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Crunching a few numbers, just for fun, and I was looking at the SuperDraco engines on the Dragon V2. Their quoted ISP is really, really low...like, monopropellant low. If the SL impulse is 240 seconds and the SL thrust is 68,170 N, then the mass flow at full thrust is going to be about 29 kg/s. The quoted vacuum thrust is 73,000 N, so this corresponds to a vacuum specific impulse of 257 seconds. But that's crazy. The NTO/MMH hypergols used for the SuperDraco engines should have a SL specific impulse of 288 s and a vacuum specific impulse of 336 s. What gives? At first I thought that perhaps the pressure-fed SuperDraco engines were running with a lower-than-ideal chamber pressure. But they have a chamber pressure of 1000 psi, which gives the expected ~280 s SL specific impulse according to this chart. Apparently the SuperDraco engines have really, really underexpanded nozzles to allow deep throttling. This got me thinking. If SpaceX designed attachable nozzle extensions that mounted to the outside of the Dragon V2, the vacuum specific impulse could theoretically come right up to 336 s. The combined thrust from the eight SuperDraco engines would be a little over 760 kN. The thrust of the single Merlin 1D Vacuum engine in the Falcon 9 second stage is 934 kN...just 22% higher. The crewed Dragon V2 masses an estimated 9,200 kg including onboard propellant, full cargo capacity, and the trunk. The second stage has a dry mass of 3.9 tonnes, a specific impulse of 340 seconds, and 92.67 tonnes of fuel. So the second stage can deliver almost 7 km/s. Pretty impressive. But suppose we equip the Dragon V2 with nozzle extensions, raising its vacuum isp to 336 s and its total thrust capacity to 760 kN, and replace the entire second stage with a drop-away NTO/MMH tank feeding up to external couplings on the Dragon V2. We can assume a tank dry mass fraction of 4.6% for that propellant combination. If the total mass being lifted by the first stage is kept the same, then the tank will contain 92.13 tonnes of fuel and you end up with a dV of 6.74 km/s without using any of the Dragon V2's internal propellant. The first stage should easily be able to manage an extra ~200 m/s and still RTLS. So you have an almost fully reusable launch system with only a single drop tank being discarded. And sure, I know that NTO/MMH is expensive and toxic. But surely it's cheaper than throwing away a Falcon 9 second stage with each flight, right?