An interesting comment here introduced me to the concept of Thrust-augmented Nozzles aka Afterburning Rocket Engines. The pubs I've read on it so far introduce the concept as a way to provide good performance throughout ascent. The nozzle performance is enhanced at low altitudes by engaging the afterburner at ignition, possibly turning it off later at altitude.

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This kind of sounds like a good idea, but all the pubs I've see so far are from the originators of the idea, and as far as I know, it's never been adopted. (F-1s dumped the gas generator exhaust into the nozzle - it was fuel-rich so presumably burned, but that's not the same thing at all).

What are the downsides to this concept? (I mean, besides the obvious "the benefit wasn't worth the cost", but why?)


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    $\begingroup$ "The TAN exhaust gases flowing in the nozzle reduce the exit flow area available to the core flow, thereby reducing overexpansion and thereby further reducing the core-thrust loss associated with overexpansion." This is fundamentally different from a afterburner, i.e. over-expansion is not solved by adding more gas to the nozzle, but by shrinking the nozzle, by the means of creating a thick boundary layer over the original bell which inhibits the expansion of the main flow. $\endgroup$ May 5, 2022 at 16:52
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    $\begingroup$ @user3528438 probably why they went with the "thrust augmented nozzle" name $\endgroup$ May 5, 2022 at 16:58
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    $\begingroup$ To make a truly “after-burning” rocket engine you would want the flow in the bell to expand but not lose much pressure at the same time, so that the pressure at the end of the bell isn't lower than atmosphere pressure. This is achieved by adding more hot gas or/and make existing gas hotter. But remember the flow in the bell is super-sonic and today we still aren't quite sure how to maintain a stable flame in a super-sonic flow, i.e. the exact same problem as a scramjet. $\endgroup$ May 5, 2022 at 17:00
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    $\begingroup$ @user3528438 injecting the fluid would cause a shock near the surface like in this TVC method i.stack.imgur.com/4HSGA.png but since it's an oblique shock the flow could still be supersonic below it...so maybe it does not help much. $\endgroup$ May 5, 2022 at 17:16


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