In a system of nuclear propulsion thermal or nucleothermic a propelling fluid, in general of the Hydrogen, is heated at high temperature by a Nuclear reactor and is ejected by a Tuyère by creating a Poussée.

The energy of nuclear fuel replaces that of the chemical reactions used in the traditional launchers. Thanks to the more important energy density of fissile materials, approximately 10⁷ higher than the chemical reagents, it results from it a more effective mode of propulsion (specific Impulsion at least twice better) even in spite from the important weight of the engine.

Such an engine was considered with the E. - U. to replace the J-2 of stages software firm and S-IVB of the rockets Saturn V and Saturn I. Originally envisaged like a " exchange standard" aiming at improving the performances, more important versions of replacement of the S-IVB were studied for advanced or Martian lunar missions. In the same way, the Soviet Union considered this option for the last stage of the lunar rocket N-1. However, no prototype was really at the point before the frenzy of the Course to space is completed.

To date, no nucleothermic rocket still flew.

Design

The systems of nucleothermic propulsion can be catagorized according to the principle of their nuclear reactor, which goes from the solid simple heart (similar to that of a Nuclear plant) until the more complex but more effective concepts such as the gas hearts.

Solid heart

The simplest model uses a conventional engine (although smaller) functioning at high temperature. This design puts a ceiling to the maximum temperature of operation to that of fusion of construction materials. As the specific impulse strongly depends on the temperature of the propelled fluid, one then seeks to use the most resistant materials. But even the least fusible of advanced materials have a melting point much lower than that which the nuclear reaction could give to the fluid, which returns very under-utilized the potentialities of nuclear energy. The concept of heart solid is generally supposed to deliver I_sp of 800 with 900  S, is twice that of the best chemical engines LOX - LH2.

The weight of the engine is so important that it would reach only hardly a Rapport weight-push of 1:1, essential to plan to tear off itself with the terrestrial attraction with launching. Nevertheless, for a ΔV given, the total mass of the engine and fuel would be less. That implies that this type of engine can be used only for the upper floors as launchers when the vehicle is already orbits about it or almost and that the necessary push is weaker.

In the United States, this concept was tested in a thorough way within the framework of the program Rover/NERVA of 1959 to 1972. Many other variations of the principle were also investiguées, of which:

• Dumbo , precursor of the concept, it differs by the design from its heart made several tubes themselves made up from stackings from flat or undulated rings creating channels by which hydrogen circulates of the interior of the tube towards outside. The rings are out of refractory metal (tungsten or Molybdène) impregnated of UO₂. Inside each tube, therefore in the cold zones of the engine, is another made tube of stacking of discs of moderating Polystyrène and Dural undulated for spacing. The gases heated by this device leave to 2500°K. When it was abandoned with the profit of NERVA and its graphitic heart, the principle of the heat exchanger of Dumbo was adapted and transformed into thermal system of drilling.
• Small Nuclear Rocket Engine (SNRE), a derived version from NERVA, more compact and with divergent of retractable conduit intended on the last floors of launchers and the space shuttle, of a push of 73  kN and with an impulse of 875  S (announced like easily improvable with 975  S).

• ASPEN , nuclear launcher monoétage imagined by Robert Harrier, functioning in Ramjet and Superstato ( ramjet and scramjet , uses the air of the atmosphere like propelling fluid).

• Russian IRGIT

Refractory metal heart

The advantages of using a metal fuel are the best held with the high temperatures and the mechanical deformations, a better corrosion resistance of hydrogen, the more effective retention of the fission products and the possibility of functioning in fast spectrum (thus without regulator within the heart and more compact design).

Cermet

The first model Cermet was born from the fusion of the activities of the National laboratory of Argonne (ANL) with the beginning of the Rover program and the studies of plane with nuclear propulsion carried out at General Electric (known under the name of Programme 710 ). It is very similar to NERVA by its hexagonal prismatic combustible matter with 19 channels, but the material used is a Cermet Tungstène - UO₂ (60% - voluminal 40%) protected by a coating tungsten - Rhénium. The tests did not exceed the stage of the evaluation of combustible matter and predicted I_sp of 832  S, a push of 100  kN, a power of 2  GW and of the operating time of tens of hours. However, the successive Recuit S of fuel cause a Croissance of the grains which involves the increase in the losses of fuel and fission products. Moreover, the neutron properties of tungsten are not very interesting for this type of configuration. The ANL and GE continued their improvements of the engine Cermet (stabilization of the grain by traces of Gadolinium, reduction of the variation in temperature thanks to a preheater,…) while the other implied teams directed themselves towards other concepts based on a metal heart.

