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Old 05-17-2018, 12:18 PM   #71
Fred Brackin
 
Join Date: Aug 2007
Default Re: Why Dirigibles?

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Originally Posted by Anthony View Post
That might be limits of VE; with properly designed engines, your fuel requirement will be Drag * Distance / Efficiency, and if you go slowly enough the dirigible is lower. The problem is that 'slowly enough' is really really slow.
It could have been my methodology With GVB it was easy to design a dirigible and then re-name it, delete the gas bag and replace it with wings while keeping the cargo, engines, fuel and everything else the same.

The dirigible was of course slower and could not travel as far on the same amount of fuel.

I never tried getting from point a to b on the smallest amount of fuel, Fuel is generally not that valuable and if you're trying to amortise your vehicle cost over the same period of time lower speed means fewer trips and higher vehicle cost per trip.
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Old 05-17-2018, 12:21 PM   #72
Anthony
 
Join Date: Feb 2005
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Default Re: Why Dirigibles?

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Originally Posted by Fred Brackin View Post
It could have been my methodology
Well, your first error is using GURPS Vehicles. It's good for what it does, but it's hardly a complete aerodynamics simulator. The problem is likely that Vehicles lacks different fan speeds, it gives them all the same thrust to power ratio.
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The dirigible was of course slower and could not travel as far on the same amount of fuel.

I never tried getting from point a to b on the smallest amount of fuel.
Those two are equivalent tests.
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Old 05-17-2018, 12:42 PM   #73
Fred Brackin
 
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Default Re: Why Dirigibles?

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Well, your first error is using GURPS Vehicles.
<shrug> It's the one that's relevant to Gurps and without it I'd have nothing but equations from you that I can't test the validity of. Your math probably doesn't constitute a full aerodynamics simulator either.
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Old 05-17-2018, 01:12 PM   #74
Anthony
 
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Default Re: Why Dirigibles?

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<shrug> It's the one that's relevant to Gurps and without it I'd have nothing but equations from you that I can't test the validity of. Your math probably doesn't constitute a full aerodynamics simulator either.
Sure, but we can get closer. We'll start with basic drag equations:
Parasitic Drag = Cd * A * .5 * ρ * V^2
Lift-Induced Drag = L^2 / (0.5 * ρ * V^2 * π * b^2)

Lift-induced drag is 0 for LTA craft, and since it's the dominant term at sufficiently low speeds, once velocity is low enough, an LTA craft will have lower total drag. Unfortunately, the A (area) term for LTA craft is much larger, so we need to get to a very low velocity before the reduced lift-induced drag helps us.

Now for power. If we were pushing on an unmoving medium, power requirement = force * velocity. Since we're actually pushing on air, power requirement is higher, roughly force * (airspeed plus fan delta-V/2). As thrust = air flow rate * delta-V, we minimize delta-V by maximizing air flow rate, which requires extremely large fans.

The ducted fans in VE2 have a fixed efficiency of 4 lb/kW, corresponding to pushing on an unmoving medium at 56 m/s (125 mph). That's probably a plausible approximation for aircraft that commonly use fans, though it's too efficient for something like a DC-3 and too inefficient for a blimp.
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Old 05-17-2018, 02:29 PM   #75
AlexanderHowl
 
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Default Re: Why Dirigibles?

Since dirigible provide their own lift, they can potentially use sails to provide propulsion. Since they can go very high, that means that they can benefit from very fast winds, which would allow them to travel with practically no fuel.
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Old 05-17-2018, 02:33 PM   #76
Anthony
 
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Since dirigible provide their own lift, they can potentially use sails to provide propulsion.
Sails require a medium that you can push against with your keel (which airships don't have). Otherwise, you just move with the wind.
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Old 05-17-2018, 03:52 PM   #77
RogerBW
 
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Default Re: Why Dirigibles?

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Sails require a medium that you can push against with your keel (which airships don't have). Otherwise, you just move with the wind.
At which point you don't need a sail, but you certainly can take advantage of winds - it seems that in the 1920s it was quite usual to fly airships close to tropical storms (on the appropriate side) in order to benefit from the air movement.
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Old 05-17-2018, 04:25 PM   #78
AlexanderHowl
 
Join Date: Feb 2016
Default Re: Why Dirigibles?

I think that you could use a set of smaller lower sails and a set of smaller upper sails to function as an aerial keel, allowing you to direct the dirigible by applying a counterforce in the form of additional air resistance that does not produce thrust. In addition, you would probably require a set of tail sails to assist in stabilizing the dirigible. The set of larger middle sails would provide thrust and, since we are talking about an advanced society, the four sets of sails would be controlled by a computer.

The dirigible would not have to be completely unpowered though. Using a photovoltaic surface, a dirigible the size of the Hindenburg would probably generate the equivalent of 900 kW of electricity (assuming similar light levels as the Earth), which would be 1/4 of the power available to the Hindenburg. You could use it to power small duct engines so that the dirigible could furl its sails to land or moor with another airship. It would also provide sufficient electricity for the electronics of the dirigible.
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Old 05-17-2018, 04:32 PM   #79
Anthony
 
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Default Re: Why Dirigibles?

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I think that you could use a set of smaller lower sails and a set of smaller upper sails to function as an aerial keel
To get thrust in a direction other than straight downwind, you need to have your keel (or equivalent) in a region moving at different speed from the wind. That doesn't have to be the water (various land-based setups can also use sails), but it can't be the air.
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Old 05-17-2018, 04:42 PM   #80
dcarson
 
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Default Re: Why Dirigibles?

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One thing that is interesting is that, in Venus' dense CO2 atmosphere, oxygen and nitrogen are lifting gases. A floating city above the clouds at 1 atm is awesomely steampunk but plausible, with outside work done in shirtsleeves and a face mask (and perhaps an acid resistant umbrella).
Geoffrey A. Landis has done work for NASA on floating cities on Venus. At least two SF stories have used his work, Tobias S. Buckell's novel Sly Mongoose and Charles Stross's novel Saturn's Children. Poul Anderson has the on Earth in Orion Shall Rise. The Clouds of Saturn by Michael McCollum has them on Saturn.
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