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I can't find this information anywhere for the single or double engine variant. could it be variable? Or is it a secret?

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Since we know the Centaur on Atlas 5 uses RL10C engines, we can look that up.

The current RL10C engine mixture ratio is 5.5.

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    $\begingroup$ According to Spaceflight101, Atlas V Centaurs are now using the RL10-C-1 which is also 5.5:1. spaceflight101.com/spacerockets/atlas-v-401 $\endgroup$ – Russell Borogove Jun 27 '18 at 19:36
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    $\begingroup$ Oh, sure, like ULA knows anything about it. $\endgroup$ – Russell Borogove Jun 27 '18 at 19:41
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    $\begingroup$ Yes, the convention is oxidizer mass : fuel mass. $\endgroup$ – Russell Borogove Jun 27 '18 at 19:44
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    $\begingroup$ Various production versions of the RL10 have run anywhere from 5:1 to 6:1, and it's been tested across much broader ranges, even oxidizer-rich (i.e. above the 8:1 stoichiometric ratio). "An amazingly tolerant engine" per Henry Spencer. yarchive.net/space/rocket/rl10.html $\endgroup$ – Russell Borogove Jun 27 '18 at 19:47
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    $\begingroup$ The RL10A-3-3 had a propellant utilization control valve (like the J-2) that could vary the mix ratio between 4.4:1 and 5.6:1 from a nominal 5:1; I haven't been able to confirm that later versions also support variable ratios. $\endgroup$ – Russell Borogove Jun 28 '18 at 23:35

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