Ramjet Cycle (propulsion/ramjet/ramjet-cycle)
Use when the task is ideal ramjet cycle evaluation: stagnation
temperature, total temperature ratio, specific thrust, total thrust,
specific impulse, and thermal efficiency for an airbreathing hypersonic
engine.
Domain quick reference
Units are SI throughout:
- Static temperature in K, speeds in m/s.
- Mass flow in kg/s, thrust in N, specific thrust in N/(kg/s).
- Fuel air ratio f dimensionless, LHV in J/kg, cp in J/(kg K).
- Specific impulse in seconds, efficiencies dimensionless.
The ideal ramjet cycle (isentropic inlet, constant-pressure heat
addition, fully expanded nozzle, perfect gas):
- Speed of sound a0 = sqrt(gamma * R * T0), with gamma = 1.4 and
R = 287 J/(kg K) for air.
- Flight stagnation temperature Tt0 = T0 * (1 + (gamma - 1)/2 * M0^2).
- Combustor total temperature ratio tau_lambda = Tt4 / Tt0 = 1 +
eta_b * f * LHV / (cp * Tt0).
- Specific thrust F / m_dot_a = a0 * M0 * (sqrt(tau_lambda) - 1),
from the fully expanded jet speed v9 = v0 * sqrt(tau_lambda).
- Total thrust F = m_dot_a * (F / m_dot_a).
- Specific impulse Isp = (F / m_dot_a) / (f * g0), with g0 = 9.80665
m/s^2.
- Thermal efficiency eta_th = (F / m_dot_a) * v0 / (f * LHV).
- Ramjet cycle practice sits in the FAR-33 engine design context.
Workflow
- Collect the static temperature, gas properties, and Mach number;
compute the speed of sound and the stagnation_temperature.
- Collect the fuel air ratio, LHV, cp, and combustor efficiency;
compute the total_temperature_ratio.
- Combine speed of sound, Mach number, and total temperature ratio
into the specific_thrust.
- Multiply by the captured mass flow for the thrust and by the fuel
air ratio for the fuel_mass_flow.
- Compute the specific_impulse and thermal_efficiency from the fuel
flow terms.
- Gate the airbreathing engine assessment on the four outputs.
Pitfalls
- Using a total temperature ratio at or below 1.0; the ideal ramjet
needs heat addition, so sqrt(tau_lambda) - 1 turns negative.
- Confusing static and stagnation temperature; the combustor ratio
and the energy balance use Tt0 from the flight Mach number.
- Using total thrust where specific thrust belongs; specific impulse
divides specific thrust by the fuel flow per unit air, not the
total fuel flow.
- Reporting thermal efficiency above 1.0; thrust power cannot exceed
the fuel chemical power input.
Behavior contract (gate 3)
The stagnation, ratio, thrust, impulse, and efficiency logic is
exercised by the gate 3 contract test: scripts/test_ramjet_cycle.py
against scripts/ramjet_cycle_logic.py (stdlib unittest, offline). Run:
python3 scripts/test_ramjet_cycle.py
Compliance
- Standards referenced, not reproduced: FAR-33 is US government work
(public domain); the ideal ramjet relations are common propulsion
methodology, summary-only per standards-map.yaml.
- compliance: STANDARDS-REF, gated: false.
1---2name: ramjet-cycle3description: Use when you must analyze the ideal ramjet cycle: compute the flight stagnation temperature from the speed of sound and Mach number, the total temperature ratio across the combustor from the fuel air ratio and lower heating value, the specific thrust from the Mach number and total temperature ratio, the total thrust from the captured mass flow, and the specific impulse and thermal efficiency from the fuel flow. Produces the specific thrust, thrust, specific impulse, and thermal efficiency that gate the airbreathing hypersonic engine assessment. Trigger: ramjet, fuel air ratio, total temperature ratio, specific thrust, specific impulse.4license: Apache-2.05---67# Ramjet Cycle (propulsion/ramjet/ramjet-cycle)89Use when the task is ideal ramjet cycle evaluation: stagnation10temperature, total temperature ratio, specific thrust, total thrust,11specific impulse, and thermal efficiency for an airbreathing hypersonic12engine.1314## Domain quick reference1516Units are SI throughout:1718- Static temperature in K, speeds in m/s.19- Mass flow in kg/s, thrust in N, specific thrust in N/(kg/s).20- Fuel air ratio f dimensionless, LHV in J/kg, cp in J/(kg K).21- Specific impulse in seconds, efficiencies dimensionless.2223The ideal ramjet cycle (isentropic inlet, constant-pressure heat24addition, fully expanded nozzle, perfect gas):2526- Speed of sound a0 = sqrt(gamma * R * T0), with gamma = 1.4 and27 R = 287 J/(kg K) for air.28- Flight stagnation temperature Tt0 = T0 * (1 + (gamma - 1)/2 * M0^2).29- Combustor total temperature ratio tau_lambda = Tt4 / Tt0 = 1 +30 eta_b * f * LHV / (cp * Tt0).31- Specific thrust F / m_dot_a = a0 * M0 * (sqrt(tau_lambda) - 1),32 from the fully expanded jet speed v9 = v0 * sqrt(tau_lambda).33- Total thrust F = m_dot_a * (F / m_dot_a).34- Specific impulse Isp = (F / m_dot_a) / (f * g0), with g0 = 9.8066535 m/s^2.36- Thermal efficiency eta_th = (F / m_dot_a) * v0 / (f * LHV).37- Ramjet cycle practice sits in the FAR-33 engine design context.3839## Workflow40411. Collect the static temperature, gas properties, and Mach number;42 compute the speed of sound and the stagnation_temperature.432. Collect the fuel air ratio, LHV, cp, and combustor efficiency;44 compute the total_temperature_ratio.453. Combine speed of sound, Mach number, and total temperature ratio46 into the specific_thrust.474. Multiply by the captured mass flow for the thrust and by the fuel48 air ratio for the fuel_mass_flow.495. Compute the specific_impulse and thermal_efficiency from the fuel50 flow terms.516. Gate the airbreathing engine assessment on the four outputs.5253## Pitfalls5455- Using a total temperature ratio at or below 1.0; the ideal ramjet56 needs heat addition, so sqrt(tau_lambda) - 1 turns negative.57- Confusing static and stagnation temperature; the combustor ratio58 and the energy balance use Tt0 from the flight Mach number.59- Using total thrust where specific thrust belongs; specific impulse60 divides specific thrust by the fuel flow per unit air, not the61 total fuel flow.62- Reporting thermal efficiency above 1.0; thrust power cannot exceed63 the fuel chemical power input.6465## Behavior contract (gate 3)6667The stagnation, ratio, thrust, impulse, and efficiency logic is68exercised by the gate 3 contract test: scripts/test_ramjet_cycle.py69against scripts/ramjet_cycle_logic.py (stdlib unittest, offline). Run:70python3 scripts/test_ramjet_cycle.py7172## Compliance7374- Standards referenced, not reproduced: FAR-33 is US government work75 (public domain); the ideal ramjet relations are common propulsion76 methodology, summary-only per standards-map.yaml.77- compliance: STANDARDS-REF, gated: false.
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