The recent demise of NASA's X-33 advanced technology demonstrator and its would-be follow-on Lockheed Martin's VentureStar fully reusable single-stage-to-orbit launch vehicle is only the latest, although the most dramatic, in a series of cancellations of reusable launch systems. Most of these ill-fated RLV projects were officially designed to take over the commercial launch market from vintage expendable launchers.
While some may blame the failure of the X-33/VentureStar development on Lockheed Martin's management of the project, private entrepreneurs are accusing the U.S. space agency of killing their "emerging industry" by distributing budgets to large aerospace corporations and zeroing their chance to finance their own fancy projects.
Actually, none of them, either large or small, seems to have fully taken into consideration the incredibly demanding requirements for the development of a commercially-oriented RLV beyond the already very impressive technical challenge.
Selected in 1996, the wedge-shaped lifting-body design for the X-33 was supposed to be a half-scale version of an operational commercial RLV to be later developed by Lockheed Martin, the VentureStar.
Since the introduction of NASA's Space Shuttle, it has become obvious that a reusable launch vehicle is not cheaper than an expendable one. This would be true only if the two were of similar nature. Unfortunately, to be reusable, a launch vehicle has to be more robust in order to sustain launch loads as well as reentry and landing loads, not only once but many times. Moreover, it has to be designed to allow an easy access to all its components to enable maintenance and replacement when needed. It also has to incorporate smart health monitoring systems and recovery systems able to bring back the vehicle safely on the ground. This is especially true in case of launch abort where an expendable launcher needs only a flight termination system.
At a similar technological level, such constraints increase the launch mass, leading to a well larger and heavier reusable launcher compared to an expendable vehicle of identical payload capacity. Being larger and more complex, it is also more expensive.
Beyond these basic costs, we need to consider those related to recovery, maintenance and refurbishment. Reusability has a very high cost.
Stiff Competition to be Expected from ELVs
To access the commercial launch market, a RLV will have to be competitive with the ELVs whose specific cost is decreasing while they benefit (at a smaller scale) from the same technological breakthroughs such as advanced avionics and lightweight materials. Moreover, the RLV will have to maintain its competitiveness all along its lifetime. If we consider a 10-year development and a 10-year lifetime, this means that it will have to be competitive with ELVs available 20 years after its inception while these new launchers will incorporate advanced technologies introduced even after its own entry in service.
Another major challenge comes from the launch market itself. The recent years have shown that it is changing fast and that launchers have to adapt to new requirements in terms of mass, volume and availability. A series of expendable launchers can be upgraded by some design changes, such as the introduction of stretched stages and larger payload fairings. Propulsion and avionics upgrades are possible on a reusable launcher but the vehicle's basic design will remain as it can hardly be re-built to new standards several times during its lifetime. Thus, necessarily, the RLV will not be optimized for the market during part, if not most, of its operational life. It will then have to be operated under non-optimum conditions, likely leading to an increased specific launch cost.
Which Market to Address?
Reusability makes sense only with frequent flights. Some studies gave the rate of 20 flights per year and per vehicle as a threshold. As of today, Arianespace, considered as the leading commercial space launch provider, conducts barely 10 to 12 flights per year and market forecasts show that the market will not increase significantly in the foreseeable future.
A commonly accepted idea is that significantly reducing the launch costs would allow more space projects to emerge. This is true for small science payloads from cash strapped research institutes but they can hardly be considered as a reliable and wealthy market. However, this is no longer the case for large satellite operators. Launch costs for large commercial satellites have largely decreased during the last decade, compared to the cost of the spacecraft themselves and their ground segments, and they are no longer considered as decisive in a business plan.
From a technical point of view, an RLV seems to be more suitable for low-Earth orbits rather than higher ones such as geostationary or medium Earth orbits. Accessing the latter requires an upper stage which can be considered as another, smaller, launch vehicle, integrating costly propulsion and avionics systems. To be reusable too, such a vehicle would have to carry twice more propellant to be able to get back to low-Earth orbit. It could be deployed only from an orbital vehicle able to recover it at the end of the flight. If it is kept expendable, it will add to the operating cost of the RLV.
Unfortunately, the market for low-Earth orbit launches looks rather small. As the number of observation satellites may increase in the short and medium-term future, so is their operational lifetime which delays significantly any need for replenishment. Even if constellations were to be resurrected, they would likely be deployed by clustered launches. Consequently there is no foreseeable significant commercial market in low-Earth orbit requiring more than 20 flights per year for a given launch vehicle.
All these considerations seem to leave little hope for a commercial RLV in the short to medium-term future unless, of course, a major technological breakthrough changes the whole picture.
Nevertheless, this does not mean that there will be no need for RLVs. Actually, they could have a major role to play for the launch of very large and costly payloads to low-Earth orbit and for manned space flights, the latter requiring a vehicle and its passengers to return to Earth. Although the Mir/Soyuz/Progress troika has demonstrated that a limited space station could be operated with expendable launch vehicles at relatively low cost, the launch of elements and crews for a large orbital infrastructure or the assembly of large ships heading to the Moon, Mars or the asteroids would require frequent flights which could be performed by future RLVs. They could be the key to man-tended capabilities in Earth's vicinity.
Then the decision to develop them should not be taken based on false assumptions regarding a would-be commercial market, they should be decided as a political tool for man-tended activities in space and designed as such.
Stefan Barensky is senior space editor for Space-Launcher.com.
React to this column.