re: Shuttle Derived Vehicles
The point is not reliability - but price.
The concept that the SRB's are reusable is - at best - creative use of the word reusable - and at worst, an outright lie. They are totally refurbished between flights, a process that costs a fortune.
The space shuttle ET costs more than a complete EELV - where is the sensibility in that!!
The ET's are expensive, the SRB's are expensive, and if used on an expendable mission, the SSME's are very VERY expensive. There are much more viable components available to build a heavy lift capacity - Energia would be a start.
Doug