A brief overview of aircraft engine types and what makes the SR-71 Blackbird so special

Over the years engineers have devised many different and creative solutions to our transportation problems. This article will be delving into the engineering and applications of jet engines from commercial examples to the legendary US military’s SR-71 Blackbird.

Firstly, to grasp what happens inside these marvelous engines it is best to understand some key concepts regarding the operations under the chassis of the machine. Jet engines utilise a series of stages to provide thrust; the intake is followed by a compressor stage which is followed by a combustion stage and a turbine. Shortly after, the air is ejected out the exhaust. The air intake is self-explanatory but to comprehend the compressor stage visualise a piece of paper you wish to burn to start a campfire; it is going to provide a more focused and effective fuel source when you crumple it up into a more compressed form. Compressing air also has the effect of making it more efficient as a fuel source. The combustion chamber is then used to ignite the air similar to a car engine (fuel is mixed in and an electric spark ignites the mixture), propelling it out at a much higher speed for maximum thrust. The turbine that the flow passes through before the exhaust is used to turn the air intake fan and the compressor blades. All the mentioned stages above are key components of modern-day jet engines utilised by the commercial jets that carry you to your destination; some may vary as certain compressors use the airflow to turn themselves without much or any help from the turbine. However, all jet engines have the concept of compressing air to increase its efficiency and then combusting it for maximum thrust.

Secondly, the jet engines that have been developed all have their limitations, advantages, and disadvantages which can be overcome in some circumstances. The common measurement of speed for jet engines and supersonic aircraft is Mach where Mach 1 is the speed of sound and Mach 3 is three times the speed of sound. To note, 3 Mach and Mach 3 possess the same value. The classic turboprop engines, such as those on the propeller planes post-WW2, can propel a plane up to around 1000 km/h. Turbofan engines such as those on almost all modern airliners can reach up to 2.5 Mach (around 3000 km/h) making them far quicker. However, it must be noted that modern day airliners cruise at around 1000km/h for reasons such as fuel efficiency. To make aircraft that could fly as fast as the SR-71 Blackbird, engineers needed another engine type that can reach higher than Mach 2.5 which is where the Ramjet engine comes into the equation. Ramjets are jet engines that can reach up to Mach 4 or 5000km/h, beaten only by scramjets – short for Supersonic-Combustion Ramjets- which have yet to be fully tested but are projected to reach Mach 10 or over 12000km/h. From slowest to fastest; Turboprop, Turbofan, Ramjet, Scramjet is the order we have to date. All of the listed engines utilise Newton’s 3rd law by ejecting air out on one side fast enough to move the aircraft in the other direction.

 

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Turbojet

Found widely on older aircraft. For military planes, an afterburner is placed after the turbine and before the exhaust nozzle, providing extra thrust for manoeuvres and take-off on certain planes eg. The F-4 Phantom II.

 

 

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Turbofan 
Found predominantly on commercial airliners due to its fuel efficiency and noise-reduction capability. This occurs as the bypass air ‘cushions’ the hot air from
the exhaust nozzle thus masking some of the noise. For military applications, an afterburner is placed similar to the Turbojet engine and this can be seen on famous aircraft such as the F-22 Raptor. 

 

 

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Turboshaft
Found on helicopters, the
Power Shaft is what can be seen turning the propellers on top of the helicopter.

 

 

 

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Turboprop
Famously found on the large propellor jet aircraft of the Cold War such as the Soviet TU-95 Bear which had 4 such engines. They are fuel-efficient but lack in terms of speed and the altitudes they can reach which is why they were used for slow bombers and other aircrafts such as the iconic Cessna or other applications. eg. Tourist planes for sightseeing. 

 

All the engines listed above vary in application and advantages but follow the same base principles that we covered at the start. Interestingly, the Ramjet and Scramjet engines have no moving parts while being jet engines. This means that, while Scramjets have reached up to 9.6 Mach (over 11,000Km/h) on aircraft such as NASA’s X-43, the engine is incapable of taking off a runway. Without any moving parts, a Ram and Scramjet engines will not generate ‘static thrust’, meaning they will not provide thrust to a stationary aircraft. Therefore, these engines need help to get going; in the case of the X-43, it was taken up attached to another aircraft with rockets on its wings before detaching and flying on its own. To note, the X-43 was an unmanned experimental flight and many articles are available online for further information. 

A question you may have right now is ‘why not implement scramjets in more vehicles?’. One major hurdle that the SR-71 engineering team at Lockheed Martin had to overcome was the intense heat that an aircraft moving at supersonic speeds for prolonged time experiences. In movies, this is frequently shown in meteors, satellites and alien ships burning up in the atmosphere. Although the Blackbird did not have to experience exactly that, it is still a decent visualisation of the issues created by such speeds. Furthermore, airliners today use aluminium alloys for their strength-to-weight ratio but the SR-71 used titanium just because the aluminium could not withstand the heat which could reach over 500 degrees celsius. To clarify, the melting point of Titanium ranges well over 1500 degrees whereas Aluminium is just below 700 degrees meaning that the aluminum may not only melt on certain hotter areas, but also deform and lose strength or undergo metallurgical changes from heating and cooling. Imagine swords that are quenched in oil or water after being heated to increase hardness or to be tempered. This effect on an aircraft could lead to more brittle wings that snap or break near the landing phase. In addition, while the Concorde failed as a supersonic consumer plane for many reasons, one of them was that people on the surface of its flight paths complained about the noise of sonic booms. Our advancements in technology are many but the laws of physics persist, meaning that new metal alloys would have to become affordable for customers with a budget far smaller than that of the US Military for Ramjets and Scramjets to be implemented on a larger scale. 

Finally, the SR-71 was an aircraft built in the 1960s and flown successfully until the 1990s. It sustained Mach 3 flight with a 2-man crew for significant amounts of time, finally crossing the US from coast to coast within 67 minutes before retiring. More importantly, the aircraft took off and landed unaided meaning that unlike the X-43 its J-58 engines could provide the static thrust for take-off while also being able to exceed turbojet engine speeds far longer than other afterburning turbojet engines. In short, the SR-71 had an engine that is described as a turbo-ramjet meaning it is a turbojet on takeoff and adjusts into almost a ramjet to maintain the over Mach 3 flight at over 80,000 Ft or over 24Km in altitude. The end product of seemingly impossible engineering for its time was an aircraft that could simply fly past any enemy airspace without the risk of being shot down as it was too fast and flying too high for a surface-to-air missile to hit it. In addition, the aircraft possessed an RCS (radar cross-section) smaller than a man meaning for the enemy radar it could be seen as an unusually large bird. Many articles and documents exist online about the SR-71 as to why it was painted black, why it used a different fuel, how it cooled itself and many more fascinating aspects of the legendary vehicle. The Blackbird stands today as a testament to human engineering pushing the limits and overcoming a multitude of challenges to make the perfect vehicle for a task. We may be lucky in our lifetimes to see the rumoured successor SR-72 and possibly the implementation of Scramjets on new vehicles to deliver payloads to the international space station as a possible replacement for rockets.

Overall, the future is certainly an exciting place that is made more exciting by understanding some of the brilliant work that has occurred over six decades ago.

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