Moon Landing using Inheritance

1 Body

In the MoonLanding example from Chapter 3 the declaration of certain fields like mass and name were repeated in each of the classes CelestialBody and Rocket. This can be avoided by collecting those field declarations into a generic body class called Body, and reusing these by inheriting Body into CelestialBody and Rocket.

We have replaced the general keyword class by the restricted class keyword model which has the same semantics as class apart from the fact that it cannot be used in connections. The reason is that it is more common practice to use the keyword model for modeling purposes than to use the keyword class.

model Body "Generic body" Real mass; String name; end Body;

2 Celestial Body

model CelestialBody "Celestial body" extends Body; constant Real g = 6.672e-11; parameter Real radius; end CelestialBody;

3 Rocket

class Rocket extends Body; Real altitude(start = 59404,fixed=true); Real velocity(start = -2003,fixed=true); Real acceleration; Real thrust; // Thrust force on the rocket Real gravity; // Gravity forcefield parameter Real massLossRate = 0.000277; equation (thrust - mass*gravity) / mass = acceleration; der(mass) = -massLossRate * abs(thrust); der(altitude) = velocity; der(velocity) = acceleration; end Rocket;

4 Moon Landing

model MoonLanding parameter Real force1 = 36350; parameter Real force2 = 1308; parameter Real thrustEndTime = 210; parameter Real thrustDecreaseTime = 43.2; Rocket apollo(name = "Apollo13", mass(start=1038.358,fixed=true) ); CelestialBody moon(name = "moon", mass = 7.382e22, radius = 1.738e6); equation apollo.thrust = if (time < thrustDecreaseTime) then force1 else if (time < thrustEndTime) then force2 else 0; apollo.gravity = moon.g*moon.mass/(apollo.altitude + moon.radius)^2; end MoonLanding;

5 Moonlanding Simulation

simulate( MoonLanding, stopTime=230 )
plot( apollo.altitude )

Diagram 1: Altitude of the Apollo rocket over the lunar surface

plot( apollo.thrust)

Diagram 2: Thrust from the rocket motor, with an initial high thrust f1 followed by a lower thrust f2.

plot( apollo.mass )

Diagram 3: Rocket mass decreases when the fuel is consumed

plot( apollo.gravity)

Diagram 4: Gradually increasing gravity when the rocket approaches the lunar surface

plot( apollo.velocity )

Diagram 5: Vertical velocity relative to the lunar surface