The United States is opening the throttle in its push to develop and procure hypersonic missiles after falling behind key foreign adversaries China and Russia in the race to field a potentially game-changing defense system.

The U.S. is pushing to procure at least 24 hypersonic missiles in the near future, according to the fiscal year 2024 budget proposal for the Pentagon released Monday. And earlier this month, the Biden administration invoked the Defense Production Act to boost the defense industrial base to “meet the hypersonic warfighting mission.”

While hypersonics have been well-developed and prototyped, the Defense Department has not fielded the weapons yet, and there remain challenges in the industrial production base and with testing infrastructure. 

George Nacouzi, a senior engineer at the RAND Corporation, said the U.S. is now in a pivotal stage where it will soon transition to initial production.

“One of the things that’s limiting us [now] is how fast we can develop and how fast we can test them,” Nacouzi said. “But those are going to be increasing and they are increasing, so I expect to see more deployment of a larger number of these in the future.”

But the pursuit comes with deep risks, according to critics who contend the U.S. has not crafted a strategy for the use of the highly expensive weapons, which may raise the risk of nuclear conflict since they are difficult to detect or take down.

Shannon Bugos, a senior policy analyst with the Arms Control Association, said after a major nuclear pact between the U.S. and Russia was suspended last month, any new negotiations must include hypersonics.

“A lot of these questions about mission sets numbers, it’s coming out a lot later, a lot farther down the development and production line than I think is useful — because then we’re just making these systems without any clear guidance on what they’re going to be used for,” Bugos said. 

Most ballistic missiles fly at hypersonic speeds — defined as Mach 5, or five times the speed of sound — but true hypersonics are maneuverable and can travel entirely within the Earth’s atmosphere to reach its velocity.

Since 2019, the Defense Department has spent $8 billion on hypersonic weapons systems development, and it wants an additional $13 billion for development and another $2 billion for procurement, according to the Congressional Budget Office.

At a House Armed Services Committee hearing last week, several Pentagon and military officials outlined ambitious plans to accelerate development. They argued the weapons will serve as a deterrent to foreign rivals and eventually become a crucial component of the military arsenal.

Emerging as a skeptical counterpoint, Rep. Seth Moulton (D-Mass.) grilled defense officials at the hearing for developing the weapons simply because U.S. adversaries possess them.

“There are plenty of weapons our adversaries are developing that we are not,” Moulton said. “What matters is how we will use them, not chasing after what our adversaries have just because they have it.”

Michael Horowitz, the director of the Emerging Capabilities Policy Office at the Defense Department, demurred, saying the weapons could help the U.S. “prevail” on the battlefield and increase “options” for nuclear deterrence.

“The intersection of the speed, maneuverability and range of hypersonics provides capabilities that are simply not provided by ballistic missiles,” Horowitz told lawmakers.

There are two types of hypersonics: hypersonic glide vehicles, which are released from a rocket booster, and hypersonic cruise missiles, powered by a scramjet, or air-breathing engines that use oxygen to combust fuel at high speeds.

The closest hypersonic missile to deployment in the U.S. is the Air Force’s Air-launched Rapid Response Weapon, a boost-glide vehicle that could be operational by the fall, according to the Congressional Research Service.

Concerns on the use of hypersonic missiles in conventional warfare is only growing after Russia has deployed its Kinzhal hypersonic missiles in Ukraine, with Kyiv saying it is unable to defend against the strikes.

Not even the U.S. currently has an adequate defense system to take down hypersonic missiles. Air defense systems, such as Patriots and Terminal High-Altitude Area Defense, are capable of taking down ballistic missiles that reach hypersonic speeds, but only over small areas. 

Russia’s Kinzhal strikes have also sparked a debate about how much of a game-changer the advanced weapons are when equipped with a conventional warhead.

“There’s the connotation that the use of hypersonics in warfare will fundamentally change the course of the war,” said Bugos, of the Arms Control Association. “That mindset I don’t think has actually played out [and] I don’t view hypersonics as game-changing systems.”

Another concern is the price to develop the hypersonic missiles, which deploy sensitive technology that can be threatened by high speeds and high heat. 

Hypersonic weapons cost one-third more than ballistic missiles, according to the Congressional Budget Office, which noted ballistic missiles can be just as difficult to shoot down.

The U.S. developed the early technology for hypersonic speed testing in the ‘50s and explored the technology for years. But Washington largely abandoned the pursuit of fielding hypersonic weapons over time, not seeing an immediate military application for the technology.

In the early 2000s, the U.S. took note of Chinese and Russian advancements in the field and began slowly increasing investment, with research quickly picking up steam in the last few years after Russia fielded a hypersonic glide vehicle called the Avangard in 2019. 

China displayed its own hypersonic glide vehicle during a military parade that same year and may have fielded the system in 2020, according to a report from the Arms Control Association.

Beijing has also tested a nuclear-capable hypersonic weapon that could evade U.S. defense systems.