It's too soon to say what sort of rocket, or family or rockets, might be needed to support NASA's new moon-Mars initiative, officials said today. But engineers across the nation are studying a variety of options, including use of upgraded Atlas and Delta rockets, development of new, "clean sheet" designs and use of existing shuttle hardware and infrastructure.

The moon-Mars initiative was unveiled by President Bush in January. It calls for the establishment of a base on the moon by the end of the next decade and eventual flights to Mars. As part of that program, he directed NASA to retire the space shuttle by 2010, after completion of the international space station, and to develop a new Crew Exploration Vehicle, or CEV, that could carry astronauts into orbit and, in some form, on to the moon.

What sort of rocket might be needed to boost the CEV into space an open question. But based on presentations before the 41st Space Congress today in Cape Canaveral, a shuttle-derived vehicle appears to offer the fastest path toward developing a new heavy-lift launch capability, one that supporters say could be scaled down to carry astronauts to and from low-Earth orbit.

Mike Kahn, vice president of ATK Thiokol's reuseable solid rocket motor program, said "the soonest thing you could do is take the orbiter off and bolt on a cargo carrier and bolt it onto the tank the same way the orbiter bolts on."

"Of course, it has no wings or tail or tiles or any of the other complex items that the orbiter has," he told the annual gathering of aerospace engineers and managers. "But it does have engines on the back and they would most likely be the same engines we have (in the shuttle) today to minimize requalification costs.

"The goal would be to take the orbiter off and bolt something else on that could put up a pretty good amount of payload. The motors and the tank and most of the infrastructure is already in place and it's just a matter of working that cargo carrier. So that's a nearer-term approach."

Using existing boosters, external tank and shuttle main engines, an unmanned shuttle-derived vehicle could lift 160,000 pounds to low-Earth orbit and be operational as early as 2008, Kahn said. Using five-segment boosters and a stretched eternal tank, up to 200,000 pounds could be delivered to LEO by the 2011-2015 timeframe.

Beyond that, major upgrades would be required, including shifting the cargo carrier to the top of the vehicle. While he did not provide details, Kahn said up to 225,000 pounds could, in theory, be launched using shuttle-derived hardware.

At the same time, a medium-lift vehicle would be almost "a gimme."

"If you want to put up something smaller, just take one of the (shuttle solid-fuel) motors, put on a liquid second stage ... and you could put up 35,000 or 40,000 pounds," Kahn said. "You could use it as your CEV (crew exploration vehicle) launcher since most of its heritage is man-rated systems."

Thiokol has been studying the possibility of using hydrogen-fueled J2-class engines, the same powerplants used in the second and third stages of Saturn 5 moon rockets.

A shuttle-derived vehicle offers a variety of benefits, including common hardware, an already experienced workforce, existing launch pads and a long history of flight operations. Much of the hardware already is "human rated," which is a major issue. In addition, Kahn said, "if you decide you want to fly this hardware before the shuttle program ends, you could still do that."

Dan Collins, Boeing vice president and Delta program manager, agreed a shuttle-derived vehicle is a "very viable option." But Collins and Lockheed Martin vice president and space transportation manager Michael Gass believe their evolved expendable launch vehicles - the Delta 4 and Atlas 5 - offer many of the same benefits without relying on 1970s-era technology.

"Delta 4 is certainly a big part, or we hope will play a part in this," he said. "But the pieces can also play a part. We can take the Delta 4-Heavy (nose cone) fairing, which is actually taken from the Titan 4 booster ... that fairing can certainly play a part in a lot of different configurations, whether there's a Delta underneath it or something that's shuttle derived or a new clean sheet.

"Taking pieces of hardware that we know work and that are flight tested is a great way to reduce risk and to accelerate what can be done early on in the program," he said. "We're also looking at the upper stages, in this case the Delta 4 upper stage, can you get it into orbit and then use it as a transfer vehicle of some type."

Boeing and Lockheed Martin can support more than two dozen EELV launches per year, but due to a soft commercial satellite market less than a third of that capacity is being utilized.

"So there is a current capability already there to help jump-start the initiative and move quickly into some early successes," Collins said. "That may be one way to go, get some early successes without trying to hit the home run ... to get the momentum going and get the support behind the program."

The first heavy-lift version of the Delta 4 is scheduled to make its debut flight this summer. In its current configuration, it can boost about 50,000 pounds to low-Earth orbit. With modifications, use of solid-fuel boosters and more powerful upper stages, "you can take that system with its current launch pads and grow it significantly" into the 100,000 pounds-to-LEO capability, Collins said. "Beyond that, you can still use that hardware, but you're going to need a different launch pad, or a modified launch pad. Beyond that, with some modifications and civil engineering, you can get even further."

Perhaps, ultimately, to Saturn 5-class performance. Gass said Lockheed Martin's Atlas 5 also can be upgraded to increase payload capability.

"Our first step would be in widening the upper stage, widening the Centaur," he said. "Just like a Titan today has a Titan Centaur that's a 14-foot diameter (vehicle), we'd go to a five-meter diameter on an Atlas, get that real estate, get that fuel capacity. Simple, one step.

"Next thing is to put multiple engines on that upper stage, moving towards a performance capability but it's also moving toward an engine-out capability for human space travel. Next step, do the same thing on the booster: Add volume. The engines have the thrust capability, but we need more fuel. So we widen the tank to that five-meter capability. In each of these, that new vehicle becomes the core vehicle that's used on all the missions. It would be used for the DOD. ... We'd have a new common core vehicle."

That approach also would enable survivable engine-out scenarios in first stage powered flight. Gass said the Atlas 5, like the Delta 4, could be scaled up to Saturn 5-class performance by bolting five core vehicles together, each one equipped with two powerful RD-180 engines.

"Are there in-between steps? Absolutely," Gass said. "The important message on this is it can be dialed into the exploration architecture, not to the launch vehicle architecture. This is just one of the solutions that is available."

A team at Boeing also is looking at possible "clean sheet" designs that don't rely on existing systems. The company's Phantom Works team is looking at kerosene-powered first-stage engines with hydrogen-fueled upper stages or no upper stages at all.

"If you're going to go the clean sheet route, how does it work?" Collins asked. "Certainly, the development of a clean sheet rocket is a constraint that you have to be aware of as you go into this. There are some advantages to it. It really becomes a cost-benefit analysis. Can you use that money in other parts of the exploration business and rely on boosters that already exist or do you really need (a new vehicle). Tough questions.

"I think the customer's doing the right thing, keeping an open mind at this point, trying to understand what the mission is and what the options are to get there," Collins said. "If you came here looking for what Boeing thinks the launch vehicle for space exploration is, we don't know. We're not sure what the mission is yet. The requirements certainly will flow from the mission once it's there."