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Diagrams of rocket engines like this one,

rocket engine

(source)

always seem to show a combustion chamber with a throat, followed by a nozzle.

Why is there a throat? Wouldn't the thrust be the same if the whole engine was a U-shaped combustion chamber with a nozzle?

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And after looking up the venturi effect, check out Bernoulli's principle –  Kyle Kanos Apr 6 at 20:27
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Try blowing air through your lips without puckering them and see the difference the force makes –  Adsy Apr 7 at 9:27
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@Adsy With a wider throat, the opposing surface is bigger. This may cancel the effect of lower pressure or exit velocity. It apparently doesn't, and I'm interested why. The lip experiment is not really conclusive in my eyes. –  Jens Apr 7 at 10:58
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Eyes… Lips… There has to be a better way to test this at home! –  user1306322 Apr 7 at 18:18
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@user1306322 Home experiments -- balloons filled with air and not tied shut are pretty good rocket motor models. Tape a straw to it and run a string through it. Tape the string across the room and now it's a guided rocket. You can play with various nozzles but putting different sized straws into the opening in the balloon which will allow you to vary the size of the contraction. –  tpg2114 Apr 7 at 22:02
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3 Answers 3

up vote 62 down vote accepted

The whole point to the throat is to increase the exhaust velocity. But not just increase it a little bit -- a rocket nozzle is designed so that the nozzle chokes. This is another way of saying that the flow accelerates so much that it reaches sonic conditions at the throat. This choking is important. Because it means the flow is sonic at the throat, no information can travel upstream from the throat into the chamber. So the outside pressure no longer has an effect on the combustion chamber properties.

Once it is sonic at the throat, and assuming the nozzle is properly designed, some interesting things happen. When we look at subsonic flow, the gas speeds up as the area decreases and slows down as the area increases. This is the traditional Venturi effect. However, when the flow is supersonic, the opposite happens. The flow accelerates as the area increases and slows as it decreases.

So, once the flow is sonic at the throat, the flow then continues to accelerate through the expanding nozzle. This all works together to increase the exhaust velocity to very high values.

From a nomenclature standpoint, the throat of a nozzle is the location where the area is the smallest. So a "U-shaped chamber with a nozzle" will still have a throat -- it's defined as wherever the area is the smallest. If the nozzle is a straight pipe then there is no throat to speak of.

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+1 I ♥ rocket science! –  ja72 Apr 7 at 1:52
    
So, since the increased energy of the exhaust comes from the resistance of the nozzle against the expansion of the gas, in a way, this technique extracts additional thrust from the binding energy in the material of the rocket itself? –  Ryan Reich Apr 7 at 15:10
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@Ryan: No, the energy is in the exhaust gas already. You're seeing conversion of potential energy (in the form of pressurization and high temperature) to kinetic energy. Which is good because it increases specific impulse. –  Ben Voigt Apr 7 at 17:05
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Just to pick nits, even straight pipes can have throats due to friction (en.wikipedia.org/wiki/Fanno_flow) or heat transfer (en.wikipedia.org/wiki/Rayleigh_flow). –  Ghillie Dhu Apr 7 at 17:19
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@aidan It all depends on the pressure ratio between the chamber and the ambient conditions. It could be over-expanded, under-expanded or just right depending on the outside pressure. This is what causes those shock diamonds in the outflow of afterburners and rockets. You can see examples here and some notes here –  tpg2114 Apr 8 at 14:47
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As addition to @tpg2114 answer, I suggest also to read about de Laval nozzle and Rocket engine nozzle on wikipedia.

Some typical values of the exhaust gas velocity
for rocket engines burning various propellants are:

1.7 to 2.9 km/s (3800 to 6500 mi/h) for liquid monopropellants
2.9 to 4.5 km/s (6500 to 10100 mi/h) for liquid bipropellants
2.1 to 3.2 km/s (4700 to 7200 mi/h) for solid propellants

so that definitely makes sense to have nozzle)

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The question needs to be turned around. Why is the combustion chamber bigger than the throat? This is to allow time for combustion before the gas exits the engine.

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I'm not sure if this is correct. Interestingly, the F-1 engine has an almost cylindrical combustion chamber, as can be seen here: 007rb.blogspot.de/2013/04/… (2nd image from the top). –  Jens Apr 8 at 8:21
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