Mihika Basu

NITK

Japan

Mars

Indian Space Research Organisation

ISRO

Department of Aerospace Engineering

Indian Institute of Science

Bengaluru

Mariner

NASA

Reddy

Designed by scientists from IISc andin, it can prevent heat damage to aircrafthas been a target for the space scientific community for the last four decades. The biggest challenge faced by a spacecraft on expeditions like theor’s Mars Orbiter Mission (MOM) is the heat generated due to its speed. When a spacecraft starts to descend on the surface of a planet, a large amount of heat is generated due to the speed of descent. This heat could damage the spacecraft and hence the spacecraft requires an additional Thermal Protection System (TPS). Now, scientists from two institutions — Department of Civil Engineering andat the(IISc),, and National Institute of Technology, Kisarazu College, Japan — have designed a cooling system for spacecraft entering the Martian atmosphere. “The exploration of planet Mars was started in the 17th century with the invention and development of telescopes. With the advancement in rocket technology in the late 20th century, several missions to explore mars were started. The first successful mission was4. Since then, several spacecraft have been sent to the planet for its exploration, which includes orbiters, landers, and rovers, with the recent ones being’s MAVEN and ISRO’s MOM,” said the authors in the paper, which has been published in the American Institute of Aeronautics and Astronautics Journal.They further said, “For landers and rovers, the spacecraft have to pass through the atmosphere of the planet and reach its surface to start its mission. Martian atmosphere entry begins at an altitude of 125 km and typical entry velocities vary from four to nine km/s, depending on the entry trajectory.At these entry speeds, the spacecraft are subjected to severe aerodynamic forces and aerodynamic heating, of which aerodynamic heating is a major aspect in the design of the spacecraft. Generally, largescale blunt cones are used as forebodies for the spacecraft to minimise the aerodynamic heating. These configurations also enhance the aerodynamic drag, assisting in reducing the speed of the spacecraft during entry.”The additional Thermal Protection System acts as a barrier between the high-temperature gas in shock layer (which is layer of molecules formed between the outer blunt portion of the spacecraft i.e. forebody and the atmosphere like a cushion) and the spacecraft, during planetary entry. “For any successful planetary entry, there is a need for an efficient thermal protection system,” said the paper.The team says the TPS used so far on all spacecraft entering the Martian atmosphere is “ablation” cooling, where the “TPS material chars, melts and undergoes pyrolysis, and the hot gas formed gets blown away, blocking the heat transfer to the surface.” The ablation cooling, according to the research team, has several drawbacks. Ablation cooling is also very expensive when reusability of the spacecraft is concerned.“During ablation, complex hydrocarbon products are formed, and their presence in the boundary layer of the spacecraft leads to a chemically reacting boundary layer, which can have an influence on aerodynamic forces and moments. The shape change occurring during ablation can affect the aerodynamics of the spacecraft, resulting in change in the flight path. Further, if the spacecraft experiences insufficient heat flux to cause pyrolysis, then the TPS will serve as a thermal insulator rather than an ablator,” said the findings.The drawbacks of the current thermal protection system led the research team to investigate the feasibility of using an alternate system. The team led by Dr KPJ, a professor at IISc’s Department of Aerospace Engineering, conducted an experimental study to investigate the effectiveness of the “transpiration” cooling technique, an alternative to the conventional “ablation” cooling.This technique involves passing of a coolant gas through a porous wall that absorbs the heat and gets blown away. The coolant gas forms a film on the outer surface of the spacecraft, absorbing heat from the molecules that it comes in contact with, via convection. The heated coolant gas is then flushed downstream by the continuous supply of gas from the spacecraft. “In this way, the heat transferred to a vehicle traveling at hyper-velocities can be greatly reduced,” said the paper.They tested the transpiration cooling ability of two gases, nitrogen and helium. According to the research team, nitrogen and helium were chosen because of their inert nature, which would not react with the environment and result in high heat transfer rates. These experiments were carried out in conditions similar to the Martian atmosphere and the team tested the transpiration cooling under different internal energy levels, pressure conditions and volume.The team concludes that both gases can be used as coolants in the transpiration cooling process with different efficiencies. While one gas is a better coolant at lower atmospheric energy levels, another is better at higher atmospheric internal energy levels.The paper states that “with the development of ceramic matrix composites like Carbon/Carbon ceramic,” which can withstand very high temperatures and has natural porosity, transpiration cooling seems to be a promising technique.“Transpiration cooling is relatively cheaper when reusability of the spacecraft is concerned. With the recent flight test of ISRO on reusable vehicle and with the development of C\C composites within the country, we feel that transpiration cooling technique will be incorporated within next 10 years. Research is currently underway in our laboratory to generate mist using water, and use it as coolant,” said Dr Reddy.A reduction in the heat transfer rate was observed using both the coolants. At low levels of internal energy, pressure and volume, helium resulted in a better heat transfer rate reduction than nitrogen. The team explained that this is because nitrogen molecules absorb more heat for the corresponding increase in temperature as compared to helium due to its higher volume flow rate.At high levels of internal energy, pressure and volume, said the researchers, nitrogen performed better than helium “due to the diatomic nature of nitrogen molecules.” “As nitrogen is always present as a dimer along with one other nitrogen atom, it is able to store more energy, undergoes excitation and dissociation at higher temperatures. Greater reduction in heat transfer rate was observed when more coolant gas was pumped downstream,” said the findings of the research team.