Nuclear Propulsion

For those who are interested in the exploration and development of space by humans, nuclear propulsion technology is a very attractive option. Why? Compared with the best chemical rockets, nuclear propulsion systems (NPS's) are more reliable and flexible for long-distance missions, and can achieve a desired space mission at a lower cost. The reason for these advantages in a nutshell is that NPS's can get "more miles per gallon" than a chemical rockets. If NPS's were analogous to a 1998 Honda Civic, then your best chemical rocket would be a gas-guzzler from the 1950's.

Space Mission Analysis
For any space mission, there are a few basic questions that must be answered:

1. What is the destination?
   
2. What is the trip time?
   
3. Do we want to return?
   
4. What is the mass of the payload we want to send there (and bring back)?
   

Upon answering these questions, one can proceed to determine approximately what the propellant requirements are using the rocket equation: delta-V = v_exhaust*log(M_initial/M_final). The delta-V is an equivalent velocity change that depends on the destination, travel time, trajectory, gravitational force field, and the thrust mode for a given space mission. V_exhaust is the exhaust velocity of propellant coming the rocket engine; M_initial is the initial mass of the rocket vehicle, and M_final is the final mass of the rocket vehicle. The propellant used to achieve the mission is simply M_propellant = M_initial-M_final. A higher exhaust velocity is desired, because less propellant will be required for a given space mission. A performance parameter that rocket engineers like to use is the specific impulse (Isp) which is simply the ratio of the thrust to the weight consumption rate of propellant (Isp = F / ( dm/dt * g) = v_exhaust / g , g=9.81 m/s²) So, for example, if you have an engine with a specific impulse of 500 seconds, that means you have an exhaust velocity of 4905 m/s and your engine will produce 500 pounds of force for every pound of propellant it consumes per second. Figure 1 shows a plot of the mass ratio (M_initial / M_final) for various mission delta-V's and specific impulses. It is pretty clear from this diagram that a high specific impulse is desired to minimize the propellant requirements. The specific impulse, Isp, is a rocket engine's equivalent of an automobile's "miles per gallon" rating.

Limitations of Chemical Rocket Engines
In chemical rocket engines^1,2, such as the Space Shuttle Main Engine (SSME), the chemical reaction between the hydrogen and oxygen releases heat which raises the combustion gases (steam and excess hydrogen gas) up to high temperatures (3000-4000 K). These hot gases are then accelerated through a thermodynamic nozzle, which converts thermal energy into kinetic energy, and hence provides thrust. The propellant and the heat source are one in the same. Because there is a limited energy release in chemical reactions and because a thermodynamic nozzle is being used to accelerate the combustion gases that do not have the minimum possible molecular weight, there is a limit on the exhaust velocity that can be achieved. The maximum Isp that can be achieved with chemical engines is in the range of 400 to 500 s. So, for example, if we have an Isp of 450 s, and a mission delta-V of 10 km/s (typical for launching into low earth orbit (LEO)), then the mass ratio will be 9.63. The problem here is that most of the vehicle mass is propellant, and due to limitations of the strength of materials, it may be impossible to build such a vehicle to just to ascend into orbit. Early rocket scientists got around this problem by building a rocket in stages, throwing away the structural mass of the lower stages once the propellant was consumed. This effectively allowed higher mass ratios to be achieved, and hence a space mission could be achieved with low-Isp engines. This is what all rockets do today, even the Space Shuttle. In spite of the relatively low Isp, chemical engines do have a relatively high thrust-to-weight ratio (T/W)². A high T/W (50-75) is necessary for a rocket vehicle to overcome the force of gravity on Earth and accelerate into space. The thrust of the rocket engines must compensate for the weight of the rocket engines, the propellant, the structural mass, and the payload. Although it is not always necessary, a high T/W engine will allow orbital and interplanetary space vehicles to accelerate quickly and reach there destinations in shorter time periods.

