When Top Gun Maverick released in 2022, viewers were astounded by a great scene in the beginning of the movie where the main character breaks Mach 10 in a prototype hypersonic jet. This jet doesn't actually exist in real life, and hypersonic vehicles remain limited to very few testing vehicles today. The current speed record for a crewed, powered aircraft, wasn't actually set by the SR-71 Blackbird as many people assume, but the X-15 in 1967 which reached Mach 6.7. The most important difference between the SR-71 and the X-15 is the propulsion system. The X-15 used a rocket-powered engine and the SR-71 used an after-burning turbojet, but neither of these solutions are cost-effective and efficient enough for hypersonic travel. The answer lies in today's topic, ramjets and their supersonic derivative, scramjets.
In the constant race to go faster, aircraft designers have typically been restricted by a major limiting factor, the propulsion system. At first, it was the propeller engine, and then followed by the jet, but reaching higher speeds pushes existing engine technologies to their limit.
An early solution to this was using rockets to propel planes, but this was mostly restricted to test planes. This is due to their difficult maintenance and extreme fuel costs. Almost all supersonic planes nowadays use low bypass turbofans, and utilise afterburners to get brief periods of extreme speed. However, turbofans can only realistically go up to Mach 3, as any higher would cause the air to be too hot for the moving parts in the engine and would be extremely inefficient. To break into hypersonic territory (Mach 5+), engineers would have to design a completely new type of engine — one that can achieve greater efficiencies at high speeds without using moving parts.
Ramjets don't use any moving parts such as fan blades or compressors, and rely on three main parts to provide supersonic thrust at high efficiency: an air intake, a compression chamber, and an exhaust nozzle. The inlet has a special shape so that when supersonic low-pressure air enters it, the air deflects at different angles, causing shockwaves that turn the air flow subsonic. Overall, this is called the ram effect, and is the driving force behind ramjets as they slow supersonic air to subsonic speeds without spilling or using a compressor. The high-pressure subsonic air that exits the inlet is then ignited in the combustion chamber and accelerated back to supersonic speeds in the exhaust nozzle, generating thrust.
Scramjets operate very similarly to ramjets, with the notable difference being that the air that leaves the inlet remains supersonic. The original ramjet, and conventional jet engines, use a combustion chamber that requires subsonic air. On the other hand, scramjets use a redesigned combustion chamber that can work with supersonic air, giving it much higher efficiency and a faster max speed.
While ramjets and scramjets can both facilitate faster speeds, they also have limitations. For one, they can only work with supersonic air. This means that they can't be used as the only propulsion system on a plane as it won't be able to take off. Adding to this issue, ramjets and scramjets start to lose effectiveness when the altitude is too high, as the air density becomes too low. These downsides mean that, to be effectively utilized, ram/scramjets must be combined with turbine jets, creating a turbine based combined-cycle (TBCC) engine.
While Maverick from Top Gun is a hotshot pilot who's always itching for a new dogfight, it's unlikely that we'll see fighter planes use ramjets any time soon, as stealth has taken prevalence over speed due to new surface to air missiles that can shoot down even the fastest of planes. On the topic of missiles, these will likely be the first that switch to ramjet technologies. In fact, India launched the BrahMos supersonic cruise missile in 2005 featuring a combined cycle rocket and ramjet engine, and a hypersonic variant is already in development. France also has a supersonic cruise missile, the ASMP-A.
Another likely use case for ramjets are high-altitude reconnaissance planes such as the highly secretive SR-72 supposedly under development by Lockheed Martin. In fact, the Darkstar jet from Top Gun Maverick was inspired by the SR-72 and Lockheed Martin engineers even helped design the model for the movie. The real SR-72 will most likely be unmanned, as will most future reconnaissance planes. With new startups such as Hermeus and Destinus reaching successful milestones in their engine testing, the future for hypersonic travel looks bright.
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