4 ms·

FTA: “The paper defined an ideal power plan that can output 1 kW per kg of weight. This is currently well outside the realm of possibility, with the best ion

FTA: “The paper defined an ideal power plan that can output 1 kW per kg of weight. This is currently well outside the realm of possibility, with the best ion thruster power sources coming at something like 10 W per kg and even nuclear electric propulsion systems outputting 100 W per kg.”

JumpCrisscross 2y ago
1 kW/kg is an ideal power plant. Does the paper define a minimally-viable one? If we beef up the chemical stage, e.g. by launching on Starship and re-fuelling in LEO, can we make do with 100 or even 10 kW/kg? (Also, to put 550 AU in perspective, Voyager 1 is 165 AU out [1]. At 38,000 mph Voyager 1 [2] travels about 3.6 AU/year [a]. Going straight out, it would reach the Solar gravitational lens in 2131 [b]. [1] https://science.nasa.gov/mission/voyager/where-are-voyager-1-and-voyager-2-now/ https://science.nasa.gov/mission/voyager/where-are-voyager-1... [2] https://en.m.wikipedia.org/wiki/Astronomical_unit https://en.m.wikipedia.org/wiki/Astronomical_unit [a] (38026.77 x 24 x 365) / (9.2956 x 10^7) [b] 2024 + (550 - 165) / a
- perihelions 2y ago
  550 au in 13 years is a mean of 200 km/s—chemical rockets are nothing compared to that.
  - JumpCrisscross 2y ago
    And that is for a flyby. We’re not getting to 550 AU with chemical rockets alone. Nuclear, ion and/or solar sails will be needed.
    - zardo 2y ago
      The focal "point" extends indefinitely, there's no need to slow down once you get there.
      - deleted 2y ago
        [deleted]
perihelions 2y ago
- "ideal power plant that can output 1 kW per kg of weight" I wished I remember where I read about this engineering problem, because it's an entertaining one. The main constraint on your [kW/kg] past a certain point is heat dissipation—the mass of the radiators rejecting waste heat into the vacuum of space. Thermal radiation scales like [temperature^4] (a very fast-growing function), so that parameter's obvious—you have to scale your exhaust temperature as high as you can engineer. That's how you shrink the radiator size. And you still need to run a heat engine—you need a significant temperature gradient, on top of the already-high exhaust temperature, to get useful work out of it. That's the temperature output of your primary heat source. So, the high-level design solution is: you have an array of infrared radiators glowing red, and a nuclear fission reactor glowing orange. That's the way to get a high power/mass ratio in space. Also, everything's built around pipes of molten metals of different species (optimized heat transport), and the heat engine is like a steam turbine that spins on boiling molten potassium metal. (I think?) They're really exciting-looking machines. I wish someone would build one!
- kristianp 2y ago
  Is this where a Nuclear Thermal Rocket[1] is more efficient by mass as hydrogen running through the reactor might take some of the heat away, reducing the need for radiators? This is such a fun topic. [1] https://en.wikipedia.org/wiki/Nuclear_thermal_rocket https://en.wikipedia.org/wiki/Nuclear_thermal_rocket
  - wiml 2y ago
    Yes, and even chemical rockets make use of the exhaust as effectively coolant. But the problem is that then you have to carry enough mass to carry away heat. And suddenly your rocket is 99% expendable coolant and only 1% payload (or less). Or to put it another way, nuclear-thermal rockets might have an Isp of 3000 or so, which is amazing compared to a chemical rocket which might have 1/10 of that ... but an ion engine can have an Isp of 30000-70000, which is what makes a 550-AU trip even remotely practical. I don't have the math right now to estimate how hot a conventional combustion chamber would have to be to get ion-engine-like exhaust velocities through a rocket nozzle/bell, but I think the answer is probably "ridiculously hot".
hinkley 2y ago
NASA has designs for thermoacoustic power plants that are much denser than this, but I note that they seem to be conveniently ignoring the weight of the fuel tanks in those calculations. If the generator were for instance 2% of the weight of the system, that means it has to be 50x as powerful as the goal.
rurban 2y ago
With this unrealistic ion thruster method they estimate 13 years, whilst the realistic solar sail Turyshev method, which is actually planned, needs 17 years. Just the payload is much less. This is pure science fiction, with no real practical benefit.

