There are many formidable challenges to overcome before we can make a working air-breathing spaceplane engine. The most common approach to doing this is via the Scramjet engine.

The design of such an engine’s intake and exhaust is well understood and characterised - and so, of these technical challenges, the most difficult is to achieve a good air-fuel mixing (and as a result, combustion).

The reason why this is a problem, is that at high speeds, vehicle drag is very large and it is difficult to add further kinetic energy to an already hot and energised airstream. Good conversion of the fuel’s chemical energy to trust is therefore essential. Yet, at these speeds, air passes through the engine in a few milliseconds, meaning that the fuel must mix with the air, burn and release its energy in a time measured in microseconds.

To achieve maximum extraction of energy, the fuel and air must be mixed thoroughly. The mixture must then be burnt, but without the aid of the flame-holding structures used at lower speeds - as projections into the duct would cause drag.

The mixing is a particularly difficult problem because, not only does the fuel not have time to diffuse fully into the air, but there is usually a barrier (a region of compression, like shockwave) between them, formed due to their radically different speeds.

Achieving good mixing and combustion this is the main aim of the ASPIRE engine.

How Does Aspire Work?

The ASPIRE system works by directing the airflow away from the main engine-duct and then injecting the fuel in the form of encapsulated gas, liquid droplets/capsules or a pelletized solid (or a mixture of these forms).

The air is then redirected back into the duct with the appropriate timing and engulfs the fuel, which vaporizes and so releases its load evenly throughout the mixing volume. The final stage is ignition and combustion.

This cycle is shown opposite: