Routine access to space has proved elusive. As the cost becomes clear questions are being asked about the need to make spacecraft truly reusable.

Spaceflight is substantially younger than flight itself - half a century younger - but its pioneers were dreaming of voyaging to the moon and beyond even as the Wright brothers made their first flights 100 years ago. There has been significant progress in the 46 years since the Soviet Union put the first artificial satellite into orbit around the Earth, but the dreams of spaceflight pioneers such as Robert Goddard and Wernher von Braun remain unfulfilled.

The truth is that spaceflight is hard, arguably the hardest thing man has tried to accomplish. And if spaceflight is difficult, then safe, routine, commercially viable manned spaceflight is almost impossible - at least for another few decades. The disintegration of the Space Shuttle Columbia during re-entry was a setback for manned spaceflight, but it could also prove to be a milestone in the development of reusable launch vehicles (RLVs).

The Columbia Accident Investigation Board is likely to criticise NASA for handling the experimental Shuttle like an operational vehicle after only a few test flights. Columbia was the first of five orbiters to fly, in 1981, and had flown 28 times in just over 20 years when it crashed on the 113th Shuttle mission. It was the second fatal loss of the world's first reusable spacecraft, following the Challenger explosion in 1986. Aircraft, in contrast, fly for thousands of hours before they are certificated and delivered to the customer, and accumulate tens of thousands of hours a year once in service.

Aircraft-like operations are the holy grail of reusable spacecraft developers, but any designer with an RLV on the drawing board must be asking how it will be possible to prove the vehicle's safety and reliability without incurring the prohibitive cost of many development test flights. The alternative is to accept the risk of placing a relatively unproven design into service, then drive the risk down as the vehicle matures. But as two decades of Shuttle experience shows, risk reduces only slowly, if at all, when flight rates are low.

The Columbia accident came barely months after NASA had restructured its space transport plan, deciding to keep the Shuttle upgraded and in service for up to two more decades and launching development of an orbital space plane (OSP) for International Space Station (ISS) crew return and transfer, while delaying development of a second-generation RLV by at least five years. The latest plan calls for the Shuttle to operate until 2015 at least, with a decision in 2010 on whether to extend its life beyond 2020. The OSP, carried atop an expendable launch vehicle (ELV), is to enter service as a one-way crew return vehicle by 2010 or earlier, and as a two-way crew transfer vehicle two years later. The revamped next-generation launch technology (NGLT) programme could lead to the first flight of a Shuttle replacement by 2015, but could also see NASA skip a generation and pursue development of a more advanced RLV that would enter service after 2020.

All this seems a far cry from the vision of future spaceflight provided by Arthur C Clarke's and Stanley Kubrick's 1968 film 2001: A Space Odyssey. On the eve of the first manned landing on the moon, many believed it would be possible by the turn of century to have airliner-like launch vehicles flying commercially to massive space stations that would be linked by regular shuttle flights to thriving moonbases, but the transition from the expendable Saturn V to the reusable Shuttle proved more prolonged and expensive than anyone imagined.

Today, the bulk of spaceflights, manned or unmanned, are launched on expendable boosters. ELVs have become more reliable over time but, while the latest vehicles are designed to reduce launch costs, expendable launchers remain an expensive way of getting into space. The lure of RLVs has been the promise of a significantly lower cost per kilogramme of payload, thanks to greater reusability and more aircraft-like ground operations. But the road to full reusability has been a rocky one.

NASA's first attempt at replacing the partially reusable, and complex to operate, Shuttle was the Lockheed Martin X-33, a suborbital technology demonstrator for the planned VentureStar commercially developed and operated single-stage-to-orbit (SSTO) second-generation RLV. The X-33 proved ruinously over-ambitious technologically and was cancelled in 2001 before the demonstrator was completed, after technical problems led to schedule delays and cost overruns. NASA and its contractor team eventually spent almost $1.3 billion.

Next-generation RLV

The X-33 was superseded by the five-year, $4.8 billion Space Launch Initiative (SLI), the centrepiece of which was the development of technology for a second-generation RLV. NASA's original goal was to decide on full-scale development of the RLV in 2005, leading to a first flight in 2010. Development was to be funded by the US government, but plans called for the second-generation RLV to be operated commercially.

By late 2002, the three prime contractors working on SLI - Boeing, Lockheed Martin and Northrop Grumman - had selected their preferred vehicle concepts. All were two-stage-to-orbit (TSTO) designs, considered lower risk than an SSTO. But NASA and industry were becoming concerned over the potential development cost, while the need for a vehicle that would allow the ISS to be fully crewed was becoming critical, which led to the late-2002 revamp of the agency's space transport plan.

