Roc, the massive bi-fuselage carrier plane operated by hypersonic vehicle manufacturer Stratolaunch, took off from California's Mojave Desert for its sixth test flight on Thursday (June 9) but ended the sortie earlier than expected.
"During the test program, the team encountered results that determined they would not complete the full set of test objectives," the company said in a press release Thursday evening.
"We made the decision to land, review the data and prepare for our next flight," Stratolaunch representatives added in a tweet Thursday at 1:50 p.m. EDT (1750 GMT).
Related: Stratolaunch test photos: The world's largest plane in action
Thursday's test flight focused on expanding Roc's flight envelope with a newly installed pylon on the plane's center wing (between the two fuselages). This pylon will be used to carry Stratolaunch's rocket-powered Talon-A hypersonic test vehicle. Thursday's flight was expected to last around 3.5 hours but landed after only 90 minutes.
Operators were forced to land the craft ahead of schedule due to an unspecified test result, according to the release. Following the unexpected reading, teams turned their focus to general handling and performance of the aircraft with its new hardware for the remainder of the flight. Roc, which has a wingspan of 385 feet (117 meters), reached a maximum altitude of 15,000 feet (4,572 m) during Thursday's test, the company said.
The addition of the new pylon to Roc's center wing represents another step forward for the giant plane, which is designed to carry payloads to a high cruising altitude before releasing them to rocket the rest of the way to orbit (or do whatever else a customer wants it to do). Talon-A, Stratolaunch's 28-foot-long (8.5 m) hypersonic vehicle prototype, is designed to fly as fast as Mach 6, or six times the speed of sound.
A statement from Stratolaunch president and CEO Zachary Krevor in Thursday's press release voiced continued optimism for the program, saying that data from this flight would only build on previous success. "We will leverage this flight experience as we complete integrated testing in the coming months and prepare for Talon-A test flights," his statement read in part.
The company hopes to evolve future iterations of the Talon-A design into a reusable vessel and begin flying hypersonic missions for customers sometime in 2023.
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