1: The History of Rocketry and The Systems Involved

Fire is the origin of weapon development in a true sense. The throwing of fire pots, containing flammable materials like naphtha, is reported as far back as 1000 B.C. Although not rockets in a true sense, Archytas, a Greek philosopher, demonstrated the reaction principle in 360 B.C. He filled water in a hollow clay pigeon and set it over fire. The pigeon moved under its own power due to the escape of steam through strategically placed holes. In the first century AD, Hero from Alexandria demonstrated the reaction principle using an aelopile, in which a globe mounted on two central trunions rotates due to passage through tangentially placed exit points. The book by Sir Issac Newton Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) in 1687 resulted in the first scientifically defined reaction principles.

The Chinese were the leaders in the development of firearms. They contributed immensely to both the theoretical and practical development of rocketry. By 200 B.C., the Chinese are believed to have discovered black-powder, while separating gold from silver during a low temperature reaction. They added KNO₃ and sulfur to gold ore but forgot to add charcoal. They added charcoal as the last step. Unknown then, they had made Black-powder, which resulted in a tremendous explosion. Black-powder was, however, not introduced until the 13th century. In 994 A.D., the Chinese developed an attack mechanism based on artillery fire made up of catapulted stones and fire arrows launched by bows. In 1045, a compendium by Tseng Kung-Liang named as “Wu-ching Tsung-Yao” (Complete Compendium of Military Classics) was compiled, which illustrates the use of ballistic fire arrows not launched by bows but by charges of gunpowder. These fire arrows were propelled by ignited gunpowder housed in tube tied to the arrow. These fire arrows were launched in salvos from arrays of cylinders or boxes, which could hold as many as 1000 fire arrows each. In 1500, the Chinese even attempted to propel man with the help of a similar rocket-propelled vehicle, but failed.

The word “Rochetta”, which means “rocket” in English, was used first by an Italian named Muratori in 1379 by the 13th century, the armies of Japan, Korea and India are believed to have acquired a sufficient knowledge of gunpowder-propelled fire arrows. In 1285, the Arabs began using gunpowder-propelled fire arrows in combat. By 1410, briefs on the design of tube-launched military rockets were also published. During the 15th century, French troops used war rockets extensively in their attacks. In 1627, gunpowder was used as a blasting agent for recovering ore in Hungary. During 1670, the British used black powder for copper mining. Indian troops were not far behind in using rockets in battlefields. In 1788, Hyder Ali formed a rocket contingent made up of 1200 men. His son Tipu Sultan used it effectively against the British army in the Battle of Srirangapattam in 1792. The rockets disoriented the British soldiers by sheer numbers, sound and dazzling blue light, even during the night.

In 1804, William Congreve developed a variety of superior rockets with incendiary effects, conical metallic warheads, parachute mounted flares and battlefield messages distribution services. He developed a variety of rockets of different calibers, types and for various purposes. Congreve rockets were successfully used in battles for capturing Callao (1809), Cadiz (1810), Leipzig (1813), Fort McHenry in Baltimore (1814) and Waterloo (1815). In the middle of the 19th century, William Hale developed spin-stabilized rockets for better accuracy. The first use of these rockets took place in Mexican war of 1846–48. Russia test fired rockets in 1817 and their first rocket manufacturing plant was established at St. Petersburg in 1826. In the latter half of the 19th century, significant advances in conventional artillery resulted in the reduced use of rockets in wars.

