Forgive me for going on but this article really is very nice.

Figure 3 also deserves close attention. It shows a dual-mode ramjet / scramjet. Dean Andreadis, who wrote the article is obviously an experienced aeronautical engineer. A pure ramjet processes inlet air through a "normal shock wave". The flow down stream in the combuster is always subsonic. For a pure scramjet, the inlet air is processed by oblique shockwaves. The flow in the combuster of a scramjet is always supersonic. Figure 3 shows the cute tricks used to make ramjet dual mode (both subsonic and supersonic combustion). The big "gotcha" are "unstarts". When you do the math for supersonic flow, you find that for a given inlet condition there is almost always two possible solutions. One solution is what you want for a working engine while the other is typically disasterous (it'll cause a flame-out and/or wreck the engine). A functioning ramjet can transition almost instantly from one solution to another. Say the engine ingests a bug, goes through a thermal, the pilot looks at it cockeyed and then the engine transitions (or maybe not). This transition is called an "unstart". Unstarts are a very bad thing. SR-71 crewmembers have died because of unstarts (SR-71 crew members were very brave men).

Gary