An important consideration in NASA's ongoing space transportation studies is that the existing four-segment RSRB has demonstrated high reliability since its first flight in 1988. It also satisfies NASA's known performance requirements for the Space Station era. These facts, combined with the risks involved in changing to a relatively unproven booster on a manned vehicle with only minimal crew escape capability means that NASA is not likely to, and the committee agrees it should not, enter into any major new booster program without substantial national need for the performance enhancements and long-term safety and cost benefits.

A recent proposal by Thiokol Propulsion, this upgrade would add a fifth segment to the shuttle's RSRB, alter the grain of the solid fuel to provide a safer thrust profile, and modify the RSRB's nozzle and insulation. On its surface, the five-segment RSRB appears to be a relatively straightforward approach to improving the performance of the booster, but it will require substantial integration engineering and testing. Early estimates suggest at least $1 billion development cost. A thorough evaluation of the potential for separate implementation of subsets of the proposal should be included in NASA's ongoing assessment.

This NASA generated concept would replace the shuttle's solid rocket boosters with liquid-fueled boosters designed to fly back automatically to the launch site after they have separated from the orbiter. NASA believes that the LFBB would cost $4 to $5 billion to develop but would improve safety, reduce long-term operational costs, enable a higher flight rate, and increase the shuttle's payload capacity. Before proceeding with the LFBB, NASA should initiate a detailed independent assessment of configuration trade-offs, costs, and programmatic and technical risks to determine the best fundamental configurations for a new liquid shuttle booster. Should NASA proceed with this program, they should closely coordinate their efforts with other government and industry transportation initiatives.