Monday, July 4, 2022

The Functional and Economic Value of Repurposing Spent SLS Core Stages

Notional SLS derived Dry/Wet orbital habitat at LEO with more than 4000 cubic meters of pressurized volume. A notional  crewed reusable REUS-LV approaches the habitat after a voyage from a similar habitat at NRHO.

by Marcel F. Williams
Deploying large habitats, depots, and reusable interplanetary vehicles to LEO is probably the most logistical and economically efficient way to utilize the new Space Launch System (SLS). But the basic SLS Block I configuration capable of deploying up to 70 tonnes to LEO also comes with an added benefit. The spent core stage of the  SLS  could also arrive and remain in low Earth orbit fully intact. 

With a pressurized volume exceeding 916 cubic meters, $165 billion (in current dollars) the International Space Station is considered to be the most expensive structure ever built by humanity. But the pressurized volume of a spent  SLS core stage would exceed 3000 cubic meters (more than three times the pressurized volume of the ISS). 

If SLS core stages were allowed to remain in orbit, it could be retrofitted by astronauts from a nearby space station or space vehicle to accommodate enormous amounts of water, or cryogenic propellants, or the pressurized atmospheres of habitats. This, of course, would add significantly more economic value to each SLS launch.   

X-Ray of the SLS core stage hydrogen and oxygen tank (Credit NASA)


Microgravity Habitats

Polls suggest that 42% of Americans would like to travel into space. But  there are only about 52,000 people, worldwide, who could actually afford the $25 to $50 million cost of space travel. Still that suggest that perhaps 22,000 people who would like to travel into space-- actually have the funds to to do so. Annually, if just 10% of that group paid to visit an orbiting space hotel it would  require more than 400  commercial launches every year. This, of course, doesn't include the number of unmanned launches needed to provide such orbital facilities with the food and water and other supplies.  But even if a mere 1% of the super wealthy who desired to travel into space purchased rides to a space habitat every year,  that would require 44 to 55 private commercial launches annually.

Of course, additional revenues could be accrued by providing accommodations for astronauts from NASA, the Department of Defense and astronauts from foreign space agencies.  

Cramped conditions inside the interior of an ISS crew module (Credit NASA)

Relatively spacious interior of  6.6 meter in diameter Skylab space station (Credit NASA)

If the owner of a space hotel charged  $500,000 a day for each day their facility accommodated a  visiting astronauts or space tourist resided in their facility (including arrival and departure days) then they would make $5 million for a ten day stay. For a six member flight crew that would be $30 million dollars in revenue in just ten days.  If such a facility was continuously occupied by six individuals on average then  the orbiting hotel would make $1.095 billion (nearly $1.1 billion a year).

An SLS core stage retrofitted with docking adapters and airlocks could be latter attached to habitat modules deployed by the SLS or other launch vehicles. This could allow pressurized habitats deployed by private companies or national space agencies to attach their modules to a purchased retrofitted SLS spent core stage, guaranteeing them and additional 3000 cubic meters of pressurized space-- no matter how large of small their habitat modules are.

A  spent SLS core stage retrofitted with docking airlocks sold for $5 billion could be paid off in less than five years if the facility was continuously occupied by at least 6 astronauts or tourist. A 20 to 30 year lifetime might produce $21 to $32 billion of revenue for the SLS core stage enhanced habitat.


Notional SLS derived Dry/Wet orbital habitat at NRHO with more than 4000 cubic meters of pressurized volume. A notional crewed reusable REUS-LV is docked at the habitat before transferring a six person crew to the surface of the Moon.

Water & Sewage Depots

Water, of course,  is an extremely valuable commodity in extraterrestrial environments. Water can be used to wash, to drink, and to prepare food for its human occupants. Electrolysis can be used to convert water into  oxygen to create breathable atmospheres.   And the oxygen and hydrogen produced through the electrolysis of water can also be liquefied and utilized for rocket propellant. 

Since a spent SLS core stage would have two empty tanks, one tank, in theory,  could be used to store enormous quantities of water. This water  could later be transported to nearby orbiting habitats or to propellant producing orbiting depots. 

Human occupied orbiting habitats will also produce substantial quantities of waste water in the form of sewage. So the second SLS core stage tank could be used to store this sewage-- almost indefinitely. Ultimately this sewage could be used as another source of water.  Methane could also be produced from the sewage to fuel liquid methane fueled interplanetary spacecraft. 

