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Dry thrust is normal jet engine power without using an afterburner. Max dry thrust is the engine's most powerful thrust level without afterburner.

But what if the engine is at half power, or even idle power? Can you safely engage afterburner at that point?

I'm interested in turbojets and turbofans. Both can have an afterburner. I'm wondering if it's possible to engage afterburner on both of them at a time when the engine is not already at full dry thrust.

Edit: one reason it might not work is that idle thrust might not be powerful enough to ignite the fuel in the afterburner.

The reason I ask is for emergencies. Jet engines need several seconds to spool up from idle to full thrust. If you could engage afterburner from the get-go, you could be up to max speed much quicker.

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    $\begingroup$ No need to include information from the answer(s) in your question. Most people will read the answer(s) and find the info there. ;) $\endgroup$
    – FreeMan
    Commented Jun 2, 2016 at 12:37

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Afterbuner simply injects fuel directly into the jet pipe (i.e. beyond the turbine) in order to greatly increase thrust. The way throttle levers are normally set up, afterburner is engaged once the lever is advanced past maximum dry thrust, but I can't imagine why, theoretically, you couldn't set one up to be engaged at less than maximum dry thrust.

Mind you, it would be awfully inefficient to do so, as thrust output is exponential, with most of it being delivered in the top 5% of the RPM band; engaging afterburner while you still have a lot of dry thrust available would just be a massive waste.

What's more, jet engines are much quicker to respond today, and, in addition, landing approaches are not performed at idle thrust; the main benefit of flap settings greater than 20 degrees is that they slow the aircraft down, allowing for steeper approaches. In jet aircraft, this also allows the engines to be kept spooled up above flight idle, specifically in order to reduce response times in the event of a go around. In other words, throttle might be reduced to flight idle for the initial descent, but the final approach will use a higher thrust setting.

The benefits of adding afterburner to civilian transport aircraft are usually massively outweighed by their costs (see the Concorde and Tu-144).

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  • $\begingroup$ Right, but I'm wondering if idle thrust would be enough to ignite the fuel in the afterburner. Made an edit to the OP. $\endgroup$
    – DrZ214
    Commented Jun 2, 2016 at 5:30
  • $\begingroup$ @DrZ214 - We wouldn't know the answer to that unless we could get some temp readings from the exhaust pipe with thrust at idle; I do know that kerosene doesn't burn easily and that the ignition system used to start the combustion is very powerful. At any rate, I think the point is rather academic: advancing the throttle lever past full dry thrust would engage the afterburner and start fuel injection into the pipe; the fuel would start burning as soon as the exhaust temperature gets hot enough, while any unused fuel would simply be vented outside along with the exhaust gases. $\endgroup$
    – habu
    Commented Jun 2, 2016 at 11:02

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