Why F-35s can never really rust – and why it doesn’t matter if they do

In 2022 when pictures of rusty F-35s sitting on the flight deck of an American aircraft carrier hit the internet, they prompted a wave of hyperbolic news articles and social media tirades from political pundits. But then the story seemed to just… go away.

Like a lot of stories that get tons of attention and then fizzle out, this story went away because it was entirely predicated on misinformed knee-jerk reactions to something most people just didn’t understand.

The F-35 is almost entirely made out of aluminum, titanium, and carbon composites, none of which can actually rust. So, the first thing you need to know about those rusty F-35s is that the rust we can see never had a chance at compromising the jet’s actual performance. The rust was only paint deep, because what we see oxidizing in these pictures is actually microscopic iron ferrite in the outermost layers of the F-35’s radar absorbent coating.

All stealth aircraft are coated in radar-absorbent materials, or RAMs, which can be produced in different ways and with different ingredients to absorb very specific types of electromagnetic radiation (or radar waves) and then dissipate them as heat, rather than reflecting them back at the broadcasting array. The RAM coating on America’s stealth fighters has been said to be capable of absorbing upwards of 80% of inbound radar waves, and former Skunk Works director Ben Rich famously attributed about 35% of a stealth fighter’s stealth to this advanced coating. This coating is particularly important anywhere there are seams between body panels, for example on the leading edges of the wings, around jet inlets, and anywhere else that’s likely to produce a sizable radar return.

However, the F-35 takes a very different approach to its RAM coatings than previous stealth jets like the F-22 Raptor or B-2 Spirit. Those older stealth jets have layers of RAM in the form of tape, epoxies, and liquid spray applied to them after assembly; but the F-35 has its base layers of RAM baked into the composite material of its body panels.

Related: Why the Navy never put the F-22 on aircraft carriers

Aviation Boatswain’s Mate (Handling) 3rd Class Haydee RiveraContreras, from Kissimmee, Florida, signals for an F-35B Lightning II aircraft assigned to Marine Medium Tiltrotor Squadron (VMM) 262 (Reinforced) to launch aboard amphibious assault carrier USS Tripoli (LHA 7), Aug. 24, 2022. (U.S. Navy photo by Chief Petty Officer Peter Burghart)

The F-22, B-2, and F-117 need to have RAM epoxy and tape painstakingly applied to every seam between thier body panels. But the F-35’s body panels are made primarily by incredibly advanced multi-axis robots, CNC machines using lasers to measure so precisely that the gaps between the panels are known to be somewhere in the neighborhood of 50 microns across… that’s less than half the width of a human hair.

In other words, the F-35’s body panels are not only radar-absorbent in themselves, but they’re also assembled with such precision that the radar-reflecting seams between them practically aren’t even there.

And there’s data to support this claim. Before the first F-35 entered service, Lockheed Martin built what it called a “highly detailed, full-scale Signature Measurement Aircraft (SigMA),” with all the same RAM coatings found on the production F-35. The SigMA aircraft was then put in front of a radar array to get a baseline measurement of the jet’s radar cross section. Then, it was subjected to extreme abuse comparable to more than 600 operational flight hours (which, depending on the aircraft, is about three years’ worth of use). Finally, three dozen more significant defects to paint were added, representing the sort of damage that might happen on a busy flight deck, but that would all but certainly be repaired once identified.

The beaten and battered SigMa aircraft was put back in front of the same radar array to get a new radar cross section measurement and it was found that the aircraft was pretty much exactly as stealthy as it was at the start.

It turns out, that top layer of RAM coating doesn’t really do any of the heavy lifting when it comes to stealth. But, that doesn’t change the fact that rusty jets are a bad look, so Lockheed Martin set to work changing the formula of that top layer of coating to prevent the rust discoloration from happening anymore. Now every new F-35 that rolls off the line will be rust-free for the foreseeable future. As for the older jets, they’ll just receive the new top coating during their next scheduled maintenance.

Feature Image: An F-35B on HMS Prince of Wales. The F-35B was conducting flight tests as part of developmental test phase 3 (DT-3) flight trials during the ship’s deployment to the U.S. Eastern Seaboard for WESTLANT 2023. (Photo by Kyra Helwick/F-35 Lightning II Pax River IT)

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