I hope that my readers had an amazing summer, and I look forward to discussing more aerospace engineering topics this fall. In this week’s blog post, I will explore a topic that I began to research this June: propulsion mechanisms for high-speed aircraft. In particular, we will explore ramjets and scramjets, which can propel aircraft and missiles into the hypersonic regime, or beyond five times the speed of sound.
Before we can understand high-speed engines, it is important to examine lower-speed engines. For the last sixty years, turbojet engines have powered many standard commercial aircraft. These engines, which look like massive tubes from the outside, have three primary components on the inside. The first of these is the compressor blades. As air enters the engine, the compressor blades bring it to a higher temperature and pressure. Now the air is ready to enter the combustion chamber, the second inner component. Here, the air is mixed with fuel and ignited. This ignited air then passes through the turbine blades, the third inner component, before it exits out the back of the engine. The exiting air turns the turbine blades, which by means of a shaft, turns the compressor blades at the front of the engine. At speeds up to three times the speed of sound, this system can effectively provide the thrust an aircraft needs to stay in the air.
However, at speeds beyond Mach 3, air friction causes turbojet engines to become dangerously hot. At these high temperatures, compressor and turbine blades can start to bend and melt, which would be catastrophic for the plane they are powering. Therefore, at these speeds, a new engine design is needed.
Ramjets can solve this problem by removing the compressor and turbine blades completely. Indeed, a ramjet is a glorified pipe, an engine with no moving parts that are in danger of melting. Instead of compressor blades, ramjets have an intricate intake duct that manipulates supersonic shockwaves to slow incoming air down to subsonic speeds and bring it to a higher pressure. At this point, the air is ignited in a combustion chamber and exits out of the engine. Turbine blades are no longer needed because there are no longer any compressor blades that need something to power them.
Ramjets, which slow incoming air down to subsonic speeds, are perfect for flight between Mach 3 and Mach 6. However, ramjets are not able to slow air moving any faster than Mach 6 down to subsonic speeds for combustion. Hence, there is a need for a third type of engine for aircraft that fly faster than six times the speed of sound. This new type of engine, called the scramjet, is just like a ramjet in most respects. The only difference is that a scramjet allows incoming air to remain supersonic all the way through combustion. This is an impressive feat because it is very difficult to combust air that is moving so quickly. A close analogy to scramjet combustion would be trying to light a match and keep it lit inside a supersonic hurricane.
Because scramjets are so advanced, aerospace engineers have been unable to fly a working model in any experimental scenario until 2004. Even now, there are challenges to be worked out before the military or civilian companies can implement one in an operational plane. The 2020s are an exciting time though, and secret development projects could bring a workable scramjet to market by the end of the decade. It will be exciting to remain updated on such technology that will propel humans to unprecedented speeds.