Nearly a tonne a day of waste water could be produced for six occupants aboard a space habitat saving more than 300 tonnes of water a year, avoiding perhaps $1.5 billion a year in cost if such water had to be brought from the surface of the Earth.  Assuming a 20 to 30 year lifetime for the depot, that could be $30 to $45 billion in water transport savings from Earth.                                                                                                                                                                                                                                                                                               

Liquid Oxygen and Liquid Methane Depots

Nine tonnes of water for a propellant producing depot would produce one tonne hydrogen propellant plus six tonnes of oxidizer-- leaving two tonnes of oxygen as a waste product. However, a spent SLS core stage could be used to store the excess oxygen.  The spent SLS core stage could be retrofitted with solar panels to keep the oxygen refrigerated. Interplanetary  vehicles launched from Earth could arrive in orbit with only fueled with hydrogen or methane, utilizing the excess oxygen stored by the SLS core stage for propellant oxidizer.

The second tank in the SLS spent cores stage could be used to store liquid methane produced from the sewage stored in water and sewage depots.

So every 90 tonnes of water sent into space for propellant production (~$450 million),  20 tonnes ($100 million) would be wasted-- if the excess oxygen produced through electrolysis  is not stored away to be later utilized. So 900 tonnes of water sent into orbit a year for propellant production could save $1 billion a year in propellant for extraterrestrial missions.

 Rotating Crop and Orchard Plantations 

With an interior diameter of approximately 8.4 meters, the two empty tanks of a spent SLS core stage tanks could accommodate two large areas for growing crops or trees. The cylinders would have interior surfaces of 285 meters squared for the empty oxygen tank and 886 meters squared for the empty hydrogen tank. The domed end caps could be used for vineyards (growing grapes). Even with soil (presumably originating from the lunar surface) a meter deep, the two tanks should be able to grow enough food to feed at least 12 people a year. 

Dwarfed trees between 2.5 to 3 meters tall could be used to grow apples, oranges, lemons, peaches, pears, plums, cherries, olives, etc. Banana plants could also be grown. 

Crops such as wheat, corn, rice, potatoes, yams, sugar beets,  tomatoes, lettuce, pineapples, etc. could also be grown.

The bio-waste from the agricultural foliage could also be of value. Pyrolyzed into syngas,  hydrogen or methane could be produced for rocket fuel and the waste CO could be converted into CO2 for growing trees and crops. 

A thruster module could be attached to the top of the spent core stage in order to make the structure spin along its axis, producing artificial gravity within the interior of the tanks. Remote controlled robots could be used to manage the orchards and crops. Humans in pressure suits could periodically visit the space farms under microgravity conditions to retrieve fruits and vegetables or to bring in more water and equipement by simply using the thrusters to hault the structure's rotation. Entry docking ports into the two tank areas could be retrofitted into each tank area the same way it would be done for the habitat core stages that were retrofitted to be habitats.   

Solar panels and cooling fins  could be attached to the thruster unit for providing thermal control and  electricity and interior lighting for a space farm.

If eight tonnes of food could were grown a year to feed up to 12 people, $40 million a year could be saved in food exports from Earth. This is perhaps $800 to $1.2 billion --in food import savings over the course of a lifetime for the spent SLS core stage tanks.

Notional EUS derived REUS-OTV with connecting ring for operations with a second REUS-OTV vehicle


Exploiting Residual Fuel

Several tonnes of propellant would be required to de-orbit the SLS core stage. But the residual fuel used to de-orbit the core stage sending it into the Earth's atmosphere could be extracted by orbiting propellant depots to be stored and saved to fuel vehicles destined for beyond LEO missions. This could amount to $5 million per tonne of fuel saved.


Beyond LEO Deployment 

While the SLS could deploy spent core stages to LEO, a notional REUS (Reusable Exploration Upper Stage) used in pairs could be used to redeploy the core stage practically anywhere within cis-lunar space (NRHO, DRO, L3, L4, and L5) and even into orbit around Mars and Venus.

Links and References

Deploying a Ginormous SLS Derived Dry/Wet Workshop Habitat with a Single SLS Launch

Space Launch System

Commercial Launch Demand to Private Microgravity Habitats at Low Earth Orbit

The Logistical Viability of an SLS EUS Derived Reusable Lunar Crew Lander

All about dwarf fruit trees

Cis-Lunar Gateways and the Advantages of Near Rectilinear Orbits

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