Boeing X37 remains most successful space plane in last decade. Due its designation as military technology demonstrator it is not well known construction.
Since 2010 X37, unmanned military test space plane is performing test missions in space. Its success is not well known, but is undisputable. In all test missions two operational X37B spent 1367 days - only in 2015 it spent on orbit 200 days without any known issues, which is outstanding result for test construction. In spite of test flights, purpose of X37 missions is the subject of speculations. According to official statement from DoD, X37 is part of testing program of new technologies, avionics and reentry systems. In spite of this, X37 is suitable platform for imaging and reconnaissance satellites tests with particular emphasis on new generation sensors. If it is truth, it means that X37 took over part of the Space Shuttle tasks. Some sources claimed that X37 is part of new antisatellite weapon program, but due the small dimensions and low maneuverability of X37 it is not very probable. Another explanation could be using space plane as a conventional spy satellite with possibility of returning to earth - it could be useful in case when military would like to avoid transferring data with onboard transponders, or when rapid change of observation object is demanded. In such case, X37 would be unmanned successor in 1969 Manned Orbiting Laboratory (designed generally as demonstration manned space station showing possibility of utilizing it for military purposes), but still conventional imaging satellites are performing their function well enough. It is not possible to utilize X37 for lifting military satellites because of small dimensions of payload tray. Another possibility is using X37 for observation other spacecrafts - in 2012 some sources claimed that X37 was spying Chinese space station Tiangong (it becomes not possible because of fact that orbits of Tiangong and X37 practically made any observation impossible). Reasonable explanation is that X37 is part of larger program aimed at creating manned spaceplane with possible of full independent flight if necessary; something like Buran space shuttle in smaller scale. There are few reasons to think in this way. Firstly, Boeing is considering bigger manned version called X37C (announced in 2011 two versions - 165% and 180% of present vehicle) with capability of lifting up to six astronauts and payload. It will be utilizing same launch vehicle (Atlas V) but without fairing. Secondly, military space plane projects have long history, but always without any final steps. After couple attempts in sixties and seventies (for example X20 Dyna-Soar strongly resembles X37 - it was designed to be lifted with rocket and land on conventional runway) projects were halted because of possibility of using Space Shuttle for military purposes. Unfortunately, U.S. Air Force cooperation with NASA was not extremely happy and with disaster of Challenger in 1986, DoD again started to consider necessity of possessing own space plane. It resulted in X30 project (hyper-sonic space plane with possibility of reaching LEO, starting and landing with utilization of conventional runways) with DARPA participation. After cancellation and particular successful using Space Shuttles during nineties, everything changed after Columbia disaster in 2003. After this event it was quickly decided to move X37 project from NASA to DARPA in 2004 after five years of development (NASA awarded Boeing Integrated Defense Systems four years contract for X37 in 1999 for $125 million, in 2002 next contract was signed for $301 million). After many years of attempts U.S. Air Forced reached ultimately goal - possessing own space plane and strengthening separation military space program from NASA. It should not be surprising that DoD would like to develop their space plane not least because of will to remain independent from NASA. Third reason is included in X37 - avionics from X37 was utilized by Boing in their manned CST-100 spacecraft. It seems that X37 consists many features which could predestine it to role of base for designing manned space plane in future.
Since now, four missions were performed by X37B. All were described with "OTV" shortcut which stays for: Orbital Test Vehicle. First, OTV-1 started 22 April 2010 23:58 UTC. Space plane (first manufactured X37B) was put in to orbit 401 km x 422 km with inclination at 39.99°. X37 landed after successfully reentry on 3 December 2010 09:16 UTC with minor problems on runway (one tire failed). Second mission (OTV-2) started for second built X37B on 5 March 2011 and was planned to last 270 days. On 29 November 2011 it was announced that mission duration will be extended to 2012. After 469 days on 315 km × 341 km orbit with inclination at 43°, space plane landed on 16 June 2012. Third mission (performed by first X37B) took place on 11 December 2012 and finished after 674 days on 343 km × 360 km orbit (inclination at 43.5°) on 17 October 2014. Fourth mission (OTV-4) started for second X37B on 20 May 2015 and is planned for at least 200 days on 312 km × 325 km orbit with inclination at 38°.
X37 is unmanned, robotic space plane designed as reusable, second stage of launch vehicle. It is suitable for long duration missions and with its onboard computers and avionics could reenter autonomously and land horizontally on conventional runway. Present operational version of space plane is X37B which is operating from Cape Canaveral Air Force Station and land on Vandenberg AFB, California. Last landing took place on 17 October 2014 after 674 days in space, last start on 20 May 2015. Length of X37B is 8.9 m, height is 2.9 m, wingspan is at 4.5 m. Launch weight is 4990 kg. Propulsion is one AR-2 engine by Aerojet, fueled with hydrogen peroxide/JP-8 and thrust with 29341 kN. Construction is based on advanced airframe with improved thermal protection system based on silica ceramic tiles to help surviving reentry with 25 Ma speed. Onboard avionics, computers and instruments are powered with Gallium arsenide solar cells with lithium-ion batteries. Lifting rocket is Atlas V in 501 version - with 5 m fairing, no boosters and one engine in Centaur stage. During launching on atop Atlas V, X37 remains covered with rocket fairing, which is preventing rocket from any aerodynamic interferences generated by shape of X37 (at the beginning, when X37 was still under NASA, utilizing Delta IV rocket without any fairing).