The Falcon 9 first-stage landing tests were a series of controlled-descent flight tests conducted by SpaceX between 2013 and 2016. Since 2017, the first stage of Falcon 9 rockets are routinely landed if the performance requirements of the launch allow.
The program's objective was to reliably execute controlled re-entry, descent and landing (EDL) of the Falcon 9 first stage into Earth's atmosphere after the stage completes the boost phase of an orbital spaceflight. The first tests aimed to touch down vertically in the ocean at zero velocity. Later tests attempted to land the rocket precisely on an autonomous spaceport drone ship (a barge commissioned by SpaceX to provide a stable landing surface at sea) or at Landing Zone 1 (LZ-1), a concrete pad at Cape Canaveral. The first ground landing at LZ-1 succeeded in December 2015, and the first landing at sea on a drone ship in April 2016. The second landed booster, B1021, was the first to fly again in March 2017, and was recovered a second time.
Landings of Falcon 9 first-stage boosters
Overview The first landing test occurred in September 2013 on the sixth flight of a Falcon 9 and maiden launch of the v1.1 rocket version. From 2013 to 2016, sixteen test flights were conducted, six of which achieved a soft landing and recovery of the booster: flight 20 (Orbcomm OG2 M2) safely touching down on the LZ-1 ground pad upon first attempt in December 2015; flight 23 (CRS-8) achieving a stable landing at sea in the Atlantic on the drone ship Of Course I Still Love You in April 2016 after four previous attempts ended in destruction of the booster upon impact; flights 24 (JCSAT-14) and 25 (Thaicom 8) returning at higher speed from GTO missions at sea on a drone ship in May 2016; flight 27 (CRS-9) returning to LZ-1 in July 2016; flight 28 (JCSAT-16) landing on a drone ship in August 2016.
Since the January 2017 return to flight, SpaceX has stopped referring to landing attempts as "experimental", indicating that they have become a routine procedure (see Iridium-1 and CRS-10 press kits of 2017, compared with CRS-9 and JCSAT-16 of 2016). By , 14 routine landings had been performed ( success) and three missions were launched in expendable configuration, not attempting to land.
The first-stage descent tests were part of the larger SpaceX reusable launch system development program, which included a large amount of new technology development activities and earlier low-altitude test flights at the SpaceX facility in McGregor, Texas in preparation for the high-altitude high-velocity testing of landing test phase of the program. The overall objective of the program is to privately develop reusable rockets using vertical-landing technology so as to substantially reduce the cost of space access.
Traditionally, the first stages of orbital carrier rockets have been discarded in the ocean once the ascent was complete. Achieving routine recovery and reuse of the launch vehicles could substantially reduce the cost of access to space.
History
From the beginning, Elon Musk wanted the first stage of SpaceX launch vehicles to be recoverable, and all Falcon 1 launches and the first two Falcon 9 launches had parachutes. However the boosters burned up on reentry, before the parachutes even deployed. After seeing the success of Masten Space Systems Xombie in-air relight and vertical landing, SpaceX began evaluating this technique for use on Falcon 9. Experimental prototypes were built and flown during 2012 to 2014 to test the idea of propulsive landings and to gain experience.
SpaceX first announced in March 2013, that it would instrument and equip subsequent Falcon 9 first stages as controlled-descent test vehicles, able to propulsively decelerate to a soft touchdown over the water surface. The company expected to begin these flight tests in 2013, with an attempt to return the vehicle to the launch site for a powered landing no earlier than mid-2014.
In the event, SpaceX performed its first controlled-descent test flight in 2013 but continued the over-water testing well into 2015. Following analysis of telemetry data from the first controlled descent in September 2013, SpaceX announced that a large amount of new technology passed their real-life test objectives, and that coupled with the technology advancements made on the Grasshopper prototype, they were now ready to test the full EDL process to recover the first stage. The rocket was "able to successfully transition from vacuum through hypersonic, through supersonic, through transonic, light the engines all the way and control the stage all the way through [the atmosphere]".
This second EDL test took place during the third cargo resupply mission for NASA in April 2014. SpaceX attached landing legs to the first stage, decelerated the stage through atmospheric re-entry and attempted a simulated landing over water, following the separation of the second stage carrying the Dragon capsule to the ISS. The first stage was slowed down sufficiently to perform a soft touchdown over the Atlantic Ocean. SpaceX announced in February 2014 that they intended to continue over-water tests of the first stage until mastering precision control of the vehicle from hypersonic speed all the way through subsonic regimes.
Subsequent tests, starting with the CRS-5 mission in January 2015, attempted to land the first stage on an autonomous spaceport drone ship stationed off the Florida coastline or in the Pacific Ocean depending on launch site. The ships were used for six landing attempts, two of which succeeded in April and May 2016. Meanwhile, the first attempt to land on solid ground at Cape Canaveral occurred on December 21, 2015, and succeeded.
