Rocket Cars

2025-12-26.03-37-00.mp4

Longer video here

Rocket Cars is a free open-source server-authoritative physics-based multiplayer car game sample, inspired by Rocket League. It’s made in Unity using Netick, a powerful free networking solution that makes creating complex multiplayer games like Rocket League simple and easy for indie developers.

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Features

• Clean and documented code
• Fully server-authoritative simulation (cheating is impossible)
• Fully predicted physics for all cars and the ball
• Custom vehicle physics model, inspired by Rocket League
• Goal replay system
• Full-match replay mode (using Netick's built-in full-game replay system)
• Custom field shader
• Camera Effects
• In-Game Text Chat

Controls

Rocket Cars uses Unity’s legacy input system (Input API).

Car Controls

Steering / Movement

• A / D – steer left / right
• W / S – accelerate / brake (also controls pitch up / down while airborne)
• Q / E – air roll left / right

Actions

• Left Mouse Button (button) – rocket
• Right Mouse Button (button down) – jump

Camera Controls

• Space: switches between car-oriented and ball-oriented camera modes
• Middle Mouse Button (while grounded and on car-oriented camera): reverses the camera direction

Replay & Spectator Controls

When the game is in replay mode (Sandbox.IsReplay == true), normal gameplay input is disabled and the following controls become available:

F Key – Toggle replay camera between:

• Free camera
• Following a selected player

Number Keys (1–9) – Select a specific player to spectate

• 1 selects player index 0
• 2 selects player index 1
• etc.

If the selected player is no longer valid (e.g. disconnected or not spawned), the replay camera automatically falls back to free mode.

Testing

• Running a client player: start the game and connect to a server
• Running a host player: start the game and choose host. Now, with proper port-forwarding, other people can join the game by using your IP address
• Running a dedicated server: by simply starting the game in batch-mode, it would automatically run the game as a dedicated server

Code Overview

• Soccer: Implements the Soccer game mode and handles overall game management
• CarController: Contains the vehicle simulation logic, using a custom lightweight physics model
• Ball, GoalBox, Booster: Core interactive game elements
• GoalReplaySystem & GoalReplayable: Handles server-side goal replay system (recording and playback), using Netick state storing/reseting API
• UI Folder: Contains all user interface logic, designed to be independent from the gameplay layer

Networking

In most games, only the local player character and some environmental objects are predicted. However, in Rocket Cars, we predict everything: the local player car + other players and the ball (remote/proxy objects).

However, due to latency, it's impossible to properly predict other (remote) players, since we can't predict their inputs. Thus, the client's prediction of other players will often be wrong, and it gets much worse at higher pings (>100ms). To mitigate this problem, we use a feature of NetworkTransform/NetworkRigidbody called Prediction Error Correction Smoothing to smoothly correct the error over multiple frames. Without that, the game would appear jittery because each time a correction comes in from the server, we would reconcile with the server and appear in a vastly different location instantly (prediction error magnitude scales with ping).

Read Netick's Prediction In-Depth article to learn more about Rocket Cars networking.

In addition, this is a nice video about Rocket League that describes the same networking-related issues inherent to proxy/remote prediction.

Performance

The game is written performantly, and it should run over 300 FPS even on low end PCs.

PhysX Resimulation Overhead

Because Rocket Cars uses physics prediction, it must resimulate the physics engine (Unity's 3D physics engine PhysX) during reconciliation. Unfortunately, PhysX is:

• Slow during resimulation: problematic for lower-end or mobile devices.
• Not deterministic: leading to potential mispredictions that shouldn't happen otherwise, especially at high ping.

Future improvement suggestion: Use a deterministic and CSP-ready third-party physics engine for better performance and accuracy.

Credits

Massive thank you to Steak and Milk-Drinker01 for being invaluable during the development of this sample. Steak completely cleaned up the project and overhauled the graphical style, and Milk-Drinker01 created a beautiful field shader and fixed the camera jitter issue.

Third-Party Assets

Asset	Source	License / Terms
DOTween	Demigiant	Standard EULA
Low Poly Car Pack 1	Designer Soup	Custom
Toon Shader	Erik Roystan	MIT
Textures	FreePBR & Free3DTextures	Custom
ParticlePack	Unity Technologies	Unity Asset Store EULA
Game Countdown 62	Pixabay	Pixabay License
car-tire-squeal-skid-loop	opengameart.org	CC BY 3.0
Vehicle - Essentials	Nox_Sound	Unity Asset Store EULA (permission granted for this project)
Font Electrolize	Google Fonts	Open Font License (OFL)