[Note: I haven't quite finished with my Venus ISRU series, but some of the articles I've read over the past few days drew me back to this series on propellantless lunar launch approaches that I started writing four years ago but never finished. While Venus ISRU is interesting, I still think it's pretty likely that the first operational ISRU (ie beyond demo or pilot-plant scale) will be on the Moon.]
One of many important issues that doesn’t get enough airtime when discussing lunar ISRU is how to efficiently get the propellants and other materials off the lunar surface. There seems to be a line of thinking that could be called “all we need is ISRU” that says that lunar ISRU is the most critical technology and everything else is just a distraction.
While it is possible to take propellant produced on the lunar surface up to LLO or to one of the Earth-Moon Lagrange points using similar rockets to what you landed with, and then deliver this to LEO using entirely propulsive tugs with no new technology, this isn’t very efficient. You end up spending a significant fraction of the lunar derived propellant lifting both the delivery propellant and the landing return propellant, as well as the propellant to ship the cis-lunar tanker back to LEO and bring it back for refueling near the Moon.
To give you an idea of how inefficient, I’m attaching a spreadsheet with some back-of-the-envelope level calculations to illustrate this point. In the spreadsheet I model a Lunar Surface to LLO or EML-2 and Back tanker, and then an LLO or EML-2 to LEO and Back tanker. In both cases, I assumed they were about Centaur size (~23tonnes), and used RL-10 based propulsion. For the reusable lunar surface tanker, I gave two propellant mass fractions–90% (aggressive once you factor in landing hardware) and 85% (more conservative). For the cislunar tanker, I assumed a 90% propellant mass fraction, and also analyzed cases where an aerobrake was provided that weighed 5% of the GTOW and 10% of the GTOW.
In the most extreme case of “all you need is ISRU” thinking, where you use entirely existing chemical propulsion systems for getting propellants from the lunar surface to LEO, only 9-11% of the propellant produced on the Moon actually makes it to LEO. Alternately, this means you have a “gear ratio” (ratio of propellant extracted on the Moon to propellant delivered to LEO) of 9-11. Not only is this very wasteful, but it means that you would need to size your ISRU capacity significantly higher than if you had a more efficient system.
Of the approximately 12km/s of round-trip Delta-V from the lunar surface to LEO and back, there are several options you can use to improve your gearing ratio, each of which attack a different leg of the journey:
- Stage and refuel in LLO or EML-1/2 (which was already assumed for this analysis).
- Aerocapture/braking to go from your Trans-Earth Injection trajectory into LEO
- Propellantless methods for launching from the lunar surface to LLO, EML-1 or 2, or even directly to LEO.
- Propellantless methods for landing on the Moon from LLO or EML-1 or 2
- Propellantless or high-Isp methods for traveling from LEO to LLO or EML-1 or 2.
This series is focused on options #3 and #4, though #2 is also low-hanging fruit (and provides about a 2-3x gear ratio improvement over the baseline “all we nee is ISRU approach).
Next up: Five Propellantless Lunar Transportation Approaches