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As a follow up to Are there liquid fueled rocket boosters having coaxial fuel/oxidizer tanks? and thinking about JCRM's answer pointing the issue of having a huge contact surface between inner and outer tank, requiring efficient insulation between the two (which means relatively thick low density material)

Why did Orbex choose to use propane in combination with liquid oxygen, instead of for example methane, which boiling point is very close to oxygen's boiling point, allowing use of very thin insulation or even only structural skin?

  • How are the two coaxial tanks structurally linked together so that the entire system saves weight compared to classic stacked one above the other tanks? (maybe another question, even if it's all about coaxial tanks architecture, in this case.)
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Why did Orbex choose to use propane in combination with liquid oxygen, instead of for example methane, which boiling point is very close to oxygen's boiling point, allowing use of very thin insulation or even only structural skin?

Methane's freezing point, 90.7K, is practically the same as oxygen's boiling point 90.2K. Liquid oxygen could easily freeze methane in a shared-wall tank.

Propane stays liquid down to 85.5K, so both it and LOX can stay liquid over a 4 degree range.

(All figures from Wikipedia, I assume for sea level pressure, so pressurized in tanks the ranges are probably a little different.)

Liquid propane is also slightly denser than liquid methane; this leads to more compact tankage and a smaller rocket, which, depending on the engineering assumptions you make, can outweigh the slight specific impulse advantage of methane/LOX.

The gray lines on the chart from this answer represent payload mass fractions as a function of density and specific impulse given one particular set of assumptions -- you get more payload as you either go up (higher ISP) or to the right (higher density):

enter image description here

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    $\begingroup$ Methane's boiling point is 111K, not that far above oxygen's, and very different from propane's 231K. $\endgroup$ – Russell Borogove Aug 16 at 21:03

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