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Capsule Systems - Thermal Control Systems

Table of Contents


External Thermal Control System
Internal Thermal Control System



Man has evolved to thrive and survive on Earth within a temperature margin. The human body is delicate and when placed outside of its safe temperature range, quality of life, and life itself, is threatened. The human body is 70-75% water, and therefore survival of man is closely related to water¡¯s properties and reactions to different temperatures. Standard room temperature on Earth is a comfortable 20 ¢ªC or 293 K compared to the 3 K present in the vacuum of space. The extreme cold temperatures of space do not permit human existence due to water¡¯s freezing point, 273 K. For space travel to become a viable form of transportation man has to conquer the extreme climate of the final frontier. In the Mercury program this was done. (Boynton)

The internal and external control systems of the Mercury capsule helped protect the astronauts from the harsh temperatures of space. The internal thermal control system (ITCS) regulated the astronauts¡¯ environment and maintained optimum operating temperatures for capsule components. The External Thermal Control System (ETCSS) regulated heat absorbed and radiated by the capsule. The ITCS and the ETCS are two intertwined systems that consisted of many active and passive components that worked together as the thermal control system (TCS). The system became active prior to launch, and remained in service throughout reentry until landing and completion of the mission.

External Thermal Control System

The external thermal protection was based around the semi-monocoque, titanium structure.

The external thermal protection consisted of beryllium and Rene alloy shingles and insulation layers for the afterbody and an ablation shield to dissipate the high temperature on the forebody. The ablative heat shield was a dish-shaped structure that forms the large end of the reentry module. (Mercury Manned Orbital Capsule Detail Specification, 20)

Mission Objectives

Upon launch the ETCS was necessary to protect the astronauts from the deafening noise of the engines. In orbit the ETCS was used to maintain safe operating temperature of internal systems by expelling heat generated by astronaut and instruments. The beryllium and Rene alloy shingles of the ETC acted as the skin of the capsule. The Thermoflex 300 blankets and the Johns-Mansville MIN-K insulation were used to thermally isolate the inner structure from the shingles. Beneath the beryllium and Rene alloy skin the Thermoflex 300 insulation was held in place by a titanium mesh (See Figure 1). The titanium was held in place by stringers and rings that were protected by MIN-K foil (.015 inches thick) that ran through a fiberglass channel. The ETCS¡¯s ablative shield located on the forebody was designed to dissipate energy generated by the impact of reentry. (Project Mercury Familiarization Manual)

Figure 1: Insulation on the Mercury capsule (WWW-4)

The ablation shield was a blunt plate made of a fiberglass-resin matrix that creates a shockwave/buffer of air between the ablation shield and the highest temperatures of reentry. When the Mercury capsule entered the atmosphere the velocity decreases becomes it comes in contact with the atmosphere. The atmosphere acts as a frictional force to the capsule and heat was generated. Temperatures could reach to as much as 3000 F on reentry. With the shockwave present and thermal insulation, heat flow to the titanium and honeycomb structure was less than 600 F. (See Figure 2).

Figure 2: Heat of re-entry (Boynton)

On January 16, 1959, design and fabrication of the Mercury capsule was assigned to McDonnell as the primary contractor. The capsule could be outfitted with either a beryllium heat sink or an ablation shield. McDonnell subcontracted the ETCS to three different companies and Andre J. Meyer Jr. from the Space Task Force was given the task of collecting the data from the separate companies for comparison and final decision on the ETCS configuration. Brush Beryllium Company of Cleveland was to forge six heat-sink heat shields; General Electric Company and Cincinnati Testing and Research Laboratory (CTL) were to fabricate 12 ablation shields (NASA). Brush was having major problems with the beryllium heat sink. Their scientist failed to produce beryllium ingots to a degree of purity useful to the Mercury mission. Beryllium heat sink was also an expensive exotic metal that would be a costly toll of the Mercury budget. The beryllium heat sink also was creating design problems for reentry and recovery. Upon entry the beryllium heat sink will absorb the heat from friction and then hold onto it creating a hazard for the astronaut in the capsule. Questions on whether or not the cabin could be cooked or whether a forest fire could be started upon dry landing also plagued Andre J Meyer Jr. The ablation shield could not be jettisoned prior to landing either due to fear that it might fall like a leaf and endanger the capsule. These problems ultimately killed Brush¡¯s beryllium heat sink and sealed the contract for Cincinnati Testing Laboratory. Cincinnati Testing Laboratory was headed by John H. Winter, who was the heat shield project coordinator. They successfully delivered the first heat ablation shield to NASA on June 22 of 1959. (WWW- 1).

