The premier example of the piggyback ride to orbit, and one which has been demonstrated over 120 times, is the Space Shuttle. Let us take a look at where the idea came from, how successful it has been, and where it might lead. As noted earlier, the Space Shuttle represents the vertical takeoff horizontal landing concept rather than the relatively unexplored horizontal takeoff and landing mode. If the Shuttle were equipped with jet engines, it could indeed take off from a runway and operate as a jet airplane. This capability was demonstrated many times by the Russian Buran test vehicle, which was fitted with jet engines for this purpose.
The original plan for the Space Shuttle used a huge winged booster that would be flown back to Cape Canaveral after every launch. The manned orbiter would ride piggyback on the booster, and both would lift off from a launch pad much like the Shuttle does today. While the booster's crew was returning to the spaceport, the orbiter's crew would continue into space. There would be no drop-off stages or throwaway tanks. This was to be a fully reusable system, as originally conceived. Owing to cost concerns, the fully reusable concept was vetoed in favor of the present partially reusable design. Ironically, the less costly system actually wastes far more costly hardware on every launch than the more costly system would ever have done. It also has proven far more costly in terms of vehicle attrition. But the real price has been paid in lost lives.
Developed in the 1970s following the Apollo Moon program, the Space Shuttle is the most important spaceplane to date. Despite the loss of two vehicles - each with a crew of seven - the Shuttle has continued to provide heavy-lift launch capability in a reusable winged spacecraft. It is the Shuttle that has delivered, or is scheduled
to deliver, 27 of the 31 planned major components to the International Space Station. The other four components are being sent, or have been sent already, by Russian rocket. Every Shuttle flight has returned to a precision landing on runways at the Kennedy Space Center in Florida, Edwards Air Force Base, California, or White Sands, New Mexico. Of all the spaceplanes that have been conceived, only the Space Shuttle has proven itself capable of regular orbital spaceflight operations.
The Space Shuttle "stack" is actually a cluster of four components specifically designed to break apart on the way into space (Fig. 7.2). The largest and heaviest of these pieces is the huge bullet-shaped external tank (ET) containing the liquid hydrogen and oxygen propellants for the Space Shuttle main engines. This tank has no engines of its own, but serves as the structural core of the launch configuration. Attached to the tank is the double-delta-winged orbiter and two huge solid rocket boosters, the largest of their kind ever built. The two SRBs burn a solid fuel and oxidizer propellant in a rubber matrix for 2 min, giving a terrific boost to the stack in the initial part of the ride to space. Once they are ignited, they cannot be shut off under any circumstances. When their propellants are expended, the SRBs are jettisoned and recovered in the ocean for refurbishment and reuse. Meanwhile, the orbiter and tank continue to accelerate toward orbital velocity, with liquid propel-lants feeding into the three Space Shuttle main engines. When its propellants have been expended, the ET is also jettisoned so that it follows a ballistic trajectory and impacts in the Indian or Pacific Ocean. The Shuttle orbiter coasts to apogee and uses its orbital maneuvering system engines to insert the Shuttle into orbit.
By employing powerful solid rocket boosters and carrying the bulk of its liquid propellants in a throwaway gas tank, the Shuttle system can haul large and heavy pieces of hardware into low Earth orbit. The Space Shuttle is perfectly suited to serve as the space truck to deliver space station modules to the International Space Station. The seven crew members provide the human element, ensuring that Space Station assembly is performed professionally and proficiently. Space Shuttle crews have also performed important maintenance on the Hubble Space Telescope, including installing corrective optics. And they have launched probes into deep space, including Magellan, Ulysses, and Galileo.
How reliable is the Shuttle as a launch vehicle? In 120 missions, we have lost two orbiters. This translates to a loss rate of 1 in 60, or 1.7%, which is typical of unmanned space launch vehicles. The remaining Shuttles - Discovery, Atlantis, and Endeavour - are due for retirement in the year 2010 upon completion of the International Space Station. But they still have another 20 or so missions to perform. If the statistical loss rate of 1.7% applies to these 20 missions - and everyone hopes it does not - then there is a 33% probability that we will lose another Shuttle before the program ends. If the unthinkable happens and another Shuttle is lost, then the program will be finished at that point. The remaining two Shuttles will be grounded permanently, and America's manned space program will have to wait until the Ares launch vehicle and Orion spacecraft are ready to fly, sometime around the years 2012-2014.
To the public, the Space Shuttle is an operational "man-rated" spaceship. Despite the loss of two orbiters, it is deemed safe enough to fly every few months with crews of seven. Three spaceworthy orbiters remain after the losses of Challenger in 1986 and Columbia in 2003. Before looking at each of these incidents, the alert reader would do well to understand that the Space Shuttle has made far fewer flights in a quarter century than the X-15 made in 9 years. Each Shuttle flies twice a year, on average. The X-15 flew 199 times, but was never considered an operational vehicle. It was a research aircraft only, flown by professional test pilots only. Two of its pilots were Neil Armstrong and Joseph Engle; both later became NASA astronauts. Astronaut Engle later flew the only completely manual reentry in the Space Shuttle. Let us now take a look at what happened to Challenger and Columbia, and see what lessons we can learn from these events.
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