While Rocket League is a clear evolution of developer Psyonix's 2008 game Supersonic Acrobatic Rocket-Powered Battle Cars, its origins lie even further back, in a mod for Unreal Tournament 2004.

Psyonix founder Dave Hagewood recalled how the idea for knocking a ball around with rocket-powered cars first came about more than a decade ago in a new interview with Gamasutra. He talked about how, as a modder, he added vehicles to Unreal Tournament 2003. This led to him working on Unreal Tournament 2004's vehicle-based Onslaught mode.

"That was kind of my big break in the industry, and me and Ben Beckwith, who was a level designer at the time, were really having a lot of fun taking one of the cars in the expansion pack and making it like, jump and spin in the air and do tricks and stuff like that," Hagewood said of his time after working with Epic Games. "After we started doing our own thing, we were trying to figure out how to make something really cool out of that car stuff."

When trying things like racing and car combat didn't catch on, someone had the idea to throw a ball into the arena, and that instantly captured people's attention.

"The rocket booster on the cars was really originally intended to just be like, a turbo option," he said. "But we discovered, completely by accident, that you could jump, keep boosting straight up, and literally fly across the arena. It was the kind of emergent gameplay we had to keep around, and a few weeks later people were hitting goals by just rocketing straight across the maps."

In 2008, Psyonix released SARPBC on PS3. And while it was good in its own right, the game was never the huge success that Rocket League appears to be. Hagewood says SARPBC was "too hardcore, so we slowed down the gameplay a little bit" for Rocket League. He also pointed out how online play wasn't perfect, something that--server instability aside--is improved in the new game.

No doubt, being a PlayStation Plus game has helped Rocket League a good deal. While a beta earlier this year created some buzz, being handed out to all Plus members has helped to ensure its popularity (though it's also selling well on Steam). While Psyonix is making money as part of the Plus deal, there is the question of how it would've done if it weren't offered for free on PS4.

"I will always wonder; given how much it has taken off, you have to wonder. You look at the amount of downloads and you go, 'Wow, that would have made a lot of money if we'd had even a tenth of that in paid sales,'" Hagewood said. "But we knew what we were doing when we made that deal. We knew that was a possibility."

Rocket League has been downloaded more than 2 million times so far, and seen more than 183,000 players online at the same time.

"I'm a very patient person, and I'd rather build this brand and make it become a thing than make piles of cash," he continued. "I look at it as a long-term strategy: even if we have to wait until we make Rocket League 2, if it's become a thing, that's the most important thing for me. That it becomes this phenomenon. That people realize we've been making this really cool game, and now everyone's playing it."

Rocket League launched earlier this month on PS4 and PC. Free and paid DLC is on the way, and we may also see mutators for private matches. There won't, however, be weapons or power-ups added to the game.