Orbital Payload Calculator

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Introduction

In Kerbal Space Program (KSP), designing rockets is often less engineering and more alchemy — slapping on fuel tanks, guessing stage counts, praying it doesn't explode at launch, and only realizing after orbit insertion that:

• Either you have too little Delta-V, missing the Mun by a hair and watching your Kerbals cry;
• Or you have absurdly too much Delta-V, arriving at the Mun with enough fuel to tour the solar system;
• Most awkwardly, you finally reach orbit only to find the payload feels like a lead block, watching the fuel gauge drop bar by bar along with your heart.

Surprisingly, despite KSP being a decade old, few tools have specifically addressed this problem. As a rocket designer myself, I've felt the pain: constantly checking Delta-V charts and calculating payloads is brain-draining and error-prone. These needs and pain points led to the birth of Orbital Payload Calculator.

Now, Orbital Payload Calculator upgrades your rocket alchemy to rational engineering. It calculates payload mass and required Delta-V in advance, so you no longer pile on fuel by feel or carry extra emotional support fuel tanks.

From now on, your rockets won't be almost there or overflowing with fuel, but rather — just right, reaching orbit elegantly and heading to the Mun with style.

Orbital Payload Calculator is a utility mod for Kerbal Space Program (KSP) that estimates the maximum payload mass your rocket can deliver to a specific target orbit. It performs a simulation-based calculation to account for gravity, atmospheric drag, and steering losses, helping you design efficient launch vehicles without the guesswork.

It works in both the Editor (VAB/SPH) and in Flight (for vessels that are Landed or PreLaunch).

Features

• Payload Estimation: Calculates the maximum payload (tons) to your target orbit; iteratively matches available Delta-V against required Delta-V.
• Advanced Loss Modeling: Ascent simulation based on actual atmospheric curves; Pessimistic / Normal / Optimistic modes; bottom-stage sea-level TWR and turn start speed auto-scaled by selected body and vehicle; Advanced parameter overrides.
• Multi-Body & Rotation: Supports any celestial body and launch latitude; accounts for prograde/retrograde launch.
• Staged Analysis: Multi-stage support; automatic engine role classification (Main / Solid / Electric / Retro / Settling / EscapeTower); separation group detection; manual engine role overrides; fuel line (asparagus) modeling for accurate payload estimates on Kerbal X and similar staged boosters.
• Configurable Targets: Apoapsis, periapsis, inclination, launch latitude; units m / km / Mm; automatic plane change Delta-V.
• Ideal Delta-V Model: Surface-to-orbit ideal Delta-V (two-body, impulsive); auto-selection between energy-optimal and Hohmann models by orbit parameters.

Technical implementation details → Technical Description.

Dependencies

• Click Through Blocker

Compatibility

• Celestial bodies & aerodynamics mods: Stock bodies, RSS (Real Solar System), FAR (Ferram Aerospace Research), etc. are theoretically supported for calculation. Actual payload capacity depends on real flight trajectory; results are for reference only. Prediction accuracy is generally acceptable in most scenarios.
• Launch pad mods: This mod has minimized the impact of AlphaMensaes Modular Launch Pads and similar launch pad mods on calculations (though some cases may still slip through). For more accurate payload estimates, do not include launch clamps, launch pads, or other rocket-unrelated dead weight in your vessel when calculating.

Installation

1. Download the latest release.
2. Copy the GameData/OrbitalPayloadCalculator folder into your KSP installation’s GameData directory.
3. Ensure Click Through Blocker is installed.

Usage Guide

Open the Calculator

• Hotkey: Press Left Alt + P (default) to toggle the window.
• Toolbar: Click the Orbital Payload Calculator icon in the AppLauncher.

Workflow

1. Select Body: Choose the celestial body you are launching from (e.g., Kerbin).
2. Set Orbit: Enter your desired Apoapsis, Periapsis, Inclination, and Launch Latitude.
3. Launch Latitude:
   • In Editor: Enter the expected launch latitude manually (range -90° to 90°). Invalid values will be flagged.
   • In Flight: Automatically reads the vessel's current latitude; displayed in degrees-minutes-seconds (DMS) with N/S.
4. Loss Settings:
   • Pessimistic / Normal / Optimistic: Choose one; all use the built-in ascent simulation. Normal is the default; Optimistic assumes a more optimized ascent; Pessimistic gives conservative, higher-loss estimates.
   • Advanced Settings: Expandable anytime. Set turn speed, Cd coefficient (range 0.3–2.0), turn altitude, and gravity/atmosphere/attitude overrides; Advanced parameters override Pessimistic/Normal/Optimistic.
   • Treat cargo bay as fairing: When enabled, cargo bay mass is excluded at its jettison stage; disable if the cargo bay stays on for orbit.
   • Attitude loss reference: Excellent trajectory 10–30 m/s, average 30–80 m/s, aggressive turn 100–300 m/s.
5. Engine Classification: In the stage breakdown, click Engine Classification to switch each engine's role (Main / Solid / Electric / Retro / Settling / Escape Tower (LES)); overrides are persisted.
6. Calculate: Click the Calculate button.

Payload Calculation Methods

• Incorporate Payload: Treats the payload as part of the total rocket mass. As long as the estimated payload is greater than 0, the rocket still has transport capacity.

• Pure Rocket: Sets the payload aside (does not affect rocket calculation) and calculates the rocket's own Delta-V directly to determine transport capacity.

Incorporate Payload

Pure Rocket

Notes

• SSTO Support

  • Pure Rocket SSTO: Theoretically well-supported.
  • Air-breathing / Hybrid SSTO: (e.g., RAPIER) Use for reference only, as propellants like IntakeAir (not stored in tanks) are not included in the calculation.

• Calculation Accuracy

  • Actual launches are affected by factors like gravity turn speed vs. start height, flight trajectory, vessel aerodynamics, and piloting style. Therefore, the calculated payload is a theoretical estimate.
  • If actual flight results deviate from calculations, try optimizing gravity turn speed, turn height, and related parameters in MechJeb (MJ) to get closer to theoretical performance.
  • This mod is still being improved — it can help you calculate Delta-V, but it's not smart enough to save you from every aerodynamic disaster.
  • If it miscalculates, explodes, or behaves like an intern engineer, feedback and suggestions are welcome. Let's polish it into a true Chief Engineer together.