As of today (March 25), the X-37B has spent 675 days on its latest Earth-circling mission, which is known as Orbital Test Vehicle-4 (OTV-4). The previous record was 674 days, set during OTV-3, which lasted from December 2012 to October 2014.
The ongoing mission of the U.S. Air Force's robotic X-37B space plane is now the longest in the clandestine program's history.
The X-37B is a prototype of a reusable unmanned satellite. The first prototype is for atmospheric testing, while the second has a strengthened structure to support an unshrouded launch, which was originally planned to be launched in the Shuttle payload bay, the transferred after the Columbia accident to a Delta-7920 launch vehicle. Due to problems with the aerodynamics of an unshrouded launch, the first space test was be launched shrouded on a Atlas-5(501) or on a Delta-4M+(5,2) in 2006. Multiple flights with mission durations up to 21 days were planned. NASA asked Boeing to design the X-37 orbital vehicle for a 270-day stay on orbit.
The original X-37 was to feature an AR-2/3 engine for maneuvering and deorbiting, which uses Hydrogen Peroxide and JP-8 as oxidizer and fuel. Later the propulsion system has been changed to a hypergolic nitrogen tetroxide / hydrazine version with a robust propellant load for maneuvering in space and for the deorbit burn.
The X-37's shape is a 120 percent scale derivative of the Air Force's X-40A, also designed and built by Boeing, which was released from a helicopter and glide-tested in 1998. The X-40A, which lacks the X-37's advanced thermal protection materials, rocket engine, experiment bay and other spacecraft systems, was released from a U.S. Army Chinook helicopter in a series of free flight tests in 2001 to reduce technical risk before flight testing the X-37.
NASA has directed Boeing to throttle back on development of the orbital variant of the X-37 prototype space plane until more money is found for the program, an action likely to delay a re-entry and landing demonstration that was planned for 2006.
Following transfer from NASA to DARPA in October 2004, the X-37A technology demonstrator, the Approach and Landing Test Vehicle, recently completed a series of captive carry and free flight tests from the White Knight aircraft, which was also used to launch the SpaceShipOne. The ALTV validated the X-37 program flight dynamics and extended the flight envelope beyond the earlier low speed/low altitude tests conducted by NASA. The ALTV is not capable of space flight.
The OTV program will focus on risk reduction, experimentation, and operational concept development for reusable space vehicle technologies, in support of long term developmental space objectives. The first orbital test flight of the OTV was planned for FY08, with a launch from Cape Canaveral Air Force Station on an Atlas-5(501) launch vehicle. Key objectives of the first flight include demonstration and validation of guidance, navigation and control systems to include fault tolerant, autonomous reentry and landing as well as lightweight high temperature structures and landing gear. Vandenberg Air Force Base will conduct reentry and recovery activities. The first launch did finally take place in 2010.
The first vehicle, X-37B OTV-1 was launched on April 22, 2010 from Cape Canaveral. Although orbital data was classified, the OTV-1 was found in an 401 km × 422 km, 40° orbit by amateur observations. The Centaur upper stage was reportedly disposed off in an escape trajectory to solar orbit. The X-37B OTV-1 conducted several orbit changes during its mission. It landed successfully on December 3, 2010 after 225 days in space. The vehicle was refurbished for a second flight.
The first flight of the second vehicle, X-37B OTV-2, began in March 2011. This time it stayed 469 days in space, more than doubling the endurance of the first mission.
A second flight of the first vehicle, X-37B OTV-3, was launched in December 2012. It returned on 17 October 2014 to the Vandenberg AFB after 675 days in space.
A fourth mission, X-37B OTV-4 was launched in 2015 from Cape Canaveral. The individual orbiter used in this mission has not been identified, but is likely the second one. It conducted an Hall thruster electric propulsion experiment to enable in-space characterization of design modifications that are intended to improve performance to the units onboard AEHF military communications spacecraft. Produced by Aerojet Rocktdyne, the AEHF satellites’ Hall thrusters are 4.5-kilowatt units that use electricity and xenon to produce thrust for maneuvering satellites in space. The on-orbit test plans for the experiment were developed by Air Force Research Laboratory and administered by the Rapid Capabilities Office, which runs the X-37B program. The experiment included collection of telemetry from the Hall thruster operating in the space environment as well as measurement of the thrust imparted on the vehicle.
It's unclear what the new duration record will end up being; most X-37B activities and payloads are classified, and the Air Force has historically been tight-lipped about landing plans.
The Air Force is known to have two X-37B vehicles, both of which were built by aerospace giant Boeing. The X-37B launches vertically and lands horizontally on a runway, just like NASA's now-retired space shuttle orbiters did.
The Air Force vehicle also looks like a miniature version of the space shuttle. Each solar-powered X-37B is just 29 feet (8.8 meters) long and 9.6 feet (2.9 m) tall, with a wingspan of about 15 feet (4.6 m). For comparison, the space shuttle was about 122 feet (37 m) long, with a wingspan of 78 feet (24 m).
Together, these two X-37Bs have flown four space missions to date, each of which has set a new duration record. OTV-1 launched on April 22, 2010, and landed on Dec. 3 of that year, spending a total of 224 days in orbit; OTV-2 lifted off on March 5, 2011, and wrapped up on June 16, 2012 (468 days); OTV-3 launched on Dec. 11, 2012, and landed on Oct. 17, 2014 (674 days); and OTV-4 lifted off on May 20, 2015. (The mission lengths of OTV-3 and OTV-4 may seem equivalent as of today, but the former landed about 2 hours shy of the full 675-day mark.)
The latest three missions have therefore blown past the space plane's designed orbital lifetime, which is just 270 days, according to the Air Force's X-37B fact sheet.
The secrecy surrounding the X-37B program has led to some speculation that the vehicle may be a space weapon. But this is unlikely to be true, experts say; for example, the space plane does not seem big enough or maneuverable enough to be used to manipulate other satellites.
Indeed, the Air Force has always stressed that the X-37B has two primary objectives: testing reusable spacecraft technologies, and helping to conduct experiments that can be returned to Earth for examination.
"Technologies being tested in the program include advanced guidance, navigation and control; thermal
protection systems; avionics; high-temperature structures and seals; conformal reusable insulation, lightweight electromechanical flight systems; and autonomous orbital flight, re-entry and landing."
All four X-37B missions have launched from Florida's Cape Canaveral Air Force Station.
