Sustaining propulsion burns Mesosphere Stratosphere Ozone layer Jet aircraftN Mt.Everest Troposphere Range (scale is compressed) Figure 6:Principle idea of the HyperSoar trajectory Figure 7:Artists impression of the HyperSoar 2000 configuration However,this preliminary evaluation should be regarded with a high degree of skepticism.Almost no data can be found on the propulsion system performance characteristics of HyperSoar.The concept developer at LLNL Preston Carter,published a paper on periodic hypersonic cruise [6].This study used an ejector RAMJET/- SCRAMJET rocket engine.Therefore,it seems quite reasonable that a similar RBCC is the basic propulsion system of HyperSoar.The specific impulse model as presented in [6]seems to be very simplified,at least on the ejector rocket side.Very limited historical data of experiments or flight tests on air-augmented rockets have been published. Although the SART assumptions for the recalculation in [2]are less euphoric,the performance data still represent ideal values because actual intake,combustion chamber,and nozzle characteristics were not taken into account D.LAPCAT-M8 The new generic hypersonic cruise airplane for LAPCAT has been initially based on the HyperSoar 2000 shape despite some concerns on its flyability.However,for defining the size and the propulsion system thrust requirements this approach is still acceptable.LAPCAT-M8 is considerably enlarged compared to HyperSoar to accommodate the passengers and the cabin and to meet the ambitious LAPCAT range requirement.Since the beginning of the study the hypersonic-M8 configuration had to be redefined already two times. The TRL of RBCC propulsion is low and a high degree of uncertainty exists on its actually achievable performance in ejector-rocket and SCRAM-mode.Therefore,an iterative approach in defining the thrust requirements and subsequent calculation of the mission performance has been chosen.As the LAPCAT study is in focus of far-term propulsion concepts [1],usually optimistic engine performance assumptions are drawn.This approach is maintained not only in case of missing validation by an actual flight test but also if the efficiency could not yet be demonstrated in a ground experiment.This important difference should be kept in mind if obtained data are compared with the other two transportation options of the chapters II and IV which are based on propulsion systems with much more advanced TRL All variants described here are based on LH2 propellant and on LOX as the oxidizer in rocket mode.Hydro- carbon propellants had also been regarded at an early phase but fuel consumption was found tremendously high due to the poor specific impulse in ejector-rocket operation mode.Therefore,all hydrocarbons were dropped quite early in the study as a feasible propellant for LAPCAT-M8 [2].The following paragraphs give a brief overview on the evolution process In the first iteration cycle with the configuration status of June 2005 (see Figure 8)the lifting body's projected area had been increased to 3000 m2(+150 compared to HyperSoar 2000)to keep the wing loading within an acceptable range.The span grew to 34.2 m,while the total length reached 106.1 m (+74 %when compared to HyperSoar 2000 [7]. The preliminary RBCC performance assumptions of the generic LAPCAT hypersonic cruise airliner were more conservative than those of HyperSoar [7].However,engine Isp was used according to ideal RAM and SCRAM performance not taking into account any actual intake and nozzle geometry.Thrust of the propulsion system was selected as required for simulation of a sustainable trajectory.This very optimistic approach was justified at this design stage because the major intention had been in finding the reference operation conditions of the propulsion system. American Institute of Aeronautics and Astronautics Paper 2006-7984American Institute of Aeronautics and Astronautics Paper 2006-7984 7 Figure 6: Principle idea of the HyperSoar trajectory Figure 7: Artists impression of the HyperSoar 2000 configuration However, this preliminary evaluation should be regarded with a high degree of skepticism. Almost no data can be found on the propulsion system performance characteristics of HyperSoar. The concept developer at LLNL, Preston Carter, published a paper on periodic hypersonic cruise [6]. This study used an ejector RAMJET/- SCRAMJET rocket engine. Therefore, it seems quite reasonable that a similar RBCC is the basic propulsion system of HyperSoar. The specific impulse model as presented in [6] seems to be very simplified, at least on the ejector rocket side. Very limited historical data of experiments or flight tests on air-augmented rockets have been published. Although the SART assumptions for the recalculation in [2] are less euphoric, the performance data still represent ideal values because actual intake, combustion chamber, and nozzle characteristics were not taken into account. D. LAPCAT-M8 The new generic hypersonic cruise airplane for LAPCAT has been initially based on the HyperSoar 2000 shape despite some concerns on its flyability. However, for defining the size and the propulsion system thrust requirements this approach is still acceptable. LAPCAT-M8 is considerably enlarged compared to HyperSoar to accommodate the passengers and the cabin and to meet the ambitious LAPCAT range requirement. Since the beginning of the study the hypersonic -M8 configuration had to be redefined already two times. The TRL of RBCC propulsion is low and a high degree of uncertainty exists on its actually achievable performance in ejector-rocket and SCRAM-mode. Therefore, an iterative approach in defining the thrust requirements and subsequent calculation of the mission performance has been chosen. As the LAPCAT study is in focus of far-term propulsion concepts [1], usually optimistic engine performance assumptions are drawn. This approach is maintained not only in case of missing validation by an actual flight test but also if the efficiency could not yet be demonstrated in a ground experiment. This important difference should be kept in mind if obtained data are compared with the other two transportation options of the chapters II and IV which are based on propulsion systems with much more advanced TRL. All variants described here are based on LH2 propellant and on LOX as the oxidizer in rocket mode. Hydro￾carbon propellants had also been regarded at an early phase but fuel consumption was found tremendously high due to the poor specific impulse in ejector-rocket operation mode. Therefore, all hydrocarbons were dropped quite early in the study as a feasible propellant for LAPCAT-M8 [2]. The following paragraphs give a brief overview on the evolution process. In the first iteration cycle with the configuration status of June 2005 (see Figure 8) the lifting body's projected area had been increased to 3000 m2 (+ 150 % compared to HyperSoar 2000) to keep the wing loading within an acceptable range. The span grew to 34.2 m, while the total length reached 106.1 m (+ 74 %) when compared to HyperSoar 2000 [7]. The preliminary RBCC performance assumptions of the generic LAPCAT hypersonic cruise airliner were more conservative than those of HyperSoar [7]. However, engine Isp was used according to ideal RAM and SCRAM performance not taking into account any actual intake and nozzle geometry. Thrust of the propulsion system was selected as required for simulation of a sustainable trajectory. This very optimistic approach was justified at this design stage because the major intention had been in finding the reference operation conditions of the propulsion system