The X-37B is derived military variant from the NASA’s X-37 Space Vehicle program authorized by US Air Force. NASA initially constructed ALTV variant without propulsion and this glider variant is referred as X-37A.
In 2004, NASA transferred X-37 program to US Air Force under classified project category. In 2006, US Air Force announced for a derived Orbital Test Vehicle or OTV variant of X-37A spacecraft called X-37B.
The design of X-37B resembles NASA’s famous space shuttle but it has very smaller dimension. It has a length around 8.8m. , a height of the vehicle is about 2.9 m with a wingspan around 4.6 m. Besides, it has internal cargo bay area is around 2.1 by 1.2m with launch weight is 11,000 lbs. or 4,990 kilograms.
This solar powered X-37B space plane launches vertically with the aid of a rocket booster. It has designed to operate at altitude range 110 to 500 miles. Then it cruised back to earth and landed on the runway using own landing gear.
According to Space, this classified X-37B already completed four clandestine missions to date with secret payloads. These missions are titled as OTV1, OTV2, OTV3, and OTV 4 all these launches are boosted by using Atlas 5 engine.
However, OTV1 blasted off at April 2010 for the mission duration of 224 days in space. OTV2 is allotted for twice of mission duration of OTV1 and launched in March 2011. While OTV3 launched in 11th December 2012 using the same X-37B vehicle of OTV1 for 674 days mission.The OTV3 program ended in October 2014, reported by Mail Online.
Eventually the last OTV4 is still ongoing mission which began on May 20, 2015. Additionally, the OTV4 mission is second flight of X-37B vehicle used in OTV2. It breaks the record of the longest duration of OTV3 on May 20, 2015.
As per X-37B fact sheet, this program has two primary objectives. Initially, they are testing the reusable capability of a space plane for future US space program. While another one is operating experiments which can be returned to, and examined, on Earth.
Air Force orbital analyst Brian Weeden emphasized that X-37 used as a testbed for next-generation sensors and satellite technologies. He added X-37 is less-than-ideal space weapon because of its less maneuverability. Additionally, it acts like a slow moving glider that can easy to track down by enemy forces.