Conclusions

“Nothing ever built arose to touch the skies unless some man dreamed that it should, some man believed that it could, and some man willed that it must.” – Charles Kettering

The ‘golden age’ of the rocket plane, whether it is defined in terms of the number of aircraft, speed of progress or number of flights, kicked off with the He-176 in 1939, essentially at the same time as the jet age, and arguably ended with the final flight of the X-15 in 1968. Successful rocket aircraft projects of that period were based upon three vital ingredients: a good aircraft design, a good rocket engine, and a great pilot. If any part of this fundamental triangle was lacking, the outcome was often disaster: aircraft pitched over due to Mach tuck, engines blew up, and pilots overshot landing fields and crashed their expensive aircraft. The extreme speeds that rocket aircraft achieved and the new aerodynamic territories they ventured into meant things could go wrong very fast and very unexpectedly. Pilots who let their powerful, sleek planes get ahead of them often did not make it back. And extensive flight experience did not mean that pilots were safe from making mistakes. For each new, experimental rocket aircraft every pilot was essentially inexperienced. The same applied to the designers, but at least they rarely lost their fives due to a fault in their aircraft or engine.

However, even while they were in the limelight, airplanes with rocket propulsion were rapidly rendered obsolete by improved turbojet engines. The Me 163B was the only pure rocket fighter that ever entered military service, while the only operational mixed-power fighters were the Mirage IIIC, – E, and – S, and for most of the time even these flew without their optional rocket packs. Altogether the rocket propelled fighter plane does not have a very impressive track record when taking into account all the development effort on experimental aircraft and prototypes.

Rocket planes were soon realized only to be really useful as research aircraft to fly at extremely high speeds and altitudes. The X-15 set incredible records for aircraft speed and altitude but the data it collected at the extremes of its flight envelope was so far beyond what was required for operationally useful manned aircraft that there was no need to make a successor to push the boundaries even further. Orbital rocket planes, the logical next step, proved to be too complex, too costly, and ultimately not really needed. Rocket aircraft development therefore stopped at the end of its

infancy and at the peak of its success, and so never matured into really operationally useful series-produced planes. Instead, new military planes relied on advanced jet engines and spacecraft kept using vertical take-off launchers that were usually expendable or at best included a reusable shuttle that was able to glide back from orbit.

By the mid-1980s it seemed that a second golden age was about to begin, with a number of ambitious spaceplanes and hypersonic airliners such as the Sanger-II and HOTOL following up on the experimental rocket planes of the 1950s and 1960s. But these new vehicles would not be pure rocket planes, as they were to rely on airbreathing propulsion for the first part of their flight. Indeed, NASP was initially expected to do without rocket motors. In that respect, they were more similar to the various mixed-power interceptors of the 1960s.

But the revival proved to be a false start. While routine hypersonic flight into orbit appeared to many people to be imminent, the unforgiving numbers in the engineers’ weight budgets and the managers’ cost estimates said otherwise. In part the optimism appears to have been inspired by the ease of imagining a spaceplane flying into orbit as a natural extension of high-speed and high-altitude aviation: an X-15, just flying a bit faster. Looked at Uke this, the intrinsic difficulties seemed smaller than they really are. Spaceplanes are inherently large because of the enormous volumes of propellant required. It is possible to make a small, relatively low cost aircraft, but not a small, cheap orbital spaceplane (indeed, people build simple aircraft in their garage but it is very unlikely that one day your neighbor will roll a hypersonic satellite launcher out of his shed).

Dr. Richard HalUon, a former Chief Historian of the US Air Force recently said of the apparent lack of progress in hypersonic flight (and thus the spaceplanes discussed in this book): “The hope of hypersonics thus became inextricably caught up in what might be termed a hypersonic hype. This led, over time, to a cycle of fits and starts that has largely worked to discredit the potential of the field and taint it with an image of waste and futility. Typically, a program has begun with great fanfare and promise, increased in complexity, and when realistic performance, schedule, and cost estimates are derived, its appeal quickly fades.”

