Might want to look into Pulse Detonation Engines, these are basically the pulsejets of WWII but enhanced to work for more than 30 minutes before the engine shatters itself appart, and the fuel is burnt supersonically to create a shock wave that effectively increases the compression pressure, making for a more efficient burning of fuel. These kind of engines have arguably made more progress in the past 10 years than scramjets have made in 30 or so, thanks primarily to computing methods that let us explore the realm of unsteady flow in engines. Nozzle design for this kind of engine flow has only recently recieved great investigation, and General Electric is keeping very quiet about their efforts to make a pulse-detonating engine for jet airliners. These engines can be mechanically much simpler than turbojets, lighter, more efficient, yet can potentially operate from stand-still to mach 4, even as airbreathing systems! There were some NASA articles about investigating using these engines with oxygen as spacecraft engines, opening the posibility of making a dual-mode engine that could work in the atmosphere at lower-than-hypersonic speeds, then switch over to onboard oxygen for launch into orbit. Who knows, maybe evolution of this technology could make an SSTO possible? If not, it certainly has potential to cheapen spaceflight in general, maybe allowing multi-stage systems with stages that can be landed and reused almost immediately, which could be nearly as elegant as SSTO concepts. Ultimately, something needs to bridge the gap between our current chemically-fueled systems and the fantastic dreams of fusion-powered or anti-matter-powered vehicles of tommorow. As of yet, nothing really bridges this gap, and it's one hell of a gap to fill.