by Marcel F. Williams

“I’m taking nothing off the table, and we’re not compromising safety. Anything we don’t need to do we can delay. There’s future launches, there’s future things we can test, but right now, how do we get boots on the moon in 2024?” (NASA Administrator Jim Bridenstine)

It is now a directive of the Executive Branch of the United States for American astronauts to return to the surface of the Moon by 2024. But the type of transportation infrastructure developed for a US return to the lunar surface could largely determine whether, or not, America will strategically and economically dominate the Moon, cis-lunar space, and the rest of the solar system.

It is estimated that between 100 million to one billion metric tons (tonnes) of water ice may exist at the Moon's north and south poles. Exploiting polar ice deposits on the lunar surface for the production of rocket fuel is one of the principal arguments for returning to the Moon. Lunar hydrogen and oxygen propellant would make it much easier to send humans to Mars. And lunar propellant and propellant dept technology could also give astronauts easy access to the surfaces of Mercury and Jupiter's Galilean moon, Callisto, two additional worlds that could be potentially colonized by humans someday.

Liquid oxygen comprises nearly 86% of the mass of LOX/LH2 propellant and nearly 89% of the mass of water. So even if there were no ice deposits on the Moon, the extraction of oxygen directly from the lunar regolith would provide humans with an almost endless supply of oxygen for utilization as propellant.

So any reusable spacecraft developed to return humans to the surface of the Moon should also be inherently designed to utilize potential lunar propellant resources-- once such lunar resources become available. But until lunar ice and regolith resources can be exploited hydrogen and oxygen, or water, will have to be launched into cis-lunar space from the Earth's surface.

The primary purpose for a Lunar Gateway at NRHO (Near Rectilinear Halo Orbit) is to make it simple and easy to routinely visit the lunar surface from that delta-v bridging location. Yet NASA is currently advocating a highly complex and inherently more dangerous transportation infrastructure to operate out of the NRHO Gateway. NASA's current gateway transportation architecture requires two or three different spacecraft in order to transport astronauts on a simple round trip between NRHO and the lunar surface. And the elements are not even completely reusable.

Notional Lockheed Martin reusable lunar landing spacecraft on the lunar surface (Credit: Lockheed Martin)

Lockheed Martin, on the other hand, has proposed a simple-- single stage-- spacecraft that can operate out of NRHO. And its completely reusable. The Lockheed Martin's reusable spacecraft concept is derived from the ULA's future Centaur V and ACES rocket technologies. These cryogenic oxygen and hydrogen fueled upper stages will be used in the ULA' new Vulcan rocket system-- which is supposed to go into operation in 2021.

Lockheed Martin's Notional Reusable Crewed Lunar Landing Vehicle

Propellant: 40 tonnes of LOX/LH2

Inert Weight: 22 tonnes

Engines: Four RL-10 derived engines

Maximum delta-v capability: 5.0 km/s

Maximum number of crew: Four

Two of the 22 tonne Lockheed Martin lunar landing vehicles, which I will refer to as the R-LL (Reusable Lunar Lander), could easily be deployed to LEO by a single Block I SLS launch within the 8.4 meter (7.5 meter internal) payload fairing equipped with an extra barrel section. The notional lunar spacecraft, however, would have to be fueled by propellant depots. But propellant depots would be essential if NASA is really serious about exploiting lunar resources to produce hydrogen and oxygen. So there's no logical reason not to develop cryogenic depots now!

The optimal propellant depot design would be a-- water depot-- that simply uses solar electricity to convert liquid water into hydrogen and oxygen though electrolysis and then into liquid hydrogen and oxygen through cryo-refrigeration. However, much simpler depots could be directly derived from the propellant tanks of existing upper stages and could utilize NASA's new helium or nitrogen cryorefrigeration technology.

Propellant could be easily transferred to a spacecraft by docking the spacecraft to the propellant depot, automatically connecting the spacecraft fuel hoses, and then firing thrusters to create simulated gravity through acceleration. Useful acceleration for propellant transfer can be as little as 0.00004 g.

