The X-21 Space Saucer is a replacement model for the copters in the DLC campaign Invasion! They are encountered fequently in all 3 levels and come in great numbers ranging from a measley 10 to a whopping 50. They are one of many whacky model replacements in Invasion! that make the campaign so enjoyable.
- An achievment is awarded to you after shooting down 40 UFOs with your P-51 Mustang in a single level.
- The box art for the Space Saucer says "will not fall off table" indicating that the real life toy persona of the Saucer is a battery operated wheeled toy either being switch operated, wind up, or pull-back.
- The X-21 is based off real life tin saucer toys, one of which is known as Satelite X-11 which sounds very similar to the X-21 Space Saucer.