What is an afterburner and how long can a jet fly on afterburner?

Question

I heard this word so many time in military documentary about jets, especially fighter jets, but I don't fully understand what it is or how it works. In a documentary from the Discovery Channel, all but one fighter needed to engage afterburner to reach supersonic speed. Could you tell me why is that?

The documentary also states that a fighter jet rarely engages this system. If the afterburner system can make a jet fly faster, why don't they use it all the time? Is that because of the airframe cannot handle supersonic airflow for a long period of time?

Answer 1

An afterburner is a secondary combustion system which burns additional fuel downstream of the combustion chamber, to further increase thrust at the expense of much higher fuel consumption.

This is the Pratt & Whitney F100 afterburning turbofan, variants of which power the USAF's 4th-generation fleet of F-15s and F-16s:

The final spoke-looking thing just beyond the turbine fins, plus all the space between the turbine core and the exhaust nozzle, is the afterburner. In this area, fuel is sprayed directly into the exhaust stream from the turbine core, where the heat from the air leaving the core is enough to ignite it. This additional pressure adds to the thrust produced by the turbine.

As I said, though, the tradeoff is increased fuel consumption, sometimes usually dramatically so. The F-16 at full military power and low altitudes burns about 8000 pounds of fuel an hour, which with a full droptank configuration gives it about 2 hours' flight time. Cruising at higher altitudes, that flight time can be further extended as both the higher altitude and the lower throttle setting (about 80%) reduce fuel flow rate by up to 40% versus low-altitude flight.

In full afterburner at low altitudes, the F-16 can burn in excess of 64,000 pounds an hour. At full throttle, a U.S.-variant F-16 with maximum external fuel stores has about 20 minutes until it's on emergency reserves (which would only last an extra minute or so at full afterburner). The speed gain is minimal; the F-16 cruises at between 450-550 knots, while full afterburner only increases that to about 700-800 knots with a typical underwing loadout. So, burning 8 times the fuel, you get around a 50% speed boost.

Answer 2

By using the afterburner, fuel is injected into the downstream of the turbine. Exit velocity gets higher -> More thrust.

Comparison of the generated thrust in an F/A-18C Hornet:

• Maximum thrust without afterburner 10,440 daN (each 5'220 daN)
• Maximum thrust with afterburner 15,660 daN (each 7'830 daN)

(The F/A-18C Hornet uses 2 General Electric F404-GE-402 turbofans)

Some airplanes need the afterburner to reach supersonic speed because the "normal" use of the jet turbine doesn't generate enough thrust. Using the turbine in "normal" mode (without afterburner) is also called "military power" or "dry". Using the turbine with afterburner is also called "full power" or "wet".

From this wikipedia article:

Due to their high fuel consumption, afterburners are usually used as little as possible; a notable exception is the Pratt & Whitney J58 engine used in the SR-71 Blackbird. Afterburners are generally used only when it is important to have as much thrust as possible. This includes during takeoffs from short runways, assisting catapult launches from aircraft carriers and during air combat situations.

It's true that a fighter jet rarely engage the afterburner because it uses extreme amounts of fuel. Sometimes up to the factor 10 to the normal fuel useage. Thats also why they don't use it all the time: The operating-range of the fighter-jet is drastically reduced by using the afterburner.

The pilot can use the afterburner in different stages to find the perfect ratio of fuel usage / speed / range..

Source (In english): http://www.lw.admin.ch/internet/luftwaffe/en/home/dokumentation/assets/aircraft/fa18.html

Answer 3

It is possible to design an aircraft that can cruise at supersonic speeds without using afterburners (for example Concorde, the British TSR-2 strike/reconnaissance aircraft, and the Tu-144). The aerodynamic drag force is higher at transonic speeds than when supersonic, and using afterburners to accelerate through the transonic speed range qucker may actually reduce the total fuel burn. That was definitely the case for Concorde. The afterburners were also used to shorten the takeoff roll on Concorde.

Most jet fighters are not designed for "efficient supersonic cruising in a straight line at constant speed," so supersonic flight without afterburners is not the main design consideration.

