五角大楼4月24日宣布：美国空军将在5月20日执行第4次X-37B太空战机任务，据称本次任务的重点在于载荷测试。美空军官员拒绝透漏第4次太空战机任务会持续多久。

USAF to begin fourth X-37B space plane mission in May

                               
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Marina Malenic, Washington, DC - IHS Jane's Defence Weekly
27 April 2015
   
X-37B OTV-1 is shown here shortly after landing at Vandenberg Air Force Base in California after a successful maiden flight that lasted more than seven months. The USAF has not disclosed how long OTV-4 is expected to last. Source: AP/Vandenberg AFB

The US Air Force (USAF) plans to launch its Boeing X-37B unmanned space plane to begin a fourth mission next month, the Pentagon announced on 24 April.

The X-37B, which looks like a miniature version of NASA's space shuttle, is again expected to be launched from a United Launch Alliance Atlas V rocket. The launch from Cape Canaveral Air Force Station, Florida, is scheduled for 20 May.

"We are excited about our fourth X-37B mission," Randy Walden, director of the USAF's Rapid Capabilities Office, said in a statement. "With the demonstrated success of the first three missions, we're able to shift our focus from initial checkouts of the vehicle to testing of experimental payloads."

The Rapid Capabilities Office plans to test the performance of an experimental propulsion system developed by the Air Force Research Laboratory (AFRL) and Space and Missile Systems Center (SMC), among other experiments, according to the statement. A NASA "advanced materials investigation" is also planned for the mission.

"We're very pleased with the experiments lined-up for our fourth [Orbital Test Vehicle] OTV Mission, OTV-4," Walden said. "We'll continue to evaluate improvements to the space vehicle's performance, but we're honoured to host these collaborative experiments that will help advance space technology."

Boeing's Phantom Works advanced technology division has built two X-37B OTVs for the USAF. The spacecraft are about 29 feet (8.8 m) long. The X-37B completed its last mission, OTV-3, on 17 October 2014, spending 674 days on-orbit. One of the two vehicles completed the first and third missions, while the second spacecraft was used for OTV-2. OTV-1 launched in April 2010 and stayed on orbit for 225 days; OTV-2 launched in March 2011 and orbited for 469 days. USAF officials declined to disclose how long OTV-4 is supposed to last.