A theoretical approach of UNIT (Unified Nuclear Integral Technology) propulsion and its potential for future applications in space exploration.
Space exploration is the present inevitable challenge for researchers. Various theoretical propulsion concepts have been evolved over the past years for space missions. Their potential remains as a key factor for the spacecraft to travel deeper into space in a shorter mission duration. The propulsion concept UNIT is an integrated nuclear propulsion technique that provides high entry, descent and landing (EDL) performance in such short duration to conquer other galaxies. This paper describes the theoretical approach of the UNIT propulsion system in detail. UNIT produces the highest energy possible by consuming nuclear fuel and possess the highest potential that opens new opportunities for space exploration. The principle is that the neutrons from the fusion are deliberately allowed to induce fission. It uses National Ignition Facility's laser beam for inertial confinement fusion followed by utilizing the power from tubular solid fuel cell. Thus, the net thrust is produced from the expansion of the combined plasma of the nuclear fusion and nuclear fission reactions through the nozzle.
KEYWORDS: Nuclear fusion, Nuclear fission, Nuclear propulsion, Space exploration, Inertial Confinement Fusion, Lawson criterion, Rayleigh Taylor instability
CITATION: Dran, S., "A Theoretical Approach of UNIT (Unified Nuclear Integral Technology) Propulsion and its Potential for Future Applications in Space Exploration," SAE Int. J. Aerosp. 8(1):2015, doi: 10.4271/2015-01-9004.
With the technologies available today, high amounts of energy could be produced only by nuclear reactions, either fission or fusion. Fission produces lower energy when compared to fusion. Either breaking up of a heavy molecule called fission or combining the lighter molecules called fusion individually can provide energy many times greater than chemical propulsion systems. The fusion process can be carried out only at extreme temperature of the order of [10.sup.8] K, because only at these temperatures the nuclei can overcome coulombic repulsion which hinders the fusion of nuclei. Also, generally, fission reaction occurs only when triggered by energized neutrons. The heavier nucleus which is stable initially is triggered by energized neutrons that make it unstable and thereby induce the fission reaction. Current propulsion techniques that are under implementation in space missions have the potential of reducing the mission duration to other planets.
But they add more complexity to the mission including fuel quantity, duration and increased number of stages in accomplishing the mission. This is due to their low potential to produce larger thrust and low fuel to thrust ratio.
This paper deals with the combined performance of these nuclear processes in a single system in producing the highest energy and hence greatest thrust. In UNIT, nuclear fusion reaction is carried out at the main chamber and fission reaction at the sub-chamber. Fusion
reaction is initiated by the laser beam method (Inertial Confinement fusion-ICF) and the fusion plasma is directed towards the main nozzle with the help of magnetic mirrors. Neutrons obtained as a byproduct along with the release of fusion energy is filtered from the fusion plasma and reflected back into the sub-chamber through the neutron port using super mirrors. Sub-chamber containing the uranium inside becomes unstable on accepting the neutron and fissions to produce stable fission fragments. The fission reaction is maintained subcriticai to ensure that when fusion is shut down, fission is also stopped (correlative fission). UNIT thruster is designed preliminarily comprising the major components using Pro-E Wildfire since the practical development of the thruster is only possible with fundings and support from well-established research center (Design Overview). Super mirrors, Magnetic mirrors, ICF are commercially available technologies and are adapted in UNIT to increase the chances of its commercial development (Adaptation of existing technologies). Technology synergism illustrates the detailed working principle of UNIT thruster and the possible thrust is found to be greater than [10.sup.14] N. This thrust is double the thrust available from Nuclear fission propulsion and is much greater compared to chemical & electric propulsion.
