One of the high-performance features of SpaceX’s methane/LOX fuel pair is the supercooling of the fuels.
Methane boils at -164 C at a density of 425.5
kg/m3. At its melting point of -184 C that is 447.7 kg/m3. That means the fuel tank will hold 105% fuel by mass compared to the usual liquid at boiling point.
For oxygen the effect is even greater. At boiling point, the liquid’s density is 1.14 kg/m3. The company chills it to -206 C, and the density is 1.27 kg/m3. That allows 112% fill relative to boiling oxygen.
Smaller tankage means higher propellant mass fraction = more delta V.
The third increment comes from specific impulse Isp, expressed in seconds. (A pound of propellant will provide a pound of thrust for ____ seconds.) Higher ISP means higher velocity at staging.
Saturn V had a LOX/RP-1 (a refined kerosene) ISP of 263 seconds at sea level.
Falcon 9 first stage has a sea-level Isp of 282 s. The vacuum-tuned Merlin 1D+ in the second stage yields 348 seconds.
The planned “BFR” (Big Rocket) first stage for Starship runs the Raptor motor; the second Starship stage uses the Vacuum Raptor. The company is saying that Isp (sea level) will be 330 s, and a stunning 380 s in vacuum. (I had an expression of disbelief here earlier. Since I made a mistake on Falcon’s fuel, these numbers seem likely to me now.)
The gold standard is oxygen/hydrogen. Sea level 390 s and 450 s in vacuum. The downside is liquid hydrogen’s very low density of 71 kg/m3. This means that LH2 tanks are necessarily large, which means more dead weight in the rocket stage, largely undoing the Isp advantage for launches from sea level.
This means that the BFR/Starship stack (390 feet tall, which is the limit for the Vehicle Assembly Building on the Cape) can lift from 90 to 130 metric tons to LEO. This would realize Von Braun’s original DIrect Ascent architecture. In direct ascent, the complete spacecraft leaves Earth, lands on and takes off from the moon, then lands back on Earth without dropping pieces on the way, like Apollo, and dies away with a dedicated lander like the Lem.
Bummer the test article blew up this morning. Better now than later. Spares us an Apollo 1 or a Challenger.
www.google.com
