Barring additional difficulties getting a new launch pad ready at Wallops Island, Va., Orbital Sciences Corp. expects to begin fit checks and other pad work with its first Antares cargo launch vehicle late next month or early in April.

That would clear the way for a first launch of the liquid-fueled rocket late in June or early in July, and its first flight to the International Space Station with a load of cargo late in August or in September. Conceivably the first mission under the company’s eight-flight, $1.9 billion commercial resupply services (CRS) contract with NASA could come before the end of the year.

“I want to stress that this sequence of events assumes that all major activities proceed smoothly without major technical problems, which if they were to occur would likely delay subsequent milestones from these targeted dates,” Orbital CEO David Thompson said Feb. 21 in reporting company earnings for 2011.

Problems getting the greenfields Antares pad at Wallops certificated for propellant handling and pressurization delayed the schedule “eight to nine months,” Thompson said, and traffic at the ISS could thwart the notional plan to begin docking at the station at the end of the year.

The first of those flights would complete Orbital’s Commercial Orbital Transportation Services (COTS) milestones and clear the company to begin delivering cargo under the CRS contract.

Orbital developed the Antares (formerly Taurus II) and the pressurized Cygnus cargo vehicle under a COTS space act agreement, which pooled company and NASA funds to develop private vehicles for ISS resupply.

The Cygnus vehicle for that flight is complete and in thermal vacuum testing at the company’s Dulles, Va., satellite factory.

It should be delivered to Wallops Island in June, Thompson said. The first Antares reached the “98% complete point” at the end of December, he said, and will support the new launch schedule.

But the equipment needed to handle liquid oxygen and liquid nitrogen for the vehicle has needed redesign and repair, and won’t be tested and certificated until the end of April.

Thompson said the first vehicle may be rolled from its integration facility to the pad and erected early in April for fit checks and other work, and then returned to the integration building to await a hold-down host-fire test after the pad is fully tested and certificated.

That test should come in May, followed by the test flight with an instrumented Cygnus mass simulator by midsummer. The COTS demonstration flight to deliver the first Cygnus to the ISS for grappling and berthing with the station’s robotic arm would come next, station traffic permitting, followed by the first CRS flight.

“Assuming the first three go well, then I think the fourth one is a little bit anticlimactic because we will have fully demonstrated on multiple occasions most aspects of the cargo delivery system and at least on one occasion all aspects of it,” Thompson said. “So our focus right now is on those things we can control centered on those first couple of big milestones coming up in the – mostly in the second quarter.”