a piece of cardboard, 2 Two-liters and flames of felt...

Supplies:

• Two 2-liter bottles, empty and clean
• Silver spray paint
• Cardboard
• Webbing or Ribbon
• Hot glue/gun
• Felt in Red, Yellow and Orange
• Scissors
• Hobby Knife

Step One:
 Begin by cutting a piece of cardboard that fit the width of the back.  Cut a slit in the top edge and bottom edge of the cardboard.  Slide ribbon or webbing in the slits and hot glue or staple them to themselves.  Use some old backpack webbing and staple or hot glue in place to go around arms.
You could use hot glue instead of staples...we just went for quick!
You can add some ribbon or webbing across the middle to tie together or use a buckle to help stabilize the rocket on the back so it doesn't slip off the shoulders while running.

Step 2:
Clean and dry the soda bottles.  2-liter bottles are the best, but you could use any size of plastic soda bottles.
 Lightly mist the tops of the soda bottles with silver spray paint.  I didn't bother to paint the backside of the bottles, because they will be glued to the backboard and won't show.

Step 3:
Cut flames out of felt in red and orange and yellow (if you have it) for each rocket engine...turbine...soda bottle.
this could get fancier, with streamers and tinsel...but it doesn't have to.  You could even use old grocery bags from the grocery store.

Hot glue them in the spout

Last Step:
Hot glue the bottles onto the cardboard.

Source: Super Sci-Fi Rocket Fueled Jet Pack--Upcycled Craft DIY