Falcon 9 is a partially reusable two-stage-to-orbit medium-lift launch vehicle designed and manufactured by SpaceX in the United States. It is powered by Merlin engines, also developed by SpaceX, burning cryogenic liquid oxygen and rocket-grade kerosene (RP-1) as propellants. Its name is derived from the fictional Star Wars spacecraft, the Millennium Falcon, and the nine Merlin engines of the rocket's first stage. The rocket evolved with versions v1.0 (2010–2013), v1.1 (2013–2016), v1.2 "Full Thrust" (2015–present), including the Block 5 Full Thrust variant, flying since May 2018. Unlike most rockets, which are expendable launch systems, since the introduction of the Full Thrust version, Falcon 9 is partially reusable, with the first stage capable of re-entering the atmosphere and landing vertically after separating from the second stage. This feat was achieved for the first time on flight 20 with the v1.2 version in December 2015. Falcon 9 can lift payloads of up to 22,800 kilograms (50,300 lb) to low Earth orbit, 8,300 kg (18,300 lb) to geostationary transfer orbit (GTO) when expended, and 5,500 kg (12,100 lb) to GTO when the first stage is recovered. The heaviest GTO payloads flown have been Intelsat 35e with 6,761 kg (14,905 lb), and Telstar 19V with 7,075 kg (15,598 lb). The latter was launched into a lower-energy GTO orbit achieving an apogee well below the geostationary altitude, while the former was launched into an advantageous super-synchronous transfer orbit. In 2008, SpaceX won a Commercial Resupply Services (CRS) contract in NASA's Commercial Orbital Transportation Services (COTS) program to deliver cargo to the International Space Station (ISS) using the Falcon 9 and Dragon capsule. The first mission under this contract launched on October 8, 2012. Falcon 9 has been human-rated for transporting NASA astronauts to the ISS as part of the NASA Commercial Crew Development program. Currently, Falcon 9 has been certified for the National Security Space Launch program and NASA Launch Services Program as "Category 3", which can launch the priciest, most important, and most complex NASA missions. Five rockets of the version 1.0 design were launched from June 2010 to March 2013. Version 1.1 conducted fifteen launches from September 2013 to January 2016. The "Full Thrust" version was in service from December 2015 into 2018, with several additional upgrades within this version. The latest variant, Block 5, was introduced in May 2018. It features increased engine thrust, improved landing legs, and other minor improvements to help recovery and reuse. The Falcon Heavy derivative, introduced in February 2018, consists of a strengthened Falcon 9 first stage as its center core, attached to two standard Falcon 9 first stages used as boosters.