Vehicles propelled by rocket engines are commonly called rockets. Rocket vehicles carry their own oxidizer, unlike most combustion engines, so rocket engines can be used in a vacuum to propel spacecraft and ballistic missiles. Compared to other types of jet engines, rocket engines are the lightest and have the highest thrust, but are the least propellant-efficient (they have the lowest specific impulse). The ideal exhaust is hydrogen, the lightest of all elements, but chemical rockets produce a mix of heavier species, reducing the exhaust velocity.
Rocket engines produce thrust by the expulsion of an exhaust fluid that has been accelerated to high speed through a propelling nozzle. Most rocket engines use the combustion of reactive chemicals to supply the necessary energy, but non-combusting forms such as cold gas thrusters and nuclear thermal rockets also exist.
A rocket engine uses stored rocket propellants as reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. The fluid is usually a gas created by high pressure (150-to-4,350-pound-per-square-inch (10 to 300 bar)) combustion of solid or liquid propellants, consisting of fuel and oxidiser components, within a combustion chamber. As the gases expand through the nozzle, they are accelerated to very high (supersonic) speed, and the reaction to this pushes the engine in the opposite direction.
Liquid-fuelled rockets force separate fuel and oxidiser components into the combustion chamber, where they mix and burn. Hybrid rocket engines use a combination of solid and liquid or gaseous propellants.
Combustion is most frequently used for practical rockets, as high temperatures and pressures are desirable for the best performance. Both liquid and hybrid rockets use injectors to introduce the propellant into the chamber. However the fuel and oxidizer need ignition to start combustion until it gets that bump of an insertion of energy, there is no combustion. Ignition is defined as the transformation process of combustible material, from an unreactive state to a self-propagating state, where the ignition source can be removed without extinguishing the combustion process.
Ignition can be achieved by a number of different methods; a pyrotechnic charge can be used, a plasma torch can be used, or electric spark ignition may be employed. In most cases when lighting liquid propellants directly, the components on rocket engines used to make electrical sparks are not a whole lot different than higher-energy, more robust, and more reliable versions of the spark plugs that you’ve got in your automobile. They use a high-voltage electrical circuit to make a spark jump across a gap thereby exposing whatever is around that gap, namely vaporized propellants, to ionizing electrical energy. The second method also uses electrical energy but in this case rather than making a spark, you use it to make heat. The intent of the wire filament is to produce light. And it does. But is also produces heat. What if you apply that heat directly to a combustible mixture.
Some fuel/oxidiser combinations ignite on contact (hypergolic), and non-hypergolic fuels can be “chemically ignited” by priming the fuel lines with hypergolic propellants (popular in Russian engines). These are propellants that combust spontaneously when they come into contact with each other. They don’t need any energy boost to start reacting. The most common hypergols for this purpose are triethylborane (a.k.a., triethylboron), triethylaluminum, or some mixture of the two.
Solid propellants are usually ignited with one-shot pyrotechnic devices. Once ignited, rocket chambers are self-sustaining and igniters are not needed. Indeed, chambers often spontaneously reignite if they are restarted after being shut down for a few seconds. However, when cooled, many rockets cannot be restarted without at least minor maintenance, such as replacement of the pyrotechnic igniter.
With liquid and hybrid rockets, immediate ignition of the propellant(s) as they first enter the combustion chamber is essential. With liquid propellants (but not gaseous), failure to ignite within milliseconds usually causes too much liquid propellant to be inside the chamber, and if/when ignition occurs the amount of hot gas created can exceed the maximum design pressure of the chamber, causing a catastrophic failure of the pressure vessel. This is sometimes called a hard start or a rapid unscheduled disassembly (RUD). Gaseous propellants generally will not cause hard starts, with rockets the total injector area is less than the throat thus the chamber pressure tends to ambient prior to ignition and high pressures cannot form even if the entire chamber is full of flammable gas at ignition.
