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First of three Commentaries
President Bush has called on NASA to implement a human lunar exploration program with the objective of both supporting operations on the Moon and developing the technologies to enable piloted Mars missions. The question is: how should this be done? Three central issues that need to be addressed are launch strategy, l unar mission mode and method of evolution from Moon to Mars exploration capabilities.
With respect to the launch issue, the key question is whether or not we need a heavy-lift launch vehicle.
Currently, those opposed to such development have advanced an argument for a quadruple launch, quadruple rendezvous mission architecture employing medium-lift launch vehicles. As the success or failure of the program depends upon the practicality of its launch strategy, this concept needs to be carefully scrutinized.
Quadruple Launch, Rendezvous
In the quadruple rendezvous mission plan, a Crew Excursion Vehicle (CEV) is launched into orbit where it rendezvous with an Earth d eparture s tage capable of delivering it to l ow l unar o rbit.
Separately , a l unar s urface and a scent m odule and another Earth d eparture s tage are launched. They also rendezvous and head for l ow l unar o rbit.
Once in lunar orbit, the CEV rendezvous with the l unar s urface a scent m odule so the crew can take it down to the lunar surface. Once their excursion on the M oon is over, they use their ascent module to rendezvous with the CEV in low l unar orbit. They can then take the CEV all the way back to Earth’s surface.
If we choose liquid oxygen/hydrogen propulsion for the Earth d eparture s tage and the l unar s urface a scent m odule descent stage and then use a space-storable propellant of liquid oxygen/methane for the trans-Earth injection and lunar ascent stage, the mass that has to launch from Earth is 12 metric tons for the CEV (including the t rans-Earth i njection stage), 15 metric tons for the lunar module (including ascent and descent stages), 27 metric tons for the Earth d eparture s tage used by the CEV and 33 metric tons for the departure stage used for the lunar module.
So, quadruple mission could indeed be launched by two medium-sized launch vehicles capable of lifting 30 metric tons to low Earth orbit and two medium launch vehicles capable of launching 15 metric tons to low Earth orbit.
However, packaging concerns have been ignored in this scenario, and it is not clear that the small-launch fairing of a 15 ton to low Earth orbit medium-launch vehicle would be sufficient for the lunar module , so it is possible a bigger medium-launch vehicle might be required. But four medium-launch vehicles are required for each mission.
The above four launches must be done quickly, since the Earth departure and lunar surface ascent vehicles are carrying cryogenic liquid oxygen /H2 hydrogen, and the piloted CEV is launched last. In the quadruple scenario, the crew also flies to the Moon apart from the lunar module. These features are all causes for great concern.
Using multiple m edium-l aunch v ehicles to launch a heavy-lift payload is not cost effective.
It is a well-known feature of launch-vehicle economics that larger boosters are more economic than smaller boosters. Thus, by dividing the launch mass into four parts, the overall launch costs per mission roughly doubles.
The quadruple scenario requires four launches of m edium l aunch v ehicles within a very short period of just a few weeks. Three of those launches involve cryogenic upper stages and the fourth involves a piloted vehicle, all launched from Cape Canaveral. Such a launch rate has never been accomplished by m edium l aunch v ehicles with any payload, and to assume that it can be done repeatedly with payloads of this complexity is wildly optimistic.
In contrast to the Apollo mission plan, which only required one launch and a single rendezvous, the quadruple plan requires four mission-critical rendezvous and four launches to all occur successfully. That’s eight big chances per mission (in addition to l unar landing and ascent) for an operational failure that would cause loss of mission.
The mission would also fail if a launch delay caused any of the three launches after the first launch to stall too long for the cryogenic propellant aboard the initial orbiting payload to last until trans-lunar injection , or if a ny of the four orbiting payloads were to take an orbital debris hit while waiting in low Earth orbit for trans-lunar injection.
The mission also fails if any of the four spacecraft malfunction, or if either of the two trans-lunar injection or two lunar orbit capture burns should fail, or if any of the four orbital rendezvous operations should fail, to name just a few additional sources of potential mission failure that multiply in proportion the number of flight elements and critical operations.
This mission architecture is supposed to support not a single l unar mission, but routine, repeated access to the Moon. Inserting so much complexity and vulnerability into such a transportation system is an open invitation to program failure.
In fact, an elementary calculation using very optimistic assumptions (presented in detail at www.spacenews.com ) shows that, at best, the the quadruple plan using m edium l ift v ehicles might obtain a mission reliability of about 0.75. This means that roughly one out of every four missions could be expected to fail. If three missions are flown per year, there would, on average, be mission failure roughly every 1.3 years. Assuming a typical suspension of operations of two years after each mission failure, the program would need to be shut down for failure investigations at least 60 percent of the time.
This is not a good way to design a program.
Apollo traveled to the Moon with the l unar e xcursion m odule attached to the c ommand m odule. The availability of the lunar excursion module during transit proved essential to saving the lives of the Apollo 13 crew. The quadruple plan lacks this important safety feature.
The reason the quadruple mission scenario has such low reliability is because of the incredible proliferation of critical events that occurs if four launches, four rendezvous and four spacecraft are required for each mission. The way to solve this problem is simple: develop a heavy-lift vehicle that allows the entire mission to be launched with a single booster, just as was done during Apollo.
This will cut program launch costs in half, and reduce the risk of mission failure by a factor of four. It also creates and exercises a system that is directly useful to enable human Mars exploration, which is the primary purpose of the l unar program as stated in the p resident’s directive.
Some people within the aerospace establishment understand that the development of a heavy-lift vehicle is essential for a successful l unar program, but wish to postpone consideration of the issue for political reasons.
This is very unfortunate. One of the cheapest options to create a heavy-lift launch vehicle is by converting the s huttle. The s huttle launch stack has the same takeoff thrust as a Saturn 5 , and if we delete the orbiter and add a hydrogen/oxygen upper stage, we can create a launch vehicle with similar capability.
However, under NASA’s current plans, only about 25 more s huttle launches are contemplated, and absent a plan for s huttle conversion to a heavy-lift launch vehicle , much of the industrial infrastructure for manufacturing key s huttle-system components (such as external tanks) will soon be dismantled. Recreating such capabilities after they have been lost will cost the taxpayers billions of dollars.
If such massive waste is to be avoided, NASA needs to make the case for heavy lift immediately.
Next article: The Question of Mission Mode
PRIVATE tabstops:<*t(0.0000,0,” “,327.2500,0,” “)> Robert Zubrin, an astronautical engineer, is p resident of the Mars Society, and the author of the books The Case for Mars, Entering Space, and Mars on Earth. Mr. Zubrin’s technical paper can be found at www.space.com/spacenews/pdf/zubrin.pdf.