As Stratolaunch Systems Corp. has rolled its super-massive aircraft out of the hangar during the last year and performed some ground-based tests, there has been one big unanswered question—what is the purpose of building the largest airplane in the world?

Yes, the company had signed an agreement with Orbital ATK, a Dulles, Virginia-based company, for its aircraft to serve as the first stage for launching Pegasus XL rockets from the air. But these are relatively small rockets, with a diameter of just 1.27 meters—compared to the 117-meter wingspan of the Stratolaunch aircraft—and capable of hefting less than half a ton into low-Earth orbit. This is a bit like using a Falcon Heavy rocket to launch a few cubesats into space.

We may finally have some clues. As part of his forthcoming book The Space Barons, the Washington Post's Christian Davenport got Stratolaunch founder Paul Allen to open up about his ambitions for the giant aircraft. And it turns out that Allen, a cofounder of Microsoft, wants to launch a reusable space shuttle into orbit.

Internally, the company calls the shuttle proposal "Black Ice." In an interview, Allen told Davenport, “I would love to see us have a full reusable system and have weekly, if not more often, airport-style, repeatable operations going."

Allen apparently offered few technical details but did explain that the space plane would be about the same size as NASA's space shuttle, which had a 24-meter wingspan. Because of the Stratolaunch aircraft's mobility, the shuttle could be launched from virtually anywhere in the world where the large aircraft could take off. And the system would be fully reusable, with the airplane serving as the first stage and the space plane only needing to be re-fueled.

Initially, the space plane might fly up to the International Space Station (although this facility may go away by the end of 2025) or take satellites into orbit. Eventually it might become capable of carrying people, although there are no immediate plans for that.

Stratolaunch also has no immediate plans to develop the "Black Ice" vehicle. Building a new space plane from the ground up would likely require multiple years and hundreds of millions, if not billions, of dollars—and it is not clear whether Allen is committed to funding such an initiative. For now, Stratolaunch wants to make sure its massive airplane can fly. A first test flight could perhaps come in 2019.