In January, Elon Musk mused that the design goal for Starship was 3 flights per day, for about 1000 flights per year per Starship (assuming for the sake of simplicity he’s talking about a whole stack) with a payload of about 100 tons to LEO. That’s 10 Starships flying a total of 10,000 flights per year to reach 1 megaton in orbit. He points to 100 megatons annually to orbit (100 Starships per year for 10 years… 1 million Starship flights per year) as the goal.
Just how big is that? Is it realistic?
Note: I decided to make a couple images to illustrate the difference between 1 Megaton IMLEO/year (corresponding to roughly 1000 larger Starships heading off to Mars each synod) and 100 Megaton IMLEO/year (roughly 100,000 larger Starships heading off to Mars each synod). There are no stars in these images, just Starships:
1000 Starships doing trans-Mars-insertion simultaneously:
(alternately, for 100,000 Starships, they might do this once per orbit for a week straight)
Going by rough figures, there’s about 3 billion tons of methane (in the form of 4 trillion cubic meters of natural gas) produced every year. Each Starship takes about 1000 tons of methane (very rough numbers…). So you’d need 1 billion tons of methane every year to reach Elon’s 100 Megaton-to-orbit goal.
1 billion tons of methane is approximately 0.75 billion tons of carbon. About 5 billion tons of carbon is absorbed total by land and sea every year (Candela and Carlson 2017), so if all the world was doing as far as carbon emissions was launching SpaceX’s rockets, this wouldn’t be a problem. 1 gigaton of payload to LEO (so about 7.5 billion tons of carbon), however, WOULD be beyond the current ability of the land and sea to absorb carbon, slightly less than the current global carbon emissions from human civilization.
That puts current Starship launch efficiency to about 580MJ/kg, compared to the absolute minimum of ~32MJ/kg, or about 5% efficient. This is remarkably efficient if you think about it, but it’s nowhere near the efficiency that’s *possible*. Stretch estimates for what SpaceX hopes to eventually achieve with Starship might be 2320 tons of methane for 380 tons of payload (propellant in this case), given the 2016 ITS tanker figures (from “making_life_multiplanetary_2016.pdf“). They had a higher O:F mixture ratio (3.8), higher Isp, and lower dry masses. That’s just under 10% efficiency. Significantly better. That would mean half as much methane would be needed, perhaps lowering the carbon emissions to 4 or so gigatons of carbon for 1 gigaton to orbit, below the 5 gigatons the land and sea can absorb… Back to the 100 megaton goal, that’s 340MJ/kg times 100 billion kilograms… divided by about 31 billion seconds in a year, and you’re talking about a terawatt of methane per year.
Of course, you could switch from fossil methane to CO2-direct-air-captured methane. That’s about 50% efficient, so it’d take about 2 terawatts average per to produce. Or, given about a 20% capacity factor, about 10 terawatts of solar nameplate capacity.
But I think they’re potentially still leaving energy on their payload. (See previous posts) If they operate at yet higher O:F, even deeply oxygen-rich, for the first stage, they can get closer to an optimum Isp for early in flight. They can switch to a near-stoich hydrolox upper stage. Maybe we continue making advances in structural materials. Maybe there’s a small launch assist in the beginning (at least getting the vehicle to an altitude where vacuum-optimized first stage engines are feasible). Or they use gigantic expansion nozzles on the upper stage; higher chamber pressures; adjustable Isp. I can imagine achieving 20% or even 30% efficiency. Perhaps 100MJ/kg could be achieved without miracles. That lowers to about 300Gigawatts of chemical energy per year. Hydrogen may be more efficient to make (75%?), so maybe 400Gigawatts of average electricity per year… The US electric grid produces about 475Gigawatts average, so for the first time we’re below the US’s electrical output to power Elon’s 100 megaton/year dream.
However… Pumping all that water vapor up in the atmosphere could cause problems, too. But let’s say we avoid that somehow. There are other problems:
NOx emissions (nitrous oxide and similar) in the high atmosphere cause several problems. One is breaking down the ozone layer. Another is acid rain (although this is also part of the normal nitrate cycle on Earth, where lightning fixes nitrogen into the soil). Another is greenhouse effects… NOx emissions are approximately 250 times worse than CO2, pound-for-pound. It’s estimated that the Space Shuttle produced about 5% (but perhaps up to like 15%) of its reentering mass in NOx emissions (see: Global atmospheric response to emissions from a proposed reusable space launch system). Air-breathing rockets would make this worse by also producing NOx on the way up (see Skylon). We currently emit about 13 megatons of NOx every year from burning fossil fuels (compared to another 8 megatons annually from lightning). Considering the current fairly high dry mass of Starship, there’s basically about a 1-to-1 ratio of Starship mass reentered to payload delivered. So 1 megaton of payload would produce about 50,000kg of NOx. Not nothing, but not a showstopper. 100megatons, however, would produce about 5 megatons of NOx emissions… almost half of what we already make, but could be even higher, if the higher estimates of reentry NOx production are accurate (I don’t think they are).