¡°with a load-carrying Fiberglas sandwich structure consisting of two 5-ply faceplates of resin-impregnated glass cloth separated by a 0.65 inch thick Fiberglas honeycomb core, an additional Fiberglas honeycomb is bonded to the convex side of the sandwich and filled with Dow-Corning DC-325 ablative material. The entire shield is encircled with Fiberite ring.¡± (WWW-3)

The ablative shield was successful in protecting the Mercury capsule from the re-entry temperature throughout the Mercury The ablative shield design was used in five Little Joe flights, eight Redstone, two Jupiter, ten Atlas flights, and two balloon ascents by 1961. (WWW-2). During these flights the temperature measurements were recorded for analysis. The temperature gradient can be seen in the figure below

Figure 3: Ablative shield temperatures (WWW-3)

Even though Brush Beryllium Company lost the bid for the ablation shield design, they still contributed the race to space by developing the skin of the capsule. Mercury capsule¡¯s skin was comprised of beryllium and Rene 41 alloy shingles. (WWW-2)

The Rene 41 alloy shingle was a nickel alloy (see Figure 2). Rene 41 was a nickel base, high strength alloy that could withstand high temperatures up to 1800 F. After the first test flight of the Mercury capsule using the Atlas launch vehicle, test data lead to replacement of the shingles with thicker shingles to improve heat protection. (Manned Satellite Capsule Technical Proposal)

Figure 4: Shingle composition (Manned Satellite Capsule Technical Proposal)

Internal Thermal Control System

The ITCS system was designed to remove excess heat from the environmental control system, summarily maintaining nominal temperature for astronaut and cabin components. Heat was dissipated by evaporative cooling in the two heat exchangers. The heat exchangers channel heat via cooling loops that cycle through the astronaut¡¯s suit and cabin. The heat was dissipated into space by a heat sink. The cabin and suit temperature could be regulated by the astronaut to their comfort. (WWW-3).

Figure 5: Coolant valve control panel (WWW-3)

Design and Technology

The heat for the cabin and the astronaut¡¯s suit was regulated. If the cabin atmosphere was compromised the astronaut¡¯s suit could operate as a closed system and provide temperature and pressure control. The cabin temperature was regulated by the astronaut adjusting the cabin temperature valve. The cabin air was then circulated around the cabin and through the electronic equipment by the equipment blower. The heat was removed from the capsule by the two heat exchangers, and two cooling loops. One of the coolant loops was for the cabin (exterior) and one was for the astronaut¡¯s suit (interior). (WWW-3).

Figure 6: Mercury capsule coolant loops (WWW-3)

There were two cooling loops, an interior and an exterior, each with separate water reservoir. The exterior cooling loop, cycles water from the water storage tank. Water was cycled in to the heat exchanger and absorbs heat from the cabin. The heated water was then vented in to space. The interior cooling loop used water from pressurized tanks. The pressurized tanks used a pressurized bladder to reduce sloshing of the water in the zero-g environment of space. The increase in flow rate of the water increased the cooling capacity of the ITCS. During ground, and operations below 115,000 ft, the capsule exterior coolant loop cycled Freon instead of water through a heat exchanger. The operation of the heat exchanger was controlled by a temperature sensor located on the steam exhaust duct. (Project Mercury NASA Capsule Flight Operations Manual).

The temperature sensor on the steam exhaust duct was insufficient in monitoring the heat loads of the capsule and regulation of the heat exchanger. This design fault resulted in increased suit inlet temperatures, wet undergarments, and uncomfortable conditions for the astronauts. This problem was caught during ground testing after MA-7, and a temperature probe was introduced in the heat exchanger dome. The suit temperatures were reduced from 75-80 ¢ªF to 60-70 ¢ªF. (WWW-3)

Figure 7: Heat Exchanger (WWW-3)

The temperature sensor, when triggered would increase or decrease the flow through the heat exchanger. Testing on the effectiveness of the coolant system was done during the flight of MA-9. The cabin fan and coolant loops were turned off and the cabin conditions were monitored under varied electrical loads. The temperature remained between 85-95 ¢ªF and it was concluded that the capsule did not require a coolant loop with the electrical power system powered down. The majority of the heat generated in the capsule was by the electrical components. (WWW-3).

Figure 8: Temperature of MA-9 Cabin (WWW-3)


In conclusion the ITCS and the ETCS work to reduce heat produced by the astronaut and capsule components. The heat generated from electrical instruments requires the Mercury capsule to have an active coolant system. The active coolant system carries the heat away to be dissipated through the heat exchangers. The heat was ejected into space. The ETCS protects the astronaut from the harsh temperatures of space and re-entry to the Earth¡¯s atmosphere. The ITCS collects the heat generated by the internal components and astronauts and vents excess heat in to space. With only a few minor failures that were corrected, the TCS of the Mercury capsule was a success. The technology developed was a vital stepping-stone to sending man to the Moon.


Mercury Manned Orbital Capsule Detail Specification. McDonnell Aircraft Corporation. World Spaceflight News Special Report. (1961).

Project Mercury NASA Capsule Flight Operations Manual. McDonnell Aircraft Corporation World Spaceflight News Special Report. (1960).

Manned Satellite Capsule Technical Proposal, NASA Report 6483, McDonnell Aircraft, 1958.

Project Mercury Familiarization Manual, McDonnell, 1961, NASA CR 555570

WWW-1, Swenson Jr., Loyd. This New Ocean.

NASA, 1989.

WWW-2, Swenson Jr., Loyd. This New Ocean.

NASA, 1989.

WWW-3, Boynton, John H., Mercury Project Summary.

WWW-4, Project Mercury Familiarization Manual. NASA.
(1962). 4/15/04