Practical Airship Design Part 5c

Sunday, April 21, 2013

A self mobile cloud!

In my last technical post I talked about the layout of the engine room of our airship and hinted at some of the controls that would allow the officers on the flight deck up forward to send commands to the engine room.  Since then I have received a couple of questions about how the lift system would be controlled in practice. I was also asked about how the steam lift system would actually operate, given the enormous volume of the lift bags inside the hull.  I will try to answer those queries in this post.

First I'd like to remind everybody of the scale of vessel we are talking about here.  Our airship is about the same size as the Hindenburg.

  • Length: 245 m (803 ft 10 in)
  • Diameter: 41.18 m (135.1 ft 0 in)
  • Volume: 200,000 m3 (7,062,000 ft3)
  • Powerplant: 4 × Daimler-Benz DB 602 diesel engines, 890 kW (1,200 hp) each
  • Maximum speed: 135 km/h (85 mph) 
  • Dead Weight: 149 T

That is only slightly shorter than the RMS Titanic at 269 m (882 ft 6 in).

We are almost twice as long as the largest Victorian Battleship the HMS  Magnificent built in 1894

  • Length: 128 m (421 ft)
  • Beam: 23 m (75 ft )
  • Powerplant: 2 × 3-cylinder triple expansion steam engines, twin screws
  • Maximum speed: 30 km/h (18 mph)
  • Dead Weight: 15,810 T
Note the difference in weight though!

Even though we are the largest moving man made object ever constructed, we are essentially a self propelled cloud.

Here is an image of our Airship that I put together:
As you recall from Part 3b The Case for Steam the static lift for our ship is provided by low pressure steam generated by our fantastical power Core. In that post I also described why we wanted to use steam as our lift gas and how steam, along with our core, eliminated some of the constraints traditional rigid airships faced in operation.  But steam has some constraints of its own that a traditional hydrogen or helium lift airship doesn't have.

The biggest constraint being that steam must be maintained at a temperature of at least 100C or it quickly turns back into liquid water!

Once generated, the steam immediately begins to loose heat by conduction through the piping and the envelope of the lift bag. The inner surface of the envelope will be streaming with water constantly. Unlike a traditional lift gas system, where once inflated a lift bag will not loose volume unless intentionally vented, (ignoring for the moment the unavoidable leakage through the envelope) a steam lift system will rapidly collapse by condensation unless additional steam is added or some mechanism for adding heat directly to the volume can be found.

This is why steam as a lifting gas is in reality not very practical, the amount of fuel needed to reboil the condensate makes it impossible to maintain flight for very long. Our airship doesn't have that problem of course due to its power core, which doesn't use any fuel. So this means that the losses due to condensation can be readily made up. The addition of fresh steam also serves to add heat to the total volume reducing the condensation rate from convection losses as the steam circulates inside the volume and flows across the cooler envelope.

One of the benefits of using steam is the ability to control the gross lift by varying the volume of the lift bag. The volume can be varied by adjusting the balance of steam flow to condensate rate. If we add more steam  than is lost through condensation the volume increases and we get additional lift without having to drop any ballast. If we allow the rate of steam flow to be less than the condensation rate the volume has to decrease and we loose lift. Unlike in a traditional lifting gas ship, we are not permanently loosing the lift gas as we have an essentially unlimited supply.

To see how our steam airship controls the lift we first need to understand how a traditional gas lift airship does it.
In a traditional rigid airship the lift gas bags are very close to the atmospheric pressure at the altitude she is flying at. The ship is "weighed off" such that she is neutrally buoyant at the starting elevation with the gasbags inflated to 80% or so of their total volume. This neutral condition continues to exist at any elevation above the starting point, because as the airship ascends the volume of gas expands. The expanded gas volume displaces the equivalent weight of the lower pressure atmosphere so the buoyancy is the same. Thus at any elevation above the starting point the pressures inside and outside the gas bags continue to be equal. Obviously there is a limit to how high the airship can go before the gas can no longer expand inside the hull structure. This is known as the pressure height and is the maximum elevation the ship can attain for a given weight.  In a traditional rigid airship exceeding the pressure height is problematic because the gas must be released from the lift bags to prevent over pressure. Once that happens the ship will descend because there is no longer enough volume of gas to sustain the weight aloft. As the ship descends the gas bag volume decreases and the ship continues to descend! Ballast must be dropped to allow the ship to remain aloft at all. 

