Jet VS Rocket nozzle in space

[Spacescifi]

 

In visual scifi, jet nozzles are quite popular on spaceships.

Now from my limited understanding (feel free to educate me if I am wrong), jet nozzles are ideal for subsonic exhaust flows, but for supersonice exhaust flows rocket nozzles are ideal.

Rocket nozzles diverge at the throat where jet nozzles converge.

In space, I am not sure what advantage a jet nozzle would have, in fact I have noticed that rockets made only for space vacuum operation tend to have larger nozzles than the ones for launch, at least with current rocket technology.

So I guess my question is this, is visual media scifi getting it wrong like they so often do?

Should not those sleek SSTO spaceships have rocket nozzles instead of the jet nozzles pictured below?

Note: I am aware of adjustible fightet jet nozzles that can mechanically converge or diverge, yet rockets are so often not reusuable so it makes little sense to make them extra complex. A fighter jet most definitely does have a multi-mission lifetime as opposed to a staged rocket's all or nothing launch. Not to mention that adjustible mechanical nozzles are heavier and heaviness is something rocket designers dread.

Last: if you can give me one legitimate reason for jet nozzles on spaceships, other than it looking cool in scifi, I am fully willing to hear it.

Unless I have overlooked something, rocket nozzles are superior in space and even for an SSTO that flies in an atmosphere as well.

Edited May 19, 2019 by Spacescifi

[Dundral mk2]

Jet engines have supersonic exhaust as well, excluding turbofans. (partialy)

What jet engines get out of their nozzles being able to expand or contract is an efficient expansion ratio at a wide variety of altitudes. Like a low expansion nozzle being able to become a high expansion nozzle.

Rocket engineers WISH they could do this on rockets, but the mechanism that accomplishes this is prohibitively heavy when scaled up to operates at the stress and temperature levels in rocket engines.  They do however come up with other ways to get similar effects on some engine designs.

 

Most commercial airliners skip this entirely when they can, simply optimizing for a given altitude and speed to save fuel (money) and making the rest of the required flight envelope "good enough to get by".

Where this is usually seen is in military aircraft, where you need to be able to perform a well at whatever altitude/speed the situation requires you to be.

Edited May 19, 2019 by Dundral mk2

[Spacescifi]

18 minutes ago, Dundral mk2 said:

Jet engines have supersonic exhaust as well, excluding turbofans. (partialy)

What jet engines get out of their nozzles being able to expand or contract is an efficient expansion ratio at a wide variety of altitudes. Like a low expansion nozzle being able to become a high expansion nozzle.

Rocket engineers WISH they could do this on rockets, but the mechanism that accomplishes this is prohibitively heavy when scaled up to operates at the stress and temperature levels in rocket engines.  They do however come up with other ways to get similar effects on some engine designs.

 

Most commercial airliners skip this entirely when they can, simply optimizing for a given altitude and speed to save fuel (money) and making the rest of the required flight envelope "good enough to get by".

Where this is usually seen is in military aircraft, where you need to be able to perform a well at whatever altitude/speed the situation requires you to be.

 

Hmmm... is not expansion and contraction based on the surrounding air pressure and also how high velocity your exhaust is?

For example, near the ground air pressure will keep the plume rather tight so you will lose some thrust efficiency with a wider plume right?

But at higher altitudes the air pressure drops so it makes plenty of sense to expand the nozzle like this.

Also is not high velocity exhaust less damaging to expanded nozzles?

In space would a contracted or expanded nozzle win out? Or would it make no difference at all?

I mean, there is no air pressure so it would seem that expanded nozzles are preferred for thrust efficiency. Squeeze more exhaust out than less concept, with no air pressure dictatinf how wide your exhaust plune actually is.

Correct me if I am wrong. Thank you.

Edited May 19, 2019 by Spacescifi

[Dundral mk2]

Quote

Nozzles can be (top to bottom):

• underexpanded
• ambient
• overexpanded
• grossly overexpanded.

If a nozzle is under- or overexpanded, then loss of efficiency occurs relative to an ideal nozzle. Grossly overexpanded nozzles have improved efficiency relative to an underexpanded nozzle (though are still less efficient than a nozzle with the ideal expansion ratio), however the exhaust jet is unstable.^[7]

^-wikipedia

 

 

 

Any time you burn fuel and that exhaust goes anywhere but straight back, you lose efficiency.

