# How hot do rocket engine nozzles get?

What temperatures are encountered by the nozzles of rocket engines like those in SpaceX's Merlin series? Radiative or regenerative cooling, atmosphere or vacuum.

There's plenty of info about exhaust temperatures, but I can only guess at nozzle temperatures from their materials. For example, Inconel's upper bound is about 1200 °C. But I don't know how much safety margin there is before it starts to soften or melt or otherwise malfunction or even become nonreusable.

• Are you asking for the highest recorded value, or a range of typical values? Commented Oct 28, 2020 at 0:39
• Only typical hottest values for particular engines, because data seems scarce. Commented Oct 28, 2020 at 3:31

Direct measurement is difficult; I've seen some optical methods used but can't put a hand on them at the moment.

Here are some calculated inner and outer wall temperatures for the Space Shuttle Main Engine, a regeneratively-cooled booster engine. The X axis is axial distance from the throat. I am pleased to see that both metric and English units are provided.

The source paper, Wall temperature distribution calculation for a rocket nozzle contour, is very informative.

We can sanity check these numbers using some data from the SSME INTRO presentation.

Given a bypass flowrate of 73 lb/sec at -367 deg F, a nozzle cooling flowrate of 47 lb/sec, and a mixed exit temperature of -193 deg F, we can do a mass-flowrate-weighted average calculation to get a nozzle cooling exit hydrogen temperature of 77 deg F, or 536 deg R, roughly matching the graph above for the nozzle wall temps at the exit.

$$T_{\mathrm{mixed}} = \frac { T_{\mathrm{bypass}}\times\dot m_{\mathrm{bypass}} + T_{\mathrm{nozzle}}\times\dot m_{\mathrm{nozzle}}} {(\dot m_{\mathrm{bypass}} + \dot m_{\mathrm{nozzle}})}$$

• That's far cooler than I expected Commented Oct 27, 2020 at 22:45
• Degrees Rankine? And an actual FORTRAN listing? Wow! I only wish they'd validated the model against some measurements. Commented Oct 27, 2020 at 22:47
• @CamilleGoudeseune I edited in a sanity check with some fluid temperature data. Commented Oct 27, 2020 at 22:57
• @CamilleGoudeseune Rankine is actually common in thermodynamic systems like power stations and jet/rocket propulsion, especially in areas (locales) that otherwise deal in Fahrenheit. Had it be an EU designed rocket, you'd probably encounter Kelvins instead.
– Mast
Commented Oct 28, 2020 at 7:42
• @Mast : Kelvins are present (right axis). Commented Oct 28, 2020 at 16:37

Some of the hottest engines around are the Merlin 1D Vacuum.

Radiative cooled, and from the color of it reaches about 1600-1800K And that is WITH the use of film cooling by pumping the gas turbine's exhaust along the inner nozzle wall.

The Nozzle is made of Niobium. A handy metal that handles heat well, and only melts at about 2750K (but doesn't get near that in use, as it loses strength)

The combustion chamber itself is much hotter, but is also regeneratively cooled to levels far,far below this.

• Can you explain how to estimate temperature from black-body color? Does the in-flight video suffer from pure-white pixels that would hurt such estimating? Commented Oct 28, 2020 at 19:44
• Examine color of nozzle in video. Examine color of sunlit earth surface in background to get current level of camera light compensation. Look up result on blackbody color wheel. Very simple. Not very accurate, hence the rather large range.
– user38044
Commented Oct 29, 2020 at 5:56