The British defense ministry for years has insisted it ultimately will buy 138 F-35 stealth fighters for the Royal Air Force.

Sticking to its original requirement has helped the United Kingdom to preserve its industrial workshare in the U.S.-led F-35 program. But the ministry has declined to specify when or why it would acquire such a large fleet of the radar-evading planes.

As of mid-2019, the United Kingdom has ordered just 33 vertical-landing F-35Bs against a near-term inventory goal of 48 of the fighters. The F-35s would embark on the Royal Navy’s two Queen Elizabeth-class carriers and also deploy for land-based operations.

Many observers believe London has no intention of ever paying for 138 F-35s. But David Simpson, a former RAF pilot and defense ministry manager, has an idea for why it just might do so.

As of June 2019, the RAF has 102 Typhoons and 17 F-35B stealth fighters in seven front-line squadrons. The last squadron of 1980s-vintage Tornado fighter-bombers disbanded in February 2019.

The RAF’s 119 fighters represent a 40-percent reduction compared to the air force’s fleet in 2007. That year, the RAF possess a little more than 200 Tornados, Jaguars and Typhoons. In 1989 the RAF possessed around 850 fighters including Tornados, Jaguars, Phantoms, Harriers and Buccaneers, according to the Daily Mail newspaper.

As recently as 2015 the RAF planned to operate F-35Bs in just four front-line squadrons, each with 12 ready fighters, plus one training unit, for a total of 60 ready jets.

“This in theory allowed for up to three squadrons to be carrier-deployed in extremis when required, with one or two still being the norm,” Simpson wrote at U.K. Defense Journal. “A minimum of one would then be available always for a land expeditionary deployment or any permutation between the one active carrier and land deployment depending on the actual operational demand at any given time.”

Each of the navy’s two Queen Elizabeth-class carriers has space for around 40 fighters. For the next decade or so as the RAF F-35 fleet slowly expands, it’s unlikely the RAF will have the capacity simultaneously to embark fighters on both carriers. It’s not for no reason that the Royal Navy plans to embark on its carriers contingents of U.S. Marine Corps F-35s.

The RAF might acquire as many as 30 extra F-35s as an attrition reserve to complement the 60 ready planes. But even then, the fleet would number just 90 F-35s, 48 short of the 138-plane objective.

“So in reality 138 F-35Bs ... is simply too many,” Simpson wrote.

But the RAF also could acquire conventionally-landing F-35As in order to partially replace the venerable Tornado bombers that the service finally retired in early 2019. The defense ministry in 2015 decided to retain in service around 30 early-mode “tranche one” Typhoon fighters that it originally had planned to dismantle, sell or place in storage.

The early Typhoons have helped to preserve mass in overall RAF fighter fleet, freeing up late-model Typhoons to handle ground-attack missions in the Tornados’ absence. But the older Typhoons lack key upgrades and likely will retire from service before 2030, Simpson asserted.

“This also implies a likely commitment to buy about 40 F-35As by 2030 to replace those two tranche-one Typhoon squadrons alongside the required F-35B buy now to provide additional airframes.”

Acquiring 138 F-35s would make the United Kingdom the third-largest customer for the fighters after the United States with its roughly 2,300-plane order and Japan, which so far has ordered 147 F-35s.

The RAF’s F-35s flew in combat for the first time on June 16, 2019.

Taking off from the British airbase at Akrotiri in Cyprus, a pair of the vertical-landing stealth fighters patrolled over Syria alongside RAF Typhoon fighters, the defense ministry announced.

The F-35s did not drop bombs or fire missiles or guns during their combat patrol. Still, the mission made the United Kingdom the third country after Israel and the United States to deploy F-35s in wartime.

David Axe serves as Defense Editor of the National Interest. He is the author of the graphic novels  War Fix, War Is Boring and Machete Squad.