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".[17] 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.[1][17] 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.[7]
This first experimental descent was considered successful, achieving substantial test milestones and collecting engineering data, despite losing the stage into the ocean.[18] 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".[8][18][19]
Flight 9
The second test of controlled-descent hardware and software on the first stage occurred on April 8, 2014,[9] and became the first successful controlled ocean soft touchdown of a liquid-rocket-engine orbital first stage.[20][21] 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.[10][22]
During the second test, the first stage was traveling at a velocity of 10 Mach[22]
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."
Following the first stage loft of the second stage and payload on its orbital trajectory, SpaceX conducted a successful flight test on the spent first stage. The first stage successfully decelerated from hypersonic speed in the upper atmosphere, made a successful reentry, landing burn, and deployment of its landing legs, and touched down on the ocean surface. The first stage was not recovered for analysis as the hull integrity was breached, either upon touchdown or on the subsequent "tip over and body slam".[29] Results of the post-landing analysis showed that the hull integrity was lost as the 150 ft-tall first stage fell horizontally, as planned, onto the ocean surface following the landing.[30]
Flight 13
The fourth test flight of a returned first stage, with a planned ocean touchdown, occurred on Falcon 9 flight 13 which was launched on September 21, 2014. The first stage flew a profile approaching a zero-velocity at zero-altitude simulated landing on the sea surface. SpaceX made no attempt to recover the first stage, since earlier tests had confirmed that the 14-story tall first stage would not survive the tip-over event into the sea. The booster did run out of liquid oxygen.[31]
One month later, detailed thermal imaging infrared sensor data and video were released of the controlled-descent test. The data was collected by NASA in a joint arrangement with SpaceX as part of research on retropropulsive deceleration technologies in order to develop new approaches to Mars atmospheric entry. A key problem with propulsive techniques is handling the fluid flow problems and attitude control of the descent vehicle during the supersonic retropropulsion phase of the entry and deceleration. All phases of the night-time flight test on the first stage were successfully imaged except for the final landing burn, as that occurred below the clouds where the IR data was not visible.[32] The research team is particularly interested in the 70 - 40 km altitude range of the SpaceX "reentry burn" on the Falcon 9 Earth-entry tests as this is the "powered flight through the Mars-relevant retropulsion regime" that models Mars entry and descent conditions.[33]
Flight 15
SpaceX had planned to make the sixth controlled-descent test flight and second landing attempt on their drone ship no earlier than February 11, 2015. Landing a returning rocket at sea would have been a "potentially historic rocket launch and landing", as such a feat "was unheard of" five years earlier.[34][35]
According to regulatory paperwork filed in 2014, SpaceX plans called for the sixth test flight to occur on a late January 2015 launch attempt. However, after the completion of the fifth test flight, and with some damage being incurred by the drone ship in the botched landing, it was not clear whether the sixth test would still be feasible only a few weeks later.[36] This issue was resolved within days of the ship's return to Jacksonville, and by January 15, SpaceX was unambiguous about its plans to attempt a landing of the first stage following the boost phase of the Deep Space Climate Observatory mission.[37]
However, in a statement by SpaceX, the drone ship was in conditions "with waves reaching up to three stories in height crashing over the decks". Additionally, one of the four thrusters that keep the barge in a constant position had malfunctioned, making station-keeping difficult.
Flight 46 and 48
Flight 46 and 48 were both boosters on their second flight that were not recovered due to the older Block 3 design only being capable of two flights. Instead of having an uncontrolled descent, SpaceX softly landed both boosters in the water to test high energy landing techniques without the risk of damaging a drone ship.[40][41] On flight 48, the booster survived landing and stayed intact after tipping over. Unplanned recovery was discussed but the booster broke up before it could be attempted.[42]
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".[17] 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.[1][17] 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.
Flight 9
The second test of controlled-descent hardware and software on the first stage occurred on April 8, 2014,[9] and became the first successful controlled ocean soft touchdown of a liquid-rocket-engine orbital first stage.[20][21] 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.[10][22]
During the second test, the first stage was traveling at a velocity of 10 Mach[22] at an altitude of 80 km
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."
