Burst Effect?

I have some balls which are arranged in a circle. I draw them to screen with the following:

	Function drawBalls()
	
		For Local b:ball = EachIn ballList
			SetRotation(BallRotationAngle)
			DrawImage balls,b.xpos+BallXoffSet+Sin(b.angle)*distanceFromCenter,..
			b.ypos+BallYoffSet+Cos(b.angle)*distanceFromCenter,b.ballFrame
			SetRotation(0)
		Next
		
	End Function

I want there to be a sort of burst effect which causes the balls to burst outward then return inward to the center of the screen.

I thought I could use something like this.

Function testBurst()
	
	Local temp:Float = distancefromcenter
	BurstDistance = 100
	
	If KeyDown(KEY_SPACE)		
	distanceFromCenter = Sin(temp)*temp	
	temp:+1
	If temp <=0 Then temp = 360
	EndIf
	
	DrawText "temp = " +temp,0,0
End Function

It craetes a nice smooth movement inwards but then it stops and hangs.

Any ideas on how I can proceed?