The above image shows the velocity profile of the Saturn 5 vehicle. The blue line represents altitude and the red line represents velocity with the horizontal line demarcating the point up to which the atmosphere plays a significant role.
At this point, a rocket is ascending at about 3 km/s. While descending, the rocket hits speeds of up to 10 km/s at this point. As we know, energy varies with the square of velocity, so this means that a velocity ratio of 3:10 results in an energy ratio of 9:100 between ascent and descent. This can be used to explain why a space vehicle does not burn while leaving the Earth but does so on re-entry. It has been recorded that the spacecraft reaches up to temperatures of 1600˚C
This heat produced during re-entry can be highly destructive, both to sensitive instruments on board and any potential humans on board. So, it becomes imperative for spacecraft designers and engineers to come up with a method to protect the contents of the spacecraft. This is achieved by using heat shields. Re-entry is the most dangerous part of a spacecraft’s journey, so a heat shield is a highly important aspect of spacecraft design.
Heat shields used are mainly of two types. They can be an ablative heat shield or a heat sink heat shield.
Heat sinks work on the principle that certain parts of the spacecraft absorb most of the heat and divert it away from the main body of the spacecraft. These materials then dissipate it to the surroundings. Sinks were initially made of titanium and beryllium alloys but were later replaced with lighter and more advanced composite.
These heat sinks are shaped like flooring tiles and are then lain on to the body of the space craft like normal flooring. The composition of heat sinks is not constant throughout the body of the spacecraft. For example, the heat sinks at the bottom of a space shuttle is made of silica which is about 90% porous. This porosity ensures that a large amount of heat can be absorbed by the sink without letting it reach the main body. The wingtips of a space shuttle experience the highest temperatures, so the heat sink here is made of carbon fibre reinforced carbon.
The main problem with the heat sink is that these tiles are very brittle, fragile and can easily be broken off by debris. This remains the cause of the fatal Columbia disaster where the space shuttle Columbia carrying Kalpana Chawla and 6 others with her disintegrated on re-entry after a 16-day scientific mission in 2003.
The Columbia Accident Investigation Board determined that a hole was punctured in the leading edge on one of the wings. The hole was formed when one of the insulating tiles peeled off during launch and struck the left wing. During re-entry, hot gasses penetrated this hole, compromising the internal hydraulics leading to a failure of control surfaces. This led to the loss of control over the shuttle and its eventual destruction.
A more advanced form of heat shielding is using ablative materials.
Ablative materials are materials that are so designed to slowly burn in a controlled manner, such that the heat can be directed away from the spacecraft interiors and the leftovers of the burnt ablative materials remains on the surface of the spacecraft exterior insulating the spacecraft from the surrounding heat. An ablative is generally a composite material made of a combustible matrix and a passive reinforcement material built like a mesh. The ash from the burnt combustible matrix is captured in the reinforcement mesh allowing the ash to insulate the spacecraft.
Another method employed is one where the matrix burns to produce a layer of non-combustible gasses over the surface of the vehicle which protects the spacecraft. A commonly used composite for ablative shields is a matrix of epoxy novolac resin with fibreglass meshing. This method of heat shielding was used in the Apollo, Mercury and Gemini spacecrafts.
There is an entire branch of spaceflight research involving the search for new fireproofing materials to achieve the best ablative performance; this function is critical to protect the spacecraft occupants and payload from otherwise excessive heat loading. The same technology is used in some passive fire protection applications, in some cases by the same vendors, who offer different versions of these fireproofing products, some for aerospace and some for structural fire protection.
While these shields are good at protecting spacecrafts from burning up, they cannot be intricately fabricated for payloads that are not conventionally shaped like rovers. Also, these heat shields must be sent as a single unit around the spacecraft which causes an unwanted increase in weight.
This has lead NASA to develop inflatable shields which can be used both to prevent burning on re-entry and to slow down descent. Such an inflatable shield is Low Density Supersonic Decelerator(LDSD) which is going to be used on future missions to Mars. The LDSD is covered by a heat resistant fabric.
For even more extreme uses, NASA is developing something called the Hypersonic Inflatable Aerodynamic Decelerator(HIAD) made of braided Kevlar fibres – a material used in bullet proof vests.
Deep space journey is the next step for humans and seemingly insurmountable challenges will be encountered along the way. But the undying will of the humans combined with a deep knowledge of science can take humans to stars and beyond.