Some commenters have raised a very reasonable question: Why doesn't NASA just human rate an expendable launch vehicle like the Atlas V or the Delta IV? It seems reasonable because they fly all the time, they are big, they are reliable and they are *already built. *So what is the problem?

Weight- The Orion Crew Exploration Vehicle is too heavy for the Atlas V and the Delta IV (I will deal with other capsules after the jump). You also have to account for other mission constraints. For example, you have to include the loss of performance you get from going to the high inclination orbit of the International Space Station. ISS doesn't just fly directly over the equator but at an angle that allows it to pass over Russia. That orbit takes more energy to get to. In addition you will also lose performance because you have to fly a less aggressive launch profile. The current profile subjects payloads to 6 G's (Space Shuttle is 3 G's). Human payloads don't respond well to sustained 6 G loads. Flying a less aggressive profile will mean you will be able to lift less.

So all in all, you would need to develop a "triple-core" (three Atlas' strapped together) to be able to lift Orion. A triple core Atlas has never been flown. You would also need a new upper stage for an extra bit of kick at the end of the flight. The Delta IV, although larger and more mature, would also need a new upper stage.

Triple Redundancy- All human-rated vehicles are designed to be triply redundant. If something fails, there has to be two other options for the crew. It's like when the power goes out at your house, you have a back-up system which is a flashlight. If the flashlight fails, you have a second back up system which is a bunch of candles and matches. Now, to be stuck in the dark you would need to have all three systems fail. Cargo rockets are not designed with that much redundancy in their systems. Adding in that kind of redundancy after the fact would be like getting under the hood of your car and installing a back-up set of brakes. You have to create feed throughs for the wires, try to figure out where you will put the extra brakes etc.

Now to be fair, you could launch a smaller human vehicle on a current expendable rocket...(Orion does seat six).

In fact, before the Columbia disaster NASA teams were working on an Orbital Space Plane (OSP) designed to do just that. At the time, OSP was maligned for being small and limited in capability. (It is ironic that the Orion is now being maligned for being too large and capable.) OSP was a smaller vehicle designed to launch on an Atlas V or a Delta IV and seat "no less than four" crew members. It was eventually overcome by events when Constellation and the Moon, Mars, and beyond program kicked off and NASA transferred the knowledge gained on the OSP to the development of Crew Exploration Vehicle.

Then there is Robert Bigelow, a self made millionaire, who is building an inflatable space module hotel (they already have a scale model on orbit now). He is converging on a deal with Lockheed Martin to human-rate a version of the Atlas V. This is plausible, given that they are looking for an OSP-class capability. Bigelow's main interest is getting clients to and from his future orbiting facilities.

SpaceX is also building a small capsule that could launch people to ISS or Bigelow's space modules hotel. Their Dragon capsule, being built to take cargo to the International Space Station, will fit on their Falcon 9 rocket and be able to launch humans. Although it will not fly on an Atlas or a Delta, it is in that same class of vehicles.

Perhaps in the end it is a good idea for NASA to focus their efforts on building the new, heavier Orion capsule and the Ares I and V. These vehicles are suitable for the longer, more demanding trips to the moon. Then NASA could leave the lighter, smaller low Earth orbit capsules to commercial space to work out. That would serve both the goal of NASA being an exploration agency and of evolving our commercial space sector.

So the short answer is: NASA is not human-rating an Atlas V because that system will not be sufficient for exploration beyond low Earth orbit (though Bigelow Aerospace is looking at human-rating the Atlas V).

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Image courtesy of NASA