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Laser-induced ignition has a game-changing technological potential for re-startable upper stage engines and reaction control thrusters of spacecrafts. A laser pulse of approximately 10 nanoseconds of duration with a wavelength of about 1000 nanometers is focused into a small spot of 10-100 microns of diameter in the rocket combustor, delivering a power density of 10- 100 gigawatts per square centimeter and breaking down the gaseous fuel/oxidizer mixture.

The photons from the laser beam are absorbed by the gas molecules, which get ionized and produce more electrons. These electrons collide with other molecules, are decelerated and absorb photons (a process called inverse bremsstrahlung or radiation absorption by deceleration). Upon colliding with a neutral molecule, each fast electron leads to two slow electrons, which become fast again upon colliding with other molecules, thereby leading to a cascade or electron avalanche. This results in a localized, hot plasma kernel in the first 0.1-1 microseconds after the laser- energy deposition along with an outgoing spherical blast wave. The ions and electrons in the plasma serve as early precursors for the oxidation kinetics of the fuel, which ultimately leads to a propagating flame kernel and a stable diffusion flame anchored near the injector after times of order 1-10 milliseconds.

Lased-induced ignition has a number of advantages over traditional ignition technologies like pyrophoric, pyrotechnic, or augmented-torch igniter systems.

Figure 2: Green flash produced by the TEA/TEB pyrophoric ignition of Falcon 9's Merlin 1D VAC engine (Credits: SpaceX)

Pyrophoric ignition consists of mixing hypergolic fluids to produce ignition and has been used widely (Falcon-9, Soyuz, Apollo Lunar Excursion Module, etc.). However, it requires utilization of highly toxic propellants that are expensive and can be deadly (triethylaluminum-triethylborane or TEA/TEB, unsymmetrical dimethyl- hydrazine, nitrogen tetroxide, etc.). Pyrotechnic ignition is limited to rocket motors like Space Shuttle’s solid rocket boosters. Neither pyrophoric nor pyrotechnic ignition methods are capable of multiple ignition cycles. Augmented spark ignition has also been used widely (Saturn V’s J-2 engines, Starship’s Raptor engines, etc.), but requires a pre-chamber with a spark igniter that is mounted fixed in space and releases inefficiently a large amount of energy over a large gas volume.

In contrast, laser-induced ignition technology:

(a) is re-startable: A large number of ignition cycles can be achieved, thus providing multiple engine re-start capabilities in flight. This is particularly relevant for upper stages in space missions requiring several burns for orbit insertion, and for reaction control thrusters to be able to reposition the spacecraft in orbit over its lifetime.

Figure 3: Laser beam focused on an outer injector in a multi-element full scale rocket combustor (Credits: Airbus and DLR).

(b) is non-intrusive: No invasive systems like torches or pre-chambers are needed, and no additional propellants have to be injected into the combustor to produce ignition. In addition, flame quenching is reduced by eliminating the need for massive electrode components in spark plugs that act as heat sinks.

(c) is localized both spatially and temporally: The laser energy can be delivered efficiently in a highly localized manner both in space and time in the combustion chamber. Lasers allow for more flexibility in positioning the ignition kernel in regions of the flow in the combustor where the fuel/oxidizer mixture flammability characteristics (composition, temperature, and turbulent fluctuations) are favorable for ignition.

(d) is lighter and simpler: The miniaturization of modern lasers have resulted in ignition systems weighing about 5-10 kg. In contrast to pyrophoric, pyrotechnic and spark ignition systems, laser-induced ignition does not require any additional supply systems or hypergolic propellant feed lines.

(e) is decoupled from engine transients and instabilities: The laser-ignition system operates independently from flow transients and resonant instabilities that can appear in propellant feed lines during start-up of the engine.

Laser-induced ignition systems are also applicable to other propulsion systems like internal combustion engines, gas turbine engines, and supersonic combustion ramjets (scramjets), in which spark plugs tend to be traditionally used. In high-pressure environments, including supercritical combustors, laser ignition is favored because of the correspondingly lower breakdown thresholds. In contrast, capacitive spark plugs require increasingly higher voltages with increasing pressure, which shortens the lifetime of the ignition system. Lasers also enable the ignition of leaner fuel-air mixtures because of the extra reactivity added by the hot plasma, and therefore can lead to lower NOx pollutant emissions and higher fuel efficiency in internal combustion engines and jet engines.