By Jonathan Fildes
Technology reporter, BBC News
Nasa has until 29 October to launch the Ares I-X
A rocket designed to replace the aging space shuttle is set for its first test-flight, despite questions over the future of the programme.
The 100m-long Ares I-X has a four-hour launch window for blast-off from Nasa's Kennedy Space Center.
The two-minute flight will allow Nasa to test technology crucial for the development of the manned Ares I craft.
A high-profile report has cast doubt on the future of the Ares rocket, which is intended to enter service in 2015.
The Augustine panel, which had been asked to review the US human spaceflight programme, published its report last Thursday just days before the scheduled launch.
Although the panel supported the Ares I-X test flight, it questioned the need to develop the Ares I rocket, part of the Constellation programme commissioned by the previous administration and intended to return the US to the Moon by 2020.
In particular, the panel queried the cost and design of the craft as well as its development time.
"With time and sufficient funds, Nasa could develop, build and fly the Ares I successfully," the report said, "the question is, should it?"
The $450m (300m euros; £275m) Ares I-X is what Nasa describes as a "pathfinder" vehicle.
"It is a chance for the agency to remind itself what it takes to build a vehicle," explained Robert Ess, Ares I-X mission manager.
Nasa has not designed a new launch vehicle for more than three decades and has lost much of the expertise in the area, according to some critics.
"Ares I-X is all about information; about gathering data," Mr Ess told BBC News.
"We have a lot of computer models that we think show it all works. We're very confident we can do it but the proof is actually doing it for the first time."
The Ares I-X is the longest, thinnest vehicle ever designed and built by the US space agency.
Its shape has been determined by the design of its solid rocket booster - itself a modified version of the units used to lift the shuttle into orbit.
"[We wanted] to use as much existing technology as possible," said Mr Ess. "Given that we have the space shuttle booster and given that it is 14-and-a-half-feet in diameter, we didn't want to change that. That architecture drives the length of it."
The final Ares I design calls for the shuttle booster to be upgraded from four to five segments for the first stage of the rocket. The Ares I-X, though, will fly with just four segments and an additional unit that merely simulates an extra portion of booster.
Engineers saw value in flying the more limited configuration to get engineering flight data as soon as possible rather than wait the extra year or two before a five-segment booster became available.
The I-X vehicle is 100m (320ft) tall and has been designed to be as close as possible to the eventual Ares I rocket.
However, the top half of the launcher, like the fifth segment of the booster, is a dummy. What would be an upper-stage, with a crew capsule and its emergency escape mechanism, are mock-ups made to the correct shape and weight.
"We have been building the simulators for over two years now. It has been a very long and very intense process," explained Jonathan Cruz, deputy project manager for the Ares I-X crew module and launch abort system.
"[They] are incredibly accurate. One reason why it has been so difficult and taken so long to build this is because we are building this to exact tolerances."
The test launch - scheduled for Tuesday between 1200 and 1600 GMT (0800 and 1200 EDT) - will take place from Kennedy Space Center in Florida.
If weather delays the flight, the team has a four-hour launch window each day until 29 October.
When given the go-ahead, the rocket will blast off from pad 39B, a space shuttle launch site that has been modified to handle the slender rocket.
"It has a little higher thrust-to-weight ratio than the space shuttle, so you'll see it come off the pad a little quicker than you're used to," explained Mr Ess.
The flight will last for 120 seconds and will see the demonstrator climb to about 40km (25 miles). At that point, the lower and upper stages will separate.
"After the separation we basically do a kick manoeuvre," Mr Ess told BBC News.
Motors will be used to make the lower-stage spin, creating drag and helping the booster slow down enough to allow it to deploy its parachutes so that it can be recovered.
"The front part of it just carries on going and splashes into the ocean."
The vehicle has also been wired with sensors from top to bottom, allowing engineers to monitor all aspects of the flight, including pressures, accelerations and temperatures.
The team is particularly interested to see how such a slender and flexible vehicle behaves during flight.
"Our flight control team has spent a long time designing a flight control system that can handle it," said Mr Ess.
"So being able to do a test flight and prove that a concept is going to work the way we think it is going to is a big one. That is our primary objective."
In addition, the team is keen to gather data on so-called "thrust oscillations".
The solid rocket booster is known to shake towards the end of its burn, which might cause problems if the Ares ever becomes a manned space vehicle.
"Getting some actual data on that phenomenon during Ares I-X is going to help those designers know how much of an attenuation system they are going to need for Ares I," said Mr Ess.
These oscillations are just one of the problems with Ares I and the I-X, highlighted by critics of the design.
"I look at it as a suicide launch vehicle," said Charles Vick, an aerospace analyst, "I would not fly on that vehicle."
He added: "The upper-stages and spacecraft are essentially alright but it's that first stage that bothers everybody - it's entirely too tall," he continued.
Mr Vick said there were a number of options that could replace Ares I and provide a safer vehicle. These included designs using technology more heavily derived from the shuttle or the Saturn V rockets of the Apollo programme, he said.
Other technology or vehicles could be adapted from the Delta IV rockets currently used to loft satellites into orbit, he argued.
"The reality of it is that there are many other alternatives," he claimed.
The Augustine panel has examined many of these different approaches, including a design dubbed Ares V Lite.
Under current plans, Ares I would be used to carry astronauts into orbit, whilst a heavy-lift Ares V rocket would loft hardware.
The Ares V Lite would be a smaller version of the freight carrier, capable of carrying astronauts.
Crucially, the design would use the solid rocket booster flown on the Ares I-X, meaning years of costly engineering and testing would not be wasted.
However, the decision on the future of the Ares I and the rest of the Constellation programme now rests with President Barack Obama who is expected to give a response in the coming weeks.
According to Mr Vick, the success, or otherwise, of the Ares I-X test flight could have a significant bearing on his decision and the future of the programme.
"I really think everybody is waiting to see what happens with this flight - a lot is dependent on that flight," he said.