Following the Columbia accident, continued work on a second-generation RLV under the NGLT programme has taken a back seat to NASA's efforts to return the Shuttle to flight and develop the OSP to support the ISS. The OSP has drawn criticism as being too limited in its capability as it will only be designed to carry four crew to and from the space station. Designs being considered range from an Apollo-type capsule to a small reusable spaceplane, launched atop a Boeing Delta IV or Lockheed Martin Atlas IV.

To meet NASA's tight deadline, the OSP would be launched, initially unmanned, to dock with the ISS and act as a one-way lifeboat - a role now performed by the same Russian Soyuz TM craft that ferry crews to the station. Manned flights to the ISS are to begin two years later, and will require man-rating of the ELV/OSP combination. NASA plans to achieve this through the development of launch abort and crew escape systems for the OSP, rather than making major changes to the ELVs, which should have demonstrated their reliability over about 10 years of flying by the time crew transfer flights are due to start.

Critics of the OSP are concerned that NASA's near-term focus on supporting the ISS will delay development of a second-generation RLV to replace the Space Shuttle in the longer term. When the Shuttle returns to service, it is likely to be restricted to assembly and resupply flights to the ISS, aside from the occasional Hubble space telescope servicing mission. While ELVs including Europe's Ariane 5 can be used to fly cargo to the ISS, the Shuttle will be the only heavylift vehicle available to complete assembly and enable future expansion of the ISS.

While NASA's NGLT programme could lead to a decision to start full-scale development of a second-generation RLV as early as 2009, leading to a first flight - as an unmanned cargo vehicle - in 2015, most observers view this schedule as optimistic. Sceptics believe NASA could choose to bypass the conventional rocket-powered TSTO designs now being pursued in favour of a third-generation RLV that has an air-breathing hypersonic first stage. This would bring the US space agency more into line with its European and Japanese counterparts, which envisage continuing to develop their current generation of ELVs well into the next decade while maturing technology for a future generation of RLV.

Europe missed out on the first generation of reusable spacecraft, cancelling the Hermes spaceplane and concentrating instead on development of the Ariane family of expendable launchers. This has culminated in the heavylift Ariane 5, which has established itself as a formidable competitor in the commercial launch market. There are growth plans in place that would see the Ariane 5 evolve well into the next decade, and Europe does not see a compelling need to begin development of a reusable replacement in the near future.

NASA's original SLI programme worried Europe because it offered the prospect of a government-developed, commercially operated second-generation US RLV entering the satellite market with significantly lower launch costs as early as 2010. But the revised programme presents less of a threat to Ariane, which with European government support will able to hold its own against the latest generation of USELVs.

Europe has looked at repackaging Ariane 5 elements into a reusable spacecraft, but sees greater sense in waiting for technology to mature to the level where a third-generation RLV is feasible. This aligns closely with Japan's latest thinking, which involves continuing to upgrade its current generation expendable booster while working on technology development for an aircraft-like, air-breathing RLV that can offer a truly revolutionary reduction in launch costs.

Hypersonic work

Work on potential hypersonic air-breathing propulsion systems is under way in Europe and Japan as well as in the USA, where an array of programmes is pursuing development of different engine types and vehicle configurations. NASA is working on rocket-based and turbine-based combined-cycle (RBCC and TBCC) engines for a third-generation RLV, while the USAir Force is pursuing development of hydrocarbon-fuelled supersonic-combustion ramjets (scramjets) for hypersonic aircraft and missiles.

RBCC and TBCC propulsion systems combine scramjets with either rocket motors or turbine engines to allow the third-generation RLV to take off from a runway. The leading candidate is an unmanned, hydrogen-fuelled hypersonic aircraft with a propulsion system that uses turbine engines from zero airspeed to Mach 4, where ramjets take over and transition to supersonic combustion as speed increases, and with integrated rockets to take the vehicle out of the atmosphere to release its payload, which could be a manned spaceplane.

The concept looks promising, but high-speed turbine, ram/scramjet and combined-cycle engines have yet to be tested in flight. That is planned for later this decade, using a series of X-43 demonstrators in NASA's case. Even if these tests are successful, a hypersonic air-breathing RLV would not fly before 2022, more than 40 years after the Space Shuttle first flew, and more than half a century after the Pan Am Clipper Orion SSTO docked with the space station in 2001:A Space Odyssey.

There are many ideas for even more advanced methods of getting into space, including craft that would ride a beam of laser light or microwave energy, or climb a ribbon-thin space elevator thousands of kilometres long, but the reality is that development of new technology will take far longer, cost far more and involve far greater risk of failure than the pioneers of spacecraft could have ever imagined.

Source: Flight International