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SpaceX’s third Falcon Heavy rocket took off from the Kennedy Space Center in a predawn launch Tuesday, delivering two dozen research and weather observation spacecraft into orbit on a marathon three-and-a-half mission for the U.S. Air Force.

The mission included the successful landing of the Falcon Heavy’s two side boosters back at Cape Canaveral, and a SpaceX recovery boat netted part of the rocket’s payload fairing for the first time after trying for more than a year.

SpaceX’s fairing recovery boat, recently renamed from “Mr. Steven” to “Ms. Tree,” is fitted with a giant net to catch the rocket’s fairing shell as it descends under a parafoil. The payload fairing protects satellites from aerodynamic friction during the first few minutes of launch, then jettisons in two halves in a clamshell-like fashion to lighten the rocket’s load once it has climbed into the rarefied upper atmosphere.

Since early 2018, SpaceX has tried using the fast-moving boat to steer underneath a fairing. But the efforts chalked up a series of near-misses, prompting engineers to evaluate reusing fairings that fell into the sea, which require more refurbishment after exposure to salt water.

But Tuesday’s catch shows there is promise for SpaceX’s preferred method of recovery.

SpaceX has outfitted its fairings with avionics, thrusters and steerable parachutes to make a soft landing. The company wants to reuse the fairing, eyeing it as the next step in reducing launch costs after proving the landing and reuse of Falcon booster stages.

Elon Musk, SpaceX’s founder and CEO, told reporters last year that the fairing costs around $6 million. Musk has identified the payload fairing as the next component that could be recovered and reused, following SpaceX’s pioneering achievement in landing and re-flying first stage boosters.

SpaceX eventually aims to catch both halves of the fairing.

The U.S. military’s Space Test Program booked the mission, named STP-2, on SpaceX’s Falcon Heavy to exercise the launcher and gauge its suitability to loft high-value national security payloads. The mission also offered a ride to space for a suite of backlogged research satellites.

The intricate series of orbital maneuvers, and the long duration of the launch sequence, prompted SpaceX chief executive to Elon Musk to label it the most difficult launch in the company’s history.

“It’s been a bit of a marathon, but when you finish a marathon, you really feel good,” said John Insprucker, principal integration engineer at SpaceX, after the final satellite payload separated from the rocket. “We’ve had an outstanding mission tonight. We lifted off from 39A, turned night into day with the Falcon Heavy, over 5 million pounds of thrust.”

“The center core went downrange, and we knew it was going to be the toughest mission ever for a center core,” Insprucker said on SpaceX’s launch webcast. “We did get the visibility of the drone ship camera, but we just missed the drone ship, “Of Course I Still Love You.” However, the side cores made up for that with a great return to Landing Zones 1 and 2.”

“The second stage went into orbit,” Insprucker said. We did four burns of the second stage, all of them were right on target. We did 24 of 24 payload separations, and then the icing on the cake tonight was the ability to get one half of the payload fairing, as we were targeting, into the net above the ocean on our recovery ship known as ‘Ms. Tree.'”

The Air Force confirmed in a tweet Tuesday afternoon that all of the payloads launched by the Falcon Heavy were alive and transmitting signals.

“All satellites are on orbit and have made contact!” the Air Force Space and Missile Systems tweeted.

Falcon Heavy launches on action-packed mission

Running three hours late after a ground system hydraulic held up the countdown Monday night, the Falcon Heavy lit its 27 Merlin main engines at rocketed into a moonlit sky at 2:30 a.m. EDT (0630 GMT) Tuesday.

The Falcon Heavy’s three boosters drove the rocket off the pad with some 5.1 million pounds of thrust, more than any other launcher currently in service, and steered the vehicle toward the east.

The window-ratting launch was the first nighttime takeoff by a Falcon Heavy, coming after two daytime departures SpaceX’s heavy-lifter in February 2018 and on April 11.

Around two-and-a-half minutes into the launch, the side boosters shut down and separated from the Falcon Heavy’s center core to begin a series of propulsive maneuvers guiding the twin rockets back to Landing Zone 1 and Landing Zone 2 at Cape Canaveral Air Force Station, around 9 miles (15 kilometers) south of pad 39A.

Speeding back from the edge of space, the rockets reignited their engines for a re-entry braking burn, and a final maneuver to slow for nearly simultaneous landings at Cape Canaveral.

A pair of double sonic booms echoed across Florida’s Space Coast as the rockets returned.

