by Marcel F. Williams

Artist rendition of ULA's future upper stage precursor to the IVF modified ACES 68, accommodating 68 tonnes of LOX/LH2 propellant (Credit: United Launch Alliance)

NASA currently envisions three launches for the SLS (Space Launch System) using a Block I configuration and consisting of an ICPS (Interim Cryogenic Propulsion Stage) for its upper stage. The basic Block I vehicle will be capable of deploying at least 70 tonnes to LEO. However, by utilizing the ICPS as an upper stage that can accommodate 27 tonnes of propellant, the Block I configuration with an ICPS upper stage will be able to deploy at least 95 tonnes of payload to low Earth orbit.

A Block IB configuration with an EUS (Exploration Upper Stage) is expected to be introduced by 2024. The EUS will be able to accommodate 128 tonnes of LOX/LH2 propellant. And this will enable the Space Launch System to deploy up to 105 tonnes of payload to LEO. NASA currently plans to use the Block IB configuration to assemble the future Lunar Gateway at NRHO (Near Rectilinear Halo Orbit). The Lunar Gateway will serve as a bridge for Lunar and Martian operations, substantially reducing the delta v requirement to reach the orbits of the Moon and Mars.

NASA's Current Launch Sequence for the SLS

2020:

The SLS Block I launch vehicle will deploy an unmanned Orion spacecraft to a Distant Retrograde Orbit (DRO) before returning the capsule to the Earth. DRO is interesting because of its lack of station keeping requirement. Such a distant equatorial orbit of the Moon might make DRO a prime location for massive rotating artificial gravity habitats perhaps sometime in the second half of the 21st century. It might also be a good location for small asteroids imported into cis-lunar space for potential exploitation.

2022:

The second SLS Bloch I launch is scheduled to send a crewed Orion spacecraft around the Moon as an inaugural human occupied beyond LEO spaceflight for the SLS system.

2023:

The third scheduled SLS Block I launch will be used to deploy the Europa Clipper to Jupiter orbit in order to study the surface of its icy moon, Europa.

2024:

The first launch of the SLS Block IB is scheduled for 2024. It will consist of an Orion crew of four plus a ten tonne component of the Lunar Gateway which will be assembled at NRHO . This location will eventually give crewed vehicles weekly access to the lunar surface with as little as 12 hours of travel time to and from the surface of the Moon.

2026:

Four subsequent SLS Block IB flights will be required to completely assemble the Lunar Gateway by 2026.

SLS Block I Cargo with ICPS upper stage capable of deploying 95 tonnes to LEO (Credit: NASA)

SLS Block I without upper stage, capable of deploying 70 tonnes to LEO (Credit: NASA)

The Flaws in NASA's SLS Plans

One. Only one or two SLS launches are required to deploy the Lunar Gateway to NRHO-- not five SLS launches. Of course, the amount of mass that can be deployed to NRHO by the SLS is substantially reduced under the NASA scenario because of the joint launch of the Orion spacecraft and Service Module. This would be an unnecessary joy ride for NASA astronauts that would greatly inflate the cost of deploying the Gateway while also significantly delaying its full implementation.

Why spend more than two to five times as much as you have to deploy the Lunar Gateway when dollars for NASA's human spaceflight related programs are so hard to come by? Plus NASA needs more funding to help private companies develop lunar crew landing vehicles, space propellant depots, extraterrestrial habitats, and interplanetary crew transport spacecraft.

Two. Under NASA's current scenario, the Lunar Gateway wouldn't even start to be assembled until 2024 with a completion date around 2026. Again, a totally unnecessary delay in deployment.

An SLS Block I with an ICPS upper stage could probably deploy a complete SLS derived Deep Space Habitat to NRHO as early as 2022 that weighs about 20 tonnes-- without consumables. Food and water could subsequently be launched to the Gateway by commercial launch vehicles.

