Re:The Tyranny of the Rocket Equation (2012)
by Robotbeat on Friday June 29, 2018 @09:20PM
(#17428996)
Honestly, we do. The rocket equation is not just exhaust velocity, it's ALSO mass fraction. And material advances can make a huge difference there.
Chemical rockets are also much more efficient devices than is given credit for. A good, very optimized two stage rocket like SpaceX BFR (and the ITS concept before it) can get payloads to orbit using just ~300 Megajoules per kg (250MJ/kg in the more efficient tanker configuration). The specific kinetic energy of orbit is about 25MJ/kg (40MJ/kg if we include gravity and aero losses and changes in altitude), so rockets are total 10% efficient at converting chemical energy directly into orbital energy, and with an optimized, high pressure and variable mixture ratio hydrogen rocket engine architecture with some improvements in materials science, we could improve that to 20% or more. That beats the chemical to mechanical efficiency of the typical car.
That translates to between $1 and $10 per kg to orbit. That's 3 orders of magnitude less cost than today, if we can master reuse. So arguably, chemical rockets are perfectly fine for achieving orbit cheaply.
Chemical rockets are also much more efficient devices than is given credit for. A good, very optimized two stage rocket like SpaceX BFR (and the ITS concept before it) can get payloads to orbit using just ~300 Megajoules per kg (250MJ/kg in the more efficient tanker configuration). The specific kinetic energy of orbit is about 25MJ/kg (40MJ/kg if we include gravity and aero losses and changes in altitude), so rockets are total 10% efficient at converting chemical energy directly into orbital energy, and with an optimized, high pressure and variable mixture ratio hydrogen rocket engine architecture with some improvements in materials science, we could improve that to 20% or more. That beats the chemical to mechanical efficiency of the typical car.
That translates to between $1 and $10 per kg to orbit. That's 3 orders of magnitude less cost than today, if we can master reuse. So arguably, chemical rockets are perfectly fine for achieving orbit cheaply.