The Energy Consulting Group

Management consultants for upstream oil and gas producers and service companies

Knowing that methane (liquified natural gas-LNG) will fuel most next generation rockets, the natural questions to ask are: how much methane and how will it be sourced?

To help answer these questions, let's start by looking at how SpaceX plans to use methane to fuel Starship, its next generation rocket.

How Much Fuel in the Form of Methane Will the Starship Use?

Our analysis indicates that a fully configured Starship launch (booster and Starship) will use about a 1000 tonnes of methane in the form of LNG as fuel.  This is equivalent to approximately 50 million standard cubic feet (mmscf) of methane. In addition, each such launch will also use nearly 4000 tonnes of liguid oxygen.

According to Darrell Etherington of TechCrunch, Elon Musk, in response to questions during a Twitter interview, indicated "that the spacecraft is being designed with the plan of flying it for an average of three flights per day, each carrying over 100 tons of payload per flight, for a total of more than 1,000 flights per year, per vehicle."  Mr. Musk also stated in the same interview that he is working towards having a fleet of 1000 such spacecraft by the end of this decade.  The spacecraft referenced in the quote is the Starship currently undergoing development and testing at SpaceX's  Boca Chica, TX facility.

This works out to be about 150 mmscfd per Starship, or about 150 billion cubic feet of methane/natural gas per day (bcfd) for a fleet of 1000 rockets.  150 bcfd of natural gas is roughly equivalent to 25 million barrels a day of oil.  For reference, recent (2019) US demand for methane in the form of natural gas averaged about 82 bcfd.  In essence, Mr. Musk is suggesting that US demand for natural gas could grow dramatically over the next 10 or so years due to SpaceX activity.

Space May Be the Final Frontier for Natural Gas Demand Click on Image For Higher Resolution Version

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How Will The Methane Be Sourced for Starship?

Information related to supplying methane fuel for the Starship fleet:  Potential approaches include buying natural gas on the open market, supplying from SpaceX owned/operated oil and gas wells or manufacturing methane using a methanation process, such as the Sabatier process.

Elon Musk and SpaceX acquire oil and gas leases with the apparent goal of drilling gas wells to supply growing space ambitions.

To supplement gas from natural gas wells, Elon Musk seeks to encourage research into "best carbon capture technology" with the ultimate goal being to transform the captured CO2 into methane for use as a rocket fuel.  MIT Technology Review

No, You Don’t Have To Worry About Emissions From SpaceX’s Mars Rocket  The author of the article posits that SpaceX can launch its Starship fleet in a carbon neutral fashion using Direct Air Capture (DAC) of CO2 and conversion of that CO2 to methane.  To perform the methanation of CO2 into a rocket fuel will require at least three steps, with DAC being the first step of the process and likely have a large, dedicated plant for that purpose.  The second step is to manufacture the hydrogen required, which will also likely require a  large dedicated plant.   And in the final step, a third plant that houses the Sabatier conversion process, which combines the CO2 and H2 into methane, will likely be required. At this time, we envision each of these plants being of similiar scale and cost.

Elon Musk:  SpaceX is starting a program to take CO2 out of atmosphere to turn into Starship rocket fuel  Per the linked twitter submission, Mr. Musk has announced a program, under the sponsorship of SpaceX, of turning "CO2 into rocket fuel".  The following commentary documents the likely techonological path and energy costs of doing just that.

Power Usage
An important aspect in manufacturing methane is the power required. Lets's start by focusing on hydrogen, which needs to be added to carbon to form methane.  Currently, hydrogen is sourced primarily via steam methane reforming (SMR) technology, which strips hydrogen from methane molecules. However, an alternative is to use the electrolysis of water, which breaks the water molecule into its constituent components-hydrogen and oxygen-with the capture of each.  We think it reasonable to assume that Musk/SpaceX expects to source hydrogen via electrolysis.