Material to be used for the commercial development of UNIT thruster has to possess melting point greater than the temperature likely to prevail inside. The sub-chamber and sub-nozzles have to possess higher melting point than that of main chamber wall because it carries fusion plasma on the outer side and fission plasma on the inner side. Similarly, main nozzle also requires the highest melting temperature as it expands the combined fusion and fission plasma through it. Hence, a double layered solid wall of graphite coated with starlite is preferred to meet the melting point requirements (Heat dissipation and other Challenges). The potential of UNIT thruster is analyzed theoretically for possible thrust neglecting the losses of energy of neutrons (Energy Calculation). Results showed that UNIT possess highest thrust to fuel weight ratio. UNIT possess the advantage of carrying low fuel weight. UNIT also shortens the mission duration because of its highest thrust compared to other propulsion systems which is shown in table and graph (Fig.3).
II. INERTIAL CONFINEMENT FUSION
The UNIT propulsion system consists of two chambers which act as a fusion reactor (main chamber) for neutronic fusion and a fission reactor (sub-chamber) for binary fission, respectively. Neutronic fusion is achieved by the Inertial Confinement Fusion (ICF) method. In general ICF initiates the fusion reaction by heating and compressing the fuel capsule in a pellet form. To compress and heat the fuel, energy is delivered to the outer layer of the capsule either with the help of high energy laser beams, or electrons or ions. When the laser beams are focused on the capsule, due to some imperfections on capsule there are possibilities of non-uniform heating and Rayleigh Taylor instabilities to occur. Hence, fuel capsule is placed inside a hollow area which converts the laser beam into X-rays resulting in uniform heating of the capsule. For confinement fusions, sufficient density and confinement time are required as specified by Lawson criterion. In ICF, the confinement times are extremely short ([10.sup.-10] s) and the particle densities are greater than [10.sup.25] [cm.sup.3]. This solves the demand of our UNIT propulsion system which is focused on shorter confinement time that in turn proliferates the plasma production. Thus, UNIT induces fusion in its chamber, which contains a capsule filled with fusion fuel, by ICF.
III. CORRELATIVE FISSION
Nuclear fission is a man-made deliberately-produced nuclear reaction, achieved with the help of neutrons and hence UNIT also follows the same methodology for inducing fission reaction in the fuel atoms filled in a portion of the sub-chamber. Here the neutrons from the fusion are utilized to induce the fission reaction and thus, both the fusion and fission in UNIT are correlative. The sub-chamber containing the nuclear fission fuel, when induced by neutrons, releases large amount of energy along with the release of free neutrons and gamma rays. Fission reaction continues as a chain reaction if the byproduct free neutrons are not absorbed. It is because of the free neutrons moving in random directions have a mighty chance of inducing fission in other fuel atoms. This in turn leads to an uncontrolled chain reaction producing multiple neutrons in increasing order leading for infinite fission stages. UNIT illustrates the reason for its fusion-fission correlation by the chain reaction. Shut down and uncontrollability of the fission reaction can be controlled with this correlative approach along with the use of Carbon control rods.
IV. DESIGN OVERVIEW
The unit propulsion has two chambers, a main chamber and the sub-chamber. Fusion is carried out in the main chamber and fission in the sub-chamber. Neutrons from fusion are deliberately allowed to induce fission in the sub-chamber. They are filtered and reflected into the sub-chamber by super mirrors. Control rods absorb the free neutrons from the fission. Both fusion and fission plasma are directed with the help of magnetic mirrors and reflectors. Combined hot plasma is expanded through the nozzle to produce thrust (Tig. 1).
V ADAPTATION OF EXISTING TECHNOLOGIES
For the choice of driver and fuel for fusion and reactions, UNIT adapts with currently existing technologies so that complexity of the system is reduced. For ICF which occurs at the main chamber, UNIT prefers laser driver to initiate fusion by exciting the fuel to high plasma temperature of about [10.sup.8] K.
