The B-28 mission patch. Photo: NRO/Vandenberg AFB

Efforts to repair a leaky steering device aboard a Titan 4B rocket has forced the U.S. Air Force to delay the upcoming launch to deliver a top-secret spy satellite into orbit.

The rocket is now slated for liftoff on the night of August 10 from Vandenberg Air Force Base in California carrying a classified cargo for the U.S. National Reconnaissance Office. Launch will occur sometime within a four-hour period extending from 7 to 11 p.m. local time (10 p.m. to 2 a.m. EDT; 0200-0600 GMT on 11th).

Officials had hoped to fly the mission on July 31, but during recent pre-flight processing at the oceanfront Space Launch Complex-4 East pad, a hydraulic actuator on one of the rocket's twin solid-fueled rocket boosters was found to have a leak.

There are two such actuators on each solid rocket motor nozzle, which steer the vehicle during flight and keep it on the proper course by controlling pitch, yaw and roll.

Acting like a complex piston, the device works by receiving a signal from the rocket's guidance computer and in turn applying high pressure hydraulic fluid to pivot the nozzles.

The Air Force said in a statement Thursday the faulty actuator had a "small" hydraulic leak in one of its seals of approximately one-drop of oil every three-minutes while at flight pressure.

"This leak is in violation of the requirement for zero leakage through this seal," the statement read.

As a result, the Air Force and Titan-maker Lockheed Martin slipped the launch so technicians could remove and replace the actuator, and give time to retest the new one.

The delay is the second to occur recently. The Titan 4 launch had been scheduled for a July 17 date, but that was slipped previously so Lockheed Martin to double-check wiring harnesses aboard the rocket to ensure workers hadn't damaged them by using the wrong tool.

The two pads of Space Launch Complex-4. Photo: Spaceflight Now

Meanwhile, the slips have had a ripple effect by also postponing the scheduled launch of a new U.S. government weather satellite aboard a Titan 2 rocket from neighboring Space Launch Complex-4 West.

Slated for August 18, the launch of the NOAA-L polar-orbiting weather spacecraft has slipped to September 12 during a window of 3:22 to 3:32 a.m. local time (6:22-6:32 a.m. EDT; 1022-1032 GMT).

The reason: Officials don't want the crucial weather satellite at the launch pad, attached to the Titan 2 rocket, until after the Titan 4 flies. If the massive Titan 4 were to fail moments after liftoff, the powerful explosion and raining debris could also destroy the $156 million NOAA-L.