The second stage will have a liquid engine capable of restarting multiple times. The payload will sit atop this second stage inside a large aerodynamic payload shroud.
During the study several shuttle-derived heavy launch vehicle options were considered. An old favorite, designed in the late 1980's would have replaced the shuttle orbiter with a payload canister which would more or less replicate the existing orbiter's payload interfaces - sans the orbiter. Existing launch infrastructure would stay mostly the same. This configuration has its limitations in terms of the size of payload that could be launched and was rejected in favor of the in-line design, which has greater capacity for growth and performance.
The in-line design will be a rather immense vehicle more on the scale of a Saturn-V. It will require substantial modifications to the existing launch pads and payload handling facilities.
The second vehicle to be pursued is based on a 5 segment Solid Rocket Booster (SRB). Atop the SRB will be a new liquid-fueled upper stage and the CEV. While this vehicle is being developed for CEV launching, Mike Griffin has spoken of a cargo version of the CEV as well. With the announcement of this new space architecture, no one can say that Mike Griffin is not serious about conceiving and building the systems needed to get humans back to the Moon and on to Mars ASAP.
("Light This Candle")