A principal objective of the new design was to facilitate booster reusability for a larger range of missions, including delivery of large commsats to geosynchronous orbit.
Modifications included liquid oxygen subcooled to −206.7 °C and RP-1 cooled to −7 °C for density (allowing more fuel and oxidizer to be stored in a given tank volume), several size and volume changes to the first- and second-stage propellant tanks, and several small mass-reduction efforts. The modified design gained an additional 1.2 meters of height, stretching to exactly 70 meters including payload fairing.
Two key improvements were the replacement of the first-stage engine with the full-thrust variant of the Merlin 1D and the replacement of the second-stage engine with the Merlin Vacuum (1D). The new first-stage engine featured an increased thrust-to-weight ratio while the new second-stage engine was optimized for higher performance in vacuum through modifications such as a larger exhaust nozzle and improved attitude control system. First stage booster can reach low Earth orbit as a single-stage-to-orbit if it is not carrying the upper stage and a heavy satellite
In late 2012, Elon Musk tweeted an image of the Merlin 1D-Vac firing on the test stand and stated “Now test firing our most advanced engine, the Merlin 1D-Vac, at 80 tons of thrust.” Currently the official SpaceX’s Falcon 9 product page lists the thrust of the Merlin Vacuum on the second stage of the launcher at 934 kN (210,000 lbf) and specific impulse of 348 seconds in vacuum conditions. The increase is due to the greater expansion ratio afforded by operating in a vacuum, now 165:1 using an updated nozzle extension
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