The launch manifests of NASA's two commercial cargo service providers will bump up against one another in December when Space Exploration Technologies (SpaceX) and Orbital Sciences Corp. are slated to send competing rockets and spacecraft to the International Space Station (ISS).

NASA says both missions were intentionally scheduled for the end of 2013 in anticipation that one will experience delays in meeting target launch dates.

“We think one or the other will slightly fall behind and allow us then to fly a cargo flight in the December time frame,” William Gerstenmaier, head of NASA's Human Explorations and Operations Directorate, said July 29. “So there's an advantage of having the two providers in that area for cargo.”

SpaceX's Falcon 9 rocket and Dragon cargo capsule conducted their first two runs to the ISS, in October 2012 and March of this year, respectively. The latter marked the Hawthorne, Calif.-based SpaceX's first mission under a fixed-price Commercial Resupply Services (CRS) contract with NASA valued at $1.6 billion. However, the company is now completing work on a larger version of the launch vehicle that will deliver the next Dragon to the space station, an upgrade that has suffered repeated delays.

Known as Falcon 9 v1.1, the new rocket will feature a more powerful engine, a larger payload fairing and extended fuel tanks, all of which will enable it to reach Geostationary Transfer Orbit (GTO), the destination of most commercial communications satellites.

Initially slated to debut last fall, the v1.1 is not expected to launch before September, when it will loft a small Canadian science and technology demonstrator to an elliptical low Earth orbit from SpaceX's new launch pad at Vandenberg AFB, Calif. If successful, the demo is to be followed by SpaceX's first mission to GTO, during which the rocket will launch a commercial communications satellite from Cape Canaveral for Luxembourg-based fleet operator SES. In addition to the upcoming SES-8 mission, commercial launches for Thailand's Thaicom and Fort Lee, N.J.-based Orbcomm are also scheduled this year.

Orbital, meanwhile, continues preparing for its first station demonstration visit under NASA's Commercial Orbital Transportation Services (COTS) program. Slated for mid-September from NASA's Wallops (Va.) Flight Facility, the launch of Orbital's Antares launch vehicle carrying the Cygnus cargo vehicle marks the final milestone in the company's COTS agreement. A successful first flight paves the way for the Dulles, Va.-based company to begin routine resupply runs to the ISS as soon as December under a separate, fixed-price CRS contract valued at $1.9 billion.

Orbital's COTS demo schedule is contingent on the early-September liftoff of NASA's Lunar Atmosphere and Dust Experiment Explorer atop a Minotaur 5 rocket, the next mission slated to fly from Wallops.