United Launch Alliance (ULA) is targeting Dec. 24 for the first launch of its Vulcan-Centaur rocket from Cape Canaveral SFS, potentially to be followed by the debut flight of Sierra Space’s Dream Chaser cargo ship to the International Space Station (ISS) in April aboard Vulcan’s second flight.

The missions, known as Certification-1 and 2 (Cert-1 and 2), are required before ULA can begin flying National Security Space Launch (NSSL) payloads for U.S. military organizations.

Vulcan’s debut was most recently delayed by a problem with the rocket’s Centaur V upper stage, which is nearly twice the size of the current Centaur III that flies on ULA’s Atlas rockets. Designed for high-energy, long-duration missions, Centaur V is 40 ft. long, 18 ft. in diameter and built from stainless steel sheets thinner than a dime. The metal is welded to form a cylinder shape with domes at the top and bottom. The vehicle is propelled by liquid hydrogen and liquid oxygen.

ULA was about halfway through its 15th Centaur V structural test at NASA’s Marshall Space Flight Center on March 29 when a hydrogen leak developed near the vehicle’s forward dome due to a previously unknown loads issue and weaker-than-expected joint welds. ULA is reinforcing the upper stage to address the issues.

Cert-1 will carry a commercial lunar lander for Astrobotic. Cert-2 would launch Sierra Space’s first Dream Chaser cargo ship to the ISS for NASA.

ULA plans to use the Centaur V earmarked for the first NSSL mission (Vulcan’s third flight), which was the first to undergo modifications, for Cert-1. The upper stage previously planned for Cert-1 will be retrofitted for Vulcan’s third flight, the NSSL mission.

Cert-2 is expected to launch Sierra Space’s first Dream Chaser cargo ship to the ISS. The flight, previously targeted for December and then delayed to early 2024, is now tentatively slated for April 2024, Janet Kavandi, Sierra Space president and chief science officer, told Aerospace DAILY at the AIAA’s Ascend conference in Las Vegas.

The Dream Chaser’s launch date is contingent on certification and successful launch of ULA’s first Vulcan rocket, launch vehicle integration and testing, and ISS schedules. Boeing’s long-delayed crewed flight test of its Starliner capsule is also slated for April.

Aerospace DAILY understands Sierra Space plans to ship the Dream Chaser vehicle in coming weeks to NASA’s Neil Armstrong Test Facility in Ohio for thermal vacuum, vibration and other testing. The spacecraft would then be shipped in January 2024 to Florida for launch preparation.