How is efficiency defined in rocket propulsion?

[Jurgen M]

Whicle has 4 wheels, inside is oval closed room with fan that blow inside air to the wing. Wing produce difference in static pressure in direction of travel.
Will vehicle move forward or not?

If I look just at pressure difference at wing, vehicle will move forward but I know from Newton 3 law, you can't move object with "internal forces", just like you can't move car forward if you you are on seat and push front window with your legs..

If vehicle will not move how explain this with pressure distribution?

Top view

 

[Dale]

If I look just at pressure difference at wing, vehicle will move forward but I know from Newton 3 law, you can't move object with "internal forces"

Exactly. So that means you must look beyond just the pressure difference at the wing. Where else do you think you should look?

 

[Jurgen M]

Exactly. So that means you must look beyond just the pressure difference at the wing. Where else do you think you should look?

At the room walls and at every object inside the room..
Interesting how nature distribute all these surface pressures in such way that resultant force is allways zero.

Newton 3 law is in same time so simple and so brilliant.

 

[Dale]

At the room walls and at every object inside the room..

Exactly, yes.

 

[Jurgen M]

Exactly, yes.

Is photon drive one way to achieve propulsion with closed system, because photons don't have mass?
If I point powerful laser to the back, will vehicle move forward?

 

[Ibix]

Is photon drive one way to achieve propulsion with closed system, because photons don't have mass?

They carry momentum. If they escape the system they carry away momentum and the system moves but is not closed. If they don't escape, the system doesn't move.

 

[Jurgen M]

They carry momentum. If they escape the system they carry away momentum and the system moves but is not closed. If they don't escape, the system doesn't move.

So laser point to the back will move vehicle forward?

 

[Ibix]

So laser point to the back will move vehicle forward?

As long as there's a window for the light to go out of, yes. Not in a closed system.

 

[Jurgen M]

As long as there's a window for the light to go out of, yes. Not in a closed system.

Hmm that sounds impossible because multiplication by zero gives zero!

How many watts laser must have to get 10N of thrust?

 

[Ibix]

Hmm that sounds impossible because multiplication by zero gives zero!

The momentum of light is $E/c$, not $mv$.

How many watts laser must have to get 10N of thrust?

The momentum of light turns out to be its energy divided by $c$. So you'd need a continuous output 3GW laser to get 10N thrust.

 

[Jurgen M]

. So you'd need a continuous output 3GW laser to get 10N thrust.

So this is very very inefficient type of propulsion

 

[jbriggs444]

So this is very very inefficient type of propulsion

With the kind of energy density we have in chemical rocket fuel, yes. You burn through the fuel to get energy and throw the energy away in the photon exhaust stream for little benefit. You are left with the expended fuel and throw that away pointlessly.

One is better served using the expended fuel as reaction mass. Which is how chemical rockets work.

If you had better energy density in your fuel (like matter anti-matter annihilation) then the a photon exhaust would be as efficient as rocket propulsion can be.

 

[cjl]

With the kind of energy density we have in chemical rocket fuel, yes. You burn through the fuel to get energy and throw the energy away in the photon exhaust stream for little benefit. You are left with the expended fuel and throw that away pointlessly.

One is better served using the expended fuel as reaction mass. Which is how chemical rockets work.

If you had better energy density in your fuel (like matter anti-matter annihilation) then the a photon exhaust would be as efficient as rocket propulsion can be.

You have to be a little careful with how you define efficiency here.

If, as Jurgen seems to be assuming above, we define efficiency as thrust per unit energy, a photon drive is phenomenally inefficient no matter where we source that energy from. Using the same amount of energy to accelerate mass out the back will always give you more thrust, and it turns out that the more mass you accelerate per unit energy (and thus the lower the exhaust velocity), the more thrust you get.

This is why for example jet engines are going to larger and larger fans relative to their thrust. That allows them to interact with (and therefore accelerate) a larger mass flow of air, which increases the thrust produced per unit of fuel burned.

However, when we start talking rockets, there's an obvious problem with the above: you have to carry all that mass with you. It might take a tiny amount of energy to generate a large amount of force if you throw a huge mass out the back pretty slowly, but then you need to carry that huger reaction mass.

As a result, rocket efficiency is usually defined not as thrust per unit energy, but as impulse (thrust multiplied by how long that thrust is applied) per unit mass of fuel. This is also equivalent to thrust divided by the fuel mass flow rate. As you can see, this efficiency is trying to minimize the amount of fuel mass you have to carry to achieve a certain amount of thrust for a certain time.

Unfortunately, this pretty much goes exactly backwards from the energy efficiency used above. If you want to maximize the thrust you get from each piece of fuel, you need to throw it backwards as fast as you possibly can. This takes a ton of energy, but since you can keep the rocket lighter by not having to carry as much propellant, it's very much the way you want to go. In addition, with chemical rockets, the source of energy is also the reaction mass, so you don't need to try to source the energy anywhere else anyways. This leads to looking for fuels and oxidizers that burn as hot as possible and have as light of a molecular mass of their reaction products as possible, since that leads to the highest molecular speeds (which can then be directed out the back with a nozzle).

This is why a photon rocket is often referred to as the "most efficient" - it's the way to achieve the highest overall impulse from the smallest mass of propellant possible.