>>Using the SRB as the CLV first stage maintains production and expertise from the end of STS until start up of Ares V. <br /><br />If the stick were cut, they could use the money to start work on the heavy lift vehicle now, at least at a low level. The best way to develop and maintain expertise for a system is to build prototypes and technology demonstrators directly related to the ultimate system, not something else that uses somewhat similar components.<br /><br />Moreover there are alternatives to the SRBs; post Challenger a variety of liquid fuel boosters were considered as replacements for them. Parallel-staged LRBs have been used on the Soyuz, Ariane 4, Long March and Energia; the latter were similar to the first stage of the Zenit, which remains in production, and thus could probably be procured quickly and at reasonable cost.