• Ukraine is using "FrankenSAM" air-defense systems developed with the US.

• The US making experimental weapons reveals its lack of ground-based defenses, an expert told BI.

• Russia's invasion tactics show how much the US needs those weapons in its arsenal, he said.

Ukraine's use of experimental "FrankenSAM" defense systems has highlighted gaps in NATO's own arsenals, according to a military expert.

The weapons, named for a combination of Frankenstein's monster and the acronym for surface-to-air missiles, are produced by the US and Ukraine and made by cobbling together different systems to create new weapons that can take down missiles, drones, and aircraft.

They often combine US weaponry with Soviet-era equipment, and had their first success in January, according to a Ukrainian official, when one was used to shoot down a Russian drone.

Some have involved converting American-made AIM-9M Sidewinder missiles typically shot from planes so that they can be fired from a ground-launching system.

But the fact that this strategy is being pursued shows just how much NATO countries, including the US, are missing ground-based air defense systems, according to Mark Cancian, a retired US Marine Corps colonel and a senior advisor at the Center for Strategic and International Studies.

"The FrankenSAMs fill a critical gap" for Ukraine as its allies don't have enough ground-based air defenses to give it, Cancian told Business Insider.

He said Ukraine desperately needs ground-based air defenses, with Russia launching major drone and missile strikes on cities and towns across the country.

The way Russia is fighting now also shows how NATO needs these kinds of weapons if it wants to be ready for future conflicts, Cancian said.

But it simply doesn't have enough, after member states stopped investing in the weaponry after the Cold War, he told BI.

Equipment like the surface-to-air missile system NASAMS "is a great system against cruise missiles and drones. The problem is that we just don't have very many of them," he said.

The US has committed 12 NASAMS to Ukraine, but as of December only two were known to have been delivered. The Pentagon said more would be delivered as they are built, in a reflection of how few the US has available.

Yet demand for these systems from Western countries is soaring, creating a yearslong backlog in their production.

This means that for now, the US and Ukraine are making multiple types of FrankenSAMs to try to fill the gap.

The US and other NATO countries had strong ground-based air defenses during the Cold War, but changed focus afterward, according to Cancian, seeing their main adversaries as countries like North Korea and Iraq, which didn't require having such strong defenses.

That changed in 2014, when the possibility of conflicts with major powers arose, after Russia annexed the Ukrainian region of Crimea and China became more aggressive, he said.

The Pentagon changed its strategy and embraced ground-based defenses again, Cancian said, but he described that realization as taking place "probably belatedly."

Those new systems have, for the most part, not yet reached the battlefields, so they are of little help to Ukraine. "That's why it's been so hard to get air defenses to Ukraine because the US and NATO had eliminated most of that," Cancian said.

With the defenses it does have, Ukraine has been able to shoot down many Russian attacks, and has forced Russian aircraft to stay far from the front lines, Cancian said.

This shows the US what is needed to counter Russia, or any other country with a powerful air force, he added.