In 1855, the first two-stage rocket was developed for the transport of heavier cord and in rescue line applications. In 1881, Russian Nikolai Kibalchich is believed to have designed the first rocket-propelled aircraft and the first gimbaled engine. Although unconfirmed, but Peruvian chemical engineer Pedro A. Paulet proposed the first liquid-fueled rocket; he used nitrogen peroxide and gasoline for propulsion. However, by the middle of the 19th century, the limitation of black-powder as a blasting explosive became apparent. In 1846, the Italian professor Sobrero discovered liquid nitroglycerin (NG). A few years later the Swedish inventor, Nobel developed a process for manufacturing NG. Nobel began to license the construction of NG plants, which were built near the site of intended use, as transportation of NG tended to generate a loss of life and property. The Nobel family suffered many setbacks in marketing NG. One of the accidental explosions destroyed the Nobel factory in 1864 and killed Alfred's brother Emil. After another explosion in 1866, which demolished another NG factory, Alfred turned his attention to safety issues for transporting NG. Alfred mixed NG with “Kieselguhr”. This mixture was known as guhr dynamite and was patented in 1867.

Along with NG, the nitration of cellulose to produce nitrocellulose (NC) was being studied by different workers. With the announcement of Schonbem in 1846 (and by Bottger) that NC had been prepared, its utilization began. Many accidents took place during the preparation of NC and many plants were destroyed in France, England and Austria. Abel (1865) showed that through the process of pulping, boiling and washing, the stability of NC could be greatly improved. In 1875, Alfred Nobel discovered that on mixing NC with NG, a gel was formed. This gel was used to produce blasting gelatine. Later in 1888, ballistite, the first smokeless powder consisting of NC, NG, Benzene and camphor was discovered. The British called it “Cordite”. In various forms cordite remained the main propellant of British force until 1930. The British established a cordite factory in India, close to Otty (Arvankadu) to manufacture various types of cordites.

Before WWII, propellants were established mainly for small arms in cannons and for sporting ammunition for civilian uses. The advent of rockets in WWII and the use of extruded double base propellants in rockets as early as the 13th century by the name of “Fire Arrow” propelled by rockets increased their range. In the latter part of the 19th century, the development of artillery with a high accuracy and long range was due to improvements in propellant characteristics, which continue till now. The two main classes of propellants, solid and liquid have some characteristics in common but there are many more that are quite different. Today, hybrid rockets using a liquid oxidizer and a solid fuel, are gaining importance with respect to their high performance and high safety level.

Although a number of improvements in gunpowder were made, it still had many undesirable properties like bright muzzle flash, a large quantity of smoke and hygroscopicity. The solid residue formed was also very corrosive and had to be removed after each firing. The introduction of NC smokeless powder by Vielle in 1886 marked a significant advancement in propellant history. After a few years Nobel introduced NC–NG based double base propellant. As a result of increased research and development activities during WWII, a group of solid propellants called composite propellants emerged. The first composite rocket appeared somewhere during 1945. Since then composite propellants have assumed a major role in the propellant field. Earlier propellants were used mostly for military applications, but the advent of sputnik and explorer satellites opened the way for greater usage of propellants in space. There has been considerable propellant applications for industrial use but when compared to military use, these applications are limited. Oil-well perforating guns, industrial cannons for quarries are a few examples of industrial use. Jet-assisted take-off (JATO) rockets have been introduced for aircrafts.

In the early 20th century, the preparedness for world wars resulted in the introduction of several new technologies in rocketry. The first guided missiles were introduced in the form of the British A.T. (Aerial Target) and the US Kettering Bug. The feasibility of radio guidance was established during this project. The Kettering Bug, a bi-plane bomber, was successfully demonstrated in 1918. However, these could not be placed into production due to the end of the hostilities of WWI. In between the two world wars, the development and evaluation of several rockets, based on solid and liquid fuels continued in different countries, e.g. Larynx, a radio guided mono plane, Queen Bee and Queen Wasp, both radio guided bi-planes (British), GIRD-X and Aviavnito (Russia), Mirak-I (Minimum Rocket–I), Huckel-Winkler-I (powered by liquid oxygen and liquid methane), Repulsor of Germany etc. In fact, major development in this vital field took place in Germany after the establishment of a rocket production facility at Peenemunde and production of the ‘V’ series of rockets. JATO, based on solid and liquid fuels, was developed and introduced by the US During WWII, the US introduced the Bazooka (a rocket-powdered grenade), the Barrage rocket (an air-to-surface missile with many variants like: M-8, super 4.5-incher, spinner, HVSR or high velocity spin-stabilized rocket, Tiny Tim, Bat, T-22, Little Joe, Lark etc.). Parallel to this, the British rocket development program included a finned version of barrage rockets, Snare and winged missile “Stooge”. The Russians deployed their barrage rocket named “Katyusha”. The Japanese developed the surface-to-air missiles Funryu-2 and 4 and the solid propellant-propelled suicide plane “Ohka” during this period. After WWII, the allied forces captured the Peenemunde plant of Germany and further development of rockets were mainly offshoots of the technical expertise and knowledge gained due to the concentrated efforts of Von Braun of Germany.

