4 ms·

I've been wondering about that limit. Haven't found any good sources so far, do you have any handy?

• pfdietz 4y ago
  I don't have a reference handy, but one can ballpark this by looking at (1) the concentration of water in the stratosphere, (2) the lifetime of a gas parcel there, and (3) the mass of the atmosphere there. This gives the natural rate of injection of water, and I think it would be a good idea to keep manmade injections below that level. Some numbers I've pulled out of various places (not guaranteeing these are accurate): Earth's atmosphere is 5e18 kg, 99% of mass of atmosphere is below 30 km (so mass above that is 5e16 kg), lifetime of gas in upper stratosphere is ~8 years, concentration of water vapor there is about 7 ppmv. Given this, natural water injection into the upper part of the stratosphere is maybe 2e10 kg/year. A Falcon 9 produces about 2e5 kg of water vapor per launch (much at lower altitude, granted). So, it's looking like at most 1e6 F9 launches/year, perhaps less, would be over the limit I set. Concerns from water vapor in the stratosphere would be ozone destruction and contribution to global warming (I understand 15% of the GWP of methane comes from the water vapor it produces when oxidized in the stratosphere.)
  • DennisP 4y ago
    Thanks, that makes sense! Not as limiting as I'd feared. StarShip uses 1,200 tonnes of propellant, SuperHeavy uses about 3,300 tonnes. 45% of that will become water vapor, the rest CO2. Couldn't find what altitude SuperHeavy reaches, let's say 1e6 kg water vapor to upper atmosphere and we get 20,000 Starship launches per year, lifting between two and three million tons. Probably a good bit less water vapor actually gets that high, but on the other hand it's fairly generous to allow a doubling of the water vapor.