CammNut said:
The theory is PDEs are simple. They know how a turbojet works and they know it will cost billions to develop a full-scale Mach 4-plus turbojet. They think - and they hope Vulcan will prove - that combining an off-shelf Mach 2-ish F100, F110, F119 or F414 with a PDE will get them to Mach 4-plus more cheaply. But they don't know, and that's why its a DARPA program. On paper it looks easier to combine a turbojet with a PDE externally, the way Vulcan would, that it will be to replace the core of a turbine engine with a PDE, which is what GE and P&W are working towards. It is also easier than trying to make a PDE work from zero airspeed all the way up, without the turbojet. Make sense?
Well, no. Conceptually PDEs are simple, but the issues that have kept them from being practical are big ones.
- It's difficult to initiate a detonation in real-world fuels like JP.
- Thermal, structural, etc. loads are thought to be very high. Honestly though, I do not myself think these are insurmountable.
- High energy required for ignition/detonation. Your initiator shouldn't weigh more than the engine!
- Minimizing the detonation to deflagration transition length has been a challenge
- Nozzle integration. Having several PDE tubes discharging into a shared nozzle brings another set of challenges
- Inlets, backflow, etc. - depending on which PDE school of thought you subscribe to, valved or valveless, this is a major issue. The leading valveless designs use the incoming flow of air itself to prevent backflow, which does lead to all kinds of constraints on inlet geometry. Valved designs have less limitations in this area, but tend to have problems with cycle rate, mechanical stress, etc.
Unless something has solved much of the above in the past few years, I do not think PDEs are simple at all. This is why I this program has me scratching my head. Without major breakthroughs in several areas, VULCAN will produce a lot of paper, but not much thrust.
