WASHINGTON — NASA intends to pick at least two commercial crew hopefuls next summer to share in the $1.6 billion the agency intends to spend through mid-2014 to help the chosen firms get their space taxi designs ready for initial production.

NASA’s Commercial Crew Program released a draft request for proposals Sept. 19, the same day NASA Administrator Charles Bolden said the agency expects to pump an additional $45 million into the Commercial Crew Development (CCDev) initiative by funding optional milestones included in a pair of awards made this past spring.

Both announcements, however, come with caveats about funding uncertainty.

NASA has awarded a total of $320 million since it began the CCDev initiative in early 2010. Currently, four companies — Boeing, Blue Origin, Sierra Nevada and Space Exploration Technologies Corp. (SpaceX) — are refining their designs under a second round of funded CCDev agreements that run into next year.

During the next round of the program, NASA intends to award at least two Integrated Design Contracts (IDCs) that would run from July 2012 through April 2014, culminating in a critical design review of the proposed end-to-end system. A follow-on contract would be used to carry the most promising design, or designs depending on funding availability, through testing and first flight to the international space station in 2017.

Phil McAlister, NASA’s acting director of commercial spaceflight development, told Space News that the $1.6 billion the agency hopes to spend on the IDC contracts through mid-2014 assumes Congress funds the program at the levels laid out in the budget request President Barack Obama sent lawmakers back in February.

That request included $850 million a year for the Commercial Crew Program through 2016. The NASA spending bill moving through the GOP-controlled House of Representatives includes just $350 million for the program next year. The Senate’s bill would provide $500 million, the same amount included in the NASA authorization bill Obama signed into law last October.

Of the $1.6 billion laid out for the first phase of the IDC, $720 million is marked for 2012, $690 million for 2013 and $200 million for 2014. Most of Commercial Crew’s annual appropriation for 2014 is being reserved for the second phase of the IDC, McAlister said.

Should funding fail to materialize, the start of commercial crew flights could be delayed unless NASA thins out the ranks of the aspiring providers.

Meanwhile, Bolden said Sept. 19 that NASA is adding additional milestone-based payouts to CCDev contracts held by Sierra Nevada Space Systems and Boeing Space Exploration. He made the announcement during a speech at the Air Force Association’s annual Air & Space Technology Exhibition just outside Washington.

Sparks, Nev.-based Sierra Nevada is working on a seven-passenger reusable space plane called the Dream Chaser. Houston-based Boeing is working on a space capsule known as the CST-100.

The CCDev awards are funded Space Act Agreements that pay out money upon completion of negotiated milestones. NASA made additional money available to Sierra Nevada and Boeing by activating “optional milestones” that were part of the original agreements.

Sierra Nevada got four additional milestones, worth up to $25.6 million, which makes the company’s total potential CCDev award $105.6 million. Boeing got three additional milestones, worth up to $20.6 million. Its CCDev award is now worth up to $112.9 million.

However, the amendments to the company’s awards, which were posted online by NASA, indicate that the agency currently has only enough money to fund one optional milestone for each company. Boeing’s funded milestone, worth $15.2 million, is a preliminary design review for its capsule’s flight and ground software. Sierra Nevada’s, worth $4.5 million, involves testing Dreamchaser’s landing gear on an engineering mockup of that vehicle.

The IDC, in contrast to CCDev awards, is being administered as a fixed-price contract under the Federal Acquisition Regulations. Switching the commercial crew program to a traditional government contract has been criticized by some commercial spaceflight advocates, who say that onerous federal contracting rules blot out the savings that are possible when industry, rather than the government, manages a project.

One clause in the IDC said that NASA would select multiple providers in the first phase of the acquisition, but that afterward, a “single contractor for Phase 2 will be chosen from among these contractors after a competitive down-selection.”

McAlister said the hackle-raising clause was stock contract language that NASA is seeking to alter — if not drop — in the official solicitation, which is due by the end of the year.

“NASA’s goal is to have multiple commercial partners,” McAlister said. “The Commercial Crew Program is investigating getting a waiver or deviation from this standard clause language for the final [request for proposal].”

He added that language elsewhere in the contract allows NASA to make multiple awards in the second phase of the IDC, as long as there is enough money to support multiple industry partners.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.