Conventional propulsion systems based on gasoline or jet fuel need atmospheric air for their operation. However, rocket or gun propellants do not need air because the required oxygen is contained within the propellants. The propellant system is a balanced source of potential energy containing oxidizer and fuel for combustion or conversion to kinetic energy. When the propellant has an oxidizer and a fuel in one molecule, like NC or nitromethane, it is called a “mono-propellant”. However, if the fuel and the oxidizer remain separate and are then mixed in a combustion chamber, they are called “Bi-propellant” systems. Composite propellants have their fuel and oxidizer in separate solid phases. The solid propellants can be broadly classified into three separate groups: (1) homogeneous propellants, containing NC, NG, stabilizer etc. also known as double base propellant (DBP); (2) composite propellants (CP) consist of an oxidizer (AP), a binder (polymeric material) and a metallic fuel (Al); and (3) composite-modified double base (CMDB) propellants take advantages of both the DBP and CP systems.

Solid propellants undergo decomposition by a deflagration process. Sufficient heat is generated above the propellant surface, which is transferred back to the surface by conduction and thereby causes further decomposition of the newly exposed surface. This reaction is self-propagating. Propellants perform their work by the slow liberation of energy characterized by high temperature gases pushing against the surrounding air. High explosives, on the contrary, perform their work by sudden shattering, as in the case of rock breaking. Solid propellants were used in the early rockets and have always been used in guns.

The development of propulsion units for spacecrafts has been inseparable from the development of rockets. Although not starting until the mid-20th century, space vehicle development has in fact surpassed the wartime uses of rockets. The most popular among US space vehicles is the space shuttle approved officially in 1971 as a “Space Transportation System” (STS). The argument put forth during the sanction about its utility was to ferry people and supplies; to act as orbiting scientific laboratory; and to place, repair and recover satellites in orbit. Its first flight took place in 1981. For the upper stage launch system, Centaur and Agena have been developed. Vanguard, started in 1955, placed the first satellite in orbit in 1958 only after several unsuccessful attempts. The Titan rocket was developed by the US as a powerhouse for an ICBM (Intercontinental Ballistic Missile). Titan-II, developed in 1962 as a 2-stage vehicle, was successfully produced. However, obsolescence resulted in diversion of these vehicles for space applications as launch vehicles. Later the Titan-III series and Titan-IV were also developed for specific applications. A similar history surrounds the development of “Thor”, an intermediate range ballistic missile. The US developed it for deployment in England, in 3-stage and 4-stage configurations. Deployment started in 1958 but later on it was diverted to space application and during 1962, it was used for high-altitude tests. Several versions of hybrid launch vehicles by combining different stages of Thor with stages of Vanguard, Delta, Agena were made and used for space applications. Solid fuel rockets “Scout”, instrumental in successful launch of Explorer-9 in February 1961 was grounded by NASA (National Aeronautics and Space Administration) in 1994 after around 118 flights. The development of the Saturn V class of launch vehicles resulted in the successful lunar mission. The 6th Saturn-V propelled Apollo-11 for the first landing of humans on the Moon surface on 20 July 1969. Furthermore, the Saturn class of vehicles was used for the launch of the manned Skylab during 1972–73. The first American satellite, Explorer-I used a Jupiter-C rocket, which was conceived by adding a fourth stage to the Redstone rocket, first launched in 1961. A highly successful but lesser known rocket developed by the US was Delta, which had several feathers to its caps like the launch of the first communications satellite to Earth orbit, the launch of the first geostationary satellite, the launch of the first Intelsat (International Telecommunications Satellite Organization) besides the launch of the Explorer satellites, pioneer interplanetary probes and most of the satellites in the TIROS and Landset series.

