With winter approaching, Ukrainian officials are desperate for more air defenses to protect their power grids from Russian strikes that could plunge the country into freezing darkness.

So desperate, in fact, that they are willing to experiment with a monster of a weapons system that was the brainchild of Ukraine and is now being pursued by the Pentagon.

American officials call it the FrankenSAM program, combining advanced, Western-caliber, surface-to-air missiles with refitted Soviet-era launchers or radars that Ukrainian forces already have on hand. Two variants of these improvised air defenses — one pairing Soviet Buk launchers and American Sea Sparrow missiles, the other marrying Soviet-era radars and American Sidewinder missiles — have been tested over the past several months on military bases in the United States and are set to be delivered to Ukraine this fall, officials said.

A third, the Cold War-era Hawk missile system, was displayed on Ukraine’s battlefield this week for the first time, in an example of what Laura K. Cooper, a senior U.S. defense official, had described this month as a FrankenSAM “in terms of resurrection” — an air defense relic brought back to life.

Together, the FrankenSAMs are “contributing to filling critical gaps in Ukraine’s air defenses, and this is the most important challenge that Ukraine faces today,” said Ms. Cooper, the deputy assistant secretary of defense for Russia, Ukraine and Eurasia policy.

Almost since the start of the war, Ukraine has tinkered with commingling offensive weapons — its aging Soviet-era stockpiles and the ones it has gotten from the West — in unexpected but, in many cases, successful ways. American military officials spoke admiringly last year of Ukraine’s ability to “MacGyver” its arsenal, a metaphor for the 1980s TV show in which the title character uses simple, improvised contraptions to get himself out of sticky situations.

The FrankenSAMs project is now trying to do the same for Ukraine’s air defenses.

Over the past 20 months, the West has supplied a range of air defenses to Ukraine, including state-of-the-art Patriot and IRIS-T systems, tanks fitted with antiaircraft guns and more than 2,000 shoulder-fired Stinger missiles.

This past week, Chancellor Olaf Scholz of Germany announced that his government would provide Ukraine with three more batteries of sophisticated air defenses, including another Patriot system, as part of what he called a nearly $1.5 billion “winter package.”

“As winter approaches, we are putting up a protective shield against renewed Russian attacks on energy, water and heating infrastructure,” Mr. Scholz said on Tuesday. “This is because it is becoming apparent that Russia will once again use cold and energy shortages as a weapon against the civilian population.”

The air defenses are part of the close to $100 billion in military aid that Ukraine has received from allies since Russia’s full-scale invasion in February 2022. The United States, which has already sent more funding for weapons than any other single nation, is considering donating $60 billion more as part of a new Biden administration emergency spending plan.

On Thursday, the administration announced another $150 million in military aid for Ukraine, a package of weapons that included additional munitions for three kinds of air defense systems — including Sidewinder missiles for one of the FrankenSAMS.

Now that it has Western tanks, armored vehicles, air defenses and long-range attack missiles in its arsenal, and with fighter jets on the way, officials said Ukraine largely needs more of the same weapons it has already received as opposed to systems that have yet to be sent.

FrankenSAMs are a mix of both. The program’s origins date to late last year, when Ukrainian officials asked the allies to help them find missiles for around 60 Soviet-era Buk launchers and radars that were sitting idle in Kyiv’s arsenal. Knowing it would be difficult for the West to obtain Russian-manufactured munitions to fit the Buk systems, the Ukrainians instead suggested refitting the launchers to use NATO-caliber antiaircraft missiles donated by the United States.

“We realized we needed to come up with some solutions,” said Oleksandra Ustinova, the chairwoman of a commission in Ukraine’s Parliament that oversees arms transfers from the West. She said Ukrainian officials offered to jury-rig the weapons themselves, in the interest of time, “because for the winter period we need desperately the air defenses, and this is what is going to be used.”

But American engineers insisted on doing the work, and they needed more than seven months to test and approve the mash-up after the Pentagon agreed in January to provide Sea Sparrow missiles for the project. The first few refurbished Buk launchers and missiles arrived in Ukraine only recently, Ms. Ustinova said.

She said Ukraine was prepared to send 17 more Buk launchers to the United States to be refitted, but American engineers had been able to turn around only five each month.

Ukraine has also had to wait for the older Hawk systems to get up and running after they were initially pledged by Spain in October 2022. A month later, the United States said it would pay to refurbish older Hawk missiles for the donated Spanish systems. But at least some of them were delivered to Ukraine without the necessary radar equipment. That took another nine months to arrive.

By Monday night, the Hawks were fully operational, shooting down targets alongside more modern air-defense systems, the commander of Ukraine’s air forces, Lt. Gen. Mykola Oleshchuk, said on Telegram. Hitting 100 percent of the targets “is not easy, but we will get closer to it every day, strengthening our air defense,” General Oleshchuk wrote.

Another creation — an improvised ground launcher that uses Soviet-era radars to fire old American missiles that are usually used on fighter jets — was revealed in tandem with a $200 million security assistance package that the Pentagon announced on Oct. 11.

That FrankenSAM uses American-made supersonic AIM-9M Sidewinder missiles, which were developed in the 1950s and are used on F-16 and F-18 fighter jets. They are now part of the improvised ground-launching system, which Ms. Cooper previewed in Brussels as “a real innovation” that she said would help speed air defenses to Ukraine, “instead of it being, you know, years and years of development time.” It is not clear precisely when it will arrive in Ukraine.

American defense officials and engineers are also still testing what may be the most powerful FrankenSAM yet: a Patriot missile and launching station that operates with Ukraine’s older, domestically made radar systems.

A Pentagon official said on Wednesday that a test flight of the system this month, conducted at the White Sands Missile Range in New Mexico, successfully hit the drone it had targeted. The system is scheduled to be sent to Ukraine this winter, the official said, accompanied by donated missiles and other Patriot parts from multiple allies.

Can Kasapoglu, a defense analyst for the Hudson Institute in Washington, praised the idea of integrating the Soviet-era equipment with more sophisticated Western missiles as a way to help Ukraine “maintain its arsenal for the long war ahead.”

It also “provides an opportunity to put weapons that are collecting dust on NATO capitals’ shelves,” Mr. Kasapoglu said, “into practical use.”

Christopher F. Schuetze contributed reporting from Berlin, and John Ismay from Washington.