On Friday, the Dragon CRS-5 mission lifts off from Space Launch Complex 40 at Cape Canaveral Air Force Station atop a Falcon 9 v1.1 rocket. If the launch is successful, the unmanned Dragon spacecraft will carry supplies and experiments to the ISS, but that part is almost routine. What is unusual is that SpaceX has confirmed that it will not only attempt a powered landing of the Falcon 9 booster, but will do so as a precision landing on a robotic sea barge.
SpaceX says that the attempt has only a 50 percent chance of success, yet marks a major advance in the company's program to create a fully reusable launch system. The barge, called an "autonomous spaceport drone ship," is a custom-built platform measuring 300 by 100 ft (91 by 30 m) with wings extending the width to 170 ft (52 m). Because the platform will be stationed far enough out in the Atlantic to avoid danger, it will not be anchored, but will rely on computerized thrusters similar to those used for station keeping by ocean-going oil drilling ships.
As it re-enters the atmosphere, the Falcon 9 will execute a series of engine burns, beginning with a "boostback" burn to set it on course for the landing site. This will be followed by a supersonic retro propulsion burn to help slow it, then a final burn to slow it to 2 m/s (6.5 ft/s) as the landing legs deploy for (hopefully) a soft touchdown on the barge.
SpaceX has already made two ocean soft landings on previous Dragon launches, as well as many land-based test flights, but Friday's test will involve a much greater risk and degree of accuracy than before. Where the last two attempts were made with an accuracy of 10 km (6 mi), the next one will require landing with 10 m (33 ft). This is particularly important because the landing undercarriage of the rocket spans 70 ft (21 m), which doesn't leave much margin for error on the barge's limited deck space.
To aid in making the landing, SpaceX has equipped the Falcon 9 with a set of four independent hypersonic grid fins set in an X-wing configuration to help slow its descent from hypersonic speeds and to control its roll, pitch, and yaw along with the thrust from the gimbaled liquid-fueled engines.
As it descends, the 14-story rocket will be moving at 1,300 m/s (about 1 mi/s), slowing to 250 m/s (820 ft/s). SpaceX says that controlling the spacecraft under these conditions will be "like trying to balance a rubber broomstick on your hand in the middle of a wind storm."
SpaceX says that though the odds of success for Friday's landing are not good, it will still collect valuable information whatever the outcome.
The video below shows a land-based test of SpaceX's landing system.