2.2 NUCLEAR STAGE 2.3.1 PARAMETRIC DATA The high-thrust nuclear COTV considered here is a NERVA-engined vehicle. The primary data source for this stage was the Boeing FSTSA study results. Figure VI-D-2-5 presents the payload capability of this stage as a function of stage mass fraction and engine Isp. The characteristics of this stage appear to place it in the knee of the Wp/Wpl curves when operated in a reusable mode where small changes in engine Isp or stage mass fraction result in a large effect on capability. It is doubtful that the cost of the nuclear power plant could be reduced to the point where operation of this stage in an expendable mode would be attractive, but for the expendable mode, a Wp/Wpl of 1 to 1.3 would be expected for the reusable case. Boeing's values give a stage mass fraction of .75 at an impulse propellant mass of 220,000 Ibm; at the predicted effective delivered Isp of 768 seconds, approximately 2.7 Ibm of propellant are required per pound of payload. Thus the stage could deliver a payload of 81,500 Ibm to GSO and return itself to LEO. However, operation in a reusable mode would require greater engine life than has been possible to date. It is possible that new materials or techniques could increase engine life. But if engine life requirements forced a reduction in operating temperature, the effect of lower Isp on the payload capability could be disasterous.
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