Indirect TMF Panel Test Strain Range Variation

Abstract

Inner liner materials of combustion chambers of liquid rocket engines are exposed to extreme thermomechanical conditions. The combination of high temperature and the pressure difference between the cooling channel and the hot gas on its own would still result in an elastic deformation of the hot-gas side wall ligament. However - the load carrying jacket prevents the chamber wall material from thermal expansion. Moreover - the abovementioned outer jacket (designed for thrust and chamber pressure carrying purposes) even contracts in case the wall-integrated regenerative cooling is realized by a cryogenic propellant. This imposes some large hoop stresses and large hoop strains to the inner liner material - causing some excessive inelastic straining of the hot gas wall. Some deformations remain after shut down and accumulate with each operational cycle of the liquid rocket engine. As result of this ratchetting effect, the rectangularly shaped cooling channels of the inner liner of the combustion chamber thin-out to a roof like geometry, commonly referred to as doghouse. Usually this dog-house effect is stronger than effects such as creep, fatigue and thermal ageing of the material. Finally, crack formation in the hot wall ligament leads to a direct flow of the coolant from the regenerative cooling system into the combustion chamber (without the intended diversion of the coolant through the injector head of the liquid rocket engine). To replicate this chamber wall cracking without having to pay for the vast liquid rocket engine propellant consumption (exceeding the 100-ton-level already for a single full duration hot-run of a single core-stage engine such as Vulcain 2.1 of the European Ariane 6 launcher), TMF panel tests are used. A TMF panel can be seen as an actively cooled chamber wall cut-out with less than 10 cooling channels (in comparison to the more than 400 cooling channels of the abovementioned Vulcain 2.1 combustion chamber. The cyclic hot-gas loading of the liquid rocket engine chamber wall due to engine start-up and shut-down is replaced by cyclically switching on and off some high optical output power laser. In the TMF panel, the hoop stresses are missing and have (for this reason) to be replaced by bending stresses of some solid material blocks outside of the laser loading surface of the TMF panel.