Patent Number: 
Section: claims

1. A nuclear thermal propulsion system comprising:a nuclear reactor core including:an array of fuel elements; andan array of tie tubes adjacent the array of fuel elements, each tie tube including:a propellant supply passage to flow a propellant, anda propellant return passage;a first propellant density control valve system that includes a first regulator valve between a pump and the tie tubes to regulate density of the propellant flowing through the tie tubes, wherein the propellant flows in series from the pump, through the first regulator valve, then to the tie tubes; anda second propellant density control valve system that includes a second regulator valve between a turbine and the fuel elements to regulate density of the propellant flowing through the fuel elements, wherein after the tie tubes, the propellant continues to flow in series to the turbine, through the second regulator valve, then to the fuel elements. 2. The nuclear thermal propulsion system of claim 1, wherein the propellant is hydrogen. 3. The nuclear thermal propulsion system of claim 1, wherein:each tie tube further includes:an inner tie tube layer surrounding the propellant supply passage,a moderator sleeve surrounding the inner tie tube layer, andan outer tie tube layer surrounding the propellant return passage;the propellant return passage surrounds the moderator sleeve. 4. The nuclear thermal propulsion system of claim 1, further comprising:a propellant tank to store the propellant;a nuclear reactor core inlet directly or indirectly connected to the nuclear reactor core;a propellant line directly or indirectly connected to the propellant tank to flow the propellant to the nuclear reactor core inlet; andwherein the first propellant density control valve system is directly or indirectly connected between the propellant line and the nuclear reactor core inlet to regulate density of the propellant flowing into the nuclear reactor core. 5. The nuclear thermal propulsion system of claim 4, further comprising:a turbopump assembly comprising at least one turbopump that includes the turbine and the pump;wherein the pump is configured to cool the nuclear reactor core during a burn cycle by flowing the propellant from the propellant tank through the propellant line to the first propellant density control valve system, and then through the nuclear reactor core inlet. 6. The nuclear thermal propulsion system of claim 5, wherein:the nuclear reactor core inlet includes a tie tube inlet to flow the propellant to the array of tie tubes and is directly or indirectly connected to the first propellant density control valve system and a respective propellant supply passage of a respective tie tube in the array of tie tubes; andthe pump is further configured to flow the propellant through the tie tube inlet, through the respective propellant supply passage of the respective tie tube in the array of tie tubes, and then through a respective propellant return passage of the respective tie tube. 7. The nuclear thermal propulsion system of claim 6, further comprising:a propellant heating line directly or indirectly connected to the turbine and the respective propellant return passage of the respective tie tube; andwherein:the second propellant density control valve system is directly or indirectly connected to the turbine and the nuclear reactor core inlet to regulate density of the propellant flowing back to the nuclear reactor core inlet,the pump is configured to flow the propellant returned by the respective propellant return passage to the turbine via the propellant heating line, andthe turbine is configured to flow the propellant back to the second propellant density control valve system and the nuclear reactor core inlet. 8. The nuclear thermal propulsion system of claim 7, wherein:the nuclear reactor core inlet further includes a fuel element inlet to flow the propellant to the array of fuel elements and is directly or indirectly connected to the second propellant density control valve system and a respective propellant passage of a respective fuel element of the array of fuel elements; andthe turbine is configured to flow the propellant to the second propellant density control valve system, through the fuel element inlet, and then through the respective propellant passage of the respective fuel element of the array of fuel elements. 9. The nuclear thermal propulsion system of claim 5, further comprising:a pressure vessel housing the nuclear reactor core that is directly or indirectly connected to the first or second propellant density control valve system and including:a pressure vessel inlet connected to the periphery of the pressure vessel that is outside of the nuclear reactor core, anda pressure vessel outlet to return the propellant passing through the periphery of the pressure vessel; anda propellant heating line directly or indirectly connected to the turbine and the pressure vessel outlet;wherein the pump is further configured to flow the propellant to the pressure vessel inlet, through the periphery of the pressure vessel, and then flow the returned propellant from the pressure vessel outlet to the turbine via the propellant heating line. 10. The nuclear thermal propulsion system of claim 9, further comprising:a nozzle;a plurality of control drums;a neutron reflector; anda third propellant density control valve system directly or indirectly connected between the propellant line and the pressure vessel inlet to regulate density of the propellant flowing into the nozzle, the periphery of the pressure vessel, the neutron reflector, and the plurality of control drums. 11. The nuclear thermal propulsion system of claim 4, wherein the first propellant density control valve system includes:the first regulator valve to adjust density of the propellant passing through the first regulator valve from an initial density when entering the first regulator valve to a regulated density when exiting the first regulator valve; andan actuator to actuate the first regulator valve by adjusting the regulated propellant density upwards during each subsequent burn cycle to maintain constant reactivity at a beginning of each of the subsequent burn cycles. 12. The nuclear thermal propulsion system of claim 11, wherein the actuator is electric, mechanical, thermal, magnetic, or a combination thereof. 13. The nuclear thermal propulsion system of claim 12, wherein the first propellant density control valve system further includes a flow control circuit to control speed of the actuator and the flow control circuit is a bleed-off circuit, a meter-out circuit, or a meter-in circuit. 14. The nuclear thermal propulsion system of claim 11, wherein the first regulator valve and the actuator form a solenoid valve or an electrohydraulic servovalve. 15. The nuclear thermal propulsion system of claim 14, wherein the first regulator valve and the actuator form the solenoid valve and the solenoid valve is controlled by electric signals conveyed from an external computer, a digital circuit, an analog circuit, or a combination thereof. 16. The nuclear thermal propulsion system of claim 11, wherein:the propellant is hydrogen; andthe first regulator valve is configured to adjust density of the hydrogen from the initial density to the regulated density by varying pressure between 100% and 200% of nominal hydrogen pressure, inclusive, to increase reactivity of the nuclear reactor core. 17. The nuclear thermal propulsion system of claim 16, wherein the nominal hydrogen pressure is 5 to 15 megapascals (MPa). 18. A nuclear thermal propulsion system comprising:a nuclear reactor core including fuel elements and tie tubes;a propellant tank to store a propellant;a nuclear reactor core inlet directly or indirectly connected to the nuclear reactor core;a propellant line directly or indirectly connected to the propellant tank to flow the propellant to the nuclear reactor core inlet; anda first propellant density control valve system that includes a first regulator valve between a pump and the tie tubes to regulate density of the propellant flowing through the tie tubes, wherein the propellant flows in series from the pump, through the first regulator valve, then to the tie tubes; anda second propellant density control valve system that includes a second regulator valve between a turbine and the fuel elements to regulate density of the propellant flowing through the fuel elements, wherein after the tie tubes, the propellant continues to flow in series to the turbine, through the second regulator valve, then to the fuel elements. 19. The nuclear thermal propulsion system of claim 18, further comprising:a turbopump assembly comprising at least one turbopump that includes the turbine and the pump;wherein the pump is configured to cool the nuclear reactor core during a burn cycle by flowing the propellant from the propellant tank through the propellant line to the first propellant density control valve system, and then through the nuclear reactor core inlet. 20. The nuclear thermal propulsion system of claim 19, wherein:the nuclear reactor core inlet includes a tie tube inlet to flow the propellant to the tie tubes and is directly or indirectly connected to the first propellant density control valve system and a respective propellant supply passage of a respective tie tube; andthe pump is further configured to flow the propellant through the tie tube inlet, through the respective propellant supply passage of the respective tie tube, and then through a respective propellant return passage of the respective tie tube.