Seems like all the pieces for a privately-built space renaissance are starting to fall into place. When the Falcon Heavy goes operational to provide cheap launches, we could finally have the spaceflight boom that space enthusiasts have been dreaming of."
1. Usage of the Shuttle main engines. "Those were developed to be reusable, and they are expensive because of that. They are in fact magnificent engines and thoroughly reusable if operated at 90-95% of rated capacity; it’s not their fault that they had to be run at 103% and above to fly Shuttle. But they were developed to be reusable, and that adds greatly to their cost."
2. Usage of SRBs. "You don’t want recoverable solid rockets in the first place. The operations are a nightmare, and the design has to be compromised so that the impact on the water does not destroy the thing, and it has to float. All that changes the design and affects performance. There is no good reason ever to recover a solid booster, which is, after all, a big sewer pipe stuffed with guncotton and leached with nitroglycerine....
The only reason we ever came up with any notion as mad as a segmented solid booster was that the SRB had to be made in Utah because of political constraints. If you make a booster that size in Utah it has to be segmented because you can’t ship it by rail or on the highway – the curves are too sharp and the tunnels are not big enough. You would have to make it in Michoud Louisiana and ship it by barge to Canaveral. That is possible but Louisiana isn’t Utah. Apparently the new NASA design is worried about the Utah Senatorial votes to this day."
His widely read 2000 paper titled How To Get To Space provides excellent insights into the X-programs and is well worth reading.