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Old 06-25-2018, 03:44 PM   #61
boogabooga
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So, I've been working on adding a ballonet implementation to my workbook. Spoiler alert: if you have neutral buoyancy at one altitude, you basically have neutral buoyancy at all altitudes (within a range). It's a consequence of the fact that maintaining constant relative pressure is very close to maintaining constant relative density.
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Old 06-25-2018, 03:55 PM   #62
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Quote:
Originally Posted by boogabooga View Post
 So, I've been working on adding a ballonet implementation to my workbook. Spoiler alert: if you have neutral buoyancy at one altitude, you basically have neutral buoyancy at all altitudes (within a range). It's a consequence of the fact that maintaining constant relative pressure is very close to maintaining constant relative density.
The classic consequence of Boyles law.
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Old 06-25-2018, 04:53 PM   #63
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Note quite, temperatures not constant.
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Old 06-25-2018, 05:30 PM   #64
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 Note quite, temperatures not constant.

Thats the reality there, but for small altitude ranges, your assumption can be derived from Boyles law alone.
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Old 06-25-2018, 08:24 PM   #65
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Quote:
Originally Posted by boogabooga View Post
 So, I've been working on adding a ballonet implementation to my workbook. Spoiler alert: if you have neutral buoyancy at one altitude, you basically have neutral buoyancy at all altitudes (within a range). It's a consequence of the fact that maintaining constant relative pressure is very close to maintaining constant relative density.

That had me scratching my head a bit earlier this evening but now I've got it!
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Old 06-27-2018, 01:14 PM   #66
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New version of the airship is working. The volume now remains constant and the ceiling can be increased of decreased before takeoff by varying the weight of the ballast.

I'll make sure the code is readable then upload the the new version.
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Old 06-27-2018, 02:19 PM   #67
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I just go the ballonets implemented in Excel. Complexity goes way up here.

First complete the inflation worksheet. You tell it what gauge pressure to inflate to and the percentage of the envelope taken up by ballonets. It outputs the mass of loaded lifting gas, which needs to stay constant unless vented. (Venting is meant for emergencies and not as part of normal landing.)

The ballonet model tries to maintain a constant envelope gauge pressure by varying the ballonet volume. The ballonet is inflated with the outside air (could be Mars air or Earth air, etc., as set in the parameters worksheet). It is assumed to have the same pressure as the lifting gas in the main envelope; skin tension is ignored. The envelope temperature is assumed to be the same as the outside temperature (thermal equilibrium assumption) unless the user specifies an offset.

The ballonets have limitations in volume range and both under-pressure and over-pressure are modeled. Cell format of the pressure-related cells change to green-on-green- in the former case and red-on-red in the later case. The pressure ceiling is calculated for reference, but is calculated based on the current envelope temperature, so it won't be accurate unless the current envelope temperature is the same as that at the eventual pressure ceiling.

The Excel "solver" is also set up to control the venting, if you are able to use it. Run the solver if you have an envelope over-pressure, and it will vent lifting gas to bring it back under control. You risk under-pressure on the way down, though!
Attached Files
File Type: zip AirshipCalcs_0_2_0.zip (16.4 KB, 2 views)
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Old 06-27-2018, 02:20 PM   #68
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I still find it mindblowing that the Hindenburg only got 5 tons of duraluminum from the R101 wreck because the R101 was mostly made of stainless steel tubes...
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Old 07-14-2018, 08:25 PM   #69
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I think I'll have a new version to upload in a few days. I have been busy with these things which all took some time to do:

1. Organising and commenting the code to make it easy to read
2. Playing around with boogabooga's worksheet and learning about ballonets
3. Implementing boogabooga's work in the code (probably will need some revision)

I find precision landings are easier to do now. I managed to land quite nicely in the area next to the runway at Canaveral as shown in the image below.

Click image for larger version

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Old 07-15-2018, 01:47 AM   #70
boogabooga
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Any questions?
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Old 07-18-2018, 11:40 AM   #71
markp
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Thanks boogabooga I do have had quite a few questions but I manage to answer them after typing a few key words into the internet ... there seems to be a lot to learn though so still plodding along. You do seem pretty proficient with excel and physics!

I'm a bit puzzled on how you derived the formula to calculate the pressure ceiling?

Last edited by markp; 07-18-2018 at 12:34 PM. Reason: Ask a question
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Old 07-18-2018, 07:39 PM   #72
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Hello, markp.

This website might help you regarding standard atmosphere:
http://nebula.wsimg.com/ab321c1edd4f...&alloworigin=1

Note that I only model the first layer of the atmosphere (one lapse rate).

Regarding pressure ceiling:
At some point, the ballonet system can't deflate any more, and the gauge pressure will exceed the threshold if the airship continues to rise.

The strategy to calculate the pressure ceiling is to first figure out the air pressure and density where that will happen. The standard atmosphere equations can be solved for h.

There is a curve ball in that the pressure ceiling depends upon the temperature of the lifting gas, which may depend on your current altitude h. By default, I assume thermal equilibrium such that the lifting gas is at the same temperature as the outside air. If you do that, then the pressure ceiling is an approximation that is more accurate as you approach the associated altitude h and the lifting gas temperature approaches its true value. I also include an offset for other lifting gas temperature assumptions. You might assume that the airship's envelope is a perfect insulator and the lifting gas always remains at T0. Then use "-lambda0*h" for the offset, and the pressure altitude prediction will be accurate regardless of h.
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Old 07-18-2018, 07:54 PM   #73
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Well, practically, even the gas inside the gas bags of Zeppelins heated, despite being protected from direct heating by the outer skin. Just a bit slower than if would have direct contact with the skin.

It was also a matter of airspeed - higher airspeed meant more circulation inside the hull, preventing heating.
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Old 07-19-2018, 12:27 PM   #74
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I think I understand the equations and worksheets now, thanks.

I am now thinking how best to operate the airship.

So I understand that if for some reason ballast is dropped, e.g. cargo is released, from the airship then it would rise, obviously. What I'm not clear on is would the airship have to vent (or maybe compress) gas so it doesn't rise up beyond its pressure altitude or could it adjust its ballonet volume?
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Old 07-19-2018, 12:52 PM   #75
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Quote:
Originally Posted by markp View Post
 So I understand that if for some reason ballast is dropped, e.g. cargo is released, from the airship then it would rise, obviously. What I'm not clear on is would the airship have to vent (or maybe compress) gas so it doesn't rise up beyond its pressure altitude or could it adjust its ballonet volume?
Welcome to the problems that Cargolifter had.

Practically: you would need to take in air / inflate ballonets so that the aircraft becomes heavy in advance of dropping a lot of mass.
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