After Combustion:Detonation! At first glance, the engine bolted to the test stand looks like an unlikely candidate to lead an aerospace revolution. Its size is unimpressive: At about four feet long, it's dwarfed by the machinery that feeds it air and fuel, machinery that fills a house-size structure at the China Lake Naval Air Warfare Center in California. And its appearance is unremarkable: This machine has none of the grace of the high-bypass turbofans that power modern jetliners, with wide, sweeping inlets and delicate blades. From the outside, it's simply a collection of metal tubes, one large cylinder feeding into...