'Futurists' have been predicting flying cars since shortly after the first cars started rolling out of Benz's factory in the 1890s. They haven't happened. Several reasons
CAVEAT1: I am neither an aeronautical engineer nor a designer of cars. But until someone who is one or the other tells me that I'm wrong and why I'm wrong, here is my take on the difficulty of building what most people probably think of as a flying car.
CAVEAT2: When I talk about flying cars, I'm talking about them in the more or less usual sense of ubiquitous flying devices similar in use to today's passenger cars. A few vehicles that can both fly and travel the roads have been built. More will be. But the hurdles that have to be overcome before a vehicle like that can be found in most every driveway are enormous.
Fixed wing aircraft consist of a fuel carrying airframe wrapped around an engine. It is possible to build heavy aircraft wrapped around high powered engines (the SR71 "Blackbird" for example). But they tend to use outrageous amounts of fuel. Based on data at http://www.aerospaceweb.org/ the SR71 probably gets about 4 gallons per mile. A practical flying car will need to do about 100 times as well -- 25mpg. It will probably be a lightweight vehicle with a small engine. Aircraft of that general type can deliver mileage comparable to an automobile. For example the popular Cessna 172 Skyhawk gets around 15mpg. That's better than many SUVs and is point to point without the twisting and winding that roads do.
A Cessna 172 weighs about 1470 lb (667kg) empty, has a fuel capacity of 181 liters (about 295 lb -- 134kg), and has a maximum takeoff weight of 2300 lb (1043kg). A four cylinder Toyota Camry Sedan with an automatic transmission weighs in at around 3100 lb(1406kg) empty. That's roughly half again the maximum takeoff weight of Cessna 172. And that is without fuel, driver, passengers, dogs, luggage, wings, propeller, transfer case to hook the engine to the propeller, rudder, instrumentation, radio gear, parachutes -- etc, etc, etc. Even the Chevrolet Aveo -- one of the smaller cars currently available in the US -- weighs in around 2350 lb(1066kg) empty. Still a bit more than a fully loaded Cessna.
If we were new to aircraft and automobiles, it would be reasonable to dismiss this weight issue as something that can probably be resolved over time with a lot of engineering and hard work. But both kinds of vehicles have been around a long time. Aircraft are already built as lightly as possible in order to increase range. Cars are already built rather lightly in order to increase fuel economy. A good deal of the weight in each is dedicated to components that are neither needed nor present in the other. Cars do not have wings. Aircraft do not have automatic transmissions. Combining the features of the two into a single vehicle is certainly going to be very difficult. If it is achievable at all, after all the engineering compromises, the resulting vehicle is likely to be a mediocre aircraft and a worse car.
I think that about the best that can be hoped for after much engineering is a mediocre car that can be converted to a so-so conventional aircraft. This might be popular with hobbyist pilots since the vehicle could presumably be stored in a garage, driven to the airport, and avoids a car rental at the destination. It'll still require pilot training, licensing and real airports -- albeit small ones. This probably won't be a vehicle that most folks would commute to work in and it won't be one they would want to drive to the supermarket unless their real car is broken or has been repossessed.
There are a number of technologies that can -- at least in concept -- be used for a personal aircraft. These include:
Lighter than air vehicles have been around a long time. Longer than ordinary aircraft in fact. The first known human flight was in 1709 using a hot air balloon. Of course, we will want the gas bag to fold up and store in the car. And we'll need to be able to propel the thing and steer it where we want to go. But overall, this may be a viable concept.
One key question looks to be how big a gas bag we need. That, of course depends on how much we are trying to lift and what we are trying to lift it with. Let's assume that we are trying to lift that Toyota sedan along with 1500 lb of payload and additional equipment -- gasbag, propeller(s), passengers, etc. Take off weight = 4600 lb = 2087 kg. Pragmatically, there are four current choices for what to lift with.
Lifting our 4600lb (2087kg) payload with Helium needs a balloon roughly 50 feet in diameter. A Hydrogen balloon would be a bit smaller -- about 45 feet. A hot air balloon would have to be around 65 feet in diameter. Taking off or landing any one of these things from a suburban yard might be tricky in calm air -- quite possibly impossible in any sort of wind.
