Modern air travel is a dream sequence wrapped in nightmarish service. Between manhandled baggage, Kafkaesque security, and boarding procedures to shame a ballet production, there is enough to make us forget that every single day, millions of land-based mammals fly across entire oceans and this is considered perfectly humdrum.
It wasn’t always so. In the fifties and sixties, a new form of engine adapted from the military was revolutionising commercial travel, allowing for radically shorter travel times and some very novel vacationing options for the well-heeled. The jet set became a byword for high society, cosmopolitanism, corporate power and la dolce vita, all blended into a single image of speed and efficiency. It isn’t very often that a piece of internal machinery catches the popular imagination so vividly. This is an exploration of that miraculous engine in all its avatars.
At one level, all jet engines are merely genteel rockets, since rockets are in fact an aggressively straightforward subset of jet engines. In all jet engines, a rapidly-expanding volume of air is directed one way, and the resulting thrust moves the aircraft* in the opposite direction.
The oldest form of jet engine is the turbojet, essentially a pump that compresses incoming air and ejects it at high pressure. The incoming air is pushed into a reaction chamber where it is mixed with fuel and ignited. The resulting explosion (now isn’t that a comforting thought for nervous flyers?) is directed out the back of the chamber, pushing the aircraft forward.
The turbofan, a newer variant used in almost all modern aircraft, adds an additional source of thrust. Since the engine can only handle so much air at once, the turbofan places an enormous fan in front of the engine to push air both into and around the reaction chamber. The air sent into the chamber reacts with fuel in the usual way. The air that bypasses the engine adds in some extra thrust.
This bypassed air can account for as much as eighty percent of the total thrust in some cases. Turbofans are quieter than turbojets, and given that fuel for the reaction chamber can account for nearly a quarter of the operating costs of airlines, much more economical as well.
So much for turbines; not all jet engines use them, and so not all jet engines have the turbo- prefix. This brings me to my two favourite jet engine models: the minimalist ramjet and its hush-hush sibling, the scramjet.
The ramjet is the Hulk of this Avengers team: a simple, single-minded engine that does not trouble with the niceties of compressors and prissy moving parts, choosing instead to intake air by smashing right into it. The engine’s motion pushes air in, and from there on it’s a regular jet engine. Since ramjets require the engine to already be in motion, they can’t get an aircraft off the ground by themselves. This has limited their practical use so far to ballistic missiles and artillery shells, but they retain their niche by helping achieve speeds of up to Mach 6.
Attaching tiny ramjet engines to the tip of helicopter rotor blades has been successful as well. The idea is to get the rotors spinning under their own power, then switch on the ramjets, which make the entire arrangement like a spinning Catherine wheel.
One extreme variant of the ramjet is the scramjet, or supersonic combusting ramjet. The scramjet has an inlet shaped to compress air as it enters the engine, allowing for a larger throughput and higher speeds than the regular ramjet.
Scramjets are currently at the leading edge of jet technology, developed by few countries, and in considerable secrecy at that. The upper limit of scramjets is rumoured to be around Mach 12. That’s still on paper, since the fastest any scramjet has actually gone in real-life testing is Mach 9.68, and commercial hypersonics are 25 years away at best. If they do come to fruition, it will probably be at speeds of around Mach 6, which is still a 35-minute flight from Vancouver to Halifax, or around the world in less than six hours.
There is almost nothing humans will not attach a rocket to just to see what happens. The list includes, so far, a leather recliner, a lawnmower, a wheelchair, a shopping cart, and a portable toilet that can reach highway cruising speeds.
The jet-augmented Volkswagen Beetle is (a) street legal, since there is no actual law against attaching a jet engine to a small car, and (b) allowed in the hybrid lane since it does, technically, have two engines.
There are certain questions which define any sufficiently advanced industrial civilisation: “Will it blend?”, “Does it go with Sriracha?”, and most importantly, “Can we strap a jet engine to this?” Some people are interested in P versus NP too, but these are the really important ones.