3 ms·

No, you're comparing the method of applying the energy required, which is painfully inefficient (rockets), to the energy actually required. But I did touch on

No, you're comparing the method of applying the energy required, which is painfully inefficient (rockets), to the energy actually required. But I did touch on that in the last paragraph -- the energy required on a rocket to get into LEO is $20/kg, or $2k/person. It's a tiny amount of the total cost of launch.

pfdietz 4y ago
Rocket engines are actually extraordinarily efficient. They're the most efficient heat engines we have. More than 90% of the thermal energy produced in the thrust chamber is converted to kinetic energy of the exhaust jet (especially in high expansion ratio engines). The fraction of chemical energy of the propellants of a launch vehicle that ends up in the kinetic energy of the final stage is surprisingly high (well, it was to me, initially.)
- iso1631 4y ago
  They are not efficient at doing their job (moving things upwards), hence we don't use them to get from the bottom of a skyscraper to the top. They have to carry their entire mass of fuel with them. A large crane is far more efficient at lifting items. But even if you accept you have to use a rocket, the cost of the fuel is trivial compared with the cost of launch.
  - pfdietz 4y ago
    Of course they are not efficient for that use case. That use case is not the same as the use case of getting to orbit. When the delta-V involved is similar to the exhaust velocity of the rocket the overall efficiency goes way up. If you could vary the exhaust velocity of a rocket continuously, so it was equal to the total delta-V so far, the efficiency of converting jet kinetic energy to vehicle energy would be 100%, as the jet would be left stationary in the reference frame of the launcher. (This ignores gravitational potential energy and also that the mass ratio would diverge to infinity at zero velocity, but never mind that.) In practice, using a lower Isp first stage and a higher Isp upper stage partially implements this, and the overall efficiency isn't too bad.

pfdietz 4y ago

Rocket engines are actually extraordinarily efficient. They're the most efficient heat engines we have. More than 90% of the thermal energy produced in the thrust chamber is converted to kinetic energy of the exhaust jet (especially in high expansion ratio engines). The fraction of chemical energy of the propellants of a launch vehicle that ends up in the kinetic energy of the final stage is surprisingly high (well, it was to me, initially.)

iso1631 4y ago
They are not efficient at doing their job (moving things upwards), hence we don't use them to get from the bottom of a skyscraper to the top. They have to carry their entire mass of fuel with them. A large crane is far more efficient at lifting items. But even if you accept you have to use a rocket, the cost of the fuel is trivial compared with the cost of launch.
- pfdietz 4y ago
  Of course they are not efficient for that use case. That use case is not the same as the use case of getting to orbit. When the delta-V involved is similar to the exhaust velocity of the rocket the overall efficiency goes way up. If you could vary the exhaust velocity of a rocket continuously, so it was equal to the total delta-V so far, the efficiency of converting jet kinetic energy to vehicle energy would be 100%, as the jet would be left stationary in the reference frame of the launcher. (This ignores gravitational potential energy and also that the mass ratio would diverge to infinity at zero velocity, but never mind that.) In practice, using a lower Isp first stage and a higher Isp upper stage partially implements this, and the overall efficiency isn't too bad.