During the study phase for the Ariane 6 follow-on to the Ariane 5, one of
the configurations examined would add a second Vulcain engine to the Ariane
5 core. According to a CNES study this version without solid side boosters
could only get 2,200 kg to GTO.
However, when I did a calculation I was surprised to see this
configuration without side boosters could get in the range of 5,000 to
7,000 kg to GTO.
This would be a quite cheaper development to only add the second Vulcain,
in the range $200 million, compared to the multi-billion dollar development
cost of the current proposed version of the Ariane 6 that uses solid side
boosters.
Moreover this version could be reusable a la the Falcon 9, thus
potentially keeping pace with the SpaceX cost-cutting approach through
reusability.
However, the payload result is surprising I would like to see confirmation
on this.
Bob Clark