NASA was constantly working on propulsion system mods to increase the payload capacity of the shuttle. Some were put into service and some were only ground tested.
- The super lightweight (Al-Li alloy) external tank saved about 7,000lbs and was needed to get some of the heavier ISS module payloads to the required orbit.
- There was a lightweight filament wound SRB case developed but never flown.
- There was a 5 segment SRB developed for the shuttle that would have increased max weight of ISS payloads by about 10 tons. The 5 segment SRB is being used on the new SLS vehicle.
- I recall reading that NASA was looking at super cooling the LOx to reduce volume and allow a smaller, lighter tank to be used.
- The shuttle used the
payload assist module (PAM) to boost payloads into higher orbits.