How do heat shields on spacecraft work?
Jake Port explains how heat shields protect astronauts from the extremes of re-entry and looks at NASA's experimental lightweight options.
Travelling at more than seven kilometres a second and generating temperatures of over 1,600 °C, a spacecraft hurtles towards the ground carrying people and equipment. Its body glows red hot from the heat generated by friction with the atmosphere.
It should be melting, but thanks to its heat shielding, it isn't.
So how do these heat shields work?
Re-entry is the most dangerous part of a spacecraft’s mission, where temperatures and forces push materials and technology to the limit. So heat shielding is one of the most intensely investigated areas of space travel.
There are many ways to reduce the heat force on a craft. One is the use of heat sinks, which absorb huge amounts of heat then dissipate it to the surrounding atmosphere.
Sinks were originally made from titanium and beryllium but were soon ditched for lighter, more advanced materials.
Space shuttles use heat sinks in the form of a wide array of different tiles, each made from a particular blend of elements and compounds. The black tiles on the underside of the craft, for instance, are made of silica, the same stuff as you find in sand. It's so porous that around 90% of its volume is air. This porosity meant that it could absorb huge amounts of heat before it reached the underlying structure.
Other plates were made from heat-resistant materials such as carbon-fibre-reinforced carbon, used to protect the wing tips from some of the highest temperatures.
These plates have a problem, though. As tragically demonstrated by the Columbia disaster in 2003, they are very fragile and can break if knocked by debris.
A solution now being developed by NASA involves the use of inflatable shields.
The focus is now on the technology originally used during the early space missions – ablative heat shields.
These shields work by covering a large rounded surface with compounds that, when superheated during re-entry, burn off. This dissipates the heat as the material is burnt away. A gassy barrier forms that further insulates from heat.
NASA has been using the same ablative coating made from epoxy novolac resin mixed into a fibreglass honeycomb since the Apollo missions. Called Avcoat, it’s so effective that it will be used on the Orion spacecraft for many years to come.
While these shields are good at protecting the spacecraft from burning, they are carried into space in a single piece. This creates problems when sending large payloads such as rovers or manned missions, especially to other planets. They will need massive heat shields and that means extra weight – the bane of space travel.
A solution being developed by NASA involves the use of inflatable shields. These are either used to slow a spacecraft during part of its descent, or for the full duration of the re-entry.
In the video below, you can see NASA testing a type of inflatable heat shield called the Low-Density Supersonic Decelerator (LDSD) for use in future Mars missions. The LDSD is covered in heat-resistant fibres and fabrics. This makes it suitable for decelerating craft at high speeds, but only for a short period of time. For more heat-intensive missions we need to examine another inflatable shield, also being developed by NASA.
Called the Hypersonic Inflatable Aerodynamic Decelerator (HIAD), it’s inflated just before re-entry and is designed to protect spacecraft headed to Mars.
A thermal protection system made from a concoction of advanced materials including Nextel, Pyrogen and Kapton provides heat shielding up to the 1,260 °C expected to be generated in the Martian atmosphere.
The strength to resist airspeeds as high as 42,000 kilometres per hour comes from inflatable rings made from braided Kevlar, a material normally found in bulletproof vests.
The whole shield can fit inside a bag around half a metre in diameter, which, when deployed, expands to more than six metres. This makes it far easier to package and can be popped in the nose of a rocket.