The United States Air Force’s X-37B spaceplane will begin its sixth mission with launch aboard a United Launch Alliance Atlas V rocket Saturday. Atlas is due to lift off from Cape Canaveral during a two-hour, 29-minute window that opens at 08:24 EDT (12:24 UTC), with deployment of the X-37B into low Earth orbit likely to occur about twenty minutes later.
Built by Boeing, the X-37B Orbital Test Vehicle (OTV) is a recoverable, reusable, platform that can operate in low Earth orbit for months or years at a time. The spacecraft facilitates on-orbit research and experiments, as well as other military applications. Most of the specifics of each mission are classified.
The US Air Force has a fleet of two X-37B spacecraft both of which have already performed multiple spaceflights. Saturday’s launch will begin the first X-37B mission operated by the US Space Force – although overall control of the X-37B program and ownership of the vehicles remains with the Air Force.
This sixth flight of the X-37B is designated United States Space Force 7 (USSF-7) – formerly Air Force Space Command 7 (AFSPC-7) – part of a series of generic designations that are increasingly being used to identify US military space launch missions. As the sixth X-37B mission, the flight is also designated OTV-6.
Once on orbit, the X-37B will acquire another public designation, under the USA series that is used for American military satellites. Each X-37B receives a new USA designation each time it enters space. USA designations have been assigned sequentially since 2006, so USSF-7 is expected to become USA-299 on orbit.
The X-37 program was started as a joint venture between NASA and the US Air Force, aimed at developing a recoverable multi-mission spacecraft which could be used for research or satellite servicing and repair. An X-37 would have been carried into orbit in the payload bay of the Space Shuttle before being released to begin its own operations. The Shuttle and X-37 would then have returned to Earth separately after completing their respective missions. NASA awarded an initial development contract to Boeing in 1999.
Following the loss of Space Shuttle Columbia in February 2003, and the subsequent decision to retire the Space Shuttle as soon as its remaining International Space Station assembly missions had been completed, meant that the Space Shuttle was no longer available to launch the X-37. After NASA left the program in November 2004, X-37 was transferred to the remit of the Defense Advanced Research Projects Agency (DARPA).
The X-37 would launch atop a Delta II rocket, without a payload fairing, to conduct missions for the US military. The prototype, X-37A, first flew on a captive-carry flight in June 2005 before beginning drop tests in 2006. X-37A was used to test the design’s performance during atmospheric flight – its glide in to landing at the end of a mission – but was never designed to operate in space.
The Air Force began development of the spacegoing X-37B in late 2006. Another change of launch vehicle saw it manifested to fly aboard an Atlas V rocket – whose five-meter payload fairing could enclose the spacecraft – owing to aerodynamic concerns about launching the spaceplane unencapsulated.
The first X-37B began its maiden flight on 22 April 2010, remaining in orbit for over seven months before making a successful landing in early December of the same year. Four more successful missions have followed, with increasingly long durations. On its most recent mission, which began in September 2017, the X-37 remained in space for over two years before returning to Earth last October.
As well as flying on Atlas V, OTV spacecraft can be deployed by SpaceX’s Falcon 9 rocket, a capability that was demonstrated with the OTV-5 launch in 2017. For Saturday’s launch, the X-37B spacecraft is back atop an Atlas V.
USSF-7 is believed to be using the first X-37B spacecraft, which will be making its third trip into orbit, although this has not been confirmed. After carrying out the program’s maiden flight, OTV-1, in 2010, this spacecraft returned to space in December 2012 as OTV-3, conducting a 22-month mission that ended in October 2014. The spacecraft has likely spent some of the four and a half years since its last flight undergoing maintenance and modification to enable operations with a new service module which will make its debut on this mission. The OTV-2, OTV-4 and OTV-5 missions were flown by the second X-37B vehicle.
On its USSF-7 flight, the X-37B is expected to demonstrate new capabilities – including flying with the service module for the first time. This module, which is attached to the aft end of the spacecraft, hosting additional equipment and experiments for the mission. Ahead of Saturday’s launch Barbara Barrett, the Secretary of the Air Force, stated that this mission would carry out more experiments than any previous OTV flight. The X-37B spacecraft incorporates a payload bay that can be opened in orbit to expose experiments to space. A solar panel, deployed from the bay, provides power to the spacecraft and its experiments.
