Sunday, September 7, 2008
Building the Airship II
Laying out a model for the airship has its uses, but we must also plan for how to actually build it. That means some thought must be given to the building/hangar and the structures need to support the construction and operation of the airship.
Hangars for airships face a definite challenge that drives up the cost of building them. The roof must be relatively high AND wide yet have no support columns except at the walls. Airplanes face similar issues, but they are generally a little smaller and the wing plan permits some interior support for a room if the airplane is pulled out the same door it enters. Airships tend to be tall and wide at all points, so supporting a roof can be problematic. Supporting a door is also a big issue, but that is no different for airplanes. Add it up and an airship hangar can be a little bit more expensive to build than one for a large airplane.
We also face the issue that an airship is a bouyant structure. When deflated, only the rigid airships maintain their shape. We don't intend to build one of those, so our load envelope will sit on the floor when not filled. That changes the loads on the trusses within the airship significantly. One can design for this fact, but the components added are likely to be dead weight once the airship is actually in flight and that limits payload masses, flight durations, and increases power requirements for movement.
One way around adding extra components to survive sitting on the floor is to avoid sitting on the floor. Hanging the truss structures from the roof places them in a situation not unlike what they face while in level flight. The deflated envelope may still reach the floor, but the trusses will be supported from above and not inclined to be torqued out of alignment.
What I've drawn up in Second Life is a sketch of a hangar structure that would allow this approach withing actually using the roof for the support. The hangar is tent-like, so all loads are transferred to poles/masts along the walls. Cables passing across the span would support winches. Exactly how this is done doesn't matter much to me, but I want to represent it this way in order to help figure out the possible costs and construction complexities.
The version I'm working with now uses a long axle at the top of the masts. It need not be contiguous, I suppose, though I have modeled it that way. My picture shows the axle on the exterior of the wall, but the one I've actually modeled is interior and hanging from the support plate in order to lower the roof and simplify the walls. Some kind of winch would be needed to monitor and maintain tension on the spanning cables while others would be needed for the drop cables themselves.
In-hangar operations would occur with the airship 'tied up' to the drop cables. We can probably even bring power to some subsystems nearest the air duct trusses by bring it down along-side the drop cables. This will help us un-clutter the hangar floor, but we must go too far in this direction for it will drive up the costs for building the hangar.
On a side note, the airship model is now located on the Space Island sim in Second Life. Feel free to drop by, have a look, and kibitz. I'll spend more time on the hangar and related structures next and then return to the lift systems on the airship itself later.