Other than the sheer size problem, there are many other problems with lighter than air personal vehicles. Altitude control is tricky. Power lines, etc are a problem. The air resistance of a 45-65 foot in diameter gas bag is substantial, it is easy to visualize a sudden windstorm blowing thousands or tens of thousands of commuters off to some place they do not want to be. Out to sea for example.
But there are a few virtues of lighter than air vehicles. For one thing, collisions -- while probably more likely to occur than with other personal air vehicles -- are possibly more likely to be relatively gentle bumps rather than fatal mangling. A collision between a balloon and some other kind of air vehicle may not be so benign. Another possible virtue is that malfunctioning balloons may often land relatively gently rather than crashing with probably fatal results.
The Graf Zeppelin was actually used in commercial air service for a number of years in the 1930s with no significant problems other than an emergency landing in France when four of the five propulsion engines failed.
Personally, I can conceive -- without too much difficulty -- fleets of cargo carrying blimps riding the jet streams West to East in the mid latitudes and the trade winds East to West in the tropics. But their virtues would be low cost, not speed. It might even happen if future fuel costs make current means of moving cargo substantially more costly. I can also envision lighter than air vehicles replacing helicopters in some "flying-crane" applications. I can not envision lighter than air vehicles as personal aircraft for everyman. Too big. Too much potential for trouble in bad weather.
This category includes paragliders and other very light personal aircraft. I think that the key word is 'ultralight'. It's one thing to strap on a parachute, 80 pound backpack with a motor and fuel and fly off at 30 or 40mph. It's said to be great fun and safer than it sounds. It would be quite another to hang a paragliding sail on your Toyota sedan, load it up with family and picnic stuff and fly off into a sky full of other flying cars. The parachute would have to be huge, the weather calm, and the force with you. The only estimate I have found says that ultralights are about 6 times as likely to crash as small fixed wing planes.
Ultralights might be a viable commuting/shopping option for a few athletic folks living in semi-rural areas in areas with moderate climates. Less work and more range than say bicycles. But it is not easy to imagine tens of thousands of these things converging on Tokyo or Sacramento at the start of the work day. I can not envision a older person setting off in a NorthEaster for a medical center 10 miles away for a cancer treatment with the temperature around 0F, a howling wind, two feet of snow on the ground and more snow coming down an inch an hour. I did exactly that in a compact car in February 2007 with no particular problem other than the 2 hours it took to get the car 12 feet to the street.
I haven't done a quantitative analysis on ultralights a personal air vehicles, but I really think it very unlikely that the engineering problems can be overcome.
VTOL aircraft are aircraft that direct their propellers upward and take off like a helicopter, fly to their destination like normal aircraft, then descend suspended from their propeller.
This is a perfectly viable concept. Britain's Harrier Jump Jet has been able to do this for decades. The US Osprey tiltable rotor helicopter also is considered to be a VTOL craft. The great advantage of VTOL in a Personal Air Vehicle is that the vehicle need not act like a car ever. It thus does not need bumpers, brake lights, a multiple speed transmission or any of dozens of other heavy, complex components. Of course, the vehicle must still have all the features required in a civilian aircraft.
The problem, as usual, is weight. It takes a very powerful engine and/or a large propeller for an aircraft to lift itself and a payload into the sky. Existing VTOL aircraft work by brute force. I haven't been able to find fuel mileage figures for the Harrier jump jet (yet), but they almost certainly are abominable. The VTOL Moeller sky car which has never flown and is known for its creator's unlikely promises is projected to generate 1200hp from a bank of eight Wankel engines in order to lift a payload of 800lb. Moeller projects a gas mileage of 20mpg. [https://web.archive.org/web/20201018030328/http://moller.com/dev]. Having once owned a Wankel engine vehicle, I can attest the the engines can generate a remarkable amount of power from a very small engine. But their unimpressive gas mileage is one of the reasons they have never caught on in cars. I think it is a safe bet that if the Moeller thing ever flies, it won't come close to 20mpg. I think the prudent futurist should bet that way also.
The TV program Mythbusters attempted to build a VTOL flyer from plans obtained from the Internet. After a great deal of work, they managed to achieve marginal lift. They pronounced the concept unworkable.
VTOL flying cars with acceptable fuel usage are unlikely without some sort of major breakthrough in design.
Helicopters are aircraft that are supported by one or more rotating horizontal propellers. All helicopters include some mechanism -- a second main rotor, a tail rotor, fan driven airstreams -- for preventing the craft from obeying Newton's Third Law of motion and rotating in the opposite direction from the rotor.
Helicopters have been around about as long as fixed wing aircraft. Helicopters can take off and land in very small areas and can hover in place. The hovering capability makes them extremely valuable as 'aerial cranes' and other specialized applications. Problems include noise, vibration, poor fuel economy, and poor stability compared to cars and fixed wing aircraft. The Wikipedia gives the fuel economy of helicopters as 4 mpg but that appears to be per occupant and may be generous for a personal vehicle. Piloting a helicopter is not easy. It has been compared to trying to balance while standing on a beach ball. In fairness, a helicopter designed as a Personal Air Vehicle very likely would get better than mileage although I suspect that gas-guzzling would still be a problem.
(In the mid 20th Century, the Russians experimented with helicopters that also had fixed wings. These were demonstrated and had, as desired, better speed and range than conventional helicopters. They were extremely complex devices and a distressing number of them ended their life in crashes.)
Another possible problem is that the lift achieved by a helicopter is a function of rotor length and the power applied to it. Widespread use of vehicles with rotor lengths greater than a couple of meters would present a substantial parking problem. Considering the premium associated with parking space in modern cities, this rotor length thing may be a non-trivial problem. Experimental designs for human powered helicopters involve rotor diameters greater than 30 meters -- close to the a third the length of an American football field. Obviously, real PAV helicopters would have much smaller rotors driven with much greater power. Still, though, very short rotors may not be compatible with fuel economy.
I have not been able to find many statistics on helicopter safety. The best I can do is 8.74 accidents per 100000 flight hours in the 1991-1998 period http://www.flightsafety.org/pubs/hs_2001.html. Assuming commute times comparable to today's cars -- about 30 minutes each way 200 days a year. That would make a commuter's chance of an accident about one in 57 for each year spent commuting. About 19% of the accidents resulted in fatalities. Make that one fatality for 250 commuter years.
Automobile fatalities are measured in fatalities per passenger mile and most years come in around 1.4 fatalities per 100 million passenger miles. For commuting at an average speed of 25mph and the same 30 minute commute used above, that would work out to 10000 miles a year and a fatality rate of 1 per 7100 commuter years.
That makes helicopters about 28 times as lethal as automobiles and does not bode well for the safety of helicopter commuters. (Don't like someone, give them a helicopter and let nature take its course).
In fairness, the statistics above do not address how helicopters would be used in commuting all that well. On the one hand, commuting would presumably be controlled flight on routes with obstructions and other menaces removed. On the other, the helicopters data is for professionally maintained vehicles driven by trained pilots. There are lots of other considerations. Helicopter safety is a major consideration and might well prevent or limit their use for personal air transport even if all the other factors were resolved.
Helicopters certainly have a place in the world, but there are a bunch of problems that would have to be overcome before they can be personal air vehicles for the masses. The three biggest problems are the need for simplified controls that everyman can handle, poor fuel consumption, and safety. Probably none of these will be easily overcome.
Autogyros are aircraft that look like helicopters, but aren't. They have no wings, but instead are supported by passive rotation of the helicopter-like blade. The blade gets its rotation from the lateral motion of the vehicle. Autogyros are moved by a separate powered propeller or (presumably) jet. Advantages include the ability to take off from very short runways and to land in even smaller areas. Disadvantages include poor fuel economy, difficulty in control, low speed (cars are faster), and inability to operate safely in even moderate winds.
Currently, this is a fringe technology. Barring an unexpected breakthrough, there seems to be little reason to believe that autogyros will be a major Personal Air Vehicle technology. There is at least one available commercial product (a kit) for those who can't live without an autogyro.
Gliders are unpowered fixed wing aircraft designed to maximize lift at the cost of other capabilities like speed. They are typically taken aloft by tow planes. Skilled pilots with Well designed gliders can find regions of rising air (thermals, etc) and ride them to great height. Gliders can achieve remarkable ranges -- hundreds of kilometers -- by flying from one area of air uplift to another. Interesting, but largely irrelevant to the issue of personal air travel. Unless and until someone can figure out how to get gliders to altitude cheaply and efficiently, they simply won't work as commuter craft. that is not to say that there are not a bunch of other problems besides launch that would need to be solved as well because there certainly are.
