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The efficiency of rocket engine nozzles depends greatly on their expansion ratio an how well the ambient pressure matches the rocket nozzles exit pressure. An optimal expansion ratio means that there is a straight stream of exhaust leaving the rocket, while under- or overexpansion (picture) are inefficient, possibly fatal if the exhaust is grossly overexpanded at launch.

Sea- level optimized engines (with smaller nozzles) seem to be very inefficient during most of their flight, except for the few seconds of being close to sea level. However, I have seen that jet engines like those of the F-16 can contract their 'nozzles' to achieve different expansion ratios (see it on youtube).

Are there any rocket engines in use that can change their expansion ratio by contracting their nozzles? And how come, we don't see contractable nozzles more often on rocket engines - are there any major disadvantages?

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    $\begingroup$ The picture to which you linked has things bass-ackwards. Here is a much better picture. An over-expanded nozzle is pinched while an under-expanded nozzle flares out. With regard to fatality, that's only the case for grossly over-expanded nozzles. With regard to launch vehicles that start at sea level, they are typically slightly over-expanded at launch so as to not have the vehicle suffer too much under expansion (and hence too much loss of efficiency) at MECO. $\endgroup$
    – David Hammen
    Commented Sep 16, 2020 at 15:03
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    $\begingroup$ @DavidHammen I see, I actually took the first picture from this answer ... I edited the post anyways to use your graphic $\endgroup$
    – publicdomain
    Commented Sep 17, 2020 at 6:14
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    $\begingroup$ The "likely fatal" comment is incorrect. Launch vehicles with a fixed length de Laval nozzle are typically slightly overexpanded at launch, optimally expanded at some design altitude, and underexpanded above that design altitude. Problems only arise when the exhaust is grossly overexpanded at launch. $\endgroup$
    – David Hammen
    Commented Sep 17, 2020 at 10:16

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If you were able to change a booster rocket engine nozzle's area ratio at will, you would want it to expand, not contract1. Best performance is achieved when the exit plane pressure matches the ambient pressure. As the rocket ascends, the ambient pressure drops, and more expansion is needed, not less.

Why don't they do this? The usual aerospace reasons: cost, weight, and complexity.

A few engines have used extendable exit nozzles. The RL-10B2 has one.

enter image description here

Note that this design allows exit plane pressure matching at two points instead of one, not continuous matching. This graph from Aerothermodynamics of Gas Turbine and Rocket Propulsion (Oates, 1984) shows the advantage of this design. The dashed line is the ideal thrust coefficient. You can see how the solid "actual" line starts to bend away from the ideal line as the vehicle ascends, and then starts trending back for a time when the extendable nozzle is deployed.

enter image description here

The Peacekeeper ICBM had extendable exit cones on its 2nd and 3rd stage solid rocket motors. I am not 100% sure but I believe this drawing (also from Oates) shows one of these motors. The drawing shows two deployable cones allowing exit plane pressure matching at three points.

enter image description here

Extrapolating from this, one could visualize a kind of Venetian blind nozzle with many extendable cones. Visualize, but probably never sell it.

I am sure there are other examples, but "the usual aerospace reasons" keep this from being a common feature.

The "holy grail" of self-adjusting nozzles is the aerospike engine. This uses the aerodynamic properties of the exhaust flow to create a self-adjusting virtual nozzle of sorts. Problems with cooling the centerbodies have so far relegated this to the realm of "will be flying soon" for large engines.

There are a number of informative questions about aerospike engines and expansion/deflection nozzles on this site already.

1Unless you are doing rocket powered landings in the Earth's atmosphere. Clearly a sci-fi concept.

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    $\begingroup$ I actually spent all of yesterday afternoon learning about aerospikes and and optimizing expansion ratios - that's how I came to ask this. Thank you for this extensive answer! $\endgroup$
    – publicdomain
    Commented Sep 16, 2020 at 13:59
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    $\begingroup$ @JCRM 3rd link, last line before the footnote. I'll make it more explicit though, thanks. $\endgroup$
    – Organic Marble
    Commented Sep 16, 2020 at 15:02
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    $\begingroup$ Note: all rocket engines that fly in vacuum are underexpanded. Period. $\endgroup$
    – Abdullah is not an Amalekite
    Commented Sep 17, 2020 at 11:13
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    $\begingroup$ @Abdullah except for the ones with infinitely long nozzles, of course. $\endgroup$
    – Organic Marble
    Commented Sep 17, 2020 at 11:47
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    $\begingroup$ I've heard those are coming but are seriously behind schedule and there's no reliable completion date. At least something is taking longer than JWST. $\endgroup$
    – uhoh
    Commented Sep 18, 2020 at 3:10

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