OSC-COTS  Taurus-II (now Antares)

NASA established the Commercial Orbital Transportation Services (COTS) program to develop a competitive market for crew and cargo flights to the International Space Station (ISS). The program breaks from the traditional government contracting approach by allowing the private sector to develop, own, and operate its own launch vehicles in service of NASA's ISS delivery needs. Unlike today's Space Shuttle, the reusable launch vehicles (RLV's) developed under COTS will also be able to serve commercial customers.
The original COTS announcement listed several capability requirements. These included the ability to lift a combined total of "up to" 16 tonnes of cargo per year to ISS using two to eight flights per year. Thus, the minimum launch vehicle/cargo carrier would need to be able to haul at least 2 tonnes of cargo per flight.

In August 2006, NASA announced its competitive selection of Space Exploration Technologies (SpaceX) and Rocketplane-Kistler (RpK) as partners in the COTS program.

In November 2007, PlanetSpace company, announced that it had teamed with aerospace giants Lockheed Martin and ATK to submit a proposal response for NASA's COTS (Commercial Orbital Transportation Services) program. The proposal was triggered by NASA's October 18, 2007 decision to cancel Rocketplane-Kistler's original COTS contract.

Meanwhile, it can be contemplated alongside other proposed launch vehicles like Orbital Sciences Taurus II and SpaceX Falcon 9.

Orbital is currently in early stage development of his medium-class launch vehicle, dubbed Taurus II, to extend its family of small-class Pegasus, Taurus and Minotaur launchers. The Taurus II design adapts elements from these proven launch technologies along with hardware from one of the worlds leading launch vehicle integrators to provide low-cost and reliable access to space for civil, commercial and military Delta II-class payloads.
Taurus II will be designed to achieve a 98% or greater launch reliability. It will be developed, manufactured and launched using identical management approaches, engineering standards and production and test processes used in Orbitals other major launch vehicles Pegasus, Taurus, Minotaur and OBV missile defense interceptor.
 

The first stage structure will be manufactured by Ukrainans Yuszhnoe company. The stage structure is based on the Zenit launch vehicle and uses two AJ-26-62 (NK-33) engines. The second stage is a Castor-30, which is based on a shortened Castor-120 solid rocket motor. The optional third stage called ORK (Orbit Raising Kit) is based on the propulsion system of Orbitals Star-2 satellite bus. For high energy orbits a Star-48V can be used as third stage.