Demands for newer, more advanced forms of ignition, are increasing as individuals strive to meet regulations that seek to reduce the level of pollutants in the atmosphere, such as CHx, NOx, and SO2.
Many aviation gas turbine manufacturers are interested in increasing combustion efficiency in engines, all the while reducing the aforementioned pollutants. There is also a desire for a new generation of aircraft and spacecraft, utilizing technologies such as scramjet propulsion, which will never realize their fullest potential without the use of advanced ignition processes. These scenarios are all limited by the use of conventional spark ignition methods, thus leading to the desire to find new, alternative methods of ignition.
Laser ignition technology
Renewed interest in the use of high-speed ramjets and scramjets and more efficient lean burning engines has led to many subsequent developments in the field of laser ignition for aerospace use and application. The method is based on laser ignition devices that produce short but powerful flashes regardless of the pressure in the combustion chamber.
LASER stands for Light Amplification by Stimulated Emission of Radiation. A laser is a device that produces highly directional light. It emits light through a process called stimulated emission of radiation which increases the intensity of light. A laser is different from conventional light sources in four ways: coherence, directionality, monochromatic, and high intensity. The laser beam is very narrow and can be concentrated on a very small area. This makes laser light highly directional. The laser light spreads in a small region of space. Hence, all the energy is concentrated on a narrow region.
Laser ignition, or laser-induced ignition, is the process of starting combustion by the stimulus of a laser light source. Laser ignition uses an optical breakdown of gas molecule caused by an intense laser pulse to ignite gas mixtures. The beam of a powerful short-pulse laser is focused by a lens into a combustion chamber and near the focal spot and hot and bright plasma is generated. It is well known that short and intensive laser pulses are able to produce an “optical breakdown” in air. Necessary intensities are in the range between 1010 to 1011W/cm2. At such intensities, gas molecules are dissociated and ionized within the vicinity of the focal spot of a laser beam and hot plasma is generated. This plasma is heated by the incoming laser beam and a strong shock wave occurs. The expanding hot plasma can be used for the ignition of fuel-gas mixtures.
According to Ronney, laser ignition can be characterized into four categories. Thermal initiation is the first of these types, where no electrical breakdown of gas occurs. In thermal initiation, the laser source heats up a solid target or excites vibrational/rotational modes of different gas molecules. The second type is non-resonant breakdown, which is considered by Tauer et al. to be the most appropriate mechanism for ignition. In non-resonant breakdown, the electrical field strength of a focused laser beam is enough to cause an electrical breakdown of a gas, and is most similar to standard electric spark discharges. A main difference between laser ignition and spark ignition is that small amounts of vapor, dust, and microparticles can reduce the breakdown field strength by orders of magnitude, whereas electrical spark discharges are not usually affected by such phenomena.
The third type of laser ignition is resonant breakdown, which is similar to non-resonant breakdown, but includes a non-resonant multiphoton photodissociation of a molecule, followed by a resonant photoionization of atoms created by the previous photodissociation. The final type of laser ignition is photochemical ignition, in which a single photon, typically in the UV range, undergoes dissociation after being absorbed by a molecule. Direct heating of the gas may occur, but due to the species being in thermal non-equilibrium, they may recombine with themselves or other molecules.
When applying the concept of laser ignition to actual application, laser-induced optical breakdown for the use of igniting combustible gas mixtures may be done by using laser ablation ignition (LAI) or laser plasma ignition (LPI). In laser ablation ignition, a laser beam focuses on a target, generating optical breakdown once the intensity in the focal region exceeds a certain threshold. In laser plasma ignition, the concept is the same, but the laser beam is tightly focused into the combustion chamber, and the plasma is formed in free space as opposed to a target in laser ablation. Typically, this threshold for plasma formation in LPI is one hundred times greater than target surfaces in LAI. Laser ablation is typically employed for use in rocket engines and spacecraft designs because, just as in chemical rockets, thrust is produced from the resulting reaction force. Plasma ignition on the other hand, is typically found in internal engines, since ignition of the plasma drives moving parts, such as in a cylinder. In either case, plasma formation is based on ionization of atoms and molecules around the focal point followed by acceleration of ionized electrons. This causes collisions with more neutral atoms of molecules.
