The Manhattan project had the benefit of most of the physics being at least fairly well understood. Everyone knew there was energy to be liberated from the atom

The Manhattan project had the benefit of most of the physics being at least fairly well understood. Everyone knew there was energy to be liberated from the atom, the question was whether a bomb would be practical: could it be small enough to fit in a plane? Could the material be enriched and mined practically? All of that stuff was engineering problems, absolutely massive ones. There were big uncertainties about the physics, but they were about the size of the effect, not about whether it existed at all. But EmDrive ideas are putting the engineering before the horse, the physics supposedly underlying them are not mainstream at all. The Manhattan project got off the ground because all the heavyweights in nuclear physics at the time thought it was possible. Nuclear chain reactions were demonstrated first, before the Manhattan project was started. (Self-sustaining chain reactions did only come as part of the Manhattan project)