Notional reusable EUS derived REUS-LV/Crew vehicle descending to the lunar surface using side thrusters for the final descent and soft landing

by Marcel F. Williams

The current development of the Exploration Upper Stage (EUS) for NASA's Space Launch System offers Boeing Aerospace an opportunity to produce reusable variants of the spacecraft that could greatly enhance the capability of Boeing's super heavy lift vehicle system.

Reusable Lunar Crew Lander

By simply adding a pressurized crew module at the top of the spacecraft and landing gear at the bottom of the vehicle could allow the EUS to land humans on the surface of the Moon. I will refer to this notional crew landing EUS variant as the REUS-LV/Crew.

Replacing gaseous helium with gaseous oxygen and hydrogen for pressurizing liquid oxygen and liquid hydrogen tanks should allow the REUS-LV to be reused at least 50 times. The gaseous hydrogen and oxygen could also power thrusters for attitude control. However, with RL 10 engines capable of only 50 starts, reusability for such a vehicle might be limited to only six round trips between LEO and the lunar surface.

Artist depiction of an expendable EUS for the Space Launch System (Credit NASA)

NASA is intent on establishing its deep space Gateway at an NRHO (Near Rectilinear Halo Orbit), a seven day polar orbit around the Moon. The human occupied Gateway habitat would allow 12 hour trips to the lunar surface every seven days. A weeks stay on the lunar surface would also allow a return to NRHO in just 12 hours time.

Assuming a dry mass of 23 tonnes for a crewed version of the REUS-LV, the space vehicle should be cable of round trips to the lunar surface utilizing less than 70 tonnes of propellant. So substantial amounts of additional payload could be deployed with crewed missions to the Moon if 113 tonnes of the vehicle's total fuel capacity is utilized.

Private commercial launch vehicles could transport passengers to the REUS-LV/Crew spacecraft orbiting independently at LEO or docked at a LEO orbiting space station.

Notional reusable REUS-LV/Crew landing vehicle for lunar operations

Once the REUS-LV/Crew vehicle is on the Moon, a davit crane system could be used to lower astronauts, vehicles, and other equipment to the lunar surface and to retrieve astronauts and lunar material later for transport back to the NRHO Gateway. Solar panels positioned on four of the walls surrounding the LOX tank would provide electricity for the crew module plus electric power to keep the hydrogen and oxygen liquefied using the thermal radiators to assist its cryocooler refrigeration systems.

Orbital Propellant Depots

Two REUS-LV/Crew vehicles could be deployed to LEO with a single SLS launch. But propellant depots would be required to fuel the vehicles at LEO and at NRHO in order to conduct crewed missions to and from the lunar surface and to return the spacecraft back to LEO. So the deployment of two propellant manufacturing water depots at LEO and at NRHO would be necessary for crewed lunar missions.

Two REUS derived vehicles (REUS-OTV/Depot) could be utilized as propellant producing water depots capable of storing up to 150 tonnes of water for the production of 113 tonnes of LOX/LH2 propellant. 9 tonnes of water contains approximately 8 tonnes of oxygen and one tone of hydrogen. But only 7 tonnes of propellant could be manufactured from 9 tonnes of water since rocket fuel would require a ratio of 6 tonnes of oxygen per ton of hydrogen. So each depot would be capable of storing approximately 16 tonnes of LH2 plus 97 tonnes of LOX) while wasting 32 tonnes of liquid oxygen.

However if only 113 tonnes of water is converted into 97 tonnes of liquid oxygen and 13 tonnes of hydrogen then 100% of the water could be utilized as fuel if an REUS-LV vehicle initially arrives at LEO from Earth with at least 3 tonnes of liquid hydrogen propellant. Two REUS-LV/Crew vehicles would only weigh 46 tonnes. And with an additional six tonnes of liquid hydrogen propellant, would still only be 52 tonnes of payload mass for a basic SLS vehicle capable of deploying at least 70 tonnes to LEO. Fully fueled with LH2, the first REUS-LV/Crew vehicles launched to LEO could require no liquid hydrogen from the LEO depot at all, only its liquid hydrogen.

Water could be continuously supplied to propellant producing depots at LEO by various private American launch systems (Space X, the ULA, and Blue Origin) who have vehicles that are either currently operational or are very close to being operational:

Falcon Heavy (Space X) - 63 tonnes to LEO

New Glenn (Blue Origin) - 45 tonnes to LEO

Vulcan-Centaur (ULA) - 27 tonnes to LEO

During the first SLS launch of two propellant depots to LEO, private launch companies could supply one depot with enough water to produce propellant that can be transferred to the second depot to deploy itself plus its solar array to NRHO.


EUS derived propellant producing water depot approaching orbiting solar power plant @ NRHO where it will use photovoltaic power to electrolyze water into LOX and LH2.

Private launch vehicles could also be used to deploy water directly to NRHO once the propellant producing depot arrives. But it would be much cheaper and efficient for private launch companies to simply focus on supplying water to LEO. Large reusable REUS-OTV (Orbital Transfer Vehicles) could transfer water from LEO to depots located at NRHO much more efficiently. A single SLS launch would be required to deploy two such EUS derived vehicles into orbit probably with their LH2 tanks already filled with liquid hydrogen.

