SpaceX launched its Falcon 9 rocket for the US military Thursday, carrying the X-37B spaceplane to orbit for its fifth mission. Liftoff for the OTV-5 mission occurred at the first attempt, with a T-0 of 10:00 AM Eastern time (14:00 UTC) out of the Kennedy Space Center’s Pad 39A in Florida. The booster returned to LZ-1 for another successful landing.

Falcon 9 OTV-5 Launch:

Thursday’s launch began the X-37B OTV-5 mission, the fifth flight of the US Air Force’s X-37B Orbital Test Vehicle (OTV), an unmanned reusable spaceplane capable of carrying out operations and experiments in low Earth orbit before returning to Earth.

The Air Force operates a pair of X-37B spacecraft, with missions assumed to alternate between the two vehicles. On this basis, OTV-5 will be the third trip into space for the first X-37B spacecraft.

Developed by Boeing, in partnership with the Air Force and the Defense Advanced Research Projects Agency (DARPA), the X-37B evolved from NASA’s X-37A vehicle.

The X-37A, which conducted a series of glide tests in 2006 but never flew in space, was designed to be deployed from the Space Shuttle to carry out independent satellite servicing or scientific missions, capable of staying on orbit beyond the end of the Shuttle’s mission. NASA began the X-37 program in 1999, however transferred it to DARPA in 2004.

The military X-37B was originally planned to launch atop the Delta II rocket, without a payload fairing. This was abandoned amid concerns over the aerodynamic properties of the rocket with its unencapsulated payload and instead the first four X-37B missions launched atop United Launch Alliance’s Atlas V rocket, inside a five-meter payload fairing.

Thursday’s launch was the first time the X-37B has flown aboard Falcon 9, which also encloses the spacecraft within its fairing.

The X-37B is used to conduct research, technology development and potentially intelligence missions in low Earth orbit. Designed to fly missions of up to 270 days at a time, X-37B’s previous flights have pushed it well beyond this limit, with the recently-completed OTV-4 mission remaining on orbit for 718 days: less than a fortnight short of two years.

Once in orbit, X-37B will deploy a gallium arsenide solar array to charge its batteries and provide power throughout the mission. The spacecraft will also open its cargo bay, allowing experiments to be performed.

The spacecraft measures 8.9 meters (39 feet, 3 inches) in length with a wingspan of 4.5 meters (14 ft, 11 in) and stands 2.9 meters (9 ft, 11 in) tall. At launch it will have a mass of approximately 5,000 kilograms (11,000 lb). The Air Force has confirmed that several miniature satellites are also aboard Thursday’s launch. However, it has not clarified whether these are to be deployed from Falcon 9, or the X-37B itself.

X-37B began its maiden flight in April 2010, boosted to orbit atop an Atlas V 501 rocket flying from the Cape Canaveral Air Force Station.

The OTV-1 mission, also designated USA-212, lasted 220 days before a successful landing at Vandenberg Air Force Base in California.

The spacecraft that flew the OTV-1 mission was later reused for the OTV-3 flight, and is expected to be the one being carried aboard Thursday’s launch for OTV-5.

The second X-37B vehicle began its 468-day debut mission in March 2011. Six months after its June 2012 landing, the first X-37B lifted off to begin the OTV-3 mission in December 2012. This 675-day mission concluded with another Vandenberg landing in October 2014.

The most recent, and longest, mission was the 718-day OTV-4, which began in May 2015 and ended four months ago Thursday. OTV-4, which is presumed to have used the second X-37B spacecraft, was the first X-37B mission to land at the Kennedy Space Center’s Shuttle Landing Facility.

X-37B processing operations have been established at the Kennedy Space Center, using OPF-1, the first bay of the Orbiter Processing Facility which was originally built for the Space Shuttle program.

Ahead of Thursday’s launch the spacecraft was integrated with Falcon’s payload fairing within the OPF, before transport to SpaceX’s integration building at Launch Complex 39A.

Complex 39A (LC-39A) – which is one of two SpaceX launch pads on Florida’s Space Coast, alongside Space Launch Complex 40 (SLC-40) at the nearby Cape Canaveral Air Force Station – is another facility which was previously used by the Space Shuttle.

Built in the 1960s, the launch pad was first used during the Apollo program by the Saturn V rocket. Twelve of the thirteen Saturn V launches, including all of the missions which landed men on the Moon, took place from LC-39A. Apollo 10 was alone in launching from the backup pad, LC-39B. The last Saturn V launch from LC-39A was the rocket’s final flight in May 1973, carrying the Skylab space station into orbit.

At the end of Apollo, LC-39A was rebuilt for the Space Shuttle, with modifications including Fixed and Rotating Service Structures (FSS and RSS) to provide access to the Shuttle stack at the launch pad. SpaceX plans to reuse the Fixed Service Structure for crew access on future manned Dragon missions. However, the RSS is currently in the process of being disassembled.

The Space Shuttle began eighty-two of its 135 missions from Launch Complex 39A, including its first twenty-four and its last eighteen launches. The remaining missions lifted off from pad 39B.

The Shuttle, like Saturn V before it, was integrated in a vertical configuration atop a Mobile Launch Platform, in the Kennedy Space Center’s Vehicle Assembly Building. The assembled stack was then transported to the launch complex atop a Crawler Transporter. While NASA intends to retain this approach for future launches of its Space Launch System from LC-39B, SpaceX has modified LC-39A to use horizontal integration.