UNIT prefers deuterium and tritium as fuel mixture to be filled in the capsule of the ICF operational setup. This is the easiest method of producing fusion which also releases an enormous amount of energy among neutronic fusions. Currently, the largest operating laser is the Omega laser which consists of 60 Nd-glass (neodymium ions embedded in a matrix of yttrium aluminum garnet [Y.sub.3][A1.sub.5][O.sub.12] crystal or glass) laser beams delivering a total energy of 40 kilojoules is utilized in the UNIT propulsion, since it is capable of releasing high power in a short time. The laser setup is designed for meeting the energy needed with as minimum dimensional parameters as possible. Nd-YAG laser which is a gas laser have been preferred to solid laser for its high directionality and highly monochromatic characteristics.
Fission of a heavy nucleus needs high input energy to overcome the force which holds them into a definite spherical shape. Uranium-235 is the fission fuel being used in the UNIT system for the reason that it is fissionable with neutrons of about 2MeV. Thus, energy loss in neutrons from the fusion would have no effect on the fission reaction in the sub-chamber. A neutron supermirror is a highly polished surface used in connection with neutron beams. Supermirrors are produced by depositing and polishing large numbers of layers of a reflecting substance, such as silicon, nickel, titanium or nickel/titanium composite, on a substrate. It can be used as both beam filters and focusing device, and are used at appropriate positions on the outer fission sub-chamber such that the neutrons from the fusion can be reflected and thus transmitted to the target, fission fuel Uranium-235.
VI. TECHNOLOGY SYNERGISM
UNIT combines fusion and fission under a single propulsion system, designed to produce the highest possible thrust which could be never achieved by other propulsion systems. The preliminary design layout of UNIT developed using Pro-E Wildfire is given in Fig.1. To describe the operation of UNIT'S net energy release and thrust, single atom of deuterium & tritium for fusion and Uranium for fission is considered. Initially, in the main chamber, the nuclear fusion reaction between deuterium and tritium contained in a target capsule is induced by the ICF method. Energy from the Neodymium Yttrium Aluminum Garnet laser simply called Nd-YAG laser is applied to the outside of a spherical capsule containing deuterium and tritium. The capsule which is held inside a hollow area coverts the high energy laser beams into X-rays and causes uniform heating on its outer layer. The energy of X-rays makes the capsule to ablate outward. As a result, reaction forces are produced towards the inner portion of the capsule causing it to implode. This implosion compresses and heats the fuel sufficiently inside the capsule. This initiates the fusion reaction with the release of energy.
When this occurs, the products of the fusion reaction have a smaller total mass than the total mass of deuterium and tritium. The mass difference between them is converted to energy which can be determined by Einstein's famous formula, E = [mc.sup.2]. Both deuterium and tritium are filled in the target capsule simultaneously for sustaining the confinement fusion. Thus, the total energy released from fusion in the main chamber for the total fuel in the capsule can be determined knowing the volumetric fusion equation, [P.sub.E] = [N.sub.1] * [N.sub.2] * [sigma] * V * E. Here [P.sub.E] is the energy from the hot cloud, [N.sub.1] and [N.sub.2] are the number density of the light atoms being fused, [sigma] is the nuclear cross section of the fusion reaction at that temperature, V is the average velocity of the light atoms when they collide, and E is the energy made per fusion reaction.
The magnetic mirror in the main chamber is generated by conducting magnetic coils around the main chamber. The function of magnetic mirror is to direct the plasma towards the main nozzle. The magnetic coils are supplied power drawn from the NIF 192 laser system. Generally, a magnetic mirror is a magnetic field configuration where the field strength changes when moving along a field line. The mirror effect results in a tendency for charged particles to bounce back from the high field region. Lithium liquid from the radiator in the rocket can be used as a coolant which proves good for our UNIT as in a nuclear fusion propulsion system.
