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Model limitations and appropriate use

EnSim is a preliminary engineering simulator. Numerical convergence is not proof that a result represents a buildable or safe engine.

Thermochemistry

  • ideal-gas mixture; no fugacity correction or real-fluid feed-system state;
  • optional Pitzer-Curl second-virial and Redlich-Kwong utilities are preliminary gas-phase corrections, not a multiphase propellant-property package; mixture estimates use Kay pseudo-critical mixing and reject missing critical data;
  • only packaged gas species participate;
  • no condensed carbon, metal oxides, ions or plasma chemistry;
  • bundled engine and staging cases are editable numerical inputs, not replicas, performance specifications or validation records;
  • chamber equilibrium does not model injector mixing, ignition or finite-rate combustion.

Nozzle and thrust

  • one-dimensional, steady, adiabatic and attached flow;
  • chamber composition and gamma are frozen for the standard performance result;
  • equilibrium-shifting nozzle expansion is not implemented; EnSim does not infer recombination from temperature thresholds or apply an empirical Isp increment;
  • conical momentum divergence uses only (1 + cos(alpha))/2; no boundary-layer, erosion, two-phase or separated-flow loss;
  • reported ideal thrust must not be used as an acceptance prediction without independently justified efficiencies.
  • the MOC contour is a two-dimensional planar, sharp-corner minimum-length solution; it is not an axisymmetric bell nozzle and contains no viscous, boundary-layer, variable-gamma or separated-flow correction;
  • in the planar MOC result, throat_radius is a backward-compatible API name for throat half-height and the area ratio is the exit-to-throat height ratio;

Cooling and structures

  • Bartz and Gnielinski are engineering correlations with restricted domains;
  • geometry is reduced to local hydraulic and wall dimensions;
  • no conjugate finite-element conduction, film cooling, boiling, coking, roughness evolution, fatigue, creep or transient start/shutdown analysis;
  • material limits are screening values, not allowables for certification.

Combustion stability

Acoustic eigenfrequencies identify possible resonances. EnSim cannot infer a universal stable/unstable verdict from geometry alone. Growth and damping inputs must come from a defensible model or experiment.

Flight

  • rigid body with preliminary aerodynamic coefficients;
  • no flexible modes, propellant slosh, actuator dynamics, sensor model or closed loop guidance/navigation/control unless explicitly supplied by another model;
  • atmosphere and wind are engineering profiles, not a launch-day forecast;
  • the implemented 1976 Standard Atmosphere layers end at 84.852 km; above that boundary pressure and density are explicitly set to zero for aerodynamic propagation, without claiming an upper-atmosphere composition model;
  • the general trajectory is not validated against telemetry.

Uncertainty and optimization

Outputs are conditional on the selected distributions, bounds and objective. Monte Carlo frequency is not evidence that the assumed distributions are true. Reduced-order optimizers do not enforce manufacturing, combustion stability, structural dynamics, range safety or all thermal constraints.

Decision policy

Use EnSim to understand trends, screen concepts and reproduce documented trade studies. For hardware decisions, establish an independent model-validation plan, use traceable test data, quantify model-form discrepancy and obtain review by a qualified propulsion/flight-safety team.