How much mass can we put in orbit before running into atmospheric constraints?

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.

Building 100 Starships/year gets to 1000 in 10 years or 100 megatons/year or maybe around 100k people per Earth-Mars orbital sync

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:

1000 Starships doing TMI
(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, 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: 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:,%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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16 Responses to How much mass can we put in orbit before running into atmospheric constraints?

  1. gbaikie says:

    Musk doing this many rockets because lifting rocket fuel into orbit.
    In beginning stage of mining lunar water and making rocket fuel, I don’t think you will ship lunar rocket fuel to LEO. Instead to you would ship LOX to Low lunar orbit and be enabling reusable rockets to be used to get the the Moon.
    But after decades to lunar water mining, you would or could be shipping lunar rocket fuel to LEO.
    But kind of wasteful energy-wise bringing rocket fuel down from high orbit Earth to low orbit. And I generally think one should to stage to Mars, from from High earth orbit to go Mars- high earth to highly elliptical and with burn at low perigee.
    But back to Musk, he mostly bringing LOX to LEO.
    And I think one needs fair amount of water to go to Mars. Or 100 tons payload with 100 people would have large portion of 1 ton per passenger being the water the passengers would need {and I don’t think he is allowing for enough water}.
    So could bring less LOX and water to LEO, go to high earth, get Lunar LOX and lunar water.
    Another aspect is a lot rocket fuel being used for the gravity loss.
    Quite awhile ago NASA was claiming a Mag Lev {zero stage/launch assist} could reduce rocket fuel use by 30% {as I recall}.
    It would be wild Mag Lev to lift a Starship. And like my idea, which I call pipelauncher. And what pipelauncher does is lower gravity loss. Though main thing is it launches rocket from the ocean.
    And I think if going to launch such large rockets and so many of them, one should be launching them from the ocean.

  2. Jardinero1 says:

    Sorry. Your facts are wrong.

    Total production of terrestrial CO2 production per year, human and non-human is actually 33 gigatons(yes gigatons). Over 99 percent of that is absorbed via natural processes. A fraction of a percent is not absorbed, hence slowly increasing CO2 levels in our atmosphere.

    Human related production of CO2, mainly from fossil fuels, is currently around 40 Billion tons, per year. In the scheme of things, another .75 billion tons of CO2, on top of 33 gigatons, won’t make any difference.

  3. Chris Stelter says:

    Another .75 Gigatons makes a difference over a long period of time if it’s ABOVE the amount that is naturally produced and absorbed. There is a natural yearly, seasonal cycle of CO2 emission and absorption which is larger than the net emissions, BUT the net effect means CO2 increases over time.

    Humans are becoming enormously powerful. So powerful that some of our processes can exceed natural processes’ ability to compensate, over a timeline of decades and centuries. It’s a good idea for us to face that fact, not pretend it doesn’t exist…

  4. gbaikie says:

    .75 billion tons of carbon is 2.75 billion tonnes of CO2
    US current CO2 emision is
    “In 2019, around 5.13 billion metric tons of CO2 emissions were produced from energy consumption in the United States.
    In 2018, around 36.6 billion metric tons of carbon dioxide was emitted globally.”

    5.13 + 2.75 = 7.88 billion metric tons of CO2 emissions US emission.
    Or would increase Total US Co2 emisison by more than 50%.
    That alone should cause greenies to murder Musk.
    In terms of world and 10 times as much: 27.5 billion tonnes of CO2
    36.6 + 27.5 = 64.1 billion tons of CO2 {more than 70% increase}
    But in terms of US Emission
    5.13 + 27.5 = 32.63 billion tonnes of CO2
    623% increase in US CO2 emiision

    I would say if Musk increases US CO2 by 10%, he will be prevented by politic forces.
    Or he will not be allowed to emit 500 million tonnes of CO2.
    “Each Starship takes about 1000 tons of methane (very rough numbers…)”

