WASHINGTON — For the second consecutive year, the U.S. Defense Advanced Research Projects Agency’s top-funded space program is an experimental spaceplane intended to make frequent trips to orbit.

DARPA asked for $50 million in the Pentagon’s 2017 budget request for its Experimental Spaceplane 1, or XS-1 program. That’s up from a $30 million the agency asked for during the fiscal year 2016 budget cycle.

XS-1 aims to develop a reusable first stage that could carry an expendable upper stage capable of placing payloads weighing up to 1,800 kilograms into orbit. DARPA said the vehicle could ultimately fly 10 times in 10 days and boost payloads into low Earth orbit for less than $5 million per launch.

Three industry teams are working on the program: Boeing and Blue Origin; Masten Space Systems and XCOR Aerospace; and Northrop Grumman and Virgin Galactic.

In July, all three teams received funding to continue design work and risk reduction activities in preparation for a production contract.

DARPA said in 2014 it intended to pick one team in 2015 to work toward demonstration flight in 2018, but now it is unclear when such a downselect will occur.

DARPA said in budget documents that it plans to complete system and subsystem designs later this year, as well as coordinate with the Federal Aviation Administration for preliminary flight test planning.

A critical design review is planned for fiscal year 2017, the documents said.

In October, the Government Accountability Office said none of several Defense Department efforts to field quick-reaction launch vehicles, including XS-1, have advanced past the development stage.

In its 2017 budget request DARPA asked for $175 million for its space programs and technology office, significantly higher than the $127 million budget for 2016.

In addition to $50 million for XS-1, next year’s budget would also include: