Here is a bit more background from a physics and sci-fi perspective from wikipedia:

Jet pack, rocket belt, rocket pack and similar names are used for various types of devices, usually worn on the back, that are propelled by jets of escaping gases (or in some cases water) to let a single user propel him or herself into the air or fly.

The concept emerged from science fiction in the 1920s and became popular in the 1960s as the technology became a reality. The most common use of the jetpack has been in extra-vehicular activities for astronauts. Despite decades of advancement in the technology, the challenges of Earth’s atmosphere, Earth’s gravity, low energy density of available fuels, and the human body not being adapted to fly naturally remain an obstacle to its potential use in the military or as a means of personal transport. To compensate for the limitations of the human body, the jetpack must accommodate for all factors of flight such as lift and stabilization.