A separate chronological look at the launch of satellites is a true depiction of the development and progress of launch vehicles. The Russians launched the first Earth orbiter (Sputnik-I) in 1957 and the first animal in space went in Sputnik-II in the same year. Luna-I, II and III were launched in 1959 to land and take photographs on the moon surface. In 1961, the first manned spaceflight took place in Vostok-I with Yuri Gagarin. The first British-built satellite (Ariel) was launched by the US in 1962. Humans landed on moon in 1969, collecting lunar samples. The first Japanese Earth orbiter (Lambda) was launched in 1970 and the same year the Chinese and French also attained success in sending their orbiters. In 1975, the Venera-9 lander from the Soviet Union reached the surface of Venus. Voyager-I of the US passed by Saturn in 1980 and sent spectacular pictures. In 1983, Pioneer-10 became the first probe to venture into interstellar space, when it crossed Neptune on 13th June. However, it slowed down later and Voyager-I passed Pioneer-10 on 17th February, 1998 to become the most distant human-made object in space. Voyager-I and Voyager-II crossed terminal shock on 15th December 2004 and 5th September 2007, respectively. On 25th August 2012, Voyager-I entered interstellar space, a remarkable achievement for humankind.

With the launch of Sputnik and Explorer, we are now in a space age. What was pure fantasy is now getting serious attention by scientists and technologists. Each day there are new speculations about spacecrafts, particularly after the successful landing on the moon. Oberth published the theoretical basis for space flight in 1923. In 1944, high altitude research studies were launched in the US. An altitude of 65 km was reached in 1945 with a rocket. In 1946, the German V2 rocket reached an altitude of 170 km. In 1952, the school of aviation medicine sent rats and monkeys to an altitude of 60 km. The space programs got a big boost in view of our curiosity about unexplored space and a desire to go where no one has gone, so far. If space is to be used for military purposes, countries of the world should be prepared to protect themselves. A strong and advanced space technology shall place the concerned nation as world leader. Moreover, space missions will help in advancing our knowledge about our solar system, universe and also about our own planet.

Spacecrafts (and rockets) use solid, liquid and hybrid propellants (liquid oxidizer, solid fuel) for propulsion. DBP use NC/NG as the main ingredients. They are also referred to as “Colloidal Propellants”. A typical single base (NC-based) propellant used in gun powder consists of NC – 60–99%, Diphenylamine (DPA) – 0.2%, K₂SO₄ – 1–3% and candelila wax – 1–2%. They are thermoplastic in nature, which mean they soften at high temperature and have a waxy appearance. Compositions made by a solvent-less processes are hard and horn-shaped. Numerous shapes like cruciform, tube, rod and tube, internal star, multi-perforated etc. have been used for rocket application. Single base, double base and triple base (NC, NG, nitroglycerin or picrite) propellants have been used for gun propulsion. Double base propellants are processed into desired shape by either extrusion technique or by casting process. The casting process uses small granular (1 mm × 1 mm) casting powder. These granules are poured into a mould (Al, steel) of proper size with mandrel (core) to obtain desired shape and then desensitized NG is added from the bottom under vacuum. Then cast propellants are cured and mandrel is extracted.

