Starship lunar lander
A lunar lander proposed by SpaceX and based on its Starship vehicle is one of three concepts selected by NASA for its Human Landing System program. Credit: SpaceX

On April 30, NASA announced the award of close to $1 billion in funds split between three companies to develop human landing systems for its Artemis moon base program. The companies were SpaceX, which proposed a variant of its Starship reusable launch system, and teams led by Blue Origin and Dynetics, which each offered much smaller dedicated lunar landing and ascent vehicles. Accepting these as now the primary options for Artemis, how might they best be used?

The SpaceX Starship, now aggressively being developed with the company’s own funds, is the obvious choice for the Artemis launch vehicle, as it offers a payload capacity similar to the NASA Space Launch System at less than one-tenth the cost. To go beyond low Earth orbit, however, it would need to be refueled in space. Under its Artemis contract, NASA is funding the company to create this additional capability. Accordingly, SpaceX has released artwork showing the booster being used to land and take off from the moon.

This latter application of Starship is not a good idea. Starship is estimated to have a dry mass of 100 metric tons. To refuel it is LEO to fly to the Gateway with its payload of 100 tons would require 300 tons of liquid oxygen and methane (LOx/CH4) propellant. This could be supported with three tanker Starship flights to LEO. But then to land on the moon, discharge its cargo, and return to the Gateway would require a further 400 tons of propellant delivered to the Gateway, or four trans-lunar tankers. Each of these would need to be enabled for flight from LEO to the gateway by three further tanker flights, for a total of 20 Starship launches for each piloted lunar mission. That doesn’t make sense. Furthermore, landing a Starship on the moon with 100 tons of cargo and the 120 tons of propellant it would need to return to the Gateway (without its cargo) would require at least 120,000 pounds of thrust. Unless a prepared landing pad were provided, this would blow a large crater below the Starship during landing. This rules out landing Starships on the moon until astronauts can prepare the necessary facilities.

But there is an alternative Artemis application for Starship that is extremely attractive. If it can be refueled in LEO, a Starship tanker could be put in orbit about the moon and serve as an excellent refueling station for much smaller landing and ascent vehicles. Such a standalone orbital propellant depot would not need to be at the Gateway. With its extensive propulsion capability, a tanker Starship could be stationed anywhere, including a much more favorable low polar orbit that would provide ready access to the entire moon.

To avail themselves of such support, the smaller landers would need to use LOx/CH4 propellant. The proposed Blue Origin lander is an awkwardly tall, three-stage expendable system employing LOx and liquid hydrogen (LOx/H2) propulsion. This would need serious redesign. The Dynetics configuration is more attractive, as it is a clean single-stage (with drop tanks) system, employing dense propellants, with its hab or other payload delivered close to the ground. This could readily make use of LOx/CH4 propulsion.

Let’s assume a three-ton dry mass for the small lander. (This appears to be the case for the Blue Moon system. The Apollo Lunar Module upper stage and cabin had a dry mass of 2 tons.) Employing LOx/CH4 propulsion with an exhaust velocity of 3.7 kilometers per second (or 378 seconds of specific impulse), 12 tons of propellant would be needed to descend, land, and return the small lander to the Gateway. If the tanker placed itself in a low polar orbit, however, only 6 tons of propellant would be needed, and expendable staging or drop tanks would become unnecessary. A tanker carrying 100 tons of propellant as cargo could thus support up to 16 sorties to the Lunar surface. This could be greatly expanded further by producing oxygen on the moon, which can be done at the poles by splitting water ice or nearly anywhere by reducing iron oxide. LOx/CH4 propellant is 78 percent oxygen by mass. If the oxygen required for travel between the surface and orbit is produced locally, then the tanker will only need to supply methane, and the number of sorties flights it could support would more than quadruple.

Using such a coherent transportation system, the moon could be thoroughly explored. Sortie missions could be sent to many locations, and landing pads could be prepared at the best. Then Starships could be landed on the moon, not to use as ascent vehicles — their high mass makes them unattractive for that application — but to stay as bases, for which purpose their large size is a plus. In a short amount of time we could have a network of capable bases around the moon, producing oxygen to support an extremely cost-effective lunar exploration and development program.

The pieces to enable such a plan are there. NASA just needs to make sure they all fit together.

Dr. Robert Zubrin, an aerospace engineer, is the founder of the Mars Society and the president of Pioneer Astronautics. His latest book is “The Case for Space: How the Revolution in Spaceflight Opens Up a Future of Limitless Possibility.” Follow him on Twitter: @robert_zubrin