Metallic TPS solutions for increased heat loads

For future reusable launch vehicles, the thermal protection system (TPS) has to be both durable and easy to maintain, apart from the basic function of providing thermal protection at low mass. Metallic materials offer the promise of improved durability and reduced maintenance effort, but have a limited application range with respect the thermal loading. In order to improve the applicability of metallic TPS materials, two concepts of cooling metallic hot structures have been developed at the Faculty of Aerospace Engineering of Delft University of Technology. The first concept is called Improved Radiation Cooling and is based on the principle of providing a cool interior background for the hot outer skin to radiate to, thereby increasing the radiation cooling effect. The cool inner background is made up of a porous material saturated with water, ensuring a temperature no higher than the boiling temperature of water. This system was tested in laboratory experiments and shows an increase of allowable heat flux for the PM1000 metallic skin from some 250 to almost 500 kW/m/sup 2/. A flight experiment is planned on ESA's Expert re-entry module. For higher heat loads, a direct water cooling system is proposed for the nose of a small re-entry module. Here water is in direct contact with the outer skin of the vehicle, is held in place by deceleration forces, and cools the skin by evaporating. The vapour is vented through the base of the vehicle. This system cools the outer skin to a very low temperature of about 250/spl deg/C, and requires some 8 kg of water for a ballistic re-entry of the baseline DART vehicle of Delft University. The limit of allowable heat flux for this system depends on pressure inside the water tank and deceleration forces.

Background

To dramatically lower the cost of launching payloads into space, the launch vehicles of the future need to be reusable, and have operating costs much lower than those of today's semi-reusable launch system, the Space Shuttle, and lower than that of the current expendable launchers (Davis et al). One of the key technology areas is that of thermal protection systems. On the space shuttle, this requires an estimated 17,000 man-hours of maintenance for every flight (Blosser 1996). The requirement for the TPS for a Reusable Launch Vehicle (RLV) includes operability and robustness as prominently as thermal performance and low mass. The desired RLV should be able to fly within days after returning from its previous mission, and maintenance needs should be minimised.