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Comment by _Microft | Hacker News Reader

Am I correct that the interesting thing about subluminal warp drives is that they do not require working mass for propulsion? So if we had such a warp drive, we could put energy in and get motion out of it, without being subject to the tyranny of the rocket equation?

It seems like this idea is not correct. The authors write in section 5.2: "Warp drives, being inertially moving shells of normal or exotic material, do not have any natural way of changing their velocities. They are just like any other types of inertially moving objects. Similarly, just like for any other massive objects, achieving a certain velocity for a warp drive requires an externally applied force or, more practically, some form of propulsion.", from https://arxiv.org/pdf/2102.06824.pdf#page=17

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The tyranny of the rocket equation isn't some fixed property of orbital mechanics, it's a limitation of our method for converting energy from chemical propellants to propulsion. Specific impulse for nuclear rockets designed decades ago (i.e. nuclear lightbulb) is in the thousands, which is enough to reach orbit in a single stage without dumping extraneous mass.

The original Alcubierre drive required the energy equivalent of the mass of Jupiter to create the warp field, with an impossible shortcut in the form of exotic particles with negative mass. The sun burns loses about 6x10^9 kg of mass a second which is the upper limit for the amount of energy a Type I civilization can extract in our solar system. Jupiter's mass is roughly 2x10^27 kg, so assuming there are 4x10^11 stars in our galaxy and our star represents the average, a Type II civilization in our galaxy wouldn't have enough energy to produce a warp field. The drive doesn't just need a little bit of working mass to expel, it needs enough energy/mass to bend space time around it!

The rocket equation doesn’t only apply to chemical energy propulsion - it applies to any propulsion system that conserves momentum and uses reaction mass - which nuclear drives do too.

Chemical rockets are limited in how much energy they can put into the reaction mass (by the energy density of their fuel) and therefore the maximum exhaust velocity they can achieve - which is the input into the rocket equation

But while nuclear rockets get away from the constraint of chemical energy density they still have to operate by putting energy into reaction mass and shooting it out the back to create a momentum change.

And the rocket equation still applies to the reaction mass and its exhaust velocity.

In fact in theory you can build a chemical rocket engine that burns a fuel for heat, then uses that heat to energize some other reaction mass that you eject - it’s just that that is inherently less efficient than using the spent fuel as reaction mass since otherwise you’re needlessly pumping momentum into spent fuel. Far better to throw the exhaust products away as reaction mass.

Ah, my mistake - I thought the "tyranny of the rocket equation" was referring to the rocket equation's consequences on our attempts to reach orbit (i.e. multistage rockets), not the equation in general.

Slight quibble, but the original needed more negative mass than the universe, it was one of the subsequent improvements which reduced it to only Jupiter.

Thank you for the correction! Though, I hope you don't mind that I quibble the quibbler: the original needed more regular mass than the universe - negative mass exotic particles were proposed as a more "practical" alternative, if they exist.

Likewise, thank you — what you say is new knowledge for me :)

Yup.

But also they're interesting as a theoretical construct for better understanding the whole idea of intentionally warping space.

Just a causal reminder that solar sails, beamed propulsion, mag sails, photon rockets, etc are all examples of propulsion methods free from the tyranny of the rocket equation which require no onboard reaction mass. There is really no need for exotic physics if that's all you're looking to accomplish.

I don't understand why it would be called a warp drive then. If it doesn't cause movement it's not driving anything.

If general relativity is an accurate model of gravity then distortions in space-time are caused by mass. Traveling at the speed of light can be shown to require an infinite amount of energy to accomplish for a finite mass. However space-time itself could, according to our current understanding, travel faster than the speed of light.

So if you could 'warp' space around you rather than travel through it, you could go at an arbitrary speed. Unfortunately warping space-time is typically accomplished by large amounts of mass. Hence a large amount of energy. If I remember correctly it also requires we bend space in a way we have never seen nor have any reason to believe is possible to accomplish beyond it being allowed by general relativity. It's quite a pickle.

Without such a device humanity will still populate the galaxy but it'll take hundreds of millions of years. Assuming we don't wipe ourselves out.

Also the travelers are essentially in an inertial frame, so it acts kind of like "inertial dampeners" in Star Trek.

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