How F-15s Work

The F-15 can carry extra fuel in three external tanks. One mounts under each wing and one attaches to the fuselage.

Photo courtesy U.S. Air Force

Mechanical and Electronic Systems

The engines are outfitted with afterburner nozzles, which can provide an extra kick of thrust when necessary. The afterburner simply injects fuel into the hot jet exhaust stream. It ignites, adding to the hot gases shooting out the back of the engine (see this Question of the Day for details on afterburners). At full force, the plane can get up to more than Mach 2.5 (approximately 1,854 mph / 2,984 kph).

The high engine power does come at a price -- poor fuel economy. Of course, the F-15 was designed with this limitation in mind. In order to extend its un-refueled range, it was built with large internal fuel tanks in the fuselage (the main body) and in the wings. It can also carry three external tanks, as well as a pair of aerodynamic form-fitting tanks under the wings that generate some lift of their own. Fully fueled, the F-15C can fly 3,450 miles (5,550 km), and the F-15E can fly 2,400 miles (3,860 km).

The other problem with the engines is that they wear out pretty quickly. This is to be expected, given the amount of work they do. Fortunately, they're very easy to replace -- an Air Force ground crew can do it in less than an hour!

The F-15 doesn't just take off quickly, it stops quickly too. It has its own extendable air-brake, a hydraulically operated panel that dramatically increases the aircraft's drag to slow it down (just like a parachute).

How F-15s Work

An F-15 extends its air brake before landing.

Photo courtesy U.S. Department of Defense

The main thing that sets the F-15 and other modern fighters apart from their predecessors are their electronic systems. Early fighter pilots controlled their planes mechanically, by moving linkages, and they mainly used their own eyes to target enemy planes. In stark contrast, nearly every aspect of the F-15 is computerized.

The plane is essentially a robot. It has a central computer, which is connected to an array of advanced sensors. Based on input from the inertial guidance system (which contains highly sensitive gyroscopic sensors) and the pilot, the computer activates hydraulic actuators to adjust the wings and rear stabilizers. The pilot doesn't actually fly the plane directly: He or she gives instructions and the computer decides how to carry them out. The computer is constantly making flight adjustments on its own to improve flight performance -- the computer artificially creates a relatively smooth ride. The F-15 computer can make necessary adjustments in milliseconds, about a hundred times faster than a human being.

The plane's main "eye" is its computer-controlled radar system, mounted in the nose. The radar's job is to locate other aircraft and generate ground maps. The dish is mounted on moving gimbals, so it can pivot to scan different areas or follow a moving target. The radar figures out which way targets are moving using the pulse-Doppler system -- essentially, shifts in the reflected radio wave frequency indicate whether the target is moving toward the radar system or away from it (see How Radar Works for more information.)

The F-15 Strike Eagle has additional scanning equipment called the low-altitude navigation and targeting infrared for night (LANTIRN) system. The LANTIRN system is housed in two pods mounted to the bottom of the plane, near the engine inlets.

The navigation pod holds another radar unit that is optimized to map the ground terrain, and a forward-looking-infrared (FLIR) night vision scanner that picks up the infrared heat energy from surrounding objects. Together, these sensors generate a detailed image of the ground below, allowing the pilot or computer to fly in total darkness.

How F-15s Work

One of the LANTIRN pods on an F-15 Strike Eagle

Photo courtesy U.S. Department of Defense

The targeting pod houses a powerful laser and another FLIR scanner, mounted to a swiveling turret. The laser works as a range-finder, calculating the distance to targets based on how long it takes a laser beam to bounce off of them, and also as a target designator, marking targets for laser-guided missiles. The targeting system is designed to pick out ground targets, but it can also be used in air-to-air combat.

The central computer processes data from the radar and the LANTIRN system and presents targeting and navigation information to the crew. In the next section, we'll look inside the cockpit to see how the crew accesses this information, flies the plane and targets the enemy.

The original F-15 was designed for a single-person crew. The pilot flies the plane and targets enemy aircraft at the same time. The F-15 Strike Eagle has an additional station in the back of the cockpit for a weapons systems officer, or WSO (pronounced "wizzo"). In the Strike Eagle, the WSO is in charge of selecting and eliminating ground targets while the pilot concentrates on maneuvering the plane and fighting enemy aircraft. Both stations are housed in a sturdy "bubble" canopy on top of the plane. This canopy design gives the crew a full 360-degree view of their surroundings.

How F-15s Work

The F-15 bubble canopy gives the crew a wide view of the sky. This plane is preparing to refuel.

Photo courtesy Department of Defense

The pilot's station is designed to make flying and targeting as easy as possible. The computer presents most relevant information on the heads-up display (HUD), a monitor that projects an image onto a transparent screen at the front of the cockpit canopy. With the heads-up display, the pilot can monitor the flight data and the radar information while keeping an eye on the sky. This is crucial in combat -- a pilot can't keep looking down at gauges and instruments while evading or chasing enemy fighters. The Air Force is planning to eventually replace this system with a helmet-mounted monitor that projects flight data onto the pilot's visor.

How F-15s Work

An infrared terrain image displayed on an F-15's heads-up display

Photo courtesy U.S. Department of Defense