4 ms·

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 propellan

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!

jameshart 5y ago
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.
- akiselev 5y ago
  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.
ben_w 5y ago
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.
- akiselev 5y ago
  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.
  - ben_w 5y ago
    Likewise, thank you — what you say is new knowledge for me :)

jameshart 5y ago

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.

akiselev 5y ago
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.

ben_w 5y ago

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.

akiselev 5y ago
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.
- ben_w 5y ago
  Likewise, thank you — what you say is new knowledge for me :)