The Boeing Defense X-37 serves the United States Air Force (USAF) and NASA as a reusable, long-duration, unmanned spacecraft and has, to date, completed several low- and high-profile flights to and from Low Earth Orbit (LEO). The craft is categorized as an 'Orbital Test Vehicle' and therefore takes on the acronym of 'OTV' in publication. The X-37 now stands as the dimensionally smallest and lightest of all of the orbital-minded spacecraft ever flown (it is decidedly smaller than the original NASA space shuttle fleet).
Boeing's work on the earlier X-40 glide vehicle no doubt influenced the design of the X-37 (the X-37 is slightly larger than the X-40). The X-37 was born through the X-37A form which was the initial design pushed by NASA. In 2004, the craft passed to DARPA under classified terms and the Approach and Landing Test Vehicle (ALTV) was derived from the earlier work, ultimately used for drop-glide testing during 2005 and 2006. The X-37B marks the current (2017) operational model of the series and is largely based on the X-37A with modifications made to suit growing requirements. A scaled-up version is also planned as the X-37C by Boeing, nearly 200% larger than the B-model currently in use. The X-37C will be mated to the Atlas V 'Evolved Expendable Launch Vehicle (EELV) also being planned.
Externally, the X-37B shares some visual similarities with previous STS space shuttle craft (beyond the traditional black-and-while color scheme). The nose is blunt yet aerodynamic for atmospheric travel and integrated into the dorsal spine of the vehicle (since no manned flight deck is needed). The fuselage is nearly-slab-sided with a curved top and near-flat bottom. The wing mainplanes are positioned low along the fuselage sides (as in the space shuttle) and at midships with sweep found along the leading edges only. No horizontal tailplanes are used - instead, outward-cranked vertical fins serve a dual purpose during atmospheric flight. The undercarriage is of a tricycle arrangement and fully-retractable with a double-wheeled nose leg and single-wheeled main legs. These are used for traditional runway landings.
Internally, the craft features precision maneuvering thrusters at both its nose and tail sections. Avionics are positioned near the nose. Aft of the avionics fit is the JP-8 kerosene-based fuel-tank and a hydrogen peroxide tank is positioned further aft (electrical power is by way of a Gallium arsenide solar cell-based unit with lithium-ion batteries). Separating the two tanks is the payload bay area making up the central portion of the fuselage. The main engine exhausts through a nozzle at the extreme rear of the spacecraft.
Dimensions of the vehicle include an overall length of 29.2 feet, a wingspan of 14.10 feet and a height of 9.5 feet. Maximum Take-Off Weight (MTOW) is 11,000lb. The payload measures 7 feet x 4 feet. Orbital speeds (Low Earth Orbit) reach 17,426 miles per hour.
Launching of the X-37 is accomplished by way of an Atlas V 501 series rocket (by United Launch Alliance) with a boosted 'Centaur' rocket stage adding additional propulsion during the voyage. The X-37 sits within a payload fairing prior to launch and is then released when the desired altitude is achieved. Landing is autonomous and speeds nearing Mach 25 are reached during this action. The X-37 is the only space plane, after the Soviet-era 'Buran' shuttle, to feature this autonomous landing capability. The craft is reusable.
To date (2017), there have been four major missions involving the X-37B (flights OTV-1 through OTV-4). OTV-1 was launched on April 22nd, 2010 and conducted mission USA-212 which lasted over 224 days. It completed the first American autonomous orbital runway landing upon its return. OTV-2 launched on March 5th, 2011 for mission USA-226 and remained in space for over 468 days. This mission marked the first-flight of the second X-37B example. OTV-3, launched on December 11th, 2012, went on mission USA-240 which lasted over 674 days. it marked the second flight of the first X-37B example. OTV-4 was launched on May 20th, 2015 for mission USA-261 and marked over 717 total days in orbit. This was the second flight of X-37B and the first flight to land at the Shuttle Landing Facility of Kennedy Space Center in Florida.
Two flyable X-37B spacecraft have been completed to date (2017). Their missions remain largely secretive to the public so speculation abounds as to their true purpose. Some view the series as nothing more than test vehicles collecting data on different components during missions. Others suspect the series to be in line with USAF dominance of space - attempting to remain steps ahead of a rising Chinese space program.