Composite propellants (CP) are a mixture of finely granular oxidizer, mainly ammonium perchlorate (AP), and a binder (polyurethane, HTPB, CTPB). Since the oxidizer is the major constituent of a composite propellant, it contributes most to the burning process. The various oxidizers used include ammonium nitrate (AN), potassium perchlorate (KP), and ammonium perchlorate (AP) etc. For binders starting from asphalt, phenolic resin, synthetic rubbers etc., the modern hydroxyl terminated polybutadiene (HTPB) have been used. When elastomeric binders are used, flexible grain results, which can be directly cast into motor casing. Such propellants are called “case bonded rocket propellants”. A typical composite propellant contains 60–75% oxidizer (AP), 15–20% binder (HTPB), 15–18% metallic fuel (Al) along with additives (5%) to impart desired properties to the propellant.

Liquid propellant based rockets differ from solid rockets in that they need a combustion chamber and a feeding system for propellants from storage tanks. Liquid propellants have been extensively used for missile and spacecraft propulsion. Figure 1.1 describes the basic features of solid and liquid propelled rockets.

Both mono and bi propellants can be used. Typical examples of monopropellants include nitromethane, ethylene oxide, ethyl nitrate etc. In bipropellant systems, the fuel and oxidizer are stored separately and mixing takes place only in the combustion chamber. Self-igniting or spontaneously igniting bipropellants are called “hypergolic”. Those that need an external source of ignition, are called non-hypergolic propellants. The various oxidizers used include HNO₃, liquid oxygen, N₂O₄, H₂O₂ etc. The various fuels used are alcohols, gasoline, hydrazine, etc.

Any rocket motor has five major components namely: (i) rocket motor casing, (ii) propellant, (iii) ignition system, (iv) inhibitor, insulator and liner, and (v) nozzle.

The casing material of rockets has to withstand both high pressures and the hot combustion gases generated by the burning of the propellants. The strength of the casing material at high temperatures is an important criteria for material selection. In the cartridge loaded mode type system, where propellants are separate from the casing and are loaded in the casing like bullets of a gun projectile, the rocket motor casing is painted with a heat resistant coating to restrict the rise in temperature of the casing. In case-bonded mode type systems, a thermal insulation layer is pasted at the inner surface of the casing to restrict the temperature of the casing to 100–150 °C. AISI4300, Ladish D6 low alloy steel, Maraging Steel, 15CDV6 are the mostly used casing materials for rockets and missiles. Another criterion is the search for a low-density material, so that the mass fraction of the propellant can be increased. This includes composite materials like Kevlar, resin impregnated glass fiber etc. With development of better resins and fiber strands, the future of casing materials can be made stronger and stiffer to produce a more efficient pressure vessel (defined by pressure–volume product per unit weight of case). For high energy, highly loaded fully stress-relieved propellants, development work has been initiated to develop the technology for wrapping the motor case over propellant grain, nozzle and ignition system. For repeated use, especially in the case of launch vehicles, material selection criteria includes water impact load bearing capacity and corrosion resistance.

Propellants are the power behind rockets, missiles and launch vehicles. They are energetic materials that are ejected as the hot gaseous products of combustion from the nozzle to produce forward thrust. They can be liquid, solid or gaseous. Fluids as propellants have complications during storage, actuation (feeding, piping, valves) and environmental exposures. However, liquids produce specific impulse of the order of 400 s, which is higher than solid propellants. The perpetual search for ingredients of solid propellants has resulted in improved specific impulse, density, burn rate and mechanical properties. But the upper limit of delivered specific impulse for solids has to cross the 300 s barrier to match the performance of liquid and hybrid propulsion systems. Efforts are also being made to improve the mechanical properties in addition to achieving higher performance. This needs higher solid loading, resulting in susceptibility to structural failure, granulation and deflagration to detonation transition (DDT). A very low binder content leads to extensive fracturing of propellants and burning front proceeds towards detonation waves. Hydroxy terminated polybutadiene (HTPB) binder based propellants with AP oxidizer and high solid loading (86–89%) have higher performance, mechanical properties and lower thermal coefficient of expansion. Another requirement put forth for propellants used in tactical missiles is to produce minimum exhaust smoke and low IR signature of the exhaust product of propellant combustion. This ultimately needs reduced aluminium content or non-aluminized propellants, good physical properties over a wide temperature band and high safety. These issues are being debated for achieving better high energy non-smoky propellants. Ageing characteristics and reproducibility and reduced hazard and sensitivity parameters are assuming high importance.