Following the first stage loft of the second stage and payload on its orbital trajectory, SpaceX conducted a successful flight test on the spent first stage. The first stage successfully decelerated from hypersonic speed in the upper atmosphere, made a successful reentry, landing burn, and deployment of its landing legs, and touched down on the ocean surface. The first stage was not recovered for analysis as the hull integrity was breached, either upon touchdown or on the subsequent "tip over and body slam".[29] Results of the post-landing analysis showed that the hull integrity was lost as the 150 ft-tall first stage fell horizontally, as planned, onto the ocean surface following the landing.[30]
Flight 13
The fourth test flight of a returned first stage, with a planned ocean touchdown, occurred on Falcon 9 flight 13 which was launched on September 21, 2014. The first stage flew a profile approaching a zero-velocity at zero-altitude simulated landing on the sea surface. SpaceX made no attempt to recover the first stage, since earlier tests had confirmed that the 14-story tall first stage would not survive the tip-over event into the sea. The booster did run out of liquid oxygen.[31]
One month later, detailed thermal imaging infrared sensor data and video were released of the controlled-descent test. The data was collected by NASA in a joint arrangement with SpaceX as part of research on retropropulsive deceleration technologies in order to develop new approaches to Mars atmospheric entry. A key problem with propulsive techniques is handling the fluid flow problems and attitude control of the descent vehicle during the supersonic retropropulsion phase of the entry and deceleration. All phases of the night-time flight test on the first stage were successfully imaged except for the final landing burn, as that occurred below the clouds where the IR data was not visible.[32] The research team is particularly interested in the 70 - 40 km altitude range of the SpaceX "reentry burn" on the Falcon 9 Earth-entry tests as this is the "powered flight through the Mars-relevant retropulsion regime" that models Mars entry and descent conditions.[33]
Flight 15
SpaceX had planned to make the sixth controlled-descent test flight and second landing attempt on their drone ship no earlier than February 11, 2015. Landing a returning rocket at sea would have been a "potentially historic rocket launch and landing", as such a feat "was unheard of" five years earlier.[34][35]
According to regulatory paperwork filed in 2014, SpaceX plans called for the sixth test flight to occur on a late January 2015 launch attempt. However, after the completion of the fifth test flight, and with some damage being incurred by the drone ship in the botched landing, it was not clear whether the sixth test would still be feasible only a few weeks later.[36] This issue was resolved within days of the ship's return to Jacksonville, and by January 15, SpaceX was unambiguous about its plans to attempt a landing of the first stage following the boost phase of the Deep Space Climate Observatory mission.[37]
However, in a statement by SpaceX, the drone ship was in conditions "with waves reaching up to three stories in height crashing over the decks". Additionally, one of the four thrusters that keep the barge in a constant position had malfunctioned, making station-keeping difficult.
Flight 46 and 48
Flight 46 and 48 were both boosters on their second flight that were not recovered due to the older Block 3 design only being capable of two flights. Instead of having an uncontrolled descent, SpaceX softly landed both boosters in the water to test high energy landing techniques without the risk of damaging a drone ship.[40][41] On flight 48, the booster survived landing and stayed intact after tipping over. Unplanned recovery was discussed but the booster broke up before it could be attempted.[42]
Landing attempts
As of 28 January 2023, SpaceX has attempted 178 landings of a first stage on a solid surface, 167 of which have succeeded (93.8%), with 139 out of 144 (96.5%) for the Falcon 9 Block 5 version.
In July 2014, SpaceX announced that the fifth and sixth controlled-descent test flights would attempt to land on a solid surface, merging the lessons from the high-altitude envelope expansion of the first four controlled-descent flights over water with the low-altitude lessons of the F9R Dev1 testing in Texas.[43] At that time, the "solid surface" was not further described, and was later revealed to be a seafaring barge dubbed an autonomous spaceport drone ship.
Many of the test objectives were achieved on the first attempt, including bringing the stage to the specific location of the floating platform and collecting a large amount of test data with the first use of grid fin control surfaces for more precise reentry positioning. However the touchdown on the corner of the barge was a hard landing and most of the rocket body fell into the ocean and sank; SpaceX published a short clip of the crash. It would take four more attempts to achieve the first barge landing at sea on flight 23. Meanwhile, ground landing succeeded on the first attempt with flight 20 on December 21, 2015.
In October 2014, SpaceX clarified that the "solid surface" would be a floating platform constructed from a barge in Louisiana, and confirmed that they would attempt to land the first stage of the fourteenth Falcon 9 flight on the platform.[44]
Flight 14
This fifth controlled-descent test flight was anticipated by the specialized press as a historic core return attempt.[46] It incorporated for the first time in an orbital mission the grid fin aerodynamic control surfaces that had previously been tested only during a low-altitude, low-speed test with the F9R Dev1 prototype vehicle in early 2014. The addition of grid fins, with continuation of the control authority obtained from gimbaling the engines as on previous test flights, was projected to improve the landing accuracy to 10 m, a thousand-fold improvement over the four previous test flights which landed within 10 km of their target coordinates.[47] Prior to the flight, SpaceX projected that the likelihood of success on the first try was 50 percent or less.[48]
The first test flight for this new hardware occurred on January 10, 2015, on the CRS-5 mission for NASA. The controlled-descent flight started approximately three minutes after launch, following the second stage separation event,[46] when the first stage was approximately 80 km high and moving at a velocity of 10 Mach.[49]
Flight 17
A seventh test flight of the first stage controlled-descent profile occurred on April 14, 2015, on Falcon 9 flight 17, which carried CRS-6 to the International Space Station. This was SpaceX's second attempt to land on a floating platform. The first stage was fitted with grid fins and landing legs to facilitate the post-mission test.
