Hypersonic vehicles

Hypersonic vehicles are perhaps a dream of the future, but if they have a chance of happening, research is important to develop materials that will handle the high temperatures created at hypersonic speeds.

What defines a hypersonic vehicle?

The speed of sound at sea level at 15°C is 1226 km/h. Anything that travels at the speed of sound is said to be travelling at Mach 1. Anything travelling greater than the speed of sound is called supersonic and has a higher Mach number. For example, the SR-71 Blackbird plane could travel at Mach 3, which means it could travel three times the speed of sound.

Anything travelling faster than Mach 5 is hypersonic. Some vehicles have travelled at hypersonic speeds, but only for brief periods of time, for example, the Space Shuttle orbiter was hypersonic during re-entry through the atmosphere.

Developing a hypersonic scramjet vehicle

There is a group of researchers trying to develop air-breathing hypersonic vehicles that will work at low altitudes for sustained periods of time. These air-breathing engines are called scramjets (supersonic combustion ramjet).

Air is forced into a reaction chamber and then reacts with the fuel. The combusted material is propelled out the back end to make the vehicle experience a reaction force, which pushes it forwards. A scramjet vehicle is much lighter than an equivalent rocket because it doesn’t need to carry a supply of oxygen to react with the fuel.

Hypersonic test vehicles

In 2004, an experimental vehicle called X-43a was dropped by a bomber and accelerated to Mach 5 by a rocket before separating and igniting the scramjet engine. It operated for 12 seconds and reached a speed of Mach 9.8 (12 144 km/h).

In 2010, another experimental vehicle called the X-51 Waverider had a scramjet burn time of 140 seconds and reached a speed of Mach 5.

If scramjet technology can be developed to produce a vehicle that can fly for extended periods of time, it may be possible to fly to the other side of the world in 2 hours! There are many challenges that need to be met before this could ever become a reality.

Why a new protective material is needed

One of the challenges to be solved is to develop a material that will handle the 2000 °C temperatures that would be generated at those speeds. These temperatures are created because the vehicle is slamming into the air particles faster than the particles can get out of the way. Current hypersonic experimental vehicles are only able to travel at these speeds for a short time before the material on the outside breaks down.

Metal would not be suitable for a hypersonic vehicle because the melting point is lower than the 2000°C that would be encountered. For example, titanium was used for the supersonic SR-71 Blackbird, but it has a melting point of 1668°C.

The vehicle material also needs to be able to resist violent chemical attacks. At the high temperatures and pressures encountered, oxygen will break apart the molecular bonds of any structure that is not strong enough to handle it.

Ablation, insulation and active cooling

There are several ways of protecting space vehicles from extreme temperatures. Even if a new heat-protective material could be developed, other technologies would still be needed to keep a hypersonic vehicle from overheating:

• Ablative materials – a sacrificial layer is placed on the outside of the structure. As it heats up, tiny bits of the ablative material break away and carry the heat energy with it. Ablative materials are not reusable.
• Insulation – thick ceramic tiles were placed over the Space Shuttle orbiter (SSO) to stop the heat from reaching the inside. Thick tiles will not be suitable for a hypersonic vehicle because of a need for lightness.
• Active cooling – for a liquid fuel engine, the cold fuel can be pumped through pipes near the hottest parts of the rocket engines before it is pumped into the reaction chamber. A different method is to use heat pipes near the surface on the front edge of a vehicle. These pipes use evaporation and condensation cycles inside the pipes to transfer the heat away from the hottest part.