The Advantage of Nuclear Propulsion Systems
Nuclear propulsion systems have the ability to overcome the Isp limitations of chemical rockets because the source of energy and the propellant are independent of each other. The energy comes from a critical nuclear reactor in which neutrons split fissile isotopes, such as 92-U-235 (Uranium) or 94-Pu-239 (Plutonium), and release energetic fission products, gamma rays, and enough extra neutrons to keep the reactor operating. The energy density of nuclear fuel is enormous. For example, 1 gram of fissile uranium has enough energy to provide approximately one megawatt (MW) of thermal power for a day.³

The heat energy released from the reactor can then be used to heat up a low-molecular weight propellant (such as hydrogen) and then accelerate it through a thermodynamic nozzle in same way that chemical rockets do. This is how nuclear thermal rockets (NTR's) work. There are two main types of NTR's^4,5,6: solid core and gas core. Solid-core NTR's (See Figure 2) have a solid reactor core with cooling channels through which the propellant is heated up to high temperatures (2500-3000 K). Although solid NTR's don't operate at temperatures as high as some chemical engines (due to material limitations), they can use pure hydrogen propellant which allows higher Isp's to be achieved (up to 1000 s), since Isp is approximately 1/M_propellant^0.5, where M_propellant is the molecular weight of the propellant. In gas-core NTR's, the nuclear fuel is in gaseous form and is inter-mixed with the hydrogen propellant. Gas core nuclear rockets (GCNR) can operate at much higher temperatures (5000 - 20000 K)⁴, and thus achieve much higher Isp's (up to 6000 s). Of course, there is a problem in that some radioactive fission products will end up in the exhaust, but other concepts such as the nuclear light bulb (NLB)⁴ can contain the uranium plasma within a fused silica vessel that easily transfers heat to a surrounding blanket of propellant. At such high temperatures, whether an open-cycle GCNR, or a closed-cycle NLB, the propellants will dissociate and become partially ionized. In this situation, a standard thermodynamic nozzle must be replaced by a magnetic nozzle which uses magnetic fields to insulate the solid wall from the partially-ionized gaseous exhaust. NTR's give a significant performance improvement over chemical engines, and are desirable for interplanetary missions. It may also be possible that solid core NTR's could be used in a future launch vehicle to supplement or replace chemical engines altogether⁴. Advances in metallurgy and material science would be required to improve the durability and T/W ratio of NTR's for launch vehicle applications.

An alternative approach to NTR's is to use the heat from nuclear reactor to generate electrical power through a converter, and then use the electrical power to operate various types of electrical thrusters (ion, hall-type, or magneto-plasma-dynamic (MPD)) that operate on a wide variety of propellants (hydrogen, hydrazine, ammonia, argon, xenon, fullerenes) This is how nuclear-electric propulsion (NEP) systems work.^4,5,6

To convert the reactor heat into electricity, thermoelectric or thermionic devices could be used, but these have low efficiencies and low power to weight ratios. The alternative is to use a thermodynamic cycle with either a liquid metal (sodium, potassium), or a gaseous (helium) working fluid. These thermodynamic cycles can achieve higher efficiencies and power to weight ratios. No matter what type of power converter is used, a heat rejection system is needed, meaning that simple radiators, heat pipes, or liquid-droplet radiators would be required to get rid of the waste heat. Unlike ground-based reactors, space reactors cannot dump the waste heat into a lake or into the air with cooling towers.

The electricity from the space nuclear reactor can be used to operate a variety of thrusters. Ion thrusters^1,2 use electric fields to accelerate ions to high velocities. In principle, the only limit on the Isp that can be achieved with ion thrusters is the operating voltage and the power supply. Hall thrusters² use a combination of magnetic fields to ionize the propellant gas and create a net axial electric field which accelerates ions in the thrust direction. MPD thrusters² use either steady-state or pulsed electromagnetic fields to accelerate plasma (a mixture of ions and electrons) in the thrust direction. To get a high thrust density, ion thrusters typically use xenon, while Hall thrusters and MPD thrusters can operate quite well with argon or hydrogen. Compared with NTR's, NEP systems can achieve much higher Isp's. Their main problem is that they have a low power to weight ratio, a low thrust density, and hence a very low T/W ratio. This is due to the mass of the reactor, the heat rejection system, and the low-pressure operating regime of electrical thrusters. This makes NEP systems unfeasible for launch vehicle applications and mission scenarios where high accelerations are required; however, they can operate successfully in low-gravity environments such as LEO and interplanetary space. In contrast to a chemical rocket or an NTR which may operate only for several minutes to less than an hour at a time, an NEP system might operate continuously for days, weeks, perhaps even months, as the space vehicle slowly accelerates to meet its mission delta-V. An NEP system is well suited for unmanned cargo missions between the Earth, Moon and other planets. For manned missions to the outer planets, there would be a close competition between gas-core NTR's and high-thrust NEP systems.

