火箭设计-火箭推进的进展

Advances in Rocket Propulsion Dr K Ramamurthi, Non-member The state-of-the art technology in propulsion is briefly reviewed in this article. The application and future growth of rocket propulsion for launch vehicle and spacecraft applications are also disscused. The power density of rockets is seen to be a limiting factor and the growth with energetic materials, vortex combustion and water for propulsion is discussed. The application of nano-technologies, non-chemical propulsion and propellantless propulsion is also discussed. Keywords: Rocket propulsion; Launch vechicle; Spacecraft application; Chemical propulsion; Nano-technology Dr K Ramamurthi is with Liquid Propulsion Systems Centre, ISRO, Trivandrum 695 547. This paper was presented and discussed at the All-India Saminar on ‘Aircrafts and Trans-atmospheric Vehicles : Missions, Challenges and Perspectives’ held at Kolkata during May 28-29, 2004. INTRODUCTION Advances signify improvements and this review of ‘Advances in Propulsion’ deals mainly with methods of improving the performance of rockets used in launch vehicles and satellites. Improvements are mainly aimed to make the launch vehicle and satellite less-expensive to fly and to achieve better reliability. Additional factors, such as, safety and an eco- friendly propulsion system are also important. The advances, are not meant to have a sophisticated or complex system and the pursuit is essentially to have a simple and rugged system that propels efficiently. The pioneers in rocket propulsion, such as, Tsialkowsky, Goddard and Oberth dreamt of and conceived very innovative propulsive devices for space applications in the early part of the twentieth century. These devices have been developed in the twentieth century and scaled-up and also applied successfully for various space missions including planetary missions. India has also successfully employed solid and liquid propellant rockets for launching satellites in different orbits. Liquid- fuelled rockets have been used for station keeping and orbit control of satellites. Advanced propulsion involving electrical propulsion is to be employed in some of the upcoming Indian geostationary class of satellites. The application of revolutionary thoughts, based on advances in physical sciences, could assist in radical changes in the propulsion systems and help in major innovations. Innovative research using new concepts is to be preferred over the conventional mundane method of improving by incremental stretching of performance using small modifications and improvements over the present propulsion modules. The scope of such innovations are specifically addressed in this article. Propulsion involves locomotion or movement. Everyone is familiar that a less efficient system can do the job of locomotion of a more efficient one if the less efficient one is made larger, that is, stronger. This is a damper for innovative development in propulsion and the objective of propelling satellites into orbit can do a large extent be met by larger and somewhat less efficient propulsive devices compared to advanced propulsion systems, which are possible to be developed based on current technologies. However, the missions using less efficient devices become not only unduly costly but have limitations in terms longer durations and smaller pay-load capability. For certain high demanding missions involving interplanetary or interstellar travel, a very high performance propulsion system having grossly improved characteristics is required. These aspects dictate the direction of the advances in propulsion. CHEMICAL PROPULSION AND POWER DENSITY OF COMBUSTION In this section, classification of chemical propulsion, along with the details of solid and liquid propellants liquids is discussed. Classification of Chemical Propulsion The heat release from the chemical reactions between a fuel and an oxidizer has invariably been used for generating high pressure and high temperature gases which produce the thrust or locomotion for propulsion. The solid propellant rocket are fed with fuel and oxidizer in a solid form, whereas if the fuel and oxidizer are stored as liquids before being brought together for burning in a combustion chamber, the rocket is known as a liquid propellant rocket. Certain fuels and oxidizers (which are in gaseous form at ambient temperatures and pressures) need to be stored at very low temperature in order to use them as liquids. Rockets using these low temperature liquid fuels and oxidizers are called cryogenic propellant rockets. These low temperature cryogenic fuels are stored in insulated containers. Application of Solid and Liquid Propellant Rockets A large number of rockets have been developed using solid, liquid and cryogenic propellants. Rockets generating very large thrust (thrust of several thousand kN) and very small thrust (thrust about a fraction of a Newton) have been made. Rockets having large values of thrust are used for boosters of launch vehicles whereas very small thrust rockets are used for spacecraft propulsion and control. New types of fuels have been tried and very high performance cryogenic propellant rockets using liquid hydrogen and liquid oxygen have been developed. 