The initiation of propellants needs external heat flux, which is given by pyrotechnic or pyrogen igniters. In pyrotechnic igniters, currently boron-potassium nitrate based pellets and powders are filled in a container. This is initiated by a squib using electric discharge, which produces hot combustion products for initial pressurization of the propellant port and bringing the surface temperature of the propellant to auto-ignition temperature to establish self-sustained combustion. The quantities of igniter composition and container design are important factors for an efficient pyrotechnic igniter. The ignition delay, rate of rise of pressure and initial pressure peaks are the signals, indicating adequacy of ignition system. Pyrogen igniters are used in large size motors and are itself a small motor initiated by pyrotechnic compositions. Safety and arming devices for ignition systems becomes a prime requirement for reliability and survivability. To reduce EMI (electromagnetic interference) and stray voltage from ground loops, inductive coupling and short circuit, air-borne systems need solid-state switching devices with multiple electric interlocks. The firing circuit for igniters may shift to remote, low voltage (28 V dc), capacitor-discharge hot bridge wire devices. Capacitor discharge needs smaller batteries, no high-voltage components, minimum packaging space and no heat sink requirements. Laser beam based initiation systems are also being studied.

There are certain non-energetic, inert (but essential) components in rocket motors. For the cartridge loaded class of motors, propellant grains are inhibited to selectively restrict the burning surface and thereby getting desired thrust time profiles for a given configuration. There are many methods of inhibiting solid propellants like casting, tape winding, cloth winding, brush coating etc. The inner surface of cartridge loaded rocket motor's combustion chamber is also coated with heat resistant paint. For case-bonded solid propellant motors, a thermal insulation layer lies between the propellant and motor casing. This prevents the rise in skin temperature of the casing, acts as a cushion to allow the propellant to withstand various handling and flight loads and also absorbs the heat of combustion gases by endothermic pyrolysis. A very good interface strength is always desired for case-bonded motors in case of casing–insulation and insulation–propellant contact zones. For better adherence, a thin layer of liner exists at the propellant–insulation interface. The liner material prevents plasticizer migration also. In the case of case-bonded motors, inhibition lies at the ends of the propellant grains. These inert materials are a must but are also undesirable for the propulsion system's performance. They add extra weight and are non-energetic in nature. Therefore, improvements in ablation rate and a reduction in density are two major research areas in this field.

The nozzle gives way to the high pressure propellant combustion gases, which are discharged at a high velocity in a rearward direction to produce forward thrust to the rockets and missiles by reaction forces. An optimum expansion ratio of combustion gases to the outside pressure becomes a prime requirement and effective exhaust velocity is considered a true measure of the propulsion system performance. Two extreme ends in the design of nozzles are being worked out. A simple nozzle has a convergent and divergent portion in which exhaust gases expand. However, extendable exit cones for strategic motor nozzles are complex in design and operation. The selection of nozzle material, especially throat inserts, becomes significant due to the fact that very high temperatures, coupled with very high heat transfer, exists. Carbon–carbon and graphite–graphite families of composite materials have been an ideal choice for nozzle material. High modulus, increased density (2 g cm⁻³) and pyrolytic graphite with new orientation are technology improvements in nozzle materials. Nozzle performance is also improved by operating the motor at higher chamber pressure, high expansion ratios and reduced erosion. Two-phase flow losses can be further reduced by short-sharp throat and reduced exit-cone particle impingement. Instead of having conical nozzles, bell shaped nozzles are reported to be efficient, although are difficult to manufacture.

The components are shown in Figure 1.2. The volume fraction occupied by various components in a typical rocket is also given in Table 1.1.

