Is an afterburning turbojet louder than a non-afterburning turbojet?

Factual Questions
1

Subject line is pretty much it. Suppose I have a turbojet engine which produces 30,000 pounds of thrust (and plenty of noise) when the afterburner is on.

Suppose I have another turbojet that produces 30,000 pounds of thrust, despite having no afterburner. Is one likely to be louder than the other, or is it going to come down to the specific design details of each engine?

My guess is that the afterburning engine will be louder because it’s likely to have a smaller, higher-velocity outlet plume - afterall, turbofan engines are quiet because they have the exact opposite, i.e. a large, low-velocity outlet plume. But that’s just a guess. Anyone got authoritative info?

2

I don’t have any authoritative info, but my gut tells me AB will always be louder than non-AB (regardless of thrust) due to lighting off the fuel on the aft side of the turbine. The turbine should act as a noise baffle for the combustion chamber.

3

Oh. Hell yes! Ive been on the runway when our F106b fighter had to bail on the landing and light off the AB and do an extreme high angle climb-out. It lights with a huge “Poom” and gets deafening after that.

Heck, just use an ox-acetelyene cutting torch. When you hit the cutting trigger the high velocity burning is much louder then the pre-heat flame.

Dennis

4

An afterburning turbojet is louder than everything.

5

Here is a good visual representation.

The combustion in an afterburner is not contained or constrained like the turbine's combustion is, it will always be louder.

That channel will also have teardown of jet engines which will possibly make this far more obvious.

6

I’ve worked on aircraft with afterburners (F-111) and those without afterburners (Sabreliner/T-39 and DC-9/C-9A). While the F-111s engines put out much more power than the other - so not a fair comparison according to Machine Elf’s criteria - I can certainly say that an engine with after burner is much louder. MUCH LOUDER! Even the T-39 with its relatively small afterburning engines is louder than some larger aircraft engines. Discussions of the old T-37 (AKA The Six Thousand Pound Whistle) is more about the sound frequency than decibels.

Of course almost all commercial aircraft now have to meet noise restrictions so even much larger turbofan engine are much quieter than the turbojet engines of old.

As far as military jets having to worry about sound, if they can go supersonic, they get to the target before the sound does. Even subsonic bombers sound is greatly delayed if they are at a high enough altitude.

7

As has been pointed anecdotally and otherwise it’s always going to be louder. A big source of noise from jet engines is shear noise. It’s representative of the auditory battle of high speed flow from a jet engine trying to mix with the slower or even stationary air of the surroundings. Reducing this difference of flow speeds is the reason that high-bypass engines are quieter: The really high speed flow from the core of the turbofan is shrouded in an intermediate speed flow from the bypass flow of the fan. There’s a pretty good wiki article that’s a good start under “Jet Noise”. With the pure high-speed flow of an after-burning engine coming out through the nozzle and trying to mix with the slower air of the surroundings you have what is essentially the recipe for maximum shear noise.

8

No authoritative info on that kind of a pound for pound comparison. But it would seem to almost have to be true for a given thrust and a given technology or design emphasis in the nozzles of the engines, how shrouded and shielded etc. Less gas going faster, more noise.

On practical comparisons of actual a/c, not correcting for the power output or specific design features, there’s seems both anecdotal and measurement evidence suggesting a/c with multiple non-afterburning J57’s hung on nacelles, ie the original KC-135A and pre-H model B-52’s, might have been the loudest US a/c ever, especially where the engines were fitted with water injection for takeoff (not that that itself made them noisier, rather limiting internal temp to allow more fuel flow, more power, more noise).

Many sources say non AB water injected J57’s at takeoff power put out 165 decibels though a distance isn’t quoted with that commonly seen number AFAIK. But see these two papers the first a test of various turbojet transport sound measurements from 1971 and other a 2000 summary of current fighter noise levels. Page 32 of the first shows KC-135 reaching 130db at almost 400 ft nearest point of approach on takeoff (doesn’t say water injection or dry). The second shows you have to get within 150 ft of an F-22 at least from the side to get to that level. The levels at 50’ for the modern US fighters are 148-150, see third link pg. 6.
file:///C:/Users/User/Downloads/AD0740563.pdf

file:///C:/Users/User/Downloads/JSF%20News%2020090415%20-%20F-35%20Acoustics%20Test.pdf

9

Where does the B-1 (Bone) fit into that? Anecdotally, it’s supposed to be unGodly loud at full burner. The A-6 was another a/c that was supposed to be a whole lot louder than you’d think, although, that’s probably due to carrier guys having to stand next to the thing while it took off. IME, the C-5 before they re-did the engines, had a perhaps the most piercing high pitched annoying wail I’ve ever heard from an airplane doing a low pass. Even if other jets, like the F-18 zipping over the stands in full burner, back when they could do that sort of maneuver, were louder in the NHRA-feel-it-in-your-chest volume.

Never heard the F-104 live, which was another that was supposed to be really piercing and loud.

10

I mis-typed something above. It should have read “Even the T-39 with its relatively small **non-**afterburning engines is louder than some larger aircraft engines.”

11

The Joke was that the F-4 was the “triumph of thrust over aerodynamics”.

I grew up in a tiny town that the 419th used for training runs for a short couple of years in the early 80’s and they actually use to go supersonic way to close to town until (I am told) that the USAF got sick of paying for peoples windows. If you were out hiking in the hills they would often be below you.

I know that they ran F-105, F-4 and were just getting the F-16, while I don’t remember the F-105 the F-4 was the loudest thing that I had ever heard up to that point in my life and the F-16 didn’t even compare.

Note I was a child, so my perception may have been way off but it was a very memorable event.

12

I worked briefly at Dulles airport in the early 1990’s as an aircraft fueler for Ogden Allied Aviation Services. Most of my work consisted of driving tanker trucks to the “puddlejumper” side of the tarmac to fuel prop planes over the wing, like Brasilias and such. I did however get a chance to fuel some commercial jets with a pumper truck (underground tanks), all from under the wing.

Those planes always obviously took much, much longer due to quantity of fuel. I always seemed to work as a part timer (job was unionized except for us part time lowlifes) on undesirable January days in extreme cold, with freezing strong winds blowing across the airfield while I held a metal deadman in my hand for the entire fueling operation for these under the wing jets. The gloves I had to wear were necessarily thin so I had manual dexterity with my hands that I required for my job.

At that time, Dulles had a weekly Sunday flight of the Concorde. I was part of the three man crew to fuel it. It took an hour and a half (me holding one of the deadman switches the entire time) to fuel it, and it took 380,000lbs of fuel.

The British Airways crew was really nice, let me tour the plane, sit in the cockpit, etc. It was a real treat and a rare opportunity. I was all of 21 years old. They told me that the aircraft used 1/3rd of that fuel just to take off. I was astounded by that.

Anyway, we were always back to our HQ before the plane took off after we fueled it, I’m guessing due to pre-flight checks, boarding, etc. I was eating lunch about an hour after we fueled it when it took off, and that motherfucker was the loudest plane I have EVER heard taking off. It was RIDICULOUSLY loud. Our building shook, the windows rattled, I could feel the bass in my chest, etc. And I was a pretty far distance away.

Later I found out that they used afterburners only upon takeoff to reach Mach 1.7, and once at cruising altitude, they used far less fuel.

I’ll never forget it, but that thing gets my vote. I can’t imagine being near it, even with hearing protection, when it took off. Freaking amazing.