ParticleSystem.RotationBySpeedModule.xMultiplier

Description

Speed multiplier along the x-axis.

Changing this property is more efficient than accessing the entire curve, if you only want to change the overall speed multiplier.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    private ParticleSystem ps;
    public float hSliderValue = 1.0f;

    void Start()
    {
        ps = GetComponent<ParticleSystem>();

        var rotationBySpeed = ps.rotationBySpeed;
        rotationBySpeed.enabled = true;
        rotationBySpeed.separateAxes = true;

        rotationBySpeed.xMultiplier = 1.0f;
        rotationBySpeed.yMultiplier = 0.0f;
        rotationBySpeed.zMultiplier = 0.0f;
        rotationBySpeed.range = new Vector2(1.0f, 5.0f);

        var psr = GetComponent<ParticleSystemRenderer>();
        psr.material = new Material(Shader.Find("Sprites/Default"));    // this material renders a square billboard, so we can see the rotation
    }

    void Update()
    {
        var main = ps.main;
        main.startSpeed = hSliderValue;
    }

    void OnGUI()
    {
        hSliderValue = GUI.HorizontalSlider(new Rect(25, 45, 100, 30), hSliderValue, 1.0f, 5.0f);
    }
}