Although chemical propulsion has been the power behind almost all rockets, missiles and launch vehicles, several other concepts are also being investigated for their potential use in propulsion. There are several ways in which higher performance can be obtained but at the same time getting high thrust has been a major challenge. Propulsion system performance has been conventionally expressed in terms of thrust delivered by unit mass flow rate, popularly known as specific impulse and is expressed in seconds (s). Solid propellants are capable of developing specific impulse (Isp) of 250 s, while liquids can deliver up to 400 s (Cryo). Fuel-rich propellants can deliver Isp of more than 1200 s in a secondary chamber. Various options for propulsion are discussed in this section.

Energy of the order of 10⁶ to 10⁸ times than that created by chemical reaction, is released by splitting (or fission) of a nucleus. It is feasible to utilize this controlled energy by means of a neutron-induced fission chain reaction in U²³⁵. The second type of nuclear reaction, thermo-nuclear or fusion involves the release of energy from joining (or fusing) the charged nuclei of light elements. The reaction results in the formation of new elements with mass equal to the sum of masses of the reacting elements. The problem of controlling and containing such high temperatures is a challenge before propulsion engineers use nuclear energy.

The most likely method of using fusion for propulsion are the production of thrust by thermodynamic expansion of hot reaction products and the use of a heat transfer system for conventional propulsion, such as heated or ionic rocket propulsion. By adopting the principle of cathode ray tubes, an ion rocket could be created. The thrust would be generated from the ions and electrons, which are accelerated to velocities, approaching the speed of light. Although specific impulses are extremely high (millions of seconds), the total thrust is small and would only be suitable for outer space, away from gravitational forces.

Photon propulsion is the most promising from a performance point-of-view. However, it is technically very difficult. Particles of finite mass can not achieve the velocity according to the theory of relativity. Both fusion and fission can provide an excellent source of high-energy photons. However, the problem of lining up these high-energy photons is an insurmountable task, currently.

When free radicals recombine, they release enormous amounts of energy. The basic problem associated with free radical propulsion is to formulate, stabilize and evaluate free radicals. If a mixture of 75% hydrogen free radicals is used as a working fluid of hydrogen, a Isp of more than 1000 s would be observed. Furthermore, an extreme temperature will be created from the fuel consisting of atomic hydrogen combustion and the chamber may evaporate at a very high temperature of 4000 °C; considerable re-dissociation may also occur.

In nuclear propulsion, criticality lies with thermal resistance of reactor material and methods adopted for conservation of neutrons for self-propagating fission. In initial stages, graphite has been used as the moderator, but this resulted in the formation of acetylene and other gaseous compounds. So proper protective coating over graphite becomes an essential requirement. In chemical propulsion, the highest achievable exhaust velocity is 4000 m s⁻¹, whereas in nuclear propulsion exhaust velocity as high as 7500 m s⁻¹ is possible. Since nuclear rockets needs shielding, they are very heavy and are never preferable for smaller missions.

There are several variants for nuclear propulsion. Fluidized suspended particle bed type reactors have solid fuel particles moving in the stream of high velocity propellant gas. This results in very good heat transfer, small time in solid gas boundary layer and potentially high power density. Fuels used in this case include uranium carbide/niobium carbide, uranium carbide/zirconium carbide or uranium carbide/hafnium carbide and Isp obtained is around 1100 s. In liquid core reactors, gas bubbles through the central core of molten fuel are used for efficient heat transfer but loss of fuel and limitations of gas flow rate restricts the thrust level possible by these reactors. This system can deliver specific impulse of the order of 1200–1400 s. Similarly, gas core reactors can deliver around 2500 s of Isp. With the introduction of pulsed-rocket firing, the pulsed-nuclear propulsion concept is also presented for researchers, where heated propellant charge transfers momentum over a short interaction. The pulse repetition rate may vary from 1 s⁻¹ to 1000 s⁻¹ and a Isp of the order of 1800 to 2500 s is possible by this method.

