One of the more annoying “arguments” against low-cost space travel goes like this:  “Low-cost space travel has been predicted for decades. So have rocket belts and flying cars. Where’s my flying car?”

The person making the statement will then lean back and smile smugly, as if he’s said something very clever and original.

What’s annoying about this argument is that it’s neither clever nor original (although the person saying it always seems to think that he’s the first to think of it). In fact, it isn’t really an argument at all. It’s a logical fallacy. The fact that A and B have been predicted for decades and haven’t occurred does not prove that C, which has been predicted for decades, cannot occur. If it did, then nothing which had been predicted for decades could ever occur!

Beyond the general logical fallacy, there are specific errors in the claims that are being made here.

Flying cars have not only been predicted for decades, they have been around for decades. So have rocket belts (also known as rocket packs or jet belts). Neither of those devices have become popular (or even practical), however, for technical reasons that are specific to those technologies and tell us absolutely nothing about the feasibility or practicality of low-cost space travel.

Flying cars (also called roadable airplanes) have been around since the 1930’s, and they’re still around today. If you visit the Experimental Aircraft Associations’s Airventure Oshkosh fly-in, you’ll almost always find someone who’s working on a flying car. The problem with flying cars is that aircraft and automobiles operate in very different environments. Any vehicle that’s well-optimized for one environment is going to be poorly optimized for the other.

Walking around Airventure Oshkosh, you will see plenty of airframes that are still airworthy after 30, 40, 50, or more years. You can even buy a ride in a Ford Tri-Motor that’s over 80 years old and still approved by the FAA to carry passengers. Older airplanes are more common than older cars. The reason for that is quite simple. The roadway is a much harsher environment, mechanically, than the sky. We’re used to trading in our cars after 10 years or less, but aircraft owners would not be happy with a plane that lasted only 10 years – which is almost inevitable with a roadable airplane that’s driven on a regular basis.

Flying car / readable airplane

Bell Aircraft built a working Jet Belt (actually a rocket belt) in the 1960’s. Derivatives of the Bell design are still occasionally flown by stuntmen today. Building a jet belt is not impossible (or even particularly hard), but the concept has little practical utility due to its technical limitations. The jet belt uses a hydrogen-peroxide monopropellent rocket to provide the thrust it needs to fly. Unfortunately, hydrogen-peroxide monopropellent has poor performance (low specific impulse), which limits the time of flight to about 30 seconds with the amount of propellent a pilot can wear on his back – not really long enough to fly any useful distance.

Why not switch from hydrogen peroxide to a higher-performance propellent? That seems like an obvious solution, but hydrogen-peroxide monopropellent was chosen for a reason. Peroxide decomposition not only has low specific impulse, it also has low flame temperature. Remember that this is a device the pilot will be wearing on his back. Also note that on real rocket belts, the rocket exhausts are located on arms that jut off to the side, not strapped directly to the small of the pilot’s back as shown in comics and cartoons. These are necessary safety precautions. You could improve the performance of a rocket belt simply by switching from hydrogen-peroxide monopropellent to a bipropellent (even hydrogen-peroxide/LOX would work), but you could not modify the pilot to survive the flame temperature.

Another solution is to replace the rocket engine with a turbojet, which has lower fuel consumption and reasonable exhaust temperatures. True jet packs have, in fact, been built. The problem with this concept is that, while turbojets have better fuel economy than rocket engines, their thrust-to-weight ratio is poorer. So, while it’s relatively easy to design a turbojet that can carry a man for several minutes, the device becomes impractically heavy to wear on the ground. Without some sort of support or stand, the pilot would be in constant danger of toppling over. That limitation has led designers away from jet belts to flying platforms on various types. Such platforms are still being worked on, occasionally, today and may still find some practical use in the future.

Another problem with rocket/jet belts is the virtual impossibility of an effective escape system if the engine quits. A pilot who’s already wearing a rocket or jet pack on his back would have a hard time wearing a parachute. Furthermore, rocket and jet packs normally fly at altitudes that are too low to allow time for a parachute canopy to open. They do fly high enough to seriously injure or kill a pilot, though.

All of which tells us nothing about the feasibility of low-cost space travel.


Written by Astro1 on May 4th, 2012 , Space Exploration (General)

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    Gerard Martowlis commented

    Unlike most conventional aircraft such as helicopters, piston driven or jet turbine powered airplanes the Rocket-belt has less than 10 moving parts. With such few parts there is only a small chance of a malfunction. It has a 40+ year safety record that confirms its reliability. It was so safe and so reliable it was seriously considered by NASA as an option to the Lunar Rover as a better form of Lunar transportation on the Moon during the famous Apollo missions in the early 70’s. It was also considered as a viable Astronaut escape pack in the event the Lunar Excursion Module(LEM)ascent engine failed to fire.
    It does however have a limited flight duration here on Earth and it does fly in the “death zone”. That zone’s under 500′ which is not high enough for a parachute deployment yet it’s high enough to kill if a problem should occur. That being said,most of the very few recorded pilot injuries were and are still likely to be just twisted ankles or knees with an occasional fractured ankle or knee-cap due to hard landings. This is to be expected considering the landing gear are your two legs.
    To date there has never been a fatality with a Rocket-belt or jetpack and there are those like myself, who are involved with these fantastic flying devices that aim to keep that long safety record intact. Safety has to be #1 when you have 800hp strapped to your back!

    May 7, 2012 at 3:10 pm