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In this paper about the SABRE engine, there is the following paragraph (emphasis mine):

The water vapour in the atmosphere up to an altitude of around 12 km is a problem for precooled engines, causing them to block with frost in a matter of seconds. A great deal of this water is precipitated in the liquid phase during the cooling of the air and has to be rejected from the engine before it can freeze. Provision then has to be made to stop the build up of ice within the matrix as it precipitates directly from the vapour. A major part of the experimental program has been to demonstrate that this can be achieved.

How might this be achieved? If there is more than one possibility, is there a most likely or an easiest way of doing this?

This should be seen as more of a brainstorming question than as an opinion-based one. After all, Reaction Engines was most secretive about the mechanism, so there will probably not be a definitive, public answer any time before 2013.

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  • $\begingroup$ Related space.stackexchange.com/questions/26689/… $\endgroup$
    – Steve Linton
    Commented Nov 15, 2019 at 21:02
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    $\begingroup$ It is explained, in great detail, in patent GB2519153, also published as US patent US20150101334A1 TL;DR: this answer has the jist of it. $\endgroup$
    – user20636
    Commented Nov 15, 2019 at 22:15
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    $\begingroup$ You might also care to look at how it was achieved for Bond's previous air breathing hydrogen rocket engine, "Swallow" (or Rolls-Royce RB545) -- Due to this method being declared an Official Secret by Her Majesty's Government he had to come up with a different solution for SABRE, as in several other areas the one he came up with was better than the old one. $\endgroup$
    – user20636
    Commented Nov 15, 2019 at 22:21
  • $\begingroup$ @JCRM The more I read about him, the more fascinated I am. Will check out the documentary on him at some point. $\endgroup$
    – Everyday Astronaut
    Commented Nov 17, 2019 at 21:16

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According to an article in the July 20, 2015 issue of Aviation Week, they spray methanol into the precooler to defrost it.

"We inject the methanol at one of the coldest points and we effectively get the mixture of water and methanol to flow forward in the matrix - against the direction of the airflow.... We have multiple injection and removal points in the matrix....Eventually you end up with a situation where you have extracted all the water vapor as liquid from the airflow."

-- Reaction Engines Chief Engineer Richard Varvill as quoted in the article

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Available in the archive but may be paywalled

h/t to JCRM who provided a link to the US patent which has additional detail.

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  • $\begingroup$ Interesting... is the methanol a consumable which must be re-provisioned for each flight, or is there some sort of recovery cycle for it? $\endgroup$
    – Anthony X
    Commented Nov 17, 2019 at 0:34
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    $\begingroup$ TL;DR: In Skylons A-C it would be a consumable, in LAPCAT it would be topped up (probably after each flight). In more detail @AnthonyX, paragraph 238 the patent suggests the methanol/water could be added back into the airflow and could add 2% to the thrust, in those cases where losing the weight of the methanol is desirable, thus making it a consumable, and paragraph 245 discusses separating the methanol to reduce the total weight of methanol carried, but that would be at the cost of the permanent weight of the distillation (or other) equipment. $\endgroup$
    – user20636
    Commented Nov 17, 2019 at 10:45

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