Reusable Launch Vehicles and Lunar Return

Space enthusiasts and the media were thrilled when SpaceX returned Falcon 9’s first stage to its launch site for a soft landing. Their dream is that this event heralds a new age of cheap access to space, whereby reusable launch vehicles will continuously deliver various payloads to orbit, and in time, we will proceed to colonize the universe. Although this event was a significant achievement, there is good reason to question its long-term significance for cheap space access. Many unanswered questions remain about the true economics of Falcon 9 reusability, including the reliability of reused stages, the costs in money and time for refurbishing and preparation for re-launch, and the flexibility of manipulating manifests and schedules to make a workable space transportation system.

That said, there is something significant about the development of this capability relevant to the future use and development of cislunar space. With the Falcon 9 first stage recovery, SpaceX developed the capability to safely soft-land a throttleable, cryogenic engine system—a key technical development needed for the creation of a permanent space-based transportation system. Although there are differences between landing the Falcon 9 stage and a lunar soft-lander, if one can be done, so can the other. All lunar landers to date, both robotic and manned, have used storable propellants (usually hydrazine and nitrogen tetroxide) and then, after a single use, were discarded. To return to the Moon permanently, we must develop reusable propulsion systems that use the propellants that we are able to manufacture on the Moon (cryogenic liquid oxygen and liquid hydrogen and/or liquid methane).

A comparable technical project toward achieving these ends was completed two decades ago—the Department of Defense Delta Clipper (DC-X) project. This effort was part of the research program by the Strategic Defense Initiative Organization, whose efforts required developing reliable and routine access to space. The Delta Clipper used LOX-hydrogen rocket engines that could be throttled between 30 and 100 percent of their rated thrust. This particular vehicle was designed not to achieve orbit, but to prototype the various systems and technologies needed to build a single-stage-to-orbit (SSTO) launch vehicle in the future.

The DC-X launched vertically like any rocket—it was able to maneuver in attitude during flight, then re-orient to nose-first attitude for reentry, and soft-land vertically at launch site. The vehicle successfully flew eight times under the remote control of a human pilot. Astronaut Pete Conrad, who had previously conducted the first precision soft-landing on the Moon during the Apollo 12 mission in 1969, was one of the DC-X remote control pilots. The DC-X was a one-third-scale version of an actual SSTO. It only flew to an altitude of a couple thousand feet, but it certified the systems that would be needed later for a full-size DC-Y launch vehicle.

The idea of a SSTO launch system is as old as spaceflight itself and has been proposed in many guises over the years. Always it has been assumed that the development of such a vehicle would make space access routine and cheap. But in fact, flight through Earth’s atmosphere on both ends of a mission imposes significant stress on vehicle systems and thus, difficulties (read: costs) during preparation for reuse. The greatest value to be realized from a reusable cryogenic space vehicle would come from developing a version that is permanently based in space, one that is not subjected to the extreme thermal environments of Earth orbital re-entry.

Four years ago, Tony Lavoie and I developed an architecture whose aim was to establish a resource-processing outpost on the Moon. In order to maximize efficiency and minimize cost, we imagined a lunar soft-lander designed to 1) use the propellant that we planned to make on the Moon from lunar polar water; and 2) be permanently space-based, traveling only between low lunar orbit and the surface, and reusable for multiple trips (it was for this reason that we based our orbital node in low lunar orbit (100 km), where lunar single-stage-to-orbit is possible). This hypothetical lander would be able to throttle its engines for terminal descent, and be of modular construction for between-flight servicing and engine change-out on the Moon.

The efforts undertaken to softly land rocket stages by both SpaceX and Blue Origin are directly relevant to the system requirements of a reusable lunar lander. The SpaceX Falcon 9 first stage uses LOX-kerosene while the Blue Origin New Shepard is a LOX-hydrogen vehicle, so both use cryogenic propellant. Both vehicles are suborbital, with the Falcon 9 stage delivering a second stage and payload to an orbital path before return, while the New Shepard is designed to deliver a human spacecraft to suborbital “space tourism” journeys. The velocities and accelerations associated with such flights are coincident with those experienced in flights to and from lunar orbit. Unlike the DC-X, both vehicles have automated control systems and minimal interaction with ground crews during flight; this experience is relevant to our need for an automated lander designed to deliver cargo to the lunar surface.

A return to the Moon involves not only technology and vehicles but human capital as well. The current generation of space engineers have limited experience with deep space systems and none whatsoever with lunar landers. All lunar and planetary lander spacecraft developed to date use storable propellant and are discarded after one use. Developing a reusable cryogenic space vehicle will be challenging, but with these efforts, the necessary experience base is beginning to take shape. While the value of reusability in Earth-to-orbit systems is yet to be demonstrated, the technology and experience base used to make both the SpaceX and Blue Origin landings possible is directly relevant to future efforts on the Moon and in cislunar space. By developing such capabilities, we can build a permanent human cislunar transportation system—a space-based infrastructure that will enable both large-scale activity on the Moon and, in time, missions to the planets.

Paul D. Spudis | READ MORE

Paul D. Spudis (1952-2018) was a senior staff scientist at the Lunar and Planetary Institute in Houston, Texas, and a prolific author on the subject of the moon. His books include The Value of the Moon: How to Explore, Live, and Prosper in Space Using the Moon's Resources.