for presentation to the at the
         American Nuclear Society 1998 Annual Meeting
                                                             Annual Meeting, June 6-10, 1998 Boston, MA

 
Figure 1. Nuclear-heated steam rocket uses water directly as propellant to launch payloads from the lunar surface.
 

Figure 2. A nuclear heated steam rocket uses bleed steam to drive the turbine-turbopump assembly.

The Lunar Prospector spacecraft found water ice at permanently shadowed, 80 Kelvin, regions inside crater basins at the North and South Poles of the moon. Either a PWR or a BWR would power a nuclear heated steam rocket, using water from the moon as propellant. A nuclear heated steam rocket offers an exceptionally simple and presumabley inexpensive system to shuttle people and payloads to and from the moon surface, as suggested in Figure 1. Previous work¹ determined the specific power required to develop sufficient thrust for liftoff: each ton of nuclear rocket engine must deliver at least 150 megawatts into 1100 Kelvin, or hotter, steam.

The design point² uses a 300 Megawatt reactor to produce about 100 kg/s of steam. Nuclear fuel is allocated 1000 kg, and pressure vessel, control rods, pipes, etc are allocated another 1000 kg.

A PWR reactor design features single phase coolant flow and high heat transfer rate. High pressure also permits a reduced mass rocket nozzle. However, PWR requires supercritical steam, with pressure greater than 22 MPa.

The BWR features extensive operating experience and pressures considerably lower than supercritical conditions. Steam dryness and heat transfer rate are significant issues.

The propellant pump, its driving turbine, gear box and the rocket nozzle are allocated an additional 500 kg, of which the pump is allocated 300 kg, based on chemical rocket experience. System margins may permit a factor of two higher pump mass.

The nominal containment vessel would contain 22 MPa in a 1 meter diameter cylinder with wall temperature less than 150 Celsius. Coolant would flow radially inward from the tank walls and axially outward to the rocket nozzle. Nickel chromium steel, S.A.E. No. 3250, with a 1.37 GPa yield strength was specified as vessel material, with a safety margin of 2.

These specifications permit a 520 kg vessel to contain 22 MPa, which is within mass budget for both PWR and BWR.

The mass of a turbine/turbopump needed to raise 100 kg/s of water to 22 MPA from an initial 100 kPa was estimated. Figure 2 shows schematically that bleed steam would drive a turbine to power the pump. A 90% efficient pump would require about 2.3 Megawatts. A "10 hp/lbs" rule-of-thumb results in drive turbine mass less than 150 kg --within mass budget. However, no single commercial pump exists to supply both the 22 MPa pressure and the 100 kg/s flow rate³. This pump must reliably start and stop thousands of times, unlike turbo-pumps used in rockets currently in use. The mass of a commercial pump to deliver 100 kg/sec at 9.3 MPa is about 1600 kg and requires 1.1 MWatts.

Linear scaling of this pump mass to achieve required pressure gives a lower limit PWR pump mass of 3850 kg. Linear scaling to give a pump mass of 300 kg gives a BWR pressure of 1.72 MPa at 100 kg/s, and a PWR flow rate of 19 kg/s at supercritical pressure.

The scaled pump mass (3850 kg) for PWR exceeds budget (300 kg). This implies the PWR can not be used unless pump specific mass can be lowered by an order of magnitude.

The scaled pump mass for BWR would provide acceptable pressure (1.7 MPa).