Paraboloid Nozzles... The Most Efficient?

[Spacescifi]

 

 

This is a paraboloid shape, which looks roughly like a shortened standard shape for bell nozzles.

 

My understanding is that this shape is most efficient, as it pushes all exhaust backward in one direction,  moving the rocket forward. Flashlight mirrors at times employ the same design to focus a light beam as it spreads out in one direction.

The only reason rocket designers lengthen the nozzle has much to do with accelerating the exhaust as much as possible, something flashlight nozzles are unconcerned with, as those nozzles only reflect and focus light which cannot be accelerated further.

 

Is my understanding of rocketry nozzles correct?

Edited December 3, 2019 by Spacescifi

[Wjolcz]

17 minutes ago, Spacescifi said:

My understanding is that this shape is most efficient, as it pushes all exhaust backward in one direction

In this case the most efficient shape is Orion's (the boom-boom one) pusher plate.

[kerbiloid]

53 minutes ago, Spacescifi said:

it pushes all exhaust backward in one direction

from the focal point.

Chemical engines don't need a focal point in the nozzle, as their reaction zone is inside the combustion chamber.
They need a nozzle visually close to parabola (or sometimes a cone), and nothing more.

Edited December 3, 2019 by kerbiloid

[magnemoe]

1 hour ago, Wjolcz said:

In this case the most efficient shape is Orion's (the boom-boom one) pusher plate.

No the orion pusher plate is not very effective, this is on purpose as the ship can not contain an nuclear bomb so it just uses an part of the energy. 
However as this is an nuclear reaction you still get lots out of it compared to chemical reactions. 

[Wjolcz]

5 hours ago, magnemoe said:

No the orion pusher plate is not very effective, this is on purpose as the ship can not contain an nuclear bomb so it just uses an part of the energy. 
However as this is an nuclear reaction you still get lots out of it compared to chemical reactions. 

I know. Should've said "following this logic" first, I guess.

[Rakaydos]

In a vacuume, an infinite length parabolic nozzle is the most efficient.

In variable pressures, ignoring thermal management and combustion efficiencies, an aerospike is theoretically the best possible nozzle type. (practical concerns mean we use staged optimized parabolic nozzles instead.)

An orion pusher plate is very inefficient as a nozzle, but allows the use of a "combustion efficiency" orders on magnitude beyond any chemical engine while keeping the "thermal management" survivable. In effect, the combustion is so powerful it reduces the need for efficiency.

An Orion-Medusa Sailcraft  combines Orion and a parabolic nozzle- a magsail  that catches the detonation at a safe distance and directs the products backward, with the actual spacecraft suspended by tethers.

[magnemoe]

9 hours ago, kerbiloid said:

from the focal point.

Chemical engines don't need a focal point in the nozzle, as their reaction zone is inside the combustion chamber.
They need a nozzle visually close to parabola (or sometimes a cone), and nothing more.

Vacuum engines like the upper stage of falcon 9 looks pretty parabolic, size is limited by engine weight and diameter, also interstage length. 

[mikegarrison]

Rocket exhaust is not made of light. It is a fluid, and subject to fluid mechanics.

Rocket nozzles are also material things, which must resist tremendous heat and force while still being as low-mass as possible.

And as @kerbiloid mentioned, the exhaust does not come from a point source at the focal point of the parabola but rather the entire throat of the nozzle. (The combustion chamber behind the throat doesn't really matter, though, if the throat is sonic, as it usually is.)

All of these are reasons why a simplistic analysis claiming they should all be built in a perfect parabolic shape is too simplistic.

Edited December 3, 2019 by mikegarrison

[Rakaydos]

50 minutes ago, mikegarrison said:

Rocket exhaust is not made of light. It is a fluid, and subject to fluid mechanics.

Rocket nozzles are also material things, which must resist tremendous heat and force while still being as low-mass as possible.

And as @kerbiloid mentioned, the exhaust does not come from a point source at the focal point of the parabola but rather the entire throat of the nozzle. (The combustion chamber behind the throat doesn't really matter, though, if the throat is sonic, as it usually is.)

All of these are reasons why a simplistic analysis claiming they should all be built in a perfect parabolic shape is too simplistic.

Your 1st and 3rd points cancel each other out. While your second point remains true, a parabola remains the optimum shape for a bell to turn pressure into thrust. Other considerations, like the SSME avoiding flow separation despite it's expansion ratio, call for a non-optimum bell structure, reducing the theoretical vacuum thrust.

[Pds314]

On 12/2/2019 at 11:02 PM, magnemoe said:

No the orion pusher plate is not very effective, this is on purpose as the ship can not contain an nuclear bomb so it just uses an part of the energy. 
However as this is an nuclear reaction you still get lots out of it compared to chemical reactions. 

We should however remember that Orion pulse units are shaped charge devices. The intent is to redirect >80% of the energy into a ~25-degree cone centered on the ship. So that reflector is hardly terrible in efficiency the way it would be with a raw fission or fusion explosive with no shaped charge system.

It's more complex. For one thing, the mean collision distance of particles in rocket exhaust is tiny. Whereas that of light in a flashlight is huge (light doesn't work that way at reasonable energies). For another, we also need to remember that there's a hole at the back for a combustion chamber, and that too rapid expansion early on would likely cause significant loss of energy. The optimal rocket bell is supposedly about 3% more efficient than a 12-degree cone though, so it's hardly a massive difference, if not a totally insignificant one considering the tyranny of the rocket equation.

Edited December 5, 2019 by Pds314

[RCgothic]

The main difference between a cone and a bell with the same area ratio and exit angle is that the bell diverges quickly to begin with, allowing it to equal the cone within a shorter overall length.

This is lighter. And it allows other structures such as interstages to be lighter as well.

Edited December 5, 2019 by RCgothic

[Rakaydos]

6 hours ago, Pds314 said:

We should however remember that Orion pulse units are shaped charge devices. The intent is to redirect >80% of the energy into a ~25-degree cone centered on the ship. So that reflector is hardly terrible in efficiency the way it would be with a raw fission or fusion explosive with no shaped charge system.

It's more complex. For one thing, the mean collision distance of particles in rocket exhaust is tiny. Whereas that of light in a flashlight is huge (light doesn't work that way at reasonable energies). For another, we also need to remember that there's a hole at the back for a combustion chamber, and that too rapid expansion early on would likely cause significant loss of energy. The optimal rocket bell is supposedly about 3% more efficient than a 12-degree cone though, so it's hardly a massive difference, if not a totally insignificant one considering the tyranny of the rocket equation.

40+%, not 80+%. Half the energy goes the opposite direction and doesn't interact with the plate. That's just how Conservation of Momentum works.

A combustion chamber catches that extra 50%, and squeezes it through a nozzle to reach maximum efficiency. But Orion is too powerful for any combustion chamber we can build.