Project Hybrid airship

[boogabooga]

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.

 

[Urwumpe]

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.

 

[boogabooga]

Note quite, temperatures not constant.

 

[Urwumpe]

Note quite, temperatures not constant.

Thats the reality there, but for small altitude ranges, your assumption can be derived from Boyles law alone.

 

[markp]

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! :tiphat:

 

[markp]

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.

 

[boogabooga]

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!

 

[Urwumpe]

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...

 

[markp]

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.

 

[boogabooga]

Any questions?

 

[markp]

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! :thumbup:

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

 

[boogabooga]

Hello, markp.

This website might help you regarding standard atmosphere:
http://nebula.wsimg.com/ab321c1edd4...D67CEBF3A194F66A3&disposition=0&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.

 

[Urwumpe]

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.

 

[markp]

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?

 

[Urwumpe]

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.

 

[boogabooga]

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?

Or it could use it's propulsor, COM shift, and elevator to control altitude. That's why I had you make the propulsor swivel 180 degrees .

The primary job of the ballonet (as I understand it) is to maintain a constant gauge pressure in the envelope. I guess that you have the option of inflating the ballonet more, if the envelope can handle a higher pressure. That would depend on the airship design, I guess. Probably there would be a safety margin to work with between optimal pressure and bursting.

I would say that venting should be considered a last resort to keep from damaging the envelope in cases where you have already exceeded the pressure ceiling. At least modern helium designs seem to be that way. In the old days of cheap hydrogen, they may not have minded venting so much. I'm not sure, because venting can give you problems when you try to come back down.

 

[Urwumpe]

Well, the Germans vented a lot of hydrogen for keeping the airship level, they only reduced this near thunderstorms. On the other hand, this improved safety also a lot, since it reduced the contamination of the hydrogen with air.

Of course, Helium is too expensive for this kind of procedures.

 

[markp]

Or it could use it's propulsor, COM shift, and elevator to control altitude. That's why I had you make the propulsor swivel 180 degrees .

ok good to know there's a method in the madness!

 

[markp]

Update:

I have been working on some operational aspects for the airship. These have been 1) Landing gear and 2) Collecting and delivering cargo.

1. Landing gear

For this I experimented with adding wheels to the gondola. I also came up with a few ideas such as deploying some kind of rigid structure from the nose. In the end I simply added an animation to rotate the wings so the tips ended up a bit forward of the centre and level with the lower back fins. Not sure the wings and fins would be strong enough to support the airship in reality but it looks good ...

Image of wings rotated for landing


Another perspective of the rotated wings


2. Collecting and delivering cargo

An autopilot has been added so the airship positions itself over the cargo. The pilot can then decrease the altitude until the airship docks with the cargo. A marker can be used by the airship's autopilot to deliver cargo to a precise location where it can then be deposited on the surface by the pilot. The building used in the addon and featured in the images below now automatically fixes itself to the surface so it doesn't slide about as it previously did.

In the image below the cargo, a building, has been collected from a landing pad and is now being delivered to a location occupied by a marker called "foundation1"


The cargo is positioned over the marker.

 

[markp]

Update:

Making some progress.

At the moment the ballonet is automatically adjusted to keep the pressure difference inside the envelope at 500 Pa above the outside pressure. A warning is displayed at 600 Pa and the airship starts venting gas at 750 Pa. These 600 and 750 Pa values are guesses so I'll have to dig out some references and maybe adjust these.

I was rather surprised when warnings were given when the airship climbs faster than a few metres per second. This is because the simple algorithm I have used doesn't change the ballonet size quickly enough to maintain the correct pressure. I am wondering if real airships climb or descent rates are have to be limited in a similar way due to a limit on the rate of air pumped into or out of the ballonet. Something to look into.

First picture below shows when the airship reaches its ceiling and a warning message is displayed. This is from a high altitude test in the alps. All my tests have recently been at Canaveral so I needed to check everything works from some higher ground.

Second picture shows the base from where I took off. It's the antarctic base copied to the alps ...

Third picture shows an example of the airship data that I display to help me figure out what is going on in the code.