Guest Post: SpaceX Starship as a Lunar Transport

A couple weeks ago, I wrote a piece on the next steps for American space travel, and speculated about a permanent lunar base within the next two decades. Today I have another guest post from my friend Eamon Minges, following up on my speculation with some hard numbers—he definitely has a knack for that sort of thing! Read his thoughts on a Starship-based lunar infrastructure below:

Introduction:

Successful development of large-scale transportation infrastructure in orbital and cislunar space is extremely expensive. This is largely a result of the inefficiency of liquid rockets—for a fully reusable, surface-to-LEO rocket like SpaceX’s Starship, only ~2% of the gross liftoff weight of the rocket can be payload, with the rest of the precious mass consigned to fuel by the tyranny of the rocket equation. Indeed, the deep gravity well of the Earth as well as its thick atmosphere is the main impediment. Given those constraints, what if we were able to build an infrastructure using the resources of the moon to perform in-space operations, and eventually facilitate space industries?

Map showing the required delta-v’s for a lunar Starship mission, undertaken as part of NASA’s Artemis program. Credit: Eamon Minges.

I would argue that, in the current landscape of the launch market, the SpaceX Starship is the best-equipped vehicle to face such a challenge. Not only is Starship the first fully and rapidly reusable orbital launch vehicle, but its adaptability and iterative design process allows adaptation for lunar operations and eventually the build up of large-scale infrastructure to facilitate the development of space through lunar resources.

Here I want to give an outline for how that might be accomplished over the next 25 years, using Starship and its confirmed derivatives as way points on a quarter-century road map.

Near Term:

Starship and Starship HLS docked to NASA’s Gateway Station. Credit: Deep Space Courier.

It goes without saying that before we are able to build significant space-based infrastructure and industrial capacity using lunar resources, we have to return to the Moon first. Fortunately the good folks at NASA and SpaceX have a plan. NASA’s Artemis lunar program intends to build the first way-station in cislunar space. The Artemis program is an ecosystem of vehicles centered around a central space station, the lunar gateway, or LOP-G, which stands for Lunar Orbiting Platform Gateway.

The next Americans to land on the moon aren’t going to go about it in the exact same way as the Apollo program in the 1970’s. While Apollo represented a successful architecture for landing humans on the moon and returning them safely to the Earth, the Apollo architecture can be viewed as a brute force technique, in many ways sacrificing efficiency to achieve a lunar landing as soon as possible. Artemis is building what is analytically speaking a more efficient means of transporting people and supplies to the Moon. Instead of going from a trans-lunar injection straight to a low lunar orbit of 50 km, craft (including SpaceX’ lunar starship, pictured below) will opt for a slightly longer albeit more efficient trajectory, taking five days, instead of three, like Apollo. SpaceX’s Lunar Starship Human Landing System (HLS) will make its way on trans-lunar injection and then use a close flyby of the moon and a retrograde burn to put the HLS Starship into something called a Near Rectilinear Halo Orbit, or NHRO for short. NHRO is a highly elliptical orbit that has an perigee of around 3,000 kilometers and an apogee of 70,000 km (past the Moon’s Hill sphere and into Earth’s gravitational sphere of influence).

Starship variants, from left to right: Lunar (HLS), Crew, Deep Space, Tanker & Cargo. Credit: Eamon Minges.

The advantages of this are several-fold. For starters, when NHRO is used as a staging point for lunar missions there is continuous access to solar power, which is very important for Lunar Starship. Along with a continuous supply of energy, it also has a constant communication line of sight with Earth—unlike the Apollo missions, which spent an hour in comms silence every time they orbited the Moon’s far side. Since NHRO passes within the Earth’s Hill sphere, it costs some 800 meters per second to return from the LOP-G, making access to Earth less costly when compared to the Apollo flight profile.