However, I think we can do much better. The 2016 ITS tanker had a propellant payload to reentry mass ratio of about 4, reducing the amount of NOx production for 100 Megatons by a factor of 4 again. And we can maybe do better by changing the staging situation… Because NOx production and reentry temperature are really non-linear, there would be very little NOx production from a reentering 1st or 2nd stage in a 3-stage-to-LEO rocket. Falcon Heavy, in expendable mode, has an upper stage dry mass of around 4.5 tons (guesstimate from spacelaunchreport.com), and a payload of about 63.8 tons. That puts the ratio at about 14! Over an order of magnitude better than first-generation-Starship. Maybe knock that back to 10:1 for a reusable upper stage (but still using really advanced structures) for a launch vehicle optimized for this constraint, and we could be talking only 500,000tons of NOx per year. MUCH more manageable. To equal the current 20 megatons tons (combined human and lightning) NOx per year, we can reenter about 400 megatons of material, or launch (with an upper stage empty mass to payload ratio of 10) about 4 gigatons.
It may also be possible to scrub NOx from the atmosphere. This concept (backup link: https://doi.org/10.1007/s11356-016-6103-9 by Ming et al) suggests using a solar tower to help scrub NOx from the atmosphere and generate solar electricity at the same time. At really high launch rates, that might be necessary. In fact, any plan to use space resources to “deindustrialize” Earth (like Bezos and O’Neillians like to mention) would have to deal with the problem of (re)entry of massive amounts of material to Earth and the NOx emissions that causes.
It is also possible to pump Ozone into the stratosphere or maybe even suppress lightning to compensate.
Thinking long-term, what is the ultimate limit to ability to launch stuff with rockets, of any type? Current anthropogenic global warming from the greenhouse effect from fossil fuel emissions is much larger than, say, fundamental waste heat from any energy usage whatsover. Waste heat is on the order of 18 TW (same as primary energy usage), with global warming effect from fossil gas emissions (and land use changes) about 100 times that, so about 1-2 Petawatts. If we take current global warming level to be the ultimate limit that we could safely pursue long-term, then human society could grow to use approximately 100 times as much energy as it does right now relying on fossil fuels, or about 1-2 Petawatts. If all of that was used for chemical rockets with each kg of payload into LEO requiring 100MJ/kg, then we could get about 300 gigatons of payload into LEO per year before producing too much waste heat. Maybe with perfect launch systems, about 1 trillion tons per year.
So there are a lot of constraints. We, long-term, probably want to off-load much of that into space. That means maybe using solar-electric propulsion eventually. Before we get much beyond 1 megaton per year, I hope we’re looking seriously at scaling up solar electric propulsion, asteroid mining for propellant, and tethers. Using rotovators (discussed elsewhere on this blog), we could drastically reduce the amount of energy needed to be expended on Earth to launch payloads. And maybe just important (at that scale), we don’t need to use the atmosphere to slow down payloads to the surface of the Earth, either. Tethers combined with megastructures ~100km tall would allow payloads to be launched at higher efficiency and returned to Earth without massive aerobraking… in fact, even reducing reentry from 7.8km/s to 5.5km/s using a modest rotovator would halve the orbital energy input into the atmosphere and probably would non-linearly reduce NOx emissions as well.
Humans move on the order of 50-100 gigatons of material per year (with trucks, bulldozers, etc). See: https://www.sciencedaily.com/releases/2004/07/040709083319.htm#:~:text=In%201994%2C%20Hooke%20published%20the,%2C%20glaciers%2C%20oceans%20or%20wind.
That’s more than all the sediment moved by all the rivers of the world every year.
To move that much of material *to space* every year would require some clever thinking, but wouldn’t be impossible. It’d just take on the order of 100 Terawatts. Current solar cell prices are just 5.5 cents per watt… if they were illuminated constantly, that’s less than $10 trillion dollars worth of solar cells to move more material into space than all the material that all of humanity moves anywhere every year. And we might not even have to cook ourselves to do it.
However, Musk’s 100 Megatons to LEO every year would use up about a third of the world’s annual natural gas production. Might want to move beyond fossil fuels (and maybe optimize launch vehicle efficiency) if we’re going to really launch that much stuff…
And to continue the crude visual thought experiment from earlier, this is what it might look like if those 100 Megatons IMLEO were used to send 100,000 Starships to Mars at once each synod, with a total power output of approximately 2 Petawatts for 8 minutes, maybe even regionally outshining the Sun for a few minutes:
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