During the course of its mission, X-37B will deploy the small FalconSAT-8 satellite for the US Air Force Academy. The latest in a series of experimental satellites built by the Academy for technology demonstration and to give cadets experience constructing and operating spacecraft. FalconSAT-8 carries eight experiments. Two of these are being carried on behalf of NASA, investigating the effects of the space environment and radiation on material samples in one experiment, and on seeds in the other. FalconSAT will also conduct an experiment for the Naval Research Laboratory to investigate wireless power transfer, generating electrical power through the satellite’s solar panels and transmitting it to the ground as microwave radiation.
The planned duration of the X-37B’s mission has not been announced, although the spacecraft has already shown that it is capable of remaining on orbit for over two years. It is not clear whether the new service module will remain attached to the X-37B for the duration of its mission, or whether it will be jettisoned partway through. When it is time for the X-37B to return to Earth it will fire its engine for a deorbit burn, lowering the perigee – or lowest point – of its orbit into the Earth’s atmosphere. Following re-entry into the atmosphere the spacecraft will glide down to a runway landing at one of its three designated landing sites.
USAF photo of X-37B post landing safing.
The first three X-37B missions landed at the Vandenberg Air Force Base in California, however since Boeing moved processing operations to NASA’s Orbiter Processing Facility 1 (OPF-1) at the Kennedy Space Center (KSC) beginning at the end of 2014, the two most recent missions have landed at Kennedy’s Shuttle Landing Facility. KSC is likely to be the primary landing site for the USSF-7 mission, with the west coast Vandenberg and Edwards Air Force Bases available as backup sites if weather or other factors prevent a landing in Florida.
USSF-7 will launch aboard United Launch Alliance’s Atlas V rocket, which is flying in the 501 configuration for this launch. This will be the eighty-fourth flight of an Atlas V rocket, one of the most proven and reliable rockets currently in service. Atlas is a two-stage rocket, consisting of a Common Core Booster (CCB) and a Centaur upper stage. It can fly with up to five AJ-60A solid rocket boosters to provide additional performance for heavier payloads or where the rocket is targeting a higher-energy orbit. However, with lightweight payloads like the X-37B Atlas flies without these additional motors.
The three-digit configuration number – in this case 501 – denotes the diameter of the rocket’s payload fairing, number of solid rocket motors and number of engines on the Centaur upper stage. In this case, it means a five-meter fairing, no solid rocket motors, and a single-engine Centaur (SEC). Atlas V can fly with a four or five-meter fairing – with three different lengths available at each diameter – or with no fairing when carrying payloads such as Boeing’s CST-100 Starliner which do not need to be encapsulated. Saturday’s launch will use the shortest version of the five-meter fairing, which is 5.4 meters (17.7 feet) wide and measures 20.7 meters (68 feet) in length. The composite structure is produced by Swiss manufacturer RUAG, who also makes a similar fairing for the European Ariane 5 rocket.
This Atlas V has tail number AV-081, a unique designation assigned to each individual Atlas rocket which began with Atlas-Centaur rockets in the 1960s. The Atlas-Centaur tail numbers (beginning with the letters AC) were continued by the Atlas I, Atlas II and Atlas III rockets which evolved the Atlas-Centaur design, before being replaced with the “AV” series for Atlas V.
Atlas V on SLC-41 – via ULA
Saturday’s launch will take place from Space Launch Complex 41 (SLC-41) at the Cape Canaveral Air Force Station. SLC-41 is the East-Coast home of Atlas V, with the USSF-7 mission marking the rocket’s sixty-ninth launch from this pad. Prior to its role in the Atlas V program, SLC-41 served the Titan family of rockets. Originally constructed as a Titan IIIC launch pad in the 1960s, Launch Complex 41 (LC-41) as it was then designated was part of the Integrate-Transfer-Launch (ITL) complex alongside nearby Launch Complex 40 and a shared vertical integration building.
In the mid-1970s a series of Titan IIIE rockets launched from LC-41 bearing interplanetary missions to study the solar system and beyond – these included the Helios spacecraft that studied the Sun, the Viking missions that orbited and landed on Mars, and the Voyager probes that explored the outer planets and continue to return data from interstellar space. From 1989 until 1998 the pad was used by Titan IV vehicles. In total twenty-seven Titan rockets were launched from the pad before it was torn down and rebuilt for Atlas.
Atlas rockets flying from Space Launch Complex 41 are assembled in the nearby Vertical Integration Facility (VIF), located about 550 meters (1,800 feet) away from the pad. The Common Core Booster – the first stage – of AV-081 was hoisted into position on its mobile launch platform on 27 April – designated as the start of Launch Vehicle on Stand (LVOS) operation. Two days later the interstage, Centaur upper stage and the lower section of the rocket’s payload fairing – which had already been mated together and are known as the base module – were brought to the VIF and lifted atop the first stage. The X-37B spacecraft, encapsulated in the upper section of the payload fairing, was added on 5 May.
