Multidisciplinary Design Analysis & Optimization of the SpaceLiner Passenger Stage

Kurzfassung

The SpaceLiner is a futuristic concept of a suborbital space plane intended to provide ultra-fast intercontinental passenger transport by means of a fully reusable, rocket powered, winged architecture. In its latest version, the passenger stage of the SpaceLiner 7 (SLP7) is designed for sustained gliding flight thanks to its large hypersonic lift-to-drag ratio. On its reference mission, it would be capable of transporting 50 passengers from Australia to Europe in less than 90 minutes. Vehicles traveling at hypersonic speeds inevitably produce shock waves that are perceived at ground level as sonic booms, with associated disturbance of the overflown populations. These disturbances can be minimized either by flying over less-populated areas (exploiting the increased cross-range capabilities of lifting reentry vehicles) or by flying at altitudes in excess of 80 km, at which the shock wave propagation reaching the ground is negligible. This second solution cannot be achieved by means of a gliding atmospheric flight, but rather with so-called skip reentries: trajectories going in and out of the atmosphere alternating ballistic arcs and skip phases. This works aims at performing a redesign of the aerodynamic shape of the SpaceLiner passenger stage so to extend its reentry capabilities and perform both gliding and skipping trajectories, while at the same time minimizing the reentry peak heat flux and the overflown population disturbance. For this purpose, a multidisciplinary design and optimization methodology was laid out with the tasks of (1) effectively exploring the design space and (2) finding a promising SLP8 configuration that would outperform the SLP7 along intercontinental routes. First, a Python toolbox (SLOT) was developed to compute vehicles performances from a wing shape parametrization. Using fast estimation methods (surface-inclination methods for hypersonic and handbook methods for sub- trans- and super-sonic velocities) the aerodynamic datasets can be computed, which are then pitch-trimmed using the information on the wing-shape dependent COG position. Then, the vehicles characteristics positively affecting trajectory performances were identified by means of analytical tools (equations of motions and heat-transfer relationships), and later cross-checked with simplified trajectories optimizations (launching from the equator towards a prescribed cardinal direction) minimizing the stagnation point heat flux and maximizing the achievable downrange. These preliminary results were later used to drive the choice of the objectives for the wing-shape optimization. Afterwards, SLOT functionalities were systematically exploited to perform parametric studies on thousands of different wing shapes, allowing an effective design space exploration which provided a number of insights on the problem at hand, and that ultimately allowed the simplification of the forthcoming optimization. The wing shape was then optimized using a three-objective evolutionary optimization, minimizing the vehicle mass and maximizing the aerodynamic performances that would result in an improvement of both gliding and skipping capabilities, namely the lift-to-drag ratio and the lift coefficient, while enforcing a feasible flight path through the entire velocity regime and constraining the maximum value of landing speed. Simplified trajectory optimizations were then systematically run on the set of non-dominated solutions obtained from the wingshape optimization to locate the region of the pareto front associated to the best trajectory performances. Once a promising vehicle was selected for further investigations, multi-objective evolutionary optimizations were run for point-to-point trajectories, minimizing at the same time the stagnation point peak heat flux and the integral of overflown population. Two routes of interest have been analyzed: a reference mission EuropeAustralia, and an Asia-Europe mission where a skipping trajectory could drastically reduce the population disturbance while flying over populated landmasses. The results were compared with the SLP7 performances on the same routes, showing a significant improvement in terms of a reduction of both the descent stagnation point peak heat flux and the overflown population disturbance.