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?