KENNEDY SPACE CENTER, Fla.— Two companies vying to build space taxis to fly NASA astronauts to the international space station plan to offer unmanned versions of their vehicles in what is likely to be a heated competition for cargo resupply services.

NASA said it will keep its commercial crew and cargo resupply initiatives separate, but two programs give the U.S. space agency a back door to financially support multiple vehicles for a wider scope of missions.

NASA currently has contracts worth a combined $3.5 billion with Space Exploration Technologies Corp. and Orbital Sciences Corp. to fly cargo to the station initially through the end of 2015. Two-year extensions were announced March 31.

A draft solicitation for a follow-on contract is due to be issued June 16, according to documents posted on NASA’s procurement website.

SpaceX also has a separate partnership agreement and funding through NASA’s commercial crew program to upgrade its Dragon cargo capsule to carry people. The California-based firm, founded and run by technology entrepreneur Elon Musk, is competing against Boeing and privately owned Sierra Nevada Corp. for a fourth and final round of space taxi development funds. NASA expects to make one or more awards in late August or September.

Boeing, which is developing the Crew Space Transportation (CST)-100 capsule, “absolutely” will bid for the upcoming Commercial Resupply Services (CRS) 2 contract, said John Mulholland, Boeing commercial crew program manager.

“The CST-100 has been designed from Day 1 to fly any combination of crew and cargo,” Mulholland said.

With no crew, the capsule will be able to carry more than 1,100 kilograms of cargo, and — like SpaceX’s Dragon — be able to return experiments and equipment from the station to Earth. SpaceX completed the third of 12 resupply missions in May.

Orbital Sciences’ Cygnus spacecraft, which is being prepared for the second of eight station cargo runs, is designed to incinerate in the atmosphere during re-entry, like Russia’s Progress, Europe’s ATV and Japan’s HTV cargo vehicles.

Sierra Nevada is expected to offer another supply line option with its Dream Chaser spaceplane.

“We can fly the Dream Chaser unmanned,” Mark Sirangelo, vice president of Sierra Nevada Space Systems, said. “We strip out all of the human factors stuff and we have a pretty capable cargo vehicle.”

Without crew, Dream Chaser, which resembles a miniature space shuttle, can haul about 1,350 kilograms into low Earth orbit. “The factor is how long we might have to stay in orbit,” Sirangelo added.

NASA planning documents, posted on the agency’s procurement website, specify a desired on-orbit stay of 45 to 75 days. NASA is looking for flight services four to five times per year and said it has a “strong preference for use of existing launch facilities.”

In a response to a written question from a prospective bidder, NASA said, “The crew and cargo flights will remain separate.”

NASA expects to follow its draft solicitation with a final request for proposals Oct. 1. Bids would be due Nov. 14. The agency said it expects to award one or more contracts covering flights between 2017 and 2024.