    Starship, wiki:
    First stage – Super Heavy
    Propellant mass: 3,400,000 kg
    SpaceX Raptor, wiki:
    Mixture ratio: 3.54 or 3.8
    So going to have 3.54 kg of LOX per 1 kg methane or 3.8 kg of LOX per 1 kg of methane
    “When methane burns, CH4 + 2O2 gives CO2 + 2H2O, about half of the energy comes
    from the burning of the hydrogen, and that produces no CO2.”
    We going to look at both:
    Carbon is 12, 4 hydrogen is 4 + oxygen is 4 x 16 = 64
    Methane is 16 and oxygen is 64 {16 x 4 is 64]
    or 1 kg of methane needs 4 kg of O2
    If rocket fuel mixture is 3.54 If you have 4.54 kg rocket fuel you have 1 kg of methane
    3,400,000 / 4.54 = 748,898.678 kg methane
    3,400,000 – 748,898.678 = 2,651,101.32 kg LOX
    748,898.678 x 2.75 = 2,059,471.365 kg of CO2

    And I say it breaks {politically speaking} at 500 million tonnes or 500,000 million kg of CO2 per year

    Each starship does about 2 million kg of CO2 so, 250,000 launches per year
    250,000 / 365 = 648.9 launches per day.

    Not saying Musk will be allowed to do 10% of US total CO2 emission.
    But merely the sound of +100 launches a day anywhere around people would another political problem- so, I say launch from a vast and empty ocean- where there are no people to hear you scream.
    Though some might worry about what the whales hear. But hear that whales make huge loud racket under water, and it might be like rock concert for them.

  5. Jardinero1 says:

    I want to clarify something: The premise of this article is that SpaceX needs a billion tons of CH4, per annum, to fly 10,000 flights per year. Right?

    10,000 flights at 1000 tons of CH4 per flight is 10 million tons of CH4, not a billion tons. That is a couple orders of magnitude difference. Correct me if I misunderstand.

    With regard to the CO2 this 10,000,000 tons of CH4 would create: The weight of CH4 is 16.04 g/mol. If it oxidizes completely, then each molecule of CH4 becomes four molecules of CO2, and some water. Each molecule of CO2 weighs 44.01 g/mol. Burning one ton of CH4 produces CO2 weighing almost 11 times as much as the original CH4. So 10,000 flights produces just shy of 110 million tons of CO2.

    Please correct any errors I may have committed.

  6. gbaikie says:

    “I want to clarify something: The premise of this article is that SpaceX needs a billion tons of CH4, per annum, to fly 10,000 flights per year. Right?”

    The premise is 1 starship flies 3 times a day and 1000 flights per year
    10 starship flies 3 times a day and 10,000 flights per year.

    And if build 100 starships a year and get 1000 starship in 10 years, then:
    1000 starships 3 times a day is 1 million flights per year.
    [Or 3000 launches per day times 365 days is 1,095,000- and calling it 1 million.]
    Of course how would do this, exactly.
    Maybe have 6 launch sites, all in same inclination spaced 100 km apart {and they all go to same trajectory/orbit {??}, second one catches first one, docks refuels, deorbits, 3 rd one docks, refuel, deorbits, etc. And repeats 2 more times in the day and 6 sites are launching 500 launches per day with 5 pads or 100 per pad- no that’s going to work- 50 pads, and 10 launches from each pad each day}.
    As I said greenies will murder Musk at somewhere around 648.9 launches per day and/or rest of US would stop him. Assuming there still was the global warming {caused by CO2 emission} religion. But that was at adding 10% to US CO2 emission}.
    Now maybe Musk can get some off-sets by making some nuclear powerplants- need to say, double US current nuclear powerplants}. Though growing trillion trees in Sahara desert, might cost less, and be faster.

    –10,000 flights at 1000 tons of CH4 per flight is 10 million tons of CH4, not a billion tons–
    Yes, but 1 million flights per year is roughly billion tons of CH4.

    ” The weight of CH4 is 16.04 g/mol. If it oxidizes completely, then each molecule of CH4 becomes four molecules of CO2, and some water.”
    No, one 1 molecule of CO2 and 2 molecules of H2O.
    But one can simply times the weight methane by 2.75 to get resulting mass of CO2.
    Or I said: 1 kg of methane needs 4 kg of O2.
    the combined mass of 5 kg gets 2.75 kg CO2 and 2.25 kg of H2O