Telegraphic heart

Atomics International thus continued the studies of nuclear plane on the way of the engine with telegraphic heart ( wire core reactor ). In this type of fast reactor, hydrogen radially crosses a cylindrical heart made up of an interlacing of wire nouveau riches out of fuel and wire of spacing.

The wire nouveau riches are manufactured starting from particles of 0,1  mm of Nitride of uranium ONE stripped of tungsten and packed in a braiding out of tungsten, then this sheath is drawn until forming a cable of 0,9  mm in diameter.

As the engine with bed of particles, this structure has a heat-transferring surface much more important than a prismatic heart, but moreover, the technique of braiding makes it possible optimalement to distribute the power of the engine while varying the spacing of combustible wire according to the local hydrogen flood. Such an engine could reach a temperature of 3030°K and a specific impulse of 930  S.

Moderation water

On its side, the LeRC is directed towards the development of a thermal neutron reactor said engine tungsten to moderation water ( TWMR , Tungsten Toilets-Moderated Reactor ). The heart in Aluminum is composed of moderating a water tank surrounded by reflectors out of beryllium and crossing by tubes containing each one a combustible matter in which hydrogen passes.

Water circulates in closed circuit: it cools the reflectors while crossing it and is cooled by hydrogen coming from the circuit of regeneration of the conduit in six exchangers surrounding the reflectors (as opposed to what lets suppose the general diagram opposite).

Combustible matter is a tungsten sheath enriched in ¹⁸⁴W (much more transparent with the neutrons) containing segments prismatic W-UO₂ cermet. The segments are separated by a small space allowing the redistribution from the fluid between each segment (what prismatic fuels Cermet and NERVA do not do). The sheath “is suspended” on the reflectors superior (admission) and is assembled loose in its tube to isolate the tank from heat and of the vibrations which combustible matter undergoes, water is thus heated primarily by the neutrons and gamma rays. Several types of fuel segments were conceived and tested, retaining for more promising the sheets of 0,51  mm obtained by rolling of the W-UO₂ mixture between two layers of pure tungsten to avoid the loss of fuel, and welded in honeycomb.

This type of engine is supercritical. To maintain it at rest, of the small tubes intercalated between those of fuel contain a circuit of solution of Sulfate of cadmium CdSO₄, poison neutron. To start the engine, the solution circulates by an ion exchanger impoverishing its concentration in poison. The opposite operation is carried out more simply and more quickly by reinjecting sulfate concentrated in the circuit. Experiments of Neutronique were made on models to validate the parameters of the concept. The model of reference to 121 elements weighs: 1130  kg, can produce 360  MW over one total duration of 10  H and ejects hydrogen with 2480  K.

Engine with bed of particles

A means of increasing the temperature and the specific impulse would be more to impose itself that the fuel is in a large-sized rigid form and thus subjected to important mechanical constraints. It is the principle of the engine to bed of particles (also called with reads fluidized , reads dust , or reads rotary ) in which the fuel is in the form of spherical particles which bathe in hydrogen to heat.

Precursors

This principle invented by James R. Powell was explored at the same time as NERVA by the National laboratory of Brookhaven. Particles of 500 with 700  µm of diameter was made up of core carbide UC₂ a uranium coated with porous carbon (retention of the fission products), with pyrolytic carbon and an anti-corrosion layer out of carbide of Zirconium ZrC. He was proposed two designs of the engine: the engine with bed fixes FBR , in which the particles are stored between two Fritte S porous cylindrical, and the engine with rotary bed RBR which does not have interior calcining (hot) and maintains the particles against calcining external (cold) by centrifugation with ~1000  tr/min. As the RBR does not have hot calcining, it is freed from the problems involved in this part and can produce a temperature of higher exit. Moreover, the engine can be purged at the end of the operation then reloaded later, this possibility avoids a heating prolonged of the engine after its extinction (due to the decomposition of the unstable fission products) and allows the easier maintenance of the engine. The performances considered were I_sp of 1000  S and a push of 90  kN for a mass of 1370  kg. Many mechanical aspects remained unsolved.

Because of important report/ratio/volume of the particles surfaces, systems FBR and RBR were considered to operate an excellent transfer of heat with hydrogen and announced able to reach temperatures of 3000 with 3750°K and I_sp of 1000 with 1300  S. The zone of exchange being very short, such a system has a great energy density authorizing a configuration more compact to him than NERVA and thus reaching a better weight ratio/thorough.

The studies of these systems were not very advanced when they were stopped in 1973 at the same time as the other programs of nuclear propulsion.

