When the Space Shuttle Atlantis touched down for the final time on July 21, 2011, it looked as if the notion of a manned spacecraft capable of going into orbit and then landing like a conventional airplane had been abandoned. The US government appears to be in favor of returning to Apollo-style space capsules with anything like the Shuttles being relegated to the private sector. But at the American Institute of Aeronautics and Astronautics' (AIAA) recent Space 2011 conference, Arthur Grantz, chief engineer of Space and Intelligence Systems' Experimental Systems Group at Boeing, delivered a paper indicating that the U.S. Air Force and Boeing are already on the way toward developing a manned Shuttle replacement based on the X-37B robot space plane.

X-37B

The X-37B is one of the US Air Force's most highly visible yet most secret projects of recent years. A robot spacecraft that looks like a miniature space shuttle without a flight deck was bound to attract public attention, but its mission has remained hidden under the blanket word "classified." The government has released some information about the X-37B. In part, it's an experimental test bed based on the Boeing X-40 lifting body. With an overall length of a little under 30 ft (9 m) and a wing span just under 15 ft (4.5 m), it's small enough to fit easily into the Shuttle's cargo bay, but it's still capable of acting like a robot version of the larger, older spacecraft. Launched on top of an Atlas booster at Vandenberg Air Force Base, it can carry payloads into space, return them to earth and then land like a conventional aircraft. The difference is that it doesn't require a pilot or ground control because it can land by itself.

It also has much more endurance than the old Shuttle. While the Shuttle never remained in orbit for as long as three weeks, the X-37B has already broken the record for a reusable spacecraft in orbit: 244 days. It's rated to remain on station for 270 days if needed.

Shrouded in Secrecy

But for all that the public knows about the X-37B's capabilities, its mission remains a secret. Part of the X-37B's purpose is experimenting with new technologies and it's clear that the Air Force wants its own way of getting into and coming back from space. What they intend to do once in orbit is another question. One thing that the X-37 is designed for is to release satellites that it can rendezvous with at a later date and retrieve. With today's cyber-heavy battlefields, that is a considerable advantage. Beyond that, the space plane configuration echoes the Air Force's earlier Dyna Soar program of the early 1960s, which was also a space plane (in this case manned) intended to be launched from atop a booster rocket. Its purpose was supposed to be as a hypersonic reconnaissance platform and bomber-roles that a variant of the X-37 could also fulfill.

X-37C

The paper that Grantz delivered to the AIAA, "X-37B Orbital Test Vehicle and Derivatives" [PDF summary], provides further insights into the X-37 including the possibility of a manned version. Grantz says that the X-37B's successor, the larger X-37C, could be used as a cargo ship for the International Space Station. There are already several spacecraft in service and under development that could do that, but Grantz says that the X-37C could also be easily modified to carry up to six passengers. Unlike other cargo carriers slated to become manned spacecraft, this doesn't necessarily require major design changes. One proposed version of the X-37 shows the interior significantly altered with the fuel tanks and operating systems pushed aft to make way for a traditional flight deck in the bow, but the alternative is simply to place a self-contained pressurized cylinder in the cargo bay and install television cameras fore and aft so the crew can see what's going on.

X-37B size comparison (Image: USAF)

This "plug and play" feature speaks volumes about the X-37 and the X-37C in particular. Human beings are fragile creatures and space engineers have to bear in mind that the human body can only tolerate a narrow range of variables. Too heavy acceleration on takeoff, too sharp a turn, too much vibration or too hard a landing can injure or kill a person. The X-37 is what is called a "1.5 g" spacecraft. In other words, it operates only within an acceleration range of one and a half times the pull of Earth's gravity. This means that it can carry delicate instruments into space and return them safely to the ground. It also means that it operates safely within the range of what is called "man rated" or "human rated" flight.

What's also implied by being human rated is that the craft has a mandatory failure ratio of less than one percent. Unmanned spacecraft are allowed a failure rate of ten percent. Moreover, the ability of the X-37 to launch and retrieve satellites as well as to land autonomously suggests a navigation and guidance system sophisticated enough for manned flight that can be adapted for the option of manual control that a human rating requires.

The future of the X-37 program is not certain, but the fact that a new manned spacecraft can be the result of modifying existing technology rather than starting from scratch shows that the grounding of the Shuttle fleet wasn't just the end of an era, it was the start of a new one.