Hydrogen is the most efficient fuel and the one they claim to work with. We are already within 25% of the maximum theoretical ISP rocket fuels can provide, so t

Hydrogen is the most efficient fuel and the one they claim to work with. We are already within 25% of the maximum theoretical ISP rocket fuels can provide, so their 25% claim is highly suspect. https://thephysicsofspacex.files.wordpress.com/2016/07/isp-upper-limits.pdf https://thephysicsofspacex.files.wordpress.com/2016/07/isp-u...