Post-mission test plan
The post-mission Falcon 9 test plan for the earliest flight tests called for the first stage to perform a retro-propulsion burn in the upper atmosphere to slow it and put it on a descent ballistic trajectory to its target landing location, followed by a second burn in the lower atmosphere before the first stage reached the water. SpaceX announced in March 2013 that it intended to conduct such tests on Falcon 9 v1.1 launch vehicles and would "continue doing such tests until they can do a return to the launch site and a powered landing". The company said it expected several failures before it could land the vehicle successfully.
In detailed information disclosed in the Falcon 9 flight 6 launch license for the CASSIOPE mission, SpaceX said it would fire three of the nine Merlin 1D engines initially to slow the horizontal velocity of the rocket and begin the attempt at a controlled descent. Then, shortly before hitting the ocean, one engine would be relighted in an attempt to reduce speed so that the stage could be recovered. , SpaceX said the experiment had approximately a ten percent chance of success.
SpaceX did not perform controlled-descent tests on all Falcon 9 v1.1 flights, as payloads going to GTO did not leave enough fuel margin. In September 2013, SpaceX announced that the CRS-3 mission of April 2014 (fourth flight of Falcon 9 v1.1) would be the second test of the descent test profile.
Whereas the early tests restarted the engines only twice, by the fourth flight test, in September 2014, SpaceX was reigniting the engines three times to accomplish its EDL test objectives (although only three of the nine engines were used): a boost-back burn, a reentry burn, and a landing burn. The boost-back burn limits downrange translation of the used stage; the reentry burn (from approximately altitude) is used to control the descent and deceleration profile at atmospheric interface; and the landing burn completes the deceleration from terminal velocity to zero at the landing surface.
Test flights
Ocean touchdown attempts
Flight 6 The first propulsive reentry, descent, and ocean-surface touchdown test occurred on September 29, 2013, on Falcon 9 flight 6, the maiden launch of the Falcon 9 rocket, version v1.1. After the three-minute boost phase and separation of the second stage with the CASSIOPE and nanosat payloads, the rocket's first stage was reoriented backwards and three of the nine Merlin 1D engines were reignited at high altitude to initiate a deceleration and controlled descent trajectory to the surface of the ocean. The first phase of the test "worked well and the first stage re-entered safely". However, the stage began to roll because of aerodynamic forces during the atmospheric descent and the roll rate exceeded the capabilities of the first stage attitude control system (ACS) to null it. The fuel in the tanks "centrifuged" to the outside of the tank and the single engine involved in the low-altitude deceleration maneuver shut down. SpaceX was able to retrieve some first-stage debris from the ocean. The company did not expect to recover the first stage on this flight, nor on the first several powered-descent tests, as predicted in their March 2013 announcement.
This first experimental descent was considered successful, achieving substantial test milestones and collecting engineering data, despite losing the stage into the ocean. SpaceX tested a large amount of new technology on this flight, and, combining those results with the advances made on the Grasshopper demonstrator, the company believed it had "all the pieces of the puzzle".
Flight 9 The second test of controlled-descent hardware and software on the first stage occurred on April 8, 2014, and became the first successful controlled ocean soft touchdown of a liquid-rocket-engine orbital first stage. The first stage included landing legs for the first time which were extended to simulate a landing upon touchdown, and the test used more powerful gaseous Nitrogen control thrusters to control the aerodynamic-induced rotation that had occurred on the first test flight. The first stage successfully approached the water surface with no spin and at zero vertical velocity, as designed.
During the second test, the first stage was traveling at a velocity of at an altitude of at the time of the high-altitude turn-around maneuver, followed by ignition of three of the nine main engines for the initial deceleration and placement onto its descent trajectory. The "first stage executed a good re-entry burn and was able to stabilize itself on the way down. ... [The] landing in [the] Atlantic [ocean] was good! ... Flight computers continued transmitting [telemetry data] for eight seconds after reaching the water" and stopped only after the first stage went horizontal.
The major modifications for the second first stage controlled-descent test flight included changes to both the reentry burn and the landing burn as well as adding increased attitude control system (ACS) capabilities.
SpaceX had projected a low probability of stage recovery following the flight test due to complexity of the test sequence and the large number of steps that would need to be performed. The company was careful to label the entire flight test as "an experiment". In a press conference at the National Press Club on April 25, Elon Musk said that the first stage achieved a soft touchdown on the ocean but due to rough seas, the stage was destroyed.
Flight 10 The third test flight of a returned first stage was July 14, 2014, on Falcon 9 flight 10. Whereas the previous test reached a target landing area some hundreds of kilometers off the Florida coast, this flight aimed for a boost-back trajectory that would attempt the ocean touchdown much nearer the coast, and closer to the original launch location at Cape Canaveral. Following the third controlled-descent test flight, SpaceX expressed confidence in their ability to successfully land in the future on a "floating launch pad or back at the launch site and refly the rocket with no required refurbishment."