Sample Calculations for a Mars Mission
So how do nuclear propulsion systems stack up against chemical systems for a particular space mission? Table 2 shows the results for a Mars mission comparing a high-performance chemical system (H2/O2) with a solid core NTR operating with hydrogen propellant. The payload mass is 100 tonnes and the round-trip travel time is 1 year (6 months to Mars, 6 months return to Earth). The structural mass fraction e is assumed to be 0.05 for the chemical system, and 0.1 for NTR system, to account for the extra mass associated with the reactor and shielding¹. The final results are striking. The chemical system has a payload fraction of about 17%, while the NTR system is 40%. More than three times as much propellant is needed for the chemical system. This will translate directly into higher mission costs since all the propellant must be launched into orbit. If we assume that it costs about $5000 per kg to put hardware and propellant into orbit, the chemical system will cost at least 3 billion dollars, while the NTR system would cost about 1.3 billion dollars. So, on the basis of launch costs, one could have two nuclear missions for the price of one chemical mission. Indeed, nuclear propulsion gets "more miles per gallon".

What Progress Has Been Made in Space Nuclear Power and Propulsion?Both the United States and Russia (formerly the Soviet Union) have actively pursued research and development in space nuclear power and propulsion. In the United States, radioisotope thermoelectric generators (RTG's) were developed in the SNAP program (Space Nuclear Auxiliary Power)^5,6 and used in early test satellites in the 1960s, the lunar landing missions of the early 1970s, and most planetary space missions since then (Pioneer, Viking, Galileo, Ulysses, and Cassini). The Russians built and launched a variety of nuclear power sources and reactors on military satellites up until the fall of the Soviet Union in 1991. Their most famous and successful space reactors were the Topaz I and II.^5,6

In the realm of NTR technology, the U.S. had made significant progress in the period of 1955-1973 with the Rover and NERVA programs^4,5,6. Over 20 full-scale reactors were built and tested, and met the performance requirements for various space missions, including the use of NTR's for the upper stage of a launch vehicle, such as the Titan-III or the Saturn. Due to down-sizing in the 1970's, the NERVA program was cancelled before any flight tests could be performed. The advent of the Strategic Defence Initiative (SDI)⁶ in 1983, and later the Space Exploration Initiative (SEI)⁶ in 1990 had renewed the interest in nuclear propulsion (both NTR and NEP), and extensive R&D began at various universities, government labs, and aerospace companies; however, Congress indefinitely shelved these programs in 1992, and only a small level of research has been carried out since. Like the U.S., the Russians built and ground-tested several NTR engine designs, operating on a variety of propellants, including hydrogen, ammonia, and alcohol. Although their work continued relatively steadily up until the mid-1980s, there is no evidence yet to confirm whether or not the Russians actually flight-tested any of these engines.

In contrast, electric propulsion technology has been developing steadily since the 1950s. Ion and Hall-type thruster technology have matured to the point that they are now being used on operational commercial and military satellites with photo-voltaic power sources. MPD thruster technology is still under development, although many systems have flown on test satellites. For the purpose of maintaining a satellite's orbit, the solar-electric propulsion system is quite adequate.

Conclusions
The performance gain of nuclear propulsion systems over chemical propulsion systems is overwhelming. Nuclear systems can achieve space missions at a significantly lower cost due to the reduction in propellant requirements. When humanity gains the will to explore and develop space more ambitiously, nuclear propulsion will be an attractive choice⁸. Robert H. Goddard, an American pioneer in astronautics and rocketry, made the following prophecy on October 3, 1907:

"In conclusion then, the navigation of interplanetary space depends for its solution on the problem of atomic disintegration... Thus, something impossible will probably be accomplished through something else which has always been held equally impossible, but which remains so no longer".

References

1. Hill, P. and Peterson, C., Mechanics and Thermodynamics of Propulsion, 2^nd Edition, Addison Wesley, Reading, Massachusetts, 1992.
   
2. Sutton, G., Rocket Propulsion Elements, 6^th Edition, John Wiley and Sons, New York, 1992.
   
3. Lamarsh, J.R., Introduction to Nuclear Engineering, 2^nd Edition, Addison-Wesley, Reading, Massachusetts, 1983.
   
4. Clark, J.S., An Historical Collection of Papers on Nuclear Thermal Propulsion, AIAA, Washington, D.C., 1992.
   
5. El-Genk, M.S., A Critical Review of Space Nuclear Power and Propulsion 1984-1993, AIP Press, New York, 1994.
   
6. Pike, J. , Nuclear Thermal Propulsion, http://www.fas.org/nuke/space/index.html, World Wide Web, 1998.
   
7. Campbell, W.E. et al., "NPSP Selection for A Nuclear Rocket", Paper 67-476, Collection of Technical Papers on Nuclear Propulsion, AIAA 3^rd Propulsion Joint Specialist Conference, Washington D.C., 1967.
   
