SkyLifter is an eVTOL air crane(skylifter.eu) |
SkyLifter is an eVTOL air crane(skylifter.eu) |
250 tonnes is A LOT to lift. Look how big a hot air balloon needs to be to lift a few passengers in a basket. Sure helium has more lifting power but it doesn't create miracles. That means the balloon would have to be absolutely massive. Far bigger than the CGI renders they show on their site. Helium is very expensive and a lot of it will leak out of a balloon like that even if you can reclaim the bulk of it back at the storage site.
I'd like to be proven wrong because the idea seems wonderful but the only thing they may be successful at doing is lifting some money from the hands of some gullible investors.
Agreed, per my calculations it'll need to be around 120m wide, and 30 deep (lifeisstillgood comes up with slightly smaller numbers of 100 x 25, possibly because they used 1.1kg / m3 where I used 1 for the lifting power of helium). That's more or less a baseball field.
And it's just for the naked payload, you still need to account for the envelope itself, the solar panels, the cabin, the engines, the batteries, and the fuel (as, per site, it has a biofuel backup). That probably adds a dozen tonnes at least (the solar panels alone would be 5-10 depending how much of the top's surface is covered). By the end you're probably looking at a men's cricket field, one on the large size.
Even if we ignore the lifting gas requirements just the structural requirements to be able to tie 250tons to the lifting body make this pretty much impossible.
This makes solar roadways and the air condenser solar water bottles seem viable.
For comparison the Airlander and P-791 only lift about 10-20 tons…
https://en.m.wikipedia.org/wiki/Lockheed_Martin_P-791
https://en.m.wikipedia.org/wiki/Hybrid_Air_Vehicles_Airlande...
TBF it was 10t cargo on top of crew (40), passengers (50), and luggage. So probably closer to 15~20 useful lift.
https://www.skylifter.eu/air-crane/
So is this reaction just getting caught up on combining images of the small model, with the load capacity of the (theoretical) biggest model? There's lots of things, like cranes, that have a fairly standard size and a few mega sized examples used for very specific tasks.
This whole thing, like that flying wind-turbine thing or the solar road thing, is just bordering on plausible but so far away from feasible that one has to doubt the whole effort.
Its hard to imagine, in todays world of information at our fingertips that anyone could fool or misguide someone else, but as the current state of the world is, not only is a con job possible, but its getting easier and more profitable. The trick seems to be to be to make an idea, as I said above, bordering on the plausible but far away from feasible.
Not sure if there is an alternative to helium or hydrogen, like maybe a void container, but it would still be heavy.
I guess they will suffer from bad press if they can't prove this can work. Easy to lie to investors but unless they have a prototype, I'm still very skeptical.
Does it scale? Can you strap 10 of those things together (perhaps using some sort of scaffolding) to lift 10x the weight?
If you're strapping 10 things together, you're now dealing with a much less rigid structure, the straps/scaffolding are significant points of failure, and the attachment points don't scale the same way so you have to add an even more complex (and tangle-able) web of cabling for your payload.
The lift is not so much the problem (although that's a big balloon) - fighting wind sheer will require stupendous amounts of power. Especially as wind is pressing not just at that cargo but at the balloon itself. And anywhere that's remote / inaccessible almost always comes with lots of wind.
Hmm, actually lift might be a problem ...
So to get 250 000 kg off the ground takes a volume of helium balloon (if the below site is right) of 250,000m3 (it seems to be fairly linear)
(this seems intuitive - air "weighs" about 1.2kg /m3 sea level, so a vacuum of 1m3 could lift 1.2kg. )
that is 100mx100mx25m ! that's the inner part of any athletic stadium. That's insane surely?
https://www.engineeringtoolbox.com/hot-air-balloon-lifting-f...
That’s huge!
I wondered how big was the hindenberg, wiki claims 245m long and 41m wide, and 41m tall (excl. cabin). Only 200k litres of gas capacity. 232k tonnes lift.
But back-of-the-envelope lighter than air is just hard. It's certainly possible to see this working in niche areas, but boy, it's a tough sell compared to "get a giant Sikorsky"
Turns out, so far, it's all been pretty shit, because outside of super specific niches airships are so slow they need to be much cheaper than aircraft, but the upfront costs to create massive-enough airships to even have a chance makes it a difficult proposition.
It's a very sad read.
