That s what I googled. J-2 pogo problems.

yale

Well, we are still using them!

ESA's page on the Vulcain/Vulcain-2.

http://www.esa.int/SPECIALS/Launchers_Access_to_Space/ASELVQI4HNC_0.html

Top photo - Test firings of the Ariane-5 cryogenic Vulcain engine in Vernon (France) and Lampoldshausen (Germany).

Credits: SNECMA/SEP/DLR/DASA

Bottom photo - DLR ( auf Deutsch - http://www.dlr.de/ )
( in English - http://www.dlr.de/en/desktopdefault.aspx )

I got a personal kick out of the RS-68 decision for the Cargo vehicle, or , apparently, the Ares V.  I wish they would quit trying to borrow Apollo nomenclature - it seems a bit self-conscious.  Crew launch Vehicle is Ares I, cargo Ares V, and the official release on the names called it a tribute, I think, to the Saturn IB and Saturn V.

I personally think Von Braun would be spinning in his grave if he knew that somebody was comparing that SRB pencil to his Saturn IB.  Even though the Saturn I family was an expedient that was built around clustered Jupiter and Redstone tankage as a quick and dirty way to gain heavy lift capability, I always found the unusual first stage contours rather elegant...

Anyway, back to the RS-68 decision.   I am fairly familiar with both the RS-68 and the SSME from manufacturing experience on both and long personal interest led me to become fairly familiar with operational characteristics. 

When they first announced the intention to power the upper stage of the crew vehicle with SSME, I recall wondering about implementing an air-lit SSME variant, given the rather extensive prestart conditioning requirements of the current engine - there's a great deal of ground equipment whose functions would have to be served by a flight-mass system, a real challenge if you want to preserve your Isp advantage for payload rather than trading it for subsystem mass.  Well, folks, it turns out that J-2X is looking like a better idea now, but it took a very long time to get to that decision - TOO long IMHO.

I was also frankly  incredulous about the idea of trying to make an lower-cost expendable SSME variant for the cargo vehicle, and I think I posted a speculation that they HAD to be talking about a recoverable engine module - there is just no way to build a truly cheap SSME variant.  I believe I opined at the time that RS-68 would make more sense for the cargo vehicle from a life-cycle cost AND operational flexibility standpoint. 

Now, I'm fairly bright, but certainly no engineering genius - yet my antennae were twitching on these issues over a year ago if memory serves.  It disturbs me that NASA took so long to reach these conclusions.

I fear that the vast inertia of the shuttle program once more reared its head in this process.  SSMEs are expensive, complex, very high performance machines, and they require all sorts of TLC to turn around between flights.  This employs a large number of people at KSC, Marshall, and Stennis (though not many at Rocketdyne these days - no more overhauls being done there...), and this interests the congressional representatives of these places.  An entire generaton of NASA program people have made their whole careers on shuttle, ET, SRB, and SSME.    They have not had to actually develop new major booster systems designs, just refine the one system they operate.  That's a mite unsettling to me if I think about it too long!  The idea that "shuttle-derived" equated to lower cost vehicles was sheer fantasy when one looked at STS program costs - yet the NASA folks clung to this straw as long as possible.  They wouldn't jump to RS-68 because the lower Isp meant either larger tanks or less payload than required for the CaLV baseline lunar missions.   Larger diameter tanks meant shuttle ET tooling couldn't be adapted at Michoud, and this was resisted until the absurd cost of tossing 5 SSMEs a launch forced them to reconsider - and perhaps recall that Michoud produced 10 meter diameter S-IC tankage 40 years ago and is quite roomy enough to do so again!

Anyway, I hope they move out now that the last vestige of drop-in shuttle hardware is off the table.  Even the 5 segment SRB is a new piece, though it is derivative.  They tried to hang on to the 4 segment RSRM for the crew vehicle, but they couldn't make that happen without a lightweight, air-lit SSME for the second stage. I'm hoping for good things, but I'm a concerned observer - we've already wasted more than a year squirming around with shuttle-legacy hardware designs.

Jim