8. Ragheb, M., NucE/ME 302 Nuclear Power Engineering, Fall 1998, http://www.staff.uiuc.edu/~m-ragheb/NucE302/index.htm, World Wide Web, 1998.
   

Table 1: Types of Propulsion Systems

Type	Isp	T/W
Chemical (H2/O2)	400 - 500 s	50-75
NTR - Solid Core	500 - 1000 s	1-20
NTR - Gas Core	1000 - 6000 s	1-10
NEP - Ion Thruster	2000 - 10000 s	<<>
NEP - Hall Thruster	1000 - 5000 s	<<>
NEP - MPD Thruster	1000 - 8000 s	<<>

Table 2: Mars Mission Comparison - Round Trip

Parameter	Chemical (H2/O2)	NTR - Solid Core
Payload Mass	100 tonnes	100 tonnes
Travel Time	1 year	1 year
Mission Delta-V	7.7 km/s	7.7 km/s
Isp	500 s	1000 s
Mass Ratio	4.806	2.192
Structural Mass	25 tonnes (e=0.05)	15 tonnes (e=0.10)
Propellant Mass	475 tonnes	137 tonnes
Total Initial Mass in LEO	600 tonnes	252 tonnes
Payload Fraction	0.167	0.397

Figure 1: Mass Ratio Dependence on Mission Delta-V and Specific Impulse

Figure 2: Schematic Diagram⁷ of a Solid Nuclear Thermal Rocket (NTR) Engine

President of Kazakhstan visited the King Abdullah II Design and Development Bureau Industrial Park

President of Kazakhstan visited the King Abdullah II Design and Development Bureau Industrial Park and got acquainted with a motor-car repair factory in Amman. King Abdullah II speaks for the establishment of military-technical cooperation between the two states and suggests holding joint modernization of military technique of Kazakhstan and Jordan. In this context the defense ministries and special services of the two states establish contacts. In particular, there was reached an agreement on cooperation in the sphere of security. In 2008 Kazakhstan send official representative of the National Security Committee to Jordan for coordination of joint operations on combating terrorism, extremism and organized crime.

Finmeccanica Inks Wide-Ranging MoU With Kazakhstan

Italy's Finmeccanica industrial group on Nov. 5 signed a memorandum of understanding (MoU) with a Kazakhstan sovereign wealth fund to reach industrial cooperation deals covering helicopters, railways and electro-optics, including tank upgrades.

Kairat Kelimbetov, chairman of the Kazakh fund Samruk-Kazyna, signed the MOU with Finmeccanica CEO Pier Francesco Guarguaglini as Italian Prime Minister Silvio Berlusconi met Kazakh President Nursultan Nazarbayev here.

In a statement, Finmeccanica said the MOU covers "the possibility of setting up technological centers of excellence," and "the establishment of a working group to analyze Kazakhhstan's changing needs and the business opportunities for Finmeccanica Group companies."

As part of the deal, Finmeccanica unit Selex Galileo signed an electro-optics cooperation deal for civil and military applications with Kazakh firm KazEngineering. The agreement involves the use of Selex Galileo electro-optical systems to upgrade T-72 tanks for the Kazakh armed forces and for re-export.

The statement said plans were being evaluated for a joint venture between Finmeccanica unit AgustaWestland and Samruk-Kazyna to build a civil helicopter training and maintenance center, as well as a joint venture to build natural gas-powered buses in Kazakhhstan.

Guarguaglini and the head of Italy's public rail operator, Ferrovie dello Stato, also signed an MOU with the head of Kazakh firm Temir Zholy to develop the Kazakh rail sector.

Temir Zholy also signed a deal to create a joint venture with Finmeccanica rail unit Ansaldo STS to work on rail signaling, electrification systems and command-and-control centers for railway stations in Kazakhstan and neighboring countries.

Finmeccanica reported on Nov. 5 a net profit of 364 million euros ($537.6 million) for the first nine months of 2009, and confirmed its 2009 revenue target of 17.1 billion to 17.7 billion euros.