    As far amount Natural Gas:
    “U.S. natural gas production grew by 10.0 billion cubic feet per day (Bcf/d) in 2018, an 11% increase from 2017. The growth was the largest annual increase in production on record, reaching a record high for the second consecutive year. U.S. natural gas production measured as gross withdrawals averaged 101.3 Bcf/d in 2018”

    growth 10 billion per day is 3.65 trillion cubic feet per year.
    Or 2018 total was 101.3 day which is 36.974.5 trillion cubic feet
    “0.678 kilograms per cubic meter of natural gas.”
    1 billion cubic feet = 28,316,846.592 cubic meter
    1 billion cubic feet = 19,198,821.989 kg
    1 trillion cubic feet= 19,198,821,989 kg natural gas
    50 trillion cubic feet= 959,941,099 tons
    Or US does not currently make 1 billion tons of CH4 in one year.
    So, in addition to planting a trillion trees, it seems Musk will have mine Methane Hydrate from the Ocean.

  7. Jardinero1 says:

    @gbaikie The plain text says, “the design goal for Starship was 3 flights per day, for about 1000 flights per year per Starship… 10 Starships flying a total of 10,000 flights per year to reach 1 megaton in orbit.” Stelter can clarify that if he wants to.

    @gbaikie Thanks for correcting my CH4 to CO2 math. Each CH4 molecule creates one CO2 molecule(not four-blame dyslexia). So one ton of methane, fully oxidized will yield 2.74 tons of CO2. That means 10,000 flights yields 27.4 million tons of CO2. My original point, in the first comment, was that 10,000 flights contributes little more than a rounding error to the global CO2 budget of 33 gigatons.

    Total CH4 production in the USA is about 31 trillion cubic feet or about 536 million tons at 57,803 cubic feet per ton. The 110 million tons which SpaceX requires for 10,000 flights is a big chunk of total domestic production. That’s a big problem. But the bigger problem is not the volume but the delivery capacity. There does not currently exist the pipeline capacity to carry that much CH4 to any single existing launch site or sites in the world.

  8. Chris Stelter says:

    @Jardinero1: So, I apologize for the lack of clarity. I’m talking about two different flight levels: 100 Megatons IMLEO/year and 1 Megaton IMLEO per year, requiring approximately 1 billion tons and 10 million tons (respectively) of methane. (Each flight requires about 1000 tons of Methane to deliver about 100 tons of payload… Liquid oxygen requires only 1% of the energy to produce per kg of propellant, so it’s not significant here.) So the US could provide enough natural gas for half of the 1 billion tons of methane (corresponding to 100 Megatons IMLEO/yr).

    As far as capacity for 1 Megatons IMLEO/yr, there’s an LNG export terminal, Annova LLC, that’s planned to be literally only 11km away from SpaceX’s Boca Chica site. It’ll have a capacity of about 7 million tons per year of LNG… with a modest improvement in Starship efficiency (lower dry mass, higher Isp, etc), that’d be approximately enough all by itself for the 1 Megaton IMLEO/yr goal. 1 Megaton IMLEO/yr is enough to get 1 million tons of payload to Mars per synod if SpaceX leverages Solar Electric Propulsion (SEP). (What a “coincidence” that SpaceX picked Boca Chica, which has nearby access to super cheap LNG in enough quantities to establish a Mars City…)

    (They don’t currently plan to leverage SEP, but they DO have the technology for really cheap, mass-produced SEP capability with their Krypton thruster propelled Starlink propulsion system… Krypton being important since there’s nowhere near enough Xenon production capacity for their ambitions… They may even need to switch to Argon, actually, given that I think the global krypton market is probably only like 1000-10000 tons per year… plus Mars has plenty of Argon.)

  9. gbaikie says:

