Multidisciplinary design and optimisation of a pitch trimmed hypersonic airbreathing accelerating vehicle

Abstract

The global economic environment combined with the rapid pace of technologyadvancement is placing importance on reducing the size and cost of access to spacesystems. Based on decades of practical experience with rocket-only launch vehicles,current technology is operated close to theoretical limits and only marginal furtherimprovement is possible. A possible solution is to include an airbreathing stage into thelaunch system architecture. Performance wise, airbreathing hypersonic engines such asscramjets have an advantage over rocket propulsion in terms of a significantly higherspecific impulse in the hypersonic flight regime. A reusable airbreathing stage couldcontribute to an increase in payload mass-fraction by using scramjet propulsion over ameaningful proportion of the launch trajectory and provides the flexibility of aircraft-likeoperations while being inherently reusable.Researchers at the University of Queensland investigated the use of a three stage to orbitrocket-scramjet-rocket system for transporting small payloads into LEO. The second stagescramjet powered accelerator of this system was the subject of a series of MultiDisciplinaryOptimisations (MDO) which involved flying complete trajectories. Promisingresults were achieved however this work did not include the requirement for the vehicle tobe pitch trimmed, which is an important aspect which must be addressed.The purpose of the investigation described in this thesis was to gain a betterunderstanding of the impact that the second stage vehicle’s planform has on the pitch trimof the vehicle. This was addressed through a numerical study utilising and expanding onthe capabilities of the MDO system developed by Jazra (2010) by: i) Defining a more realistic mission of delivering small satellites up to 500 kg into aSun Synchronous Orbit of 566.89 km altitude suitable for several earth sciencemissions. ii) Increasing the fidelity of the MDO system to enable an accurate estimate of the trimdrag. This included refining the vehicle mass model and aerodynamic moduleupdates such as introducing a 2-D aft body exhaust expansion model pluselevon force and pitching moment inclusions. iii) Introduction of new vehicle planform characteristics that can be optimised for theireffect on trim. iv) Introduction of a high dynamic pressure stage separation and trajectory simulationof the third stage rocket.Through careful selection of vehicle geometry parameters and ranges to give the systemenough freedom to allow for the inclusion of pitch trim effects the optimisation system wasused to identify the key planform parameters that impact on the trim of the vehicle. Thisprocess revealed important design and performance characteristics of the airbreathinghypersonic accelerator design. The vehicle was optimised for maximum flight Machnumber, payload mass fraction, equivalent effective specific impulse and minimum flightaveraged absolute pitching moment. It showed that in an integrated three stage to orbitsystem, the mass and the final stage velocity of the airbreathing stage plays equallyimportant roles and that it is important to use the stages only where they have aperformance advantage over the other stages. This work indicates that the scramjet stagecontribution is the best when it accelerates and lifts a heavy 3rd stage to around Mach 9.4.It was demonstrated that the forebody and the boat tail had the largest impact on thepitching moment of the airbreather. The wing contribution was dependent on its centre ofpressure location relative to the centre of gravity position of the vehicle. Engine size andintegration had a large impact on the vehicle design affecting the pitching moment throughthrust production but also a vehicle planform change. The best performance was obtainedby matching the vehicle’s aerodynamic, pitch and propulsive properties to the flighttrajectory to enable high net specific impulse over a large Mach range. The resultsemphasized the importance of including the trajectory into the MDO design approach.A performance objective called the Airbreathing Space Access Performance Parameterwas identified that was meaningful to a small scale access to space airbreathing stage.This parameter maximised the airbreathing stage’s performance in terms of equivalenteffective specific impulse and payload mass into orbit. Therefore it accounts for payloadmass into orbit as well as for trim drag, as part of overall vehicle drag. It was found to be auseful optimisation objective parameter for the design of a hypersonic acceleratingairbreather as part of a multi stage space access system. The vehicle showed strongperformance accelerating a heavy 3rd stage to a Mach number of 9.45 and delivering387.4 kg into the desired orbit representing a payload mass fraction of 1.46 %.