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View Full Version : Difference between space shuttle and airliner exhausts?


Halfbaked_Boy
31st Jan 2013, 03:03
How come the space shuttle's exhaust nozzles diverge towards the rear, whereas aircraft's converge?

Or in other words (if my understanding is still correct after a few years!), why is it in the interests of the space shuttle for the exhaust gases to be slowed down (thus increasing static pressure), and for an aircraft's to be sped up (decreasing static pressure)?

I vaguely remember something to do with accelerating a lot of air/gas rearward slowly, or a smaller amount at higher speeds...?

Thanks in advance :ok:

Um... lifting...
31st Jan 2013, 03:24
Well, one's a rocket engine and t'other one's a subsonic gas turbine.

They really haven't much in common at all, as the subsonic one is incompressible flow and the rocket engine is expanding accelerating flow

Supersonic jet engines and rocket engines have a thingie called a de Laval nozzle, which compresses and accelerates flow to sonic speed (M 1) before it expands and further accelerates, directing lots of kinetic type energy out the tail pipe as reaches supersonic speeds.

Your assumption that the velocity decreases coming out the exhaust pipe is apparently intuitive, but incorrect. Many people have a hard time with that concept in supersonic aero. I was one of them for a while.

Loose rivets
31st Jan 2013, 05:16
Which, I suppose, is why you said Um . . .before lifting.:p

Loose rivets
31st Jan 2013, 05:26
Blackbird

http://img.photobucket.com/albums/v703/walnaze/Flying/Florida1037.jpg


and the greatest noise generator on Earth - but she's okay sometimes.


http://img.photobucket.com/albums/v703/walnaze/Flying/SuetheJohnson.jpg

Takan Inchovit
31st Jan 2013, 05:38
and the greatest noise generator on Earth - but she's okay sometimes.

... and what about the rocket engine she's standing next to?





T

Loose rivets
31st Jan 2013, 07:53
That was the . . . oh, never mind.;)

Their kids are older than them at that age. Time flies.

Stunning how big these things are, and indeed, how many of those nozzles there are. And yes, it was thought to be the loudest man-made noise on earth. I suppose they mean continuous noise.


http://img.photobucket.com/albums/v703/walnaze/KidatJohnson_zps0d72f272.jpg


http://img.photobucket.com/albums/v703/walnaze/Work1125_zpsf73f11d5.jpg


http://img.photobucket.com/albums/v703/walnaze/KidstheJohnson_zps5e325e1e.jpg

OFSO
31st Jan 2013, 08:02
For the same reason that a bugle, or an old-fashioned horn loudspeaker, has an exponentially opening end.

It's all to do with impedence-matching between at one end a very tightly confined combustion chamber, and at the other, open space. Ideally the nozzle of a rocket motor would be even larger, but since it is also used for (a) cooling and (b) steering, there are practical limits.

A jet engine (or a hair dryer for that matter) is designed for different conditions.

MG23
31st Jan 2013, 16:34
Ideally the nozzle of a rocket motor would be even larger, but since it is also used for (a) cooling and (b) steering, there are practical limits.

Also, in the shuttle's case, the engine had to be efficient at sea-level and in vacuum, so it was a compromise between the two. I believe it would have been somewhat longer if it was only used above most of the atmosphere, like the Saturn V upper stages... assuming they could make three longer and wider engines fit in the space available.

Slasher
31st Jan 2013, 17:04
The escaping gasses in a divergent duct increase in velocity in an aerosupersonic environment.

http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-KANPUR/FLUID-MECHANICS/lecture-40/images/Fig%2014.5.jpg

vulcanised
31st Jan 2013, 17:08
Hey Slash !

The picture of the tits has dropped off the bottom.

Nemrytter
1st Feb 2013, 09:38
Also, in the shuttle's case, the engine had to be efficient at sea-level and in vacuum, so it was a compromise between the two. I believe it would have been somewhat longer if it was only used above most of the atmosphere, like the Saturn V upper stages... assuming they could make three longer and wider engines fit in the space available.yes, in a vacuum you want as long a nozzle as possible. Some newer designs of rockets have an extendible nozzle that is folded away to save space and then extends once in-use above the atmosphere.

blue up
1st Feb 2013, 12:16
yes, in a vacuum you want as long a nozzle as possible.


Should this thread be merged with the one about shoving lightbulbs where the sun don't shine??

Anyone here shoved their nozz....oh never mind.