Gravity

An orbit simulation game, written in the teaching language BSL. Gravity has been developed for teaching purposes at the university of Tübingen.

The idea: Students can plan space-missions by programming a mission-control function.

Source Overview

The game is based on a numeric simulation of celestial bodies. Its implementation is divided into several modules:

Examples

examples.rkt : Gives examples on how to use the programming interface to develop own mission-control. A mission-control is a function that takes the current state of world (sensor-data) and computes a direction. If the direction (a vector) is not the zero-vector (make-posn 0 0), then the rocket will fire its bursters (if there is fuel :) ) and try to move into that direction.

Don't forget, it is a orbit simulator. Just firing into a particular direciton might not actually move you there!

The very easiest mission-control is the one, not doing anything:

; Ship Earth Sat Moon Time -> Direction
; Example mission-control, not giving any instructions to the space-ship.
(define (do-nothing ship earth sat moon wall-time)
  ZERO-VECTOR)

A mission can then be started with:

; start the mission "do nothing"
(start-mission do-nothing)

If you need more fuel for your space-mission, you can also use the alternative

(start-mission/fuel do-nothing 50000)

which equips the space-ship with 50.000kg fuel in this case.

Bodies

The datatypes Ship, Eearth, Sat and Moon are defined in bodies.rkt. There you can also find functions to access properties like velocity or fuel of a celestial body. In the very file one can also find all the constants that make up the game world. For instance, the moon orbit is defined as:

; orbital distance of the moon to the earth, as a Distance
(define MOON-ORBIT 363.3e6)

which approximates the real world.

The satellite is an example for a real world GPS satellite. The initial orbit of the space-ship is the parking orbit of Apollo 10. Also fuel and thrust is approximately the one of the Apollo 10 Command Service Module (CSM). Since Apollo 10 also had a yet another stage for transfer from earth to moon, this is not 100% accurate :)

Vectors and Time

The modules vector.rkt and time.rkt implement functions for computing with vectors and time, respectively. For instance vector.rkt provides a function for mulitplication with a scalar:

; Number Vector -> Vector
; Scale a vector by a scalar
(check-expect (v* 0 v1) ZERO-VECTOR)
(check-expect (v* 1 v1) v1)
(check-expect (v* 1 v2) v2)
(check-expect (v* 2 v1) (v+ v1 v1))
(define (v* scalar vector) ...)

Other Modules

There are also a few other modules which implement the game-mechanics, orbital physics and rendering of the game:

• orbital.rkt : Implements the "physics-engine" for orbital simulations
• world.rkt : Contains functions that simulate the interaction of four celestial bodies.
• mission.rkt : Contains definitions that model the state of a mission.
• interact.rkt : Contains functions that implement user-interactions like zooming-in.
• render.rkt : Contains functions that compute a graphical representation of the world.
• start-mission.rkt : The only "non-BSL" file: Implements the programming interface to start a mission.