Meet the Orbiters
A fleet of winged spacecraft, the likes of which we'll never see again.
- By Michael Klesius
- AirSpaceMag.com, March 01, 2011
(Page 2 of 5)
Despite the advances needed for such a revolutionary vehicle, the airframe was built from a conventional material—aluminum. More advanced metals had been used in high-speed vehicles— in the 1960s, the Mach 3 Lockheed SR-71 spyplane, for example, was built of titanium. But during a shuttle reentry, starting at about eight times the top speed of an SR-71, temperatures would quickly exceed what even titanium could tolerate. So its only advantage, compared to aluminum, was slightly lower weight. Once the shuttle was coated with ceramic and carbon-carbon, says Moser, a titanium frame’s weight savings was negligible, while its cost—2.5 times more than aluminum—wasn’t. Add to that the difficulty of working with titanium, which could be so hard under certain conditions that tooling couldn’t drill through it properly, and it looked even less attractive. Moser recalls the day that he and other structural engineers visited Kelly Johnson’s Lockheed Skunk Works in Palmdale, California, where the SR-71 was built. “At the end of a full day of reviewing the pros and cons of titanium, we asked Johnson’s people: If they were us, would they design the orbiter using aluminum or titanium? They answered, ‘Aluminum.’ ”
Shuttle engineers were creating structures that had never gone to space. “The payload bay doors are one of the more interesting structural components on the vehicle,” says Dennis Jenkins, a contract engineer at the Kennedy Space Center in Florida who has worked on the shuttle for more than three decades and who wrote a definitive reference work on the spacecraft. “They can’t even survive in 1 G. They’re extremely fragile—just one layer of Nomex honeycomb composite [a graphite epoxy material]. If you ever look at pictures of it in the Orbiter Processing Facility, you’ll see great big yellow metal frames on the doors, and that’s because we can’t open the doors down here without those. The doors just bend and twist too much, so we have to strengthen them on the ground.” (Jenkins is updating Space Shuttle: The History of the National Space Transportation System to include the program’s final missions.) “[The doors were] actually one of the first aerospace uses of complete composite construction,” he says.
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.”