JumpCrisscross 2y ago

1 kW/kg is an ideal power plant. Does the paper define a minimally-viable one? If we beef up the chemical stage, e.g. by launching on Starship and re-fuelling in LEO, can we make do with 100 or even 10 kW/kg? (Also, to put 550 AU in perspective, Voyager 1 is 165 AU out [1]. At 38,000 mph Voyager 1 [2] travels about 3.6 AU/year [a]. Going straight out, it would reach the Solar gravitational lens in 2131 [b]. [1] https://science.nasa.gov/mission/voyager/where-are-voyager-1-and-voyager-2-now/ https://science.nasa.gov/mission/voyager/where-are-voyager-1... [2] https://en.m.wikipedia.org/wiki/Astronomical_unit https://en.m.wikipedia.org/wiki/Astronomical_unit [a] (38026.77 x 24 x 365) / (9.2956 x 10^7) [b] 2024 + (550 - 165) / a

perihelions 2y ago
550 au in 13 years is a mean of 200 km/s—chemical rockets are nothing compared to that.
- JumpCrisscross 2y ago
  And that is for a flyby. We’re not getting to 550 AU with chemical rockets alone. Nuclear, ion and/or solar sails will be needed.
  - zardo 2y ago
    The focal "point" extends indefinitely, there's no need to slow down once you get there.
    - deleted 2y ago
      [deleted]

perihelions 2y ago

- "ideal power plant that can output 1 kW per kg of weight" I wished I remember where I read about this engineering problem, because it's an entertaining one. The main constraint on your [kW/kg] past a certain point is heat dissipation—the mass of the radiators rejecting waste heat into the vacuum of space. Thermal radiation scales like [temperature^4] (a very fast-growing function), so that parameter's obvious—you have to scale your exhaust temperature as high as you can engineer. That's how you shrink the radiator size. And you still need to run a heat engine—you need a significant temperature gradient, on top of the already-high exhaust temperature, to get useful work out of it. That's the temperature output of your primary heat source. So, the high-level design solution is: you have an array of infrared radiators glowing red, and a nuclear fission reactor glowing orange. That's the way to get a high power/mass ratio in space. Also, everything's built around pipes of molten metals of different species (optimized heat transport), and the heat engine is like a steam turbine that spins on boiling molten potassium metal. (I think?) They're really exciting-looking machines. I wish someone would build one!

kristianp 2y ago
Is this where a Nuclear Thermal Rocket[1] is more efficient by mass as hydrogen running through the reactor might take some of the heat away, reducing the need for radiators? This is such a fun topic. [1] https://en.wikipedia.org/wiki/Nuclear_thermal_rocket https://en.wikipedia.org/wiki/Nuclear_thermal_rocket
- wiml 2y ago
  Yes, and even chemical rockets make use of the exhaust as effectively coolant. But the problem is that then you have to carry enough mass to carry away heat. And suddenly your rocket is 99% expendable coolant and only 1% payload (or less). Or to put it another way, nuclear-thermal rockets might have an Isp of 3000 or so, which is amazing compared to a chemical rocket which might have 1/10 of that ... but an ion engine can have an Isp of 30000-70000, which is what makes a 550-AU trip even remotely practical. I don't have the math right now to estimate how hot a conventional combustion chamber would have to be to get ion-engine-like exhaust velocities through a rocket nozzle/bell, but I think the answer is probably "ridiculously hot".

hinkley 2y ago

NASA has designs for thermoacoustic power plants that are much denser than this, but I note that they seem to be conveniently ignoring the weight of the fuel tanks in those calculations. If the generator were for instance 2% of the weight of the system, that means it has to be 50x as powerful as the goal.

rurban 2y ago

With this unrealistic ion thruster method they estimate 13 years, whilst the realistic solar sail Turyshev method, which is actually planned, needs 17 years. Just the payload is much less. This is pure science fiction, with no real practical benefit.