Each of the designs was to incorporate a 34-ton cryogenic 4.4-meter-dia. (14.4-ft.) third stage based on an adapted version of the Vinci upper stage, as well as a common payload fairing similar to the Ariane 5 and a third-stage deorbiting kit included in the performance metrics.
By June, the teams had narrowed the focus to two: one with a main stage housing a large, single, solid-fueled booster; and a second design based on a cluster concept involving three solid-rocket boosters topped by a nearly identical fourth.
Fabrizi said last month that the latter option seems more affordable. “The cluster provides higher production rates for the motors, with a lower cost for each motor,” he said on the sidelines of the June 5 launch of Europe's Automated Transfer Vehicle (ATV-4) from Europe's Guiana Space Center in Kourou. “But there are integration complications with building the cluster, so we have to make the trade.”
ESA's final concept, unveiled July 9, is a non-modular, linear version of the cluster concept that would carry 3,000-6,500 kg to GTO, and is designed to maximize commonalities with the Ariane 5 ME and Italy's new Vega light launcher. In addition to the Vinci engine, the Ariane 6 will incorporate a 5.4-meter-dia. payload shroud to accommodate the same volume of satellites as the Ariane 5 and rely on P135 solid-fueled boosters that are in the range of what Vega has demonstrated with its P80 first-stage engine.
ESA says this “Multi P Linear” design could facilitate development of a more powerful P120 engine for future evolutions of Vega, lowering production costs, improving performance and making the rocket more competitive in the commercial smallsat market.
Unlike the single-booster concept, however, the Multi P Linear's P135 boosters will incorporate thrust vector control (TVC), a feature that is expected to increase costs.
“You have TVCs and all these stages, so it's inevitably a bit more expensive,” Christophe Bonnal, a technical specialist with CNES, said in Naples, Italy, last fall.
However, Fabrizi asserts that more development risk was associated with the single, larger booster, a factor that could prolong Ariane 6 development beyond the targeted service-entry date of 2021. “If something is more complex to develop, we take a risk in getting it to market,” he tells Aviation Week.
With the basic design settled, the next step is to solicit industry proposals this month based on the Multi P Linear concept in order to benchmark a competitive development cost that will be presented to ESA ministers next year.