Imagine the splash you could make at your friends yacht meeting when you float in from above. Watch emperor penguins migrate or elephants in the Okavango delta from the comfort of a couch.
What am I missing, why isn't this a thing?
I wonder how does the fuel usage compare to a plane, that's the real measure of green-ness.
Edit: yes! I was reading that wrong. To quote the relevant part: “SkyLifters fly from job to job, so they can do work in locations that would be challenging for land cranes.”
Nah they're absolutely gigantic and usually semi-rigid (especially if you have solar panels on top).
SkyLifter seems very cagey about the purported crane's dimensions (couldn't find anything on the subject), but the Airlander 10 (which I understand only lifts 14t) is 90 x 43 x 26... meters.
IIRC the rule of thumb is that you need 1m3 of helium to lift 1kg, to lift just a 250t payload you thus need 250000m3 of helium.
According to skylifter's "design" page the lenticular envelope has a height of 1/4th the diameter (3 units wide, 0.75 units deep). For a cylinder that's 108m wide and 27m high, so taking in account the lenticular shape we might be closer to 120m/30m (400ft wide by 100ft high). That's the size of a baseball field, and 14 stories high.
And that's only taking the payload in account, not the solar panels, not the cabin, not the engines, ...
The force produced is the weight of the mass displaced. So a hole in the air would lift up an equivalent mass to what the air in that hole would have weighed. For a ship this is a heck of a lot, since water is a lot denser than air. You can have a huge cruise liner because the hole it makes in the water would weigh 1T/m3, which for the size of such a ship is enough to lift up a massive chunk of metal. But air is nowhere near as dense, so even a cruise ship water-hole sized balloon with magic gas that weighs nothing would only be able to lift ca 1/1000 of the ship.
One trick you can do with air is heat it up so that the atoms push the sides out more, making your balloon bigger for the same mass. I'm not sure how to quantify the difference though. I suppose you take the ideal gas law and say it's proportional to the absolute temp? Limited how hot you want to make the thing that's naturally at about 300K.
Can't tell how big they intend to make this but it just looks like isn't big enough. A big enough balloon would also give rise to other engineering issues, like what do you do with wind and how to you keep the helium inside?
Also if this kind of thing generally worked every moving company would have one. Stick a balloon your sofa, get the kids to take it to your new house.
1) have counterweights (e.g. as mentioned above for another similar concept Cargolifter which used water);
2) expand its volume during loading and contract after unloading. E.g. pumps compress gas while unloading and release it back while loading.
These two options are obviously the same thing - change the vehicle density to adjust the buoyancy but implemented with and without external weights. Submarines do this also by filling the ballasts tanks with either water or flushing them with compressed air.
The only thing I could come up with is to have neutral buoyancy air blatters that can expand/contract. These would be inside the volume of buoyant gas but connected to the outside air. Then pump the buoyant gas into a pressure tank to reduce buoyancy, or release it from the tank to increase buoyancy.
(Not saying if any of those is viable, just mentioning in case it makes the conversation more interesting.)
I can imagine that seriously hampers it's profitability when it can't be used 90% of the time because the wind threatens to be more than 10 mph...
The wind forces on the surface would be extreme, and you’d need a lot of power to even stay somewhere stationary instead of flying away. All of this would add weight, increasing the volume of helium needed, increasing the wind exposed surface area, making the problem worse.
You’d need a huge industrial hall to store it safely on the ground, and you can’t tie it with a rope anywhere.
The helium required to fill the balloon would make the operating costs of this thing absurd, cause it’s a lot of helium, that leaks all the time and have to be replenished, and helium is super expensive (and necessary for pricy things like cooling superconducting magnets applications, which have infinite budget).
Any 3rd year University student with basic knowledge of fluid mechanics, thermodynamics, and mechanic, can draft in a napkin why we already know that this can’t work.
I’ve sat in meetings where the VCs contracted me and others to do DD on these types of companies, where every contractor told them that this can’t physically work, and the VCs with too much money take that knowledge and translate it to “very high risk investment”. These companies got 100 million dollars and burnt through them, without nothing to show, but for the VC was more important to not have that money parked somewhere and be able to tell investors that they have “mobility companies” in their portfolio, which I think is borderline misleading / scamming because everyone knew that these companies “concepts” couldn’t physically move a thing, cause that was a physical impossibility with the tech that they wanted to use.
