I'm not a aeronautical engineer but I have spent a lot of time flying at high altitudes. 20km ~ 65,000 ft and propellers just don't work up there (pure jets & ram jets not fans are best). Also, even if they did I don't see a "zero ground speed" as being possible at those altitudes (airfoils need dense air or speed).
For everything I've flown the airspeed between stall & critical mach # becomes very small with increasing density altitude. The U2 has a very thin operating envelope at those altitudes & although slow it definately doesn't hover.
Ron
--On Sunday, May 15, 2005 8:49 PM -0700 Jones Beene <[EMAIL PROTECTED]> wrote:
Is it possible to position a large unmanned aircraft in semi-permanent stationary "quasi-orbit" but at only 20-50 km in altitude?
This concept would allow such things as a really cheap replacement space telescope, or an antenna array high enough to broadcast television and broad-band communications all over a vast undeveloped area, such as the state of Alaska, say, or Afghanistan... or many other applications where satellites are too expensive an option, or don't have enough power. But it could even be a high "launching pad" for small satellites itself - IOW it would be like dispensing with the first two stages of launch.
Maybe it is possible soon. There has been a lot of interest in space tethers here, and jet-stream windmills. Both just out of reach with present technology. This concept is a little of both and I don't think this slant on tethers has ever been considered even though it is (probably) within the reach of present technology. It appears to me, having just dreamed this up - that combining two ideas into a hybrid could get this field 'soaring' in the near term without the need for a major breakthrough in structural materials (such as a stronger fiber than Kevlar/Dnyeema)...
The problem now is that the best fiber is just not strong enough for useful tethers which allow stationary earth orbit, of the geo-synchronous variety - but they are strong enough for several tens of km, now - as long as you do not have large loads like dealing with a wind-mill in the jet-stream of several hundred MPH. And tethers have been made electrically conductive with little penalty. Last month Robin estimated the longest Kevlar cord that would sustain it's own weight is 290 km. For Dyneema it is a bit longer.
Extrapolating from available information, lets look at some 'ball-park' figures. Lets say we have an electrically conductive cable-tether, actually three of them which can carry 1-10 megawatts of high-voltage three-phase AC power to 20-50 km of altitude to power the craft in question electrically from the ground-up. Is that possible?
You would have to construct the three tethers in such a way that every 10 meters or so you have a small pulsing strobe light, visible during the day. This, and an electronic warning signal, plus robotically controlled "tower operators" radioing-out to pilots in the neighborhood - would help keep the tether from being hit by other airplanes.
We want to get a large drone airplane above the weather and just slightly above the jet stream, but still at an altitude with enough air pressure for large efficient propellers to work against. Unlike the "jet-stream wind-mill" idea, the cable can be much lighter than the windmill version, even though it is higher in altitude, but of course it is using-power, not producing it it.
Let's "bootstrap" another concept in order to put a "stationary" craft and a succession of small payloads up there cheaply, and keep the craft there (above the weather) for as long as the materials and electrical motors, robotics, etc. will hold up with limited maintenance - maybe 5-10 years. We can lift additional loads up to it, assuming that the loads can be light and "KD" and assembled robotically on the craft in question. This would be beneficial for a high altitude ultra-cheap launch of payload to even higher altitude.
BTW, KD is a term used in manufacturing in the USA to mean "knocked-down" which implies the product is designed so that small parts can be disassembled and easily reassembled (usually by the customer, like the cool stuff at Ikea). It is kind of an anachronistic term nowadays as the parts were usually never assembled first to make sure that they fit, as was once done.
OK this is going to get a bit complicated. After all, it is a hybrid. First - Check out "Flying circles around the helicopter" in April 30 edition of 2005 "New Scientist" by David Hambling http://www.newscientist.com/article.ns?id=mg18624971.600
To paraphrase: As far back as1956, fixed wing missionary pilots learned this trick, which few others have heard of, even today, although there is no good reason why it cannot be scaled up to a much larger size.
Basically, in situations where you can't afford a helicopter, a low speed aircraft which can fly in tight circles over a "bull's eye" drop point where a tether (rope) is lowered. The tether naturally begins to take the vortex inverted-cone shape and eventually can touch earth at a **single stationary location** Items can be lowered or raised on this rope. The tight circular flight path combines with the forces of gravity and drag to hold the rope almost motionless at its lower extremity, reminiscent of the tornado vortex.
This "bucket drop" technique has proved invaluable for certain situations using small planes but it has been largely ignored as to scale-up - until now. Lately, a team of engineers at the Royal Melbourne Institute of Technology in Australia are exploring the same basic principles to devise a more sophisticated air delivery system. They are working on an automated device that will allow them to pick up and put down large loads - including people - with hardly a jolt. If their system is successful, it could be revolutionary... but they have perhaps missed the best use of all.
Rather than use a traditional airplane to carry a tether up for a few hours, just to replace what a helicopter can do - now we are going to power an electric, very large drone computer controlled airplane, from the ground-up, via the conductive tether, which is still strong enough to raise a succession of light KD payloads for launch form that high altitude - or many other potential uses.
Manning the craft with humans is unnecessarily risky. The payloads, perhaps one of them being the next version of Hubble in KD form, can even manage to self-propel themselves up the stationary tether using power induced from the tether itself to drive their own propellers. They will not need to be pulled up.
Of course, you can just mount items like a lower altitude space telescope on a specially designed single purpose drone aircraft, rather than launch one more cheaply, but it would be a problem to keep it focused on one spot in the cosmos while the plane was making continuous tight turns.
That 20-50 km long electrically conductive tether (actually it would have to be longer to account for the slack in the vortex), will power a rather large robotically controlled airplane, driven by electric motors once in place, and designed to automatically operate at this altitude in continuous tight circles, constantly adjusting and correcting for the wind speed etc, using a few xbox computers, of course - and the tether is always plugged into the same megawatt (could be more or less) power source, which could itself be railcar mounted.
Because so much power is available from the stationary source, there is no need for solar cells on the craft, as they can hardly carry their own weight anyway.
Jones
