# How much power is "a power unit"

#### Bill Sheil

##### Banded Mongoose
You are still confused:
The energy gained (or lost) by the ship is different in different frames of reference, but total energy is still conserved in each frame of reference.

The ship and the reaction mass forms a closed system that conserves momentum and energy.
The second sentence is correct although it is repeating what I told you. The first is (apparently) wrong. The problem is that you are jumping between referring to the ship alone where the energy is not conserved and the closed system (ship + reaction mass) where the energy is conserved. It may be that that is not what you intended to say this time but in your first post you made you made the false suppositions based on the calculations of the kinetic energy of the ship alone and this looks like more of the same.
The ship and the reaction mass forms a closed system that conserves momentum and energy. If the ship and the reaction mass starts accelerating away from each other the total energy of the system increases, that energy has to come from somewhere and the power required to maintain constant acceleration will increase with time. I don't see where the power comes from, if not the drives of the ship?
Yes it does, as I said:
The crux of this is that we always need to consider the whole mass system not just the primary (craft) but the reaction mass as well. The reaction mass has its own, mass, velocity, momentum and kinetic energy. When we do this the invariant Power Plant energy is distributed between both the primary mass and the reaction mass (both receive equal and opposing momentum). The total change in kinetic energy of both the primary and the reaction mass equals the total propulsion (power plant) energy but the proportional distribution depends on the frame of reference.
This is not the issue. The problem in your first post was that you ignored reaction mass and thus drew the wrong conclusion about constant power/acceleration and restrictions on acceleration.

In the frame of the ship, the reaction mass keeps accelerating away from the ship, it does not start from zero in every instant.
It was I think, fairly obviously an ejected reaction mass (Fusion, HEPlaR) I was referring to here.

#### AnotherDilbert

##### Cosmic Mongoose
You are still confused:
Yes, I am, and this conversation isn't helping any.

It was I think, fairly obviously an ejected reaction mass (Fusion, HEPlaR) I was referring to here.
No, it was not obvious to me. It was in the section where you were discussing the energy of the ship alone, long before you exanded the topic to rockets.

The problem is that you are jumping between referring to the ship alone where the energy is not conserved and the closed system (ship + reaction mass) where the energy is conserved.
Yes, I can see that appears so. I was trying to respond to you, where you were apparently jumping between discussing the ship alone and the closed system. I was trying to steer the topic back to the closed system. Apparently we both failed to make ourselves understood.

We are apparently talking past each other, without understanding each other. In an effort to reduce the level of confusion I will henceforth:
- Only talk about gravitic M-drives using the local planet or system as reaction mass.
- Only talk about the closed system of the ship and the reaction mass.
- Assume that any changes in potential energy is negligible.

I see:
1: The ship and the reaction mass forms a closed system.
2: In the closed system energy and momentum is conserved, in each frame of reference.
3: The only source of power, that drives the acceleration, in the system is the ship's power plant.
4: In each frame of reference, as the ship and the reaction mass accelerate away from each other with constant acceleration, the system gains kinetic energy faster and faster.
5: Hence more and more power is needed to maintain the acceleration, in any given frame of reference.
6: In an external frame of reference, as the reaction mass (a planet or solar system) is vastly more massive than the ship, as they gain equal momentum (opposite direction), the lighter object (the ship) gains much more velocity, and therefore kinetic energy, than the reaction mass. The kinetic energy gained by the reaction mass is insignificant, and nearly all energy gained is gained by the ship.

What do we disagree about here?
I believe we disagree about statement 4 but I don't think I have seen that stated conclusively.
We obviously disagree about statement 5, that I see as an obvious conclusion from statements 3 and 4?

Without referring to older posts, what do you specifically disagree with?

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