Timberwind

The concept was arisen of the paperboards for the project Timberwind initiated in secrecy in 1988 for the needs for the “Star Wars” of Ronald Reagan, then recycled in Space Thermal Nuclear Propulsion (STNP) in 1991 and finally reorientated towards Martian exploration under the administration Clinton before being abandoned in 1993. Within the framework of Timberwind, the system of propulsion called PBR ( Particle Bed Reactor ) was intended on the last floor of missiles of interception and was to combine strong push and great specific impulse.

The PBR consisted of a heart of 19 or 37 small hexagonal prismatic elements containing each one a bed of particles fixes surrounded by regulator. Hydrogen is channeled in the regulator towards the elements which it penetrates by external calcining (cold calcining), crosses the bed of particles and arises heated by interior calcining (hot calcining) before being directed towards the exit of the engine and the conduit.

The practiced tests remained elementary but promising and made it possible to consider an engine of 500  kg of mass developing 1000  MW of power and 200  kN of push.

However, the feasibility of the PBR was far from being shown because of the corrosion of the particles (in this case, the report/ratio surfaces/volume is penalizing), the design of hot calcining, the geometry or the porosity of the cold calcining which must make it possible to distribute the hydrogen flood to homogenize and maximize the temperature of exit, and the appearance of hot points which had with the instability of the laminar flow or with the obstruction of pores of calcining which can cause the local fusion of fuel.

In the years 1990, the University of New Mexico investigua a variation of concept FBR, the engine with reads ball ( PeBR , Pellet Bed Reactor ). It is of an architecture identical to the FBR, except that the particles are included per hundreds in graphite balls of approximately 1  cm of diameter, themselves protected by a film from ZrC. In fact thus larger super-particles are stored between calcinings. Such an engine would have a power of: 1000  MW, would eject hydrogen with: 3000  K and would have I_sp of approximately 1000  S.

Recently, in 2004, he was proposed the use of fuel ¹⁷⁸Hf ^m2 increasing the energy density further and breaking up primarily by gamma ray. The control of this nuclear reaction is still to deepen.

MITEE

After the end of the Timberwind program, Powell, Maise and Paniagua found the company Plus Ultra Technology Inc. and undertakes to improve the released concept of the military objectives implying a strong push. They then propose in the years 2000 various modifications making it possible to reduce the size and the power of the engine while preserving the energy density of ~30  MW/l in order to meet more immediate needs for mission.

The word family conceived, named MITEE ( Miniature reacTor Engine , pronounced as mighty ) comprises the following differences compared to the PBR:

• hexagonal combustible matter are in fact of the elementary engines with their individual conduit and pressurized enclosure;
• the bed and calcinings are composed of rolling up of perforated and stamped sheets resembling plates of precut Ravioli (the perforations ensure the radial flow while stampings are used to connect the perforations of the successive layers); cold calcining is in metal Béryllium, the bed in Molybdène and hot calcining in Tungstène -184;
• calcinings heat and cold are sown of fissile material;
• the fissile material is made up of particles or fibers of UC₂ or UO₂ included in the sheets;
• the assembly of 19 or 37 motive fluids is surrounded by a layer of hexagonal elements in reflectors neutronics;
• the system allows a bimodal operation .

The miniaturization is limited by the Criticité engine, one arrives thus at a basic configuration of 50  kg and developing 100  MW.

This type of engine is thus very indicated for missions of low mass requiring important a Delta-V, such as fast space probes (able to reach and explore external planets in years instead of decades) or for missions of intervention on objects géocroiseurs.

Engine with low pressure

To the beginning of the year 1990, INEL presents a new concept: the engine with low pressure ( LPNTR , Low Presses NTR ). Composed of a hollow spherical heart of 1  m of beryllium diameter bored of 120 conical channels containing each one a bed of particle, hydrogen circulates there since an internal admission towards the outside of the sphere, with a pressure of approximately 1  bar. This operation with low pressure allows the phenomenon of dissociation/recombination of the hydrogen which increases the temperature between 3200  K and 3600  K, and the specific impulse until 1300  S, practically the theoretical maximum for the engines in solid heart.

The concept has the merit of a great simplicity: the absence of turbo-machinery (the pressure of the tank is enough to supply the engine) and of control rods (criticality is reached by the presence of hydrogen in the engine) make the engine lighter. A system of propulsion made up of a group of 7 small engines, in addition to being more reliable, would make it possible to do without Cardan joints of orientation of the push.

