Chinese Say They're Building 'Impossible' Space Drive

[Ziggurat]

What is this "energy efficiency" you speak of?

Imparted momentum divided by energy spent.

In all designs so far considered reaction mass will still be the limiting consideration.

Because nobody is considering a photon drive, because it's far too energy-inefficient (though maximally mass efficient).

 

[neutrino_cannon]

Imparted momentum divided by energy spent.

Who cares? All that matters for mission completion is delta-v (and in a few cases acceleration), and all that matters for delta-v are fuel fraction and specific impulse.

Unless you're willing to put up with a stupidly high fuel fraction, the best way to go is to pack more KJ/KG into that fuel and just accept that you'll be making a lot of entropy on the way there.


Edit: It's pedantic and semantic, but I'm not even sure that momentum/energy counts as energy efficiency. Shouldn't we be looking at joules over joules?

 

[Ziggurat]

Edit: It's pedantic and semantic, but I'm not even sure that momentum/energy counts as energy efficiency. Shouldn't we be looking at joules over joules?

No. The former is reference-frame independent, the latter is not.

 

[Dr. Trintignant]

Who cares? All that matters for mission completion is delta-v (and in a few cases acceleration), and all that matters for delta-v are fuel fraction and specific impulse.

Acceleration only matters in a few cases? Maybe most missions have a wide range of acceptable accelerations, but it always matters. No one will wait centuries for a light rocket to complete its mission no matter how mass-efficient it might be.

Let's say that we have an idealized electric rocket--it has some mass, a given power output, and a drive capable of any desired exit velocity and mass flow rate (within power limitations). What do we choose for that velocity?

If launch costs dominate all other factors, then we might choose a very high velocity, optimizing for launch mass above all else. But that's not the case in practice; ground control costs as well. If that dominated overall costs, we would pick a velocity that optimized flight time. Because the rocket is power-limited, this corresponds to energy efficiency. Obviously, flight time is even more important for human missions.

So it all comes down to the characteristics of the mission. Photon drives aren't practical yet, since energy isn't free. Chemical drives are pigs, but acceleration does matter.

- Dr. Trintignant