Why were propeller aircraft replaced with jet aircraft?

The short answer is they weren't, at least not completely. Propeller aircraft are still commonplace today, albeit in more specialized roles.

A full discussion on the differences between these two types of engines is far beyond the scope of this article, but we'll attempt to make a quick and dirty comparison for brevity.

As a general rule of thumb, jet engines are best for longer journeys, or journeys that need to be made quickly. Propeller-driven aircraft is best for small smaller or light aircraft where fuel capacity and profitability of operation is limited. Another unofficial metric is the number of seats an aircraft has.

If under 100, or so, an aircraft will often have a turboprop engine. Above that and it is probably equipped with jet engines.

When it comes to military aircraft and commercial airliners, for the most part, jet engines have pretty much completely replaced propeller-based aircraft. This is for a variety of reasons, but the increased speed that is afforded to jet-engined aircraft is one of the most important reasons.

Another benefit of jet engines is their ability to operate at higher altitudes when compared to propeller-driven aircraft. This, combined with their greater speeds, makes jet engines the engine of choice for long-distance travel — like taking you on a holiday.

Propeller-driven aircraft also require less runway distance to take off and land and can handle a variety of runway constructions, unlike jet engines. For this reason, regional airports or airfields that might not have concrete runways will likely exclusively only handle propeller-driven aircraft rather than jets.

Propeller-driven planes are also more fuel-efficient for shorter flights, have lower running costs, are cheaper to insure, and are generally cheaper to maintain than jet engines. However, jet engines are, believe it or not, much quieter engines to run, which makes them better suited for airports near residential areas.

However, modern jet engines, most commonly the turbofan engines of today are something of a hybrid between the two engine types. In a turbofan, some air comes in the front, is compressed and mixed with fuel, which is then ignited.

The hot exhaust passes through the core and fan turbines and out of a nozzle, as in a turbojet. The rest of the incoming air passes through the fan and goes around the engine, like the air through a propeller. The air that goes through the fan has a higher velocity, from the free stream. So a turbofan gets some of its thrust from the core and some of its thrust from the fan.

All of this takes place inside the shell covering of the engine, which produces a duct through which the air can flow smoothly.

The aforementioned fan is made of many tiny blades that act as a kind of propeller.

There are also propeller engines called turboprops that are, in effect, a propeller turned by a jet engine rather than a piston-engine, as in more traditional aircraft. These kinds of engines are very reliable and efficient, and also help make up some of the noticeable difference in speed between propeller engines and jet engines.

So, in a sense, since the 1950s, rather than completely replacing propeller-driven engines, jet engines have, to a greater or lesser extent, joined forces with their more traditional counterparts.

All very interesting, but in a hypothetical death match between a propeller-driven aircraft from the 1940s and a modern jet, what would the outcome be? Apart from the inherent differences in engine capability, modern fighters have some distinct advantages over their old compatriots. One is their common use of heat-seeking missiles.

But, would they be of any use against a propeller aircraft? Let's find out.

How do heat-seeking missiles work?

More correctly termed infrared homing missiles, "heat-seeking" missiles are basically special rockets with a passive weapons guidance system that relies on infrared emissions to target and track vehicles like aircraft or tanks. Heat sources, like an aircraft's engine, kick-off a lot of infrared, hence the more colloquially used name "heat-seeking".

Such missiles are able to recognize the temperature difference between objects and so can easily detect the difference between an airframe that is above ambient temperature and its background, like the sky. (Later passive air-to-air missiles homed in on ultraviolet radiation as well.) Using this temperature difference, the "heat-seeking" portion of the missile is able to target and guide a missile towards its target.

We'll talk a little about the association between "hot" bodies and infrared light in a moment, but many things give off infrared light, including your own body, vehicle engines, etc. This phenomenon makes them stand out from the background. If a method can be found to home in on this kind of light energy, then weapons, like missiles, can self guide themselves without the need for human control.

"Heat-seeking" missiles, being passive systems, do not have a transmission that can be easily "tracked" by potential targets, making such weapons very difficult to spot and disable until it is too late. This is in contrast to more active targeting systems like radar-guided weapons, for example.

While modern military aircraft are fitted with special suites of sensors and cameras called Missile Warning Systems (MWS), such systems are only really able to optically detect a missile at range. While better than the human eye, they are not completely effective and are subject to false positives and false negatives.