    A pipelauncher is simple, a big long pipe which one end capped.
    If put a big long pipe with one end capped in water, it will fill with water, and float vertical. It’s sort of like how ship sinks with bow or stern stuck up in water before sink into ocean. Or one talk about ship designed to float vertically, RP FLIP:
    Wiki says:
    “When flipped, most of the ballast for the platform is provided by water at depths below the influence of surface waves, hence FLIP is stable and mostly immune to wave action similar to a spar buoy.”
    Which important aspect of pipelauncher, it’s very stable. until you make it unstable by launching rocket from it. Though good news it it’s only unstable for several seconds, when going up and down.
    Anyhow it’s fairly simple. But to lift 5 million kg rocket, it seemed to need it to be more complicated.
    Roughly speaking to lift a 5 million kg rocket, it seems the pipelauncher could made from steel and would weight about 2000 tons {due to mass of steel}. Steel could be needed, due to it’s density and the balancing the 5 million kg rocket which is quite tall.
    Or pipelauncher needs to be tall and need material denser than water.
    Anyway this complicated version of pipelauncher, seems it could be powered by about 100 tons of liquid air and 100 tons of hotwater. Therefore the hottest it gets is the 90 C water. And “typical verison” it uses some methane to heat air, and general idea would be not to get air much warmer than 100 C. Though if getting hotwater is hard, or this thing need it, could use some methane to heat the air. Or if throw a ton of liquid air into ocean water, ocean water turn it to gas, but it’s colder gas. But ton of 90 C water will make ton liquid air into about 30 C air.
    Anyhow it’s planned push starship to speed close to 150 mph or 67 m/s straight up.
    How much rocket fuel would this save.
    It’s not simple question. It effect max Q and one has some rocket engine improvement and largely about lowering gravity loss.
    Or would say if can lower gravity loss by say 20%, that would be significant- and think it does better than that.
    Of course major problem with pipelauncher is the infrastructural cost of every else other than this launch pad that go up.
    But this topic of this post is about lot launches. And say any over 50 launches year per pipelauncher, should work.

  10. Chris, I suggest that you might want to look at this from the standpoint of a carbon tax. I think this is almost inevitable in the timeframes you’re talking about, it will apply to all the CO2 equivalents for all greenhouse gas emissions, and it will be proportional to the amount of CO2 equivalent gases in the atmosphere above some baseline.

    I’m way too lazy to work through this kind of model, but it has a couple of features that seem intuitively obvious:

    1) The number of Starships (or any other rockets) you can launch will be much more dependent on the rest of the GHG-emitting economy than they are on their own emissions. That means that arguments like “the percentage of emissions from launch are so small that they’re not an issue” simply won’t fly. If a launcher is an emitter, it will get the bejeezus taxed out of it, along with all of the other emitters.

    2) The economics of lots of IMLEO will wind up being fixed at some optimum, beyond which you lose money if you try to launch more.

    I also think that the problems associated with water vapor deposition in the upper stratosphere and mesosphere have been given short shrift, and may make any high-scale launch industry much more problematic. These could wind up being assigned CO2 equivalent values that were hundreds of times those of water emitted in the lower stratosphere, where stuff drops through the tropopause fairly quickly.

    One final thought: The launch industry could get an awful lot of carbon offset credits if it went all-in on space-based solar power. I don’t think SBSP is viable unless most of the mass for it is coming off the Moon, but there’s still a lot of Earth-based equipment and materials involved. A credit scheme for the zero-emission power could change the economics of launch considerably, even in an environment where all GHG emissions are stringently taxed.

  11. Chris Stelter says:

    That’s a good point about water vapor, but I think you’re somewhat over-stating the case. For one, it’s dependent on time of day (at least for aircraft). If done during the day, there sometimes (and under certain conditions) can even be a negative forcing due to sunlight being reflected.

    Also, dealing with radiative forcing is probably easier than dealing with CO2’s effects. There are some things which cause negative forcings and thus could counter-act water vapor but which do not impact CO2 level.

    So we can deal with this problem in two ways:
    1) by improving the efficiency of rockets generally. Go from the current estimated 5% efficiency (for fully reusable chemical rockets like Starship) to 10 or even 20%, perhaps using some launch-assist.

    2) Ironically, if the problem is that water vapor is a MUCH stronger greenhouse gas than CO2, the solution may be to just use a CO/O2 rocket whose exhaust is primarily CO2 with no water vapor (and then capturing CO2 directly from the air equal to the amount emitted… in order to split apart into more rocket propellant, so totally carbon neutral).
    I’m going to write a blog post on this (as I have been thinking about the same problem for years), but suffice it to say that this isn’t as much of a penalty as you would think.

