As space shuttle operations begin to ramp down, all eyes are on the future of international space station (ISS) crew and logistics support. Several reports, including that of the Augustine committee, indicate that nominal ISS operations will be hampered without a system as capable as the space shuttle. So, the push is on for a reliable system to replace the shuttle.

While it may be possible to sustain payload tonnage to the ISS using a variety of expendable launchers now in service, it appears that there is a shortfall in vehicles able to bring down large payloads after experiments are completed. And this is exactly where the shuttle has proved its unique capability, besides the orbiter’s ability to haul a large crew contingent and oversize payload simultaneously into orbit.

Contenders proposed to take up the slack include the Russian Soyuz and Progress vehicles, the European Automated Transfer Vehicle and the Japanese H-2 Transfer Vehicle. But none of them has the versatility, the down-mass capability or the oversized capacity of the shuttle.

In the new arena of commercial transportation to ISS, Space Exploration Technologies of Hawthorne, Calif., plans to offer its pressurized Dragon capsule atop the company’s Falcon 9 launcher to ferry cargo and eventually crew to the station. And Orbital Sciences has plans of its own with the development of the Cygnus capsule.

In addition, the administration of U.S. President Barack Obama has directed NASA to refocus the Orion capsule from the Constellation program aimed at returning people to the Moon to now support the ISS program as a rescue vehicle. Perhaps a version of it might also be built to support ISS logistics?

With all of these alternatives, some operational and many just approaching maturity, it is clear that once ISS assembly is complete using the shuttle, in order to accommodate a smooth transition of logistics support, we will need a volumetric envelope comparable to the shuttle cargo bay or we will need to thoroughly review and redesign the acceptable sizes of ISS payloads based on these much smaller carriers that the ISS partners have developed or are in the process of commissioning. Of course, more frequent logistics missions to the station are inevitable for this sort of architecture.

That we should be left with a dearth of down-mass capability just as ISS operations begin in earnest is the irony we face. It is imperative that as we expand ISS operations to include more partners, and cycle more projects and experiments through this unique laboratory, we have access to a variety of logistics and crew support capabilities, from international partners as well as domestic, both governmental and commercial.

A system that is undergoing space mission shakedown testing in orbit right now, if successful, might offer some versatile capabilities in the near-term future for ISS operations. It is the X-37B, built by Boeing Phantom Works of El Segundo, Calif., for the U.S. Air Force in a fast-paced design-build-fly environment.

Following the lineage of some venerable X-planes, the X-37B started as a mini space plane to be carried aloft in the space shuttle cargo bay. After several iterations and program reshufflings, in an impressively short period from bending metal to test flight, it was lobbed into orbit in April on an expendable Atlas 5 vehicle within an oversized fairing. It is reassuring to know that technical agility still resides in some pockets within large defense contractor aerospace corporations.

This remotely operated vehicle now in orbit evolved from and sports many of the best features of the space shuttle and the X-40 effort, and even the failed X-33 Venture Star designs. The X-37B has the ability to launch on demand and stay in orbit for up to nine months, and is capable of orbital maneuvers that are expensive on fuel (suggested by the large propulsion tankage and AR2-3 engine with multiple in-space restart capability) for employing the element of surprise, and perhaps quite useful for surveillance.

These features, combined with modular palletized cargo and quick ground processing and turn-around capability, are really suited for military missions, but might they also be adopted for civilian purposes — for ISS crew support and logistics in particular?

A look at the X-37B architecture shows that it is a tiny version of the space shuttle, in fact so small that two of them would fit end to end inside the shuttle cargo bay. It can carry a mere 227 kilograms of modular payload in its cargo bay, and operates much like the shuttle.

Especially since the vehicle is already in orbit and being paced through rigorous testing, such a system may have far-reaching global game-changer implications not only for the U.S. Department of Defense but for the civilian space program as well.

