Flash prototype of a game like Balloon Invasion

Mouse.hide();
attachMovie("crosshair","crosshair",1);
attachMovie("tank","tank",2,{_x:450, _y:350});
fire_array = new Array();
fire_delay = 30;
just_fired = 0;
crosshair.onEnterFrame = function() {
	this._x = _xmouse;
	this._y = _ymouse;
};
tank.onEnterFrame = function() {
	mousex = _xmouse-this._x;
	mousey = (_ymouse-this._y)*-1;
	angle = Math.atan(mousey/mousex)/(Math.PI/180);
	if (mousex<0) {
		angle += 180;
	}
	if (mousex>=0 && mousey<0) {
		angle += 360;
	}
	if (angle>160) {
		angle = 160;
	}
	if (angle<20) {
		angle = 20;
	}
	firepower = Math.sqrt(mousex*mousex+mousey*mousey);
	if (firepower>200) {
		firepower = 200;
	}
	this.cannon._rotation = angle*-1;
};
function onMouseDown() {
	if (just_fired>fire_delay) {
		just_fired = 0;
		angle = tank.cannon._rotation;
		start_ball_x = tank._x+48*Math.cos(angle*Math.PI/180);
		start_ball_y = tank._y+48*Math.sin(angle*Math.PI/180);
		cannonball_fired = attachMovie("cannonball", "cannonball_"+_root.getNextHighestDepth(), _root.getNextHighestDepth(), {_x:start_ball_x, _y:start_ball_y});
		cannonball_fired.dirx = Math.cos(angle*Math.PI/180)*firepower;
		cannonball_fired.diry = Math.sin(angle*Math.PI/180)*firepower;
		cannonball_fired.fire_coord_x = _root._xmouse;
		cannonball_fired.fire_coord_y = _root._ymouse;
		cannonball_fired.onEnterFrame = function() {
			this._x += this.dirx/50;
			this._y += this.diry/50;
			dist_x = this._x-this.fire_coord_x;
			dist_y = this._y-this.fire_coord_y;
			dist = Math.sqrt(dist_x*dist_x+dist_y*dist_y);
			if (dist<20) {
				fire_array.push("explosion"+_root.getNextHighestDepth());
				exp = attachMovie("explosion", "explosion"+_root.getNextHighestDepth(), _root.getNextHighestDepth(), {_x:this._x, _y:this._y});
				exp.onEnterFrame = function() {
					this._width += 2;
					this._height += 2;
					this._alpha -= 2;
					if (this._alpha<0) {
						for (x in fire_array) {
							if (this.name == _root[fire_array[x]].name) {
								fire_array.splice(x,1);
							}
						}
						this.removeMovieClip();
					}
				};
				this.removeMovieClip();
			}
		};
	}
}
_root.onEnterFrame = function() {
	just_fired++;
	new_enemy = Math.random()*100;
	if (new_enemy<1) {
		enemy = attachMovie("enemy", "enemy"+_root.getNextHighestDepth(), _root.getNextHighestDepth(), {_x:-40, _y:Math.random()*200+50});
		enemy.speed = Math.random()*2+1;
		enemy.onEnterFrame = function() {
			this._x += this.speed;
			if (this._x>550) {
				this.removeMovieClip();
			}
			for (x in fire_array) {
				dist_x = this._x-_root[fire_array[x]]._x;
				dist_y = this._y-_root[fire_array[x]]._y;
				dist = Math.sqrt(dist_x*dist_x+dist_y*dist_y);
				if (dist<(this._width+_root[fire_array[x]]._width)/2) {
					this._alpha -= 5;
					if (this._alpha<0) {
						this.removeMovieClip();
					}
				}
			}
 
		};
	}
};