The SABRE engine is designed to be used on a single-stage-to-orbit type of spaceplane. What sort of delay might occur between shutting down the air-breathing operation and switching to a closed-cycle operation (including any spin-down and cooling)?

An exact number may not be known yet, but certainly there must be some design goals. It's not likely to be 1 hour (the craft will lose too much speed and altitude) nor 1 second (there is a substantial configuration change between modes), so the answer will likely be somewhere between those two values.

Are there design goals published, or at least discussed? Is there some way to estimate this based on first principles, current theory, and/or and expected engineering limitations?

Question: Would use of the SABRE engine also require reconfiguration back to air breathing sometimes? Might it have a similar delay? Also, what is likely to be the primary reason or limiting factor for the delay? Heat dissipation?

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    $\begingroup$ Might be difficult to answer at the moment, as no complete SABRE engine exists yet. $\endgroup$
    – Hobbes
    May 26, 2018 at 10:00
  • $\begingroup$ I'll be happy with an order-of-magnitude estimate based on current theory and expected engineering limitations. $\endgroup$ May 26, 2018 at 10:35
  • $\begingroup$ @PatrickMackey It's an interesting question! I've reworded your question a bit to make it fit better into a more standard Stack Exchange question format, and included a revision based on your comment. Since someone has already voted once to close for "primarily opinion-based" I've clarified that you are looking for the engineering and science behind the limitations, and not just a guess. $\endgroup$
    – uhoh
    May 26, 2018 at 12:20
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    $\begingroup$ I dispute the "nor 1 second", @uhoh - the older engine design might have been able to continue running as the mix was altered and the newer one could even (briefly!) overlap the modes $\endgroup$
    – user20636
    May 26, 2018 at 12:47
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    $\begingroup$ Thanks @uhoh, the question is far better articulated now! $\endgroup$ May 27, 2018 at 8:28

1 Answer 1


That is unknown, and is likely to be unknown until it is tested. The currently announced engine tests do not include a transition.

The SABRE engine design goals have been extensively discussed over the years, from the first public mention in May 1993's Spaceflight, through papers in JBIS and other publications, many conference presentations and innumerable talks over the decades. There have been a number of threads at NasaSpaceflight.com's forums.

Before SABRE 4, the simplified cycle diagrams show the same piping being used for air and oxygen, so the change over from air to oxygen could occur by the valve being opened instantly or over a few seconds, involving no delay.

The SABRE 4 engine uses separate combustion chambers so oxygen could be run through while the air breathing engine is still running - although starting the hydrogen side might depend on the plumbing. ("[0097] Optionally, during transition from the first mode of operation to the second mode of operation, both the air-breathing combustion chamber and the rocket combustion chamber are operated.")

The Skylon design did not envision switching back to air breathing, so even less is known about that, although some TSTO mission designs do call for it to occur, although several minutes after the LOX stage has shut down (mostly because the craft needs to slow down to speeds the engine can operate at).

  • $\begingroup$ I've adjusted the wording of the question because it's already picked up a close-vote for "primarily opinion-based". It's a great question and should stay open. Looks like you have a great answer as well! $\endgroup$
    – uhoh
    May 26, 2018 at 12:21

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