An SLS Block IB, would be able to deploy SLS derived Deep Space Habitat concepts weighing 22 tonnes for 353 cubic meters of habitable pressurized volume and 28 tonnes for 519 cubic meters of habitable pressurized volume with a single launch in 2024. That would still be two years earlier than NASA's current Lunar Gateway completion plans.

However, a large private commercial upper stage is currently being developed by the ULA (United Launch Alliance) that will be used to deploy payloads for the Air Force (Space Force?). And the ULA's Centaur V could be certified for military payloads in 2021 and ready for utilization in 2022.

If the SLS was used to deploy a large commercial upper stage (Centaur V) to LEO, NASA could deploy a 28 tonne SLS derived Deep Space Habitat to NRHO with just two launches. Assuming a maximum dry weight of no more than seven tonnes for the Centaur V, an SLS Block one launch could deploy the Centaur V to orbit with at least 63 tonnes of propellant. An SLS Block I with an ICPS upper stage could deploy the Centaur V with 68 tonnes of propellant to LEO plus 20 tonnes of additional payload for, perhaps, another ICPS with at least 16 tonnes of additional propellant.

After docking with an SLS payload deployed to LEO by a previous SLS launch, the Centaur V should be easily capable of transporting more than 30 tonnes of payload to NRHO.

However, the preceding SLS Block I launch would be able to deploy up to 70 tonnes of payload to LEO. So you could actually deploy two 28 tonne habitats to LEO (just 56 tonnes) with one destined for NRHO after the next SLS upper stage launch-- while the other 28 tonne habitat remains at LEO as a potential replacement for the ISS.

Replacing the ISS with a new space station would possibly save NASA up to $4 billion a year!

An SLS derived LEO habitat wouldn't be a space laboratory, it would simply be a habitat used for NASA and Space Force astronaut training and for the training of astronauts from foreign space agencies. It could also be a used as a destination for wealthy space tourist and space entrepreneurs. Small commercial launched space laboratories could be co-orbited near the LEO habitat while lab specialist use the SLS derived habitat as a hotel, visiting their floating laboratory only when necessary to add or retrieve materials.

Alternatively, NASA could deploy three 22 tonne SLS derived habitats to LEO with one destined for NRHO with the other two remaining at LEO. One could be used exclusively for NASA and the Space Force with the other being auctioned off to a private space company for space tourism or to accommodate the needs of foreign space agencies.

While the above scenario might delay the first crewed flight of the SLS until 2023 (just one year), it would actually give astronauts a place to go on their first SLS flight in 2023. Of course, NASA could still have a test flight of a crewed Orion in 2021 instead of 2022. NASA astronauts could use commercial crew vehicles to travel to LEO to inspect the Gateway habitat before its deployed to NRHO.

Three. There's no logical reason to waste an SLS launch for a flyby mission of Europa. The Europa Clipper can probably be deployed by commercial launch vehicles. However, an orbital mission to Europa is questionable since the moon Callisto would be much easier to access. Callisto is the only place in Jupiter space where human outpost could be set up for potential colonization. A human outpost on the surface of Callisto would make it substantially easier to explore Europa, Ganymede, and Io (all within Jupiter's deadly radiation belt) with robots remotely controlled from the surface of Callisto. A better near term use for a non human spaceflight related launch of the SLS would be the deployment of space telescopes with mirror diameters even larger than the James Webb.

16.5 meter long 8.4 meter in diameter SLS derived orbital habitat (Credit: NASA)

13.5 meter long 8.4 meter in diameter SLS derived orbital habitat (Credit: NASA)

Propellant Depots and the Future of the Orion

The utilization of reusable vehicles for the human exploration, pioneering, and exploitation of the lunar surface is one of the primary reasons for having a Lunar Gateway at NRHO. Reusable spacecraft will, of course, require propellant depots.

Once the Lunar Gateway is deployed, co-orbiting propellant depots can also be deployed to NRHO by private commercial launch companies. NASA breakthroughs in zero boil off (ZBO) liquid hydrogen storage should make it possible for commercial launch companies to deploy propellant tank derived LH2 (liquid hydrogen) and liquid oxygen (LOX) storage tanks to NRHO that don't leak any hydrogen or oxygen. More sophisticated technologies could allow the solar power production and liquefaction of hydrogen and oxygen from water in space which could substantially reduce launch complexity and cost for commercial launch companies.