54 IE (I) Journal—AS Figure 1 shows the evolution of satellite launch vehicles in India. Solid rockets were used for sounding rockets and for the rocket stages of the Indian launch vehicles SLV-3 and ASLV. Liquid rockets stages were used along with solid propellant rockets in the polar satellite launch vehicle (PSLV). The liquid rockets used earth storable propellants di-nitrogen tetroxide for oxidizer and dimethyl hydrazine for fuel. The requirements of larger orbital velocities for geo-synchronous missions and the pay-load requirements demanded the higher performance of the cryogenic propellant rockets in order to avoid a large lift off mass and an inefficient and costly vehicle. The geo- synchronous satellite has cryogenic propellant rocket for its upper stage, earth storable propellant rockets for the second and strap on stages and a high thrust solid propellant rocket for the booster stage. Implications of High Power Density The high performance rockets release their chemical energy in a small combustion volume giving very high values of power density of the chemical energy release rates. The power density of a rocket has gone up substantially during the last sixty years. At present, it is some thousand times higher than it was for the first operational liquid propellant rocket (V2) developed in the 1940’s and used during World War II. Figure 2 compares the power density of rockets with those of other propulsive devices. A typical automobile engine such the Maruti 800 has a power density of about 0.04 kW/cm3. The power density of a high performance large rocket, such as, the RD170 Zenith rocket, which produces a thrust of 7250 kN at a chamber pressure of 24.5 MPa is 103 kW/cm3. This is very near to the power density of a detonation and is indicative of the almost explosive nature of the heat release. High values of power densities signify that larger amounts of energy are released faster in a smaller volume. The temperatures and heat transfer rates would, therefore, be significantly higher. Better methods of cooling the rocket chamber and use of special materials of construction become essential to contain the high power densities. Means of controlling the combustion also become necessary since even if a small fraction of the high power were to excite the chamber acoustics and vibration, it would spell disaster. If advances in high performance rockets are required with an increase of the power density through use of higher pressures and temperatures, one needs to specifically address the possibilities of combustion control and improved methods of cooling. The subject of active combustion control is currently of topical interest and is vigorously pursued. Vortex Rocket Cooling of the rocket chamber, however, invariably leads to losses. A new concept of a Vortex rocket in which the fuel is introduced in a rotating or vortex motion is shown in Figure 3. The fuel cools the chamber walls before being burnt with oxygen in the core. The centrifugal forces associated with the higher density low temperature fuel keeps the hot gases away from walls. The subject of vortex combustion with the Figure 1 Solid and liquid propellant rocket applications Vol 85, November 2004 55 Figure 2 Power densities in different propulsive devices associated large characteristic lengths and improved flame stability is exciting and is a promising contender for the future. IMPROVING PERFORMANCE OF CHEMICAL PROPULSION The efficiency of conversion of chemical energy of fuel into heat is of the order of 99% in the modern day rockets. If the performance of the chemical propulsion rocket is to be improved, then the important step is to improve the energy content of the fuel since the conversion efficiency is already high. Metal Addition to Improve Performance The cryogenic propellant combination of liquid hydrogen and liquid oxygen gives high performance since the molecular weight of the products of combustion is low leading to low densities and high exhaust jet vehicles. Metals have high values of stored energies and the possibilities of adding metal powders to rocket fuels to improve the performance have always been of interest. With solid propellants (which have somewhat lower performance) the metal addition leads to enhanced values of the performance. However, the enhancement in performance of the liquid propellant rockets is not very significant since the amount of increase in molecular weight of the combustion products is higher as compared to solid propellant rockets. The changes in performance of liquid propellant rockets from 56 IE (I) Journal—AS V2 Rocket Turbojets