In my last post I discussed the Falcon Heavy (FH), which SpaceX is planning to use to launch payloads into LEO of up to 53 tonnes. I suggested modifying the design slightly, so it can carry that same mass, or a bit more, to LEO as propellant in its tanks. Such a FH Tanker can then be mated to various payloads and send them to the Moon, via the Earth-Moon Lagrange-1 (EML-1) point.
But how much payload? Assuming the vacuum version of the Merlin rocket engine being used in the Falcon 9 and FH, and its vacuum Isp of 342 seconds, that allows reasonable delta-vees for decent payloads.
If we assume the total fuelled mass of the FH Tanker is 56 tonnes, and 53 tonnes of that is fuel, then 22.5 tonnes of payload can be delivered to EML-1. However the FH Tanker arrives empty. While it might be cannibalised for its large pressure volume, cluttering up the EML-1 is probably short-sighted. Also SpaceX is committed to making mostly reusable rockets, so we might have to return the FH Tanker to LEO. That requires it to arrive at EML-1 with 6.232 tonnes of propellant still in its tank, reducing the payload to just 13.25 tonnes. Tweaking the mass of the FH Tanker means we might manage it for ~2.5 tonnes dry mass and ~15 tonnes can then be delivered.
Interesting the Project Apollo’s Lunar Excursion Module, which successfully landed 12 men on the Moon over 6 missions, massed just 15 tonnes. It used a lower performing fuel combination and thus a modified version using the LOX/RP-1 of the Merlin might deliver an LEM to the Moon and back with more payload allowing weeks of stay-time. Alternatively a larger lander could touch down using 22.5 tonnes of propellant delivered by an FH Tanker. The first option requires two launches, one an FH Tanker and the other to deliver the Lunar Lander to LEO. Alternatively two FH Tankers might combine in LEO to push a larger load to the EML-1, requiring 3 launches in total.
Assuming a linear scaling, a bigger LEM with 6 passengers might mass ~45 tonnes, requiring two full payload deliveries to EML-1 and four launches to LEO. All this could cost ~$1 billion ($1 Gigadollars or $1G) – four FHs at $0.125G each and $0.5G for the Heavy LEM. That’s half the “Apollo” programme’s achievement delivered to the Moon for a tiny fraction of its $30G budget (in 1969$.) What’s more with modern inflatable space-habitat technology we’re talking about a relatively large Base being landed, not a mere two-man “sky-car”. With touch-down near one of the Lunar Cold-traps and access to the ice there, plus solar-power, then the 6-person crew could conceivably stay for months, setting up a semi-permanent Base. For a full $30G budget – worth roughly ~1/5 of what it was in 1969 – some 180 people can visit the Moon, stay six-months each and thus spend ~90 person-years exploring and expanding the Base. If anything is worth doing on the Moon, then such an effort will go a long way towards determining what it might be and if it can turn a profit. Platinum-group metals recovered from meteorites? Exotic minerals? Fusion fuels? Unimaginable breakthroughs?
We won’t know unless we go and stay for bit longer than the ~fortnight spent on the Moon by all the “Apollo” missions.
NB: I haven’t discussed how the astronauts get to EML-1 and back to LEO/Earth. The Lunar Landers are assumed to launch their Ascent Stages back to EML-1, which isn’t much different to going into Low Lunar Orbit like the “Apollo” LEM AS did. But a refuelable LEM system is preferable once an actual Base is operating and extracting propellant from the Moon. This should be factored into Base planning and begun as soon as possible so more payload can actually be delivered to the Moon. Launching 7 astronauts to EML-1 in a fully loaded Dragon Capsule, using the FH Tanker system described is incredibly easy. And Dragon is designed with a heat shield sufficient for Earth-Moon return. A six-person ferry, with minimal life-support, for delivering crew to the Base will probably mass a lot less than the 45 tonne Lunar Lander outlined above.