Add report link to Manual

Author: arda-guler

docs/MANUAL.md

@@ -42,6 +42,8 @@ Controls can be edited via 'Configure OrbitSim3D' option at startup.
 
 ## Working Principles
 
+See the [short technical report](https://github.com/arda-guler/orbitSim3D/blob/master/docs/OrbitSim3D.pdf) for the math involved, and the design philosophy of OS3D.
+
 Instead of using Kepler's Laws, which are quite useful and accurate for most two-body systems, this simulation calculates the motion of celestial bodies and spacecraft in small time steps and integrates gravitational acceleration from all celestial bodies currently in the simulation. (Spacecraft maneuvers work the same way too.) All geometrical and mathematical relations are based on numerical/infinitesimal-like calculations. This way, no movement happens "on rails", and perturbations from distant bodies and other effects are always accounted for. (However, if you wish, you can still make Kepler style orbit projections, determine apoapsis and periapsis and ascending nodes and all that; but if the perturbations are too high, the spacecraft wouldn't necessarily follow the calculated 2-body orbit path.)
 
 For numerical integration, OS3D uses the simple and fast **Symplectic Euler** by default, and also has built-in **Velocity Verlet**, **Yoshida 4th Order** and **Yoshida 8th Order** solvers for those who wish better accuracy without giving up time step size. This choice of built-in methods were due to their energy conservation characteristics, so that simulations that run for very long durations will still give plausible results - though *some* inaccuracies are inevitable regardless. (The default solver can be changed via the 'Configure OS3D' option at start-up menu. If you want to use something like RK89 for a short trajectory simulation, you can implement it in solver.py with relative ease.)