Lol fuel.
A manned fighter jet operated by GMP reactors is tested and proves to be quite an interesting and efficient concept, but the nose of the jet will have to be remodeled to have better heat shields. Due to the jet flying extremely fast and the gravitational fields pulling the surrounding gasses around it to a higher speed around the jet, it counteracts the gravitational stress on the walls at a moderate-high speed - the lesser pressure on the outside walls relieves stress on the structure, but the added friction on the nose causes it to heat up, and flying at Mach 2.4+ causes the nose to become unstable, so ESA will be providing heat shield technology to the jets.