Both water and propellant could be easily deployed to LEO and NRHO by commercial launch vehicles. The Falcon Heavy should be able to deploy more than 15 tonnes of propellant to NRHO and the future Vulcan Heavy rocket systems should be capable of routinely deploying more than six tonnes of propellant to NRHO per launch. Monthly propellant launches by each system could deploy enough liquid hydrogen and oxygen to NRHO for at least six R-LL round trips to the lunar surface per year. NASA only sent astronauts to the moon six times from 1969 to 1972 during the entire Apollo program.

Lightweight, disposable, propellant tank derived from Centaur 3 LOX tank capable of storing 26 tonnes of liquid oxygen (Credit: ULA)

If the R-LL uses ULA's future IVF technology, then only hydrogen and oxygen would have to be transported to NRHO. However, if the R-LL uses existing Centaur rocket technology then gaseous helium will also have to be deployed to NRHO. While the helium itself would represent less than 2% of the total propellant mass, the tanks needed to deliver the helium to NRHO would be heavy and would require the helium to be launched to NRHO by a Falcon Heavy or Vulcan Heavy rocket. But using IVF technology would, obviously, make the R-LL simpler to fuel.

Much larger depots, directly derived from the ULA's Centaur V or ACES upper stage rockets, could be deployed to LEO with the ability to self deploy themselves to NRHO. Such vehicles could store up to 68 tonnes of LOX/LH2 propellant. So Falcon Heavy and Vulcan Heavy launches to NRHO could transfer their propellant directly to the large depots for long term storage. Reusable ACES tankers could also transport propellant originally deposited by commercial launchers to LEO to NRHO. This could allow technology such as Boeing's Phantom Express to continuously deploy propellant to LEO that could later be exported to NRHO.

Total mass of a water or propellant that can be deployed to LEO via daily launch of a single Phantom Express space plane:

Daily - 1.36 to 2.27 tonnes

Monthly - 40.8 to 68.1 tonnes

Yearly - 496.4 to 828.6 tonnes

Yearly amount of water or propellant that could then be transported by reusable ACES spacecraft to NRHO by a single Phantom Express space plane: 200 to 330 tonnes

Once lunar water and propellant are being manufactured on the lunar surface then the R-LL could also be used as a reusable lunar tanker. Simply replacing the crew transport module with a water tank, a single R-LL tanker could transport more than 40 tonnes of water to NRHO from the lunar surface. And after 12 round trips, a single R-LL tanker could deploy more than 480 tonnes of water to NRHO before its RL-10 derived engines would have to be replaced.

Of course, propellant depots deployed to both LEO and NRHO would also make it easy for reusable spacecraft to travel between LEO and NRHO. So an Orion/ACES spacecraft could eliminate the need of using a super heavy lift vehicle to transport astronauts to NRHO.

Under NASA's current scenario, billions of dollars would be spent developing three lunar elements with one or two of the expensive elements having-- no long term future-- as far as the pioneering of the Moon and the rest of the solar system is concerned. The complexity of a three stage vehicle also enhances the risk to astronauts. And it delays the-- inevitable development-- of propellant depots, a technology that is essential for the exploitation of lunar propellant resources.

So, under the scenario presented here, the propellant depot and reusable spacecraft architecture designed to return astronauts to the Moon would give NASA and America's launch companies almost complete strategic and economic dominance over cis-lunar space by 2025. And NASA could have astronauts on the surface of the Moon at the south lunar pole before then end of 2024.

SLS and Commercial Launch Scenario for Returning Astronauts to the Lunar Surface by 2024

2020

SLS Block I: Uncrewed test launch of Orion/SM/ICPS to DRO (Distant Retrograde Orbit)

2021

SLS Block I: Crewed launch of Orion/SM/ICPS on a trans lunar injection lunar flyby.

Commercial Launch: Propulsion and Power Bus deployed to LEO for self deployment to NRHO

2022

Commercial Launch: Remaining Gateway elements deployed and assembled LEO

Commercial Launch: Commercial Crew launch to inspect the Gateway before it is deployed to NRHO later in the year

SLS Block I + ICPS upper stage: Two fully fueled ICPS or Centaur V upper stages, or a combination of both are deployed to LEO for a docking rendezvous with the Gateway at LEO. The two boosters transport the Gateway to NRHO (More Gateway component mass can be transported to NRHO if water for radiation shielding is transported to the Gateway later by commercial launchers)

