THE MOST 21ST CENTURY HEADLINE YET: SpaceX Ponders Hypersonic Decelerator For Second-stage Recovery.
Recovery of the second stage poses greater challenges than either the booster or the fairings because of its greater reentry velocity, relatively large mass and inherent instability. SpaceX currently deorbits the upper stage into the South Pacific through a retroburn of the engine. However, the upper stage also “tumbles” during reentry, normally resulting in the breakup of the structure in the upper atmosphere.
To stabilize the second stage and reduce its ballistic coefficient during reentry, SpaceX is therefore looking at several options—including a hypersonic inflatable aerodynamic decelerator (HIAD), a deployable, inflatable aeroshell with a built-in flexible thermal protection system able to protect the entry vehicle through atmospheric entry. The concept, based on the idea of a packable heat shield, has been tested in recent years by NASA under programs such as the Inflatable Reentry Vehicle Experiment-3, or IRVE-3, which successfully demonstrated a 3-m-dia. (10-ft.) HIAD during a suborbital test in 2012.
Musk hinted that the company was evaluating atmospheric decelerators in mid-April when he tweeted, “This is gonna sound crazy, but . . . SpaceX will try to bring rocket upper stage back from orbital velocity using a giant party balloon.” Following a stabilized reentry behind the protection of the aeroshell, the second stage would then likely deploy airbags and a steerable parafoil, to be recovered by a vessel positioned downrange.
I was about to write “Godspeed,” but decided “Lord, please help them decelerate” might be more apropos.