Tuesday morning’s mission was the first time SpaceX has landed two rocket boosters at the same time at night. The side boosters both flew on the previous Falcon Heavy mission with the Arabsat 6A commercial communications satellite in April, when they also landed back at Cape Canaveral.

After release of the side boosters, the Falcon Heavy’s brand new center core throttled up its engines to full power. The core stage operated at partial power for the first couple of minutes of the mission to conserve fuel.

Around three-and-a-half minutes after liftoff, the core stage turned off its engines and separate to begin its own controlled descent to SpaceX’s offshore drone ship positioned nearly 770 miles (1,240 kilometers) east of Cape Canaveral.

Live video transmitted from the drone ship showed the core stage narrowly missed landing. The recovery vessel was parked farther downrange than for any previous SpaceX mission, and the Falcon Heavy’s core stage came down faster than any booster before.

Before Tuesday’s launch, Musk gave 50-50 odds of recovering the center core intact.

The core stage from the inaugural Falcon Heavy launch in February 2018 crashed on landing, and the center booster from the second Falcon Heavy mission in April made a successful touchdown, but tipped over before it could be secured for return to port.

Falcon Heavy’s upper stage succeeds in intricate orbital ballet

The booster and fairing recoveries were just the start of the Falcon Heavy’s mission Tuesday. The rocket’s second stage, powered by a single Merlin engine, ignited four times on the lengthy flight.

The first burn heaved the mission’s 8,157-pound (3,700-kilogram) payload stack into low Earth orbit, where 13 spacecraft deployed from adapters on the Falcon Heavy’s upper stage.

The first orbital target for the STP-2 mission ranged in altitude between about 186 miles (300 kilometers) and 534 miles (860 kilometers). The first orbit had an inclination, or tilt, of 28.5 degrees to the equator.

The first of the payloads to release from the Falcon Heavy will be Oculus-ASR, a microsatellite developed by students at Michigan Technological University in partnership with the Air Force Research Laboratory. Oculus-ASR will test the ability of ground-based observers to determine the orientation and configuration of a satellite in orbit using unresolved imagery. It will also release a pair of small spherical masses to help calibrate instruments that track orbiting space objects.

Twelve CubeSats also ejected from carrier modules on the upper stage.

The Naval Research Laboratory’s Tether Electrodynamics Propulsion CubeSat Experiment, or TEPCE, mission consists of two CubeSats that will be connected by a nearly 3,300-foot (1-kilometer) electrically conducting tether. The experiment will test the tether’s ability to provide electrodynamic propulsion in space, which future missions could use in place of conventional rocket fuel.

The FalconSat 7 satellite, a toaster oven-sized CubeSat developed at the U.S. Air Force Academy, next deployed from the rocket. FalconSat 7 will test a deployable optical solar telescope structure in orbit, a device that could be used on future military reconnaissance and surveillance missions.

The Falcon Heavy then released the ARMADILLO CubeSat developed at the University of Texas at Austin. ARMADILLO carries a dust detector to characterize the population of tiny space debris objects in low Earth orbit.

The U.S. Naval Academy’s PSAT 2 and BRICSAT 2 CubeSats, each carrying amateur radio payloads, then separated form the rocket. Then the rocket deployed a Prometheus CubeSat for U.S. Special Operations Command.

NASA’s two Enhanced Tandem Beacon Experiment, or E-TBEx, CubeSats were next to separate from the rocket. The CubeSats will transmit radio signals down to receiving stations on Earth for scientists to examine how the transmissions are perturbed by disturbances in the ionosphere, a layer in the upper atmosphere through which GPS navigation and satellite communication signals must traverse to reach users on the ground.

The final satellites to separate in the STP-2 mission’s first orbit were the Launch Environment Observer and StangSat CubeSats, which recorded telemetry and environmental data inside their deployer box during launch. The LEO CubeSat was built by students at Cal Poly, and StangSat comes from students at Merritt Island High School in Florida.

With the first batch of satellites away, a second firing by the Falcon Heavy’s upper stage engine propelled the rocket into a circular orbit around 447 miles (720 kilometers) above Earth, with an inclination at 24 degrees, closer to the equator.

Georgia Tech’s suitcase-sized Prox-1 microsatellite was the first spacecraft to deploy in the STP-2 mission’s second orbit. Prox-1, also funded through an Air Force Research Laboratory grant, will test proximity operations and in-orbit inspection techniques after releasing a daughter satellite July 1 named LightSail 2, a crowd-funded CubeSat from the Planetary Society designed to demonstrate the propulsion capability of a solar sail, which harnesses pressure from sunlight for thrust.