    Yes, the lift-off mass needs to be 3 times as great, BUT the propellant requires almost a third the energy to create as stoich hydrolox would (and, in fact, if you’re using typical hydrolox O:F mix ratios like 5.5:1 instead of near-stoich 8:1, your difference in efficiency is basically nil…). There’s a propellant density advantage with using CO/O2. It does, however, take about 0.9MJ (250Wh) to capture 1kg of CO2 from the air, but that’s only like a 10-20% energy penalty. Overall, there’s about a 50% greater propellant energy requirement for CO/O2 than hydrolox under similar assumptions (and maybe a smaller difference with methalox), which works out to an extra $1-3/kg cost per kg IMLEO (depending on if electricity is 3 cents or 10 cents per kWh, including the capital cost of electrolysis and liquefaction). That’s really pretty minimal compared to the extremely high CO2-equivalent tax you were proposing for launch vehicle water vapor.

    Plus, a high pressure, pump-fed, near-stoich (if possible) CO/O2 rocket engine is absolutely perfect for Mars. Mars rover Perseverance will be testing CO/O2 production from Martian atmosphere in hopefully less than a year from now!

  12. Chris, I think you’re conflating water vapor and contrails in your response above. The amount of water vapor emitted is constant for any particular engine. Whether it forms a contrail (which will indeed briefly increase albedo), is dependent on pressure, temperature, and humidity. But ice in the stratosphere–particularly the upper stratosphere–is going to sublime pretty quickly, leaving just good ol’ water vapor, which of course is very happy to soak up IR radiation.

    The real question is what mechanisms exist to transport water vapor out of the stratosphere. In the lower stratosphere, convection coming up through the tropopause will eventually get it to condense, but since the stratosphere temperature increases with altitude, there’s no convection in most of it. So, mod the photolytic effects that produce ozone, I think it probably just stays there. That’s a problem.

  13. Chris Stelter says:

    Perhaps you’re right about that. I was using airline discussions of water vapor climate forcings as a correlate to rocket water vapor forcings…

    …but keep in mind that the higher you get, the lower the density and the greater the mean free path. Longer mean free path (orders of magnitude greater) means greater diffusion. So it will diffuse faster up there, to the extent that diffusion becomes significant even if convection is not great.

    Third, a CO/O2 rocket addresses the problems with water vapor, as CO2 normally has a high residence time in the atmosphere, so the long residence time in the upper stratosphere will have proportionally less impact (besides being much less strong of a greenhouse gas).

    And third, if water vapor (which is indeed a much stronger greenhouse gas than CO2) DOES have a much longer residence time in the upper part of atmospheres, perhaps this could be used on Mars for terraforming…

  14. spacerfirstclass says:

    The premise of this blog article is that SpaceX will build 100,000 Starships, and send them to Mars every synod? That seems to be an incorrect interpretation of what Elon Musk said, I believe when he said “100 megatons/year”, he meant the potential LEO capability of the Mars fleet, not the actual tonnage needed for Mars colonization.

    He outlined his Mars colonization plan in previous tweets:
    Q: What about what you said about 1,000,000 tons of cargo to Mars for a self-sustaining city… real estimate, back-of-envelope calc or figure of speech?
    A: Approx min payload to Mars to nearest order of magnitude, so at $100k/ton, cost would be $100B

    Q: How many starships you wanna build
    A: A thousand ships will be needed to create a sustainable Mars city
    A: As the planets align only once every two years
    A: So it will take about 20 years to transfer a million tons to Mars Base Alpha, which is hopefully enough to make it sustainable

    Q: But Elon, Earth-Mars transfer windows only occur every 26 months, what are the missions for these starships going to be during these 2 years waiting time?
    A: Loading the Mars fleet into Earth orbit, then 1000 ships depart over ~30 days every 26 months. Battlestar Galactica …

    So to summarize, he envisions building 1000 Starships in 10 years (all Mars version, this ignores the tankers, but I guess given reuse you don’t need many tankers), and each synod send this 1000 Starships to Mars, which will land 100k tons of cargo. Keep doing this for 10 synods (20 years) will land 1 megaton of cargo at Mars (btw, this seems to assume the cargo ship will do opposition-class mission), which he estimates would be enough for for self-sufficiency. Each Mars Starship needs 1,200 tons of propellant and 100 tons of cargo in LEO, so you need 1.3 megatons in LEO every synod, not 100 megatons/year.

