feat: mission what-if sandbox — tweak vehicle parameters and see outcome change

[ghtomcat]

Summary

Allow users to adjust key mission parameters on existing vehicles and observe how the outcome changes. Keeps the real-mission foundation (Apollo 8, Starship IFT, Demo-2) while adding a "what if" experimentation layer on top.

Motivation

Prompted by community feedback: users want to explore rocket physics interactively. Rather than a full custom rocket builder (KSP territory), OpenSim's angle is recreating real missions — so the right scope is tweaking real vehicles, not designing fictional ones.

Examples of questions this would let users explore:

• What if Starship launched with 10% less thrust?
• What if Apollo 8's LOI burn was 30 minutes late?
• What if Falcon 9 carried 20 t more payload?
• What if the gravity turn started earlier?

Proposed approach

• Advanced mode toggle in the kneeboard or settings panel — hidden by default so it doesn't clutter the standard experience
• Expose a small set of high-impact parameters per vehicle:
  • Mass (payload, propellant load)
  • Thrust / throttle profile
  • fpaProfile pitch program (gravity turn shape)
  • Key event timing (stage sep, LOI, TLI burn start)
• Parameters feed directly into the existing physics engine — no new simulation code needed, just wiring UI to state
• Outcome: trajectory deviates, mission may succeed or fail, displays delta from nominal

Notes

• Core physics already supports all of this via fpaProfile, massWet, thrustSL/Vac, ignitionTime, etc.
• Delta-v sensitivity for cislunar trajectories (LOI, TLI) is already modeled — timing changes are basically free
• Main work is UI: clean parameter input without overwhelming the panel
• Could share custom configs via URL hash or JSON export for community sharing

Out of scope (for now)

Full custom rocket designer (arbitrary geometry, engine selection) — that's a much larger project and changes the product's identity.