MODEL SCOPE / ASSUMPTIONS / OMISSIONS

Know where the model stops.

The Analyzer is a transparent preliminary 2D ascent model. Its outputs are only as defensible as its assumptions, input data, and simplifications. This page states those boundaries explicitly rather than hoping nobody notices them.

Inspect model boundaries View validation

Trajectory: 2D planar
Atmosphere: Lower model → 86 km
Staging: 1–3 serial stages
Use case: Educational / preliminary

Not operational flight software

The Launch Vehicle Analyzer is intended for educational and preliminary engineering analysis. It is not flight-certified, does not reproduce a complete launch-vehicle guidance/navigation/control stack, and should not be used for real launch operations, hazardous testing, or safety-critical decisions.

01 / SUBSYSTEMS

Current model boundaries.

Each subsystem is intentionally narrower than the physical system it represents. The useful question is not whether an approximation exists. Every model has them. The useful question is whether the approximation is visible.

EARTH / GRAVITY

Spherical central gravity

Included: Earth-centred 2D geometry and inverse-square central gravity using μ.
Omitted: J2, oblateness, third-body gravity, local terrain and gravity anomalies.
Consequence: Suitable for transparent ascent/orbit fundamentals, not high-fidelity orbital perturbation analysis.

ATMOSPHERE

Standard lower atmosphere

Included: Layered temperature, pressure, density and speed of sound through 86 km geometric altitude.
Omitted: Weather, humidity, seasonal/latitude variation, thermosphere/exosphere physics and automatic wind fields.
Consequence: Above 86 km the current default is an explicit vacuum approximation, not a physical upper-atmosphere model.

PROPULSION

Performance-anchor model

Included: Sea-level/vacuum thrust anchors, pressure interpolation, effective Isp and mass flow.
Omitted: Chamber thermodynamics, nozzle geometry, mixture ratio, engine transients, ignition dynamics and detailed throttle maps.
Consequence: Useful for stage-level performance propagation, not rocket-engine design or off-nominal engine simulation.

AERODYNAMICS

Drag-only Mach-Cd model

Included: Relative airspeed, Mach, dynamic pressure, Cd(M), reference area and drag vector.
Omitted: Lift, Reynolds-number dependence, angle-of-attack dependence, aeroelasticity and CFD-level flow effects.
Consequence: The built-in Cd curve is generic and assumed. Vehicle-level drag accuracy requires vehicle-specific aerodynamic data.

GUIDANCE

Open-loop 2D thrust direction

Included: Radial/tangential primitives, generic assumed ascent profile and user-defined pitch programs.
Omitted: Closed-loop orbital targeting, attitude-control dynamics, navigation errors, launch azimuth logic and vehicle-specific GNC.
Consequence: The default trajectory is a demonstrator, not a recreation of an operational launch guidance algorithm.

DYNAMICS

2D translational motion

Included: Position, velocity, mass, gravity, thrust and drag in an Earth-centred plane.
Omitted: 6-DOF attitude, body rates, moments, structural flex, slosh, control surfaces and rigid-body rotational dynamics.
Consequence: Good for trajectory-level coupling, not stability/control or structural-load certification.

STAGING

Idealized serial events

Included: Usable-propellant depletion, burnout, optional separation delay, mass discard and activation of the next serial stage.
Omitted: Parallel boosters, plume interaction, separation impulse, pyrotechnics, collision dynamics and detailed sequencing hardware.
Consequence: Serial mass bookkeeping is modeled; physical separation behavior is not.

ORBIT / NUMERICS

Two-body state + fixed-step RK4

Included: Osculating planar orbit analysis and event-aware fixed-step RK4 state propagation.
Omitted: 3D elements, perturbations, adaptive timestep/error control and long-duration high-fidelity propagation.
Consequence: Final orbital state is a two-body interpretation of the simulated state, not a complete mission-propagation solution.

02 / DOMINANT UNCERTAINTY

The default example is not a real rocket.

Two assumptions dominate how seriously the default trajectory should be interpreted: aerodynamic data and guidance.

AERODYNAMICSGeneric Cd(M)

The included Mach-Cd table exists to exercise the model. It is not published aerodynamic data for a specific launch vehicle.

Assumed input

GUIDANCEGeneric pitch profile

The default profile is open-loop and transparent, not calibrated to a real flight and not an orbital optimizer.

Assumed input

VEHICLE DATAUser supplied

Thrust, Isp, masses and geometry can be published, derived, estimated or assumed. Input quality directly controls result quality.

Traceable

TARGETTolerance-based

A target-orbit match is a numerical comparison against requested periapsis/apoapsis tolerances, not proof of mission feasibility.

Interpret carefully

03 / INTERPRETATION

What results do and do not mean.

A number can be computed to twelve decimals and still be based on an assumption made five lines earlier. Precision and accuracy remain stubbornly different concepts.

Results can be used to

Explore how stage mass, propulsion, drag and guidance interact in a coupled ascent model.
Compare transparent modeling assumptions and observe how they propagate into trajectory and final orbital state.
Study Max-Q, staging events, mass evolution and basic two-body orbital properties.
Build validation exercises around known analytic cases and documented public vehicle data.

Results should not be used to

Plan or execute a real launch, engine firing, hazardous experiment, or safety-critical operation.
Claim flight accuracy when aerodynamic and guidance inputs are generic assumptions.
Infer structural margins, stability, control authority, thermal loads or engine internal behavior.
Replace professional high-fidelity mission-design, CFD, GNC, structural or flight-certified software.

04 / REDUCING UNCERTAINTY

What has been tested, and what still needs better data.

Atmosphere, numerical integration, timestep convergence, and staging have direct validation evidence. Vehicle-specific aerodynamics and mission reconstruction remain the largest open fidelity questions.

COMPLETE 01

Atmosphere reference validation

Nine geometric-altitude checkpoints from 0 to 86 km were compared against an independent U.S. Standard Atmosphere 1976 reference implementation; all reported quantities remained within the stated 0.05% criterion.

OPEN 02

Use vehicle-specific Cd data

Replace the generic aerodynamic curve with traceable geometry-specific or published data where available. This remains a major limitation for vehicle-level trajectory claims.

COMPLETE 03

Public-flight benchmark

The Falcon 9 CRS-5 comparison is published with its disagreement intact: the current proxy is close to the rounded Mach-1 timing reference but substantially under-predicts the published MECO state.

COMPLETE 04

Numerical convergence and conservation

RK4 energy conservation and full-simulation timestep convergence have been measured separately from physical-model fidelity, preventing numerical error from being confused with vehicle-model uncertainty.