Jet Packs are backpacks, belts, or simple platforms that will lift a passenger off the ground and convey him or her elsewhere. They do not necessarily use jets for power. Think of them as flying Segways. The idea has been around for many decades often championed by the military. (One wonders if they are planning to levitate some big rocks for their flying soldiers to hide behind if shooting starts). The jetpack type devices -- could they be tamed -- probably would have merits in allowing soldiers to travel quickly over difficult country. Prototypes have been constructed and some have actually been flown. They have proved to be heavy, noisy, often dangerous, and generally difficult to control. At least one product has a record of landing (knee and leg) injuries. Most test flights have been measured in seconds, a few in minutes.
The Jet Pack occasionally seen in movies is apparently a [https://en.wikipedia.org/wiki/Jet_pack] Bell Jet unit produced in the 1960s for the US Army. Although the unit was flyable, the project was abandoned in 1969 for a variety of reasons. (Added Jan2011)
Penn Gilette on the safety of jet packs should any component fail "There is no Plan B" (Added Oct2011)
Barring some unexpected breakthrough, this seems like a technology that is unlikely to go anywhere. Maybe some hobbyist activity. Perhaps a few narrowly focused serious applications. But it doesn't seem likely that families will strap on their jet packs and head off for grandma's place any time soon. Or any time ever.
Here's a link to a commercial product that will purportedly ship in Aug2008. Range .75mi (about 1200m). Flight time up to 75 seconds. (Personally, I'd like to know a bit more about landings before I sign on). link And here's another on 'jet packs' in general
The Coanda Effect is a phenomenon which causes fluids (e.g. air) to follow concave surfaces. Attempts were made in the 1950s to utilize the Coanda Effect to lift disk shaped aircraft that would be able to take off and land vertically lifted by airstreams flowing over their concave surfaces and directed downward. Specifications called for useful speed (480 kph), altitude (3000 meters), payload (1200kg) and range(1600km). The experimental vehicles actually did manage useful lift but proved to have serious and intractable stability and performance problems. Work on these vehicles ceased in the 1960s. There is a possibility (very remote in my estimation, but what do I know?) that advances in computer and airframe design could someday lead to a Coanda effect flying car. (added Jan 2010)
Hovercraft float on a cushion of air slightly above the surface of the ground or more often water. The first experimental vehicles date to the 1930s with more serious attempts starting in the 1950s. A Hovercraft was flown across the English channel in 1959. Hovercraft are currently used as ferries, as military vehicles and for transport in some arctic regions with poor roads and even as barges in swampy areas. Their great virtue is their ability to travel over water, ice, sand, swamp, and dry land about equally well.
Regrettably, this is a fringe technology and likely to stay that way. It is not especially fuel efficient. It moves in two rather than three dimensions as the vehicles never get far above the ground. (Could be why they are called 'ground effect machines') It often has problems with tight turns. It simply doesn't seem to offer any significant advantages over conventional cars for those who live where there are roads.
US scheduled airlines have a truly remarkable safety record. In 2003 the fatality rate for was 0.3 deaths and serious injuries per million flight hours. Commuter airlines came in at about twice that. Other commercial activities like crop dusting came in at about 10 times the scheduled airline rate. Given a usage of 200 hours per year which sort of equates to 10000 miles per year on a car, personal air vehicles with a accident rate similar to scheduled airlines would give the user a fatality rate around 1 fatality every 16667 years. That number is probably high for a number of reasons -- the most important of which is that commercial aircraft typically fly with dozens if not hundreds of people on board whereas commuters typically travel alone or, at most, with one or two passengers. Modern US passenger vehicles typically run up fatality rates of 1 to 2 fatalities per 100 million miles. Call it 1 fatality every 8000 years. Yes, the big jets really are probably a bit safer than cars. Not dramatically so, but a bit. Commuter airlines are probably a smidge worse than cars. And yes, there are other ways to figure the numbers that come up with quite different numbers. You'd have to be fairly dumb (or well paid) to buy into numbers that say that commercial aircraft are 30 times as safe as cars. Or that cars are dramatically safer than planes. Neither is true in any meaningful way.
