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ENGINEERING NOTES 1177into Earth orbit is independent of launch vehicle size and equals the cost of every 4 kw installed into an electric upper stage, also independent of size. It is important to realize that neither payload cost nor the cost of an electric stage can be expected to be independent of size; the results obtained by means of this simplification can therefore not be of general validity. However, this simple-minded assumption will be found of considerable help in illustrating the method. A circular orbit around Jupiter with an altitude of 20 planetary radii is to be established. Under these conditions, the diagram shown in Fig. 4 may be constructed. As in the previous figures, the payload is plotted along the ordinate whereas the initial mass capability of the ballistic system is plotted along the bottom abscissa. In order to obtain the performance boundary for systems of equal cost, rather than systems of equal initial mass, the horizontal scale at the top of the diagram, which is determined by the tradeoff ratio, has been used for the performance of the hybrid vehicles. Under these conditions, along any vertical line ballistic and hybrid systems exhibit the same recurrent cost. For the hypothetical cost assumptions used in this Note, a hybrid system which equals the cost of a ballistic system must be based on a launch vehicle whose initial mass capability is approximately 48% of the initial mass capability of the ballistic counterpart, i.e., the tradeoff ratio is 0.48. Comparing Fig. 4 to Fig. 1, it becomes evident that the equal performance boundary has been modified by the inclusion of the equal cost groundrule. Specifically, the area of ballistic superior performance has been enlarged in such a way as to roughly double the magnitude of the payload maximum. The shaded line indicates the physical limit of capability for the ballistic system. Below this line, the ballistic system is capable of performing the mission but, outside the area B, requires more trip time than the hybrid system. To further aid in the comparison, the hybrid performance lines for the longest (1000 days) and shortest (600 days) ballistic trip times are also shown in the diagram as broken lines inside H, the area of superior hybrid performance. Concluding RemarksA method has been developed which allows one to compare the performances of space propulsion systems of widely differing characteristics. Biased comparisons can be avoided by systematically arranging and selecting the groundrules of the comparison before they are fed into the analysis. Meaningful results can be obtained in, spite of the large range of uncertainties attached to some of the performance parameters by adopting the use of scaling laws which reflect major trends or other relationships of interest rather than scattered performance estimates. This Note has intentionally been limited to a description of the method. Quantitative conclusions may be obtained by exercising the use of realistic performance and cost inputs whenever available. NomenclatureA = constant d...