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6: IDEAL RAMJET CYCLE

Fully expanded flow at the nozzle exit: static pressure is the same as the ambient atmospheric pressure.

Relating temperature ratio and pressure ratio,

Also, the exit Mach number can be found from:

Let us pause for a moment and consider what the exit Mach number will be. Since stagnation pressure is constant throughout this ideal engine, and the specific heats are assumed constant,

Obviously, the exit Mach number will be equal to the flight Mach number!

Exit velocity is

and the flight velocity is

Thrust per unit air mass flow rate:

and Thrust Specific Fuel Consumption

, or

Notes:

1. As Mach number increases, the ramjet pressure ratio increases. The combustor walls must be stronger. For the same reason, high-Mach number flight at sea-level becomes difficult.

2. As Mach number increases, the temperature at the inlet to the combustor increases. When this reaches the highest permissible combustor temperature, no heat can be added, so no thrust can be produced. Thus this provides an absolute upper limit on the Mach number at which thrust can be produced. Of course, the vehicle may not be able to fly at this Mach number, because we have not considered how much thrust is needed to overcome drag and accelerate to this Mach number.

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