The SLS propellant tank technology derived EUS might have questionable utility if NASA is already using the Centaur V and, subsequently, the ACES-68 to deploy heavy payloads to deep space locations.

Notional ACES 68 with BE3 Engine (Credit: ULA)

With its Integrated Vehicle Fuel (IVF) technology, the ULA's ACES-68 successor to the Centaur V could be in operation as early as 2023 but is currently planned to go into operation within the 2024 to 2025 time frame. It now seems likely that the ULA will allow Lockheed Martin, one of its parent companies, to be first to develop IVF technology for its future reusable crewed lunar landing vehicles. In tandem (on either side of an orbiting payload) two such ACES vehicles could transport payloads exceeding 70 tonnes from LEO to NRHO or to Low Lunar Orbit. A single ACES 68 could deploy an equal amount of payload to Mars orbit from NRHO; so massive amounts of water or propellant deployed to NRHO could easily be later deployed to Mars orbit from NRHO.

If propellant depots are deployed at LEO and NRHO, the ACES-68 could replace the ICPS and the Service Module for the Orion space capsule. This would make the Orion a completely reusable vehicle for transporting astronauts between LEO and NRHO. So no longer would the Orion spacecraft have to be deployed by the SLS for deep space missions. The ULA's Vulcan spacecraft could deploy the reusable Orion/ACES to LEO, fueling the ACES 68 booster at a LEO propellant depot before heading for NRHO or Low Lunar Orbit.

So, in theory, astronauts could board a commercial launch vehicle (Falcon 9/Dragon, Atlas V/Centaur/CST-100, etc.) that transports them to LEO. The could then dock with an already propellant depot fueled Orion/ACES vehicle for transport to the Lunar Gateway or to Low Lunara Orbit. Again, no SLS launch would be required which means that the heavy lift vehicle could be more properly used to transport heavy payloads to LEO.

Notional Orion/ACES reusable shuttle with crew hab approaches SLS derived Lunar Gateway at NRHO (Near Rectilinear Halo Orbit). (After NASA and ULA)

The Future of the EUS

If NASA eventually uses commercial upper stages such as the Centaur V and the ACES 68 to transport large and heavy payloads initially transported to LEO by the SLS then the space agency could delay the deployment of the EUS and focus on developing a far more enhanced orbital transfer vehicle. An SLS derived EUS equipped with IVF and cryocooler technologies could make such a vehicle reusable. Such a vehicle could simply use the SLS core stage liquid oxygen tank as a liquid hydrogen tank while also deriving a smaller oxygen tank from the same technology.

Such a reusable vehicle could accommodate more than 345 tonnes of LOX/LH2 propellant. And it could be used to transport large habitats and their crews to the orbits of Mars, Venus, and to the NEO asteroids from NRHO. The performance of such a reusable interplanetary crew transport could also be greatly enhanced by coupling it with a twin transport or just a reusable ACES booster with 68 tonnes of propellant. Propellant depots in Mars orbit would be required for the the return journey to Earth. But, again, the water or propellant could be routinely transported to Mars orbit from water or propellant transported from the lunar surface to NRHO. Eventually, however, propellant depots in Mars orbit could be routinely supplied with water or propellant extracted from the regolith of the martian moons, Deimos and Phobos. And it might even be economical to transport liquid hydrogen from the surface of Mars from reusable spacecraft to orbiting depots to be used with liquid oxygen extracted from the martian moons for propellant.

An Alternate SLS Launch Scenario

2020:

SLS 1: Unmanned SLS Block I launch of Orion/SM/ICPS to DRO (Distant Retrograde Orbit)

2021:

SLS 2: Crewed SLS Block I launch of Orion/SM/ICPS to TLI (Trans Lunar Injection) around the Moon

(Beginning of deployment of small mobile robots to the lunar surface by commercial launch companies)

2022:

SLS 3: Unmanned SLS Block I launch of two 28 tonne SLS derived microgravity habitats to LEO.

