From the highs of the X-15's flights at over six times the speed of sound to the lows of the early-1990s' cancellation of the over-ambitious X-30 single-stage-to-orbit aerospace plane, the USA's "X-" series of experimental vehicles has exemplified the country's drive for dominance in the aeronautical and space technology arenas.
Over the last 50-plus years, the "X-planes" have tested thin wings, swept wings, swing wings, even laminar flow wings. Propulsion has been provided by rockets, jets and even propellers - tilting and ducted. The coveted "X-" designation has been carried by aircraft as diverse as a vertical take-off tailsitter and a gliding gyrocopter.
But times are changing for the US aerospace industry, and so, too, is the X-series. There are a dozen such projects under way and, while they are all different, they share some common features. Those are: a shift towards "space", and away from "aero"; use of government/industry cost-sharing to reduce expense; and the use of subscale unmanned research vehicles, also to reduce cost.
The shift towards space is easy to explain: US industry sees commercial space as the fastest growing, and potentially largest, market of the next century. But to unlock the potential of that market, the cost of access to space must be reduced dramatically. And the technology to achieve that reduction is still too risky for industry to embrace unaided.
Hence the emphasis on space applications among the new technology demonstrators, from the X-33 and X-34 reusable launch vehicles, to the X-37 and X-40 reusable orbit-to-earth vehicles. That emphasis is unlikely to change in the foreseeable future.
Neither is the emphasis on cost sharing. The X-series is still about testing technology that will give the USA an edge - but it is less about giving the US military a combat edge and more about giving US industry a competitive edge. Increasingly, to qualify for coveted X-plane status, a concept must now offer the prospect of commercial, as well as technical, success.
This can be expected to influence the type of project that will attract government and industry patronage. This may also be the reason why aeronautical technology demonstrators are a little thin on the ground. The X-32 and X-35 Joint Strike Fighter concept demonstrators excepted - their status as true X-planes is honorary at best - there are only three aeronautical X-series projects active: the X-31 and X-36 tailless fighter and X-43 hypersonic scramjet testbeds.
There are plenty of aeronautical concepts waiting in the wings, but whether industry can persuade government to share the cost of demonstrating them remains to be seen. Before its merger with Boeing, McDonnell Douglas talked to NASA about a subscale X-plane demonstrator for its blended wing-body transport. NASA has discussed with Boeing and Lockheed Martin a similar X-plane demonstrator for the high-speed civil transport. Both have stalled on costs, and uncertainties over the commercial returns.
While such demonstrators are likely to be expensive, projects like the X-36 have shown that not all X-planes are costly. For $20 million, Boeing built and flew two remotely piloted subscale research vehicles, successfully demonstrating tailless fighter agility. Even the $150 million cost of Boeing's next "X-plane", the orbit-to-earth Future-X, does not seem excessive.
The key lies in rapid prototyping techniques, and the latest X-projects are as much validations of design and manufacturing technologies as they are demonstrations of the vehicles themselves. The fact that advanced technologies can be demonstrated at relatively low cost - and with high visibility - using rapidly prototyped subscale unmanned vehicles should not go unnoticed by Europe's aerospace leaders.
A series of such demonstrators, backed by industry and government and addressing technologies with the greatest commercial leverage, could go a long way to giving Europe's modest R&D spending more visibility. With the 100th anniversary of flight approaching, the USA is poised to claim aerospace as its own - unless Europe can rally around a high-technology flag like its own next-century X-series.
Source: Flight International