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Figure 1. Space Shuttle Atlantis launches from the Kennedy Space Center November 16, 2009 in Cape Canaveral, FL. Image Credit: Jose Antonio Perez/Shutterstock.com
There were a vast number of challenges that NASA had to overcome when designing the Space Shuttle. This reusable spaceplane would make several trips to space carrying astronauts and payloads into low Earth orbit.
It would then re-enter the atmosphere of the Earth and glide back down to the ground. In order to withstand the high temperatures associated with re-entry, NASA created the Space Shuttle Orbiter Thermal Protection System (TPS).
A Background in Space Travel
In total, six Space Shuttles were built, of which five were launched; Columbia, Challenger, Discovery, Atlantis, Endeavour. Enterprise was the first shuttle built and was used for approach and landing tests – it had no orbital capabilities. The first flight was completed in 1977 when the Enterprise, was carried by a Boeing 747 as an unpowered glider and carried out a number of atmospheric tests. In 1981, Space Shuttle Columbia made its maiden voyage into space. This success was quickly followed by Challenger, Discovery and Atlantis in 1983, 1984 and 1985 respectively.
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Figure 2. Space Shuttle preparing for take-off. Image Credit: Alan Freed/Shutterstock.com
Endeavour was built as a result of the 1986 Challenger disaster which saw an O-ring seal in the solid rocket booster fail when the space shuttle lifted off. 73 seconds into its flight, the Challenger broke apart leaving no survivors. In 2003, Colombia exploded upon re-entry into the atmosphere; a piece of insulation foam had broken during launch and struck the left wing, the shuttle’s heat shield was compromised and caused the breakup of the shuttle.
After these two incidents the safety of this kind of space travel was called into question and on July 21, 2011, Space Shuttle Atlantis completed the last ever shuttle flight and the remaining three were retired.
Thermal Protection System - Materials
All the orbiters were covered in TPS materials which protected the shuttles from the heat of re-entry and also cold temperatures experienced when in space, a temperature range of -121-1,649°C.
There is a complicated array of materials which comprise the TPS to help keep the astronauts and payload safe during flights.
The nose cap, the area between the nose cap and the nose landing gear doors, arrowhead aft of the nose landing gear door and the outer edges of the wings are produced from a reinforced carbon-carbon (RCC) composite. This composite is able to withstand high temperatures and was used to protect areas of the shuttle that would rise above 1,260°C. For temperatures below this, NASA used rigid silica tiles/fibrous insulation.
The tiles used were based on work carried out by the Lockheed Missiles & Space Company who had a patent disclosure which described a reusable insulating tile made from ceramic fibers which could be used during re-entry as a guard against high temperatures. A reusable insulation system that could be directly bonded to a lightweight aluminium airframe was very attractive to NASA and so the focus of the generation of the TPS was diverted towards using tiles.
The large portion of the TPS is comprised of High-Temperature Reusable Surface Insulation (HRSI) and Low-Temperature Reusable Surface Insulation (LRSI). The main difference between HRSI and LRSI is the surface coatings used on them.
On the HRSI tiles, a black borosilicate glass coating was used to protect areas of the shuttle which reached up to 1,260°C. A white coating was used on the LRSI tiles which had the optical properties required to maintain on-orbit temperatures for vehicle thermal control purposes. The areas of the shuttle covered with LRSI reached temperatures up to 649°C.
The tiles were manufactured by the Lockheed Missiles & Space Company
the majority of which were two different types; LI-900 (which had a density of 144kg/m3) and the LI-2200 (which had a density of 352kg/m3). Throughout the Space Shuttle program, NASA made improvements to the tiles and continuously advanced their understanding of thermal conditions. This resulted in a number of compositional changes which made the tiles more efficient.
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Figure 3. A color-coded breakdown of the materials in the TPS.
Image Credits: Wikimedia Commons / NASA / Caltech
The LI-2200 tiles located around door penetrations were eventually replaced by Fibrous Refractory Composite Insulation. This helped reduce the overall weight of the Space Shuttle. In addition to this, and especially after the Colombia disaster, NASA wanted to decrease the shuttles' vulnerability to damage from orbital debris. This was achieved by the development of Boeing Rigidized Insulation.
As can be observed in Figure 3, a large portion of the shuttle is Coated Nomex® Felt Reusable Surface Insulation. These areas of the shuttle would experience temperatures below 371°C including the upper payload bay doors, sections of the mid-fuselage and the aft fuselage sides.
Once the Colombia had been delivered to the NASA assembly facility, engineers there had developed a superior insulation material called Advanced Flexible Reusable Surface Insulation (ARSI). ARSIs is a composite of quilted fabric insulation sewn between two white fabric layers.
Whilst providing durability, ARSIs also reduce installation times and cost of the shuttle. Replacing the majority of LRSIs, they also helped to reduce the weight on both Atlantis and Discovery, which were the first shuttles to use them. After completion of Colombia’s seventh mission, it was modified and ARSIs were added replacing the LRSIs on the upper wing. Endeavour was built with ARSIs already added to the shuttle.
Strain isolation pads and room-temperature vulcanized silicone adhesives were used to bond the tiles together. Before bonding the strain isolation pad to the tile, its inner mold lining was densified. This helped to evenly distribute the stress concentration loads across the tile-to-strain isolation pad interfaces. Gases were prevented from penetrating into the tile bond line by use of filler bars which were located beneath the tile-to-tile gaps. Gap fillers were also used in locations which would be subject to high differential pressures and extreme aero-acoustic excitations.
