NASA says planned changes in the next stage of a program to develop privately operated U.S. spacecraft will give it more flexibility amid funding uncertainties.

NASA said it is getting only half the money it wanted for the program and that lack of funding made it "really tough" to lock in a firm fixed-price contract with enough companies to keep moving forward to develop a commercial vehicle to ferry U.S. astronauts to the International Space Station.

William Gerstenmaier, associate NASA administrator for the human exploration and operations mission directorate, said the agency has decided on a more flexible contracting procedure that will keep more private companies in the running and still allow NASA to sign off when the companies achieve certain milestones.

"It gives us some flexibility from a budget standpoint to make as much progress during this time as we move forward," he told Florida Today.

NASA is not expecting a new vehicle to be ready until at least 2017, a year later than initially planned, he said.

The goal of the new contracting procedure is to advance the development work enough to reach a design certification process and the eventual awarding of a contract, NASA said.