Yes, airships are expensive, inconvenient and slow, they leak helium and they require huge hangars and mooring masts, but that's far cry from "can't work".
The FAQ and literally everything else about this project is vague, disappointing and a little fishy.
I'm not sure though that fabrics exist which are both stretchable and can hold on well to small molecules like helium. (e.g. mylar can be stretched only 4% at most.) Maybe some type of "accordion" type connection between the bottom and top halves of the saucer-shaped balloon would work.
And I doubt one would be flying something like this in bad weather anyway. One would just wait for the next weather window.
And one could easily take the ship into 4000' or something like that and still have minimal amount of other aircraft around once outside of airports (proper commercial jets fly at 40k feet or so).
Actually, no. It can't maneuver fast enough, it can't gain altitude fast enough. A downdraft or a squall and it's going into a hill or a tree from 1000ft in no time.
Only good thing about lighter-that-air is that when they crash, they tend to crash slowly. Otherwise completely useless.
Not sure how you reached 411, 245 x 41 would be 323k for a straight cylinder, which the hindenburg was nowhere near[0]. And the envelope was larger than the useful gas volume owing to the internal scaffolding, and the internal volume being subdivided into 16 cells separated by structural rings[1].
And the 232t useful lift was pure lift, before taking in account the 215t of the airship itself (at basic empty weight). The useful lift (payload) was 40 crew, 50 passengers, their luggage, and 10t cargo.
[0] https://3iz4pu1r2cxqxc3i63gnhpmh-wpengine.netdna-ssl.com/wp-...
[1] https://3iz4pu1r2cxqxc3i63gnhpmh-wpengine.netdna-ssl.com/wp-...
I got to 411k by 245x41x41 (figures in GP). Yes that's not a cylinder but I am just doing very rough maths.
I did not think about the useful lift - so while the Hindenburg is a 100 years old, a lift of 232k kg was competing with a empty weight of 215. incredible - and thanks for the links
It still seems back of the envelope that lighter than air lift is just not generally useful
So the issue with this thing is, any time you move an actual load from A to B, you need to have a weight waiting for you at point B. Or to offgas at which point you won't be able to make a second trip without regassing.
The problem is not building the thing (though it is one problem), it's that if you're using this as a crane you're moving an entire baseball field with a decimeter if not centimeter-scale precision.
I have no intuitive feel for how heavy 250 tonnes is, but even if current crane tech can move that weight with centimeter precision, they're explicitly marketing this for tasks where the current tech has issues with access, which implies they don't think it competes directly with traditional cranes on their own territory, but does work well enough in specific circumstances to be an option.
Maybe its a bad idea overall, or unrelated business model issues render it a dead-end, but beating the current product in every concievable metric in every concievable use case is not necesssry for a product to be a success.
a) powerful enough to defend against wind acting against such a huge object
b) responsive enough to change their thrust vector (power & direction) such
that the associated PID loop can operate within some reasonable set point
boundary
I guess this is a long way to say i don't think it'd be easy or cheap, but i don't think they'd have to make a break through innovation to make it possible.
This is going to be a light, fragile airframe with much bigger 'sail' in the form of the airship - its the size of a stadium.
Nothing short of a rocket engine will be able to keep it in place.
"Can't work" is a pretty good effective compression of "work on the barest of technicality while being more expensive, slower, less flexible, and less reliable than pretty much all alternatives".
Also the Hindenburg was absolutely not able to take 100+ people across the atlantic, half of that was crew which you don't usually consider "taken over" as they have to fly back. So Hindenburg took 3 days to carry 72 passengers across the atlantic, needing 40 crew to do so, at a ticket price of $7460 (one way, in modern USD).
By comparison, bloody Concorde transported up to 128, with a crew of 3, in 3.5 hours, at lower prices.
Pretty sure the crew of Hindenburg included a whole bunch of waiters, cooks, etc. just like the Concorde had flight attendants on top of its flight crew of 3. Hindenburg was basically a small cruise ship in the air.
https://www.airships.net/hindenburg/interiors/ has some nice drawings/photos about the interior.
Basically traveling on it was more like a nice sleeper train with cabins and a good restaurant than a modern airplane.
For comparison a commercial plane service didn't even exist for airfoil planes in 1937. So by your argument it was impossible to fly across the Atlantic (it wasn't actually, people had done it but your argument is uneconomical=impossible) , but nowhere close to bringing 100 passengers across.
