Hypersonic (re-)entry flight vehicle designs are dominated by the thermal protection systems (TPS) required to keep the payload safe. The knowledge of the thermal loading experienced throughout a flight trajectory is critical for the optimal design of the TPS. The thermal loading information is predominantly acquired from extensive ground testing and computational modelling. The ability to obtain high fidelity ground test data is critical for the development and refinement of the computational models used, which then allows for a more accurate and efficient design of TPS for hypersonic vehicles.
This thesis presents a new hot wall model method for testing carbon-carbon models at realistic wall temperatures (2000 – 3000 K) in hypersonic impulse facilities. This greatly extends the model wall temperature range possible in impulse facilities from the previous maximum of ~ 1200 K. This opens up a whole new range of hypersonic testing, in particular for (re-)entry vehicles which are characterised by very high wall temperatures and are commonly constructed using carbon fibre based materials.
This new methodology has been tested in the X2 expansion tube at the Centre for Hypersonics within The University of Queensland. Models have been tested in an 8.8 km/s Earth re-entry flight equivalent flow field. A set of hemicylindrical models with a diameter to length ratio of 4.5 were constructed and tested. Three different models types were used; an aluminium reference model, cold carbon-carbon models and heated carbon-carbon models.
To measure the wall temperature during testing, two high temperature measurement techniques were developed; firstly using a two colour ratio pyrometry technique with a commercially available digital single lens reflex (DSLR) camera, and secondly using a visible near infrared spectrometer. These techniques are capable of measuring temperatures beyond the range available to standard thermocouples (limited to ~1400 K).
The metric used to evaluate the effect of the wall temperature on the flow field was the formation of cyanogen due to a surface chemistry reaction between the nitrogen in the air flow and the carbon model. Measurements were taken using a Ultraviolet (UV) spectrometer that was coupled to an intensified charge-coupled device (ICCD) camera for spatially resolved data, and to a photo multiplier tube (PMT) for temporally resolved data. These measurements were taken for all three model types.
The results of the UV spectroscopy clearly indicate that the radiation of cyanogen is greatly increased due to the rise in model wall temperature. This is particularly apparent in the boundary layer where measured intensity of the cyanogen molecules is the greatest. This clearly indicates that there thermal surface effects present in this impulse facility testing which have not previously been possible in these facilities. This technique can now be used to investigate other phenomena affected by the thermal surface effects such as, for example, boundary layer development, catalycity and surface thermochemistry.
The next step for this work is to conduct (subscale) re-entry vehicle tests with realistic wall temperatures and compare this data to flight data and computational models.