The bonding surface was predominantly aluminum, but many other substrates were used including graphite epoxy, titanium and beryllium.
Additional Materials
There were some areas on the outer shell of the Space Shuttles which required further protection that the TPS could not fully provide. These were mainly gaps in the TPS tiles which experienced differential pressures and, the Reaction Control System (RCS) and the windows of the shuttle.
Thermal glass was used in the windows to withstand high pressures and temperatures whilst engineers used a combination of black and white pigmented silica cloth for thermal gap fillers around operable penetrations. These included the main/nose landing gear doors, egress/ingress flight crew side hatches, RCS thrusters, mid-fuselage vent doors and the payload bay doors. The RCS fairings were made from Inconel®.
Reinforced Carbon-Carbon
Around the nose and wing leading edge, Space Shuttles would experience extremely high temperatures (>1,260°C). Therefore, a material which was advanced and could tolerate highly varying environments, from launch to re-entry, was required.
Alongside NASA engineers, the Vought Corporation, Dallas, Texas, developed RCC. A composite material, RCC is made firstly by curing pre-impregnated (with phenolic resin) graphite fabric. Through pyrolysis, and after the graphite fabric has been rough trimmed, the polymer resin is converted into carbon which is then impregnated with furfuryl alcohol. The density of this material is increased by further pyrolysis, which also leads to improved mechanical properties.
Due to the chemical nature of carbon, a silicon carbide coating is required to prevent the carbon substrate from oxidizing. Oxidization would adversely affect the mechanical properties of the substrate and as the shuttle was consistently required to perform in a highly demanding environment multiple times, this had to be prevented. The silicon carbide coating is produced by converting (diffusion coating) the outermost layers of the carbon-carbon material resulting in a stronger coating-to-substrate interlaminar strength.
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Figure 4. A hole in the RCC leading-edge panel, a result of impact testing in the investigation of the Space Shuttle Colombia disaster. Polyurethane foam was used to impact the wing at 850 km/h. Image Credit: Wikimedia Commons/Colombia Accident Investigation Board
However, the temperatures required to form the silicon carbide coating (~1,648°C) are so high that craze cracks develop in the coating when it is cooled down to room temperature due to the differing coefficient of thermal expansions between the coating and the substrate. This problem can be alleviated by impregnating the carbon substrate with tetraethyl orthosilicate and using a brush-on sealant to provide protection against oxygen attacking exposed carbon in the cracks.
Via vacuum impregnation, tetraethyl orthosilicate is applied to the substrate which also helps to fill any remaining porosity. Upon curing of the tetraethyl orthosilicate, the pore walls contained within the part are coated with silicon dioxide which inhibits oxidation. A sodium silicate sealant is then brushed onto the surface of the RCC which fills in the craze cracks and forms a glass. At high temperatures, these cracks close and the sealant flows onto the surface, but due to possessing a high viscosity the sealant remains on the part and when the carbon-carbon cools, the sealant forms a glass again refilling the craze cracks.
RCC is used due to its ability to reject heat by external radiation and cross-radiation. As a result, heat can be cross-radiated from the lower surfaces to the cooler upper surface of the material thus reducing the temperature.
Modifications and Eventual Retirement
Throughout the program NASA made small alterations to the design of the shuttles to make them safer and more efficient. This included modifications to the RCC after the Colombia disaster where a piece of insulation material from the Space Shuttle External Tank (ET) crashed into the wing resulting in damage that caused high temperatures on re-entry to break the shuttle apart. Damaging impacts to the RCC material were not originally considered in the design specification and so the RCC was left vulnerable to such collisions.
NASA engineers learnt a big lesson from this incident; that large pieces of debris could be liberated from the ET and could hit the RCC material. They made slight modifications to the ET to ensure that large debris could not easily come off it, however smaller-sized debris could still be shed.
They therefore developed critical damage criteria using inspection data. They started testing the RCC materials in an extensive Arc Jet facility which was able to closely simulate the chemical and temperature conditions of re-entry. This enabled NASA to verify newly developed thermal math models and establish damage criteria.
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Figure 5. Space Shuttle Atlantis being rolled back to the Orbiter Processing Facility after landing at the Kennedy Space Center, completing the final flight of the Space Shuttle Program on July 21, 2011. Image Credits: Wikimedia Commons/NASA
NASA employed 44 sensors across the wings of the Space Shuttles to provide real-time data to the astronauts on board. This data could also be transmitted to ground engineers for them to monitor any damage sustained during lift off.
Apart from the Space Shuttle Colombia (2003), it is fair to say that the TPSs on the Space Shuttles were effective and safe. The development of advanced coatings and materials enabled the Space Shuttle Program to operate effectively for 30 years. Continuous improvements and modifications were made on the TPS by engineers who gathered experience after every flight carried out which led to increased safety and extenuation of the program.
An overall lack of funding led to the eventual termination of NASA’s Space Shuttle Program and Atlantis completed the last flight on July 21, 2011. President Bush ordered in 2004 that it should be terminated by 2010, which was eventually pushed back to 2011.
With a reported cost of $1.5 billion per shuttle mission, the Russian Space Agency offering flights with their Soyuz to NASA for $63 million per astronaut seemed like a bargain for the continuation of taking astronauts to the International Space Station.
In 2014, NASA announced that Boeing and SpaceX have been awarded a contract to take over the building of American space systems capable of carrying astronauts into low-Earth orbit. The program may be over, but its legacy lives on.
Sources and Further Reading
This article was updated on the 13th June, 2019.