Sub-orbital Rockets to Space:
In the late 1990's a great swell of optimism arose for the commercial development of reusable launch vehicles (RLV). Several large constellations of satellites under construction would provide worldwide telephone service and, while expendable launchers would suffice to orbit the initial sets of spacecraft, the need to replace satellites as they aged and failed would provide a lucrative and long term market for low cost launch providers.
Companies such as Kistler Aerospace, Rotary Rocket, Kelly Space, and Pioneer Rocketplane announced plans for one or two stage reusable launch systems that they claimed would provide cheap access to orbit. Some of the companies signed agreements with the constellation operators to deliver payloads once their vehicles became available. Kistler raised around half a billion dollars and built many of the components for its two stage system. Rotary raised about 30 million and flew a test vehicle to prove its innovative rotor landing technology. Other companies raised several million dollars while getting at least as far as advanced design status.
Unfortunately, the financial crashes of Iridium, Globalstar, and ICO caused the private RLV projects to fall back to earth as well. Without such a clear and convincing market, it became virtually impossible for the RLV companies to persuade potential investors that they could obtain a decent return on the several hundred million dollars needed to get a vehicle to operational status.
A Bridge Less Far
Throughout this period a few people warned that an orbital vehicle, especially a piloted vehicle like Rotary's, was too ambitious, especially for small startups. Technical problems would inevitably appear and cause delays and drive launch costs higher than promised.
Instead, they argued for development of robust, reliable and low cost piloted sub-orbital RLVs, which hold far fewer technological challenges. Reaching an apogee of 100 km and back down on a parabolic trajectory requires about 6 times lower velocity (or, alternatively, more than 36 times less energy) than what's required to attain orbit. A craft that reaches such altitudes could serve a number of applications while proving out various reusable technologies and providing experience in operating a rocket vehicle like an airliner.
Most importantly in the current tough investment climate, the price is substantially lower - tens of millions of dollars instead of hundreds of millions of dollars. As seen with the orbiter projects, such levels of funding are well within the available range of private investment sources. Furthermore, a return on investment would start arriving at a much earlier stage since sub-orbiter development could proceed comparatively rapidly (1-2 years) and much of the current sub-orbital expendable market could be quickly overtaken.
An early revenue stream not only rewards investors but can also support a second generation of vehicles that can reach orbit. Along with the technological experience gained, such as use of lightweight composite tanks and structures, a sub-orbiter business becomes a strong base on which to build a private space transportation system.
Sounding the Sky
Expendable sub-orbital rockets have long provided test beds for engineers and access to space for scientists. Many of the surplus V-2 rockets brought to the US after the war, for example, were used for both engineering studies and to do high altitude research. Usually referred to as sounding rockets (from the nautical term "to sound" meaning to measure), such expendables are especially valuable for reaching altitudes above which balloons can fly (~50km) and below where satellites orbit (~200km). The payloads carry out numerous types of missions such as meteorological measurements, micro-gravity experiments, astrophysics observations, and magnetosphere studies.
NASA currently employs 14 types of sounding rockets that launch a variety of payloads to a wide range of altitudes. The single-stage Arcas, for example, stands about 3.5m high and can deliver 4kg to over 90km. The 20m 4-stage Black Brant XII can send 70-850kg payloads to altitudes of 1500-150km, respectively.
Flying parabolic trajectories, the total mission times might last only about 30 minutes and provide only a few minutes of micro-gravity (the highest altitude Black Brant flights give around 20 minutes of micro-gravity). Nevertheless, sounding rockets offer a number of advantages over satellites or shuttle flights. Sounding rockets are much cheaper, payload development times are shorter, launch opportunities are more frequent, and the launches take place from a wide variety of geographic locations. While the launchers are not usually recovered, the payloads can parachute back to earth for possible refurbishment and reuse.
