Updated with numbers from J. Ponzo, thanks!
How much helium pressurant does a SpaceX Falcon 9 use?
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.
- 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 helium is heated to 500F (533K) and pressurized to 60 to 100 psi (414 to 689 KPa). Assume 450 KPa as the tanks empty.
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 4.5 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 533K and 450 Pa, helium density is about 0.4 kg/m³. So, the total mass of helium consumed will be about 120 kilograms.
For every kilogram to orbit, 12 grams of helium are expended. If the launch puts a 7 passenger Dragon capsule into orbit, the helium use is about 17 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 30 MRI machines, each performing thousands of MRIs per year.
Helium pressurant may increase performance, but when the geologically-trapped helium runs out, one launch may cost hundreds of future lives per passenger launched today. Nitrogen is lower performance, with a significant impact on payload to orbit, but there is a heck of a lot more of it!
Current helium prices are relatively low (about $20/kg) because the US government is selling off the helium reserve. Annual sales are 23,000 tonnes per year. SpaceX consumes a tiny fraction of that, and launch rates could saturate existing launch facilities without greatly increasing that fraction. The issue is the long term - helium collects over vast geological time in only a small fraction of gas wells, and when those are depleted, that's it. No more helium, unless we concentrate it from the tiny percentage present in Earth's atmosphere. At 5 ppm, we would need to process 200,000 kilograms of atmosphere at 100% efficiency to produce 1 kilogram of helium. Relative to that, centrifuge isotope separation seems easy. The resulting helium would be enormously expensive. This is no way to establish a long term presence in space.
BTW, a 10 inch party balloon is about 9 liters, or 1.5 grams of helium. Small compared to an MRI machine, tiny compared to a SpaceX launch, but many balloons add up. Please do not invite me to parties with helium balloons, I will see lost lives, not festivity.