Something to keep in mind here is that within the limits of the starting elevation and the pressure height the airship is free to move vertically without either dropping ballast or venting gas!  Thus altitude control during powered flight is by the use of the dynamic lift from the forward motion of the hull. The angle of the hull is under the control of the elevators on the tail planes.

Of course, as I pointed out in previous posts, this gas venting and ballast dropping is a permanent change to the airship. Since the gas cannot be replaced, and neither can the ballast, there is a limit to how long flight can be maintained before there is no longer any way to control the buoyancy.

Now lets look at how we handle this control in our steam lift airship.

Exactly the same conditions exist with respect to the volume changes due to elevation, we also start our flight "weighed off" to be neutrally buoyant at our initial elevation, and have our lift bags with some spare volume available for expansion as we fly higher. In order to maintain the volume however we must be continually adding steam to replace that lost due to condensation and to maintain the temperature of the total volume at 100C. As the ship ascends the steam filled lift bag volume increases just as the gas filled one does.  Our airship also has a pressure height above which we would have to vent steam to prevent over pressure. And just like in the traditional airship that would cause us to descend.

Here is where the benefits of our steam system really show, although the control is tricky.

Unlike the gas lift bags of the traditional airship, ours are in constant flux. The engineer (me) is constantly juggling the balance of  steam flow vs condensate rate and temperature. Now when we hit pressure height steam will have to be vented just like gas would. Only we send the steam being vented to our condenser on the top of the hull, where it is turned rapidly back into water and recovered for use later. The ship will descend of course, just like a gas lift airship would, but instead of having to drop ballast the engineer simply boosts the steam flow slightly and the volume is made up again. To intentionally exceed our pressure height we would have to drop ballast of some sort, however when we returned below pressure height we would be able to control the volume of the steam lift bags again unlike a tradtional airship which could never replace it's lift gas while in flight.

In our roleplay group, as I was describing this behaviour over coffee one day, it was pointed out that all this juggling of steam flow, volume, and temperature could lead to some nasty oscillations in elevation.  Unavoidable time delays in making adjustments could result in chaos and the inability to control the ships elevation.  (We actually role played that problem a bit for fun.)

An excellent point and one that I have been thinking about for a while.

Here is one solution to the problem. In the absence of detailed sensor information and computer control of the various valves (we are a Victorian air ship after all) there is no way to predict the required changes at any point in time. However the same could be said for driving a car in gusty crosswinds! Yet people do that all the time.  The trick is to maintain the control dynamically instead of statically. Instead of treating the steam lift bag as a static container that needs to be adjusted to match changing conditions, we make the lift bag intentionally leaky. That is we allow for the steam to be flowing into the condenser ALL THE TIME.  So in addition to the steam needed to make up for the condensation we allow more steam than needed to be flowing into the lift bag.  Since we don't want the volume to be changing there is a balance between the flow in and the flow out that needs to be maintained.  More steam would need to flow in than flow out because some would be needed to make up for the condensation losses. An additional benefit is the addition of heat to the gross volume as the new steam is admitted.

Now instead of trying to guess what changes need to be made at any point in time the engineers could adjust the balance between the two valves, inlet and condenser outlet via a single control. We control the flow not the volume. Once balance was achieved the two valves work in opposition one opening while the other is closing. Why this is easier can be seen by comparing how much harder it is to control a car with only one hand instead of two on the wheel. With one hand you have to force the wheel both ways to adjust the steering, with two you just have to shift the tension between the two hands to make the same adjustment. The engineer can then fly the ship vertically like a car driver handles a crosswind adjusting for elevation changes by shifting that balance slightly.  This only works of course since we have unlimited steam volumes to play with courtesy of our core. 

As always thanks for reading. Feedback and questions are always welcome!

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

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


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