Lockheed-Martin led defense stocks lower on Friday, after President-elect Donald Trump’s latest Twitter salvo over the cost of the company’s F-35 fighter jet, in which he said he had asked rival Boeing to “price-out” an older aircraft as an alternative.

Experts questioned the wisdom of Trump’s “impractical if not irrational” intervention in the world of arms procurement, one writing that “unless the rules of physics have changed”, the president-elect’s request made no sense.

Lockheed shares were down about 0.8% in morning trading, nearing their lowest levels since the election in November and serving as the biggest drag on a basket of defense-related stocks. Boeing stock was also down, by about 0.2%.

Earlier in December, a Trump tweet about the cost of the F-35 preceded a $13 fall in Lockheed stocks.

Trump posted his latest Twitter message late on Thursday, a day after he met the chief executives of both aerospace companies.

“Based on the tremendous cost and cost overruns of the Lockheed Martin F-35, I have asked Boeing to price-out a comparable F-18 Super Hornet!” he said.

The F-18 is an older-generation aircraft that lacks the stealth capabilities of the F-35. Most defense analysts do not consider the planes comparable.

“Impractical if not irrational,” Richard Safran, a defense analyst at Buckingham Research, said by email. “First, the F/A-18 is a carrier-based naval fighter. Certainly it could not meet the US Marine Corps need for vertical lift.

“It would not be suitable for the air force either – the extra weight of a carrier fighter makes it less than ideal for the air force.”

In a note early on Friday, Vertical Research Partners analysts wrote: “Unless the rules of physics have changed, you cannot make a non-stealthy two-engined, carrier-based aircraft from the 1980s into a single-engine, multi-role stealthy fighter from the 2000s.”

Still, Trump’s dissatisfaction with the program, which has been dogged by problems while costs have reached an estimated $379bn, is a clear risk for Lockheed. The F-35 program is a critical sales generator for the company, accounting for 20% of last year’s total revenue of $46.1bn.

Lockheed-Martin declined to comment on Trump’s remarks. The Pentagon did not respond to a request for comment.

Boeing spokesman Todd Blecher said on Thursday in an email that the company was committed to providing the capability and affordability to meet national security needs.

On Wednesday, Trump met the chief executives of Lockheed and Boeing at his resort in Palm Beach, Florida. Boeing CEO Dennis Muilenburg told reporters he had guaranteed costs would not get out of control for a replacement to Air Force One, the presidential plane, another project Trump calls too expensive.

Lockheed CEO Marillyn Hewson did not speak to reporters but said in a statement the meeting was “productive”. Trump told reporters he wanted to cut the F-35 program’s costs.

Trump’s jockeying for leverage via his Twitter account is likely to be a hurdle for all US defense contractors, Roman Schweizer, aerospace and defense analyst at Cowen & Co, wrote in a client note on Thursday.

“We have no idea how this plays out but believe ‘Twitter risk’ for defense companies could be a significant issue over the next four years,” Schweizer wrote.

“This is Lockheed Martin’s time in the barrel.”