While the common understanding of the European community is that sub-orbital high-altitude flight is technically feasible within a few years, building on the available knowledge in aviation, it has never been proven experimentally. The USA have achieved an air-launched X-vehicle with SS1, which, however, requires significant effort before becoming a commercial, routinely used transport vehicle such as SS2. Such sub-orbital flight is also understood to be on the borderline to space, since the transport of people is approaching the orbital environment without really entering it fully in the sense of having to master the harsh environment of hypersonic re-entry into the atmosphere. According to reports the interest in the USA and elsewhere in high-altitude flying is very large in spite of the high price, suggesting a profitable niche for commercial flight and triggering innovation in small industries to satisfy such demand.
The objective of the study was to identify and assess the long-term potential of commercial high-altitude flight in Europe for selected mission requirements, in view of the activities in the USA following the successful SpaceShipOne (SS1) demonstration and the efforts performed to arrive at the next generation SpaceShip2 as well as aspirations by other companies. Furthermore, it was proposed to identify for Europe missing developments in technology and address safety measures as well as needed steps to satisfy legislation. A corresponding research and development strategy to enable commercial high-altitude flights would be worked out in order to secure the international competitiveness of European industries.
Key objectives were:
- assess worldwide activities and define reasonable mission requirements;
- identify potential show stoppers, technical but in particular non-technical ones, and missing elements for carrying out commercial high-altitude flight;
- propose a way forward to achieve commercial sub-orbital flight, including potential self-sustained development steps leading to human hypersonic flight, and a funding scenario for a first experimental flight.
The topics dealt with in this project were:
- To evaluate the mission opportunities of sub-orbital, potentially commercial flights based on the review of available publications and existing in-house performed work.
- To review the technical and non-technical elements required for sub-orbital flights, and to identify problems or missing elements/technologies. This includes the investigation of what has to be achieved in terms of legal issues, and other non-technology topics to be able to fly, and without too many restrictions regarding flying over land and inhabited areas. Another topic is the estimation of costs associated with sub-orbital flight based on selected mission opportunities. This would include the rough costing of a first experimental flight.
- To investigate the possibility of an air launch for sub-orbital flight, including the effect on costs in the short and long term, and whether the launching carrier could be used for other purposes. The task would include choosing a launching strategy.
- To summarise the experiences gained in the work on topics 1 to 3 by performing a synthesis and attempting a roadmap, including a rough scheduling of necessary events.
Available/published approaches for high-altitude flight have been reviewed, and a corresponding assessment has been made. An analysis of potential market opportunities in addition to space tourism has been carried out. Descent trajectories for non-zero horizontal velocities at maximum altitude have been investigated with regard to loads on the vehicle and on the people. A tentative list of requirements for high-altitude flight has been set up.
The following list indicates some requirements for the manned suborbital missions and their preliminary justifications:
- Altitude should be at least 100 Km (because of the astronautic-like view);
- Duration of low g phase > 2 minutes (much larger than for parabolic flights);
- Acceptable g loads for passengers: < 4 – 5 g over a short time (see also the famous and often shown acceptable acceleration vs duration plot);
- Number of passengers < 10 (to guarantee the exclusivity as long as possible);
- Piloted : yes;
- Safety : high;
- Visibility of space and earth: definitely necessary;
- Short maintenance duration: > 1 flight per week (XCOR claims several flights per day for its planned 2-seated Lynx concept);
- Ticket price : probably lower than 200 K$, initially well accepted as the booking for SS2 flights and others suggest, but requires low operational costs.
One of the major driving requirements of the above list is the acceptable g-load. The acceptable acceleration level is known to be a strong function of the duration of the g load as is manifested in the mentioned well-known diagram. The acceleration level Gz in body axis is the most sensitive value (< 4-5 g), while the value Gx along the transverse axis can reach higher values (> 10 g), and be still tolerable for trained persons.
The work performed for FLACON has shown that there is no technical show stopper as was already anticipated. The propulsion system required to achieve space-like altitudes has not been considered here. For BEOS suitable Russian liquid propulsion engines were assumed available off-the-shelf. Preliminary investigations for a proposal for FP7, called future high-altitude high-speed transport 20XX, indicated that a hybrid propulsion system could be made available in West Europe, if needed, similar to the one of SS1, but not with corresponding validation history. The air launch approach was followed since in the USA this approach to suborbital flight was shown to be very successful leading to a follow-up version which will be operated commercially within the next two years. In fact, several decades ago also West Europe had some experience with this approach, however, the corresponding know how needs to be gained again.
Last not least: it is true that the investigation of additional mission opportunities was not encouraging unless the cost of suborbital flight is substantially lower than that one with presently used flight opportunities, e.g. sounding rockets. However, more recently intentions are published, e.g. by Virgin Galactic, to use either the carrier aircraft or the space vehicle as rocket base to launch small payloads into orbit. This is done hoping that the new approach turns out to be much cheaper than the classical approach with rockets fired from the ground. This requires not only a reliable design, but also high re-usability of all elements used.