DOUBLE ENERGY FOR LASER IGNITION

CHALLENGE

Laser ignition of rocket engines is rated as a promising seminal alternative to the presently used pyrotechnic igniters. Advantages of laser ignition system over pyrotechnical ignition systems are repeatability and reusability, increased operational safety, a saving of space and potentially lower recurring costs. A key challenge in the ongoing development of this ignition technology is the increase of the pulse energies of the laser system. The HiPoLas Gen IV laser system, which was developed by SAL and has been tested repeatedly on rocket engines, provides typical pulse energies of 30 mJ. This enables this laser, which is worldwide unique in compactness and robustness, the smooth and reliable ignition of engines that rely on liquid hydrogen and oxygen as fuel. When using non-cryogenic fuels, or in cases where the ignition conditions are more challenging e.g. due to higher flow velocities, however, higher pulse energies of at least 50 mJ are needed. This challenge is being addressed within the R&D efforts of the competence centre ASSIC, involving an international consortium uniting the centre organisation and Airbus Safran Launchers as the two key proponents of this research in a joint effort. The goal of the related research is a laser ignition system that is comparable to the present HiPoLas system in robustness and compactness, but able to provide pulse energies exceeding 60 mJ.

FROM FEASIBILITY TO A TESTABLE PROTOTYPE

The competence centre ASSIC’s research team of the around Gerhard Kroupa, M.Sc. and Dr. Gerald Auböck succeeded in developing and testing a laser system with pulse energies exceeding 65 mJ. The energy available for ignition could thus be more than doubled in comparison to present compact laser igniters. The fundamental scientific approach behind this achievement is a coupling of SAL’s laser technology with a downstream amplifier. This allows to significantly increase the pulse energy without impairing beam quality or pulse duration of the original laser seed pulse. The optics and the amplifying crystal were specially optimized to the purpose in both simulations and test series. A special challenge in this conceptual design phase was to construct a system that is comparable to the HiPoLas system in its robustness, and only slightly larger in size. Current work now focusses on the integration of the successfully tested experimental set-up into a laser system prototype for life engine tests. Provided successful passage of pertinent tests, in particular as to the system’s robustness, this prototype will be applied shall be applied to real world full-scale rocket engine tests. Apart from the laser system itself, the inlet to the thrust chamber is a critical point for laser ignition systems - especially when applied to rocket engines, the optical components have to resist enormous thermal and mechanical loads. Also these aspects consequently form an integral part of this highly interdisciplinary and collaborative research project within in the K1 centre ASSIC.

IMPACT AND EFFECTS

The proof of feasibility and the subsequent development of a laser ignition system with pulse energies exceeding 50 mJ creates a wealth of new possibilities in the area of laser ignition of rocket engines and beyond. Future such systems are expected to facilitate the laser ignition also of rocket engines for non-cryogenic fuels at highest reliability. In addition to the envisaged application in rocket engines, a number of further possible applications of this technology can be imagined. Advances in laser ignition systems are obviously of immediate relevance and interest for the ignition of aircraft engines or spark plug in large engines. Beyond that, other intended uses for this laser technology include the field of material processing or in medical technologies.