A satellite named NPSAT 1 developed at the Naval Postgraduate School next separated from the Falcon Heavy. NPSAT 1 carries two instruments from the Naval Research Laboratory to measure electron cloud densities in Earth’s ionosphere, a layer high above Earth that affects long-range radio communications. Engineers will also use the microsatellite to test a radiation-tolerant computer processor, experimental solar cells, and low-cost memory devices, rate sensors and a commercial digital camera.

The next event was the deployment of the Orbital Test Bed spacecraft built by General Atomics. The Orbital Test Bed, or OTB, mission hosts several payloads, including the Deep Space Atomic Clock experiment from NASA, which will test a new type of hyper-accurate atomic clock that could make it easier for deep space probes to navigate.

Another package attached to the OTB satellite carries the cremated remains of 152 people, including the late astronaut Bill Pogue and space journalist and historian Frank Sietzen. The payload, called “Heritage Flight” and arranged by Celestis, will remain in orbit with the OTB spaceraft for around 25 years.

NASA’s Green Propellant Infusion Mission also separated in the 447-mile-high orbit. Built by Ball Aerospace with a propulsion system from Aerojet Rocketdyne, the mission will test a new type of non-toxic “green” propellant that could be used on future satellites to replace hydrazine, a caustic fuel commonly used on spacecraft because it can be stored for years at room temperature.

Read our full story discussing NASA’s experiments on the STP-2 launch for details on the Deep Space Atomic Clock and the Green Propellant Infusion Mission.

The Falcon Heavy upper stage then maneuvered into the proper orientation for separation of six identical satellites for the Constellation Observing System for Meteorology, Ionosphere, and Climate-2, or COSMIC-2, mission.

The COSMIC-2 satellites, each about the size of a standard kitchen oven, will form a weather observation network collecting data on temperature, pressure, density and water vapor at various layers in Earth’s atmosphere. The COSMIC-2 mission was developed by an international consortium of institutions led by NOAA, the U.S. government’s weather agency, and Taiwan’s National Space Organization, with instrument contributions from the Air Force.

Telemetry data radioed from the Falcon Heavy rocket confirmed all six COSMIC-2 satellites separated as planned, setting the stage for the final phase of the STP-2 mission.

The rocket’s Merlin upper stage engine reignited two more times to target a unique orbit between 3,728 miles (6,000 kilometers) and 7,456 miles (12,000 kilometers) in altitude, with an inclination of 42 degrees to the equator.

The fourth burn set a record for a SpaceX mission. No previous Falcon 9 or Falcon Heavy launch had fired its upper stage engine more than three times.

At 6:04 a.m. EDT (1004 GMT), more than three-and-a-half hours after liftoff, the Falcon Heavy deployed the mission’s final payload — the Air Force Research Laboratory’s Demonstration and Science Experiments, or DSX, spacecraft.

The DSX satellite will fly in a slot region between the Van Allen radiation belts with instruments to measure the effects of very low frequency radio waves on space radiation, space weather conditions and the impact of radiation on electronics and spacecraft materials.

“The space domain has never been more important to our nation than it is today,” said Maj. Gen. William Cooley, commander of the Air Force Research Laboratory. “The DSX satellite experiment will greatly increase our understanding of the environment spacecraft operate in and will give us the knowledge to build even better satellites to protect and defend our space assets. I am immensely proud of the AFRL scientists, engineers, and technicians that conceived and built the DSX satellite.”

In addition to the Air Force experiments, DSX hosts NASA hardware to measure how radiation can corrupt spacecraft memory devices and damage electrical circuits.

With DSX off of the rocket, the Falcon Heavy’s upper stage was to be “passivated,” or put into a safe configuration, by dumping the rocket’s leftover propellant overboard. The passivation milestone was expected to mark the end of the full STP-2 launch sequence, which was expected to last between six and seven hours.

Tuesday morning’s launch was designed to exercise the Falcon Heavy to its limits, allowing SpaceX and the Air Force to collect data to ensure the rocket is ready to lift the military’s most expensive national security payloads into orbit.

“Volume-wise, the payloads take up about a third, maybe a little more than a third, of their payload fairing,” said Mike Marlow, the STP-2 mission manager at Kirtland Air Force Base, New Mexico, in a conference call with reporters before the launch. “But performance-wise, because we’re going to three different orbits, it takes up all of the Falcon Heavy’s performance, actually.”

The satellites on-board the STP-2 mission, while unique and valuable, are all experimental. The Air Force holds launch vehicles assigned to carry operational reconnaissance, communications and navigation payloads to a higher standard.

