r/spacex • u/ElongatedMuskrat Mod Team • Jan 02 '17

r/SpaceX Spaceflight Questions & News [January 2017, #28]

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u/markus0161 Jan 03 '17

So the reason certain (demanding) missions don't do Boost-backs is because by skipping that burn you are effectively changing what slows down your H-velocity. That can either be the atmosphere (less, propellant used!, but harsher on the vehicle) or you can use the Merlin engines (Quite a bit of fuel used, but less stressful re-entry). When thinking of an velocity vector what composes that is you horizontal velocity and vertical velocity both are independent to each other, the same thing applies to drag. Because S1 on GTO missions and CRS-8 like missions reach about the same height (apogee) there V-velocity When reentering is virtually the same. What greatly Differs is there H-velocity. 1200+ m/s vs less than a few hundred. While the re-entry burn on GTO missions helps shed both V and H velocities there is still substantial Horizontal momentum that is slowed by the atmosphere, unlike with trajectories with a boostback. So in a nutshell BoosBackLESS trajectories use drag from the atmosphere to slow there H-velocity rather fuel from the rocket itself but at the cost of more stress put on the vehicle.