Kazakhstan to to encourage individual purchases of light aircraft

The government of Kazakhstan, an ex-Soviet country the size of Western Europe with an average weekly wage of $114, has urged its citizens to make more use of small planes to replace the "anachronism" of long car journeys.

Deputy Prime Minister Umirzak Shukeyev on Tuesday announced new laws to cut the paperwork required for flights on private planes, some of which are "no more expensive than a jeep".

"Come to any African country and they have a small runway, they take a small plane when they need to, start it like a car and go shopping to a neighbouring village," Shukeyev told a government meeting.

"Driving a car to travel 1,000 kilometres (625 miles) is a total anachronism," he said.

Shukeyev did not say if the laws aimed to encourage Kazakhs to set up businesses with small planes to ferry passengers around or if the government's aim was to encourage individual purchases of light aircraft.

Most of Kazakhstan's 16 million citizens, whose average wage is around $455 per month, are used to long car and rail journeys, such as the 800-mile trip between main business hub Almaty and capital Astana.

But a small elite, many with close ties to the government and the oil business, use helicopters and private planes to traverse the country's vast steppe.

Technology transfer in Kazakhstan problems, prospect and possibilities for foreign companies

Kazakhstan is the young and independent state with a fast developing economy. In 1991 after the dissolution of the USSR (Union of Soviet Socialist Republics), Kazakhstan became an independent state and according to the occupying area Kazakhstan takes the 9-th place in the world. Kazakhstan is recognized as a “Country with market economy”, but nevertheless raw material sector is dominating in state economy structure.

From the one hand it is connected with the heavy stocks of oil, gas and minerals, ferrous and nonferrous metals, coal, uranium and other minerals, but from the other hand, it is because of the soviet economy structure, directed to the development of raw material sector, restructuring and movement to the processing sector has just started. The Government of the country makes considerable efforts to create favorable investment climate.

For this reason an appropriate legislative basis was created. The interests of private investors are protected by the government. All these become a result of considerable range of foreign investments, growth of hydrocarbon stuff production and all other types of mineral recourses: ferrous and nonferrous metals, rare earth elements, uranium and etc.

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Alatau technopark to become a basis for IT business expansion in Kazakhstan

According to the results of the selection 12 projects will be realized on the territory of the Alatau information technologies park, 10 – in Uralsk technologies park Algoritm and 13 – on the territory of Karaganda park UniScienTech, head of Kazyna Fund Kairat Kelimbetov has stated at the sitting of the Government.

The main task today is informatization of the created technologies parks. Alatau park has all prospects to become a platform for expansion of a long-term IT business in the region, K. Kelimbetov said.

Representatives of Microsoft and Cisco Systems are demonstrating great interest in development of this park in the aspect of investments and localization of business.

Besides that, feasibility study on formation of 4 regional technologies parks in Astana city, South, East and North Kazakhstan oblasts are being worked out, Mr. Kelimbetov noted.

Strategy for Industrial-Innovative Development of Kazakhstan

Space activity on a global scale is constantly developing as a source of innovative technologies, actually in all areas of modern life. Many industries and services of the world have a direct dependence on the development of space activity. Thus, many countries of the world, including those in the Asian-Pacific region, are consistently increasing their scientific and industrial potential in this sphere, confidently and actively developing space technologies.

Certainly, the implementation of space activities by the countries in the Asian-Pacific region is one of the major factors for assuring the world's sustainable development. As an Asian country, the Republic of Kazakhstan is actively developing its space activities.

A guiding line of this development of the Republic of Kazakhstan is the "Strategy for Industrial-Innovative Development of Kazakhstan," scheduled to run till 2015. Our country has also stated the aim of having Kazakhstan become one of the top 50 competitive countries of the world. Realizing these aims will be possible only because of scientific-technical and innovative progress, in which the important role belongs to scientific and hi-tech space activities.

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Kazakhstan's space policy in a changing national and global context

In the wake of its transition to a market economy and the political and social reforms that have accompanied this, Kazakhstan—home to the renowned Baikonur space complex—is experiencing greater demand for space services. This article reports on the drivers behind and main features of the country's current space program and analyzes Kazakahstan's space policy. Key priorities are capacity building, maximizing revenue from the lease of Baikonur, international cooperation, in particular with Russia, as a means of gaining know-how and entering the world space industry, developing Earth observations and broadcasting expertise, and placing the country's activities within a legal framework.

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