Why did the turbojet replace the piston engine?

Question

Many people say that this was because aircraft powered by turbojet engines were faster during WWII. However the Grumman F7F-4N Tigercat, a fighter powered by a piston engine, flew at a maximum speed of 460mph and the Gloster E28/39 (first British jet engine aircraft) flew at a maximum speed of 466mph, which is not that great a difference.

Others say it's because aircraft powered by piston engines cannot fly as high as jet engine aircraft, but what is the reason for this?

Why did jet engine aircraft replace piston engine aircraft?

Answer 1

Two more reasons the gas turbine supplanted the piston engine for aircraft use:

Power output. Aircraft piston engines have a practical limit on how much power they can put out, before becoming inefficient. This worked out to be around 3000hp. Two of the largest and most powerful piston aircraft engines that were also reliable enough for aircraft use are the Napier Sabre and P&W R4360, at around 3000hp, developed at the end of WW2. Yes, more power has been obtained from piston engines on land based vehicles, but they're either racing engines whose reliability is too low for aircraft use, or they're far too heavy for aircraft use, such as the 20k+ HP diesel engines that power cruise ships.

Gas turbines don't have those limits, one contributing factor being the lack of a reciprocating action. With a piston engine, each full stroke results in the piston accelerating to top speed, then coming to an abrupt halt and accelerating in the opposite direction... twice. Gas turbines that just spin can be made substantially larger while maintaining the reliability and efficiency that aircraft use demands.

Also, gas turbines tend to have a very high power to weight ratio, making them ideal for large aircraft use, where weight is very important.

Equating HP to thrust isn't simple as HP is raw power, while thrust includes altitude, velocity, and propeller/fan efficiency.

A simplified example was published on Aerospaceweb:

Luckily, we do have access to data from a NASA report that does provide all the data we need to illustrate a sample case. The data is provided for a Boeing 747-200 cruising at Mach 0.9 at 40,000 ft (12,190 m). In this example, the aircraft's engines produce 55,145 lb (245,295 N) of thrust, only a quarter of its rated static thrust, to cruise at a velocity of 871 ft/s (265 m/s). Using the equations provided above, we calculate the power generated by the 747 to be 87,325 hp (65,100 kW).

Using that very simplified example, a GE90 producing 115,000 pounds of thrust would be putting out the equivalent of around 160,000 horsepower.

Also, the maintenance requirements on gas turbines are substantially lower, especially for the high output engines. For example, the very large RR Trent series turbofans have a TBO (Time Between Overhaul) of around 15,000 hours. On the large piston engines such as the R4360, the TBO was more like 1500 hours, and the very large piston engines, especially the radial engines, had a prodigious appetite for engine oil. Plus intermediate maintenance on piston engines that gas turbines don't need, like changing spark plugs, that had to be done frequently. The Convair B36, which had six R4360's, required 336 spark plugs. Not something you could do in your driveway in an hour.

Some of the gas turbine's reliability comes from it's lack of vibration. Large piston engines, with their reciprocating action, tend to vibrate a lot, which reduces the life of engine and auxiliary components, like fuel pumps and spark plug wires.

Thus, not only did the turbojet, turboshaft, and turbofan make possible aircraft that wouldn't be practical at all with piston engines, such as large airliners flying at 30k+ feet, they also lowered the cost of maintenance and the frequency of maintenance substantially.

There is one area where the piston engine for aircraft use is still the better solution, and that is when the engine gets small, below around 500hp. Gas turbines don't scale down all that well. The small ones are not fuel efficient, nor does the cost get substantially lower.

As an example of this, consider one of my pet daydreams - a single seat Mosquito helicopter. Aside from the ultralight version, two versions with larger engines (and requiring a FAA helicopter license) are made, the XE285, with an 85hp snowmobile engine, and the XET with a 90hp gas turbine engine derived from a backup power generator. The piston engine sells for maybe 2k USD and burns around 5gph, while the gas turbine sells for 10k and burns 8.5 gph.