An early report from Elon Musk suggested that the first stage made a hard landing on the drone ship.[55] Musk later clarified that the bipropellant valve was stuck, and therefore the control system could not react rapidly enough for a successful landing.[56] On April 15, SpaceX released a video of the terminal phase of the descent, the landing, the tip over, and the resulting deflagration as the stage broke up on the deck of the ASDS.[57]
Flight 20: first landing on ground pad
The first attempt to land the first stage of Falcon 9 on a ground pad near the launch site occurred on flight 20, the maiden flight of the Falcon 9 Full Thrust version, on the evening of December 21, 2015. The landing was successful and the first stage was recovered.[58][59] This was the first time in history that a rocket first stage returned to Earth after propelling an orbital launch mission and achieved a controlled vertical landing.
SpaceX applied to the Federal Aviation Administration (FAA) US regulatory authority to perform its eighth booster controlled-descent test culminating with a landing attempt at the Landing Zone 1 facility (formerly Launch Complex 13) that SpaceX had recently built at Cape Canaveral Air Force Station.[60] The FAA cleared SpaceX to attempt this landing after assessing that it would inflict minimal damage on the environment.[61]
Flight 21
Flight 21, the final launch of a Falcon 9 v1.1, carried the Jason 3 payload. At one point this was the first possible opportunity for an attempt to land the first stage on land,[70] but the launches were reordered following the loss of Falcon 9 flight 19 in June 2015. Jason-3 was successfully launched on January 17, 2016, and while the first stage managed to slow down towards a soft landing, the lockout collet on one of the landing legs did not latch correctly, which caused the rocket to fall over and explode after touching down.[71][72] Elon Musk noted that ice buildup on the collet from the high-humidity launch conditions may have led to the failure of the latch.[73][74]
Flight 22
On March 4, 2016, Falcon 9 flight 22 launched the 5,271 kg heavy SES-9 communications satellite,[75][76] the rocket's largest payload yet targeting a highly-energetic geosynchronous transfer orbit (GTO). Consequently, the Falcon 9 first stage followed a ballistic trajectory after separation and re-entered the atmosphere at high velocity with very little fuel to mitigate potential aerodynamic damage.
Therefore, SpaceX did not expect to successfully land its Falcon 9 booster on its sea barge, the Of Course I Still Love You, positioned in the Atlantic Ocean. Elon Musk confirmed in a tweet that the landing attempt had failed.[77][78]
Flight 23: first landing on a drone ship
On April 8, 2016, Falcon 9 flight 23, the third flight of the full-thrust version, delivered the SpaceX CRS-8 cargo on its way to the International Space Station while the first stage conducted a boostback and re-entry maneuver over the Atlantic Ocean. Nine minutes after liftoff, the booster landed vertically on the drone ship Of Course I Still Love You, 300 km from the Florida coastline, achieving a long-sought-after milestone for the SpaceX reusability development program.[79]
This stage, serial number B1021, was refurbished and flown again in March 2017 for the SES-10 mission, setting another milestone in the development of reusable rockets.
Flight 24: first return from GTO mission
On May 6, 2016, Falcon 9 flight 24 delivered the JCSAT-14 satellite on a geostationary transfer orbit (GTO) while the first stage conducted a re-entry burn under ballistic conditions without prior boostback. Following the controlled descent through the atmosphere, the booster executed a short landing burn as it approached the drone ship Of Course I Still Love You, and succeeded in landing vertically. This second landing at sea was more difficult than the previous one because the booster at separation was traveling about 8350 km/h compared to 6650 km/h on the CRS-8 launch to low Earth orbit.[80] Pursuing their experiments to test the limits of the flight envelope, SpaceX opted for a shorter landing burn with three engines instead of the single-engine burns seen in earlier attempts; this approach consumes less fuel by leaving the stage in free fall as long as possible and decelerating more sharply, thereby minimizing the amount of energy expended to counter gravity.[81] Elon Musk indicated this first stage may not be flown again and will instead be used as a life leader for ground tests to confirm future first stage rockets are good.[82]
Flight 25
On May 27, 2016, Falcon 9 flight 25 delivered THAICOM 8 to a supersynchronous transfer orbit; despite high re-entry speed, the first stage again landed successfully on the SpaceX drone ship.[83] The landing crushed a "crush core" in one leg, leading to a notable tilt to the stage as it stood on the drone ship.[84]
Flight 26
On June 15, 2016, Falcon 9 flight 26 successfully delivered the Eutelsat 117W B[85] and ABS 2A[86] satellites into GTO. The first stage conducted a re-entry burn and successfully deployed its grid fins, before attempting a landing on the barge. The landing failed in its final moments due to low thrust on one of the first stage engines, caused by the exhaustion of its liquid oxygen fuel supply. That caused the engines to shut down early while the first stage was just above the drone's deck, causing a landing failure.[87][88]
Flight 27
In the early hours of July 18, 2016, Falcon 9 flight 27, carrying the Dragon spacecraft for the CRS-9 mission was followed by a successful landing of the first stage at Landing Zone 1, Cape Canaveral.[89]
Flight 28
On August 14, 2016, the Falcon 9 flight 28 successfully propelled the Japanese JCSAT-16 telecommunications satellite to a geosynchronous transfer orbit. The first stage re-entered the atmosphere and landed vertically on the Of Course I Still Love You drone ship that was located in the Atlantic Ocean.[90]
