One of the biggest vacuum chambers in the world, the 150 cubic metre chamber at TWI, has been used to tackle probably its most unusual job ever, testing protective tile material intended for use on the Mars space vehicle.
The cork based test material has to endure both extremely high outer space temperatures and the sand blasting effect of a seasonal Martian dust storm in a vacuum.
EADS ST lightweight cork ablative is a thermal protection system (TPS), used on planetary entry descent landing systems. Member company Lockheed Martin UK INSYS has been commissioned to test the material for use on the European Space Agency Mars lander which is due to be launched in 2009 and arrive on Mars in 2011.
The Martian atmosphere has seasonal dust storms in its upper layer. These cause a sand blasting effect on the TPS structure as it enters the atmosphere. It could compromise the performance of the aero structure's thermal protection.
Twenty six postage stamp sized cork tile specimens were originally supplied backed onto 2mm thick aluminium. Sixteen similar specimens were exposed in the DLR arc jet facility, in Cologne, to simulate high heat fluxes during entry into the Martian atmosphere. The remaining ten specimens were left in a virgin condition.
TWI was asked to simulate the blasting action using LM INSYS's erosion test apparatus. Martian atmospheric dust was replicated using both 50 and 100 micron diameter glass spheres. These were drawn into a high-pressure nitrogen gas stream using a venturi tube.
TWI's chamber was chosen because it has a high pumping rate, in the region of 8800 m3/hr, and can handle the high volume of dry nitrogen used.
The quantity of the spheres distributed over a 15-second interval was measured, using a filtered disc to capture them. The quantity per unit of gas flow was established at 0.5%. The objective was to achieve 0.1% flow of spheres per unit volume of gas. Consequently, the duration of the exposure was fixed at 15 seconds, which replicated an exposure time of 75 seconds.
To record the input velocity of the spheres, a pair of high fidelity laser interrupts was positioned between the throat of the nozzle and the target surface.
The chamber was evacuated to 10 millibar and dry nitrogen gas was passed though the nozzle for 15 seconds at a pressure of 4.9 bar (70 psi.). This gives a gas load of up to 50 grams, with a volume of about 6250 litres at the working pressure.
A preliminary test was performed to see if the pressurised nitrogen flow could erode the ablated specimens without dust being present. The structure remained intact and in its original ablated state. The results of erosion testing the virgin specimens clearly showed a loss of material from the original 10mm thickness. The virgin specimens show a pitting of the surface at the various angles.
To learn more about how TWI's vacuum chambers could be used to help your work, contact Bob Ward at TWI. firstname.lastname@example.org