Laser ignition applications
Several categories of engines can benefit from laser ignition. Though all engines could potentially benefit from more advanced ignition methods, such as laser-induced plasma ignition,
Researchers from Japan described in 2011, the first multi-beam laser system that could be used to ignite an automobile engine’s air-fuel mixture. The laser ignition system is small enough to screw into an engine’s cylinder head and could replace the spark plugs used for more than 150 years to ignite combustion in internal combustion engines, enabling automakers to develop cleaner, more efficient, and more economical vehicles using photonics.
Equally significant, the new laser system is made from ceramics, and could be produced inexpensively in large volumes, according to one of the presentation’s authors, Takunori Taira of Japan’s National Institutes of Natural Sciences. According to Taira, conventional spark plugs pose a barrier to improving fuel economy and reducing emissions of nitrogen oxides (NOx), a key component of smog.
Lasers also improve efficiency. Conventional spark plugs sit on top of the cylinder and only ignite the air-fuel mixture close to them. The relatively cold metal of nearby electrodes and cylinder walls absorbs heat from the explosion, quenching the flame front just as it starts to expand. Lasers, Taira explains, can focus their beams directly into the center of the mixture. Without quenching, the flame front expands more symmetrically and up to three times faster than those produced by spark plugs.
Lasers promise less pollution and greater fuel efficiency, but making small, powerful lasers has, until now, proven hard. To ignite combustion, a laser must focus light to approximately 100 gigawatts per square centimeter with short pulses of more than 10 millijoules each.
Aeronautical and space applications include Gas turbines, new laser “spark plugs” and microwave-assisted ignition; Plasma-assisted ignition, using both conventional electrical spark ignition and laser-induced ignition; Ramjet/scramjet applications; Space applications, to include studies on laser ablation, applications in rocket design and satellite microthrusters.
The concept of laser ignition and propulsion may also be applied to space travel. Multi-mode engines with built-in scramjet capabilities would allow for quick, cost-effective travel outside Earth׳s atmosphere. Small microsatellites, weighing between 1–10 kg, can also benefit by using onboard laser propulsion, such as the laser-electrostatic hybrid acceleration thruster, which is discussed later in this report. Ground-to-orbit launch vehicles are also proposed using laser propulsion, which would allow rockets a high specific impulse, higher thrust, and a lower propulsion weight.
Recent progress in the area of high power fibre optics allowed convenient shielding and transmission of the laser light to the combustion chamber. However, issues related to immediate interfacing between the light and the chamber such as selection of appropriate window material and its possible fouling during the operation, shaping of the laser focus volume, and selection of spatially optimum ignition point remain amongst the important engineering design challenges. One of the potential advantages of the lasers lies in its flexibility to change the ignition location. Also, multiple ignition points can be achieved rather comfortably as compared to conventional electric ignition systems using spark plugs.
EU-funded LASIG-TWIN project
Global concern about the negative environmental and health impact of greenhouse gas emissions has led to concerted action. The European Commission, for example, has set strict limits on emissions from industry and transport, and is investing in clean energy research. Nevertheless, it is widely accepted that engines based on burning fossil fuels will continue to be used until alternative solutions (like electric cars for example) are affordable at scale. “Even when such alternatives are widely available, internal combustion engines will still be needed,” says LASIG-TWIN project coordinator Nicolaie Pavel from the National Institute for Laser, Plasma and Radiation Physics (INFLPR) in Romania.
The goal of the EU-funded LASIG-TWIN project was to investigate more efficient techniques for igniting fuels in internal combustion engines, like those you’d find in cars. Since the beginning of the 20th century, the electric spark plug has been used for ignition. The project team wanted to find out if alternative ignition systems, such as laser ignition, could make combustion engines more efficient.