Nevertheless, the concept is far from being mature because obviously of the problems of the hearts with bed of particles, but in addition to the comprehension of the phenomenon of dissociation/recombination of hydrogen, the control of the flow in the absence of pumps, the control of the reactivity without and design control rods of an effective conduit, without counting the need for an environment of depressurized test.

Engine with fragments of fission

During their research on Laser pumped by nuclear radiation, the Sandia laboratories had the idea to apply their technique of direct heating of gas with the space propulsion. However, its operation would slacken enormous quantities of very radioactive matter, it could be used only out of the atmosphere and the Magnétosphère.

Gas heart

The last class of nucleothermic engine is the engine in gas heart . This extension of the concept of heart liquid consists of a spherical room (typically of 2,4  m of diameter surrounded by 46  cm of beryllium oxide regulator) in the center of which uranium is injected. Hydrogen is injected by the porous walls of the room, thus maintaining fuel in a central pocket where its temperature can freely reach several tens of thousands of degrees. I_sp obtained goes from 2500 to 6500  S for a push of 20 with 400  kN according to the size of the engine of 40 with 210  T, radiator included/understood (in addition to the propelling hydrogen circulation, it is proposed to cool the device by an additional circuit passing by a radiator, weighing down the engine but improving the specific impulse).

The fuel is presented in the form of pastilles which are vaporized at the time of their arrival in the room, but for the lighting of the engine, it is necessary that it is in the form of particles. So that 99,5  % of the thermal energy radiated by the gas heart is absorbed by hydrogen, it is opacified by sowing of particles of graphite, Tungstène or natural uranium during its injection. The mass proportion of particles goes from 5  % in the room with 20  % on the level of the conduit collar. According to this basic principle in open cycle , the exhaust of the nuclear fuel is difficult to control (approximately 1  % of the ejected mass comes from the fuel loss) and prohibits its use near the Earth. This brought to study a configuration in cycle closed or engine in nuclear bulb , where the gaseous fuel is confined in tubes in quartz at ultra-high temperature outside whose hydrogen circulates. This system of heat exchange approaches more that of the engine in solid heart, except that it is not limited more by the melting point of fuel but by that of quartz. Although less effective than gas in opened cycle, this design makes it possible to reach sizeable performances of impulse specific of 1500 to 2000  S.

To obtain the best surfaces radiative, the engine is composed of seven tubes of 25  cm of diameter by 1,80  m length (what represents 2,2 times more surface than only one tube of the same total volume). These tubes are placed in seven cavities practiced in a moderating material/reflecting of neutrons.

In order to limit the deposition of fuel (or residues of fission) on the walls of the tubes, it is maintained in a vorticity using a closed circuit of Néon. This neon is injected tangentially on the level of the interior walls of the tubes, forming a peripheral vortex containing nuclear fuel in its center, then it is tapped at the end of the tube, is cooled, purified by centrifugation (to separate the traces from them from fuel) and reinjected.

Each tube is traversed of a thousand of channels which a hydrogen closed circuit cools, like various other bodies and structures, in particular the radiator of the neon circuit. Hydrogen in open circuit (from the tank and intended for the conduits) passes only by one exchanger with the moderating closed circuit and materials before arriving at 3500°K in the radiant cavity (either already the temperature which it would have at exit of an engine in solid heart). It is then opacified by tungsten particles, exposed to the radiation of fuel gas with 12000°K and almost reached 10000°K at the other end of the cavity.

Nuclear propulsion vs chemical

To compare directly the performances of the engines nuclear power and chemical is not easy thing; design of a rocket being the compromise of several solutions giving the best result overall. In the example hereafter, a stage based on an engine NERVA, version 1961, is compared with stage S-IVB of the rocket Saturn V which it was supposed to replace.

For a given push $T$, the engine must founir a power definite by $P\; =\; T\; *\; V\_e/2$ (where $V\_e$ is the speed of ejection, proportional to the specific impulse: $V\_e\; =\; I\_\; \{sp\}\; *\; g$). Thus, the J-2 engine of the S-IVB develops P = 414 S * (1014 kN * 9,81)/2 ≈ 2060 MW, this power released by the chemical reaction corresponds to that of a large nuclear reactor.

According to the preceding formula, a nuclear engine of equivalent push should have a power of 4600  MW (by supposing that heat exchange is ideally effective). Let us note that this increased power is not the fact that of the more important $I\_\; \{sp\}$. The configurations suggested for NERVA go until 5  GW, which would make of them the most powerful nuclear reactors of the world. But with such a power, the engine should have a size and a prohibitory mass. As the push determines the duration of lighting but does not influence the delta-V obtained, this comparative will be based on an engine NERVA of 266  kN of thorough: the power of the engine is then of 1044  MW.