The Mobile Launch Platform, with Atlas V mounted atop it, was rolled out from the Vertical Integration Facility and transported to the launch pad on Thursday.
Saturday’s launch will begin with ignition of the Common Core Booster’s RD-180 main engine, about 2.7 seconds before the countdown clock reaches zero. The rocket will reach launch readiness at T-0, with liftoff itself taking place a fraction of a second later. The twin-chamber RD-180 was developed by Russia’s NPO Energomash and is derived from the RD-170 family of engines developed for the Zenit and Energia rockets. The first stage burns RP-1 kerosene propellant, oxidized by liquid oxygen.
About eighteen seconds into her flight, AV-081 will begin a pitch and yaw maneuver to assume the planned trajectory for the climb to orbit. Flying on a north-easterly heading, Atlas will head for an orbit with an inclination of about 44 degrees. Just over seventy seconds after liftoff the rocket will pass through the area of maximum dynamic pressure – or Max-Q – where it experiences peak aerodynamic loads.
When Atlas V flies with a five-meter fairing, the fairing attaches to the interstage between the first and second stages, completely enclosing the Centaur upper stage as well as the payload. Because of this, the fairing must be jettisoned during first-stage flight. By around three minutes, 40 seconds mission elapsed time Atlas will have reached space, and the fairing will no longer be needed, so it can be safely discarded. A few seconds later the forward load reactor will also separate. This device, which attaches at the forward end of the Centaur, helps to spread some of the payload’s weight across the lower half of the fairing.
Booster Engine Cutoff, or BECO, will mark the end of first stage powered flight, about four minutes and 24 seconds after liftoff. The RD-180 will shut down, and about six seconds later the spent CCB will separate. The Centaur’s RL10C-1 engine will enter its prestart sequence, igniting about ten seconds after stage separation. It is at this point in the flight that operations are expected to enter a media blackout – as is normal for launches with certain types of military payload such as the X-37B – with the only subsequent official announcement being a press release confirming the outcome of the launch.
Centaur’s first burn is likely to continue for about fourteen minutes, placing the X-37B directly into orbit. The RL10C-1 engine, which was built by Aerojet Rocketdyne, is an evolution of the RL10 engines developed for the original Atlas-Centaur and Saturn I rockets in the 1960s. It burns cryogenic propellant – liquid hydrogen and liquid oxygen – giving it high efficiency, or specific impulse. Although the RL10 can make multiple burns to deliver spacecraft to complex orbits, it is likely that only one burn will be needed prior to it deploying the X-37B on Saturday’s mission.
A few minutes after the end of the second stage burn – second stage cutoff or SECO – spacecraft separation will occur with the X-37B beginning its mission. While the target orbit for separation has not been announced, and will depend on required conditions for the experiments aboard the OTV, the rocket’s planned ground track suggests it will be inclined at about 44 degrees to the equator, while the altitude will likely be between 300 and 400 kilometers (190-250 miles, 160-220 nautical miles) as on previous missions.
Shortly after spacecraft separation, Centaur will burn its engine again to deorbit itself. The upper stage will reenter on its first orbit, over the Indian Ocean off the coast of Australia.
The X-37B launch is part of a busy weekend at Cape Canaveral, with back-to-back launches of Atlas V and Falcon 9 rockets from adjacent launch pads. The Falcon 9 launch from Space Launch Complex 40 (SLC-40), located 2.5 kilometers (1.5 miles) south of the Atlas pad at SLC-41, is scheduled for 03:53 Eastern Daylight Time (07:53 UTC) on Sunday morning. Falcon will deploy additional spacecraft for SpaceX’s Starlink constellation of low Earth orbit communications satellites.
Saturday’s launch is the third Atlas V mission of 2020, following launches in February and March which deployed the Solar Orbiter and AEHF-6 spacecraft. Once the X-37B launch campaign is complete, Space Launch Complex 41 will be turned over to preparations for the next Atlas launch. This will be the deployment of NASA’s flagship Mars 2020 mission, including the Perseverance rover and Ingenuity uncrewed aerial vehicle (UAV), that is slated for liftoff in mid-July. The tight launch window to get the spacecraft to Mars calls for an extended launch campaign, with additional testing and contingency time built into operations.
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