A month ago, SpaceX announced that it plans to send two paying customers around the Moon, using a Falcon Heavy and Crew Dragon. The company plans to use a free-return trajectory to send the crew around the Moon and back to Earth, a method which involves only small amounts of propulsion (for course corrections) after the launch. It’s likely* that this will be the first time any human has left low Earth orbit since Cernan, Evans and Schmitt returned to Earth at the end of Apollo 17, 44 years ago last December. Such an event invites the question – how far can SpaceX take their current architecture to explore the Moon with crew?

To answer this, we put together a simple model of the performance characteristics of the Falcon Heavy and Dragon spacecraft. The model is based the Tsiolkovsky rocket equation, and uses: estimates of launch vehicle wet and dry masses found here [1], Merlin specific impulse from our two previous posts on Merlin performance, SuperDraco Isp from an FAA document [2] and a neat calculation by a member of the /r/SpaceX subreddit (corrected for cosine losses). As for the mass of crew dragon, the DragonFly test article was about 6350kg unfuelled according to an FAA document [3], and it’s likely this would be larger for a real mission. Velocity requirements are from the Apollo 11 flight plan [4]. A few parameters are tuned with telemetry data from a launch webcast, and validated by comparing the model’s prediction of the Falcon Heavy’s payload to Low Earth Orbit (LEO) and Geostationary Transfer Orbit (GTO) with the values on SpaceX’s website. The model can be found here – comments and criticism are welcome. What follows are several scenarios that the model finds possible.

Free return:

We already know this is in SpaceX’s capability, but it’s worth looking at exactly what it takes to put a Dragon into this trajectory. According to the model, Falcon Heavy can send up to 18 tons on a lunar trajectory (even if you don’t believe the model, this makes sense: SpaceX rate the Heavy for 22.2 metric tons to Geosynchronous Transfer Orbit, a trajectory not far in velocity terms from a trans-lunar injection). The Dragon, fully fueled, still weighs less than ten tons, and so it likely that SpaceX’s upcoming free-return mission will have plenty of margin for reuse on the first (and possibly even second†) stages. Of course the two passengers, their belongings and supplies for the journey will take up some of the ~8.5 tons of spare capacity, but considering the limited space inside the Dragon, probably not all.

Lunar orbit:

A free-return trajectory only offers a single circuit of Moon before returning home. If SpaceX (or its customers) want to undo a more ambitious mission, the next logical step is to maneuver a spacecraft into orbit around the Moon, allowing for as many circuits as supplies allow, before returning home. To do this fully, you need a velocity change of about 1km/s on arrival, and other 1km/s when you wish to return to Earth. Crew Dragon presently carries two forms of propulsion for velocity changes: Dracos, which are small engines designed for course corrections, and SuperDracos, which are larger, very-high-thrust engines designed for rapidly escaping a malfunctioning launch vehicle. In principle, the high reliability and wide throttle range of these engines (the SuperDracos) would allow them to be re-purposed as maneuvering engines in an ambitious lunar mission. At present, the amount of fuel that can be stored on crew dragon is insufficient for the task, so such a mission would require, at a minimum, for a redesign of the vehicle to have substantially larger fuel tanks. However, if you did that, and carried almost the full 8.5 tons of spare capacity as fuel, you might just have enough to make circular orbit around the moon and return home.

In fact, you could probably do a little better than that, if you were willing to accommodate an elliptical orbit which is close to the moon at its lowest point but very high above it at its highest point. This would have reduced fuel requirements compared to a circular orbit (improving the overall mass budget), but likely still more than the existing tank on the Dragon could handle.

Lunar landing:

You can consider various possible mission architectures for this, but the bottom line is as follows: Dragon as it exists today is simply too heavy to put humans on the Moon. When this was done in the Apollo program, the unfueled ascent vehicle weighed just 2150 kg, around a third of the mass of the Dragon. Increasing the mass of this increases the propellant mass needed to leave the Moon, which increases the propellant needs of the descent stage, and so on. Fundamentally, the main new technology SpaceX would have to develop is a much lighter, two-stage vehicle, with one stage for landing and one for launch from the lunar surface. Without that, even options like long-term storage of cryogenic propellants and/or in-orbit refueling are insufficient on their own. How difficult developing such a vehicle is shouldn’t be underestimated – the effort could easily be comparable to that needed to develop Dragon in the first place. While undoubtedly possible with enough money and will, it would be a significant distraction for a company much more focused on visiting Mars.

But let us indulge ourselves for a moment, and imagine that SpaceX (or some eager third party) did develop such a vehicle, what then? Falcon Heavy cannot lift as much mass as the Saturn V, but this could be worked around if the landing module and the arrival/Earth return module were launched separately. The unfueled mass of the vehicle would need to be less than Apollo, but not vastly less: 1.5 metric tons for the descent and 1.75 tons for the ascent, comparing with 2.134 and 2.15 metric tons respectively on the Apollo Lunar Module.

First, a Falcon Heavy launches this vehicle towards the Moon, where it maneuvers to Low Lunar Orbit using a portion of the fuel in the descent stage. It then checks out all of its subsystems, after confirming that they work, waits for the other part to arrive. Then a second Falcon Heavy launches with a Crew Dragon, passengers, and an extended fuel tank. The Dragon maneuvers to orbit around the Moon and docks with the landing stage, where the passengers move from the Dragon to the other vehicle. This then lands on the moon with the remaining fuel in the descent stage, and allows its crew to explore the surface. Once they wish to return, the upper stage of the vehicle returns them to low lunar orbit, where they dock back with, and return home in, Crew Dragon.

Nearly a half-century since humans first set foot on the moon, the lunar surface continues to taunt us. Modern technology may bring it closer than ever before since Apollo, but the cratered terrain remains tantalizingly out of reach.

Footnotes:

*The alternative scenario is that NASA decides to make Exploration Mission 1 crewed – an option which would represent a change from current plans, but is being studied by NASA. At that point, it’s possible but far from certain that the NASA mission would launch first.

†We wrote a post about second stage reuseability here – the conclusion that only the Falcon Heavy has the margin for it seems to be holding up.

Citations:

[1] Spaceflight101 estimates of various parameters of the Falcon 9 FT

[2] FAA environmental assessment for DragonFly – appendices

[3] FAA environmental assessment for DragonFly (relevant value is in Section 2-2)

[4] Apollo 11 flight plan – tables 1-1 and 1-2.