IC Engine Rockets Gas turbine RD 170 Zenit Rocket Detonative Combustion Maruti car kW/ cm3 10–2 10–1 1 1021 0 10 3 Fuel or Oxygen Oxygen or Fuel Figure 3 Vortex rocket diagram adding aluminium to kerosene and hydrogen fuel is shown in Figure 4. The metals of interest are those of lower atomic mass, such as, beryllium, boron, lithium, magnesium and aluminium. Tri-propellant Rockets The use of hydrogen along with fuels, such as, kerosene helps in improving the performance of the kerosene fuels used for booster propulsion. The rockets employing two fuels, (namely, kerosene and hydrogen) with an oxidizer are spoken of as tri-propellant rockets. Such rockets can start in a tri-propellant mode for booster and transit to a high performance bi- propellant mode (using hydrogen and oxygen). The application of such tri-propellant rockets has been promising. However, though a large amount of initial research on tri-propellant rocket was witnessed a decade earlier, not much flurry of activity is currently seen for developing tri-propellant rockets. Increasing Energetics of Propellants A novel method of adding energy to a fuel is by straining its chemical bonds. When the standard angle of 108.50 in the hexagonal bond structure in a hydrocarbon molecule is charged to form triangles, squares and tetrahedrons, a net increase in the energy content of the fuel results. Strain energies of about 40 kcal/mol to 100 kcal/mol have been claimed. However, such fuels tend to be unstable and soot readily when burning. Further research study is required before they could be applied in liquid propellant rockets. Rather than use of strained fuels, the possibilities of using the fuel in the atomic state is of interest since the energy content is several orders of magnitude higher. Thus, if atomic hydrogens were to be used for fuel instead of molecular hydrogen, the energy release rate would be almost 20 times higher giving an order of magnitude increase in the specific impulse performance parameter. But then the storage of atomic PP V V Brayton Cycle Atkinson Cycle Figure 5 Brayton and Atkinson cycles hydrogen is difficult and special traps with high magnetic fields at temperatures close to zero Kelvin are necessary. Similarly, the use of metallic hydrogen, formed when the solid hydrogen is compressed under very high pressure at Mega bar level would have intense locked up energy and contribute to the making of a very high performance propulsion system. Even solid hydrogen could be used and a proposal was put up by a German Scientist to use it as a solid propellant. Atkinson Cycle and Pulse Detonation Engines The energy is added in the rocket at constant pressure, such as, in the Brayton cycle. Energy addition at constant volume is more efficient for a given compression. The Atkinson cycle, which uses constant volume heat addition (Figure 5) gives an improvement by about 18% to 25% over Brayton cycle depending on the pressure and temperature. In the case of the constant volume combustion rocket, the fuel and oxidizer can also be admitted into the chamber at low pressures and the associated feed system can be made much simpler. The burning at constant volume marks a jump in pressure at which the burnt gas is discharged out through the nozzle. As an extension to the constant volume combustion, the use of detonative combustion in a pulsed constant volume combustor has attracted significant attention. The rockets using this principle are known as pulse detonation engines. A number of technologies, such as, extremely high mixing rates under unsteady conditions, flow control, vibration attenuation and rapid deflagration to detonation transition need to be established to make the system viable. The products of combustion have also to be aspirated out of the chamber after the detonation. Pulse detonation engines have been demonstrated with gaseous fuels for cyclic operation up to 100 Hz. The aim of the pulse detonation engine is to have a simplified rocket system, which can operate at low feed pressures with lean fuels and high power densities and efficiencies. AIR BREATHING AND REUSABILITY In this section, air breathing and in situ production of propellants along with reusability details are discussed. Air Breathing and In-situ Production of Propellants The fractional weight of the pay-load, the structure, and the fuel and oxidizer in a typical launch vehicle are shown in Figure 6. It is seen that the oxygen content is about 70%. If this oxygen can Vol 85, November 2004 57 Lox/ Kerosene with Aluminium Lox/ LH2 with Aluminium450 400 350 300 250 200 0 1 2 3 4 5 6 7 8 Mixture Ratio P C = 100 bar Sp ec ific Im pu lse , s 10 % AL 5 % AL 2 % AL 0 % AL 10 % AL 5 % AL 2 % AL 0 % AL Figure 4 Influence of metal addition to liquid fuels be drawn from the atmospheric air, the rocket weight and, hence, the cost of the rocket can be brought down. The incorporation of air-breathing propulsion for