The Air Force announced the Falcon Heavy was certified after its inaugural flight last year, making it eligible to win contracts to launch the military’s most critical operational satellites. The Air Force signed the contract for the STP-2 mission with SpaceX in December 2012 as a purely experimental mission.

Since last year’s certification milestone, the Air Force has awarded SpaceX two launch contracts for missions codenamed AFSPC-44 and AFSPC-52, which are scheduled for launch from NASA’s Kennedy Space Center in late 2020 and early 2021.

The STP-2 mission will now be the third certification flight for the Falcon Heavy as the Air Force prepares to entrust the launcher with more important payloads.

“What we’re doing now is what we call the spaceflight worthiness process,” said Col. Robert Bongiovi, director of the launch enterprise systems directorate at the Air Force’s Space and Missile Systems Center.

“This launch, STP-2, is the third certification flight. It’s one of many sets of data and reviews that we do with SpaceX and any contractor that we’re certifying for (and) doing non-recurring design and validation on … to get to the point where (we) can certify that that launch vehicle is ready to launch the critical national security payloads that we’ll be launching on those two missions,” Bongiovi said before the STP-2 mission.

The Air Force will also use the experience gained from the STP-2 mission to help certify reused rocket hardware for national security missions. The Air Force’s launches with SpaceX, to date, have all used newly-built Falcon 9 boosters.

“The launch was originally just an opportunity to characterize the launch vehicle for future use by the National Security Space Launch program, but now it is the Air Force’s first launch using previously-flown rocket hardware,” Bongiovi said.

With Tuesday’s Falcon Heavy flight, SpaceX has re-flown a Falcon booster 24 times since March 2017, all successfully.

“The use of the previously-flown hardware is providing critical insight into reusability and quality assurance that will allow us to provide space access to the warfighter in a more cost-effective and expedient manner, and I really appreciate the efforts of our industry partner SpaceX to make this happen,” Bongiovi said.

In the AFSPC-52 launch contract announced last year, the Air Force agreed to pay SpaceX $130 million for a Falcon Heavy mission. The Delta 4-Heavy rocket, the biggest vehicle the fleet of SpaceX rival United Launch Alliance, sells for about $300 million per flight.

If SpaceX convinces the Air Force to certify reused rockets for national security missions, the price of a Falcon Heavy mission could further fall.

“The reason that we’re excited about this, and about having previously-flown hardware on (STP-2) is that we’ve been able to fo follow along as we’ve done recovery and refurbishment of those boosters,” Bongiovi said.

The STP-2 mission was originally supposed to launch with all-new boosters, but the Air Force and SpaceX agreed late last year to change plans and fly reused side boosters.

STP-2 was supposed to launch on the second Falcon Heavy mission, but the contract modification pushed the STP-2 launch behind the launch of the Arabsat 6A telecom satellite in SpaceX’s queue.

The boosters from the Arabsat 6A mission were among the most “gently-used” in SpaceX’s inventory, Air Force officials said. They encountered relatively benign aerodynamic forces and structural loads on their descent back to Florida’s Space Coast in April.

The Air Force’s launch contract with SpaceX for the STP-2 mission was previously valued at $185 million, according to Lt. Col. Ryan Rose, chief of the small launch and targets division at Kirtland Air Force Base.

The launch is now costing the Air Force around $160 million, and a “big factor” in the cost reduction was the military’s agreement to fly the STP-2 mission with reused rocket boosters, Bongiovi said.

With STP-2 in the books, SpaceX is gearing up for a pair of Falcon 9 launches from Florida’s Space Coast in July.

One of the flights, set for July 21, will launch a Dragon cargo capsule on a resupply mission to the International Space Station.

SpaceX is preparing different Falcon 9 rocket to launch the Amos 17 communications satellite as soon as late July for Spacecom Ltd. of Israel. A target launch date for that mission has not been announced.

Meanwhile, Air Force launch teams will turn their attention to three missions in July.

The Air Force is supporting an atmospheric abort test of NASA’s Orion crew capsule scheduled for July 2 at Cape Canaveral. A test version of the Orion spacecraft will blast off aboard a converted Air Force Peacekeeper missile to demonstrate the capsule’s ability to escape from a launch failure.

Air Force communications and navigation satellites are set for launch from Florida’s Space Coast on July 12 and July 25 aboard United Launch Alliance Atlas 5 and Delta 4 rockets.

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Follow Stephen Clark on Twitter: @StephenClark1.