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49 votes
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Why do rocket nozzles flare?

The purpose of this nozzle is to achieve maximum acceleration of the flow to obtain the highest possible exit velocity. The shape of convergent / divergent (de Laval) nozzles is dictated by the ...
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30 votes
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The principle behind regenerative cooling?

You are missing how heat is distributed in exhaust. Most of propellant ejected through the nozzle never makes contact with the nozzle surface or walls of the combustion chamber, and as result never ...
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28 votes
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Why is the X-37B's large nozzle offset to right from the spacecraft's mid-plane?

First off, it's absolutely an engine nozzle. Some sources say X-37B has a storable biprop propulsion system with ~900N thrust. The Aerojet R-42 is such an engine and the dimensions look about right --...
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24 votes

Why do rocket nozzles flare?

The gas at the narrowest part (the throat) of a convergent-divergent nozzle used in a rocket engine is ideally moving at the Mach 1, the speed of sound. This creates a choked flow condition. After the ...
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21 votes
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How are rocket nozzles made?

There are many kinds of nozzles, and many ways to manufacture them. Here is a sampling. Actively cooled nozzles such as the the SSME and F-1 nozzles were constructed by fabricating the individual ...
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20 votes
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How hot do rocket engine nozzles get?

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 ...
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17 votes

Why do the exhaust flames from cryogenic stage engines appear to be separated from the nozzle?

What you see is the first Mach disk (a standing shock wave), which causes a sudden increase in temperature, pressure, and density. At the nozzle exit, the exhaust gases have a comparably low ...
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  • 271
16 votes

Where does the thrust act in a rocket engine and how is it transferred to rocket structure?

The thrust acts on the nozzle and combustion chamber walls by virtue of the pressure differential they contain. Using the RS-25 (SSME) as an example, with illustrations from this pdf, I can highlight ...
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16 votes
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Red color in the nozzles of Soyuz-2 rocket engines RD-107 and RD-108, is it only a decoration?

They aren't painted red. If you zoom in on the picture, you'll see that the red parts are actually protective inserts clipped onto the nozzles to keep debris out of them during transport. It's a ...
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15 votes

Why the non-symmetric design of rocket nozzles?

According to comments on this thread, the shape of the nozzle on SpaceShip Two is to provide "free pitch-up" during the short powered flight period. That nozzle shape would direct thrust force ...
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15 votes

Why should the velocity through the nozzle throat be sonic?

Uwe's comment on the question is spot on. The characteristics of the flow through the nozzle depend critically on the pressure ratio - the two pressures being the pressure at the entrance to and exit ...
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14 votes
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Sea level static test of an upper stage possible?

You need a massive facility that can maintain a near vacuum while dealing with the engine exhaust. There are (were) a couple in the US. Plum Brook Station (Part of NASA Glenn) includes the In-Space ...
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12 votes
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Ribbed rocket nozzles

Rocket engine nozzles often are cooled by pumping (some of) the propellant through pipes inside the nozzle wall or outside of it before they are burned inside the engine. An example of this is the ...
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12 votes

Is there any rule for rocket engine nozzle proximity from each other?

Obviously there are differences between implementations that will make a general rule very hard to define. For example, on the Space Shuttle, the 3 SSME engines were close together, and the two SRB's ...
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12 votes
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How can phenolic (resin?) handle rocket engine nozzle temperatures?

There is a great writeup on this in the wonderful book "Development of the Space Shuttle 1972-1981" by T. R. Heppenheimer. Highly recommended, as is his prior volume "The Space Shuttle Decision". tl;...
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11 votes
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In space, how far from the nozzle is the exhaust from a space-craft capable of causing damage?

It varies tremendously based on the engine involved and what's being protected. Back in the day when Shuttle deployed comsats with solid fuel boost stages, the satellite would be tens of miles away ...
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11 votes
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Contracting rocket engine nozzles

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 ...
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11 votes

Is it always one nozzle per combustion chamber and one combustion chamber per nozzle?

Partial answer: This engine from the 1970s vintage Viking Lander is a liquid engine featuring one combustion chamber and many nozzles. (you can ignore the red arrow, it's from another question). The ...
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10 votes

Why are the exhaust plumes exiting the nozzles of SpaceShip Two segmented along their length?

Those are Mach diamonds. They form due to the interaction of the exhaust flow with its own supersonic shockwaves. All rockets and some jet engines produce them; their visibility varies with the ...
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10 votes
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What is the white spray-pattern residue inside these Falcon Heavy nozzles?

I believe it is residue from the TEA-TEB starting fluid. Triethylaluminum combustion produces aluminum oxides, Triethylborane produces boron oxides. Both are shades of white and grey, matching the ...
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  • 6,377
10 votes

Do converging-diverging engine nozzles need to be bell-shaped?

You don't need a "bell-shaped" nozzle for a rocket -- a simple cone (often with an included angle of 30 degrees, it seems) is sufficient. Cones are easier to design (they only have one design ...
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  • 7,726
10 votes
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Conversion of heat into electricity in rocket nozzles

This would not be useful in general. Thermoelectric converters don't convert heat into electrical power, they generate electrical power from the flow of heat across them. You still need to cool the ...
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10 votes
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Are nozzle plugs taken out before launch or are they ejected upon ignition?

In general, nozzle plugs like those depicted in the linked question, as well as other assorted engine protection setups like these nozzle FOD covers are almost always removed prior to flight, during ...
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  • 493
10 votes

Are nozzle plugs taken out before launch or are they ejected upon ignition?

Certainly all the big shuttle engines: OMS, SSME and SRB covers and plugs were, ahem, "removed before flight". However some of the shuttle Reaction Control System engines had covers applied ...
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9 votes
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TR-106 under-expansion in sea-level test?

There's no good reason to run a production rocket engine underexpanded at ground level; it loses efficiency and will only become more underexpanded as it gains altitude. If you compare the shape of ...
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8 votes

Why do rocket nozzles flare?

The combustion of propellants is an exothermic process, it mainly provides heat. Initial velocity (think of the turbo pumps) and the changed specific gas constant of the combustion product are ...
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  • 1,345
8 votes

Why is the Merlin 1D Vacuum nozzle bigger than the Merlin 1D nozzle?

For maximum thrust, the pressure of the exhaust at the "exit plane" (the end of the nozzle) should be equal to the ambient air pressure it's exhausting into. The larger the area of the nozzle's ...
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8 votes
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Are there rocket nozzles that use non-elliptical shapes?

Yes, aerospike engines don't have to be elliptical, since their "nozzle boundary" is just a line of equal pressure. Even the little nozzles at the top of the ramp appear to be rectangular. As far as ...
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8 votes
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Why does the Superdraco exhaust seem so under-expanded in this photo?

In this photo, the exhaust doesn't expand so much. The nozzle looks to be a bit longer. So they've played with various nozzles. (cropped and adjusted for visibility) And the Ars Technica article ...
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8 votes
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Aerospike with solid fuel

It's been tried at least once at the research level, as the paper Flight Research of an Aerospike Nozzle Using High Power Solid Rockets details. A comparison was made between conventional and ...
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