You can also build a car with square wheels and cross north america in one.
> I'm not sure I understand your point, are you saying that airships don't work?
No, I'm saying that companies that try to sell airships don't work.
These companies are good at raising money, because the idea they sell is simple to explain, and many people don't do due diligence.
But in the same way that a car with square wheels is a worse solution to the problem, along pretty much any axis of comparison, than any of the alternatives available in the market, so are airships. With the difference that airships are also way more expensive than the solutions that are already in use.
This is why companies that build and try to sell them, default in 3-5 years after having raised the money.
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The only types of airship that "works" are balloons able to cary 2-5 people. The weight is small, so the balloon surface is also small against the wind, the components required to drive them are simple and easy because the balloon is "small", and their use is purely occasional and recreational (or a tourist attraction where you can charge a bonus), so cost isn't a "huge" issue. People pay for the "fun", not to go from A to B.
Because the goal of small craft is to go with the flow wherever the wind brings you, because that's fun.
The goal of commercial crafts is to go or transport stuff from A to B, independently of wind, and they are really bad at that.
I don’t know for sure but i suspect the airframe of this concept is within reach of current tech.
https://www.youtube.com/watch?time_continue=57&v=iEsLQd_yY0s...
If your claim is that there is a market for luxury air yachts that are extremely expensive to maintain, require a huge crew 10x larger than a yacht, are 10x "smaller" than a yacht, and can't remain stationary somewhere nor from A to B, but instead only can go "wherever the wind goes", then I don't believe you.
Feel free to prove me wrong. But the amount of yachts in Montecarlo during the F1 grand prix suggests that a significant market of the luxury yacht market does care very much about going from A to B, and about being able to remain stationary in their yacht somewhere.
So not being able to go from A to B and having to put your "yacht" inside a crappy hangar without views to be able to stay at one particular place pretty much kill the idea.
FWIW I'd be really interested in a writeup of your actual napkin math that convinces you of the infeasibility so strongly.
Archimedes says:
m = V * (density air - density helium)
that is, the total mass that you can carry (including helium, propellers, fuel, all of it), scales linearly with the volume V of the balloon, and the difference in densities between the gas inside and outside.
There is little that you can do about the density difference beyond heating helium to make it larger, but at equal temp on the ground its 0.1, so you end up with:
m = 0.1 V
so if you want to carry 120'000 kg, then you need 1'200'000 m3 of helium.
That's a lot of helium. If you put it in a ball, the radius would be 66 m. or 132 m diameter. The wind surface of the ball would be 13'685 m2.
The force of air on a ball is obtained from the definition of the drag coefficient:
F = density_air * C_d * v^2 * A
The C_d is worse at low Reynolds number, so lets fix that at 1.0 (worst case, it gets slightly better as the flow speed increases, but whether its 1 or 0.8 it doesn't really matter).
So if you want to stay stationary at 20 m /s winds (72 km / h), then you need to apply a force of
F = 1.225 kg/m3 * 1.0 * 20^2 * 13'685 = 6.7 MN
The power required to produce similar thrust via a propeller is more or less
P = sqrt(F^3/(rho * A_propeller))
so with 10x 2m diameter propellers, you'd need
P = sqrt(6.7^3 MN / (1.225 * 10 * pi * 1^2)) = 2.8 MW
of power. If you actually wanted to move against the wind at 70km/h you'd need almost 6 MW.
That's not "a lot" but is not "nothing either".
If you take Rolls-Royce AE 2100 engine powering the C130 Hercules, each engine produces 7.5 MW. These engines aren't "silent", so riding in this thing isn't going to be nice, but no engine in this league is silent.
Considering that you also need electricity and other stuff on board for a crew of 20-40 people...
You end up that for 120 tons, you need at least one cargo aircraft propeller engine to be able to move at 70km/h against the wind (which is pretty slow). You might want two, you know, in case one fails, and the wind carries you to antarctic.
From those 120 tons, you need to subtract the helium, the propellers, the crew, etc. to get to the effective weight that you can carry.
If you wanted to go faster, notice that the drag increases with velocity square. You can try to reduce the surface like zeppelins do, but the surface is still going to be pretty big, because "volume" is what make these flight.
Also if you reduce the surface in travel direction, you are going to increase lateral surface cause that's how volume works, and that's going to make it worse when you have cross winds.
