Vacuum nozzles are huge compared to their sea-level or atmospheric counterparts, as can be seen by comparing the first stage engines on an Electron or Falcon-9 to their second stages, where a big interstage space is needed for that one, giant vacuum nozzle.

Ideally or at least simplistically the expansion ratio for a vacuum engine would be infinite, so the final exit diameter chosen depends on how generous designers want to be towards the performance at the expense of other practical issues like total height, diameter, drag, mass, etc.

Question: What have been the stingiest and most generous vacuum nozzles on the stages of a launch vehicle1? What are the records for the smallest and largest expansion ratios? Candidates don't have to be on the same vehicle, you can mix and match.

A starting point might be from answers to What is a typical sea-level engine nozzle/expansion ratio? What about vacuum engines?

From Rocket Performance and Efficiency found here:

difference between expansion of a sea level and vacuum nozzle (click for larger)

1I'm kinda interested in seeing the largest expansion ratio 2nd stage nozzle, but kick stages should count as well since they are "real rocket engines" and just as much responsible for getting a spacecraft inserted into its target trajectory as then 2nd stage, conceptually at least.

  • 1
    $\begingroup$ Do you rule out 3rd stages and higher deliberately? My bet would be the winner is a kick stage with an expandable nozzle. $\endgroup$ Commented Feb 26, 2021 at 14:23
  • $\begingroup$ @OrganicMarble Hmm... I didn't even think about kick stages, just wanted to prevent some weird little thruster as an answer preventing anyone from posting some huge 2nd stage nozzle that would be good to know about and fun to see. I certainly wouldn't want to exclude an interesting answer and the record for the largest expansion ratio ever if it's a kick stage! I'll try to reword based on all of that... $\endgroup$
    – uhoh
    Commented Feb 26, 2021 at 14:56
  • $\begingroup$ Minimum thrust groundrule? $\endgroup$ Commented Feb 26, 2021 at 14:59
  • 1
    $\begingroup$ @OrganicMarble How does it look now? I've added a foot note and moved the constraint to the bottom. $\endgroup$
    – uhoh
    Commented Feb 26, 2021 at 15:00
  • 1
    $\begingroup$ Heh, after my carping, the winner may be a 2nd stage after all. $\endgroup$ Commented Feb 26, 2021 at 15:18

1 Answer 1


Partial answer to get the ball rolling:

The RL-10B-2 and RL-10C-2-1 variants of the venerable RL10 LH2/LO2 upper stage engine feature an expandable nozzle with a 280:11 area ratio and an amazing 465.5 Isp.

Aerospaceguide.net RL10 article

enter image description here

Image source: Aerojet Rocketdyne

enter image description here

Image source: Impact of dynamics on the design of the RL10B-2 extendible carbon-carbon exit cone

1some sources say 285:1


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.