The Hypersonics Flight Demonstration Program (HyFly) is the first time a scramjet missile engine has been tested with conventional, liquid hydrocarbon fuel, which is non-toxic and safe to carry aboard ships. Ramjet may actually be regarded as the simplest form of a jet engine, having no moving parts. It is essentially a hollow tube into which fuel, mixed with air, is injected and burned in order to produce thrust. The ramjet is capable of operating only when it reaches a particular speed that permits incoming air to be compressed by being forced into the engine. This phenomenon, called the "ram effect", occurs when a volume of air is forced into a small space at a sufficiently high speed and is compressed to a higher pressure. A scramjet engine generally operates in a very similar way to a conventional ramjet, with the exception that the flow of air in the combustion of the fuel-air mixture through the engine happens at supersonic speeds, allowing the scramjet to achieve ultra-high speeds. 

Until recently, the fastest scramjet ever tested was NASA's X-43A, which reached Mach 9.6 in a test flight on November 16^th, 2004. However, in May 2007, Australia's Defense Science and Technology Organization (DSTO) announced the successful testing of an experimental scramjet based craft which was rocketed to an altitude of 530 km and reached a speed of Mach 10 during re-entry. 

Ramjet and scramjet based missiles have several important advantages over existing missile technology. They have a very simple construction (no moving parts), they have a very long range (since they use oxygen from the air instead of conventional rocket fuel) and they can reach very high speeds (giving the target little time to respond). However, air breathing missiles also have several disadvantages, including relatively poor maneuverability due to problems in the air intake at high speeds. Reaching these high speeds also usually requires a rocket booster that might increase the weight of the missile. Although several ramjet based missiles have been in use for several decades (most of them by the USSR during the cold war and now by Russia), scramjet based missiles are still in the experimental stages.  The U.S. is hoping that demonstrator programs such as HyFly will enable the creation of future ultra fast attack cruise missiles early in the next decade. 

TFOT already covered several ramjet/scramjet related technologies including an in-depth look at a student built ramjet based missile, tested in Israel in May 2006, along with a look at the history and science behind air breathing missile technology. In 2007, TFOT also covered the HyCAUSE Australian Mach 10 scramjet-based missile test which broke the world record.

More information on the HyFly can be found on Aerojet's website.