REUS-OTV plus optional interstage connection ring for joining two OTV vehicles together. Such vehicles could be used to transport 60 to 120 tonnes of water and other payloads between LEO and NRHO and to various lunar orbits and Earth-Moon Lagrange points.

Crewed Missions to the Moon and Reusability

For crewed missions to the lunar surface, An REUS-LV/Crew vehicle would be fueled with propellant at LEO and then travel four days to the orbital outpost at NRHO. After arriving at NRHO, the REUS-LV/Crew vehicle would then be fueled with additional propellant at for its round trip journey to the lunar surface and then back to NRHO. Once the crew returns to NRHO, only the minimum amount of propellant would be required to return the REUS-LV/Crew spacecraft back to LEO.

Reusable REUS-LV/Crew vehicle docked at a notional single launch SLS derived MegaStation at NRHO. The notional EUS derived vehicle would be cable of round trips between NRHO and the lunar surface on substantially less than a full tank of LOX/LH2 propellant. REUS-OTV vehicles could be used to deploy the habitat modules and the pressurized spent SLS core stage from LEO to NRHO.

RL-10 engines can be manufactured with a capability of 50 restarts. So a single REUS-LV/Crew vehicle should be capable of at least 6 round trips to the lunar surface starting and returning to LEO. The REUS-LV/Crew vehicle would be capable of 12 round trips if the spacecraft is only utilized for trips between NRHO and the lunar surface.

Two reusable REUS-LV/Crew vehicles could be deployed to LEO with a single SLS launch. And each vehicle deployed would be capable of at least 12 round trips between LEO and NRHO. So a single SLS launch could allow 12 round trips to the lunar surface-- if water can be supplied to LEO and then to NRHO. This would require two more SLS launches to deploy two solar powered propellant depots and two REUS derived orbital transfer vehicles (REUS-OTV) to transport water from LEO to NRHO. So three SLS launches would be required for 12 round trips to the lunar surface (four potential round trips to the lunar surface per SLS launch).

Lunar Depots

REUS derived propellant producing water depots with landing gear could also be deployed to the lunar surface with photovoltaic solar power units. If water ice resources are exploited at the lunar poles then propellant could be produced on the Moon. The REUS-LV/Crew vehicle could be fueled with propellant at LEO and travel directly to the lunar surface. And the same REUS-LV/ Crew could later be fueled with lunar propellant for its return trip directly to LEO.

Alternatively, propellant from Earth could be substantially reduced for lunar missions if tankers supplied water from the moon to NRHO depots. REUS-LV/Crew vehicles could then return to LEO using LOX/LH2 propellant produced on the Moon.

Reusable Lunar Hopper

The delta-v and travel times for possible crewed suborbital hops on the lunar surface. Fueled with liquid hydrogen and oxygen at a lunar base, a notional REUS-LV/Crew vehicle would be capable of traveling to any region on the surface of the Moon in less than an hour and return back to the lunar outpost on less than a single tank of fuel.

Supplied with propellant on the lunar surface, the REUS-LV/Crew could also be used as a lunar hopper that could travel to any region on the surface of the Moon in less than an hour. The vehicle would also carry enough fuel to return to the lunar outpost where it was fueled in less than an hour. With four trajectory burns for each round trip, each REUS-LV/Crew vehicle could travel to 12 regions on the lunar surface. So a single SLS launch could potentially explore 24 regions on the lunar surface if REUS-LV/Crew vehicles are refueled with lunar propellant.

Repurposing Decommissioned Landing Vehicles

REUS derived vehicles could still be put to good use after they are decommissioned from their original task. REUS-LV/Crew vehicles that are no longer safely capable of crewed flights could be repurposed for storing substantial quantities of water mined from the lunar ice and for storing sewage accumulated from the inhabitants of lunar outpost. With both a hydrogen and oxygen tank, substantial quantities of water and sewage could be stored in the two different tanks of one vehicle. The decommissioned vehicles could also be used to store excess oxygen from the production of propellant. LOX and LH2 could be stored in decommissioned spacecraft to produce electric power during the lunar night using fuel cells. Such fuel cells would not only produce electricity from the hydrogen and oxygen-- but also water.

Decommissioned vehicles could also be used as temporary outpost in lunar regions of particular interest. They could be transported to their lunar locations by electric powered lunar cranes. And regolith bags could be deployed around the vehicle's habitat modules for additional protection against cosmic radiation and micrometeorites.

Decommissioned REUS-OTV vehicles in orbit could be used in a similar fashion at NRHO for storing water, or excess oxygen from the production of propellant.

Deep Space Robotic Missions to Phobos and Deimos

At NRHO, the REUS-LV/Crew vehicle could also be used for-- unmanned-- round trip robotic missions to the moons of Mars. The delta-v requirements for such round trip missions between NRHO and the martian moons would actually be less than round trip missions between NRHO and the lunar surface. So a robotic REUS-LV would be fully capable of traveling to the surface of Deimos or Phobos and returning to NRHO with substantial quantities of rocks and regolith from those two tiny martian moons. Roving vehicles could also be left behind that could be used to extensively explore each of the martian moons. The REUS-LV davit system could easily lower and retrieve such vehicles after landing on those tiny worlds.

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