;constants
(define HEIGHT 200)
(define WIDTH 400)
(define SCENE
  (empty-scene WIDTH HEIGHT "blue"))
(define GROUND
  (rectangle WIDTH 10 "solid" "brown"))
(define ROCKET
  (overlay (circle 10 "solid" "green")
           (rectangle 40 4 "solid" "green")))
(define ROCKET-CENTER-TO-TOP
  (- (- HEIGHT 10)
     (/ (image-height ROCKET) 2)))

;functions
(define (pic-rocket h)
  (cond
    ((<= h ROCKET-CENTER-TO-TOP)
     (place-image ROCKET (/ WIDTH 2) h
                  (place-image GROUND (/ WIDTH 2) HEIGHT SCENE)))
    ((> h ROCKET-CENTER-TO-TOP)
     (place-image ROCKET (/ WIDTH 2) ROCKET-CENTER-TO-TOP
                  (place-image GROUND (/ WIDTH 2) HEIGHT SCENE)))))