When your exhaust plume doesnt match the size of your engine nozzle, some of your energy goes into creating turbulence around the outer edges of the nozzle, (over expansion) or simply pushing ambient air farther to the side (underexpansion).

So you can gain a bit of efficiency  when you can match your expansion ratio to the ambient air pressure...

but at what cost? It costs money, reliabilty and weight to add adjustable engine nozzles, so unless you are gonna be hanging around at alot of different altitudes instead of passing though, its usually better to just have a nice, cheap eliable, lightweight fixed nozzle optimized for the situation you are going to spend the most time in.

 

As for damage, its a much more hospitable environment in a jet engine exhaust nozzle than a rocket engine nozzle. One you can be near with earprotection and be fine, the other will beat you to death with a constant stream of shockwaves ("sound") at over twice the distance. Rockets are VERY violent.

Listen to it loud enough to where you can hear people talking at the beginning but keep your hand on the volume slider.

 

 

[DDE]

The aerospike is an attempt to create a rocket nozzle that automatically adjusts its own expansion ratio through interaction with outside airflow. It’s heavier than a simple de Laval nozzle, but it’s a much better option than any of the various transformers.

[sh1pman]

Also worth mentioning that in modern airliners most of the thrust comes not from combustion chamber exhaust, but from rotating fans driven by said exhaust through a set of turbines. Fans don’t need a nozzle because, as I understand it, there is little pressure difference between ambient air and turbofan exhaust.

[satnet]

An SSTO would have an altitude compensating nozzle of some kind. You are basically trying to match pressures between the exhaust and the ambient atmosphere, which is changing as you ascend. As the outside pressure drops the ideal nozzle size grows until it is infinite in a pure vacuum (in the real world we must settle for something less than infinite, fortunately the point of diminishing returns fits within a interstage). A mechanically adjusting "jet" style nozzle which allowed it to expand while in a vacuum would be an option, though generally for the reasons @Dundral mk2 mentioned the approach is usually a different geometry rocket that doesn't require significant mechanical adjustment. If you could afford the weight, had materials that could withstand the stress, and your SSTO spent a non-trivial time in atmosphere it might make some sense to have a mechanically adjustable nozzle (though probably not). It would probably be much longer and expand much wider than anything you would see on a jet, but would visually resemble it. Basically with a little hand-waving you could have a nozzle like that, but the chances of that ever being what we see in the real world are slim.

Scott Manley's video on nozzles is relevant (and as usual excellent):

 

[wumpus]

On 5/19/2019 at 8:51 AM, DDE said:

The aerospike is an attempt to create a rocket nozzle that automatically adjusts its own expansion ratio through interaction with outside airflow. It’s heavier than a simple de Laval nozzle, but it’s a much better option than any of the various transformers.

A de Laval nozzle will be more efficient for at least a single pressure (such as vacuum) than an aerospike.  I'd be curious just how high an altitude you need for a de Laval nozzle to be more efficient for the entire flight (from that altitude to vacuum).

This is mostly an argument against SSTO aerospikes (SSTO is a bad idea [at least for Isp<500s] and aerospikes can't fix this), but isn't for first stage aerospikes.

[Bill Phil]

7 hours ago, wumpus said:

A de Laval nozzle will be more efficient for at least a single pressure (such as vacuum) than an aerospike.  I'd be curious just how high an altitude you need for a de Laval nozzle to be more efficient for the entire flight (from that altitude to vacuum).

This is mostly an argument against SSTO aerospikes (SSTO is a bad idea [at least for Isp<500s] and aerospikes can't fix this), but isn't for first stage aerospikes.

If I recall the aerospikes developed for Venturestar were worse than the Shuttle SSMEs in pretty much every major performance aspect.

So apparently there’s a way to make conventional nozzles really good at most altitudes. Something to do with the geometry.

[wumpus]

16 hours ago, Bill Phil said:

If I recall the aerospikes developed for Venturestar were worse than the Shuttle SSMEs in pretty much every major performance aspect.

So apparently there’s a way to make conventional nozzles really good at most altitudes. Something to do with the geometry.

I think SSME nozzles are designed for vacuum flight with a "spoiler" that prevents overexpanding from being too much of a problem.  Since for the first five minutes or so the Solid Rocket Boosters provide about 4/5ths of the thrust, you don't have to worry much about the SSME nozzle being efficient/providing maximum thrust at sea level.