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#1 2026-07-11 18:15:23
- tahanson43206
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Methane CH4 vs Carbon Monoxide CO Propellant Competition
This topic is offered for NewMars members to build up a collection of knowledge about these two fuels.
The occasion for this topic is conversation between Void and RobertDyck in July of 2026.
I understand (from Google Search) that Dr. Zubrin studied the fuels before ultimately deciding to recommend shipping hydrogen from Earth as the best way to manage refueling at Mars.
My guess is that the recommendation makes sense in the context of the time.
Now that we know water is abundant on Mars but difficult to extract, the balance of practicality may shift a bit back toward Carbon Monoxide.
A detail of physics that I did not know is that (apparently) the density of CO in liquid form is greater than CH4, so tanks can be less massive for the same lift capability.
There are many other details like that that keep popping up as I pursue this question with Gemini.
I'll post a brief summary from Gemini in #3.
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#2 2026-07-11 18:16:21
- tahanson43206
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Re: Methane CH4 vs Carbon Monoxide CO Propellant Competition
This post is reserved for an index to posts that may be contributed by NewMars members.
Index:
#3 Brief summary by Gemini of the comparison of CO and CH4 as rocket propellant
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#3 2026-07-11 18:19:03
- tahanson43206
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Re: Methane CH4 vs Carbon Monoxide CO Propellant Competition
In July of 2026 I started what turned out to be a long series of interactions with Gemini about the merits of Carbon Monoxide as a propellant at Mars, compared to methane CH4.
Here is a brief summary:
The Great Propellant Paradox: Isp vs. Density
When comparing Methane (CH4) to Carbon Monoxide (CO), rocket designers face a fascinating paradox where engine efficiency and material density fight to a standstill:
The Engine Trade: Methane is the undisputed king of efficiency, offering an Isp of ~370 seconds compared to CO's ~270 seconds. Methane requires far less total propellant weight to launch a payload.
The Tankage Trade: However, liquid CO is nearly TWICE as dense as liquid Methane. A Methane rocket requires roughly double the fuel tank volume of a CO rocket. Double the volume means larger, heavier hulls and more structural "dead weight" to push into the sky.
The Logistics Trade: While Methane saves mass in flight, it demands an immense infrastructure footprint on the ground to mine and purify hidden Martian ice. CO demands more fuel weight, but can be skimmed entirely out of the thin air from a static box, anywhere on the planet.
In aerospace systems engineering, a lower-performing engine attached to a smaller, denser tank can sometimes beat a high-performance engine attached to a massive, balloon-like tank. The jury is still out on which approach will ultimately win the funding race!
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#4 2026-07-11 19:16:55
- RobertDyck
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Re: Methane CH4 vs Carbon Monoxide CO Propellant Competition
Definition: Specific Impulse = 1 pound of propellant can produce 1 pound of thrust for how many seconds. It's a ratio so you could say 1 kilogram mass of propellant will produce 1 kilogram-force thrust for the same number of seconds. And 1 pound propellant could produce 2 pounds thrust for half the number of seconds, but Isp works out the same.
This means higher Isp is good. Low Isp bad. CO/LOX has Isp 250 to 270 seconds. LCH4/LOX has Isp 360 to 380 seconds. Higher density means lower tank mass, but Isp that different means much greater propellant mass. Liquid methane (CH4) has higher Isp so lower total propellant mass.
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#5 2026-07-12 09:39:06
- tahanson43206
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Re: Methane CH4 vs Carbon Monoxide CO Propellant Competition
In July of 2026, discussion of propellants for use at Mars led to questions about the behavior of three materials when cooled.
I asked Gemini to try to help. First, it showed how Python can be used to create a chart visible in PNG format. Then it prepared a text summary in BBCode. To finish this post, if all goes well, we will have an image to help our readers to follow the discussion.
Propellant Storage Realities at 5 ATM
Before we can calculate the total energy required to manufacture our propellant choices, we must first understand the physics of storing them as liquids. In a real vehicle or storage depot, we do not store these gases at sea-level pressure (1 ATM). We must maintain a pressurized environment—modeled here at 5 ATM—to prevent pump cavitation and stabilize the liquids.
