The Apollo program used a lunar orbit rendezvous plan to reduce the mass of the overall payload required for each lunar mission. However, the spacecraft used for the Apollo missions all used low-performance hypergolic fuels for their propulsion maneuvers after Trans Lunar Injection, a design choice that increases the value of staging propellant in low lunar orbit.
These advantages diminish sharply, however, if higher performing propellants such as liquid oxygen-hydrogen (LOx-hydrogen) or liquid oxygen-methane (LOx-methane) are used for propulsion after Trans Lunar Injection.
Moreover, if the objective is to establish a permanent lunar base, and not just perform sorties to the Moon, then the production of lunar oxygen is feasible, and because of the numerous advantages it offers, it must be an early base priority.
Once lunar oxygen is available, Direct Return missions can be accomplished with a lower launch mass than Lunar Orbit Rendezvous .
In addition to these mass advantages, substituting Direct Return for Lunar Orbit Rendezvous enhances the overall safety of the program. In the Direct Return mode, the launch window back to Earth is always open. No waiting for phasing is required. The risk of a failed Lunar Orbit Rendezvous maneuver, which would cause loss of crew, is also eliminated.
Lunar orbits are unstable, and if the lunar-orbiting spacecraft is not used within its allotted time, it will be lost. The Lunar Orbit Rendezvous mission plan also requires unattractive choices regarding the maintenance of the orbital spacecraft.
If a decision is made to keep someone aboard the orbiting spacecraft, as was done in Apollo, that person is being subjected to flight risk, zero gravity health impacts and a radiation dose. And there is no matching addition of surface exploration capability. Alternatively, the orbital spacecraft could be left unmanned. In that scenario, it could be lost if a problem needing correction develops while it is alone. So Direct Return mission mode is clearly preferable for a Moon base program.
Developing a Lunar base also will require delivering substantial cargo one-way to the lunar surface. Here the Direct Return scenario has a large advantage over Lunar Orbit Rendezvous because the same large standard lander that is used to deliver the fully fueled Crew Exploration Vehicle also can be used to deliver substantial 23 ton habitat modules or other large cargo elements to the surface of the Moon.
Thus, starting from the very first mission, substantial facilities can be available to the crew on the Moon, enhancing their scientific capabilities and safety, and enabling longer and thus more cost-effective surface stays. In contrast, the Lunar Orbit Rendezvous system must use the Lunar Surface and Ascent Module as its lander, and its 9 ton cargo capability is much less.
Once in situ propellant production is introduced, both plans also can deliver significant cargo on every piloted flight (neither can before in situ propellant production becomes available), but here again, the delivery capability of the Direct Return plan is about triple that of the lunar orbit rendezvous hardware.
Lunar oxygen can be produced by chemically breaking down the iron oxides that comprise about 10 percent of typical lunar regolith. Some scientists believe that small quantities of water ice may exist in permanently shadowed regions near the Moon’s poles, and if this can be accessed, both hydrogen and oxygen could be produced. However since oxygen comprises 86 percent of the propellant used by a LOx-hydrogen rocket, or 78 percent of the propellant used by a LOx-methane rocket, the lion’s share of the benefits of lunar oxygen production can be obtained even if polar ice does not exist. In the plan presented here, we have assumed this less favorable scenario, with only oxygen being produced on the Moon, while the fuel required to burn with it is brought from Earth.
Achieving Long-Range Mobility
In situ propellant production also would make it possible to take long trips from one part of the M oon to another. Because it has no roads or atmosphere, true long-distance mobility on the Moon can only be achieved using rocket powered ballistic flight vehicles. A ballistic hopper with a dry mass of 5 ton s and a methane/oxygen fuel system, would need about 15 metric ton s of propellant to send it on a flight of 1,000 kilometers, land, takeoff again, and return.
But of those 15 metric tons, 12.7 would be oxygen, which can be made on the Moon, while only 3.3 would be methane fuel that has to be transported from Earth.
The standard lander I have in mind for the Direct Return missions can deliver 23 metric tons of cargo per mission. If 20 metric tons of this were methane, that would enable six such long-range excursions to be conducted from the lunar base for every launch from Earth.
If hydrogen/oxygen propulsion were used on the hopper, 10 such distant sites could be visited for every cargo flight. This compares quite handsomely to the Quadruple Launch Quadruple Rendezvous scenario, where four launches from Earth and four spacecraft rendezvous and docking maneuvers are required to visit one site on the Moon.
The use of lunar oxygen combined with a Direct Return mission architecture leads to an order of magnitude improvement in overall Moon base cost-effectiveness, enabling well-equipped long-duration surface stays, with many diverse sites visited during each mission.
Furthermore, because it minimizes the number of critical operations, the single-launch Direct Return plan is much lower risk than the Quadruple Launch Quadruple Rendezvous plan, and significantly lower than the single launch Lunar Orbit Rendezvous architecture, both of which provide much less capability.
In addition, the Direct Return mission offers lower development cost and lower recurring cost than either alternative plan, because while it must develop a 120 metric ton heavy launch vehicle to low Earth orbit (as compared to a 90 ton launcher needed if Lunar Orbit Rendezvous is employed) , the need to develop a complete manned spaceflight system, the Lunar Surface and Ascent Module , is avoided, and the need to expend such a system on each lunar mission is eliminated as well.
Trading off the Lunar Surface and Ascent Module development costs to create a heavy launch vehicle is a very good deal, because the heavy launcher needs to be developed to enable human Mars missions in any case. It also can be used to support any number of other national goals (including a truly muscular Moon base program), while the Lunar Surface and Ascent Module is only useful for supporting a severely constrained form of lunar exploration.
The Quadruple Launch Quadruple Rendezvous plan would leave us with a set of inefficient, costly and failure-ridden missions that lead nowhere, while the single-launch Direct Return option would provide us with a robust and cost-effective lunar program that develops a significant fraction of the hardware set that takes us to Mars and beyond.
Next Week: Evolution to Mars
Robert Zubrin, an astronautical engineer, is president of the Mars Society, and the author of the books The Case for Mars, Entering Space, and Mars on Earth. His technical paper can be found at www.space.com/media/pdf/zubrinpart2.pdf