According to Oleg Ventsokvsky in 2004 and 2005 Yuzhnoye Design Office “conjointly with RSC Energia carried out feasibility studies” to launch Kliper using Yuzhnoye’s Zenit two-stage rocket
The Zenit was designed to be man rated and I understand that it was a derivative of the Energia booster for the Buran Space Shuttle. The Zenit’s first-stage main engine has a “deep thrust” throttling capability to allow the reduction of longitudinal g-loads during ascent, to ensure the passenger does not experience anymore than 3g. This ascent g loading is similar to NASA’s Space Shuttle and is “significantly less than Soyuz spacecraft,” says Yuzhnoye.
