Next-generation aircraft companies are increasingly turning to the U.S. Defense Department to accelerate their path to commercial flight.

Why it matters: It can be difficult to earn regulatory approval — not to mention public acceptance — of newfangled flying machines like drones, flying taxis or supersonic jets. And investors usually aren't very patient.

Driving the news: The latest example is Joby Aviation, which said last week that it is nearly doubling its existing contract with the U.S. Air Force's Agility Prime program.

Between the lines: Joby executive chairman Paul Sciarra said the company's ultimate goal is still to launch a commercial air taxi service, but military business can help it get there by providing access to testing facilities, offering early operational experience, and offsetting some research and development costs.

Other next-gen aviation companies have also turned to Defense Department contracts while working toward commercial deployment.

The big picture: The Defense Department has long invested in experimental aviation and transportation tech that it believes could give the U.S. a geopolitical edge over its rivals.

The bottom line: When you're inventing a new type of electric aircraft, diversification makes a lot of sense.