There is a strong case to be made for downsizing the Crew Exploration Vehicle (CEV) into a much smaller, cheaper and lighter vehicle than the Orbital Space Plane (OSP) derivatives currently under widespread discussion.
The OSP was conceived of as a means of servicing the crew rotations of the international space station at lower cost and lower risk than the space shuttle. It was thus specified that it be able to carry a crew of at least five, to approach the shuttle’s crew-ferrying capability. To meet this goal, vehicle masses on the order of 12 metric ton s or more were considered acceptable, since the OSP was only going to orbit, and launch capabilities to deliver such mass to low Earth orbit are readily obtainable.
However, now NASA’s mission has changed, and instead of perpetual flights to orbit we are reaching for the Moon and Mars, and the question must be asked whether such a large crew-carrying vehicle really is optimal to support these new goals.
In fact, it is not.
The simplest, safest, least-expensive and most-capable lunar base transportation system is one based upon direct launch to the lunar surface, and direct return with no Lunar Orbit Rendezvous , using a single launch vehicle. This is so because the direct return architecture requires the least number of vehicle elements to develop, expends the fewest hardware elements per flight, has the fewest necessary operations per mission, avoids the need for unintended mission-critical liabilities in lunar orbit, always has its return launch window to Earth open and also has the lowest recurring mission launch mass once lunar oxygen production commences at the base.
Doing each mission with one launch also is extremely important, because a multiple launch mission architecture not only costs more, it greatly increases mission risk. Indeed, a multi- launch lunar mission will fail not only if any one of its several launches is lost, but also if weather or other reasons should cause any launch after the first to be delayed beyond the boil-off endurance of any of the cryogenic flight elements launched earlier.
This being the case, there is a direct relationship between the capability of the heavy- lift vehicle NASA chooses to develop and the allowable mass of the CEV. The fastest route to creating a heavy lifter at this point is by reconfiguring the hardware of the space shuttle stack, deleting the orbiter and replacing it with a fairing and an upper stage.
A variety of such shuttle-derived heavy-lift vehicles are possible, with delivery capabilities to low Earth orbit ranging from 70 to 130 ton s, with the more capable versions costing more to develop. Indications are that NASA has decided to develop such a vehicle, with the preferred variant in the mid range, offering lift capability of roughly 100 ton s to low Earth orbit. This would be a very reasonable choice.
If that is the decision made, then the math that determines acceptable CEV mass follows directly. Using a hydrogen/oxygen stage for Trans-Lunar Injection and Lunar Orbit Capture, and an hydrogen/oxygen propelled lander, a system that launches 100 ton s to low Earth orbit would also be able to deliver 20 tons of payload to the lunar surface.
If direct return is to be used, this 20 ton s must include the CEV plus its ascent stage for the flight back to Earth. Using hydrogen/oxygen propulsion for the ascent stage, an 8.6 ton CEV could make the round trip to the Moon and back. If instead, for superior long-term storability, methane/oxygen propulsion is chosen for ascent, then the CEV capsule would have to be limited to 7.4 ton s.
Such lightweight CEV capsules are certainly possible. For example, the Apollo capsule, which transported three people to Lunar orbit and back, had a mass of about 6 ton s.
Thus a lightweight, Apollo capsule-derived three-to-four person CEV would allow a direct return lunar mission with a single launch, but a heavy five-to-six person Orbital Space Plane clone would not. If the heavy Orbital Space Plane clone is chosen, then development of a l unar transportation system would require either development of a second-generation super heavy-lift booster, an entire lunar excursion module manned spacecraft system, or implementation of a costly, complex and failure-prone multi-launch mission architecture.
In short, developing a CEV that is too heavy for the heavy-lift vehicle to launch to the Moon and direct return back would be a huge mistake. If the CEV matches the direct return mission capability of the heavy lifter , then the only additional hardware elements needed to begin lunar exploration are the Trans-L unar I njection Lunar Orbit Capture stage and the lander. The same lander used to deliver the CEV and its ascent stage also could deliver heavy cargo such as a 20-ton habitation module ( space station modules weigh 20 ton s), making long-duration lunar-surface stays possible right from the start of the program.
But the small CEV not only lowers the cost of and accelerates the lunar program, it lowers the cost and accelerates the CEV program itself. The funds saved by reducing the size and cost of the CEV could be used to start heavy-lift launcher development immediately, which would save further funds, since early deployment of the heavy lifter would allow space station construction to be completed sooner, allowing early retirement of the $4 billion per year space shuttle program.
By reducing the size of the CEV to close derivative of the Apollo capsule, the CEV program could be turned from an extended developmental contractor banquet into a production procurement. With development minimized, NASA could compete a contract of the following form: The winner of this contract would be paid $300 million each for five CEVs if they are delivered in 2008, plus $200 million each for five CEVs delivered in 2009, and $100 million each for five CEVs delivered per year starting in 2010 through 2015.
Such a contract form would provide a strong incentive for early delivery of the CEV, thereby allowing early retirement of the space shuttle without any discontinuity of U.S. human spaceflight capability. Furthermore, it would eliminate nearly all NASA expenditure on the CEV program during 2006 and 2007, allowing these funds to be reprogrammed for immediate development of the heavy launch vehicle . Together with other savings obtained by canceling useless programs such as the Hubble de orbit module, these funds should be sufficient to pay for the entire heavy launch vehicle development.
So to summarize, the choice of small CEV enables an optimum single-launch, direct-return, lunar mission architecture. It also enables a reduced cost, accelerated commercial procurement of the CEV itself. The savings in the CEV program thus obtained can be used to launch the heavy launch vehicle program immediately, and together the CEV and heavy lift would allow early retirement of the space shuttle, with massive savings to the taxpayer resulting.
Furthermore, with a CEV matched to a heavy-lift launch vehicle for direct lunar missions in hand, and shuttle system retired or nearly so, outgoing NASA Administrator Griffin would be able to say to the president-elect in January 2009: “We have 80 percent of the hardware needed for human lunar missions already developed, and have freed the funds required to develop the rest. If you choose to go forward with flat funding, we can have humans on the Moon by 2012, and Mars by 2016, by the end of your second term. The choice is yours.”
It’s a winning pitch.
Robert Zubrin, an astronautical engineer, is president of the Mars Society and author of The Case for Mars (Simon and Schuster 1996), Entering Space (Tarcher Putnam 1999) and Mars on Earth (Tarcher Penguin 2003).