Commercial Launch: Two FlexCraft vehicles launched to NRHO Gateway

Commercial Launch: Beginning of commercial launches of water and other supplies to NRHO Gateway

2023

(Last use of RS-25 engines from the Space Shuttle legacy)

SLS Block I: Crewed launch of Orion/SM/ICPS or Orion/SM/Centaur V to NRHO Gateway

Commercial Launch: Vulcan/Centaur launch of ACES propellant depot to LEO

Commercial Launch: First commercial launches of liquid oxygen tankers to LEO

Commercial Launch: First commercial launches of liquid hydrogen tankers to LEO

2024

(New RS-25 engines now being produced and utilized)

SLS Block I: Two R-LL reusable spacecraft launched to LEO utilizing commercial propellant depots at LEO to redeploy to NRHO. Both vehicles are initially used to deploy robotic vehicles to the lunar surface for sample returns. One R-LL goes to the north lunar pole. The second R-LL goes to the south lunar pole.

SLS Block I: Crewed launch of Orion/SM/ICPS or Orion/SM/Centaur V or Orion/SM/EUS to NRHO Gateway

Three members of the Orion crew boards one of the R-LL spacecraft for the first human mission to the south lunar pole. Three other crew members remain at the NRHO Gateway to serve as an emergency rescue team in case the first vehicle experiences a serious malfunction while on the lunar surface.

Commercial Launch: Vulcan/Centaur Launch of ACES depot to LEO to self deploy to NRHO

Commercial Launch: Beginning of commercial deployment of liquid oxygen tankers to NRHO

Commercial Launch: Beginning of commercial deployment of liquid hydrogen tankers to NRHO

Commercial Launch: Vulcan/Centaur launch of reusable Orion/ACES to LEO for crew transport between LEO and NRHO using propellant depots

Launch Vehicles that could be used to help return humans to the surface of the Moon

SLS Block IB: 110 tonnes to LEO (operational 2024)

SLS Block I + ICPS upper stage: 95 tonnes to LEO (operational in 2020)

SLS Block I : 70 tonnes to LEO (operational in 2020)

Falcon Heavy: 63.8 tonnes to LEO (currently operational)

Vulcan Centaur Heavy: 34.9 tonnes to LEO (operational 2023)

Delta IV Heavy: 28.4 tonnes to LEO (currently operational)

Vulcan Centaur: 27.5 tonnes to LEO (operational 2021)

Upper Stages that could be deployed to LEO by an SLS Block I Launch

ICPS: Total mass: 30.7 tonnes; empty mass: 3.49; propellant mass: 27.2 tonnes (currently operational)

Centaur V: Total mass: ~ 46 tonnes; empty mass: ~5 tonnes; propellant mass: 41 tonnes (operational 2021)

ACES: Total mass: ~ 73.5 tonnes; empty mass: ~ 5.5 tonnes; propellant mass: 68 tonnes (operational 2023)

EUS: Total mass: 140 tonnes; empty mass: 15 tonnes; propellant mass: 125 tonnes (operational 2024)

With NASA's new super heavy lift capability, America will be able to deploy large and heavy structures (up to 110 tonnes in mass) to LEO with a single launch. This should enable NASA and private space companies to deploy huge reusable spacecraft with crewed interplanetary capability to LEO. Single launches of the SLS will also be able to deploy enormous microgravity and artificial gravity space habitats to LEO with pressurized volumes greatly exceeding that of the International Space Station.

With its propellant depot architecture, reusable ACES spacecraft working alone or in pairs could transport at least 40 to 80 tonnes of payload from LEO to practically anywhere within cis-lunar space. An reusable EUS that could utilize propellant depots would have substantially more capability.

Cargo landing vehicles directly derived from the notional R-LL vehicle should be able to land more than 40 tonnes of payload on the surface of the Moon.

Finally, by using commercial spacecraft to reach LEO, a propellant depot architecture could allow astronauts and tourist to easily travel between NRHO and LEO. This would make it unnecessary to launch astronauts to NRHO aboard a super heavy lift vehicle that is only infrequently used to launch passengers. At the Gateway, single stage reusable vehicles could be used to travel between the lunar surface and NRHO. And suddenly private commercial space tourism could expand beyond LEO-- all the way to the practically any place on the surface of the Moon. And a new economic age of space travel will have begun!

Links and References