A number of different organizations carry out sub-orbital launches and it's difficult to obtain a clear accounting of the industry. However, estimates of the current sounding rocket market indicate that about 30-40 flights take place each year in the US. Roughly $100 million per year is spent on scientific launches and about $300 million on military related missions. Similarly, an estimate of launch cost at around $2000 per kilogram is inexact due to big variations according to payload volume, the rocket used, maximum altitude, etc.
So what can reusable sub-orbital spacecraft offer that sounding rockets cannot? Full reusability should allow more frequent flight opportunities, substantial cost reductions, and new capabilities.
The primary goal, in fact, is to operate RLVs just like an airliner. This requires solid, robust hardware that needs only minimal maintenance between flights and infrequent overhauls. Launch preparation, the launch, and flight control should require only small ground crews and simple facilities. Turnaround times should allow for daily flights.
The current applications will certainly benefit from more frequent flights and lower prices. For example, a re-flight will be easier and quicker to obtain if the experiment fails. Furthermore, with a crew on board, the experiments become cheaper because the apparatus will need less testing and redundancy. Someone is there to provide interaction with the equipment and can fix minor problems that would otherwise cause the failure of the mission.
Clearly, though, to justify daily operations the demand for more flights must rise significantly above the current level. The expectation is that the market will respond "elastically" and expand as lower prices inspire the introduction of new applications.
For example, one of the most interesting new applications proposed for sub-orbiters is imaging for remote sensing and reconnaissance. Cameras can be taken to heights far above where airplanes do aerial photography, which is a multi-billion dollar industry, and so can view a much bigger area at one go. On the other hand, they take pictures below satellite orbit altitudes and thus obtain higher resolutions (for the same camera specifications) and are not restricted to particular orbit flyover times.
Space tourism provides another especially exciting application. A flight to 100km would take amateur astronauts above the official boundary to space and provide a crisp view of the curvature of the earth and of a black sky filled with bright stars. After engine shutdown, they will experience a few minutes of weightlessness before the reentry braking begins. Many regulatory and liability issues, of course, need resolution before the space ride business becomes a big moneymaker for sub-orbiters.
XCOR Aerospace EZ-Rocket on a test flight
with Dick Rutan at the controls.
There currently exist many sub-orbiter projects trying to proceed from blueprint to bending metal. These range from ultra-low budget X-Prize vehicles, which need at least a few million dollars, to multi-purpose vehicles intended for commercial, scientific and military markets that will require funding in the range of 30-50 million dollars.
The X-Prize contest seeks to help the cause by offering a big reward for the first demonstration of a low cost, reusable sub-orbital rocket vehicle. A ten million dollar purse will go to the team that first sends a pilot and 2 passengers to 100km and back safely and then repeats the feat within two weeks. Over twenty teams have formally entered the contest.
Sub-orbiter designs vary widely. Some are single stage winged vehicles for horizontal takeoff and landing, or wingless designs with vertical takeoff and landing. Others mix these approaches such as taking off vertically and then gliding back to a runway with a parafoil. Several proposals involve two stages.
TGV Rockets, which has long advocated the sub-orbital first step, offers a single stage, vertical takeoff and landing craft called the MICHELLE-B (Modular Incremental compact High Energy Low-cost Launch Experiment.) It will carry a pilot and a one metric ton payload to an altitude of 100 kilometers. The descent is initially slowed by a pop out heat shield and then, just before touchdown, its engines fire for a soft landing.
The project would cost about $50 million dollars and take about two years to fly from the availability of funding. They expect to reduce costs to around $200 per kilogram. While it has entered the X-Prize competition, it aims primarily to capture much of the sounding rocket market while also developing new applications such as high altitude imaging.
In the winged category, Pioneer Rocketplane is now focusing on a smaller version of the original Pathfinder called the XP. Details have not been released but it would presumably follow a similar design to the Pathfinder and use conventional turbo-fan engines for take off. After loading up LOX from a tanker during flight, it would fire a rocket engine to take it to 100km. The vehicle would then reenter the atmosphere and turn on the turbo-fans for landing. Bristol Spaceplanes of England is also proposing a turbo-fan/rocket combination vehicle called the Ascender.