For any reasonable speed you might be looking at close to 20 MW of power, which is 777 jet-turbine like amounts of power.
So why doesn't make sense to build this to go from A to B?
Cause physics. If you want this to "float" (you can't really call this flying), you need absurd amounts of volume, and it turns out, that moving things with a large volume through a fluid requires a lot of energy.
You could do a more detailed analysis, improve the performance of every single component by 10x, but you can't change the physics.
Even if you just want to lift cargo, and barely move it from A to B, this wouldn't make sense, because the surfaces created by the huge volume would mean that you need absurd amounts of power to just stay stationary in case of "mild" winds. Even if you had enough power, wind changes direction quickly, but the huge propellers required for this don't. So if you need to lift heavy cargo, you probably need to position it with < 1mm tolerance, cause by definition you can't move heavy cargo after you position it, so you gotta get it right (e.g. something like a highway bridge).
You can't do that with this thing either cause of physics (big area, movement very sensitive to wind, impossible to control).
For the luxury thing, you end up much better with a luxury private jet, or a private 787 or whatever. The C130 Hercules, can take off with 137 tons, is orders of magnitude faster and cheaper, and only needs a crew of 3!
For transporting a bridge, you end up much better with a bunch of heavy cargo helicopters.
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Instead, if you just "float" somewhere, and you don't care were, balloons and zeppelins are pretty good.
They aren't as good as a modern glider obviously, but they benefit from working with the physics and not against them.
Other vehicles that "float", like boats and submarines, exploit the fact that water is way denser than air, yet even they have pretty big submerged volumes, but luckily for them water doesn't change direction as fast as wind does.
I don't have time to work through the rest now but I'll give it a shot tomorrow.
But even with 1 order of magnitude less volume, the fluid dynamics are still really bad.
These things "fly" like a brick, we are just making this brick lighter and lighter, but a brick is a brick.
We'd have to shape the helium into an airfol (an helium airfoil) to solve the main drawback.
So here is my take on your estimate:
First of all, I was thinking more about an RV-sized gondola. So something like this [0].
The axles are rated for 30000 lbs together, so let's say 15000 kg fully loaded. This includes ridiculous stuff like marble countertops, so there is likely a lot of headroom for trimming weight.
Taking that into account, and the room temperature helium density, we get 15000 m^3 of volume.
Then I would argue that taking a sphere is really not the right approach, because it has poor drag coefficient and also having this elongated shape is what gives you control authority (try steering a circular boat with a rudder, any rudder you apply will just make you spin). The argument about crosswinds also does not really apply because you would always point the nose at the apparent wind, not dead at the direction you want to go. Same as a boat in current or an aeroplane. The exception are gusts of course, and I do see those could pose a problem.
So let's take an ellipsoid with a 1:2 aspect ratio. you need semi major axes of a=b=12m and c=24m to get ~15000m^3 volume. So this thing will definitely not fit in your garage, but it will fit into a pretty standard barn that any rural construction crew builds routinely.
The frontal area is then pi*(12m)^2 = 452 m^2. The drag coefficient for such an ellispoid is somewhere around 0.1 from what I can tell. Can get even lower for a proper airfoil shape but let's go with 0.1 to account for stuff like the gondola etc.
For 20m/s, (way faster than any water-based yacht can travel) gives a force of
1.225 kg/m3 * 0.1 * (20m/s)^2 * 452 m^2 = 22 kN
and a corresponding power needed of
sqrt((22kN)^3 / (1.225 kg*m^-3* 10 * pi * 1m^2)) = 0.5 MW = 700hp
That seems like a very realistic power requirement for such a vehicle, the RV I liked to earlier has 600hp. Might have to be a bit more because the prop area is perhaps a bit much here, but that scales linearly so I don't think thats a huge problem.
Of course if you want to maintain TAS much above 20m/s things escalate quickly. For 40m/s:
1.225 kg/m3 * 0.1 * (40m/s)^2 * 452 m^2 = 88 kN
sqrt((88kN)^3 / (1.225 kg*m^-3* 10 * pi * 1m^2)) = 4.2 MW = 5200 hp
which does indeed seem excessive. But I think for such a craft 20m/s would actually be acceptable. Can't fly it in a storm and can't compete for speed, but both those things are true for leisure sailing yachts.