This puts the total number of days spent on-orbit for the OTV program at 2,085.
The Air Force will launch the fifth X-37B mission from Cape Canaveral later this year.
The X-37B has been shrouded in mystery, leading to considerable speculation regarding its purpose as a civilian, military or surveillance vehicle.
In July of 2015, Aerojet Rocketdyne stated that the company’s modified XR-5 Hall Thruster had completed successful on-orbit operations on an X-37 space plane.
The new thruster model, an ion engine not unlike the ones aboard the Dawn deep space probe, uses ionized noble gases to generate thrust. While the thrust generated by an ion engine isn’t as forceful as that created by a traditional rocket engine, it can continue to work for years at a time.
The X-37B program is managed by the Air Force Rapid Capabilities Office, which uses the space plane to conduct “risk reduction, experimentation and concept of operations development for reusable space vehicle technologies.”
"The landing of OTV-4 marks another success for the X-37B program and the nation," said Lt. Col. Ron Fehlen, X-37B program manager. "This mission once again set an on-orbit endurance record and marks the vehicle's first landing in the state of Florida. We are incredibly pleased with the performance of the space vehicle and are excited about the data gathered to support the scientific and space communities. We are extremely proud of the dedication and hard work by the entire team."
Though the Air Force’s statement makes no mention of ion thruster testing, they do crow about the X-37B’s ability to “land, refurbish and launch from the same location.” In this post-Space-Shuttle age, I guess that’s something to hang your hat on. Whatever covert research is taking place on the X-37B is probably pretty interesting as well, assuming we ever get to hear about it.
For more USAF news, check out Boeing Air Force Trainer Completes First Test Flight.