5).

A slowly rotating Odyssey II in an interplanetary configuration to provide a simulated gravity of  0.5 g  for six crew members in each of the  IAGH habitat modules. 

Since the Odyssey is intended for reuse (ten times with its RL-10 engines) for future interplanetary missions, a trajectory burn will be required to return to the Earth-Moon Lagrange points after the trajectory burn for Trans-Earth Injection from Mars orbit. Again, this will require the Odyssey to completely dump its water shielding before the burn. Once the crew is back within cis-lunar space, they will only be a few hours away from protective shelters on the lunar surface or just a few days away from the Earth's surface.

After the 12 person crew completes their 60 day mission in High Mars Orbit, the Odyssey II converts from it's artificial gravity configuration to a trajectory burn configuration for its Trans-Earth Injection burn to begin its journey back to cis-lunar space.

 Because the major Odyssey components are reusable, the recurring cost for the interplanetary vehicle should be substantially lower that other interplanetary vehicle concepts that utilized expendable interplanetary boosters. The Odyssey's RL-10 or RL-10-like rocket engines could also be periodically replaced after perhaps ten round trips between cis-lunar space and Mars orbit which could further reduce their recurring cost.

The human safety advantages of using  lunar water resources for an SLS derived reusable artificial gravity producing  interplanetary spacecraft   deployed at one of the Earth-Moon Lagrange points should also be substantial:


1. The significantly reduced delta-v requirements at an Earth-Moon Lagrange point for fueling and mass shielding a crewed interplanetary vehicle should alleviate any pressure to significantly reduce the appropriate mass shielding of habitat modules against the dangers of cosmic radiation and major solar events. 50 cm of water shielding should also eliminate the possibility of space career ending radiation exposure during a single interplanetary mission for the most vulnerable occupants to radiation.

2. The rotating interplanetary artificial gravity habitats (IAGH) could significantly or totally eliminate long periods of exposure to the  deleterious physical effects of a microgravity environment

3. Having twin AGH habitats allows astronauts the enhanced safety of being able to seek refuge in the opposite habitat in case there is a serious life safety malfunction at the other habitat.

4. The more comfortable accommodations of the spacious SLS derived artificial gravity habitats could significantly reduce the psychological stress experienced by the drew  during several months or years of interplanetary travel.

5. The dangers and the complexity of direct, high-energy aerobraking into the atmospheres of Mars or during a return to directly to Earth would be avoided by limiting the Odyssey's flight path to travel only between the Earth-Moon Lagrange points and High Mars Orbit.

6. With at least one partially fueled  ETLV-2 (Extraterrestrial Landing Vehicle) connected to the Odyssey, any major malfunction of the OTV-400 during an orbital capture trajectory could allow astronauts to safely enter Mars orbit or cis-lunar space via the ETLV-2 vehicle or vehicles.  Even though this would mean the loss of the Odyssey spacecraft, the crew could still safely enter Mars orbit or cis-lunar space via the ETLV-2. However, Entering Mars orbit aboard an ETLV-2, without the Odyssey IAGH would require that an appropriately mass shielded space station already be pre-deployed in Mars orbit.



References and Links


Considerations for Designing a Human Mission to the Martian Moons (NASA)

A Study of CPS Stages for Missions beyond LEO (Space Works)

Mission and Implementation of an Affordable Lunar Return (Spudis & Lavoie) 

 Using the resources of the Moon to create a permanent, cislunar space faring system (Spudis & Lavoie)

Evolving to a Depot-Based Space Transportation Architecture (ULA)

 SLS Fuel Tank Derived Artificial Gravity Habitats, Interplanetary Vehicles, & Fuel Depots

 Utilizing the SLS to Build a Cis-Lunar Highway

Cosmic Radiation and the New Frontier