Answer 4

I flew B-1B's for 7 years. I've also had flights in F-15s and F-16s. The B-1 has 4 afterburners, but a lot more gas than the fighters, so I rarely had to stay out of burner due to fuel. There's lots of reasons to minimize burner usage, though:

1. Operationally, AB makes you highly visible to all. At night, you put a spotlight on yourself. Daytime, everyone on the ground can hear you. IR sensors will find you quickly and easily, and even lower tech-IR missiles will prefer your burner to flares.

2. That extra 50% beyond mil power is actually an awful lot. When you do use burner, you don't need it for long. The B-1 could accelerate in full AB from .8 to .95 mach in just a few seconds. Operationally, you just don't need AB that much or often. If you're trying to defeat a missile, you're going to use excess airspeed first to slow to cornering velocity. The B-1 can maintain cornering velocity without burner since it's at relatively low g. A fighter at 7+ g will need some burner to maintain energy especially at cornering velocity, but as it can turn 90+ degrees in just a few seconds, it doesn't need much or any burner. Regardless, in a turn to defeat a radar missile, since IR missiles detect 'passively' meaning there is little or no warning, a pilot will frequently assume there's a heat seeker in the air when turning to defeat a radar missile and will avoid burner anyway.

3. Close-in air to air dogfighting is one of the few times a combat aircraft needs extended burner. In fighter combat, energy management is very important. No one wants to be on the losing end. Get too low on airspeed, and your jet turns too slow and you lose, so fighter pilots will use whatever burner they need to keep the threat off their tail and win the fight. In the B-1 as well, in fighter intercept exercises, that was when we tended to use more burner. We tended to use it to accelerate quickly to complicate the fighter's intercept, and in some cases to bug out with a fighter on our tail.

4. The other regime where burner use is frequent is takeoff. This is statistically one of the most dangerous phases of flight, and reaching flying airspeed quickly minimizes the danger. When I was flying, the B-1 always took off in burner – not sure now. Fighters can under certain conditions take-off in mil power, but I've rarely seen it.

5. Burner use in American jets ABSOLUTELY DOES NOT ADD SIGNIFICANTLY TO REQUIRED MAINTENANCE AND DOES NOT HARM THE ENGINES. The poster who mentioned that might have seen something on the MIG-25, which will destroy its engines in high speed flight. Presumably, other Soviet fighters have some maintenance issues with burner use, but American combat aircraft are built to employ burner whenever needed without damaging engines.

6. Altitude is a very important point, as burner fuel flow will decrease with altitude. In thin air, there is less oxygen available for combustion, so the fuel controls have to adjust accordingly. As the previous poster wrote, thinner air creates less drag making it easier to go fast. But... as a commercial pilot today, I've flown with many former fighter pilots, and whenever we get to talking about it, few of us have spent time above 40,000 feet. The higher service ceiling is a nice stat for the contractor sales teams, but there's rarely an operational reason, and a lot of bad things can happen (like engine stall and physiological emergencies) up in the 40's.

Answer 5

https://www.nasa.gov/centers/dryden/pdf/88117main_H-1449.pdf

Scroll down there's some useful graphs that can give you some idea. AB increases exhaust temperature and thus allows for an increase in exhaust speed. By actuator disk theory this means the in flight thrust at MAX will be closer to the static number than in flight thrust at MIL for any given speed. That's why an F-15 at 40K ft can only fly at M0.95 at MIL but can do M2.5 at MAX with only a 63% increase to static thrust.

Answer 6

The answer is it depends on your altitude. A lot.

For example, I'll take an F-16 since I asked this to somebody that identified himself as a former F-16 crew chief online: An F-16 flying at full military thrust at sea level consumes about as much fuel as full afterburner at FL400 (40000ft). At the F-16 service ceiling of FL500, full afterburner will be using quite a bit less fuel than military thrust at sea level.

So way up high, full afterburner might be usable for even 30 minutes if the climb is done efficiently and the big centerline drop tank was used. That's how the F-16 can actually reach Mach 2. It will take a while on afterburner to accelerate that much.

This also means that full afterburner won't be producing that much extra thrust, but since the air is so thin, it will have a pretty substantial effect in real airspeed obtained.