Indie success Rocket League has raked in more than $50m since it launched in July.

The footballing car game was offered for free on PlayStation 4 at launch via PlayStation Plus.

It's also available on PC and is coming to Xbox One in February.

Rocket League cost $2m to make, Wall Street Journal has reported.

The game is actually a sequel - to the far less attractively named Supersonic Acrobatic Rocket-Powered Battle-Cars. In retrospect, its no wonder that title suffered from a lack of exposure.

Developer Pysonix has now said that its decision to release Rocket League for free to PlayStation Plus subscribers was the key to its success, and drove further sales among PS4 owners who weren't Plus owners.

And for those who played and enjoyed the game, cosmetic DLC such as the Back to the Future DeLorean pack was available for a small extra fee.

It's a remarkable turnaround for a studio which previously paid the bills with unsung contract work on larger games - including Mass Effect 3 and Gears of War.