Amid technical problems and delays with its new Minotaur 4 rocket, the U.S. Air Force has swapped payloads for the launcher’s planned April debut, opting to place a suborbital hypersonic demonstration vehicle ahead of a critical space surveillance satellite on the manifest.

The Minotaur 4, built by Orbital Sciences Corp. of Dulles, Va., was supposed to debut this past fall with the launch of the Air Force’s Space Based Space Surveillance satellite, which is designed to keep tabs on objects in geostationary orbit. That launch was put on hold when the rocket’s third-stage motor was found to be producing more thrust than expected. The Minotaur 4’s first three stages are based on excess U.S. ballistic missile hardware.

Now, the rocket is scheduled to debut April 15 carrying the Defense Advanced Research Project Agency’s Falcon Hypersonic Technology Vehicle (HTV), Air Force Capt. Christina Hoggatt said in a Jan. 11 e-mailed response to questions. Falcon HTV will be placed into a suborbital trajectory to demonstrate advanced aerodynamic materials that could be used for hypersonic flight, generally accepted to be five time the speed of sound and beyond.

The Space Based Space Surveillance System, meanwhile, is now expected to launch in June.