Why bell nozzles?

[Rutabaga22]

Why do rockets need bell nozzles? Wouldn't they expand to the ambient air pressure? I feel like letting the gas out of the nozzle would be enough to create thrust.

[Superfluous J]

The gas comes out the little hole in all directions, 180 degrees around a half-sphere from the hole. That gas then hits the bell and due to its shape, is directed as close to straight back as possible. this generates more thrust than just letting the gas out the nozzle would.

Edited June 16, 2022 by Superfluous J

[mikegarrison]

https://en.wikipedia.org/wiki/De_Laval_nozzle

It's because the nozzle accelerates the exhaust flow. You can think of it as converting the potential energy of pressure into the kinetic energy of flow velocity.

Also, you have to be careful with your terminology. The whole thing is the "nozzle". The narrow constriction is the "throat".

Edited June 16, 2022 by mikegarrison

[sevenperforce]

43 minutes ago, Rutabaga22 said:

Why do rockets need bell nozzles? Wouldn't they expand to the ambient air pressure? I feel like letting the gas out of the nozzle would be enough to create thrust.

Newton’s laws of motion say (paraphrased) that the faster your reaction mass flies away from you, the more impulse it provides to you. Equal and opposite reactions and all that.

Rocket engines produce enormously high pressures and temperatures. But high pressure alone isn’t particularly efficient. You want your exhaust gas to leave your engine as fast as possible, because the faster the exhaust leaves, the harder it pushes you in the opposite direction. And in order to get the exhaust gas moving at maximum speed, you need to find a way to force it to expand.

The best way to get exhaust gas to expand fast is to choke it — create a region of maximum pressure where the gas flow becomes locally supersonic. Since pressure waves cannot flow backwards in a supersonic flow, this directly converts pressure into speed. 

[JoeSchmuckatelli]

15 hours ago, Rutabaga22 said:

Why do rockets need bell nozzles? Wouldn't they expand to the ambient air pressure? I feel like letting the gas out of the nozzle would be enough to create thrust.

Aside from what the smart guys above wrote: it's about efficiency. 

Yeah, the hole in the back method works (see, e.g. 'Fireworks' and 'Ancient Chinese Rockets')... but the addition of the bell makes it so much more efficient and enhances controllability.

 

[Exploro]

To add. According to the Glenn Research Center's write up on rocket nozzles, alternatives such as a plug nozzle (the kind used on many types of garden hose attachments) boast some advantages over De Lavel nozzles in terms of "matching exit pressure". As I recall a function of a De Laval nozzles efficiency is in it's ability to maintain optimum exit pressure is limited by the fact that the nozzles are shaped to be optimally efficient within a specific range of altitudes, above or below which pressure effect from ambient environment erode efficient operations. According to Glenn's, Plug nozzle offer a wider flight regime for pressure matching. However, plug nozzles are heavier than De Laval nozzles. This might offset the advantages such a nozzle configuration might provide.

Edited June 22, 2022 by Exploro

[mikegarrison]

59 minutes ago, Exploro said:

To add. According to the Glenn Research Center's write up on rocket nozzles, alternatives such as a plug nozzle (the kind used on many types of garden hose attachments) boast some advantages over De Lavel nozzles in terms of "matching exit pressure". As I recall a function of a De Laval nozzles efficiency is in it's ability to maintain optimum exit pressure is limited by the fact that the nozzles are shaped to be optimally efficient within a specific range of altitudes, above or below which pressure effect from ambient environment erode efficient operations. According to Glenn's, Plug nozzle offer a wider flight regime for pressure matching. However, plug nozzles are heavier than De Laval nozzles. This might offset the advantages such a nozzle configuration might provide.

The whole thing has to do with the non-intuitive way that supersonic flow works. In subsonic flow, as the area constricts the flow speeds up, and as the area expands the flow slows down. But if you have enough upstream pressure to reach "choked flow" (Mach 1 at the throat), then as you expand the area again the flow actually speeds up. But the static pressure gets lower and lower. As the static pressure drops below the ambient pressure, it starts to run into resistance.

Normally fluids want to flow from high static pressure to low static pressure. But if it is already flowing, the fluid has some momentum that will help it push into that higher pressure.

Ultimately the flow must match pressures with the ambient, and supersonic flow does so by experiencing a shock.

Anyway, the point is that if the flow pressure at the exit plane is higher than the ambient pressure, you are wasting some of your initial energy by releasing it as pressure rather than velocity. But if the flow pressure at the exit plane is too low compared to the ambient, the flow will not be able to properly push out of the nozzle and bad things will happen.

And yes, there are other types of nozzles that are less sensitive to matching the ambient pressure (like aerospike nozzles), but they have their own issues.

[sevenperforce]

11 hours ago, Exploro said:

alternatives such as a plug nozzle (the kind used on many types of garden hose attachments) boast some advantages over De Lavel nozzles in terms of "matching exit pressure".

A quick observation: a plug nozzle is still a “bell” nozzle; it has simply been turned inside-out.

A plug nozzle will be less efficient (both in terms of specific impulse and T/W) than an equivalently-sized bell nozzle at any given pressure. Its advantage is that it can be more efficient at a broader pressure range, so it is a good sustainer design.