Status and Perspectives of Hypersonic Systems and Technologies with Emphasis on the Role of Sub-Orbital Flight ∗ S
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Aerotecnica Missili & Spazio, The Journal of Aerospace Science, Technology and Systems Status and Perspectives of Hypersonic Systems and Technologies with Emphasis on the Role of Sub-Orbital Flight ∗ S. Chiesaa, G. Russob, M. Fioritia, S. Corpinoa a Politecnico di Torino Department of Mechanical and Aerospace Engineering b AIDAA Sez. Napoletana c/o Department of Industrial Engineering \Federico II" - University of Naples Abstract The relevance and the main features of Hypersonic Flight are shortly presented and discussed. A synthetic overview of the few realisations and of many studies, projects and technological demonstrators offers opportunity of point out many interesting aspects. Then the more actual realisations and studies are considered, with particular attention to the new sector of small suborbital planes for Space Tourism. That gives opportunity of proposing a Road Map for an affordable and logic development of Hypersonic Flight, passing through the development of a suborbital Space Tourism plane on the basis of an existing subsonic business jet. Then the suborbital plane will constitute the basis of developing an Hypersonic Cruise Business Jet and, as a next step, a TSTO. 1. Introduction 2. Hypersonic Flight Concept The hypersonic flight is a subject on which the ef- It is considered appropriate to establish a definition forts of basic and applied aerospace research have been of hypersonic aircraft, useful to define the topic of dis- focused over the years. These efforts consisted in cussion; we define hypersonic aircraft a system capable the development of several projects and correspond- of taking off and landing horizontally (HOTOL - Hori- ing technological fields, such as aerodynamics, propul- zontal Take Off and Landing), and capable of flying at sion, structures capable of withstanding high thermal a Mach higher than 4.5. Please note that exceptions stress, the system design, etc. By considering such regarding the takeoff may befound, as: efforts, if great progresses have been achieved on the a) aircraft VTOHOL - Vertical Take Off and Hori- side of the technological insights in most of the areas zontal Landing - can be conceived, that is a solution covered, very poor results were obtained in terms of imposed by technological constraints and in any case realization of operational products; the only practi- not to be considered definitive cal achievements were experimental vehicles or tech- b) Always for "not definitive"aircraft (and therefore nological demonstrators, capable of performing only for many of the "technology demonstrators" realized) partially the mission of a real hypersonic aircraft, and the landing may be horizontal but not on runway, with consequently with low operational capability. The aim a landing gear like airplane, but with parachute and / of this work is to summarise the great mass of ac- or paraglide, on ground or on water. As to the abil- cumulated experiences and, considering the current ity of achieving hypersonic speed (for characterization achievements and projects now under study, to suggest of the aerodynamic behavior at high Mach numbers strategies that can promote advancement in the field, greater than 4.5, please refer to the many excellent in particular leading to the development of hypersonic texts, particularly [1]) we can make some considera- aircraft more and more complex, tending to achieve a tions: complete and mature ability to fly hypersonically. a) When hypersonic speed (Mach 4.5 or even more) has been reached, if the plane can sustain such speed for periods of the order of a few hours, it will obviously get extremely interesting autonomies of flight. The hy- ∗Based on paper presented at the XXII Conference personic cruise (for containment of dynamic pressure AIDAA, September 2013 Naples, Italy values and then for containment of structures heat- 1 c AIDAA, Associazione Italiana di Aeronautica e Astronautica 100 Status and Perspectives of Hypersonic Systems and Technologies with Emphasis on the Role of Sub-Orbital Flight 101 ing) must be carried out at altitudes of about 30 km or more, i.e. at altitudes higher than those of the sub- sonic jet airplanes (9-12 km) and of those, higher, of the SST (Supersonic Transport) like the Concorde (17- 18 km). b) By considering, with reference to the previous point, the flight altitude at which hypersonic speed can be reached, at conceptual level it can be hypothesized that the energy, in the previous case used to support the hypersonic cruise, could be instead utilized for fur- ther increasing altitude and speed. In case when both the speed and altitude increase, it can be assumed that the reduction of aerodynamic lift (because of the de- Figure 1. Kinds of Hypersonic