3 ms·

Energy is the limit. If used Wolfram Alpha right for reasonable comparison that is largest cruise ship with mass of 248663000 kg to speed up to 0.01c mentioned

by Ekaros 1y ago

Energy is the limit. If used Wolfram Alpha right for reasonable comparison that is largest cruise ship with mass of 248663000 kg to speed up to 0.01c mentioned the kinetic energy would be 2.9 times the worlds fossil fuel reserves... Then double that to slow down. And remember efficiency and that well you need to spend some to keep people alive... Seems like energy in general is one of the true problems.

WithinReason 1y ago
You wouldn't be burning coal for propulsion, you would fuse atoms: https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
- Ekaros 1y ago
  I didn't coal would be used. Just that involved amount of energy would be truly massive. Even with fusion.
- m4rtink 1y ago
  Or even better - ride a continuous fission detonation powered by the worlds biggest super-soaker! :) https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket
JumpCrisscross 1y ago
> If used Wolfram Alpha right for reasonable comparison that is largest cruise ship with mass of 248663000 kg to speed up to 0.01c mentioned the kinetic energy would be 2.9 times the worlds fossil fuel reserves This math is wrong. Back of the envelope: accelerating 250 x 10^6 kg to 1% of c over one year is about 310,000 TWh. That’s like half of current annual energy production. Even at like 10g, which would accelerate that mass to 1% c in less than half a day, you’d only use like a tenth of our 10^22 joules of fossil fuel reserves. > double that to slow down Why the asymmetry? 1x to accelerate, 1x to decelerate. (Less if you can aerobrake and/or use gravity assists.) Taking the 310k TWh from earlier and doubling it, and assuming 18 MeV for D-T direct-drive fusion, and you need 6,600 kg fuel for the journey. (Plus propellant.) 100x that to account for inefficiency and cooling and you have 660 metric tonnes of hydrogen. That, if kept in its bastard liquid form, takes up about 3 Olympic-size swimming pools in volume.
- ben_w 1y ago
  Your maths is correct. Shame we don't have a way to efficiently convert fuel energy directly into a ship's kinetic energy, and have to go via conservation of momentum… though I suppose magnetic launch of interstellar vehicles would be a neat use for a Dyson shell or Niven ring?: https://www.wolframalpha.com/input?i=0.5+*+1+gee+*+%280.01c%2F1+gee%29%5E2 https://www.wolframalpha.com/input?i=0.5+*+1+gee+*+%280.01c%...
  - jvanderbot 1y ago
    Is this the right time to mention the Atomic Rockets website? https://projectrho.com/public_html/rocket/ https://projectrho.com/public_html/rocket/ It's _full_ of good stuff.
    - ben_w 1y ago
      Project Rho is one of my references for the novel I'm trapped in an endless cycle of re-writing :)
- Ekaros 1y ago
  Yeah, stupid thing went with c^2... Not (0.01c) ^2... My bad from not reading the steps properly. Still, even half of current annual production is quite a ask. For single ship, and I might venture to guess that 250e6 kg might be light for what is needed...

by Ekaros 1y ago

WithinReason 1y ago

You wouldn't be burning coal for propulsion, you would fuse atoms: https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

Ekaros 1y ago
I didn't coal would be used. Just that involved amount of energy would be truly massive. Even with fusion.
m4rtink 1y ago
Or even better - ride a continuous fission detonation powered by the worlds biggest super-soaker! :) https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket

JumpCrisscross 1y ago

> If used Wolfram Alpha right for reasonable comparison that is largest cruise ship with mass of 248663000 kg to speed up to 0.01c mentioned the kinetic energy would be 2.9 times the worlds fossil fuel reserves This math is wrong. Back of the envelope: accelerating 250 x 10^6 kg to 1% of c over one year is about 310,000 TWh. That’s like half of current annual energy production. Even at like 10g, which would accelerate that mass to 1% c in less than half a day, you’d only use like a tenth of our 10^22 joules of fossil fuel reserves. > double that to slow down Why the asymmetry? 1x to accelerate, 1x to decelerate. (Less if you can aerobrake and/or use gravity assists.) Taking the 310k TWh from earlier and doubling it, and assuming 18 MeV for D-T direct-drive fusion, and you need 6,600 kg fuel for the journey. (Plus propellant.) 100x that to account for inefficiency and cooling and you have 660 metric tonnes of hydrogen. That, if kept in its bastard liquid form, takes up about 3 Olympic-size swimming pools in volume.

ben_w 1y ago
Your maths is correct. Shame we don't have a way to efficiently convert fuel energy directly into a ship's kinetic energy, and have to go via conservation of momentum… though I suppose magnetic launch of interstellar vehicles would be a neat use for a Dyson shell or Niven ring?: https://www.wolframalpha.com/input?i=0.5+*+1+gee+*+%280.01c%2F1+gee%29%5E2 https://www.wolframalpha.com/input?i=0.5+*+1+gee+*+%280.01c%...
- jvanderbot 1y ago
  Is this the right time to mention the Atomic Rockets website? https://projectrho.com/public_html/rocket/ https://projectrho.com/public_html/rocket/ It's _full_ of good stuff.
  - ben_w 1y ago
    Project Rho is one of my references for the novel I'm trapped in an endless cycle of re-writing :)
Ekaros 1y ago
Yeah, stupid thing went with c^2... Not (0.01c) ^2... My bad from not reading the steps properly. Still, even half of current annual production is quite a ask. For single ship, and I might venture to guess that 250e6 kg might be light for what is needed...