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Extract from a speech by French rocket scientist Hubert Curien, 1992

"Early in the life of Ariane 5 we considered three variants, the -P, the -C, and the -R, respectively standing for Poudre, Cryogenic, and Reference.

To make a long story short, the -C and -R clearly were of perfect Ariane 1 lineage, retaining the modular, flexible approach of their elder sisters.

The -P was a very different beast, rather similar to the Titan III, a tiny core with two large solid rocket motors on the sides. It was a very unflexible design and a total break from the earlier models, so one can ask why we even bothered considering that concept in the first place.

There's an historic explanation for that.

When they lost their beloved shuttle, NASA claimed they would continue studies of recoverable boosters through a simple, unexpensive method. Some Titan III would parachute their solids down in the ocean, and a recovery ship would tow the big things back to the Cape for a thorough study, and eventual reuse. We briefly considered doing the same thing, hence our study of the -P.

In the end the process proved extremely cumbersome, with the solids sinking to the bottom of the ocean, or crashing down because their parachutes did not worked correctly. Needless to say, the Ariane 5P was a short lived study, leaving the -C facing the -R.

Ariane 5 Reference was of direct Ariane 4 legacy, trying to retain the plain old Viking- powered first stage we knew so well. In the end that legacy brought more issues than cost savings, the concept turning into an unworkable, bloated monster. That made the -C a possible winner, and then the first all cryogenic launcher in the world.

It was not to be, however.

Because that all-cryo Ariane 5C was too expensive and lacked power, it was ultimately rejected and a compromise was found by blending together the 5P and the 5R.

Ariane 5P single Vulcain core returned, and it was flanked with the so-called “Propulseurs d'Appoints, Poudre or Liquide” - PAPs and PALs; little solid or Viking powered boosters as found on Ariane 44L and Ariane 5R.

Because the single 90 ton-thrust Vulcain could not by itself lift the 160 ton core, a minimum of two PAPs or PALs were necessary, each with 75 tons of thrust. Then up to eight could be added, resulting in an extremely flexible launcher– an extended family of launch vehicles that ranged from Ariane 52P to Ariane 58L.

Thanks to the high performance Vulcain the core itself reached low Earth orbit. The HM-7 would be used for geosynchronous orbit; the Agena, for polar missions and manned space flights that encompassed the space station and Solaris.

After the Ariane 5 design was frozen in 1983 and interesting debate happened on the subject of PAPs and PALs. The PAPs used low-performance, cheap solid propellant motors derived from France ballistic missile programs. Italy also had some vested interest in solids.

By contrast the PALs used the old Viking engine with liquid, hypergolic propulsion. With the sheer cost of the new, advanced Vulcain soon consideration was given to discontinuing Viking production, as liquid propulsion was more expensive. If that happened then Ariane 5 would be available in -P variants only – Ariane 52P, -54P, -56P and -58P. The Viking superior specific impulse was considered not worth the expense of running two liquid-propulsion engine production lines in parallel, so the Viking was eventually discontinued with the older Arianes.

Yet the Viking didn't died: the fledging Indian space agency secured a licence. The old, reliable Viking would power their PSLV and GSLV boosters. Things reached a point where Europe considered buying licence-build Vikings from India on the cheap, but that idea went nowhere and Ariane 5 only had solid strapons.

Excerpt from: A history of the European Space Agency, 1987 - 2007