Ever since the first probes came back from space, engineers and scientists have been looking for methods to deal with the high heat loads associated with high-speed reentry environments. Throughout the years, many different technologies have been developed to improve the survivability of reentry: advancements in high-temperature materials, improved simulation programs, and the development of active cooling systems. Under the latter category falls transpiration cooling, which has been used for active cooling of aerospace components such as rocket engine thrust chambers, turbine blades, and reentry vehicles.
Transpiration cooling is achieved by forcing a cold, high pressure fluid through a porous wall, and by such preventing the wall from overheating. The cooling is a consequence of two effects: internal convective cooling of the wall material and the forming of a cool fluid film around the wall that shields it from the high temperature flow. Transpiration cooling is a promising technology as a thermal protection system for reentry vehicles, where a cold liquid or gas can protect the vehicle’s outer walls from the high-temperature flow.
There are however many challenges associated with transpiration cooling. The cooling fluid has to be stored at a high pressure to overcome the stagnation pressure of the incoming air, and the pressure drop associated with the flow through the porous material. Additionally, transpiration cooling is not effective for at all reentry conditions. At hypersonic speeds and low altitudes (< 40 km), the turbulent flow will prevent the injected fluid from forming a cold film around the vehicle and instead mixes the hot and cold flow rapidly. At high altitudes above 40 km and high speeds above roughly 8 km/s, the transpiration cooling fluid could trigger the laminar flow around the vehicle to go turbulent. This strongly reduces the effectiveness of the transpiration cooling, and could even cause adverse heating effects. However when a vehicle flies at altitudes above 40 km and velocities lower than roughly 8 km/s, transpiration cooling can be a highly effective thermal protection system.  An illustration of the different fluid behaviour of transpiration cooling can be seen on the figure to the right.
Research towards transpiration cooling started during the 1950’s, when NASA was looking for thermal protection systems that could be used to protect future reentry vehicles. From the ‘50s to the ‘70s, many high-temperature wind tunnel tests were performed under different conditions, geometries and materials to characterise the effectiveness of transpiration cooling for reentry applications. The materials of choice for transpiration cooling in this time period were almost exclusively metals, such as stainless steel or Nickel alloys. 
Sometime around 1960, NASA launched the first flight transpiration cooled flight experiment: a 25° conical vehicle with a base diameter of 26 cm. One half of the vehicle had a solid steel wall, while the other half had a porous steel wall that was transpiration cooled. This configuration allowed for a direct observation of the effectiveness of the transpiration cooling. Although the vehicle failed prematurely due to aerodynamic instabilities, some valuable flight data on transpiration cooling was obtained. 
During the early ‘60s, NASA performed more flight tests of reentry vehicles with ablative heat shields or transpiration cooling. These flight tests were used to compare the two types of thermal protection systems against each other. These conical shaped vehicles were only 22 cm wide and were launched to altitudes of 54 km with velocities up to Mach 6.7 km/s. A schematic of the vehicle with transpiration cooling can be seen in the figure on the side. 
Between 1966 and 1968, there were four flights of the Boost Glide Reentry Vehicle (BGRV), which was one of the manoeuvring reentry technology demonstrators of the United States Air Force during the ‘60s. This 4.27 m long vehicle used a water-glycol based transpiration cooling system for thermal protection during its flights. 
During the ‘70s and ‘80s, flight testing and research towards transpiration cooling came to a minimum, as ablative heat shields proved to be the preferred thermal protection system for re-entry applications. However at the end of the 20th century, the development of porous ceramic matrix materials, together with the prospect of reusability within the space industry, sparked a new interest towards transpiration cooling.  Similarly as during the ‘50s, many concept studies and high-temperature wind tunnel tests started to be published by the scientific community.
In June 2012, transpiration cooled reentry experiment, AKTiV, was launched by the German Aerospace Center (DLR) on board of the SHEFEX II (Sharp Edged Flight Experiment) vehicle. It launched from the Andøya launch site in Norway, towards the Svalbard islands in the Arctic Ocean. The experiment was a success, although the vehicle could not be retrieved due to the harsh weather conditions. 
A student team from the RWTH Aachen University and FH Aachen University is currently (2022) developing the TRAnspiration Cooling Experiment (TRACE), which is to fly aboard of a REXUS rocket from Esrange (Sweden) in March 2024. Chutes.nl had the privilege to meet the TRACE team and discuss the prospects of the mission.
The goal of the TRACE experiment is to validate the transpiration cooling hardware by flight testing on board of the REXUS sounding rocket.
The vehicle is biconic of shape with a rounded nose tip. One patch of the vehicle’s cone is equipped with transpiration cooling surfaces, through which the cooling fluid is expelled. In order to measure the thermal effectiveness of the transpiration cooling, two sets of thermocouples are placed on the vehicle’s walls: behind the transpiration cooled surfaces and behind the non-actively cooled surfaces. An additively manufactured titanium-alloy pressure vessel holds the Argon cooling fluid at pressures up to 200 bar. The porous material used for the transpiration surfaces is made from a 3D printed high-temperature plastic, called PEEK (Polyetheretherketone).
TRACE will separate from the REXUS sounding rocket by using a clamping mechanism, after which the transpiration cooling experimental phase begins. The tungsten ballast in the front end of the vehicle makes sure that it has a stable orientation throughout the descent. At lower altitudes, the back shell of the vehicle will be ejected prior to the deployment of the drogue parachute, being an hemisflo ribbon parachute. Finally, the vehicle will land safely using an elliptical main parachute.
At the moment (May 2022), the team is working effortlessly to deal with the challenges associated with the design and manufacturing of the TRACE vehicle. Amongst these are the characterisation of the transpiration cooling flow through the porous PEEK wall, thermal warping during the manufacturing of 3D printed parts and qualification of the 200 bar pressure vessel. Regardless of the challenges, Chutes.nl is confident the team will deliver valuable data to the scientific community and we look forward to the flight results of the transpiration cooling experiment.
Transpiration cooling has generally not been the thermal protection system of choice for most reentry missions in the past. However, there is a growing interest in this technology in the recent decade, especially with the current trend of reusability. Flight tests such as SHEFEX II and TRACE aim to increase the technology readiness level of transpiration cooling for future applications.
 T. Hermann et Al. Performance of Transpiration-Cooled Heat Shields for Reentry Vehicles. Dec 2019. AIAA. University of Oxford (UK)
 H. Otsu, K. Fujita, T. Ito. Application of the transpiration cooling method for reentry vehicles. 2007. 45th AIAA. Reno (USA)
 H. Weihs, J. Longo, J. Turner. The Sharp Edge Flight Experiment SHEFEX II, a Mission Overview and Status. 2012. 15th AIAA International Space Planes and Hypersonic Systems and Technologies Conference. Dayton (USA)
 Hannah Böhrk. Transpiration Cooling at Hypersonic Flight – AKTiV on SHEFEX II. June 2014. AIAA.
 T. E. Walton Jr. et Al. Free-Flight Investigation of Subliming Ablators and Transpiration Cooling at Hypersonic Velocities. Nov 1963. AIAA Vehicle Design and Propulsion Meeting. Dayton (USA)
 T. E. Walton Jr., B. Rashis. Measurement and empirical correlation of transpiration-cooling parameters on a 25° cone in a turbulent boundary layer in both free flight and a hot-gas jet. Oct 1961. Sandia Laboratories. NASA Langley Research Center.
 Yengst, W. (April 2010). Lightning Bolts: first manoeuvring reentry vehicles. Tate Publishing.