On 2020-03-07 1:22 PM, Jeff Findley wrote:
This was the first large LH2/LOX upper stage. So I believe that it was
pretty much an engineering pathfinder for the IV-B. It provided
experience with designing, building, and flying a large LH2/LOX upper
stage. I'm sure the lessons learned went into the IV-B.

It's hard to have a discussion when we agree on almost every point. ha.

So let's expand the envelop a bit. The J2 used in the Saturn 5 third
stage had to have a restart capability to do LEO and TLI insertions. Was
the ignition system used for that significantly different than that used
for the second stage engines? I suspect not.

I seem to remember the bell housing on the third stage was longer than
that used on the second stage even though same J2 engine. Vacuum
optimization?

Are there ANY applications these days for the new and improved J2X?
I should do a Google search and see. Opinions welcome.

Dave