MOSCOW—Fighter jets roared overhead this week as Russian arms firms showcased their new wares, including drones, helicopters and surface-to-air missiles, in a biennial air and space extravaganza.

But even as the country projects a muscular image, a falling ruble and weaker economy has forced the Kremlin to scale back its ambitious plans for a multibillion-dollar military modernization.

In just one example, Russia’s Defense Ministry has cut the number of Sukhoi T-50 stealth fighters it will buy in its initial order from as many as 100 to 12, an official close to the defense industry said.

Opening this year’s International Aviation and Space Show at an airfield outside Moscow, President Vladimir Putin promised Russia would push ahead with revamping its defense sector and equipping its armed forces with better and more powerful technology. It has already restructured its army to make it a more mobile and flexible fighting force.

“We will continue developing our country’s unique capabilities in the space sector, and will bolster our leading position in the production of defense-sector aviation technology,” Mr. Putin said on Tuesday.