5
$\begingroup$

The purpose of an afterburner is to provide additional thrust, and it is obvious that more fuel is needed for that.

But that is to be expected independent of an afterburner. That more thrust needs more than a proportional increase of fuel may be the case because the thrust increase needed per speed difference is not linear¹.

The reason to build afterburners may be that stronger jet engines can not be build with the same mass or volume. Or that a stronger jet engine is not useful in standard use, for example because the fuselage would overheat.

Doubling the speed increases the fuel usage by more than double I would expect, purely based on aerodynamics, independent of the engine.

Is the fuel usage for increasing speed greater with an afterburner than with a stronger jet engine? How much? Why?


¹ For spaceflight, that is true according to special relativity.

$\endgroup$
  • 1
    $\begingroup$ The reason to use afterburner than other alternatives is alternatives don't exist. You want a fighter to run mach 2 without using afterburner? Are you willing to wait for 3 decades for that? $\endgroup$ – user3528438 Nov 25 '19 at 0:08
  • $\begingroup$ @user3528438 But an engine could be larger, and even a solid fuel booster rocket would be an alternative, and has actually been used, I think. $\endgroup$ – Volker Siegel Nov 25 '19 at 0:12
  • 3
    $\begingroup$ @user3528438, er, Concorde flew Mach 2 without afterburner 3 decades ago. Volker is right, the engine could 'simply' be larger. The limiting factor is the turbine temperature, which doesn't allow the mixture to be rich enough and 'simply' pump in more fuel without increasing the air flow (i.e. making the engine larger). $\endgroup$ – Zeus Nov 25 '19 at 0:30
  • 1
    $\begingroup$ Define "excessive". Excessive assumes the same effect can be attained at a lesser cost, or the effect is unnecessary. Neither is the case with afterburning engines. $\endgroup$ – jwenting Nov 25 '19 at 11:33
  • $\begingroup$ @Zeus Concorde did have afterburners and they were required for takeoff and pushing through the Mach barrier, after which they were turned off for supersonic cruise. $\endgroup$ – pjc50 Nov 28 '19 at 13:28
5
$\begingroup$

Yes the specific fuel consumption of the afterburner, lbs of fuel used per lb of thrust, is much higher than the core engine. This is because the fuel is being added to a part of the engine where the air is less compressed, so the energy conversion is a lot less efficient. The bright orange flame coming out the tail pipe when in reheat is pretty much all that un-oxidized carbon fluorescing - wasted energy. See this article.

Fighters use reheat because all that extra power is available with minimal extra weight (the weight of the burner and additional fuel), but they can only exploit that power for short periods if they want to have decent range or endurance, so they are generally used for dashing somewhere, or to assist with maneuvering, or taking off with a heavy load.

$\endgroup$
  • $\begingroup$ The carbon present in the exhaust convinces me that an afterburner is less efficient. Using it on takeoff means it is used subsonic - that makes the whole topic more interesting. I'll remove the assumption of supersonic flight. $\endgroup$ – Volker Siegel Nov 24 '19 at 23:31
  • 1
    $\begingroup$ @Guy Inchbald I rejected your edits because some of the changes are stylistic plus there is so much extra material in there you should really just create your own post. $\endgroup$ – John K Nov 25 '19 at 18:11
1
$\begingroup$

If you look at it in terms of thrust vs fuel flow, then yes, they're very inefficient.

However, if you just look at the amount of fuel burnt to get an interceptor from the runway to 30,000ft, then they can be more efficient. Without afterburners the same climb would take significantly longer and could use more fuel.

Without afterburners, you'd need much larger, heavier engines to reach mach2 and it might not even be possible to get to the same height and speed in the same time, which after all, is the point of an interceptor.

Think of it this way - if minimum fuel was the only criteria, we'd send the pilot by train.

$\endgroup$
1
$\begingroup$

Yes they do use a lot of extra fuel, but the amount is depending on the operating conditions (Mach flight speed and altitude)!

Comparing MIL (military) power setting (maximum power without reheat) to MAX power setting (engine will be in MIL power, but the reheat will be scheduled and the exhaust nozzle opened) you can see that it is not uncommon to have 3 to 6 times as more fuel flow depending on Mach and altitude and the gas turbine.

Note that burning fuel at a low pressure is not very effective in terms of efficiency, but in power output it is very large. Again, depending on the operating conditions it is not uncommon to double the amount of thrust.

The questions you ask about using a bigger engine is not easy to answer, increasing the engine (for a design, you cannot increase the engine simply for an existing airframe) would also increase the frontal area and as such more drag. Such questions could be answered with a gas turbine simulation program in combination or coupled with an aircraft flight model.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.