Engineering
How a Ramjet Transitions from Low to Hypersonic Speeds
Quick fact
A ramjet cannot produce thrust until the vehicle is already moving at supersonic speeds—typically above Mach 2—so it must be accelerated by a rocket booster or another aircraft before it can take over and push to hypersonic speeds.
Why this is interesting
You've probably seen ramjets in hypersonic missiles, but did you know they can't even start from zero speed? So how does a ramjet go from standing still to flying at five times the speed of sound?
Read the full explanation
Understanding How a Ramjet Transitions from Low to Hypersonic Speeds
Think of a ramjet as a pipe that uses the forward speed of the vehicle to compress incoming air instead of using mechanical compressors. At high speed, air is forced into the inlet, and the shape of the duct slows it down, increasing its pressure. To start, the vehicle needs a 'kick': a rocket booster or a carrier aircraft accelerates it to a speed where the ram effect alone provides enough compression. Once the ramjet 'lights off,' it continues to accelerate. The transition to hypersonic speeds isn't a single event; it's a gradual process where the inlet and combustion system adjust as the Mach number increases. There is no particular Mach number that marks the switch to hypersonic, but typically above Mach 5 the airflow entering the combustion chamber remains supersonic even after compression—requiring a different design, the scramjet.
A deeper explanation
The key to ramjet operation is the ram effect: as the aircraft moves forward, air is forced into the inlet and decelerated by a series of shock waves. These shocks compress the air, raising its pressure and temperature. For a conventional ramjet, the inlet slows the air to subsonic speeds before combustion. The burner injects fuel, which ignites, expanding the hot gases out the nozzle to generate thrust. The transition from low to hypersonic speeds involves managing the shock system and ensuring the engine continues to swallow the correct amount of air. As speed increases, the pressure and temperature of the incoming air rise dramatically. At around Mach 5, the deceleration to subsonic causes heat that can break the material and engine efficiency drops. At that point, engineers switch to a scramjet, which allows the air to remain supersonic through the burner. Thus, the transition from low to hypersonic speeds is a carefully orchestrated process that relies on the launch assist to get to supersonic speeds, then the ramjet accelerates to the edge of its envelope, beyond which a scramjet takes over.