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The technical challenges

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(a) Thermal and mechanical loads on the laser system: Optical elements need to be used either externally or internally to the combustor that have to withstand the thermal and mechanical loads during engine start-up. Similarly, the laser system has to bear large vibrational accelerations without losing calibration.

(b) Sensitivity of ignition to local flow conditions: The flow of propellants in a rocket combustor is complex and consists of shock waves, liquid jets, shear layers, turbulence, recirculation zones, sprays, and highly unsteady and non-uniform distributions of equivalence ratio. However, the laser energy must be delivered in regions of the flow that are conducive to flammability. In these complex flows, the flow conditions that lead to successful laser ignition remain mostly unknown. Therefore, the process involves a significant amount of stochasticity, particularly near the minimum ignition energy threshold -- a limit of practical interest for system optimization. This is in contrast to torch igniters, which dump a large amount of energy over a large gas volume, thereby making ignition much less sensitive to the local flow conditions.

(c) Effects of high altitude on ignition: The ignition of upper stages and reaction control thrusters occurs near the edge of space or in space, where the initial conditions in the combustor are near vacuum. The resulting low pressures require higher breakdown energies and complicate the dynamics of the propellant sprays, inducing flashing atomization and complex flow fields that have been hardly characterized in the literature.

Figure 4: Schematics of flow phenomena in the PSAAP-III rocket combustor upon laser-energy deposition.