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The term “scramjet” is actually a semi acronym that means Supersonic Combustion Ramjet. It essentially works on the same principles of ramjets; forward motion of the aircraft, rather than jet engine fans, compress air into a combustion chamber where burnt fuel heats the air creating thrust. This method however had limits for conventional ramjets. They have a limited window of operation between speeds of Mach 1 and Mach 5. Any slower and the necessary air compression can’t happen. If the ramjet went faster then Mach 5 – five times the speed of sound – the aircraft would go too fast to properly compress air into the engine. This heightens air friction in the engine and at high speeds creates high temperatures that could melt or affect the structural integrity of the craft and the plane would crash.
The solution to this problem is the scramjet. Its design was altered to handle air compression at supersonic speeds making it possible for a jet to reach speeds as high Mach 12 to Mach 30 which would match the speed of the Earth’s rotation.
If you have read about ramjets you may know that the technology has been with us for close to a century. Though the concept was originally created by a French inventor, the testing and research for jets and scramjets in particular was done in the UK and then in the United States. For the next couple of decades, through the second World War and afterwards, NASA and the US Air force did extensive research on supersonic flight. The results are now modern day fighter jets and commercial jumbo jets. However, the average top speeds reached by these craft is around Mach 3.3. Part of this was due to the cost of producing supersonic aircraft which ended up having to be subsidized by governments to be even made financially viable. The only commercially successful supersonic aircraft built was the Concorde. However, even this craft had limited success due to the cost of development and no new innovations on craft that were over 30 years old.
So what make a scramjet able to function the way it does? Like its slow predecessor it has few moving parts; it gets rid of costly and heavy items like the turbofan. It has fewer parts than even a ramjet: an intake, a combustion chamber, a fuel injector, and a nozzle that creates thrust. In a ramjet air is slowed down by a supersonic induced shock wave allowing for normal subsonic compression. As explained earlier, this only works at most at speeds up to Mach 5. The solution that the scramjet engine presented was to not slow down the air at all. Instead the, intake was built to channel airflow at supersonic speeds and the design of the combustion chamber made to quickly dispense and burn the fuel and air.
In the end scramjets face the same challenges as ramjets. They also have a minimum operating speed which is Mach 7 and a maximum speed where anything faster would be burned up by drag and friction. These concerns and the cost of development have prevented the design of a practical scramjet craft that can make a successful flight test. If these challenges can be overcome this could lead to the development of more efficient and lighter suborbital spacecraft that use less fuel.
If you enjoyed this Universe Today article there might be others that want to read. One article is about the US Airforce starting new testing on scramjets. There is a second article about a test already done by the Australians.