(Two Commercial Crew launches to LEO to inspect the twin microgravity habitats.)

SLS 4: SLS Block I launches Centaur V to LEO. Centaur five docks with Lunar Gateway at LEO and transports the fully complete deep space habitat to NRHO (Near Rectilinear Halo Orbit)

2023:

SLS 5: Crewed SLS Block I launch of Orion/SM/ICPS to Lunar Gateway at NRHO.

SLS 6: SLS Block I with ICPS upper stage launches a Centaur V with an 8 meter class space telescope to ESL2 (Earth-Sun Lagrange Point 2). The new space telescope will have an 8 meter plus monolithic primary mirror housed within a 12 meter in diameter payload fairing. And it will join the with join the 6.5 meter in diameter James Webb telescope at ESL2. The diameter for the mirror for the Hubble Telescope was 2.4 meters. The development of such a enormous fairing size for the SLS will greatly enhance the launch vehicles unique ability to accommodate exceptionally large payloads. NASA needs to stop entertaining smaller payload shrouds for the SLS that could nullify its advantage over other launch systems.

(Commercial launch companies begin the continuous deployment of tanks of LOX and LH2 to depot clusters at LEO and NRHO)

{NASA begins using new RS-28 engines for the SLS core vehicle. Hopefully, some meager funding to develop an SLS-B (the SLS without Solid Rocket Boosters) with a commercial Centaur V or ACES upper stage and a commercial CST-100, Dream Chaser, Dragon, or maybe even an Orion as the crew capsule. This would allow more frequent use of the SLS core stage and its RS-25 engines which should help to substantially reduce the overall cost of SLS launches.}

2024:

SLS 7: SLS Block I launches two reusable Lockheed Martin Lunar Crew Landing Vehicle (LCLV) equipped with advanced IVF and cryocooler technology. One LCLV will be launched with enough fuel to self deploy itself to the Lunar Gateway at NRHO. The LCLV already fueled with LH2, will utilize a LEO orbiting LOX depot in order to fuel itself for self deployment to NRHO.

Both Lunar Crew Landing Vehicles will be fueled and tested (unmanned) at NRHO with each traveling to opposite lunar poles to deploy mobile robots to the surface via their lift elevators. Some of the mobile robots will collect regolith samples for return to the Lunar Gateway and eventually to Earth. A few weeks later, one LCLV will be deployed to the surface of the far side of the Moon to collect regolith more lunar regolith samples while proving the vehicle's reusability.

{First NASA /ULA funded unmanned test of a reusable Orion/ACES to NRHO followed by the first crewed Orion/ACES to the Lunar Gateway at NRHO. The reusable vehicle will be launched into orbit by the ULA Vulcan rocket. The success of reusable Orion/ACES spacecraft and reusable Lockheed Martin Lunar Landing Vehicle (used for crew transport between LEO and NRHO) should end the necessity of using the SLS to deploy astronauts to NRHO Gateway}

SLS 8: Last crewed SLS Block I launch of Orion/SM/ICPS to Lunar Gateway at NRHO.

After the six member crew arrives at NRHO, four astronauts will climb aboard one of the Lunar Crew Landing Vehicles to travel to one of the lunar poles (The first Americans to land on the lunar surface since 1972). If the crew on the lunar surface should have some serious difficulties attempting to return to the Lunar Gateway, the remain astronauts at NRHO will use the second LCLV to rescue the astronauts from the lunar surface, returning them safely to the Lunar Gateway.

So under this SLS launch scenario (before the end of 2024), NASA would have a new simpler and cheaper (and possibly money making) space station at LEO, a new Lunar Gateway at NRHO, plus American astronauts and hopefully, guest astronauts from foreign space agencies, routinely traveling to and from the lunar surface from the Lunar Gateway on private commercial landing vehicles.

Links and References

Space Launch System Lift Capabilities