--- title: "Ramjet operation, cycle and limitations" subject: "Turbo Machines" chapter: 5 key: "semester_7/turbo_machines:ramjet-working-cycle-limitations" source: https://www.bctengineer.com/q/semester_7/turbo_machines/ramjet-working-cycle-limitations site: BCT Engineering --- # Ramjet operation, cycle and limitations Turbo Machines · Chapter 5: Theoretical Jet Engine - Asked in: year not given, 6 marks - Video: [Ramjet engines, How do they work?](https://www.youtube.com/watch?v=kHSJiO_Fvvw) — Sabin Civil Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/turbo_machines/ramjet-working-cycle-limitations - Whole chapter: https://www.bctengineer.com/md/semester_7/turbo_machines/chapter-5 ## Question Describe the working of a ramjet engine with a sketch and its T-s diagram. Explain the advantages, limitations and applications. Write the expression for the ideal thrust. ## Short answer A ramjet uses ram compression in the diffuser, constant-pressure combustion, and expansion through a nozzle; it has no compressor or turbine. Ideal thrust is F = m_dot_a[(1+f)Cj - V0] + (pj-p0)Aj. Advantages: no moving parts, high supersonic efficiency. Limitations: zero static thrust, poor low-speed efficiency. Used in missiles and high-speed drones. ## Answer 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. ```text flight --> +-----------------------------+ air ------->| diffuser | burner | nozzle |------> jet | (ram) | + flame | | +-----------------------------+ 0 1 2 3 4 5 ``` #### Cycle (T-s) ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does a ramjet compress the air? A. By a rotating compressor B. By the ram effect of the inlet diffuser due to forward speed C. By a piston D. By a fan driven by a turbine Answer: B. Vehicle speed decelerates the air in the diffuser and raises its pressure. (Key point: Ramjet working) 2. The ramjet ideal cycle consists of A. constant-volume heat addition and isothermal compression B. isentropic compression in the diffuser, constant-pressure heat addition, isentropic nozzle expansion C. isothermal compression and expansion D. compression by compressor, expansion in turbine Answer: B. It is a Brayton-type cycle where the diffuser replaces the compressor and the nozzle does all the expansion. (Key point: Ramjet cycle processes) 3. The ideal thrust is A. F = m_a[(1+f)C_j - V0] + (p_j - p0)A_j B. F = m_a V0 - C_j C. F = (p_j + p0)A_j only D. F = m_a (V0 - C_j) Answer: A. Thrust is momentum change including fuel mass plus the pressure thrust at the nozzle exit. (Key point: Ideal thrust expression) 4. The main limitation of a ramjet is A. turbine blade temperature limit B. it has too many moving parts C. it works best at rest D. zero thrust at rest, so a booster is needed, and poor efficiency at low speed Answer: D. Ram pressure ratio is too low at low speed, and at rest there is no ram compression at all. (Key point: Main limitation) 5. Why can a ramjet use high combustion temperatures? A. It has no turbine whose blades limit the temperature B. It uses cooler fuel C. It has a compressor D. It operates at low speed Answer: A. With no turbine, blade material does not limit the temperature; also there are no moving parts so it is simple and light. 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