6 July 2011—For 30 years, the space shuttle fleet—Columbia, Challenger, Discovery, Atlantis, and Endeavour—strutted its stuff in low-Earth orbit. The spacecraft's missions included simple payload deployments, science module sorties, and the delicate assembly and servicing of the International Space Station. They were also used for in-flight repairs to themselves and to other satellites, hyperprecision orbits for radar mapping, tethered experiments, and gentle close-in maneuvering with smaller spacecraft. Those capabilities were originally unimagined by their designers, and they firmly refute the old maxim that "form follows function." Indeed, the shuttles performed functions beyond the dreams of their builders.
The current stable of heavy-launch vehicles can carry deployable satellites and rocket stages as big as or bigger than any that the shuttles have ever launched. With replacement vehicles already being designed for specific manned missions, such as Earth-to-orbit taxi services or for beyond-low-Earth-orbit sorties, the biggest engineering questions must be these: What operational capabilities are we giving up by retiring the shuttles? And are we sure we can dispense with them? Because if the answers are "Too many" and "No!" we need to start planning how to regain them with new vehicles.
Here's what we're losing.
Lost capability No. 1: Gentle delivery of large modules for attachment to existing complexes. Compared with other means, the shuttle provides an environment in its payload bay with relatively minimal acceleration, vibration, and noise, and that means very large components can be built a lot less expensively. The savings comes from several sources. The shuttle's own hardware delivers cargo close to its destination, after which robot manipulators can install it carefully. Without this capability, the items would have to be built with structural enhancements to survive the attendant stresses, making them significantly heavier. What's worse, without the shuttle, the module might need its own maneuvering capability—resulting in significant mechanical stress as the add-on connects to the existing structure. Such stress leads to design headaches: For example, the size of any connecting pressurized tunnels must be restricted. Furthermore, in the event of a mishap, the shuttle design is supposed to allow for intact retrieval of the payload for relaunch, mitigating the need to build expensive backup hardware.
Lost capability No. 2: Bringing cargo down gently. The shuttle payload bay can carry specialized laboratory modules into orbit and then back to Earth for reuse. It can also retrieve and return large spacecraft and their components for repair or redeployment. Entry stresses do not exceed 1.5 g's, and to demonstrate that, several astronauts have remained standing throughout most of the descent. Without the shuttle, the scale of returnable objects is greatly limited, and the stress and shock of descent is severe. To replace this large-scale capability, NASA might have to develop inflatable heat shields that could be scaled up in size as needed—but even with such shields, cargo would still be subject to significant entry and landing stresses.
Photo: Nikolai Budarin/Russian Space Research Institute/NASA
Lost capability No. 3: Safe "proximity operations." The length of the shuttle allowed use of nose- and tail-mounted thrusters to provide extremely gentle maneuvering, bringing the craft right up to such small targets as "round-trip" satellites and orbital instruments in need of repair. Once such objects were over the payload bay, the shuttle switched to a control mode called z axis. In this mode, the target object was mostly out of the way of the forward and aft thruster plumes, allowing the shuttle to maneuver without pushing the target around or contaminating it with propellant. An even gentler mode called low-z-axis worked by firing counterbalanced forward-pointing nose thrusters and aft-pointing tail thrusters that are slightly canted above the horizontal. Though this maneuver looked bizarre, low-z-axis mode was one of those lucky accidents of the original shuttle design that proved really useful. No other vehicle ever built or designed had this specialized "gentle approach" capability, which was critical to a number of satellite retrievals and repairs, including the Hubble Space Telescope missions. Any other vehicle would have seriously damaged such rendezvous targets.
Lost capability No. 4: Temporary deployment of a workbench in orbit for experiments, repairs, and other assembly. The boxcar-size shuttle payload bay has been the stage for delicate repairs to satellites such as Hubble. It has also been used in the following capacities: for attaching new rocket stages to stranded satellites, for test deployments of solar panels and girders (which were later upgraded to become the backbone of the space station), as a base for deployment of payloads with 20-kilometer-long tethers, and for special-purpose space station assembly and maintenance. Repeated two-person (or once, even three-person) spacewalks gave extended "hands-on" capabilities and allowed components to be readily transferred from exterior to interior work areas and back. The shuttle's size provided flexibility in the complement of tools and spare parts you could carry into orbit, and it provided external utility power and communications that no Apollo-, Orion-, or Soyuz-class manned vehicle could ever dream of.
Lost capability No. 5: High-precision research orbits with specialized instrumentation. Several special-purpose shuttle missions required "threading the needle" in space with observational equipment that mandated incredibly accurate physical positioning. For instance, ground-mapping radar missions needed to be navigated so precisely that data from multiple missions could be overlaid as if they had been acquired by several shuttles flying simultaneously in formation. Trajectory disturbances of all types had to be counterbalanced by continual course corrections using very gentle thruster firings.
Lost capability No. 6: Flexibility of crew composition. Carrying six or seven (or once, even eight) people into orbit allows three or four career astronauts to host visits from real scientists active in their fields. Some professional astronauts are former scientists, but they must spend up to 10 years away from their labs to prepare to fly. Smaller past and future vehicles are limited to highly specialized professional crew members who, though very talented, are frankly often out of touch with advanced research or other specialized skills. A seven-person crew could even have room for occasional VIPs—politicians, teachers, or even journalists.
Many of these capabilities were expensive, and the whole program wound up costing a lot more than had been projected. Worse, when operated carelessly, the machine killed two crews. But the shuttle's capabilities were often far more valuable than expected, with many surprising uses that only became clear over the years.
That last point leads to perhaps the greatest lesson of the shuttle for future spaceship designers and space exploration theorists: If you build a spacecraft, or any other machine, with a predetermined and limited set of capabilities (as NASA is now doing), you will usually get just those predictable capabilities and little more. You will not, as happened with the shuttle, learn to use it more and more efficiently and keep discovering new ways to do new things not even imagined when the vehicle was first conceived. These capabilities, in the case of the shuttle, turned out to be the only way to respond to many unexpected problems. And nobody should be surprised that the unexpected awaits us in outer space.
Space vehicles with these next-generation designs are sure to face both unanticipated challenges and opportunities. Until we realize that some out-of-the-blue, unplanned need cannot be satisfied, we won't even know what we're missing with these new designs. As for the shuttle, we are just starting to recognize the full extent of the capabilities we gained and are now going to lose—and we'd better start thinking of how to replace them. If we do that, we can wisely build future spacecraft that will allow us to be ready when we are inevitably caught by surprise out there in space.
Sent from my iPad
No comments:
Post a Comment