Another way to indirectly warn of potential incoming "heat-seeking" missiles are sensors used to detect radar locking. Called a Radar Warning Receiver (RWR), these systems can provide a pilot with a warning they are being targeted, but do not actually detect the missile itself.

"Heat-seeking" missiles are extremely effective weapons systems, with the vast majority of United States aircraft losses from infrared-homing missiles alone. However, like most weapons systems, they are not 100% foolproof.

Some fairly simple countermeasures can be employed to throw them off course and protect the target aircraft. For example, flares can be released behind the aircraft to provide false heat sources for these kinds of missiles to lock on to and "destroy".

However, such countermeasures are only effective if the pilot of a target aircraft is aware that their aircraft is currently under threat from IR-homing missiles. Some countermeasure systems, such as planes that emit fields of IR radiation, are also able to automatically deploy too.

More modern IR-homing missiles are also subject to increasing complexity to make them "smarter" against such simple countermeasures.

Interestingly, this technology is actually pretty old. Some of the first infrared devices were experimented with during the Second World War, for example. These early devices were devised by German engineers who developed rudimentary heat-seeking missiles and proximity fuses.

Thankfully, from the Allied point of view, the war was ended before the Germans could make significant breakthroughs in this technology.

True "heat-seeking" missiles did not become possible until the 1950s, when rocketry technology, conical scanning, and miniaturized vacuum tubes become sophisticated enough to be integrated together on a missile platform. These early missiles — while technologically impressive for the times — were unreliable and achieved low success rates in the 1960s.

The technology underwent significant development throughout the 1970s and 1980s to make them highly effective weapons systems. More contemporary units are very sophisticated and have the ability to attack targets out of their field of view (FOV), coming up from behind, and even to pick out vehicles on the ground.

In modern missiles, the infrared sensor package tends to be located in the tip or head of the missile and is known as a "seeker head".

Most "heat-seeking missiles" come in a variety of types, depending on their sensitivity to infrared light. Early ones, now called single-color seekers, were most sensitive to infrared light between the 3 to the 5-micrometer range. These kinds of missiles proved to be relatively ineffective, as they would often only be effective so long as the missile could "see" the jet exhaust of an enemy aircraft.

This led to the development of new missiles, called "all aspect" that were sensitive to the exhaust as well as the longer 8 to 13-micrometer range. For reference, a human body emits infrared light at around the 12-micrometers range.

Such missiles are also able to lock on to dimmer heat sources on an aircraft, such as its fuselage. They also tend to require cooling to give them a high degree of sensitivity to allow them to lock on to the lower level signals from the sides and front of an aircraft in flight. Modern "all-aspect" missiles such as the famous AIM-9M Sidewinder and Stinger use compressed gas like argon to cool their sensors, in order to lock onto the target at longer ranges and all aspects.

Similar technology is also employed in semi-automatic command to line of sight (SACLOS) technology. In this kind of setup, the IR seeker is mounted on a trainable platform on a missile launcher that is operated by a human user.

The user will manually point the weapon in the general direction of the target manually. The seeker will then not track the target but the missile itself which is then guided by flares to provide a clean signal. Some other systems will use radio signals to guide the missile towards the target in the user's aiming telescope.

These kinds of weapons have commonly been used for both anti-tank missiles and surface-to-air missiles, as well as other applications.

Why do "hot" things emit infrared light?

As promised, we'll now discuss why "hot" things emit infrared radiation. Before we start, take a moment to think about the Sun.

The Sun transmits energy to Earth at different wavelengths of the electromagnetic spectrum. All forms of electromagnetic radiation transmit energy, and heat is simply the transfer of kinetic energy from one medium, or object, to another.

Much of the harmful ultraviolet radiation is absorbed by the Earth's ozone layer, but visible light and shortwave infrared light passes through much of the atmosphere. This energy reaches the surface of the Earth and is absorbed.

The Earth's surface emits longwave infrared energy (heat) back to the atmosphere. Some of this escapes to space, but a significant portion is absorbed by Greenhouse gases or clouds. The gases or clouds, in turn, emit heat back to the surface or into the atmosphere. This emitted radiation adds to the surface warming from sunlight.

For all other heat sources on Earth, most of this kinetic energy transfer occurs within the infrared part of the electromagnetic spectrum. The reason for this is a little beyond the scope of this piece, but it has to do with black body radiation.