The neutrons from the fusion thermal-plasma are filtered and reflected with the help of the super mirrors on the outer wall of the sub-chamber and then to the inside of it for injecting the uranium atom. The sub-chamber is conical in shape which contains the fission fuel. UNIT accelerates the flow to the nozzle as quick as possible. Filtration of neutrons from the flow by the super-mirror instruments is thus carried out very instantly, say at [10.sup.-20] s, as the confinement time for ICF is [10.sup.-10] s. The temperature and relatively pressure is also extremely high and since the process is carried out for long range, liquid lithium is used as a cooling agent for the heat transfer. The energy loss of the neutrons during reflection and energy leak into the sub-chamber accounts to the loss of total energy.
Uranium is filled in the front portion of the sub-chamber just behind the neutron port inside a capsule which has small open area as vent for the travel of neutrons, such that energy leak from the fusion does not affect uranium inside the sub-chamber. Once the neutron injects the uranium which is contained in the sub-chamber, the uranium becomes unstable and fissions into two stable daughter nuclei fragments (barium and krypton). Also, an average of 2.995 neutrons is assumed to be emitted along with the prompt gamma ray photons. Thus, the induced fission energy of this isotope U-235 is directed to the nozzle using a magnetic reflector.
The rear end of the sub-chamber is a slightly concaved cross section for the flow compromise, and consists of multiple sub-nozzles. The multiple nozzles are used all at the same cumulative thrust from the fission so that the nozzle length can be reduced. Thereby, the design complexity can also be reduced. Just before the multiple nozzles, the fission fragments are magnetized to accelerate and beam the flow of energy through them. Fission is maintained sub-critical absorbing all the free neutrons by the carbon control rods inside the sub-chamber. This is because, when the fusion is shut down, the fission also gets shut down lacking neutrons to induce the fission in uranium. This increases the reliability of UNIT.
The gamma radiations from the fission would also be comparatively low and since the fission chamber is closed except on the nozzle portions, there occurs loss of energy of many gamma radiations by collision with the wall of the chamber. Passive cooling in combination with refractory materials can be used to prevent melting and reconfiguration in the sub-chamber. Liquid sodium is utilized for the passive cooling method in the sub-chamber. Magnetic mirrors inside the fission sub-chamber is used to prevent backflow of fission fragments. Also, fusion energy flow into the sub-chamber is restricted using tampers and the position of the tamper depends on the design criterion. The tamper is considered a thin film or atomic layer (say made of metals or nonreactive gas elements) through which only the neutrons can penetrate. It is because only the neutrons possess the capability of penetrating through any atom and atomic thin layer. The energy obtained from the fusion and fission is expanded through the main nozzle to produce thrust. The main nozzle is designed using the bell configuration because it has a high expansion section and the wall contour is designed to minimize losses.
VII. ENERGY CALCULATION
It is known for a fusion of single deuterium and tritium atoms that the energy releases per fusion is calculated as [17.6E.sup.6] eV. And similarly the fission of U-235 releases [4.65E.sup.6] eV of energy. Net energy from the 192 laser beam is [1.8E.sup.6] joules and 700 shoots of the beam can be made per year. ICF focuses to produce approximately [10.sup.20] D-T reactions per shoot and hence approximately 1.000079089* [10.sup.20] fission stages happen in the sub-chamber. The target centre is focused by only four laser beams and hence the energy from the D-T fusion cloud is calculated as [4.0621E.sup.27] eV. Similarly, the energy from fission cloud is calculated as [1.4410E.sup.27] eV. Thus, the total energy meeting the nozzle is [5.5E.sup.27] eV consuming 100g of uranium for fission and number of capsules 100g weighing containing deuterium and Tritium. The loss of energy of neutron has been neglected during the calculations. Hence the net thrust from the net calculated energy through the main nozzle is estimated to be approximately [2.3E.sup.14] N.
The above graph has been plotted using Microsoft Excel for thrust available from various propulsion systems. The flat straight line coinciding with the horizontal axis shows that the thrust available from the mentioned propulsion systems are much lesser than the theoretically obtained value of thrust for UNIT. The sudden shoot up of the thrust value for UNIT immediately after fission propulsion indicates that the thrust available from UNIT [2.3E.sup.14] N is approximately double times the thrust available from Fission propulsion [E.sup.7] N.