    The 100 megatons/year is clearly not realistic, for example there’s no way SpaceX can build 100,000 Starships anytime soon. Boeing only builds 500 to 600 737 per year, even if SpaceX builds Starships at the same rate it would take 200 years to build 100,000 Starships.

  15. Chris Stelter says:


    The premise of the article is exploring the consequences of some relatively recent Mars tweets about how much mass will/can be launched into orbit. Earlier tweets about the *minimum* level of mass needed to be sent to Mars are much less ambitious than Musk’s more recent tweets. 1 megaton to Mars over 50 years is about an order of magnitude less ambitious than even the low-end 1 Megaton per year IMLEO tweet, which in turn is two orders of magnitude less ambitious than the 100 Megaton/yr IMLEO tweet.

    As far as the realism of 100 Megatons/yr IMLEO tweet, I’m merely exploring the consequences of it. SpaceX’s ambition is greater than Boeing. Elon is considering society-level capacity as well, not necessarily SpaceX alone. This is in the context of SpaceX perhaps someday pursuing larger Starships (imagine, say, 18 meter diameter instead of 8 meter…). And SpaceX hopes to build Starships for much less money than Boeing sells 737s for. Again, I’m just taking Musk’s tweets at face value, here, and seeing if the atmosphere can even sustain it. Musk is, in truth, mostly just handwaving some orders-of-magnitude, here, not making any kind of firm long-term plans or projections. Maybe even some Overton Window shifting going on, making people think beyond our measly 1000 tons to orbit per year (on a really good year) level.

    Again, you’re confusing Musk’s earlier, less-ambitious tweets with Musk’s more recent sort of crazily ambitious tweets which probably would put so much water vapor in the upper atmosphere as to cause warming equal all other current annual human global warming emissions combined… (So clearly this will require different technology than Raptor to prevent cooking ourselves…)

  16. gbaikie says:

    –(So clearly this will require different technology than Raptor to prevent cooking ourselves…)–

    It needs rocket fuel to be made in space.
    I don’t know if Moon has enough water to mine to supply the rocket fuel needed.
    Particularly, if require Methane rather then LH2.
    But I always regarded that a significant part of mining lunar water to make rocket fuel, was to make a rocket fuel market in space- so starting lunar surface and low lunar orbit, and then expanding to High Earth and Mars orbits. And evenually the Moon would not be able to compete in the high Earth orbit rocket fuel market as cheaper water could mined from “space rocks”.
    Or I was thinking we needed to start a market of rocket fuel in space, before water was discovered on lunar poles. But problem was if you bring million tons of water to high Earth orbit, how could sell it.
    But at this point maybe all Musk needs to do is say he will buy it.
    Also thinking recently, that Space Force could buy 1000 ton of water, by paying for to be delivered from Earth to say high Earth orbit. And allow private sector use the water to make rocket fuel- sell it {to Space force or anyone].
    So get market starting and then if deliver a million tons of water to High Earth, one could sell it, because the technology making rocket fuel from water in space has been demonstrated. And water in space like any commodity would be sold in Earth commodity markets.
    But Musk could be the commodity market by choice.
    So, let’s say the price would be $100 per kg for water in high earth, or $100,000 per ton, and amounts talking about is 100,000 tons of water or gross value 10 billion dollars. That leaves the task of finding enough water, delivering to High Earth.
    If that done, Moon is viable place to go {doesn’t need to mine lunar water} and Mars
    is more viable destination {even if Mars doesn’t have “mineable water”].
    Not because water stays at $100 per kg, but because the price will lower further, if one party can find some water to bring back to earth high orbit, others can. And then also delivering water to Mars high orbit {which has less sunlight, but one could bring higher volumes of water to it- or using nuclear power to split the water}.
    And of course if have cheap rocket fuel in high Earth orbit, the delivery cost to get it to LEO is not much.
    So mining lunar water was indirect path to getting a market for water in High Earth, and one could take a direct path to it. Particularly, if in hurry, like Musk is.
    But I think NASA job is to create markets in space.
    So explore Moon to see if the there is mineable lunar water, and then explore Mars to find places which which would more viable settlements {the same thing as market places} on Mars. Now, NASA should doing more to find space rocks which might hit Earth {and later, Mars}- and also would include finding space rocks which are mineable.

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