40,000 80,000 120,000
Operational empty weight (lbs)
Figure 3.29. Combined cycle propulsion has the advantage.
value as the previous rocket case. So the Mach 8 airbreather is about equal, in terms of technical challenge, to the all-rocket.
Figure 3.28 presents the solution map for the rocket plus ejector ram/scramjet operating as an airbreathing system to Mach number 12. The bottom scale is for ICI in English units for Ip and Istr and the top scale is for ICI in SI units. The left scale is in English units and the right scale is in SI units for the planform area. The vertical bar is the ICI boundary for the rocket plus ejector ram/scramjet operating as an airbreathing system to Mach number 12 is to the right of the previous two cases at a value in the 11 to 11.5 range. That is a greater industrial capability fraction of the design space is available for converged designs, but those designs require a higher value of the ICI. As with the two previous cases, most of the design space is to the right of the ICI boundary, that is, beyond the current state of the art. Both the operational example and the demonstrator example have a greater ICI value than the previous two cases. So the Mach 8 airbreather is about equal, in terms of technical challenge, to the all-rocket, but the Mach 12 airbreather is a greater challenge, especially in propulsion, as the value for the structural index can be assumed to be the same for all three cases presented.
Again, it is important to note, that the conventional aircraft design wisdom puts SSTO designs out of reach of current industrial capability. Second, the SSTO challenges are similar for all-rocket and airbreather, but increasingly difficult as the Mach number at which airbreathing propulsion must transition to rocket propulsion increases beyond Mach 8. Given the similarity of the industrial challenge, the question is, what are the differences in weight for the airbreather compared to the all-rocket vehicle. Figure 3.29 presents answers to this question. For approximately the same empty weight, the gross weight of the rocket vehicle is at least twice heavier than the combined cycle vehicle. The shaded area indicates the area to the left of the ICI boundary in Figures 3.26 and 3.28. Increasing the slender-ness beyond z ~ 0.12 offers no benefit in reducing either the operational empty weight or the gross weight.
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