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 (in tons) capable of reaching your target orbit.
  • Iteratively solves for the payload mass that matches your rocket's available Delta-V against the required Delta-V.
• 📉 Advanced Loss Modeling
  • Simulation-based: Runs a time-stepped ascent simulation using the celestial body's actual atmospheric pressure and temperature curves.
  • Gravity & Drag: Automatically estimates gravity losses and atmospheric drag based on vessel TWR and aerodynamics. CdA uses the same heuristic (Cd × √mass) in both Editor and Flight; user Cd or default 0.7/1.0.
  • Estimate Modes: Mutually exclusive Normal and Optimistic modes; Advanced Settings parameters always override both. In Optimistic mode, the ascent simulation uses a more aggressive turn exponent (0.50) for earlier pitch-over, reducing modeled gravity loss.
• 🌍 Multi-Body & Rotation
  • Supports any celestial body (Kerbin, Eve, Duna, modded planets, etc.).
  • Accounts for planetary rotation (launching East vs. West) and launch latitude.
• 🔄 Staged Analysis
  • Properly handles multi-stage vessels.
  • Blends Vacuum and Sea-Level ISP for atmospheric stages based on pressure.
  • Separation Groups: Detects decouplers in all stages after the bottom stage, so multi-pair boosters (SRBs or liquid) that separate at different times are correctly modeled. Each booster pair's dry mass is subtracted when its engines exhaust.
  • View detailed breakdown per stage: Mass, Thrust, ISP, TWR, and Delta-V.
• 🛠️ Configurable Targets
  • Set Target Apoapsis (Ap), Periapsis (Pe), Inclination, and Launch Latitude.
  • Supports units: m, km, Mm.
  • Automatically calculates Plane Change Delta-V if the target inclination is lower than the launch latitude.
• 📐 Ideal Δv Model (Surface → Orbit)
  • Two-body, impulsive; ignores atmosphere and gravity loss.
  • Model A (energy-optimal): Global minimum Δv; used for low orbits (α < 1.5) and low-eccentricity intermediate orbits.
  • Model B (Hohmann): Structured burn sequence (burn1→burn2, or +burn3 for elliptical); used for high orbits (α > 2.0) and high-eccentricity intermediate orbits.
  • Automatic selection by semi-major-axis ratio α = a/r₀ and eccentricity e; see DATA_SOURCES.md.

📦 Dependencies

• Click Through Blocker (Required)

📥 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:
   • Normal / Optimistic: Choose one; both use the built-in ascent simulation. Normal is default; Optimistic assumes a more optimized ascent profile.
   • Advanced Settings: Expandable anytime. Set turn speed, Cd coefficient (drag coefficient, range 0.3–2.0), turn altitude, and gravity/atmosphere/attitude overrides; Advanced parameters override Normal/Optimistic.
   • Attitude loss reference: Excellent trajectory 10–30 m/s, average 30–80 m/s, aggressive turn 100–300 m/s.
5. Calculate: Click the Calculate button.

Results Explained

• Vessel block (top): Vessel name, wet mass, dry mass.
• Orbit block: Launch body, apoapsis, periapsis, inclination, eccentricity.
• Estimated Payload: The max tonnage you can add to the vessel's current payload.
• Required Delta-V: Total Delta-V needed to reach orbit (Ideal Δv + Losses + Plane Change ± Rotation Assist). Ideal Δv is the theoretical minimum from surface to target orbit; see Delta-V Details for breakdown.
• Available Delta-V: Your vessel's total Delta-V. In Flight, the Delta-V Details popup also shows ground altitude above this.
• Loss Breakdown: Shows how much Delta-V is lost to Gravity, Drag, and Steering (Attitude).

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

🧩 How It Works

1. Ideal Δv (Surface → Orbit): Uses a hybrid model—Model A (energy-optimal lower bound) for near-surface and low-eccentricity mid orbits, Model B (Hohmann structure) for high and high-eccentricity orbits. Selection is automatic based on α = a/r₀ and eccentricity.
2. Vessel Analysis: The mod scans your vessel (active or editor) to build a staging list, calculating mass, thrust, and ISP for each stage.
3. Ascent Simulation: It simulates a gravity turn ascent:
   • Vertical climb until a specific velocity/altitude.
   • Gravity turn pitching over based on a power-law curve (exponent 0.50 in Optimistic, 0.58/0.70 in Normal).
   • Drag: Cd coefficient is user-configurable or heuristic (0.7–1.0 × √mass); same in both Editor and Flight.
   • Thrust and ISP vary dynamically with atmospheric pressure.
4. Binary Search: It runs the simulation multiple times, adjusting the simulated payload mass until the Available Delta-V matches the Required Delta-V, finding the limit of your rocket's capacity.

⚠️ 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.

• Compatibility & Testing

  • The current version has NOT been systematically tested with FAR (Ferram Aerospace Research), RSS (Real Solar System), Principia, or Rescale mods.
  • You are welcome to test in these environments and provide feedback or suggestions.

• Development Status

  • 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.