As I’ve illustrated in the chart below, the delta-v necessary to go from LEO to NHRO, then down to the lunar surface and back, is some 5,510 m/s, which is considerably less than what an equivalent vehicle would go through in an Apollo-style architecture. Hopefully by around the years 2027 or 2028 SpaceX should be flying the Lunar Starship out to NHRO, the lunar surface and back. With a lightened Lunar Starship (likely around 35 metric tonnes in mass), plus full tanks in LEO (eight tanker flights filling the tanks to 1,200 metric tonnes of total propellant), it would enable the Lunar Starship to leave LEO, land at the lunar south pole with some 50 tonnes of payload, and then return to the Gateway. The implications of NASA choosing SpaceX’s Starship for their human landing system are very exciting.

50 tonnes of payload to the lunar surface in one go can facilitate the creation of a permanently manned lunar base. Indeed, a permanent lunar base could be built out of retired HLS Starships, tilted on their side, with installed interior floors and airlocks alongside top regolith coverage for radiation protection. The retired Lunar Starships could become the cornerstone of an early lunar base.

I fully expect a five-to-ten-year period where HLS Starship is NASA’s go-to lander for missions from the Lunar Gateway down the surface and back, with the payload access that SpaceX would have. SpaceX could establish the beginnings of a permanent lunar base at the south pole. Upon return to the Gateway the HLS can be refueled by a deep-space variant of the Starship, which will serve as a NHRO fuel depot taking liquid oxygen and liquid methane from tanker flights from Earth. Since the tanker variant of Starship has a heat shield and aero control surfaces, it has the ability to return to Earth from NHRO for a relatively small delta-v cost of just 800 m/s. Each tanker sent from LEO to LHRO can carry a full payload of 100 or so tonnes of fuel to NHRO, and any payloads brought down to the lunar surface will be brought to the Gateway by cargo starships and then transferred to the HLS variant of the Starship to be transported to the lunar surface.

Using this architecture, the Lunar Starship (HLS) may be able to be reused between three to five times before being retired to the Lunar Surface as a permanent habitat module, by the mid 2030s I can see the potential for a lunar research station staffed by dozens of people, using Lunar Starships that ferry people and cargo between the surface and NHRO, and using either crew or cargo starships to get payloads or people between Earth and the Gateway.

Mid-Term:

In the medium Term SpaceX can begin to introduce a degree of in-situ resource utilization to the base they helped build at the lunar south pole. After hundreds of tonnes of payloads have been transported to the surface from dozens of HLS flights, the Lunar base should have the necessary resources and personnel to begin building improved launch and landing pads out of melted lunar regolith.

These improved formal launch and landing pads will allow Starship variants designed to re-enter earth’s atmosphere (outfitted with a heat shield and aero control surfaces) to land on the Lunar surface. If the crew, cargo or tanker variants of Starship are redesigned slightly to have expanded methane header tanks, carrying an extra 50 tonnes of liquid methane that can’t be made by lunar surface ISRU due to lack of carbon. This would likely stored around the central axis of the ship, inside the main methane tank.

Regular atmosphere-capable starships have the delta-v (with eight refueling flights in LEO) to send a 50-tonne payload to the Lunar Surface, refill the oxidizer tanks with lunar oxygen, and then return directly to Earth with around 25 tonnes of return payload. Overall, once the base is developed, Starship logistics to the lunar surface should be a lot easier and HLS Starship may be able to be retired. I don’t expect SpaceX’s lunar transportation architecture to look like this for much longer than some 10-15 years after people first land back on the Moon with HLS Starship.

Long Term:

The current methane-oxygen version of Starship (left), compared with a prospective hydrogen-oxygen variant (right). Credit: @StarshipFairing on Twitter.

In the long term (25+ years) I think that the smartest move SpaceX can make would be to design a hydrolox Starship variant; while they would essentially be building a new vehicle, two decades of experience flying Starship should provide SpaceX with the engineering know-how to create a hydrolox Starship. The ISP advantages—as well as the ability to completely refill the tanks on the Lunar surface, combined with the ability to return directly to the earth via the use of the heatshield—cannot be understated.

Thanks for reading! If you’d like to read more of Eamon’s analyses, you can find his previous work for this site here, here, and here.

I’ll see you all again next Sunday with a new post. It will either be a history of weird astronomy, or a review of a certain financially successful movie about blue cat people…