The first launch of Orbital Sciences’ Antares is due shortly, the last in a series of new American launch vehicles (LV) for some time to come. Others that are due shortly, notably SpaceX’s Falcon v1.1 and Falcon Heavy, are modifications (significant ones) of existing LVs.

Antares is built to launch the Cygnus, an uncrewed supply capsule, to the International Space Station (ISS) under the commercial resupply services (CRS) programme. In the wake of the Space Shuttle, NASA needed new vehicles to meet commitments for resupplying the ISS.

Antares was not NASA’s initial choice. Kistler/Rocketplane and SpaceX were both selected for development funding as part of the commercial orbital transportation services (COTS) programme, but Kistler quickly went bust and was liquidated. In its wake, with the continued desire to create competition, $171 million was diverted to Orbital.

NASA later signed for eight resupply launches, and Orbital received $1.9 billion from the deal.

The launch vehicle’s first stage is powered by two Aerojet AJ-26s, essentially dusted-off Kuznetsov NK-33s that were shelved with the Soviet Union’s moon programme. US company Aerojet bought 47 NK-33s for conversion (nine are actually NK-43s, the upper stage version of the engine), and although the deal includes rights to set up its own assembly line, it appears that the cost will make the option unlikely. In any case, at least 50 more NK-33s remain in storage in Russia.

An ATK Castor 30 solid rocket powers the upper stage, a derivative of the Castor 120, which itself is a derivative of motors that powered Peacekeeper intercontinental ballistic missiles.

Two third stages are offered for the launch vehicle if required for the payload – a solid motor based on the ATK Star 48 and an ATK liquid engine adapted from in-space satellite propulsion. NASA does not require either, and no other customers have placed orders.

During a 2011 test, one of the AJ-26s caught fire. The resulting investigation found small cracks throughout some of the engine’s components, traced to corrosion from sitting in untended storage. Aerojet was forced to go through their engine supply and repair some of the flight-worthy examples. The engines have since been tested and tested again – including a recent hot-fire test on the launch pad – and both Aerojet and Orbital are confident of success.

Significant delays were caused, partially because of an unfinished launch pad at the Mid-Atlantic Regional Spaceport on Wallops Island, Virginia. As part of the deal to launch in Virginia, the state government agreed to build the launch pad, working through the Virginia Commercial Space Flight Authority. While two LVs sat in a nearby building, all but ready for launch, the Space Flight Authority was forced to transfer construction authority to the state Department of Transportation with its long experience of building Virginia’s roads. The pad is now finished, except for a few tweaks that will have to wait until after the launch.

The April launch will almost exactly simulate the launch system up to the point of payload release. Rather than a Cygnus capsule, the April launch will loft an instrumented mass simulator, which will make a few dozen orbits and burn up in the atmosphere. If successful, the first official resupply mission will be launched in June with supplies for the ISS.

“The pad operations, the first stage, the second stage will all be virtually identical to what we will do on the first operational mission in June, including the trajectory, the altitude, the team that’s operating it across the board,” says Frank Culbertson, Orbital’s vice-president of advanced systems. “Once we achieve orbit, the mass simulator will separate from the second stage and stay in orbit a few days before it re-enters – it is designed to burn up easily. In the meantime we will get data from the sensors that are installed there, we’ll get video, we’ll be able to confirm the fairing separates as it should… this mission will essentially be, up to that point, identical to the one that goes to Station later this year.”

The fourth CRS flight – the fifth overall – is scheduled to be the debut of a larger second stage engine, the Castor 30XL, required to launch the enhanced Cygnus capsule.

Cygnus is capable of carrying around 2,000kg (4,100lb) in a pressurised cargo hold, but is unable to survive re-entry into Earth’s atmosphere. The enhanced version of Cygnus will carry an additional 700kg. The first two Cygnus capsules are all but completed; the first to fly is already filled with cargo and has arrived at Wallops Island for integration with its service module.

The design of the enhanced Cygnus is largely finished, and assembly is scheduled to begin shortly. Testing of some components, including tests of the new solar panels, is ongoing.

The launch vehicle has qualified for both the Department of the Defense’s Orbital/Suborbital Programme-3 and the NASA Launch Services II contracts, the overarching contracts through which those organisations buy rockets and launch services. The qualifications mean that Orbital is able to bid with Antares for suitable launch contracts, although these are subject to the new entrant’s criteria that limit launches of certain payloads based on the number of sequential launch successes.

“We’ve talked to many customers in the government and outside the government, but I believe people are probably waiting for us to get a couple flights off before they’ll commit to anything,” says Culbertson.

Despite Orbital’s and Aerojet’s best efforts, all involved will be holding their breaths until word of successful orbit, and with good reasons. Launching any rocket is a gamble to some degree, especially for a brand new one. Of the 20 new LVs and significant derivatives launched worldwide since 1990, just over half failed on first launch, according to Flightglobal’s Ascend launch database.

“The important thing is, this is one of the biggest programmes in the company, so it’s very, very important to succeed on,” notes Culbertson. “It’s taken longer and cost more money than we anticipated, and we’ve got a lot of our own invested in it. The significance of this going well cannot be overstated.”