I heard that Virgin Galactic's SpaceShipTwo just tested something called “feathering”. What is it? How come I didn't hear about the shuttle doing this, it is unique to Virgin Galactic's ship?
This latest (second powered) test flight of the SpaceShipTwo was really interesting to watch (YouTube video, the "feathering" aerobraking starts at 2:14 into it), but this currently unique to The Spaceship Company designs (owned by its sister company Virgin Galactic that's itself within the Richard Branson's Virgin Group) approach to aerobraking was actually already introduced with the SpaceShipOne. With the help of a side-by-side image, this is "feathering" of the SpaceShipTwo:
Nominal and "Feathering" wing positions on SpaceShipTwo, side by side comparison (Source: Space.com)
This is what Wikipedia has to say about SpaceShipTwo "Feathering":
SpaceShipTwo uses a feathered reentry system, feasible due to the low speed of reentry – by contrast, the Space Shuttle and other orbital spacecraft re-enter at orbital speeds, closer to 25,000 km/h (16,000 mph), using heat shields. SpaceShipTwo is furthermore designed to re-enter the atmosphere at any angle.
And here's another image, showing the movement of the side wings on the SpaceShipTwo (similar approach, source: The Verge):
In this YouTube video, Richard Branson explains this feathered reentry system as (starting 55 seconds into the video):
The wing and tail section would take upwards, into what we call "the feather position", allowing the vehicle to slow its rate of descent smoothly and stably, just like a shuttlecock.
And here's a technical snapshot of the Virgin Galactic's SpaceShipTwo:
The term "feathering" has different meanings, dependent on context.
In regard to vehicles such as SpaceShip Two, it refers to a high drag aerodynamic configuration which produces a slow, gentle, but steep descent. It only works for vehicles which begin their descent at relatively low airspeed and simply maintains it. Remember that SpaceShip Two ascends more-or-less vertically, and never attains a significant velocity in the horizontal. Use of the term in this context was probably inspired by the notion of a falling feather.
This would not have been practical for the Space Shuttle because the Shuttle re-entered from orbital velocity. It had to manage its drag profile very carefully to avoid excess g-forces and heat build-up. Also, the Shuttle has very high wing loading (mass per unit wing surface); it required high airspeed to maintain lift, practically the opposite of what SpaceShip Two's feathering approach is all about.
Incidentally, "feathering" has another meaning in aviation. It refers to a setting for a variable-pitch propeller. If an engine fails, the propeller can be "feathered" to minimize drag.