Even with the lower price tags, sub-orbiter funding is still difficult to come by. The X-Prize contestants are generally relying on private funding and commercial sponsors. At the current development rates, the first test flights probably won't occur before 2003.
Both Pioneer and TGV have received support from a somewhat unlikely source - the state of Oklahoma. Oklahoma wants to convert a defunct military base into a spaceport and also to attract new high-tech industries to the state. It is offering enticements such as loan guarantees and a number of tax incentives to help launch companies finance their operations in Oklahoma.
XCOR Aerospace, founded by engineers formerly with Rotary Rocket, is developing low cost rocket engine technology that other projects can use. Recently, Dick Rutan flew an EZ-Long plane powered by two XCOR 400lb thrust rocket engines fueled by isopropyl alcohol and liquid oxygen.
The project is not aimed at a market for homebuilt EZ-Rockets but to demonstrate safe and reliable rocket propulsion. The company will next develop a higher thrust engine to power sub-orbital rocket vehicles for space tourism, rocket flight exhibitions at air shows, and the X-Prize competition.
Japanese vertical takeoff and landing test vehicle during
The emphasis so far here has been on private development of space launchers. The government, however, actually had a reusable sub-orbiter design that operated for nearly a decade. The X-15 demonstrated with 1950s technology that a rocket-powered vehicle could operate repeatedly and travel routinely across the boundary of space (its pilots received official astronaut designations.)
X style programs for rocket vehicle development were largely abandoned until the early 1990s when the military developed the vertical takeoff and landing DC-X (later enhanced by NASA and renamed the DC-XA) with a budget of around $50 million. The vehicle, which was never intended for high altitudes, succeeded at its primary goal of demonstrating that a rocket vehicle could achieve frequent flights and low cost operations. This included a small ground support and flight control team. The high point of the program was when the vehicle flew twice within 26 hours.
Ideally, several copies of the DC-XA would have been built as was done with the X-15. The capabilities of the system could have then been expanded in small, incremental steps and continued even if a vehicle was lost, as can be expected in such programs. Unfortunately, the project ended when the DC-XA burst into flames in a landing accident due to a failed leg deployment.
NASA chose instead to take a giant leap with the X-33 project, which required several advance technologies to be developed successfully before it could fly. The project was eventually canceled due to structural failures of a composite fuel tank and other problems.
The Japanese recently flew a small vertical takeoff and landing craft more in tune with the DC-X spirit. The RVT (Reusable Vehicle Test) vehicle rose in June of 2001 to a low altitude and landed safely. Plans are to expand the flight envelope gradually and develop the capabilities needed later to build a high altitude reusable vehicle.
Space One Step at a Time
The collapse of the orbital RLV projects was a blow not only to the companies involved but also to those who planned to use these launchers for more than just servicing telecom satellites. Space advocates saw these vehicles as the first real step towards low cost private access to space and the realization of the many grand space schemes long proposed but always stymied by their enormous launch costs.
In retrospect it now seems clear that this step was simply too high. Instead, the next logical step for private space development is the sub-orbital launcher. Projects such as the DC-X and the EZ-Rocket demonstrate that the technology for such a vehicle is very close at hand and that the incremental, small step by small step approach to rocket vehicle development is the way to go.
Sub-orbiters will prove that rocket powered vehicles can be rugged and reliable and they will provide tremendously valuable technological and operational experience. Early revenue streams will allow the companies to fund incremental improvements internally without relying on the whim of investors or government bureaucrats. Eventually, the companies will build a new generation of vehicles that will make the final step to orbit and beyond.
Thanks to Pat Bahn of TGV Rockets for many useful discussions.
Clark S. Lindsey is a free-lance programmer and publisher of the HobbySpace.com website.
© 2002 by Clark S. Lindsey