Energy Minimum Road to Outer Space

Energy Minimum Road to Outer Space

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Childhood dreams of personal flight

Posted by Rohvannyn on June 24, 2011 at 3:07 PM

I intended returning to the Moon soon but decided to come much closer to Earth for the next couple of installments.  I’ve been taking some vacation time, during which I read an interesting if not outstanding book called Jetpack Dreams;  One Man’s Up And Down (but mostly down) Search For The Greatest Invention That Never Was,  by Mac Montandon.  This discusses the history and current state of the art of the jetpack, called Rocket Belt when I was young and to make a rather winding story direct, we seem today to be about where we were in 1961 or so when I first encountered the idea on a TV show called To Tell The Truth hosted by Bud Collier.

 

The most significant point I took from Mr. Montandon’s book, largely personal narrative of meetings with the dedicated/obsessed? group of people still trying to fly on peroxide and perseverance, is what a cultural icon the Jetpack is.  The fact that we all don’t have one in out garage or coat closet ready to rocket us to the supermarket or office appears to be in the minds of many, a singular example of how science and technology has let us down. This shouldn’t be surprising to me.  When I was about fourteen I compiled a list of the things I really, really wanted to know about and work on and the list ran about like this, Building flying saucers, matter transmission, antigravity, lasers, robotics, magnetism, controllable solid fuel rockets, alternative space propulsion/fuels/reactions, telepathy and yes, rocket belts.  I’d read in Arthur Clarke’s juvenile primmer Going Into Space that solid propellant rockets wouldn’t be of much significance in space, so of course I wanted to defend them.  I was interested in magnetism and lasers partly because I had an idea for ionizing a stream of magnetic material which a laser, sending it toward the moon and allowing a dirigible in the stratosphere, equipped with a powerful electromagnet in it’s belly, to run along the stream to the moon like a trolley on a tramline.

 

The rocket belt in my mind and those of my friends, would be something that would allow us to cruise pretty much anywhere, I guess more or less like having your own Cessna or one-person copter.  We were pretty impatient with sluggish military developers who were limited to 20 MPH speeds and less than 30 seconds of duration.  That was okay though because when we got a little money and some junk air tanks etc. we’d make something that really worked!  Well why didn’t that happen?

 

Rocket belts do work though they’re damned dangerous things to use and even if we had packs of enhanced power and endurance it’s hard to see how many people would gain the skill to use them.  They work by the decomposition of hydrogen peroxide (H2O2) in a catalytic chamber, generating live steam and superheated oxygen to provide thrust.  Peroxide isn’t a particularly powerful fuel and it’s dangerous in and of itself even at room temperature.  The real limitation of the near-ground rocket belt though is more basic.  I once published some calculations that showed that a rocket hovering just above the ground and using a pretty good propellant such as kerosene and liquid oxygen, would burn up 90 percent of it’s mass in about 17 minutes.  The same amount of fuel could easily send the remaining 10 percent into orbit.  Rocketing is very expensive in terms of energy and therefore in terms of fuel.  For hovering near to the ground, revolving blades are much more energy efficient than jets or rockets.

 

A middle choice between hovering above the ground and orbiting is the comparatively high trajectory rather like an artillery shell’s path;  put on a burst of acceleration initially, coast up, over and down, break your fall at the end of your trip.  Next time we’ll look at what are some practical limits of this strategy and see if maybe there is a jetpack in the future of not only some, but with the help of computerization and sophisticated instrumentation, perhaps many?


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