Speaker
Description
Background of the study:
For most re-entry cases, spacecraft experience radiative heating from the hot shock layer flow which envelops the vehicle during planetary entry. Due to the long length scales that are required for the flow to equilibrate at conditions near peak heating, there is often significant uncertainty in the chemical environment in many planetary entry scenarios which creates uncertainty in the radiative heat flux experienced by the vehicle.
To lower these uncertainties, experiments are performed in shock tubes and expansion tubes where generally emission spectroscopy is used to ascertain the flow relaxation in this post-shock flow with distance by looking normal to the flow. In a shock tube, for example, this involves imaging the flow as it passes a set position in the facility. This allows information about the chemical length scales to be ascertained as well as knowledge of the species present in the post-shock flow.
In flight, many planetary entry vehicles are equipped with thermocouples embedded in their heat shields or radiometers looking out into the flow to measure the radiative heat flux. These onboard sensors are vital to further understanding the planetary entry environment, however, they rely on a large amount of post-processing and simulations of the re-entry environment to interpret the data. These data are extremely useful for understanding the radiative heating that a heat shield experiences during planetary entry. However, this data is very rare due to the rarity of planetary entry missions themselves.
An important step which connects the two important scenarios discussed above is to directly measure radiative heat flux to test models being tested in hypersonic impulse wind tunnels such as expansion tubes. As mentioned above, most ground testing experiments measure radiation through the radiative shock along the line-of-sight parallel to the model, which is not exactly representative of the radiative heat flux that a heat shield experiences during the entry. Radiation measurements through the surface of the model are closer to an actual entry scenario. This is not a new technique, but its application is limited and in most cases it relies on measuring total radiative heat flux by placing standard heat flux sensors behind windows. This does not allow any information about where in the wavelength spectrum the radiation is originating from to be captured.
Instead this work aims to validate the even rarer technique of using a fibre optic cable to directly measure radiative heat flux to the surface of a test model and beam it out to a spectrometer external to the facility. While this technique has received limited use in the past, this work aims to fully validate the technique as a way to directly measure radiative heat flux for planetary entry by fully characterising the system used in terms of its transmission and angular field of view and by comparing through-model data to both standard externally measured spectroscopy results and simulations. This will allow the through-model tunnel data to be used as a way to validate predictions of radiative heat flux for planetary entry vehicles from surface mounted radiometer data because the results will be spectrally resolved and will be able to be compared to external emission spectroscopy data captured during the same experiment.
Methodology:
This work involves the placing of a fibre optic cable at the centre of a scaled planetary entry capsule in UQ’s X2 expansion tube which is piped out to a Mini spectrometer (Thorlabs CCS200) which can be triggered to expose during the X2 facility’s test time. The spectrometer has a wavelength region of 200 to 1,000 nm and the designed system allows radiation to be captured from 350 to 1,000 nm. For select validation cases, experiments will also be performed with a flat model which should have a very uniform post-shock environment.
Test conditions have been selected targeting important planetary entry cases for different planets in the solar system to show validation in different environments. These conditions include superorbital Earth re-entry, Saturn entry, and Titan entry, all cases where appreciable flow radiation over the region where the system is sensitive is expected
Results:
Experimental results include bench testing validation of the angular field of view of the fibre which is important for calculating the amount of radiation which would be incident on the fibre in simulations.
Currently experimental delays have delayed our ability to perform all of the tunnel validation experiments which we would like, but preliminary through-model spectroscopy data performed last year for a Stardust entry condition will be presented if new data is not able to be captured before the conference.
Conclusion:
Further understanding radiative heat flux experienced during planetary entry is important for the future of space travel. This work aims to increase that understanding by providing a novel method to perform direct radiative heat flux measurements for planetary entry and by characterising that method so that it can be easily compared to radiating CFD simulations of the re-entry environment. This technique is still being developed by us at UQ, but preliminary results have been presented here.
Summary
Further understanding radiative heat flux experienced during planetary entry is important for the future of space travel. This work aims to increase that understanding by providing a novel method to perform direct radiative heat flux measurements for planetary entry and by characterising that method so that it can be easily compared to radiating CFD simulations of the re-entry environment. This technique is still being developed by us at UQ, but preliminary results have been presented here.
