Practical Airship Design Part 3b

The Case for Steam

In the previous part of this series I talked about some of the details concerning how an airship flies.  In this part I will discuss the pros and cons of using steam as the lifting gas for our airship.

You can get some of the technical details of why steam makes a good lifting gas at this website:
The Flying Kettle. They are actually working on a free balloon that uses steam and have dealt with a lot of the practical details, a fascinating site definitely worth a perusal.

There are lots of different gases that can be used for generating static lift for an airship. In the real world the best one is hydrogen, followed by helium then pure methane. Of these three, hydrogen and methane are explosive when mixed with air and helium, while being non-flammable, is expensive and relatively rare. Ordinary steam is a surprisingly good lift gas being between helium and methane in lift capacity, plus steam is easy to make, cheap, and non-flammable.

This table from Flying Kettle has the properties of various lift gasses.

GAS	M.W.	Temp. (‹C)	Density (kg/m3)	Lift (N/m3) in ISA	Safety	Cost	Ease of provision	Buoyancy control
H2	2	15‹	0.084	1.140 11.19	bad	fair	fair	no
He	4	15‹	0.169	1.056  10.36	good	very high	very bad	no
CH4	16	15‹	0.676	0.549  5.39	bad	low	fair	no
NH3	17	15‹	0.718	0.507  4.97	fair	low	fair	no
hot air	29 (avg)	110‹ (avg)	0.921 (avg)	2.980.327  2.2.98 (avg)	good	very low	good	yes
steam (H2O)	18	100‹	0.587	0.638  6.26	good	very low	good	yes

From the chart you can see that pure steam at sea level and 100C only has the ability to lift 6.26 N/m3 which is better than pure methane but only about 60% of the lift available from helium. My buddy Grant, who is an engineer in real life and also a member of our crew, has calculated that, given steam's lifting capability compared to hydrogen, an airship with the weight of the Hindenburg would need to be about 25% larger in volume to fly!  That is a significant difference and could easily kill the use of steam for any "practical" design on that basis alone.

Another big disadvantage of steam as a lifting gas is that it condenses when the temperature goes below that necessary to keep it as vapour. That temperature is just over 100C at sea level of course, but lower at higher altitudes. As time goes on during a flight the steam will condense back into liquid water, primarily due to heat loss through the envelope, which will reduce the volume available to generate lift. Essentially the airship will constantly be "leaking" lift gas by this condensation.

To maintain flight this condensate must be re-boiled and returned to steam constantly, plus any leakage through the envelope that contains the steam must be balanced somehow, just like a normal gas filled airship must balance against the leakage or venting of lift gas by the dropping of ballast. In a conventional airship the energy that would be necessary to re-boil the condensate must be supplied by fuel and boilers that take up payload capacity.

So why am I proposing the use of steam given these disadvantages?

What really tips the issue in favour of steam for our airship is the power source we are using. In part one I mentioned that the main fantastical element of our airship was this power source, the exotic core of Verne's Nautilus. I prefer to think of this source as being like a fission type reactor core and will treat it as such for this design. Part 4 and 5 of this series will deal with the design decisions that such a power system requires. For the purposes of this discussion here, the key elements we are concerned with are that such a reactor uses up no fuel with time, and it generates prodigous quantities of heat continuously with a very high power to weight ratio.

This power source neatly deals with the disadvantage of condensation as it can easily re-boil any condensate and return it to the envelope.  Liquid water can be boiled to make up any leakage through the envelope to the atmosphere as well.

I am a big fan of simple systems, especially mission critical ones. Since we have an almost unlimited supply of heat available with our power core, we do not need much complxity to generate large volumes of low pressure steam. Thermo syphoning through the core may be all that is required for lift gas production. I will look at some proposed details of how the core and steam production can be controlled in following articles.

Let us now look at the three constraints to airship flight duration I discussed in the last article.

The three constraints are: lifting gas supply, ballast supply, and fuel supply. These are constraints because as a flight continues, the need to balance the buoyancy by releasing ballast and venting gas to account for changing conditions, place a limit on flight duration. Venting gas to lower buoyancy must be balanced eventually by dropping ballast to increase it again. As fuel is consumed the airship gets lighter and gas must be vented to adjust for that as well. In the case of a conventional gas filled airship both of these actions, venting gas and dropping ballast, were irreversible. Once the ballast supply was used up no further adjustments were possible. Ditto once the volume of gas vented reduced the airships buoyancy below that necessary to maintain lift. At that point the voyage was over!

So how does steam as a lifting gas, with our power core, handle these constraints?



First lifting gas supply.

In a conventional airship the volume of lift gas that a ship started with was all it had. Once vented, or leaked, it was gone. Hydrogen and helium are all gasses at normal temperatures and so keeping a store of gas for making up losses was impractical as the weight of the tanks necessary to store it relative to the volume that could be stored wasn't worth it. Water however is liquid at normal temperature and a large volume of lift gas can be stored as liquid water and turned into steam as required. When used as a lifting gas, steam is basically at atmospheric pressure so one m3 of pure water turns into 1700 m3 of steam!

As the steam condenses on the sides of the envelope it can be collected and re-boiled back into steam which has the added benefit of not changing the weight of the airship over time.  Leakage to the atmosphere must be made up from the stored water, however in the case of an airship the lifting gas envelope is inside the aerodynamically shaped hull, so some of the leakage can also be condensed and reused.

Lift gas was vented in conventional airships to prevent uncontrolled increases in altitude and to compensate for weight loss due to fuel use. In our airship the volume of lift gas can be controlled in non-emergency conditions by simply reducing the rate of condensate re-boil below that of the rate of condensation. I would also propose the use of an external air cooled condenser to force condensation of the steam when needed. See part 4 for more details.

If steam needs to be vented in the case of an emergency, the loss can be made up by the reactor from stored liquid water on board.

Next ballast supply.

In a conventional airship ballast was usually water, for the simple reason that it was cheap and easily stored and could be dropped quickly under control. In our case we have lots of water on board for the purposes of making up any steam losses due to leakage, or emergency venting, so we also have ballast handy. Balancing condensation and reboiling rates to control buoyancy means that the use of ballast to control buoyancy should only be required in an emergency.

Finally fuel supply.

Fuel usage is not an issue in our case because our power core, for all intents and purposes, does not use any! There is only a slight change in mass as the core operates but not enough to be noticed over any reasonable time frame. If adequate provision is made for retention of waste products from consumables on board there should not be much long term weight reduction. See part 7 for more details on this issue.

A further benefit to the use of steam as a lifting gas is the control of gross buoyancy.  Since we can control the increase and decrease of lift gas volume through adjusting the ratio of re-boil and condensation rates we will also be able to compensate for weight changes due to a change in personnel or the addition or removal of cargo.

From an operational perspective, replacement of our primary flight consumable, liquid water that may be used to replace steam lost to leakage or emergency venting and ballast discharge, can be accomplished pretty much anywhere fresh water is available. Try replacing hydrogen in the middle of the Pacific Ocean or in some obscure backwoods colonial outpost!

While steam has a reduced lifting capability, having only 60% of the lift available from helium, the operational benefits, when combined with our fantastical power supply, make it the best lift gas for our globe trotting airship.

Look Beyond the Horizon by *voitv

She will be splendid indeed!

The next part of this series will deal with more details of how we can propel our airship on her journeys to adventure and discovery.

In the mean time...

Keep your sightglass full, your firebox trimmed and your water iced.
KJ

Click here for the next part of this series.

You can follow the full design thread by clicking on the tag "Flight Engineer".