Riding Rockets

Before the space shuttle, every astronaut who had ever launched into space had ridden in capsules on throwaway rockets. The only thing that had ever come back to Earth was the capsule bearing the astronauts. Even these capsules had been tossed aside, placed in museums across America. While the capsules had grown in size to accommodate three men, and the rockets to carry them had grown bigger and more powerful, the basic Spam-in-a-can design, launched with expendable rockets, had been unchanged since Alan Shepard said, “Light this candle,” on the first Mercury-Redstone flight.

We would fly a winged vehicle, half spacecraft and half airplane. It would be vertically launched into space, just as the rockets of yesteryear, but the winged craft would be capable of reentering the atmosphere at twenty-five times the speed of sound and gliding to a landing like a conventional airplane. Thousands of silica tiles glued to the belly of the craft and sheets of carbon bolted to the leading edge of the wings and nose would protect it from the 3,000-degree heat of reentry. After a week or two of maintenance and the installation of another 65,000-pound payload in the cargo bay, it would be ready to launch on another mission.

The space shuttle orbiter (the winged vehicle) would have three liquid-fueled engines at its tail, producing a total thrust of nearly 1.5 million pounds. These would burn liquid hydrogen and liquid oxygen from a massive belly-mounted gas tank or External Tank (ET). Eight and a half minutes after liftoff the empty ET would be jettisoned to burn up in the atmosphere, making it the only part of the “stack” that was not reusable.

As powerful as they were, the three Space Shuttle Main Engines (SSMEs) did not have the muscle to lift the machine into orbit by themselves. The extra thrust of booster rockets would be needed. NASA wanted a reusable liquid-fueled booster system but parachuting a liquid-fueled rocket into salt water posed major reusability issues. It would be akin to driving an automobile into the ocean, pulling it out, and then hoping it started again when you turned the key. Good luck. So the engineers had been faced with designing a system whereby the liquid-fueled boosters could be recovered on land. It quickly became apparent that it would be impossible to parachute such massive pieces of complex machinery to Earth without damaging them and posing a safety hazard to civilian population centers. So the engineers looked at gliding them to a runway landing. One of the earliest space shuttle designs incorporated just such a concept. Like mating dolphins, two winged craft, each manned, would lift off together, belly to belly. One would be a giant liquid-fueled booster/gas tank combination, the other, the orbiter. After lifting the smaller orbiter part of the way to space, the booster would separate and two astronauts would glide it to a landing at the Kennedy Space Center (KSC). The astronauts aboard the orbiter would continue to fly it into space using internal fuel for the final acceleration to orbit velocity.

However, designing and building this manned liquid-fueled booster was going to be very expensive at a time when NASA’s budget was being slashed. The agency had won the race to the moon and Congress was ready to do other things with the billions of dollars NASA had been consuming. In this new budget reality NASA looked for cheaper booster designs and settled on twin reusable Solid-fueled Rocket Boosters (SRBs). These were just steel tubes filled with a propellant of ammonium perclorate and aluminum powder. These ingredients were combined with a chemical “binder,” mixed as a slurry in a large Mixmaster, then poured into the rocket tubes like dough into a bread pan. After curing in an oven, the propellant would solidify to the consistency of hard rubber, thus the namesolid rocket booster.

Because they were the essence of simplicity, SRBs were therefore cheap. Also, because after burnout they were just empty tubes, they could be parachuted into salt water and reused. There was just one huge downside to SRBs: They were significantly more dangerous than liquid-fueled engines. The latter can be controlled during operation. Sensors can monitor temperatures and pressures, and if a problem is detected computers can command valves to close, the propellant flow will stop, and the engine will quit, just like turning off the valve to a gas barbecue. Fuel can then be diverted to the remaining engines and the mission can continue. This exact scenario has occurred on two manned space missions. On the launch ofApollo 13 the center engine of the second stage experienced a problem and was commanded off. The remaining four engines burned longer and the mission continued. On a pre-Challengershuttle mission the center SSME shut down three minutes early. The mission continued on the remaining two SSMEs, burning the fuel that would have been used by the failed engine.