4 ms·

The most efficient heat engine is not the most efficient method of converting fuel to transportation though, is it? https://en.wikipedia.org/wiki/Specific_impu

The most efficient heat engine is not the most efficient method of converting fuel to transportation though, is it? https://en.wikipedia.org/wiki/Specific_impulse https://en.wikipedia.org/wiki/Specific_impulse Whether we should include Mars as a distination for transport, or indeed incrementally faster broadband as good uses for energy are up for debate. But Elon does, so here we are. Light 'er up and let the hippies argue amongst themselves! SpaceX may have the absolute bee's knees in rocket engine, but that assumes that leaving the atmosphere is what we need to do, and need to do now. People are mumbling 'mass transit' and 'intercontinental travel' with respect to Starship for crying out loud. It's completely insane.

pfdietz 5y ago
High specific impulse can actually reduce efficiency, though. Consider: a rocket is most efficient at converting jet kinetic energy into the kinetic energy of the vehicle if the exhaust velocity = - vehicle velocity (so the jet is left stationary in the chosen frame of reference). An ideal (and unrealizable) rocket would have Isp that increases during the launch. It should initially be very low, then increase with time. Efficiency can (ideally) approach 100%. But I wonder what you mean by efficiency of conversion of fuel "to transportation". Do you mean the irreducible minimum energy needed to achieve a given transportation task? Because the latter can be ZERO here on Earth: the energy needed to go from one place to another at the same elevation and latitude is zero: there is no change in kinetic and potential energy before and after. By your criterion, a terrestrial transportation system has 0% efficiency in this case (and extremely low efficiency in many more cases.) > SpaceX may have the absolute bee's knees in rocket engine, but that assumes that leaving the atmosphere Yes, tell me about what can be done in space without leaving the atmosphere. You seem to have written a veiled screed against space travel itself. It's not about efficiency at all.

pfdietz 5y ago

High specific impulse can actually reduce efficiency, though. Consider: a rocket is most efficient at converting jet kinetic energy into the kinetic energy of the vehicle if the exhaust velocity = - vehicle velocity (so the jet is left stationary in the chosen frame of reference). An ideal (and unrealizable) rocket would have Isp that increases during the launch. It should initially be very low, then increase with time. Efficiency can (ideally) approach 100%. But I wonder what you mean by efficiency of conversion of fuel "to transportation". Do you mean the irreducible minimum energy needed to achieve a given transportation task? Because the latter can be ZERO here on Earth: the energy needed to go from one place to another at the same elevation and latitude is zero: there is no change in kinetic and potential energy before and after. By your criterion, a terrestrial transportation system has 0% efficiency in this case (and extremely low efficiency in many more cases.) > SpaceX may have the absolute bee's knees in rocket engine, but that assumes that leaving the atmosphere Yes, tell me about what can be done in space without leaving the atmosphere. You seem to have written a veiled screed against space travel itself. It's not about efficiency at all.