Personal Flying Machines

If some miniature J8 jet engines can produce 34 pounds (147 N) of thrust, is it possible that four of them can lift a 120-pound (54 kg) person off the ground and allow him to fly?
- question from D-Per
At first glance, the answer would be yes, but taking further considerations into account, we have to temper that to a big maybe. Here's the crux of the problem. You are correct in your reasoning that the amount of lift force needed to fly is directly related to weight. So if we were to consider a 75,000 lb (34,000 kg) aircraft flying at 33,000 ft (10,000 m), it would need 75,000 lb of lift to remain flying at that altitude. Any more lift and the plane would climb to a higher altitude, any less and it would descend. But for our "personal flyer" example, we are still on the ground. If we exert exactly 120 lb of lift on our 120 lb pilot, he won't move. Obviously, we need more lift, and conveniently enough, the engines you mention produce 34 lb of thrust each for a total of 136 lb (605 N). Therefore, four of these engines should be able to lift our plucky pilot off the ground.

Yet there are some other very important considerations that we have not accounted for. For example, how much fuel do these engines require and how much does it weigh? What kind of structure is needed to carry the pilot, fuel, and engines and how much does it weigh? We also can't forget a control system allowing the pilot to throttle the engines and control the vehicle's ascent or descent. Also keep in mind that our pilot would need enough excess thrust to be able to climb off the ground at an adequate rate. Once we add up all these items, I suspect that we'd need far more than 136 lb of thrust to do the job.

The idea of a flying powered platform or a personal flyer is nothing new. The first examples were based on a concept developed by Hiller Helicopters, an early pioneer of vertical flight technology during the 1940s and 1950s. The Hiller Flying Platform was developed during the mid-1950s using funds from the Office of Naval Research (ONR). Powered by a vectored-thrust ducted fan, the prototype completed its first untethered free flight on 27 January 1955.

Illustration of the Hiller Flying Platform

The US Army also contracted Hiller to build two enlarged models for possible reconnaissance and infantry use. All told, a total of six platforms were built, but only two are still known to exist, both in museums.

Flying Platforms built for ONR and the US Army in flight

However, most of us were probably first introduced to the idea of a personal flying machine by our old friend James Bond who used such a device in the classic 1965 film, "Thunderball."

James Bond (and stunt doubles) using the Bell jet pack in Thunderball

The "jet pack" he wore was indeed based on an actual device, the Bell Aerospace Rocket Belt. The Rocket Belt consisted of two 3 gal (11 L) tanks of hydrogen peroxide that were reacted with a silver catalyst to produce a high-pressure, high-temperature steam. The steam was released through tubes along the sides of the pack producing 300 lb (1,335 N) of thrust to propel a pilot upwards for short hops. The pack could only operate for 20 to 30 seconds before the gas supply was exhausted.

Illustration of the Bell Rocket Pack

In more recent years, the personal flyer idea has once again come back into fashion with a return to the original Hiller concept of using ducted fans. Most modern designs look pretty similar to each other and feature twin ducted fans attached to a backpack.

Trek Aerospace SoloTrek XFV

The above image illustrates the most well-known design, a prototype vehicle built and tested by Trek Aerospace called the SoloTrek Exo-Skeletor Flying Vehicle (XFV). The craft first flew in December 2001, and the company is hoping to sell the technology to the US military with a civilian model to follow. The XFV was powered by a 350 lb (160 kg) four-cylinder engine running on regular 87-octane gasoline and producing 130 horsepower (97 kW). The company claimed a maximum speed of up to 80 mph (130 km/h), a range of about 150 miles (240 km) on 10.5 gal (40 L) of fuel, and a hover ceiling up to 10,000 ft (3,050 m). The craft was controlled using two hand grips that allowed the user to change the revolutions per minute on the ducted fans individually and tilt the fans to produce forces in different directions. In addition to the low-tech solution of shifting the pilot's weight, these controls made it possible for the craft to turn, spin, roll, climb, and descend. The XFV was also able to hover in a stationary position for up to three hours. Development of this technology continues under DARPA funding.

More information on the evolution of personal jetpacks is also available at Marshall Brain's How Stuff Works.
- answer by Joe Yoon, 8 September 2002