atw International Journal for Nuclear Power | 04.2020

More documents

Recommendations

Info

atw Vol. 65 (2020) | Issue 4 ı April FEATURE | RESEARCH AND INNOVATION 194 So … how does the rocket equation translate into interplanetary mission characteristics? It turns out that complete closed form solutions to the orbital mechanic equations for these missions are generally not possible; however, by using what are called patched conic approximations, the mission characteristics of the interplanetary voyage may be determined to a high degree of accuracy using only fairly simple geometric constructs. Using these patched conic equations in conjunction with the 16.8 km/sec total vehicle velocity change calculated above, the one way travel time between Earth and Mars can be accomplished in about 160 days as illustrated in Figure 4. The minimum velocity increment required to execute a Mars mission is through the use of a mission profile using what is called a Hohmann transfer trajectory. This minimum energy transfer trajectory requires a total mission velocity of 11.4 km/sec and necessitates a one way trip time of 259 days. If a chemical rocket engine having a specific impulse of 450 sec. were used to execute this mission, then a vehicle mass fraction of 0.075 would be required. This small a vehicle mass fraction is completely unrealistic and illustrates why nuclear rocket engines are almost mandatory for any kind of practical interplanetary travel. Nuclear Rocket Engine System Characteristics The fuel elements that comprise the reactor core in a nuclear rocket engine are generally fabricated in the shape of hexagonal prisms containing a number of axial holes through which flows the hydrogen propellant. For a rocket engine that produces approximately 100,000 N of thrust, a reactor core would be expected to contain perhaps a couple of hundred fuel of these fuel elements. In NERVA rocket engines, the fuel elements were held in place in the core by means of support elements containing moderator material. These support elements held the six adjacent fuel elements together in a grouping called a cluster. The reactor core itself is surrounded by a reflector region composed of beryllium to reflect back into the core neutrons emanating from the fuel that would normally escape the reactor. Control drums embedded in the reflector serve as a control mechanism by which the core reactivity can be adjusted. The drums are composed of beryllium cylinders with a sheet of material which strongly absorbs neutrons attached to one side. When the absorbing material (usually boron carbide) is close to the core, many neutrons which would be reflected back into the core are instead absorbed in the neutron absorbing sheet causing the core reactivity to decrease. When the absorbing material is turned away from the core, the | Fig. 5. NERVA Core and Fuel Segment Cluster Detail. | Fig. 6. Possible Radial Flow Nuclear Rocket Fuel Element Configurations. Feature Nuclear Rockets for Interplanetary Space Missions ı Dr. William Emrich
atw Vol. 65 (2020) | Issue 4 ı April beryllium portion of the control drum reflects the escaping neutrons back into the core where their availability increases the core reactivity. Figure 5 below illustrates a NERVA fuel cluster and the manner in which it is integrated into the reactor with the reflector region and control drums. While current thinking is primarily directed toward core designs composed of these hexagonally shaped axial flow fuel elements, other designs are also being considered. These designs include the radial flow Grooved Ring Fuel Element (GRFE) and the Particle Bed Fuel Element (PBFE). These designs are illustrated in Figure 6. Radial flow designs such as these have the advantage over axial flow designs in that they can be made to have higher surface to volume ratios leading to more compact reactor configurations and are often amenable to using a wider variety of fuel materials. One particular advantage of the grooved ring fuel element configuration is that it can be designed in such a way that the temperature distribution across the entire fuel ring can be made to be nearly isothermal [4]. Nuclear rockets operate using one of several types of thermodynamic cycles that vary in complexity and efficiency. For nuclear thermal rockets, these thermodynamic cycles are “open” in that during operation, the working fluid is discharged through the nozzle to produce thrust after circulating only once through the engine system. These engines typically use a turbopump to highly pressurize the propellant prior to being introduced into the reactor where the propellant is heated to high temperatures before being discharged through the nozzle. The pump is normally driven by an integrated turbine system which is powered by propellant that has been warmed somewhat using waste heat from the reactor. One of the more common rocket engine cycles is illustrated in Figure 7. This particular engine cycle is called the “Hot Bleed Cycle” and is commonly considered because of its relative simplicity and high efficiency. The hot bleed cycle characteristics are as follows: 1-2 Liquid propellant from the tank is raised to the operating pressure after passing through the pump portion of the turbopump. 2-3 After passing through the turbopump the propellant circulates through the nozzle, support elements, chamber walls, etc., gasifying the propellant. | Fig. 7. Hot Bleed Nuclear Rocket Engine Configuration. 3-4 The gaseous propellant flow splits, with the majority of the flow being directed into the reactor core where it is heated to several thousand degrees before exiting the core into the engine exhaust plenum. 3-5 The rest of the gaseous propellant flow mixes with hot propellant bled from the reactor exhaust plenum and enters into the turbine portion of the turbopump. 5-6 The mixed propellant flow, which is now at a temperature consistent with the maximum acceptable turbine blade material limits, passes through the turbine portion of the turbopump where it gives up some of its energy to drive the pump portion of the turbopump. After passing through the turbopump, the propellant flow is discharged through a small nozzle. 4-7 The remainder of the hot gaseous propellant in the engine exhaust plenum is directed through the main nozzle where the heat energy is changed to directed kinetic energy producing thrust. Advanced Nuclear Propulsion Concepts Due to the material limitations of nuclear thermal rockets having solid cores, the maximum practical specific impulse achievable by these engines is in the range of 900 seconds. To achieve significantly higher specific impulses, much higher propellant temperatures will be required, thus necessitating radically different nuclear core designs. One method that has been considered in the past to achieve these higher specific impulses is to use a nuclear pulse system. In the pulsed nuclear rocket concept, small nuclear bombs are ejected from the rear of a spacecraft and detonated after they have traveled a suitable distance FEATURE | RESEARCH AND INNOVATION 195 | Fig. 8. Hot Bleed Nuclear Rocket Engine Thermodynamic Cycle. Feature Nuclear Rockets for Interplanetary Space Missions ı Dr. William Emrich
Page 1 and 2: nucmag.com 2020 4 ISSN · 1431-5254
Page 3 and 4: atw Vol. 65 (2020) | Issue 4 ı Apr
Page 13: atw Vol. 65 (2020) | Issue 4 ı Apr
Page 59 and 60: Kommunikation und Training für Ker

atw International Journal for Nuclear Power | 04.2020

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?