How much power might be needed for the hydrogen manufacturing?  About .25 kg of hydrogen will be needed for each kg of methane, so one, two stage Starship launch represents about 250 tonnes of hydrogen in the methane fuel.  The literature indicates that the electrolysis process uses between 40-50 kWh to generate 1 kg of hydrogen.  So, using the lower end of that range, the electric power required for 250 kg of H2 is about 10 mWh, which, when scaled up to the full launch amount, is about 10000mWh, or roughly 415 mw of generating capacity working 24 hrs a day is required to supply the hydrogen for a single launch.  If we then apply the potential launch cadence suggested by Mr. Musk, that works out to about 1245 mw of electricity generating capacity to supply the hydrogen via electrolysis for each fully configured Starship launching 3 times a day. However, because the amount of hydrogen actually needed to support the Sabatier process, as described below, is twice these volumes, the actual power required is 2 times 1245 mw, or 2490 mw.  As far as we know, SpaceX plans to source the electricity needed for the methanation process from renewables and not from fossil fuel plants.

Occidental to Strip Carbon From the Air No large scale plant to directly capture CO2 from the air currently exists, so the plant that Oxy is currently building, based on Carbon Engineering technology, is a first.  As such, it represents Direct Air Capture (DAC) state of the art.  In theory, it will be capable of capturing about 1 mmtpa of CO2 (about 2700 tonnes per day) and will cost about 1 billion dollars to build.   Per this article from cleantechnica,  this Carbon Engineering designed plant will consume between 5.25-8.81 gigajoules of power per tonne of captured CO2.  Using a 7 GJ/tonne average value, the power cost will be about 1.9 kWh per kg of captured CO2. 

Another question is how much methane can be produced from 2700 tonnes of CO2?  Assuming the Sabatier process is used (see below), it works out to about 980 tonnes of methane (CH4) per day, which is about enough for a single launch to orbit of a fully loaded Starship.

The Sabatier Process:  Conversion of CO2 and H2 into methane (CH4) and water (H2O)

Schematic of Methane production system for a Single SpaceX Starship over a period of two years, at about 150 tonnes of methane per year.  Sourced from marspedia.org. Michel Lamontage created this file. https://marspedia.org/User:Michel_Lamontagne

Source:  https://marspedia.org/Sabatier/Water_Electrolysis_Process ; https://marspedia.org/File:Propellant_production.png#filelinks

Based on the information in the flow chart presented above, we estimate that it takes about 8.1 kWh per kg of methane to drive a Sabatier process.  Two items of note:  First, this energy is over and above the heat energy released by the Sabatier process, which is exothermic.  Second, the above information upon which this energy cost is based is for a relatively small Sabatier plant.  Larger, continuously operated plants may realize economies of scale and operational efficiencies, but it is unclear at this time if that will be the case and, if so, what the impact might be, so until new information becomes available, we will use 8.1 kWh per kg.

Energy Cost of CO2 Methanation Process

The chemical formulation for the methanation of CO2 is:

                                                                           CO2+4H2==>CH4+2H2O 

Our estimate of the associated energy reqirements to supply the components and drive the process are:

   1.9kWh/kg CO2 to source CO2 via DAC + 40kWh/kg H2 to source H2 via electrolysis  ==>  8.1kWh/kg methane to run Sabatier process to convert CO2 & H2 into CH4

Working through the mole/mass math, we estimate it takes a total of around 33 kWh per kg of methane to manufacture the fuel via direct air capture of CO2, electrolysis of hydrogen, and the Sabatier process.  This works out to be approximately 99,000,000 kWh per day of energy for the manufacture of methane to support one Starship with a launch cadence of 3 launches per day. For reference, the average US home uses about 30 kWh per day.  Note:  most analysts use 50 kWh/kg for hydrogen electrolysis, but we believe advancing technology will likely improve the efficiency of the process to close to the theoretical limit, which is a little over 39 kWh per kg of hydrogen produced.  Of course, if those gains do not materialize, the full cycle power cost of CO2 methanation will be higher.  Also note, this assessment does not include the energy cost for the provision of liquid oxygen, nor the energy required to cool the methane into its rocket ready, liquid form.

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