DENVER - NASA's planned Ares V heavy lift vehicle can't meet its lunar-mission requirements as currently conceived, and will need beefing up.

The agency is studying a variety of options to boost the lift capability of the big new rocket, currently scheduled to begin development before the end of 2010 under the fiscal 2011 federal budget.

Initially planned as the largest launch vehicle ever built, the Ares V has grown as engineers in NASA's Constellation Program gain a better understanding of the vehicles that will be needed to send four humans to the moon for a seven-day stay, and eventually to build on that early presence into a lunar outpost where astronauts can live for as long as six months at a time.

But even with expansion from earlier concepts to a full 10-meter diameter all the way up to the fairing that will cover the Altair lunar lander, allowing the upper stage to carry more propellant, Ares V still falls short, according to Phil Sumrall, advanced planning manager in the Exploration Launch Projects Office at Marshall Space Flight Center.

"The payload requirements are very driving and very difficult to get to, and frankly our vehicle today is close but doesn't quite meet those mission requirements," Sumrall told the Third Space Exploration Conference & Exhibit here Feb. 26.

In gross terms, the Ares V needs to send about 75.1 metric tons into trans-lunar injection, including margins. But the vehicle as conceived today can only throw 63 or 64 metric tons, Sumrall said, which leaves it "actually a couple of tons short of where we need to be, without margin."

Presently the vehicle consists of a big "core stage" powered by five RS-68 engines burning liquid oxygen and liquid hydrogen, and a cryogenic upper Earth Departure Stage powered by the J-2X upgrade of the Saturn V upper stage engine already in development for the Ares I upper stage.

To get under way, the Ares V will also use two strap-on five-segment versions of the four-segment reusable solid rocket motor (RSRM) that will also serve as the Ares I first stage.

Options under study to increase the throw weight of the boosters and core stage include variations on adding a sixth RS-68 to the core stage, and on extending the length of the solid boosters by another half-segment. That could be accomplished either with an inert "spacer" or a live half-segment. In both cases the change would extend the core-stage internal thrust structure at the top of the boosters and allow the stage to carry more liquid hydrogen for its engines. Also in the trade space are composite casings for the boosters that would save weight and could be designed to withstand higher internal pressures than the current steel cases.

If a sixth RS-68 is added and the boosters are extended with a live half-segment, the Ares V would hit 75.1 metric tons without the need for composite cases. Sumrall stressed that the options he presented are "a snapshot in time" and may be revised later.

"We're trying to identify options that will allow us to get the performance that we need," he said.

Under present thinking, on a lunar mission the Ares V would launch first from Kennedy Space Center with an Altair lander under its fairing, followed one orbit later by the Ares I carrying a four-seat Orion crew exploration vehicle. After about four days of "loitering" in low Earth orbit to dock with the Orion, the Earth Departure Stage would restart to drive the Orion/Altair combination to the moon.

Artist's conception: NASA. This shows the Ares V with one of its two boosters separating.