L24: Rocket Trajectories in the Solar System

Rocket Trajectories in the Solar System #

In this lecture, we will go over a program that computes the trajectory of rockets in a solar system.

Please go over the code in : main.c

Read about BMP image files: https://en.wikipedia.org/wiki/BMP_file_format

Try the following:

  • Modularize the code into a multifile program
  • Add a Makefile
  • Implement the File I/O features described bellow.

🚀 C Programming Exercises: Rocket Simulation & File I/O #

These exercises build on the given rocket trajectory simulation program. Each problem focuses on adding new functionality that requires students to use C file I/O (text and binary) effectively.

🧩 Part 1: Input Extensions #

Exercise 1.1 — Load Body Parameters from a File #

Goal: Replace the hard-coded initialization of celestial bodies with data read from a text file.

File format (bodies.txt):

# x  y   vx  vy   mass
0.0 0.0  0.0 0.0  100.0
2.0 0.0  0.0 0.8  5.0
-2.0 0.0  0.0 -0.8  5.0
...

Tasks:

  • Write a function int load_bodies(const char *filename, Body *bodies, int max_bodies);

  • Open the file and read lines into the bodies array.

  • Ignore lines starting with #.

  • Return the number of bodies successfully loaded.

  • Modify main() to call this function instead of init_bodies().

  • Handle file-not-found errors gracefully.

Exercise 1.2 — Initialize Rockets from a File #

Goal: Allow flexible rocket starting conditions.

File format (rockets.txt):

# x  y   vx   vy
-8.0 -6.0 0.000005 0.000004
-6.0  5.0 0.000005 -0.000004

Tasks:

  • Implement int load_rockets(const char *filename, Rocket *rockets, int max_rockets);

  • Modify init_rockets() to use this function if a file exists; otherwise fall back to default initialization.

  • Print loaded rockets to the console for verification.

💾 Part 2: Output Extensions #

Exercise 2.1 — Save Final Rocket Positions #

Goal: Write final rocket positions and velocities to a text file after the simulation.

Tasks:

  • Create a function:

void save_rocket_data(const char *filename, Rocket *rockets, int n);

It should write:

Rocket_ID   Final_X   Final_Y   Final_VX   Final_VY   Trail_Length

Call this function at the end of the simulation (e.g., save_rocket_data("final_rockets.txt", rockets, N_ROCKETS);).

Exercise 2.2 — Save Rocket Trajectories in Binary Format #

Goal: Store all positions in binary for later analysis or visualization.

Tasks:

  • Implement:

void save_rocket_trails_bin(const char *filename, Rocket *rockets, int n);

  • Store trail_length followed by (x, y) pairs for each rocket.

  • Use fwrite() for efficient binary output.

  • Add a separate program analyze_trails.c that:

    • Reads this binary file.
    • Computes total trajectory length and prints summary statistics.

Exercise 2.3 — Frame Metadata Log #

Goal: Maintain a log file that records when each BMP frame was generated.

Tasks:

  • Before the main loop, open frames.log in write mode.

  • During rendering:

Append a line:

Frame 12: Step 600, Rocket0_Pos (-1.24, 3.98)

  • Include timestamp using time.h if you want bonus points.

  • Close the file at the end.

📈 Part 3: Analysis Extensions #

Exercise 3.1 — Trajectory Summary Statistics #

Goal: Write summary information for each rocket into a CSV file.

Output format (rocket_stats.csv):

RocketID,TrailLength,FinalDistance,MaxDistance,AverageSpeed
0,3200,45.7,48.9,0.032
1,2500,22.1,40.2,0.028

Hints:

  • FinalDistance = sqrt(x² + y²) at last point.

  • AverageSpeed = total_distance / simulation_time.

Exercise 3.2 — Reload and Plot Trails #

Goal: Read the stored binary or text trajectory file and re-render them into a new BMP image (without re-running simulation).

Tasks:

  • Implement a standalone program plot_trails.c.

  • It should:

    • Read rocket trail data from file.

    • Use the same render() and write_bmp() functions to visualize the loaded trails.

🧠 Challenge Exercises (Advanced) #

Exercise 4.1 — Combine File I/O and User Interaction #

Let users choose between:

Running a new simulation (N)

Loading and re-plotting saved data (L)

Use scanf() to read the user choice and perform appropriate file I/O.

Exercise 4.2 — Save Simulation Parameters #

Write a function save_metadata() to record:

Width, Height, Steps, DT, G, N_Bodies, N_Rockets

and other constants into a metadata.txt file. Students can then write a corresponding load_metadata() function that rebuilds the same configuration later.

Exercise 4.3 — Implement a Configurable Save Interval #

Currently, frames are saved every frame_interval. Modify the code so that the interval can be read from a config file:

dt=0.01
frames=200
save_interval=50

Solution Repository #

https://github.com/cpro-iiit/rocket_sim

Calendar November 12, 2025