the space vehicles with the take-off using turbojets, turbofans or ejector ramjets followed by ramjets, scramjets and rockets has been the subject of intense debate over the last thirty years. Combined cycle engines transitioning from one phase of operation to the other have been proposed. Even the possibility of collecting air, liquefying it and then using it for a rocket mode has been suggested. This constitutes the Liquid Air Cycle Engine (LACE). The major issue is the difficulty of a low weight compact-heat exchanger for the liquefaction. The principle of collecting oxygen has been extended to in-situ propellant production for vehicle returning from planet Mars. The liquid hydrogen taken by the vehicle is used for reacting with the carbon dioxide gas atmosphere over Mars. The methane and oxygen so formed are liquefied and stored in insulated tanks for the Mars Ascent Vehicle (MAV). Reusability Figure 7 shows the approximate cost distribution in a launch vehicle. The major cost is towards the hardware (about 70%) while the balance is the cost of propellants and pay-load. A major saving in the cost is, therefore, possible if the hardwares are to be repeatedly reused. The space shuttle of the USA and Buran of Russia represented the first efforts to partially reuse the hardwares. Subsequently, a large number of experimental configurations like the use of throttleable liquid rockets air- launched from an aircraft, and use of ramjets and scramjets have been toyed with during the last twenty years. These comprise of a reusable vehicle with cryogenic rockets for vertical take-off and landing known as Delta Clipper, the take-off as a rocket and landing as an helicopter (as in Roton) and the more conventional two stage-to-orbit propulsion vehicle comprising a hypersonic aircraft with ramjet and a rocket powered upper stage (Sanger). Several such configurations have been discussed and the use of adapted nozzles, such as, plug nozzle, aerospike nozzle and linear aerospike nozzles has been experimented. The single stage to orbit does not appear to be a promising reusable vehicle. The use of air-launch, which not only gives an initial velocity to the vehicle, but also operates it in a reduced drag environment seems to be a promising option. WATER FOR PROPULSION The possibilities of using water as a source of chemical energy was first put forward by the science fiction author Jules Verne in 1870 in the book ‘The Mysterious Island’. The author wrote ‘I believe that water will one day be employed as fuel, that hydrogen and oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light ...’. Studies on efficiently splitting water into hydrogen and oxygen using different methods, such as, bond-breaking, electrolysis, thermo-chemical and hybrid processes are in progress. Certain semi-conductors, when illuminated, break-up water by photolysis. The hydrogen and oxygen, thus generated from the water, can be combined chemically to give thrust as in chemical propulsion. The use of fuel cells that store the solar energy during the day by electrolyzing water into hydrogen and oxygen and then converting the gases back into water to generate electricity is being demonstrated. Regenerative fuel cells with capacities of 600 W-h/kg are developed. Considerable developments are also made in the polymer electrolyte membrane and it should be possible to use such cells in the next generation of geo- synchronous satellites. The hydrogen and oxygen from the water can, therefore, be also used for propulsion control of satellites. PROPULSION SYSTEMS FOR SATELLITE In this section, propulsion control for both the large satellites and the small satellites are discussed. Propulsion Control for Large Satellite Chemical rockets (both mono-propellant and bi-propellant) for the Indian Remote Sensing (IRS) and Indian National Satellites (INSAT) are being used at present. The electrical propulsion 58 IE (I) Journal—AS Structure (13 %) Hydrogen (15 %) Pay-load (2 %) Oxygen (70 %) Figure 6 Typical weight distribution in a launch vehicle Figure 7 Typical cost distribution of a launch vehicle Propellant (15%–20%)Pay-load (15% – 18%) Hardware (70 %) system, namely, the stationary plasma thruster within two years in Geostationary Satellites (GSAT) can be attempted for this purpose. If one looks at the future aspects of satellites, the continution of using heavy satellites for geosynchronous and polar missions can be tried in future too. For these large satellites, the propulsion would probably continue to be with electrical and chemical nature with improvement in performance for a prolonged lifetime of the satellites. The thrust levels available with the chemical propulsion make them attractive for the heavy satellites. The use of water for propulsion is attractive since the fuel cells can use water and the hydrogen and oxygen from the water can be used for propulsion. Propulsion for Small Satellit