When we raise the pressure to 5 ATM, the boiling points shift upward, widening the liquid storage windows. However, the thermal relationships between our choices reveal a significant engineering challenge:
Liquid Oxygen (LOX): Liquid range shifts to between -218 C and -164 C.
Liquid Methane (CH4): Liquid range shifts to between -182 C and -143 C.
Liquid Carbon Monoxide (CO): Liquid range shifts to between -205 C and -174 C.
The Thermal Overlap Problem
The Methalox Advantage: Notice the beautiful thermal overlap between Methane and Oxygen. At 5 ATM, Methane remains a liquid down to -182 C, while Oxygen becomes a liquid at -164 C. They share an 18-degree window where they can coexist dynamically, allowing common-bulkhead tank designs with minimal thermal insulation between them.
The Carbon Monoxide Penalty: Carbon Monoxide is a severe cryogenic fluid. At 5 ATM, its boiling point is -174 C. This is significantly colder than the freezing point of Methane (-182 C). If Liquid CO shares a tank wall or comes into close structural contact with Methane, it will freeze the Methane solid.
The Production Chemistry Baselines
To help frame our upcoming energy calculations, here are the baseline chemical equations for both production tracks, using standard text notation:
Track 1 (Atmosphere Only): 2 CO2 + Energy -> 2 CO + O2
Track 2 (Atmosphere + Mined Ice): CO2 + 2 H2O + Energy -> CH4 + 2 O2
The attached chart, generated via Python, visually demonstrates these pressurized liquid phases side-by-side.
And here is the chart generated by Python as written by Gemini at the request of tahanson43206
In a recent post in July of 2026, Void noted the temperature of -182 Celsius as coincident for all three gases, at 5 ATM.
This series will attempt to discover and to report the practical issues that will face the business entities that will come into being on Mars to support space flight.
It seems likely to me that both approaches will be supported by investors, but I expect that stock holders will do better with the Carbon Monoxide investment.
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#6 2026-07-12 14:10:29
- SpaceNut
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Re: Methane CH4 vs Carbon Monoxide CO Propellant Competition
the issue is that Mars needs energy before we can even think about the above chart.
Compressing Martian atmospheric CO₂ (ambient ≈ 0.006 atm) to 5 atmospheres requires ≈ 0.23 to 0.25 kWh of electrical energy per kilogram of gas, depending on your compressor efficiency. Assuming an average Martian density, this means about 4.6 to 5.0 kWh are needed to pressurize 1 m³ of this
compressed gas.
Key Details & Calculations
Input Pressure: ≈ 0.006 atm (610 Pa)Target Pressure: 5 atm (506,625 Pa)Compression Ratio: ≈ 833:1Isothermal Compression Energy: ≈ 0.22 kWh per kilogram (ideal minimum).
Real-World Energy Cost: ≈ 0.25 to 0.30 kWh/kg to account for system losses, cooling, and heat rejection.Related In-Situ ProcessesOxygen Extraction (MOXIE & Plasma): NASA's MOXIE experiment split CO₂ into O₂ and CO, achieving energy costs of roughly 0.19 kWh/g of O₂. Advanced cold-plasma microwave reactors can lower this to ≈ 0.013 to 0.04 kWh/g of O₂.
Liquefaction: If you need to compress CO₂ into a liquid state for propellant (like Methalox fuel) or storage, you need to compress it to roughly 60 atm (870 psi) and cool it below 31° C, which requires roughly 0.4 to 0.5 kWh/kg.
Sabatier Process: If combining this CO₂ with hydrogen to produce methane and water, the exothermic nature of the reaction reduces net power requirements, though it is usually tied to energy-intensive water electrolysis to produce the hydrogen.
Are you planning to use this CO₂ for greenhouse pressurization, propellant manufacturing, or an oxygen generation system (ISRU)? Knowing this will help narrow down the exact reactor footprint and total power generation needed.8 sites
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