The flow of propelling fluid for a given push $T$ is defined by $d\_m\; =\; T/V\_e$. For the J-2 engine, $d\_m$ = 1014 kN/(414 * 9,81) ≈ 250  kg/s. For engine NERVA, it would be only of 34  kg/s. Taking into account the fact that hydrogen is six times less dense than the mixture oxygenates/hydrogen used by J-2, the volume throughputs are both about hundreds of liters a second.

The idea of a standard replacement of stage S-IVB implies a stage NERVA of same dimensions, i.e. same the volume of fuel . The S-IVB transports approximately 310  m ³ of fuel in a mass proportion LOX/LH2 of 5,5:1, representative thus a total mass of fuel of: 106600  kg. A stage NERVA of same dimensions (by neglecting the difference in obstruction of the engines and the space recovered by the absence of separation between two tanks) would thus transport 310  m ³ of LH2, is: 22000  kg. To divide these masses by the mass throughput of the respective engines gives the lasted of lighting of these stages, that is to say 427  S for J-2 of the S-IVB (actually, this duration is of 500  S because the engine does not turn to full mode during the 100 first second) and 650  S for a stage with engine NERVA.

The change of swiftness Delta-V can be calculated by the equation of Tsiolkowski based on the report/ratio of the masses initial and final:

$\backslash \; Delta\; v\; \backslash \; =\; V\_e\; \backslash \; cdot\; \backslash \; ln\; \backslash \; frac\; \{m\_0\}\; \{m\_1\}$

where $\{m\_0\}$ is the initial mass (with fuel) and $\{m\_1\}$ the final mass (or masses dries, or inert mass, tank (S) vacuum (S)). The dry mass of a S-IVB is of: 13300  kg of which 1438  kg of J-2 engine. For a replacement by NERVA, taking into account the 6803  kg of the engine (and by neglecting the profit of weight due to the absence of connection between tanks) one has a mass dries of: 18665  kg. The lightness of the fuel of NERVA compensates more than largely the overweight of the engine, the full stage ($\{m\_0\}$) being three times lighter than the original S-IVB.

According to the preceding equation (and in the absence of payload), the J-2 version would generate a $\backslash \; Delta\; V$ of 8930  m/s. The nuclear version would obtain only 6115  m/s. This less performance must with a mass dries more important but also with the ejection of a faster but less dense fluid certainly. The nuclear version of the stage seems a priori less interesting.

However, this simple analysis does not take into account certain important aspects:

• the new stage weighs much less than the old one, which wants to say that the lower stages will have printed more initial speed to him;
• calculations are made for a of the same replacement volume, which is a bad assumption because a stage is generally designed as heavy as the capacities of launching of the lower stages do not allow it;
• the delta-V is calculated with an absence of payload, which gives a scale of comparison but really direction does not have, more especially as the performances can be very well reversed according to this load.

Accordingly, the replacement considered must thus be done by a stage of mass equivalent to the S-IVB, therefore much bulkier (because of difference in density of the fuels). By supposing simply that the weight of the tank is of it triplet (weighing then: 35586  kg), one obtains a mass dries $\{m\_1\}$ of: 42389  kg, and for the same initial mass that the S-IVB one can transport 77511  kg of LH2. New the $\backslash \; Delta\; V$ with vacuum is 8160  m/s, always less good than that of the S-IVB. The more the payload increases, the more the report/ratio $\{m\_0\}/\{m\_1\}$ tends towards 1 (the dry mass has importance less and less) and the more determining the parameter $V\_e$ becomes. With a payload of 45  T (the weight of the mission Appolo 11), the $\backslash \; V$ Delta of the S-IVB and its substitute are respectively of 4223 and 4983  m/s, thus showing the Net favors nuclear propulsion.

Of course, the increase in size of the tank is not without posing an animal problem: the vehicle must be able to be assembled in VAB and to be able to pass under the 125  m height of the doors of the building. Blow, the configuration RIFT ( Reactor In-Flight Test ) suggested in 1961 and detailed in the last column of the preceding table, adopted more modest dimensions. The vehicle made it possible to increase the load lançable in LEO of 120 with 155  T.

Risks

There is an inherent probability of failure of the rocket in the atmosphere or in orbit which would cause the dispersion of materials of the engine and the radioactive fallout. More serious, a catastrophic dysfunction involving a contamination of the environment could occur following the explosion of the launcher, a rupture of the containment of the heart because of orbital remains, of an uncontrolled fission, tiredness of the structures or an human error.

References

Information of this article is drawn primarily from the following reference:

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