Planes. [3] crease of the atmospheric density and despite the in- crease in speed) is compensated by the effect of cen- trifugal force due to the curved trajectory (because of the terrestrial curvature), with the possibility of reach- the Vertical Take Off-VTO Systems such as rockets, ing the orbital speed [2] at a given height (generally at provides a much more long stay in the dense layers of least 100 km for containing the atmospheric drag). In the Earth atmosphere (Figure 3); this leads to the op- this case, the hypersonic vehicle, capable of achieving portunity of providing \air breathing" engines even if a low orbit around the Earth and, of course, in case it with constraint of switching on the Ramjet over Mach transports payloads, can be defined orbital transport. 3.5. It is clear that, through a de-orbiting maneuver, the In this way we confirm that three different kinds of aircraft will return to earth with a trajectory, without Hypersonic Planes can be envisaged, i.e. the ones ca- propulsion, except any action of slowing down or brak- pable of sustaining hypersonic cruise without further ing. This return trajectory will be critical in order to increase of altitude and speed, or the others capable the capacity to dispose all kinetic and potential energy of increasing altitude and speed, both till to reach the possessed. If the carried payload will consist of artifi- orbit or to perform suborbital flight aimed to cover ex- cial satellites to be released in orbit, we can define the tended ranges on the Earth surface.It should be noted vehicle a Reusable Launcher-RL. that the boundary of Mach 6.5 for the Ramjet im- c) An intermediate situation between the two above poses, for higher speed, the switch to either the Scram- considered is to climb at altitude of the order of orbital jet (Supersonic Combustion Ram-Jet) or the Rocket. ones, even without reaching the orbital speed, proceed- The Scramjet technology is not fully acquiredyet and ing then to a trajectory of descent (non-propelled), there is a doubt concerning the speed limit while, cer- with ability to return to land far from the departure tainly, there is one about the altitude as, in any case, point; with this type of \sub-orbital" flight, range and an air breathing engine cannot be able to operate in flight duration comparable with those seen in the pre- vacuum, while the rocket, as known, will not have any vious point a) can be obtained. The three types of constraint of operation.Figure 4 reminds how the Spe- hypersonic missions just described are schematically cific Impulse for various kinds of engines evolves with shown in Figure 1.The 3 different types have of course Mach. It is clear the opportunity of using the types different characteristics and utilization strategies, but with better specific impulse just till the speed at which they show affinities and a common logical ratio that is they must be substituted by another engine with lower summarized in Figure 2. In this figure we show how, specific impulse. In any case it is clear the advantage, starting from the need of increasing speed and range for each kind of engines, to use, as fuel, LH2 rather of flight for air transports (in order to make accessible than hydrocarbons. in a few hours any point on the Earth) we have to in- With regard to the aforesaid considerations about en- crease the flight altitude and to adopt different engines gines and reminding that Ramjet and Scramjet have from turbojets (which, even with after-burner, cannot also a lower limit of speed for their operation, in Figure fly over Mach 3.5) obtaining a type of machine that 5 we present the more logical combinations of engines we can define \AirSpacePlane". that can be envisaged for Hypersonic Planes with dif- Figure 2 also shows that a similar result can ferent values of maximum speed and maximum alti- be reached, by imagining that we want to make tudes that they can reach. \reusable" a Launcher by providing it with a lifting To complete the overview of theHypersonic Planes surface that allows it to return to ground safely, after possible configurations, we have to consider the possi- its mission has been performed. From this first step, ble adoption of a multi-stage solution, aiming to the the next is to use such lifting surface also for the phase same advantage in order that it is absolutely manda- of ascent, with a trajectory that, compared to that of tory for Reusable Launchers; in factit is well known Aerotecnica Vol.88, No.1/2, January-June 2009 102 S. Chiesa, G. Russo, M. Fioriti, S. Corpino that advantage lies in ridding the vehicle itself by the no longer necessary parts (i.e. \inert masses") due to the progressive exhaustion of the propellants. The downside is the criticality in the operation of separa- tion and the duplication of Crew or GNC Systems (in the case of \Unmanned" solutions) for the return to earth of the early stage (Figure 6).