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Suppose that, instead of the SRBs, something very much like the Sigma Corporation's EDIN05 pressure-fed liquid booster pack is adopted to boost the Space Shuttle. What happens?

A brief description of the EDIN05 can be found in the Jenkins book on the Space Shuttle. Since y'all probably don't have a copy, this essentially mounted the Orbiter and External Tank atop a recoverable (parachute into the sea) pressure-fed liquid booster pack utilizing three 800,000-pound-force kerosene and liquid oxygen engines*. Like the OTL Shuttle, it would use parallel-staging; the ET and Shuttle were modified slightly to provide more propellant and allow larger payloads to be launched. In reality, it was proposed in 1976, but this was much too late for it to be taken seriously (it would have required extensive and expensive pad and VAB modifications). Similar systems, however, were proposed during the late (just prior to approval) studies, so the idea here is that first a system essentially identical (instead of merely similar) to the previously described design is developed and second that the budget trades work out slightly better for NASA, enough to make developing this stage look reasonable. In theory it could be reused; probably about as practically as the SSMEs (advantage of being a simple pressure-fed liquid engine, but disadvantage of being dunked in seawater).

* You may object that 2.4 million pounds-force of thrust is much, much less than the 5.6 million pounds-force generated by the SRBs. True, but the EDIN05 stage would also weigh far, far less than solid propellant boosters. There's not enough data given for me to do any sort of accurate calculation, but a first stage booster might have a dry mass of 5-7% of its loaded weight compared to the 15% or so of solid boosters, for example.
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