5. From Earth to Orbit
Despite the fire and thunder of liftoff and the enormous power required, getting into space is in some ways the easiest part of the Shuttle’s journey. It follows a familiar pattern tested by earlier manned flights and hundreds of unmanned ones: simply dropping off parts of the vehicle, as they run out of fuel, while the rest continues into orbit.
The first to go are the Solid Rocket Boosters.
Standing 45.5 meters from nozzle to nose, and 3.7 meters in diameter (150 ft by 12 ft), the boosters are attached near their ends to the External Tank, slightly taller and twice as fat, which in turn is attached to the Orbiter. A Shuttle booster is the largest solid-fuel rocket ever flown, the first built for use on a manned spacecraft, and the first designed for reuse. It is assembled from seamless segments of half-inch steel, lined with heavy insulation, that are filled with propellant at the manufacturing site in Utah and shipped on railway flat cars to the Kennedy Space Center for assembly or, for south-north flights, Vandenberg Air Force Base north of Los Angeles.
The propellant looks and feels like the hard rubber of a typewriter eraser. It is a mixture of aluminum powder as fuel, aluminum perchlorate powder as an oxidizer, a dash of iron oxide as a catalyst to speed the burning rate, and a polymer binder that also serves as a fuel. It is not sensitive to ignition by static, friction, or impact; and it will not detonate during storage. The case segments are loaded from a single lot of raw materials to minimize any thrust imbalance between the pair of boosters used for a given Shuttle flight.
For launch, the propellant—500,000 kilograms (1,100,000 lb) in each booster—is ignited by a small rocket motor. Flame spreads over the exposed face of the propellant in about 0.15 second, and the motor is up to full operating pressure in less than half a second. As the propellant burns, at a temperature of about 3200° C, huge quantities of hot gasses speed through the nozzle, which restricts their flow and increases the pressure, producing thrust as they spew from the exit cone. The two boosters’ thrust of 5 200000 pounds augments the 1,125,000-pound thrust of the Orbiter’s three main engines through the first two minutes of ascent. The propellant is shaped to reduce the thrust briefly by about a third at 62 seconds into the flight, to prevent overstressing the Shuttle vehicle during the critical transonic period of maximum dynamic pressure.
The nozzles, each 3.76 meters (12 ft) in diameter at its opening, can be swiveled hydraulically up to 6.65 degrees on command by the Orbiter’s guidance computer to control the direction of thrust. With similar swiveling of the Orbiter main engines, this steers the entire Shuttle vehicle. The outside of the boosters is insulated against the heat of air friction and the blast of the Orbiter engines at separation with an ablative material that burns away in temperatures that reach 1260° C.
After burning out, the Solid Rocket Boosters are cut loose from the External Tank by electrically fired explosive devices and are moved away by small rocket separation motors, four near the nose of each and four aft, fired by command from the Orbiter. The spent boosters coast upward and then fall Earthward for almost four minutes, reaching a speed of 4650 kilometers an hour (2900 mph) before being slowed by atmospheric drag. From about 4.7 kilometers (3 mi) each is lowered by a succession of parachutes, the three mains 35 meters in diameter (115 ft), deployed from the nose on signal from a barometric-pressure switch, to a splashdown of about 95 kilometers an hour (60 mph).
Since the empty rocket enters the water with the nozzle down, air is trapped in the upper end to float it upright until one of two recovery vessels, summoned by a radio beacon and flashing light, attaches lines to tow it back to the launch center. There the booster is taken apart and the rocket segments are shipped to the Utah factory, where they are cleaned out, inspected for cracks, pressure-tested, relined, reloaded, and reshipped to the site. When the rocket throat and nozzle also have been relined with ablative insulation, the parachutes washed and repacked, and other parts refurbished or replaced, the booster is reassembled to fly again. The main structure, directional controls, and electrical system are planned for twenty flights, the recovery system for ten.
The second element of the Shuttle that is discarded during ascent to orbit, and the only major part not used again, is the External Tank. As tall (46.8 meters) as a fifteen-story building and as big (8.4 meters in diameter) as a farm silo, the tank contains the liquid hydrogen and liquid oxygen that fuel the Shuttle’s three main engines in the stern of the Orbiter, and forms the backbone of the entire vehicle during launch. The tanks are built in a former Saturn plant near New Orleans and shipped by barge to the launch sites, those for the West Coast going through the Panama Canal.
Made of aluminum alloy up to 5.23 centimeters (2 in.) thick, the External Tank is actually two propellant tanks connected by a cylindrical collar that houses control equipment. The nose curves to a point tipped by a lightning rod. The forward tank is loaded with 529 900 liters (140,000 gallons) of liquid oxygen, chilled to minus 147.2° C, weighing 603 983 kilograms (1,330,000 lb). The one forming the aft section, two and half times larger, contains 1,438 300 liters (380,000 gallons) of liquid hydrogen at minus 251° C. This weighs only 101,503 kilograms (223,000 lb) because liquid hydrogen is sixteen times lighter.
The tank’s outside skin is insulated with spray-on polyurethane foam that reduces heat transfer into the tanks that could cause excessive boiling of the propellants. It also helps prevent the buildup during launch preparations of ice that could shake loose in flight and damage the Orbiter. An ablating material that chars away protects the tank’s bulges and projections from friction heating during ascent through the atmosphere.
Horizontal baffles in the oxygen tank prevent sloshing that could throw the vehicle out of control, and anti-vortex baffles like fan blades in both tanks prevent the formation of whirlpools that could let gasses, rather than liquid propellants, into the 43.18-centimeter (17-inch) pipes chat carry 242,000 liters (64,000 gallons) a minute to the engines. Propellants are fed to the engine pumps by the pressure of gasses formed by controlled boiling in the tanks and, during flight, by vaporized propellant gasses routed back from the engines into the tanks.
For cost saving, most of the fluid controls and valves are located in the reusable Orbiter rather than the expendable External Tank.
After the Solid Rocket Boosters separate at 50 kilometers (31 mi) altitude, the Orbiter, with the main engines still firing, carries the External Tank to near orbital velocity at about 113 kilometers (70 mi) above Earth. There, eight minutes after takeoff, the now-empty tank separates and falls in a planned trajectory into the Indian Ocean on missions from the Kennedy Space Center or the South Pacific on flights from Vandenberg Air Force Base in California. Venting of unused oxygen controls the tank’s rate of tumbling to prevent skipping when it hits the upper atmosphere and to assure that it will break up and fall within the designated ocean areas far from busy shipping lanes.
