Bethesda's just-released Fallout Shelter for iOS is racing up the charts. Released on Sunday, the game is currently the top free app on iTunes, and fourth overall (third for games) in the revenue gross chart. The game is only behind mobile juggernauts Clash of Clans and Game of War in terms of revenue.

During a panel presentation today at E3 attended by GameSpot, Fallout 4 director Todd Howard said he was surprised to see the game at the top of the charts, especially for revenue.

"This is not about making money. This is about getting Fallout out there," Howard said about Fallout Shelter when asked to respond to the fact that it's so high on the top grossing chart.

Fallout Shelter is a free-to-play game; revenue comes from the sale of virtual lunchboxes that contain extra content. You can buy a single lunchbox for 99 cents or pay as much as $20 for a set of 40.

"It just shows that fans are smart; and if you give them something they think is fun, they probably reward the people who made it," Howard explained.

In other Fallout Shelter news, Bethesda has confirmed that an Android version is in the works and should be released in the coming months.

Yes, we're also working on Fallout Shelter for Android. Hopefully out in a few months. — BethesdaGameStudios (@BethesdaStudios) June 16, 2015

Fallout Shelter lets you build your own vault and act as its overseer. Basically, you control all the people inside the vault and must keep them productive and happy.

Meanwhile, Fallout 4 launches in November for Xbox One, PlayStation 4, and PC.