Pressurant

How much helium pressurant does a SpaceX Falcon 9 use? I would appreciate actual numbers, the following is only a wild-ass guess, and could be off by an order of magnitude. DO NOT QUOTE THIS, FIND ACTUAL SPACEX VALUES INSTEAD. I was unable to, sigh.

A Falcon 9 burns LOX and kerosene, and has a fueled pad weight of 333 metric tonnes. That puts 10 metric tonnes in low earth orbit. Those are the only solid numbers I have, from here on is rank speculation.

• The big unknowns are the temperature and pressure of the helium after expansion, hence its mass density in the propellant tanks.
• Assume that 90% of the pad weight is propellant in full tanks, which will be displaced by helium.
• Assume LOX and RP1 Kerosene with an oxidizer to fuel ratio of 2.56 (mass ratio, not stochiometric)
• Assume RP1 density of 810 kg/m³ and LOX density of 1140 kg/m³
• The heat capacity ratio of helium is 1.66, which means that adiabatic expansion makes it very cold and relatively dense
• The big assumptions: Tank pressure of 3 MPa and temperature of 290K (cribbed from Sutton, example 9.2, p228 of 5th edition).
  • This is optimistic regards temperature. I would expect the expanded helium to be much colder and denser. If it is heated during expansion, less helium will be needed, and as a bonus it will boil some LOX, adding gas pressure from that.

300 metric tonnes of propellant is 215 tonnes of LOX and 85 tonnes of RP1, or 190 m³ of LOX and 105 m³ of RP. Call it 300 m³ of tank volume to fill with helium at 30 atmospheres, including the additional volume of the high pressure tanks that supply the helium. The density of helium at STP ( 273.2K, 101.3 KPa ) is 0.1786 g/L or 0.1786 kg/m³. At 290K and 3 MPa, helium will be about 5 kg/m³. So the total mass of helium consumed will be about 1500 kilograms (!).

For every kilogram to orbit, 0.15 kilograms of helium are expended. If the launch puts a 7 passenger Dragon capsule into orbit, the helium use is about 200 kilograms per passenger.

For comparison, a Siemens "zero loss" MRI machine contains 30 liters of recirculating liquid helium, about 3.75 kg. The helium lost by one Falcon 9 launch could be used for 400 MRI machines, each performing thousands of MRIs per year.

Helium pressurant may increase performance, but when the geologically-trapped helium runs out, eventually this may could cost thousands of lives per passenger launched. Nitrogen is lower performance, with a significant impact on payload to orbit, but there is a heck of a lot more of it!