While the eyes of the world are on SpaceX and other companies building and testing new reusable rockets, a related and equally important development has been happening under the public radar. DARPA’s Experimental Spaceplane-1 (XS-1) has been gearing up its own development program, which in some ways is even more impressive than the big launchers.

Along with the Atlases and Falcons, there are many smaller rockets regularly launching small payloads. That sector of the industry is ripe for the same kind of innovation now disrupting the large launcher market. For most satellite projects, launch is the largest single expense, and the trend today is to try to reduce launch costs through reusability—building hardware that can be used over and over, instead of being tossed in the ocean after liftoff.

DARPA has asked the aerospace industry to design a small, first-stage rocket that can launch into space, deploy another stage with 3,000 to 5,000 pounds of payload attached, then return to land on a runway and be ready to do it all over again the next day—ten days in a row. That’s much more often than any of the bigger rockets launch, none of which have actually achieved reusability yet. XS-1, as DARPA calls the experimental vehicle, won’t have time for extensive refurbishment. It should essentially be able to refuel and re-fly, like an airplane. And it should do it for $5 million per flight, around a tenth of the cost of a current launch.

Spaceplanes have certain advantages in this arena. They have gentler atmospheric reentries than traditional rockets do, thus lessening aerodynamic and structural stresses. They can take off or land on any runway that’s long enough, and so can be flown from pretty much anywhere.

In 2014, three teams were selected to do advanced design work on the XS-1: Boeing/Blue Origin, Northrop Grumman/Virgin Galactic/Scaled Composites, and XCOR/Masten were all funded to develop their concepts. (Generally one company leads the overall design, while the other focuses on propulsion).

Now DARPA is putting up $146 million for Phase II of the project—building and flying an actual demonstrator. Anyone with a concept can submit, but of course the three Phase I teams have a serious leg up. Even though $146 million is a lot of money, it won’t likely cover the full cost of building a novel spacecraft, so the winning company will probably need to invest some of its own money as well. Theoretically the winner will end up with a tested spaceplane that it can enter in a lucrative marketplace. But since nothing in spaceflight (or government funding) is guaranteed, it’s still a gamble.

The two large spaceplane projects of the 1980s, the U.S. Space Shuttle and Soviet Buran, never came close to achieving airline-style operations. The Shuttle, despite its name and original design goals, required a long and extremely expensive refurbishment after each launch, and the Buran made its first and only flight in 1988. Although the industry has since turned back to conventional rockets, the spaceplane concept never really died, and many have been drawn up on paper.

Now market forces and technological advancements have rekindled interest in spaceplanes. Sierra Nevada Corporation is set to test its Dream Chaser winged lifting body to resupply the International Space Station; it requires a traditional rocket to reach orbit but lands on runways. Virgin Galactic and XCOR are selling tickets for their suborbital spaceplanes (XCOR’s Lynx even takes off from and lands on a runway; Virgin Galactic’s SpaceShipOne is air-launched but runway-recoverable). Of the paper spaceplanes, Reaction Engines’ Skylon is by far the most exciting: a single-stage-to-orbit, reusable spaceplane that detailed independent studies say is technically feasible, if immensely difficult to build and fly.

DARPA exists to try risky new things, and not all of its spaceflight ideas have worked out. A smaller project to air-launch tiny 100-pound satellites, called Airborne Launch Assist Space Access (ALASA), was canceled last December after its intended fuel kept combusting prematurely.