|
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
tonnes or more were considered acceptable, since the OSP was only
going to orbit, and launch capabilities to deliver such mass to LEO
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 (LOR),
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 untended
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 is also 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 boiloff 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 (HLV) NASA chooses for
development and the allowable mass of the CEV. The fastest route to
creating a HLV 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 HLVs
are possible, with LEO delivery capabilities ranging from 70 to 130
tonnes, 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 roughly 100 tonnes
to LEO lift capability. 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 (TLI) and Lunar Orbit Capture, and an
hydrogen/oxygen propelled lander, a system that launches 100 tonnes
to LEO would also be able to deliver 20 tones of payload to the Lunar
surface. If direct return is to be used, this 20 tonnes must include
the CEV plus its ascent stage for flight back to Earth. Using
hydrogen/oxygen propulsion for the ascent stage, an 8.6 tonne CEV
could be thus delivered round trip to the Moon. 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
tonnes.
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 tonnes.
Thus a lightweight, Apollo capsule derived 3-4 person CEV would allow
a direct return lunar mission with a single launch, but a heavy 5-6
person OSP clone would not. If the heavy OSP clone is chosen, then
development of a Lunar 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 HLV 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 HLV, then the
only additional hardware elements needed to begin lunar exploration
are the TLI/LOC stage and the lander. The same lander used to deliver
the CEV and its ascent stage could also deliver heavy cargo such as a
20 tonne habitation module (ISS modules weigh 20 tonnes), making long
duration lunar surface stays possible right from the start of the
program.
But the small CEV not only cheapens and accelerates the Lunar
program, it cheapens and accelerates the CEV program itself. The
funds saved by reducing the size and cost of the CEV could be used to
start HLV development immediately, which would save further funds,
since early deployment of the HLV would allow space station
construction to be completed sooner, allowing early retirement of the
$4 billion per year Space Shuttle.
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 will 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 US 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 HLV. Together with other savings obtained by
canceling useless programs such as the Hubble deorbit module, these
funds should be sufficient to pay for the entire HLV 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 HLV program immediately, and together the CEV and HLV would allow
early retirement of the Space Shuttle, with massive savings to the
taxpayer resulting.
Furthermore, with a CEV matched to an HLV for direct lunar missions
in hand, and STS retired or nearly so, outgoing NASA Administrator
Griffin would be able to say to the President elect in January
2009: "We have 80% 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.
Dr. 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).
**** ****
An in-depth discussion of strategies to get the Moon-Mars initiative
off the ground will be held at the 8th international Mars Society
convention, University of Colorado Boulder, August 11-14, 2005.
registration is now open at www.marssociety.org
|
