Riding Rockets - Mike Mullane [20]
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.
Solid-fueled rocket boosters lack this significant safety advantage. Once ignited, they cannot be turned off and solid propellant cannot flow, so it cannot be diverted to another engine. At the most fundamental level, modern solid rocket boosters are no different from the first rockets launched by the Chinese thousands of years ago—after ignition they have to work because nothing can be done if they don’t. And, typically, when they do not work, the failure mode is catastrophic. The military has a long history of using solid rocket boosters on their unmanned missiles, and whenever they fail, it is almost always without warning and explosively destructive.
The SRB design for the space shuttle was even more dangerous than other solid-fueled rockets because their huge size (150 feet in length, 12 feet in diameter, 1.2 million pounds) required them to be constructed and transported in four propellant-filled segments. At Kennedy Space Center these segments would be bolted together to form the complete rocket. Each segment joint held the potential for a hot gas leak; there were four joints on each booster. Redundant rubber O-rings had to seal the SRB joints or astronauts would die.
Yet another aspect of the design of the space shuttle made the craft significantly more dangerous to fly than anything that had preceded it. It lacked an in-flight escape system. Had theAtlas rocket, which launched John Glenn, or theSaturn V rocket, which lifted Neil Armstrong and his crew, blown up in flight, those astronauts would have likely been saved by their escape systems. On top of theMercury andApollo capsules were emergency tractor escape rockets that would fire and pull the capsule away from a failing booster rocket. Parachutes would then automatically deploy to lower the capsule into the water. The astronauts riding inGemini capsules had the protection of ejection seats at low altitude and a capsule separation/parachute system for protection at higher altitudes.
The shuttle design did accommodate two ejection seats for the commander and pilot positions, but this was a temporary feature intended to protect only the two-man crews that would fly the first four shakedown missions. After these experimental flights validated the shuttle design, NASA would declare the machineoperational, remove the two ejection seats, and manifest up to ten astronauts per flight. Such large crews would be necessary to perform