Can private-sector space business be profitable, or is it just an exciting way for billionaires to live out their fantasies?

SpaceX isn’t just launching its third Falcon Heavy rocket tonight—it’s also launching the second one. Two of the three rocket boosters on the US Air Force mission are being re-used after they carried a satellite into space in April.

As recently as 2015, reusable rockets were derided as impractical or unnecessary. Now SpaceX has quietly, by Elon Musk’s standards, made them part of doing business.

Musk’s space company flies two kinds of rockets: The Falcon 9 and the Falcon Heavy, which both use nine-engine first-stage boosters—the much-larger Heavy uses three. After most flights, the boosters fly back to Earth, arriving at landing sites or autonomous droneships at sea. Since 2017, SpaceX has flown new boosters 28 times, and has launched re-used, or “flight proven,” boosters 22 times. In the same period, it has only failed to recover two boosters.

SpaceX is likely to use reusable boosters more often in the years ahead. Musk says his newest rockets can be flown as many as 10 times, though none has yet been flown more than three times.

The savings provided by reusability, versus the expendable rockets used by competitors such as United Launch Alliance or Arianespace, remains unknown and hotly debated within the industry. The vehicles do require some refurbishment between launches, and the infrastructure for retrieving the rockets must be included in the final breakdown—most rocket-makers do not maintain a fleet of ocean-going vessels.

Still, industry sources suspect that reusability likely saves SpaceX around ten million dollars compared with building an entirely new booster.

SpaceX The Falcon Heavy on its launch pad.

One positive sign for SpaceX: Tonight’s reusable rockets flew just over 10 weeks ago, one of the faster turn-arounds for a booster so far. (Musk has said he wants to fly the same rocket twice in 24 hours, no word yet on how that’s coming.) The company also laid off one in 10 workers earlier this year. This was possible, in part, because the company is manufacturing fewer boosters to fly the same number of missions.

Musk’s company is currently the only rocket-maker to use reusable vehicles, one reason it has dominated the commercial market. Only Jeff Bezos’ Blue Origin is designing a large rocket booster intended to be reusable. It is unlikely to fly until 2021.