It's rocket science! SpaceX lands yet another rocket at sea

SpaceX Falcon 9 Launching at Vandenberg Air Force Base (Credit: SpaceX)

On 11 October 2017, Elon Musk's company SpaceX successfully launched a reused rocket for the third time in history.  The Falcon 9 rocket launched the Echostar 105 / SES-11 communications satellite into orbit using a previously used first-stage rocket booster.  SpaceX is able to reuse the rocket boosters due to an incredible landing and retrieval process.

Autonomous Spaceport Drone Ship

One of SpaceX's Autonomous Spaceport Drone Ships (Credit: StackExchange)

SpaceX has developed autonomous spaceport drone ships (ASDS) to recover rocket boosters for reuse.  The approximately 90 meter long ships, cleverly named "Just Read the Instructions" and "Of Course I Still Love You," help to provide a safe and feasible landing site for the rocket boosters.  The autonomous ships can be overridden by a team of engineers, but they are fully capable of maintaining their location to within 3 meters using GPS and an array of motors to carefully guide the ship.

First-stage rocket boosters are used to carry the cargo and other stages of the rocket to the upper levels of Earth's atmosphere.  Upon carrying the rocket about 80 kilometers in altitude, the rocket booster detaches from the remainder of the rocket and then initiates a landing sequence.  The rocket booster, traveling at around 4800 km/h rotates 180° and orients itself with an ASDS.  The rocket booster and ASDS communicate with each other and use computer controlled guidance systems.  These guidance systems use physics (including projectile motion) to aim and land the rocket booster.  By calculating position and velocity, the rocket booster can maintain the proper trajectory and speed.

The following video from SpaceX shows a rocket booster landing on an Autonomous Spaceport Drone Ship in April of 2016.

Why Land at Sea?

The first time SpaceX demonstrated the landing of a rocket booster, the landing site was on land.  This is much easier for SpaceX to pull off due to the removal of a moving target (due to ocean currents and waves).  As this video from The Verge explains, the rocket boosters do not have enough fuel to make it back to land for about two-thirds of SpaceX's launches.  In order to follow the trajectory of the original launch, the landing point must be at sea.  The below diagram from SpaceX helps to demonstrate the landing process.

Diagram of the landing of a rocket booster (Credit: SpaceX)

Further Use

Musk and SpaceX first plan to use this rocket landing technology to allow for frequent rocket reusability.  However, the company has recently shown off plans to use rockets for passenger transportation around the globe.  Using rockets allows for people to arrive at their destination in under 60 minutes, no matter the location on the planet.  The rocket would travel the globe from landing pad to landing pad multiple times per day.  Musk hopes the airfare (or rocket-fare) would be no more than a traditional airfare.

Sources
http://www.popularmechanics.com/space/rockets/a13927/space-reusable-falcon-9-diagram/
https://www.space.com/38434-spacex-launches-satellite-third-used-rocket.html
https://foxtrotalpha.jalopnik.com/spacexs-landing-drone-ship-is-just-as-complicated-as-th-1769987148