But the real issue here is that Commercial airliners are maintained by well supervised professionals according to meticulous schedules. Planes are checked at a maintenance facility every few days and are scheduled so as not to exceed some maximum number of hours between checks. Engines are overhauled after n hours of use. They don't wait until the engine makes a peculiar noise that can't be ignored any longer. Anyone who expects that Personal Air Vehicles can be maintained the way that automobiles are typically maintained without a dramatic increase in travel injuries and fatalities is probably in for a surprise. Once again -- Broken Cars Stop, Broken Aircraft Drop. Yes, there are a few car problems that can kill you. Having a ball joint come apart at 70mph is certainly going to make that clear. And there are some aircraft problems that are non-lethal especially for aircraft that can glide; can land in small places, etc. But for the most part, a broken car is going to end up pulled over to the side of the road. And a broken aircraft is going to end up wrapped around or inserted into some landscape feature.
Another issue is driver training. Driving a car is pretty straightforward compared to driving many kinds of aircraft. In fact, learning to drive a car is not that simple, and we generally require people to take a driver's test before we permit them to drive. Piloting most types of aircraft requires more skill and even better judgment than piloting a car. Maybe computers can take over part of the job. Maybe not. The software is going to have to be a lot better written than, for example, Microsoft Windows or the firmware that controls and reports automobile diagnostics. The phrase 'Blue Screen of Death' takes on an entirely new meaning when your flying car has pitched over to vertical and is accelerating into the ground at full speed because an overworked, underpaid programmer left a semicolon off a line of code.
Liability is yet another issue. Small aircraft manufacturing in the US has purportedly been more or less shut down by lawsuits. Without some sort of enabling legislation that moderates the lawyers, it is unlikely that flying cars (in the US anyway) could be insured. Manufacturers will surely be sued by everyone within 100 yards of each and every emergency landing. They will be sued for a lot more if there is injury or damage.
Even today, aircraft insurance is complicated and is generally more expensive than automobile insurance. That makes sense. The chances of serious injury in a crash are greater in an aircraft and the scope of things you can easily run your aircraft into and damage is substantially greater than with a car. Will insurance on flying cars for every man be affordable? I haven't got the slightest idea.
Being a little slow, it took a couple of years for me to notice that on top of everything else, there is a security issue with regard to flying cars. If conventional vehicles can regularly be loaded up with explosives and used to blow up buildings in Iraq, Pakistan, Afghanistan, Spain, Germany, and Oklahoma City, think the potential for flying cars loaded up with a few hundred kilograms of explosives. Would a world with hundreds of millions of potential cruise missiles be all that comfortable a place to live?
There is a list of over 100 old and current, proposed and actual, flying car efforts at http://www.roadabletimes.com/alphalistingpage6.html. Here are a few of the current or recent projects:
Projections are for a four passenger vehicle sometime in the next few years at around $500,000 per copy with prices dropping to the $50,000-$60,000 when (if) serious production is undertaken. A pilot license will be required and initially take off and landing will be at airports. "Highway Speed" will be around 30-35mph. Mileage and load capability are do not seem to be specified in either the Performance or FAQ sections of the website.
This is to be a two person road drivable vehicle with folding wings. It is to be about the size of a large SUV. It can take off and land at small airports. It it to be street legal and is hoped to qualify as a Special Light Sport (S-LSA) vehicle which can be flown by trained pilots under relatively unrestrictive rules (e.g. does not require a current medical exam). Payload will be around 550lb (250kg) minus weight of fuel. Gas mileage in flight around 23mpg at 75% of full engine power. Takeoff weight less than 1320lb (600kg). Cruising air speed will be around 115mph. Successful test flights have been made with a 20% scale model. Orders for vehicles at around $148000.00 are being taken for 2009 delivery.
[http://web.archive.org/web/20130129145439/http://strongware.com/dragon/] Have a hankering to build your own flying car? Check out this web site. Can you really build a flying car with this technology? I don't have the slightest idea.
A $37,000 three wheel motorcycle with a folding ten foot diameter autogyro propeller. It is asserted to have a highway speed of 55mph, a flight speed of 100mph and a lift capability of 280lb. It is sold as a kit and qualifies as an Experimental Home Built Aircraft. There are a number of options including a parachute (I personally would not leave home in one of these things without one, however, I kind of wonder how one deploys a parachute from a vehicle like a helicopter or autrogyro with a rotating blade overhead).