All objects with a temperature above absolute zero (0 K, -273.15 oC) emit energy in the form of electromagnetic radiation. A perfect black body, according to Kirchoff's law of radiation, would be one that absorbs all light in all frequencies all the time (hence being called a perfect black body). Such a body would also emit light with maximum efficiency in all wavelengths. It is a hypothetical object which is a “perfect” absorber and a “perfect” emitter of radiation over all wavelengths.

However, this is a purely theoretical object. Most stuff in real life is somewhere between a perfect black body and a white body (a hypothetical object that absorbs no electromagnetic radiation of any wavelength).

All things with mass are made of atoms that are constantly in motion, or vibrating, if you'd prefer. The hotter an object is, the more violently and frequently this occurs. This vibration releases electromagnetic waves that, for most objects we can see, are in the infrared spectrum.

A campfire, for example, emits different wavelengths of light, but the vast majority of the "heat" comes from the infrared light it emits.

In fact, if you were to place a special filter between yourself and the campfire that only allows infrared light through, it would feel just as hot to you. Such light is invisible to your eyes, but can be detected using special cameras that, depending on the way information is displayed, can make things look like they are glowing.

To make infrared images, we can use special cameras and film that detect differences in temperature, and then assign different brightness, or false colors to them. This provides a picture that our eyes can interpret.

Typically, when using an infrared camera, hot things look bright yellow and orange. Items that are colder, such as an ice cube, are purple or blue.

Most bodies radiate most of their heat in the infrared spectrum, because they don't have enough energy (heat) to radiate at a higher frequency. Any object that has a temperature will release infrared light to a greater or lesser degree including, as we've previously mentioned, your body or a running engine.

Incredibly, even objects that we think of as being very cold, such as an ice cube, will emit some infrared light. This is because, although it is "cold" to us, the atoms within an ice cube are still vibrating and releasing infrared radiation.

Any object that does not quite have enough thermal energy to radiate visible light will still emit energy in the infrared.

Take, for example, hot charcoal. This may not give off much visible light but it does emit infrared radiation which we feel as heat. In fact, the warmer the object, the more infrared radiation it emits.

For this reason, infrared radiation makes an excellent indirect way to record the relative temperature of an emitting object from a distance.

Some animals have even evolved natural infrared detectors of their own too. Take the pit viper family, for example.

Like other members of the 'pit viper' (Crotalinae) family of snakes, as well as members of the boa (Boidae) and python (Pythonidae) families, they have evolved special sensory "pits" that are able to detect infrared light. This allows the snake to detect warm-blooded animals from a distance, enabling the snake to hunt these creatures. It is even thought that snakes armed with pairs of sensory pits are able to have a rudimentary form of infrared-based depth perception.

From the perspective of this piece, jet engines might not give off a lot of light (apart from the exhaust gases) but they do produce a lot of infrared light. They are, after all, very hot things.

This is something that can be used to detect and track.

Could a heat-seeking missile track and destroy a WW2 fighter?

Not to sound too flippant, but the answer to this question is a definite... maybe. It all depends on the target aircraft in question, and the kind of missile launched.

Early "heat-seeking" missiles would likely struggle to find the infrared signature of an old propeller-powered plane and maintain a lock. This would especially be the case if the target plane had some countermeasures, like flares.

This is because a typical jet engine will burn at around 3,632 degrees Fahrenheit (2,000 degrees Celsius). While modern jets will also tend to have some form of cooling mechanism to prevent engine part wear, the exhaust and engine cowling will typically be around 1,832 degrees Fahrenheit or more (1,000 degrees Celsius). A piston-engine, on the other hand, is cooler — typically around 572 degrees Fahrenheit (300 degrees Celsius), give or take.

Flares, especially those used as countermeasures, tend to be made primarily of magnesium, which burns at somewhere in the region of 3,632 degrees Fahrenheit (2,000 degrees Celsius). This would likely confuse a heat-seeking missile, allowing the piston-engine aircraft to escape.

Flares aside, for more modern IR-seeking missiles, like a Sidewinder, it should be quite possible. In fact, it has even been done before.

During the development of the AIM-9 Sidewinder missile, it was tested against a variety of targets, including an old Grumman F6F "Hellcat" piston-powered, propeller drone aircraft.