Cryogenic slush (a mixture of liquid and solid) can potentially store propellants at higher density than cryogenic liquid. For instance, hydrogen slush is 16-20% denser than liquid hydrogen. https://en.wikipedia.org/wiki/Slush_hydrogen Hydrogen slush is stored at hydrogen’s triple point.

SpaceX could potentially achieve similar increase in methane density if its liquid methane propellant is sub cooled and stored as slush. This assumes that solid methane is more dense than liquid methane, but I could not find data on this.

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I understand that SpaceX sub-cools methane (stores it below its boiling point at tank pressure, which is about -130C). But methane slush would require “sub-sub-cooling” to its triple point of -182C.

In fact, according to https://www.wevolver.com/specs/spacexs-starship-sn24-bn7 , liquid methane in Starship is “chilled to -180C “ which is very close to its triple point of -182C. Evaporative cooling, especially near the end of flight when remaining volume is low, could refrigerate to the point of slush formation.

Does Starship run on slush? What are the challenges of using cryo-propellants near their triple point?

More info on methane slush: https://ntrs.nasa.gov/api/citations/19700009484/downloads/19700009484.pdf

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    $\begingroup$ if they were doing it they'd be bragging about it. $\endgroup$
    – Erin Anne
    May 15 at 16:27
  • $\begingroup$ This is an interesting question, but why not ask more generally if any rocket has attempted to implement this idea? $\endgroup$
    – phil1008
    May 15 at 21:21
  • $\begingroup$ Ya, "does" is a pretty restrictive question and may be hard to answer authoritatively, where as "could" or "has any ever" would open this up to potentially more, and more interesting answers, and "what are the challenges" would do so even more. "Does" constrains the answer space so tightly. $\endgroup$
    – uhoh
    May 16 at 0:17
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    $\begingroup$ @uhoh ... According to the numbers, Starship methane is within a whisker of its triple point. If solid methane is denser than liquid methane (unlike water ice), it will sink towards the intake and slush will concentrate there, This could have consequences for filters and turbines. If SpaceX is pushing the lower limit of methane temperature (say, to keep it as close as possible to LOX temperature or to increase the density of liquid methane), slush could cause significant design and operational issues. But only of slush DOES form. $\endgroup$
    – Woody
    May 16 at 0:34
  • $\begingroup$ @phil1008 ... any information on cryogenic slush Would be interesting. Particularly interactions with filters and turbines, or design trade-offs near triple points. Feel free to submit an answer. $\endgroup$
    – Woody
    May 16 at 1:32


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