Electric propulsion has been attempted for accelerating exhaust jets. For comparison, acceleration of a single charged positive ion through a potential drop of only one volt would correspond to a chemical rocket combustion temperature of 11 600 K (3 times higher). However, power plant weight offsets gain in the specific impulse and heaviness reduces the spacecraft acceleration to negligible (10⁻⁴ g). Several variants are in active consideration. In electrothermal thrusters, propellant gases are passed through electrothermal arc jet, struck between two electrodes. Energized gases expand through aerodynamic nozzles and generate thrust. Lighter gases are preferred in this case and ionization of gases has to be avoided for preventing efficiency loss. Research indicates that a propellant with high density and low vapor pressure are superior in performance. Hydrazine is found to be a better fuel as compared to hydrogen and ammonia for this application. These systems are capable of generating Isp from 800 to 2000 s. Electromagnetic propulsion utilizes accelerated plasma to generate thrust. The efficiency of conversion of stored energy into the moving plasma can go as high as 30% and very high exhaust velocities can be produced by this mechanism. Electrostatic class of propulsion needs bombardment of ions or surface ionization techniques. In bombardment ion engine, propellant vapors of mercury or cesium are fed to an electron bombardment chamber. Ions are extracted by application of electric charge and further electrons are fed for maintaining neutrality in the chamber. These systems can deliver specific impulse up to 10 000 s. Since ionization efficiency close to 100% can be realized by cesium on tungsten surface, this system also has potential practical application.

ION thruster, as ION propulsion systems are commonly known as, is a form of electrical propulsion, where electrical potential difference accelerates ions to generate thrust. Depending on type of electrical energy used for accelerating ions, these thrusters may be classified as either an electrostatic thruster or an electromagnetic thruster. For a typical ion thruster, an input power of 1–7 kW can accelerate ions to exhaust velocity of 20–50 km s⁻¹ and generates thrust of 25–250 mN. Although ION propulsion is not useful as main propulsion device, due to the availability of lower thrust and higher weight of the propulsion system, but higher specific impulse makes it a definite choice for auxiliary propulsion systems. This propulsion system may not be very useful for rockets or missiles, employed for defense applications, but spacecraft propulsion have utilized this system of propulsion. ION propulsion have been successfully used in many spacecrafts. The Deep Space 1 spacecraft, powered by an ION thruster, changed velocity by 4300 m s⁻¹, while consuming less than 74 kg of xenon (propellant). The Dawn spacecraft attained velocity rise of the order of 10 000 m s⁻¹. Applications of ion propulsion include control of the orientation and position of orbiting satellites (some satellites have dozens of low-power ion thrusters) and use as a main propulsion engine for low-mass robotic space vehicles (for example Deep Space 1 and Dawn). ION propulsion have high efficiency of 65–80%, but the thrust generated is very low.

A quick comparison of chemical rocket propulsion and ion propulsion can be made. ION propulsion systems have limited thrust density (force per cross-sectional area of the engine). It creates small thrust levels (Deep Space 1's thrust approximately equals the weight of one sheet of paper) compared to conventional chemical rockets, but achieve high specific impulse, or propellant mass efficiency, by accelerating their exhaust to high speed. The power imparted to the exhaust increases with the square of its velocity while thrust increases linearly. Conversely, chemical rockets provide high thrust, but are limited in total impulse by the small amount of energy that can be stored chemically in the propellants. Given the practical weight of suitable power sources, the accelerations given by ion thrusters are frequently less than 1/1000th of standard gravity. However, since they operate as electric (or electrostatic) motors, they convert a greater fraction of input power into kinetic exhaust power. Chemical rockets operate as heat engines; hence Carnot's theorem bounds their possible exhaust velocity. Ion thrust engines are practical only in the vacuum of space and cannot take vehicles through the atmosphere. This is because ion engines do not work in the presence of ions outside the engine. Spacecraft rely on conventional chemical rockets to initially reach orbit, but ION propulsion may be used in many other thruster applications.