VIII. HEAT DISSIPATION AND OTHER CHALLENGES
Heat dissipation is the most significant challenge for UNIT due to being fusion and fission combined together under a single system. Thus, Graphite is used as the material for building the total system as it has the highest melting temperature compared to other metals. Since the fusion and fission reactions occur only at a very high temperature of [10.sup.8] K and [10.sup.6] K, the combined heat gain inside the system will be much greater. Thus, they would be carried out only if the material has the melting point of the chamber walls much greater than this. Also, it should be capable of not transmitting out the heat. It must be so compact such that gamma radiations lose their energy and do not transmit through them. But there are chances of gamma ray transition through the nozzle of the rocket motor. This gamma ray transition through the main nozzle is quite small. It is because as the fission process occurs in the conical sub-chamber many of the gamma radiations lose their energy striking the double solid layer wall made of graphite coated with starlite. Therefore, only small quantities that escape through the multiple nozzle configuration sub-nozzles is transmitted outside through the bell configuration main nozzle.
Thus, starlite is used in UNIT which can withstand heat to extremely higher degrees and is the optimum choice to coat the walls of graphite. Since the nuclear reactions also produce temperature greater than [10.sup.8] K, double solid layer of starlite wall can be suggested to increase the reliability. The supermirrors are cooled rapidly to avoid reorientation or melting. The inner region of the chamber wall is fitted with magnetic mirrors because nuclear reactions take place in a random direction. Therefore, the hot plasma can be reflected and directed to the main nozzle.
EX. ADVANTAGES OF UNIT
UNIT propulsion might be a revolutionary propulsion system and the most reliable system of propulsion for an interstellar mission. Since the material starlite can withstand extreme temperatures and as it does not transmit radiations, there is no possibility of material melting or radiation emission across the wall. This reduces the necessity of usage of coolant and is preferred for reliability in UNIT. Availability of hydrogen fuel is in large quantity. In a longer mission, payload will be more and much expandable, our rocket can solve this problem with its much higher specific impulse, while still providing the high thrust needed for fast orbit transfers. This provides much more flexibility in mission planning and would not constrain longer time on material choice for building the system.
The use of the combination of fusion-fission energy can substantially provide higher exhaust velocity, thereby providing higher thrust compared to other systems. It could provide a greater amount of electric current for scientific instruments and communications gear once the spacecraft arrives, saving engineers the trouble of designing a separate power source. It is ultimately safer because fission reaction here is not self-sustained. If the fusion process is shut down, then fission process deliberately stops and so it is easily controllable.
X. FUTURE WORK & CONCLUSIONS
A primary effort must be made to examine the potential of UNIT experimentally and to calculate its various parameters from theory. Carrying the UNIT setup into space is difficult as it is large in size. Steps must be taken to reduce the overall size if the UNIT setup. Also starlite is not readily available as their composition is unknown. So, a suitable alternative material have to be developed to replace starlite in the UNIT setup. If these efforts prove successful in future, UNIT propulsion would represent the next inevitable phase of rocket technology for space exploration and it can help humanity to unlock the solar system. Finally, the human race is at the threshold of truly exploring, developing resources and permanently inhabiting space.
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Department of Aeronautical Engineering
Jaya Engineering College
Sarath Ramachan Dran Jaya Engineering College
Table 1. Propulsion and thrust Propulsion method Thrust Range Chemical propulsion ~[10.sup.6]N Magneto plasma dynamic propulsion ~200N LOX-Augmented Nuclear thermal rocket ~184KN Nuclear Fission propulsion ~[10.sup.7]N UNIT propulsion >[10.sup.14]N
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|Author:||Dran, Sarath Ramachan|
|Publication:||SAE International Journal of Aerospace|
|Date:||Sep 1, 2015|
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