This month we celebrate 50 years of the conquest of Moon. Our columnist examines some of the advances in material science that made space exploration a reality. New materials will continue to help us breach the frontiers of space.
This month we celebrate the 50th anniversary of one of the most courageous acts of man. 20th July 1969 is a Red Letter Day in the annals of our history as Neil Armstrong took the first human step on the moon. “A Giant Leap for Mankind” was his famous words after that heroic act. It was also a giant leap for science and engineering. 400,000 engineers, scientists and technicians from more than 20,000 companies worked on the Apollo programme. More than anything else, it was a triumph of materials and material science.
Polymer Testing
Between 1964 and 1967, Jet Propulsion Laboratory of California Institute of Technology undertook a comprehensive study of polymers for spacecraft applications. The objective of this exercise was to select polymers to be used for various components of spacecraft in missions to Moon. During these 3 years, testing equipment was designed, and testing procedures and techniques were developed to identify and certify polymers for use in spacecraft. The prospective materials were tested for their ability to withstand extreme temperatures, electromagnetic radiation and meteoroid impacts. More than 350 polymeric materials were tested in the programme. Spacecrafts encounter situations that are very different from earth. The 3 most important parameters to be considered are – radiation, vacuum and temperature. In addition, resistance to abrasion by impact with micrometeoroids and erosion from atomic Oxygen is also important.
Radiation Breakdown
10% of sunlight is in the UV and X-ray region. While the earth’s atmosphere does not allow these to pass through, they are present in full strength in space. UV irradiation of polymers leads to the formation of free radicals which breakdown the polymer chain and crosslinking and consequent loss of mechanical strength and deterioration of thermal and electrical properties. Correlations between polymer structure and radiation resistance have been established. Aromatic groups in side chains have been found to increase radiation stability, but have the reverse effect in the main chain. Chlorine and Fluorine molecules in the polymer chain reduce the radiation resistance. Reinforced plastics are 1000 times more resistant to radiation than unreinforced plastics.
Vacuum Outgassing
Vacuum in space poses another kind of threat to spacecraft – outgassing. Outgassing occurs when volatile material trapped in the polymer matrix are released due to low pressure. These volatile matters may re-condense and deposit on sensitive parts of optical elements and electronic components, causing them to malfunction. The outgassing tendency of a polymer is measured by the VCM (Volatile Condensable Material) test; it measures the volatile material given off by a polymer that can condense on cooler surfaces. Norms of acceptable VCM limits were established while selecting polymers for Mariner IV mission to Mars in 1964.
Extreme Temperatures
Spacecrafts and spacesuits are designed to protect the astronauts from extreme temperatures ranging from minus 270 degrees C of deep space to 1260 degrees C during re-entry to the atmosphere. In the late 1950s, a chemist Carl Marvel synthesised Polybenzimidazole (PBI) for the US Air Force. PBI’s excellent thermal and chemical stability attracted NASA’s attention and the tragedy of Apollo I in 1967 hastened its adoption in the space programme. NASA contracted with Celanese Corporation to develop a line of PBI textiles that could be used for spacesuits and space vehicles. PBI was used extensively in Apollo and space shuttle missions. Subsequently, PBI has been used in numerous military and civilian applications.
Applications
Polymers have a wide range of applications in spacecraft and spacesuits. These include thermal blankets, thermal control paints, adhesives, insulating coatings and lubricants. Thermal blankets are essential for regulating the temperature of the spacecraft. These are essentially Multi-Layer Insulation (MLI) consisting of several layers of polymer films. Polyethylene Terephthalate (Mylar) and Polyimide (Kapton) are commonly used polymers. The layers are separated by fine gauze cloths made from Nylon. Thermal control paints provide the same purpose as thermal blankets. Paints are either black or white and consist of pigments dispersed in polymeric binders. Most widely used binders are urethanes in case of black paints and silicones in case of white paints. Adhesives are used for structural bonding, lamination of optical elements, and thread locking to prevent loosening of fasteners under high vibration conditions. Several adhesives have been developed meeting the requirements of outgassing and bond durability over a wide range of temperature.
Spacesuits
A wide variety of polymers go into the making of spacesuits worn by astronauts. The spacesuit worn by Neil Armstrong was literally a walking encyclopedia of polymers – Nylon, Mylar, Dacron, Nomex, Kapton. The outer layer was made from a special fabric called “Beta Cloth” – fibreglass fabric coated with Teflon. The specially designed outer layer provided protection from heat, radiation and abrasive lunar dust. The spacesuits continue to evolve while the basic principles remain the same.
Nanotechnology
If the last century was all about polymers, the future belongs to nanomaterials. Much research is focused on how nanotechnology can be used to reduce the mass, volume and power consumption of different components and systems used in spacecraft. Nanomaterial based composites are being studied to achieve a significant reduction in the weight of the spacecraft. Nanotechnology promises to offer self-heating materials which can be deployed for extremely long-distance space flights of future which are vulnerable to damage from meteor impacts. Spacesuits of future may incorporate nanosensors and nanorobots. The nanosensors would be able to diagnose a medical emergency and instruct the nanorobots to deliver appropriate drugs to the astronauts.
Solar Sail
The next generation of spacecraft may well be propelled through space by solar sails and not chemical-powered rockets. Solar sails are giant reflective sheets which harness the kinetic energy of light particles. Billions of photons from the Sun impinge on the giant solar sail and the resulting momentum exchange with the reflected photons pushes the spacecraft faster and faster through space. Till date, Beryllium has been the most researched material for use in the solar sail. But there is a growing interest in a Graphene solar sail and scientists believe it will outperform the one made of Beryllium. It is estimated that a spacecraft equipped with a Graphene solar sail will take about 1000 years to reach Alpha Centauri, the star closest to us and 4.4 lightyears away.
Advances in materials have enabled us to expand our horizon in unimaginable ways and will continue to do so in future. The frontiers of space and knowledge will continue to be breached.
Readers’ responses may be sent to k.sahasranaman@gmail.com or chemindigest@gmail.com
