The standard J-2 engine measures 3.38 metres in total length, with a nozzle exit diameter of 2.03 metres, to support its 27.5:1 expansion ratio. However, these dimensions are apparently identical for the simplified, tap-off cycle J-2S. But the J-2S features an area ratio of 40:1.

How is that so?

This question might have been answered if I could find any certain information about a change in combustion chamber dimensions. Unfortunately, the only graphic I have seen portrays a 'recontoured' nozzle but does not list any chamber redesign.

Could it be down to a change in throat area for the J-2S? I seem to recall a mention of the engine sporting a De Laval type nozzle, with a smaller throat diameter permitted by the higher chamber pressure. It might help support the Isp jump, but does not appear very widely - and there are no dimension changes listed.

Or is the J-2S engine nozzle actually larger than the J-2, and misinformation took its toll over the past half-century?


1 Answer 1


It appears that the J-2S has a smaller throat diameter than the J-2, and hence a greater expansion ratio with the same final nozzle diameter.

According to Altitude Testing of the J-2 Rocket Engine in Propulsion Engine Test Cell (J-4), the 1966 J-2 had a throat area of 170.4 square inches, implying a throat diameter of 14.73". A 27.1:1 expansion ratio yields a 76.7" exit diameter.

The similarly-named 1970 paper, Altitude Developmental Testing of the J-2S, gives a throat diameter of 12.192" and 39.62:1 expansion ratio for the J-2S engine, for a throat area of 116.7 square inches and the same 76.7" nozzle exit diameter.

  • $\begingroup$ Exactly what I was looking for - exact data. $\endgroup$ Jan 5, 2017 at 2:15
  • $\begingroup$ Thank you very much, once again! Also, does this mean that the J-2S would also have had a smaller combustion chamber diameter, or did it simply contract above the nozzle for that higher ratio? Is that irrelevant? I'm not sure how far Rocketdyne went with the J-2 redesign. $\endgroup$ Jan 5, 2017 at 2:20
  • $\begingroup$ Found it in the documents you linked: chamber diameter for both models is 18.6 inches. $\endgroup$ Jan 5, 2017 at 2:27
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    $\begingroup$ @AlastairHaslam since you may be new to the site, if this is the correct answer, please accept it by clicking the grey check mark at the left. $\endgroup$ Jan 5, 2017 at 3:05

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