They weren't jet packs, though. They used a hydrogen peroxide powered rocket engine. If you wear one on your back, you're limited by the amount of weight you can carry (remember, you also have a heavy spacesuit to consider). A more promising idea was to use a 2-man rocket powered "pogo" arrangement where both men stood on a platform. The platform bore the weight of the propellants and rocket, so you had a lot longer range. The vehicle was tested on Earth but the risks were considered too high. The idea was dropped in favor of the Lunar Rover, which was both safer and lighter for a given range of exploration. Here's a little info on one proposed rocket powered hopper. Here's another, which is the one I remember reading about. Here's another one.