Just the fact that the doors were a mechanical element caused concern in the design phase. “All through the human spaceflight program, we had a lot of problems with mechanisms in orbit,” says Moser. “Closing the doors in Gemini [after a spacewalk] was a problem, as was the docking system in Apollo. Mechanical systems inherently cause problems.” Temperature swings occurring as the shuttle moved between sunlight and shadow, for example, caused surfaces to expand and contract, which interfered with the proper closing of a door or panel. The payload bay doors had to open reliably not just to release a payload, but more immediately upon reaching orbit to expose radiators that shed heat, cooling the vehicle. Worse, at the end of a mission, says Moser, “you couldn’t bring the vehicle back if the doors weren’t closed.” Aerodynamic forces would tear the aft bulkhead off the vehicle.
So engineers made the doors flexible, and pulled them shut with a series of latches along the edge of the bulkheads at each end of the payload bay. “We literally zip it closed,” says Moser. “If they were rigid, that would be a lot more difficult.” Once the doors were closed, four more latching mechanisms along the centerline, where the doors meet, secured them for reentry into the atmosphere.
The flexibility that solved one problem, however, caused others. For example, during flight, the shuttle endured a variety of “loads,” or external forces. Loads come from the atmosphere (aerodynamic loads), the weight of the vehicle and its payload configuration (inertial and structural loads), and even the noise of the engines (acoustic loads). Loads result in vibrations, oscillations, and a bending of the vehicle’s body. Wobbly payload bay doors would be of little use in strengthening the shuttle against breaking apart under these stresses. Unlike the fuselage of an airliner, which is fortified all the way around the circumference of its airframe with longerons (stiffeners that run lengthwise), the shuttle’s fuselage was a cylinder with the dorsal half removed—a convertible, so to speak. Longerons existed in the lower half of the fuselage only. These were made extra robust and, along with the vehicle’s skin attached to them, shouldered the flight loads for the whole fuselage.
Design of the crew cabin required further creative thinking. The cabin was a self-contained, pressurized vessel that sat in the forward fuselage. The only loads it carried came from its internal air pressure and its own weight during launch and landing. “It’s attached [to the interior of the fuselage] at discrete hard points,” says Moser. “It floats. No matter how the fuselage wants to bend or react, that cabin doesn’t enter into sharing the loads with the fuselage.” This design offered fewer chances for the pressurized area to leak in orbit, and simplified pressure testing of the cabin before launch. But because the crew compartment wasn’t built to the same rugged standard as the hull around it, Moser says its design refutes a possibility raised by the Challenger accident investigation: that after the orbiter broke apart, the crew cabin may have remained intact, with the astronauts possibly alive all the way to the water. “I don’t believe that for a heartbeat,” he says. “It was not designed for it. That thing could not have come out of the fuselage without ripping the cabin apart.”
The design principles that protected the crew cabin were also applied to the payloads the orbiters carried. As the vehicle encountered buffeting on the way to space and back, its exterior flexed independently around both payload and crew.
Inside, too, the orbiters were standardized, with a flight deck that sat four crew members and a mid-deck that could carry up to four more. (On only one mission, STS-61A in October 1985, did an orbiter—Challenger—carry eight astronauts; most carried seven.) Astronaut Jerry Ross had spent time in the U.S. Air Force as the chief test engineer for the B-1 bomber, also developed by Rockwell International (now part of Boeing) at about the same time the company built the space shuttle. Construction began on Columbia only six months before the B-1’s first flight. Ross recalls that the flight decks of the orbiter and the bomber had a striking commonality, with their side-by-side seating. “The cockpit arrangement, the instruments, the layout, everything—it was incredibly similar,” says Ross. “The B-1, I had 150 or 160 hours flying in it. And the first time I got into our simulators down here [in Houston], I went, ‘Holy cow.’ It was really quite amazing.”
Starting in 1998 with Atlantis, each orbiter received a major upgrade, the Multi-function Electronic Display System, a so-called glass cockpit that replaced dozens of dials and gauges with 11 flat-panel, liquid-crystal color displays. The new cockpit weighed less, used less power, and reduced glare in bright sunlight, all while offering better views of data from a wide range of angles.
Ross was the first person to be launched into space seven times, all on the shuttle. He flew five missions on Atlantis. “The thing about Atlantis is that to me it seemed to have a little bit stronger buffeting [than the other orbiters] during the transonic Mach, right around Mach 1, coming back in to land,” he says. “It felt like you had run off the tracks and were riding on the railroad ties. It was just a real rough ride for just a little bit.” He felt some of this on Columbia and Endeavour, but it was more pronounced on Atlantis, he says. The crew brought it up in debriefings, but it always remained a mystery.
Franklin Chang-Dìaz, the only other seven-time shuttle flier, flew on every orbiter but Challenger. “Columbia tended to rattle a bit more on liftoff,” he recalls. “Structural integrity was always on my mind, particularly on ascent and reentry.” After years of training as a mechanical engineer with a Ph.D. in applied plasma physics, he couldn’t help thinking about the extreme heating and cooling the orbiter tolerated as it passed every 90 minutes from day to night and back to day. “It reminded me of the way one makes a steel wire break by twisting it until it fatigues away,” he says. “But at the end of the day, I would walk away in awe at the machine that kept me warm and cozy and safe so many times in such an inhospitable place.” After all, each orbiter was designed to operate for 100 missions.
Was there anything he would have changed about the orbiter? “The idea of sending crew and cargo together has never been my favorite,” says Chang-Dìaz. “I would have concentrated on a solely human vehicle and sent the cargo in a robotic craft. But this is all 20/20 hindsight, and considering the compromises that every major space program has had to accommodate, the end result was a technological achievement of the first order.”