Transatmospheric vehicles are expected to reliably operate in a wide range of environments, from the vacuum and thermal cycling of orbital space to the extreme aerothermal and structural forces of re-entry, demanding a high degree of control from Mach 25 to subsonic speeds all the way to a safe touchdown.

The only good part of this harrowing routine for crew, vehicle and controllers alike is that, if phased properly, the entire mission duration — from the time the retroburns are initiated for re-entry in low Earth orbit to the time the vehicle comes to rest on the tarmac — can be tailored to be less than 30 minutes. 

This has direct implications for an X-37B-derived application for ISS crew and cargo return capability. First, because the change in velocity needed for re-entry is only a few hundred meters per second, the propulsion system would become much smaller, compensating favorably for an appreciably larger payload. Next, since the duration of flight is short, consumables would be reduced accordingly, again adding to payload margin. Note also that this mini space plane can land on short commercial runways. Therefore, it might be worthwhile to investigate what modifications are needed to make the vehicle work with the ISS system.

A typical ISS expedition might begin on the shuttle with full complement of international crew, hauling an X37B and other logistics to station. Alternatively, the space plane would be launched on an expendable. Upon arrival, the X-37B-derived vehicle would be berthed at the ISS. The cargo version of the craft would then be loaded with two pallets or racks of standard size and flown remotely back to the desired country of origin to touchdown on a commercial runway anywhere on the globe.

For NASA and the ISS program in particular, it offers the potential for a remarkable new dimension in international collaboration. It offers the possibility to fly international crew and cargo directly to their country of origin after their expedition.

For a crewed X-37B mission, two fully suited ISS crew members, the pilot and the crew member of an ISS partner nation, could be fastened into unpressurized pallets with glass virtual cockpits (for emergency takeover of command and control if needed) and transferred through the ISS airlock into the small cargo bay. This way the heavy-airlock and life-support issues are circumvented. Once the cargo bay doors are shut, the vehicle would initiate a retroburn and quickly descend to its destination country, to touch down on a commercial runway amid public fanfare. After crew alight from the craft, the vehicle could be whisked away from the tarmac back to the United States inside a C-5 or similar cargo aircraft.

One more subtle point about this “can do” mini space plane. In my opinion, the X-37B gets an 8 on the aerodynamic aesthetics scale for transatmospheric vehicles (in a range with the Soyuz capsule rating a 1 at the bottom and the oft-copied X-20 Dyna-Soar a 10 at the top).

Engineers sometimes forget that human space activity is as much about inspiring humanity and the next generation of explorers with daring visions of the future as it is about advancing leading-edge applications of technology. Once the shuttle executed its maiden flight in 1984, it became an icon of human spaceflight. After nearly three decades of service, the era of capsules became obsolete, at least in the eyes of the public. And yet, facing the wall of sheer economics, we chose to build the Orion as a capsule to replace the shuttle!

The aerothermodynamic physics of re-entry and limitations of materials and engineering notwithstanding, it appears to be a giant step back that we are unable to muster the resources or the sheer imagination that is the hallmark of the astronautical engineering community to push the envelope to create the next Earth return vehicle with sleeker appeal, more in keeping with the glorious dawn of possibilities of this 21st century.

With the troubled world economy, it is good to be reminded by Oscar Wilde that  “we are all in the gutter, but some of us are looking at the stars.” Or Vincent Van Gogh: “I know nothing with any certainty, but the sight of the stars makes me dream.” I think they meant to inspire. Perhaps the tiny X-37B might find a role in inspiring a new generation of human space explorers. Perhaps beautiful things still come in small packages. 


Madhu Thangavelu is conductor of the ASTE 527 Graduate Space Concepts Studio in the Department of Astronautical Engineering at the Viterbi School of Engineering and the School of Architecture at the University of Southern California. Slides from the Fall 2009 team project “The International Space Station: Investing in Humanity’s Future” may be viewed online at|TeamProject20093.