Falcon 9 is assembled in a hangar at the base of the launch ramp. The rocket is mated to a transporter-erector, or Strongback, which transports the vehicle to its launch position, rotates it to vertical and provides umbilical connections up until liftoff. The strongback retracts slightly away from Falcon in the last minutes of the countdown, before retracting fully as the rocket begins to lift off.

Thursday’s launch marked the first time the X-37B has been launched by SpaceX, with the spaceplane’s previous missions having ridden into orbit aboard United Launch Alliance’s Atlas V rocket.

Falcon 9 is a two-stage rocket, comprised of a reusable first stage and an expendable second stage, both fuelled by RP-1 kerosene propellant and liquid oxygen. First flown in June 2010, the Falcon 9 has made forty launches prior to Thursday’s mission. The X-37B launch was Falcon’s thirteenth of 2017.

Falcon lifted off during a five-hour, five-minute window which ran from 09:50 to 14:55 local time (13:50 to 19:55 UTC). Had the launch be delayed, SpaceX has a backup launch window reserved for Friday – although there were questions about standing down had SpaceX not launched on Thursday.

The launch attempt came as hurricane Irma was bearing down upon Florida, which could had necessitated the standdown and saw it returned to its hangar to allow the storm to pass.

Had a rollback been required, SpaceX would have needed twenty-eight hours to get the rocket horizontal and secured within its hangar.

In the end, this didn’t become a consideration, as three seconds before liftoff, the nine Merlin-1D engines powering Falcon’s first stage ignited and Falcon began its ascent to orbit.

The rocket passed through the area of maximum dynamic pressure, or Max-Q, about seventy-nine seconds into its flight. Falcon’s first stage burned for two minutes and twenty-three seconds before shutting down its engines, with stage separation occurring three seconds after cutoff.

The Falcon’s second stage ignited its single Merlin-1D Vacuum engine eight seconds after stage separation, continuing to power the X-37B towards orbit. The payload fairing separated from around the X-37B at the nose of the rocket shortly after second stage ignition. However, the timings for this and subsequent events involving the second stage were not been released.

The first burn of the second stage was likely to last around seven minutes. It is not clear, and unlikely to be confirmed by SpaceX or the US Air Force, whether this was the only burn prior to spacecraft separation, or whether the stage fired again to raise its orbit or increase orbital inclination.

While airspace closures are consistent with the launch targeting a low Earth orbit with inclination of approximately 43 degrees, airspace reserved for the second stage’s reentry suggested that it was to be entering the atmosphere from a more highly-inclined trajectory.

It is not clear whether this inclination change would occur before or after the X-37B separates, although the Air Force has indicated that OTV-5’s orbit will have greater inclination than any previous X-37B mission – and a 43-degree orbit was used by OTV-3.

Another purpose for the inclination change could be to deploy secondary payloads into a different orbit. After spacecraft separation, the second stage will make at least one burn, to deorbit itself. The hazard area for second stage reentry shows that this event will take place to the South of Australia. Notices to Airmen, or NOTAMS, show that this hazard area is active from 17:54 to 22:26 UTC (13:54 to 18:28 EDT), suggesting that reentry will occur around three-and-a-half to four hours after liftoff.

While the second stage was inserting the X-37B into orbit, Falcon 9’s first stage made its return to Earth, attempting a landing at the Cape Canaveral Air Force Station. The first stage is designed to make a powered landing, allowing it to be potentially reused for a future launch.

The first stage for Thursday’s launch was Core 1040, a new core that was making its first launch. After separating from the second stage, the core reoriented itself, beginning a boostback burn about thirteen seconds after staging. This engine burn reversed the stage’s downrange motion, carrying it back towards Florida.

After passing apogee, or the highest point in its trajectory, the first stage began to fall back to Earth. As the stage returned to the Earth’s atmosphere, it performed an entry burn to slow itself and reduce heating.

This short burn began six minutes and thirty-four seconds after liftoff. Inside the atmosphere grid fins near the top of the stage, deployed during the coast between the boostback and entry burns, to help to steady and steer the descending core.

The core’s final landing burn began as it approached the ground. Its landing legs deployed and the core guided itself to a precision touchdown at Landing Zone 1 (LZ-1), SpaceX’s landing pad at Cape Canaveral Air Force Station.

Landing came eight minutes and 14 seconds mission elapsed time. Recovering the first stage was a secondary objective for Thursday’s launch. However, SpaceX has had considerable success with this aspect of recent launches, as proven with yet another successful landing.

Built on the site of the former Atlas Launch Complex 13 (LC-13), Landing Zone 1 is used for missions where Falcon 9 has a sufficient performance margin to return the first stage to the launch site after separation. Where performance is at more of a premium than it is during the X-37B launch, SpaceX can deploy a barge, the Autonomous Spaceport Drone Ship (ASDS) to recover the stage downrange, or fly the rocket in a fully expendable configuration.

Following a successful first stage landing, SpaceX will need to work quickly to secure the first stage and remove it from the landing zone before Hurricane Irma arrives.

Thursday’s launch was the fifty-fifth orbital launch of 2017 worldwide, excluding an Iranian attempt in July which may have been a suborbital test flight, or failed orbital launch, of the country’s Simorgh rocket. So far in 2017 SpaceX has conducted now 13 successful Falcon 9 launches, with the rocket’s next missions slated for early October.

A launch from Vandenberg Air Force Base, currently slated for 4 October, will carry a third batch of ten Iridium-NEXT communications satellites – joining those launched in January and June. The next East Coast launch, expected to deploy the SES-11 satellite, will mark the third time Falcon flies with a flight-proven – reused – first stage.

(Images: SpaceX, USAF, ULA, Boeing and L2 imagery/Brady Kennison for NASASpaceFlight.com)