The U.S. Navy is discovering that it had not fixed the exhaust heat problem with its new F-35B vertical take-off jet. This is more than embarrassing as it means that the F-35B really can’t use its vertical landing capability in most places.
Back in 2010 the navy thought it had found a way to handle the heat. Since 2008 the U.S. Navy has been furiously working on a solution to the fact that the engine exhaust heat from its two newest aircraft, the tilt-rotor MV-22 and the vertical takeoff F-35B's, was too hot for the deck plates on some of the carriers these aircraft will operate from. The gas turbine engines of both aircraft, which blow their exhaust right on to the deck of the carrier while waiting to take off, caused high enough temperatures to the steel under the deck plates to possibly warp the understructure. The navy also discovered that the exhaust heat problem varied in intensity between different classes of helicopter carriers (each with a different deck design.)
The navy sought a solution that would not require extensive modification of current carrier decks. This included a lot of decks, both the eleven large carriers, and the ten smaller LHAs and LHDs. This began looking like another multi-billion dollar "oops" moment, as the melting deck problem was never brought up during the long development of either aircraft. Previously, the Harrier was the only aircraft to put serious amounts of heat on the carrier deck, but not enough to do damage. But when you compare the Harrier engine with those on the V-22 and F-35B, you can easily see that there is a lot more heat coming out of the two more recent aircraft. Someone should have done the math before it became a real problem.
In any event the navy soon declared that an inexpensive solutions had been found, sort of. For the MV-22, the navy developed portable heat shield mats, that deck crew could drag into place under the exhausts of the MV-22s, if these aircraft were expected to be sitting in one place for a while.
For the F-35B, the heat shield mats didn't work as well (the F-35B engines put out more heat), so the exhaust nozzle on the F-35B engine was redesigned, to spread the exhaust over a larger area, thus lowering the peak heat build up to the deck plates. This would also help solve the problem of the F-35B turning asphalt surfaces to a liquid state.
The navy expected the MV-22 and F-35B would continue creating "heat management" problems. That turned out to be an understatement, especially when it came to the F-35B. Turned out that the exhaust redesign did not reduce the heat enough. It was still more than 920 degrees Celsius (over 1,700 Fahrenheit). That was enough to cause heat resistant concrete to spall (come apart in flakes) and the heat resistant portable matting the navy developed was expensive and time-consuming to install. Turns out the matting did not always prevent spalling when installed over concrete. Solutions are still being sought.