3 ms·
- "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.
by 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 agoIs 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 agoYes, 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".
- perihelions 2y agoI think it's very roughly around 200,000 K! https://en.wikipedia.org/wiki/De_Laval_nozzle#Exhaust_gas_velocity https://en.wikipedia.org/wiki/De_Laval_nozzle#Exhaust_gas_ve... (I'm assuming hydrogen at 200,000 K becomes fully dissociated atomic hydrogen radicals, which act as a helium-like, monoatomic ideal gas—γ = 5/3).