Ramjet operation, cycle and limitations

A ramjet is a jet engine with no compressor or turbine. The forward speed of the vehicle compresses the incoming air in an inlet diffuser (ram effect), fuel is burnt in the combustor, and the hot gas is expelled through the nozzle.

 flight -->  +-----------------------------+
 air ------->| diffuser | burner  | nozzle |------> jet
             |  (ram)   | + flame |        |
             +-----------------------------+
              0  1    2     3        4   5

Cycle (T-s)

 T
 |            3
 |           /|
 |          / |
 |         /  |     p02 = p03 (constant-pressure burning)
 |        2   |
 |        |   5
 |        |  /      p0 (ambient)
 |        | /
 |        0
 +------------------- s

1. 0-2: isentropic compression in the diffuser (ram compression); the speed falls to nearly zero.
2. 2-3: heat addition at constant pressure in the combustor.
3. 3-5: isentropic expansion in the nozzle, converting enthalpy into jet velocity.
4. 5-0: heat rejection to the atmosphere by the exhaust jet.

The ram pressure ratio is $\dfrac{p_{02}}{p_0} = \left(1 + \dfrac{\gamma-1}{2}Ma_0^2\right)^{\gamma/(\gamma-1)}$; at $Ma_0 = 2$ it is 7.8 and at $Ma_0 = 3$ it is 36.7.

Ideal thrust

$$F = \dot m_a\left[(1 + f)C_j - V_0\right] + (p_j - p_0)A_j$$

The ideal cycle efficiency is the same as that of the Brayton cycle: $\eta_{th} = 1 - \dfrac{1}{\text{(ram pressure ratio)}^{(\gamma-1)/\gamma}} = 1 - \dfrac{T_0}{T_{02}}$.

Advantages

• No moving parts: simple, light and cheap.
• Efficient at high supersonic speed; the gas temperature is not limited by turbine blades, so combustion temperatures can be high.

Limitations

• Zero thrust at rest: it needs a booster or launch to reach its starting speed.
• Poor efficiency at low speed because of the low ram pressure ratio.
• Large fuel consumption below about $Ma$ 1.5; diffuser shock losses at high $Ma$.
• Intense heating of the structure at high speed; flame stabilisation is difficult.

Applications

Missiles and target drones, as the second stage in rocket-boosted vehicles, and in combined-cycle propulsion (turbo-ramjet) for high-speed aircraft. Above $Ma \approx 5$ the scramjet replaces it.