The launch of a United Launch Alliance Atlas 5 rocket carrying Boeing’s hard-luck Starliner capsule has been delayed another four days, from Friday to next Tuesday, to give engineers time to make sure a small helium leak in the crew ship’s propulsion system has been resolved, officials said Tuesday.

Liftoff from pad 41 at the Cape Canaveral Space Force Station is now targeted for 4:43 p.m. EDT May 21, setting up a docking at the International Space Station the following afternoon. The flight is expected to conclude with a landing in White Sands, New Mexico, around May 30.

Mission commander Barry “Butch” Wilmore and co-pilot Sunita Williams had hoped to take off on the Starliner’s first piloted flight last Monday. They were in the process of strapping in when the countdown was called off because of trouble with an oxygen pressure relief valve in the rocket’s Centaur upper stage.

Two days later, the Atlas 5 was hauled off the launch pad and back to ULA’s nearby Vertical Integration Facility where the suspect valve was replaced. Tests confirmed the rocket is good to go for another launch try.

The unrelated helium leak in the Starliner’s propellant pressurization system was noted during the countdown last week, but it remained within safe limits for flight. After the Atlas 5 and Starliner were rolled back to the VIF for the oxygen valve replacement, managers decided to take a closer look at the helium issue.

The leak was detected in distribution manifold inside one of four “doghouse” assemblies spaced around the exterior of the Starliner’s drum-shaped service module. Each doghouse features four orbital maneuvering and attitude control — OMAC — thrusters and four small reaction control system maneuvering jets.

Pressurized helium gas is used to push propellants to the rocket motors in each doghouse. The leak was traced to a flange on a single RCS thruster.

Bolts were retorqued and engineers believe the system is flight ready. But managers decided to pressurize the helium lines throughout the spacecraft to make sure the lines are, in fact, leak-free or within acceptable limits.

“As a part of the testing, Boeing will bring the propulsion system up to flight pressurization just as it does prior to launch, and then allow the helium system to vent naturally to validate existing data and strengthen flight rationale,” the company said in a statement.

“Mission teams also completed a thorough review of the data from the May 6 launch attempt and are not tracking any other issues.”

Wilmore and Williams, both veteran Navy test pilots and astronauts with four flights to the station between them, flew back to the Johnson Space Center in Houston last Friday for additional simulator training. They are expected to fly back to Florida late this week to gear up for another launch try.

The Starliner is one of two commercially built crew ferry ships ordered by NASA in the wake of the shuttle program’s retirement in 2011. SpaceX won a contract valued at $2.6 billion for development of the company’s Crew Dragon spacecraft and Boeing was awarded $4.2 billion to develop the Starliner.

The goal was to spur development of independent, commercially-operated spacecraft capable of ferrying astronauts to and from the International Space Station. Spacecraft from different vendors will allow NASA to continue sending crews to the space station even if a problem grounds one company’s ship.

SpaceX launched its first two-man crew in 2020. Since then, the company has launched eight NASA-sponsored crew rotation flights to the station, three commercial research missions to the lab and a privately-funded, two-man, two-woman trip to low-Earth orbit. In all, 50 people have flown to space aboard Crew Dragons.

Wilmore and Williams will be the first astronauts to fly aboard a Starliner after a series of technical glitches that included major software problems during an initial unpiloted test flight in December 2019 and corroded propulsion system valves that delayed a second uncrewed test mission to May 2022.

The second test flight was a success, but engineers ran into additional questions about parachute harness connectors and protective tape wrapped around wiring that posed a fire risk in a short circuit. Work to correct those issues and others delayed the first piloted launch to this month.

The Atlas 5 oxygen valve problem was United Launch Alliance’s responsibility. The helium leak responsible for the latest delay goes on the Starliner list, but it apparently is a relatively minor problem and would not have prevented launch last week.