Answer 2

There are several benefits:

• piston engines are best for driving propellers. At the same shaft horse power $P$, propeller thrust $T$ varies with the inverse of air speed $v$: ($T_{Prop} = \frac{P}{v}$). This means that the power requirement to keep a piston-powered aircraft flying will increase with the third power of airspeed at high speed. In order to fly 50 m/s faster, an aircraft with a top speed of 200 m/s will need an engine of almost twice the horsepower (195%, to be precise). A turbojet, on the other hand, has almost constant thrust over speed in the subsonic range, so $T_{jet} = const.$

• Turbojets can make better use of precompression at higher speed. The kinetic energy of the airflow can be used to compress the air even ahead of the intake. At Mach 0.8 this yields 50% more air pressure relative to ambient air than in static conditions.

• Especially important for military aviation: The early jets cost only a quarter of a high-performance piston engine to build in terms of man-hours. This offered a much better productivity in wartime where the available labor was a serious bottleneck. A single Jumo-004B needed only 375 hours to be built:

From Wikipedia:

Costing RM10,000 for materials, the Jumo 004 also proved somewhat cheaper than the competing BMW 003, which was RM12,000, and cheaper than the Junkers 213 piston engine, which was RM35,000. Moreover, the jets used lower-skill labor and needed only 375 hours to complete (including manufacture, assembly, and shipping), compared to 1,400 for the BMW 801.

The first point really is the most important, and it translates into much better altitude performance because the turbojet is in essence a big turbo-supercharger with a continuous combustion process at its center which produces thrust by ejecting air at high speed directly backwards rather than going through the complication of accelerating the air by rotating wings. This gave jet aircraft at high altitude (where the bombers and, consequently, the action was) both a speed and a climb advantage - they could break off combat at will, and their speed was so high that the bomber gun turrets could not follow them when they flew through a bomber formation. Jet-propelled reconnaissance aircraft could fly unharmed over enemy territory because no piston-powered aircraft could intercept them.

The Gloster E28/39 was only a demonstrator - if you want a realistic comparison, do it between the Grumman F7F and the Messerschmitt 262A, which could reach 560 MPH, 100 more than the F7F, and even topped 600 MPH in a special version with a low-drag canopy.

Regarding altitude: If you add enough turbo- and supercharging to the engine, a low-speed design can reach higher altitudes than most subsonic jets, but would be rather unpractical as a combat aircraft. In civilian use, a jet will reach higher cruise speeds which gives it an advantage over comparable propeller-driven aircraft. However, a piston engine remains the best choice for fuel efficiency.

Answer 3

One more point that I haven't seen mentioned yet: the fuel they burn.

Most piston engines burn gasoline, which requires a great deal of refinement. Aviation engines generally require gasoline with very high octane ratings (typically at least 100), which is still more expensive than the lower octane gasoline used in most automobiles and such. The requirement for high-quality crude oil was (for one example) why the war in North Africa was so important during the second world war.

Jet fuel, by contrast, is pretty much like kerosene or Diesel fuel. It does take some refinement to get from crude oil to jet fuel, but the process is simpler, and the quality of crude oil necessary isn't as high either. As a result, the higher volume of jet fuel burned is largely offset by the lower cost for a given volume, and the greater ease of obtaining crude oil of the required quality.

It is possible to power an aircraft with a Diesel engine. In the time frame in question (around the end of World War II) the only practical Diesel engines for aircraft were the Jumos. In some ways, a Diesel gives the best of both worlds--relatively low fuel consumption (even lower than gasoline engines as a rule) and the ability to use relatively low-grade fuel. There are, however, a couple of major disadvantages. The first is simple weight. A Diesel normally has a much higher compression ratio than a gasoline engine, so it has to be much stronger to withstand the higher pressures involved. The second is that some parts of Diesel engines (especially the fuel injectors) requires much more precise machining than virtually any part of a gasoline engine. This means a Diesel engine of a given capacity tends to be substantially more expensive than a gasoline engine of the same capacity.

So, for Diesels to make sense you need to use them for a relatively long-range aircraft--specifically, long enough range that the fuel savings outweigh (literally) the extra engine weight. Second, given the difficulty of manufacturing you probably want to restrict them to a relatively small number of aircraft. So, if you wanted a few long-range bombers (for example) they might well make sense. For fighters (short range, relatively expendable) they'd probably make a lot less sense.