Why do some aircraft use double engines?
Some countries don't get the most powerful engines and have to use what is available in the market. To avoid upsetting China too much, the US would not sell the F404 or the F100 to Taiwan, and a derivative of a civilian engine had to be developed.
A second reason is redundancy: When the US Navy had to decide between the F-16 and what was to become the F-18, they preferred the design with two engines, because losing your only engine over water is a much more life-threatening experience than losing it over land. Similarly, the Eurofighter was designed more for peace than for war - having two engines will reduce training losses.
Generally, a single, bigger engine will be more powerful and more efficient per unit of mass, because manufacturing tolerances will be relatively smaller in the bigger engine, if the same technology is used for both. Single engines need to have more system redundancy than one of a pair of engines, so some of that advantage cannot be transferred into the finished design. Nevertheless, from a performance standpoint the single engine fighter will look better.
Very early jet engines weren't particularly powerful; it took two in most cases to equal (or slightly exceed) the performance of a single big V-12 or large radial.
The next generation was more capable; most fighters in Korea and many in Vietnam were single-engined. Some exceptions were quite large – the F-4 Phantom and (R)A-5 Vigilante come to mind. The more combat load required, the more thrust, and a single engine able to power an F-4 Phantom would've likely been quite large.
Modern engines are much more capable; it mostly comes down to the desires of the operators. A single engine requires less maintenance (although probably not half) than a twin-engine aircraft. It is also more susceptible to mechanical failures or combat damage. The need for larger combat loads, redundancy, and —in some cases— excess performance are probably the primary reasons that not all modern fighters are single-engined.
Number and placement of engines is one of the initial trade-offs that a scratch-built aircraft design deals with.
There are several criteria for the trade-off of the basic configuration. Here are a few:
- Weight penalty
- Aircraft Performance
- Stability/Control (agility)
- Maintenance intervals
- Inlet configuration (if it's down the aircraft, then FOD problems show up)
- Stealth (a single engine is bigger than two smaller engines (more exhaust area))
- Cost of engines
- Cost of aircraft
- Provisions of the platform (to add future capabilities, and robustness to design changes)
These and several other criteria (esthetics, marketing, etc) are used to judge the different configurations, and then one of them wins.
This question is pretty complex and has to do with several factors
Design performance criteria - what are the specifications for the airplane's mission, flight envelope, onboard mission systems, payload, etc.
Available engine technology - performance criteria are largely limited by this factor. Power plants are developed in parallel with aircraft programs, often beginning 1-2 years before development on the plane begins.
Military fighters tend to favor a single engine arrangement for power to weight and specific fuel ratios which favor energy maneuvering. But performance requirement such as large combat radii or mission systems and stores carry combined with engine limitations often make a multi engine design attractive.
System redundancy is a tertiary benefit of multi engine aircraft, since losing an engine results in only a 50% loss in total available thrust plus redundant generators and hydraulic pumps to allow the aircraft to continue to fly. This is alleviated somewhat with APUs or EPUs Un single engine fighters which do provide electrical power and hydraulics but no additional emergency thrust.
In war time, one engine or two engines is immaterial.
In peace time, the number of engines is quite important.
I had two engine failures during my military career: one of the aircraft I brought home. Following engine failure on the other, the aircraft and I returned to earth separated in time and distance.
So, if you envision your airforce operating during peace time for the majority of the life of the aircraft, you'll want to protect your assets from unnecessary losses, especially if you have finite resources.