Another motorcycle autogyro hybrid similar in some respects to the SuperSkyCycle. Preliminary specifications appear to call for a single person vehicle with a range of 600km (300miles plus) either on the ground or in the air. Takeoff distance is specified as 50m=165ft. Landing distance 5m=16ft. Speed 195k/h=120mph. It will require a Sports Aviation License for flight. Pricing and availability information does not seem to be available. In fact, there doesn't even seem to be a prototype or proof of concept vehicle.
A ducted fan VTOL aircraft. Apparently working on models and prototypes. No indication of range, payload, fuel consumption, driver licensing, limitations, cost, or availability as of March 2008
A car with folding wings. Propulsion is vehicle mode is via the wheels. In Aircraft mode, a pusher propeller is deployed. Wings telescope rather than unfolding. It is to be a 2 passenger aircraft or 4 passenger car capable of flying at 150mph and driving at 70mph. Curb weight is 2400lb, payload (including fuel) is 800lb. Range as an aircraft is 500miles. Mpg is not specified. Price and delivery is unavailable. A prototype is projected three years after funding is secured and first deliveries of experimental vehicles two years after that.
http://www.roadabletimes.com/roadables-integ_wernicke.html A four to six passenger, front propeller, car with (small) folding wings that received considerable publicity in the late 1990s. Plans were for 65mph as a car and 200-400 mph as a plane. Wernike's Company -- Sky Technology Vehicle Design -- seems still to be in business, but the status of the vehicle is in doubt. Consensus seems to be that Wernicke was unable to get adequate funding to develop and sell the vehicle.
http://www.roadabletimes.com/alphalistingpage6.html Unlike most flying car companies, Macro Industries is a real manufacturing company with a number of tangible products including an unmanned aircraft called the Scout which is appears to be similar in design to their proposed aircar. The Skyrider is to be a VTOL aircraft with four ducted fan engines. Availability is five years from obtaining funding. Estimated cost $500,000-$1,000,000. No information on weight, payload, pilot licensing, mileage, etc.
http://www.nasa.gov/home/hqnews/2007/aug/HQ_07199_personal_air_vehicles.html. This is a collection of modest monetary prizes offered by NASA for competing in several areas. Most are related to space exploration, but one category is devoted to Personal Air Vehicles.
http://www.ip.nasa.gov/cc/index.htm NASA has defined seven prizes to by competed for in the 2007-2020 time span. The total budget is only $5,000,000 so no one will get rich nor, for that matter, probably cover expenses. But the list does give an idea of what NASA feels is achievable, but can not quite be done today. Your first (non-trivial) challenge would be to figure out what the challenges actually are http://www.ip.nasa.gov/cc/cc_challenges.htm#pav. I couldn't figure them out. If you have more interest and a longer attention span than I do, you may be able to.
The challenge will continue annually for four more years. This year's competition establishes baselines for more difficult standards next year, when the total prize money will increase to $300,000. The total prize money provided by NASA for all five years is $2 million.
I have no idea. If the question is when will I (or, more likely, my descendants) have a flying vehicle in their garage that they will use for commuting, trips and local errands? I'd say twenty years minimum (around 2030). More likely fifty probably (2060). Very likely never. But that is just a guess. What will it look like? My guess would be that it will be a helicopter rather than a VTOL aircraft and that it will be completely computer controlled because I just don't see the folks who have trouble managing a cell phone and conventional car simultaneously being able to safely navigate a vehicle in three dimensions -- even without the cell phone.
I think that long before we see the vehicle described above, we will see some roadworthy conventional planes that manage to fold the wings and control surfaces out of the way and have backup lights, brakes, and all the other paraphernalia of a car. I think they will probably be OK aircraft and awful cars. But they will allow pilots to drive to and from small airports and will provide short distance transportation at the destination. They will require pilot training. They will not be for everyman and will not be a general replacement for the family car. (I came to that conclusion before I became aware of the Terrafugia Transition I imagine that we will also see a wide variety weird devices that fly well enough for demonstrations, but are totally impractical for real personal transportation.
Of course I could be wrong. Even if my analysis is dead on, there is always the possibility of one of Nicholas Taleb's "black swan" events. For example, we do not know that a gravity nullification/antigravity device is impossible. The only obvious constraint on such a device if it is possible is that it must consume at least as much energy as could be generated by using the weight it lifts to generate power in a lossless machine as it comes back down. That's not a lot of energy and conceptually we could even recover the energy during landing if we chose to. The problem is that we have not the slightest idea how to build such a device.
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