Reusable Moon landing modules could depart from Apollo architecture

Since the crew of Apollo 17 left the lunar surface in December 1972, the question of when and how humans will return to the Moon has lingered. In 2017, the US government announced the ambitious Artemis program, which intends to return astronauts to the Moon by 2024 and establish a permanent orbiting 'Lunar Gateway' as a possible stepping stone to Mars. The Lunar Gateway will also deploy reusable landing modules to put astronauts on the lunar surface.

This approach will require an analysis of the optimal landing approaches for this “last mile” trip, taking into consideration factors such as performance and cost. The private companies contracted by Nasa to design the landing modules are conducting this research, but will largely keep their findings private.

The Skoltech and MIT researchers developed models to assess the most promising approaches to human landing systems for Artemis. They made the assumption that the Lunar Gateway will be located in orbit around the L2 point (one of the points at which various gravitational forces balance the motion of the station) as the preferred option for landing on the lunar south pole, and assumed a week-long expedition involving four astronauts. Based on this model, they considered the optimal number of stages and propellants for the system, going through 39 variants.

In order to assess these variants, they first defined the key set of architectural decisions to be taken (e.g. number of stages and propellant type at each stage) and organised this information in mathematical models. They then performed a computational exploration of architectures arising from combinations of the different decisions. Finally, they analysed the resulting trade-offs, identifying the optimal architectures.

They found that for expendable landing systems such as those used during the Apollo program, the two-stage architecture is preferable due to having lower masses and lower launch costs. However, for reusable vehicles planned for the Artemis program, one-stage and three-stage systems could be competitive with two-stage systems.

The overall "winner" for multiple short lunar missions is a one-stage reusable module running on liquid oxygen and liquid hydrogen.

“Interestingly, our study finds that, even with the orbiting station, if fully expendable vehicles are considered, then the two-stage landing system is still expected to have lower masses and, therefore, lower costs, which sort of reconfirms the Apollo decision,” said Skoltech graduate student Kir Latyshev.

“However, reusability changes that. Though one-stage and three-stage vehicles in this case are still heavier than the two-stage one, they allow to reuse more of the 'vehicle mass' over and over again, thus saving money on producing and delivering new vehicles to the orbiting station and making lunar missions potentially cheaper.”

The authors acknowledge that this is a preliminary analysis which does not take into account some important factors such as crew safety and probability of mission success; these factors will require more complex modelling at a later stage of the program.

“[The] safety factor can affect results in either way,” Latyshev said. “For example, multi-stage solutions might offer safer return opportunities in case of emergencies in the parking lunar orbit prior to descent to the surface than our 'winner' the one-stage system: either the descent or ascent vehicle can be used for return in the case of the three-stage and two-stage systems, as opposed to the single stage of the one-stage system.

“At the same time, two-stage and three-stage systems are expected to be more complex and therefore to have more risks of breakdowns, as opposed to the simpler one-stage solution. So, there is a trade-off again.”