“We wanted to better understand the benefits that laser ignition could bring,” explains Pavel. “To achieve this, we created a network between the INFLPR (and its Laboratory of Solid-State Quantum Electronics in Romania) and four other highly renowned institutes from France, Germany and the United Kingdom. A key focus throughout has been on training and sharing learning experiences in the field of laser ignition.” A range of techniques were investigated and trialled within the network. These included methods for packing and bonding optical and metallic materials to construct laser spark plugs.
“It should be noted that laser ignition systems will only become cheaper if they can be implemented on a large scale,” says Pavel. “Furthermore, additional tests are needed to ensure that these devices can compete with the simple and inexpensive electrical spark plug.”
Laser ignition in Scramjets
The scramjet is a kind of air-breathing engine, which uses the oxygen in the atmosphere as all or part of the oxidant, and reacts with the fuel it carries to obtain thrust. Compared with the traditional aerospace turbine engine, the structure of the scramjet is relatively simple, but the combustion process inside the combustion chamber and the flow mechanism of the flow field are quite different from the traditional aerospace engine: the entrance of the combustion chamber The air flow is supersonic, which causes the ignition and combustion processes to proceed in supersonic air flow. This new type of combustion organization will inevitably bring a series of problems to the ignition, combustion and flame stabilization of the engine.
The key is that the air flow is supersonic and the fuel residence time in the combustion chamber is very short, even reaching the amount of ms. Grade, resulting in the mixing time of the fuel and air and the contact time of the mixture with the ignition source is too short,
the fuel cannot be ignited or cannot sustain combustion after ignition.
Applying the concept of laser ignition to extreme flight regimes, such as supersonic and hypersonic flows, is one of the most challenging, yet fruitful pursuits in the field of aviation. There are many benefits to all areas of aerospace study. Possibilities include multiple ignition positions in time and space, precise ignition timing, controllability of input energy and its duration, more stable combustion, ignition of leaner fuel mixtures, and ignition of high pressure mixtures. Generation and maintenance of a laser is also much less than other forms of ignition, making the prospect of laser ignition promising from an economic standpoint.
In supersonic and hypersonic engines, such as scramjets, internal regions where combustion occurs can receive energy instantaneously through laser igniters, without using electrodes, found in conventional ignition methods. Since the laser ignition system is electrodeless, there are no cooling effects associated with electrode use. Laser use also allows the laser radiation energy to focus its intensity directly on the jet fuel, creating a spark. Laser ignition offers several advantages in internal combustion engines as well, such as the absence of electrodes which disturb the cylinder geometry (thus quenching a flame kernel), and variable ignition positions within the combustion cylinder.
A successful igniter must be small enough to fit inside the rocket engine, energetic enough to ensure a reliable start and without any components that could damage the rest of the engine when it is ejected after ignition. “Consisting of balsa wood, adhesive tape, steel wool, simple electronics and about three grams of black powder our igniter is a low tech but effective device. During the development process of our ignition system, 3D printed plastic was trialed as a replacement for the balsa wood however. This unfortunately led to an engine failure during one of the Stratos III tests, when it prematurely lost strength, deformed and lodged in the combustion chamber. This is one example of how minor changes to an ignition system can have a significant impact on an entire rocket. Of course, this is not only valid for hybrid engines as used by Stratos, but also for solid engines as used by Aether.”
A few seconds before the countdown reaches zero the igniter is triggered by passing an electric current through a resistor. This resistor is coated with similar chemicals to a typical match so that it can then ignite the black powder which in turn burns the steel wool. In case of a hybrid engine like the DHX-400 ‘Nimbus’, liquid oxidizer from the main tank is added to the mix via a bypass ignition valve, accelerating and prolonging the small fire created by the black powder. Flames from this process can often be seen wafting from the rocket nozzle during engine tests, as can be seen in the video shown below. When the countdown reaches zero and the main valve is opened to complete the ignition of engine.