In addition to the canceled X-20 Air Force project, NASA has a long history of abandoned hypersonic projects, including the X-30, X-33, X-34 and X-38. The space agency seems essentially to have given up on spaceplanes, shuttle-type space gliders, and indeed reusable launchers in general for the near future. The Russians had their single Buran flight but never progressed beyond paper studies for real spaceplane concepts. At present, neither NASA nor the Russian Space Agency, nor indeed any other space agency, is willing to risk burning its hands on another shuttle, let alone a spaceplane project.

A modest resumption of interest in the rocket plane was kicked off by the success of SpaceShipOne. Hopefully other suborbital rocket propelled aircraft will soon fly. However, it seems that brief suborbital flights represent the last niche in aeronautics for the pure rocket powered plane to play a useful role: any future hypersonic aircraft or orbital spaceplane will primarily rely on advanced forms of jet propulsion, perhaps in combination with rocket power if really necessary. In fact, even the early

X-planes like the X-l, X-2 and X-15, as well as the D-558-2 Skyrocket, were launched from large turbojet aircraft that can be regarded as airbreathing first stages.

The development of hypersonic launch vehicles will be expensive but by using a one-step-at-a-time approach, also known as ‘crawl-walk-run’, it may be technically feasible as well as affordable with or without government funding. The logic is clear: start with a suborbital aircraft such as SpaceShipTwo, advance to a suborbital hopper that can launch payloads into low orbit at the apogee of its ballistic flight into space, and finally make an orbital spaceplane. Each of these steps could be a commercially viable project in its own right, earning the money needed to fund the next step on the road to a fully reusable launch vehicle. In this regard, NASA’s Commercial Orbital Transportation Services program (which encourages private companies to introduce crew and cargo transportation vehicles to service the International Space Station) is of interest since one of the participants, SpaceDev, is developing the aforementioned Dream Chaser mini-shuttle.

At the same time, the US military’s desire for a long-range hypersonic missile (or even an attack aircraft) able to reach any place on Earth in no-time and fly too fast to be shot down is generating a lot of spaceplane technology. Perhaps in the foreseeable future the quest for the first operational hypersonic aircraft able to routinely fly into orbit and back will finally be concluded. Meanwhile the old quip in the US military remains valid that “hypersonics is the future of airpower and always will be”.

A proper airplane should have pilots on board, but the more that time passes the lower the chance that future spaceplanes will be directly piloted by anyone present. For launch vehicles flying only cargo it seems certain no crew will be required, and pilots may not be needed even for transporting astronauts. The Sanger-II and NASP spaceplanes would have had cockpits and flight crews but HOTOL was specifically intended to fly without, as indeed is its Skylon successor. The technology of orbital spaceplanes will be just as exciting as that of the X-15, but without pilots the concept loses a lot of its glamour and sense of adventure.

So what are the chances of there being a fully reusable, crewed rocket plane (with or without airbreathing engines) like the Euro 5 discussed in the Preface of this book blasting through the air anytime soon? Unfortunately, I think it looks like it will take quite while. The only operational rocket aircraft in the near future will be suborbital. When orbital spaceplanes eventually come around (hopefully) it is likely they will be fully automatic vehicles rather than resembling a hypersonic fighter aircraft piloted by gallant astronauts.

Still, the required technology and the possibilities that are on offer are extremely exciting: if spaceplanes really can dramatically reduce the cost of putting things into orbit then they will at long last open up space for large-scale commerce, production, moonbases, space solar energy satellites, space hotels and other marvelous ideas that are currently wishful artistic impressions and science fiction.

I just cannot accept that the expensive, wasteful expendable rocket as we know it today is the best that we can do and therefore represents the final answer in access to space. Spaceplanes truly represent the last great aeronautical frontier.