# How to attach and distribute suspended weight to multiple envelopes?

In my conworld I've got heavily armoured airships majestically floating through the skies with help of a science-defying lighter-than-anything material.

While I don't mind introducing a pinch of phantastium to my world, I prefer having things structurally sound and rooted in reality wherever I can. To that extent I'm doing plenty of thinking, reading up, and asking questions - such as this one on controlling altitude using my lta-material or that one on aiming flying artillery guns.

In order to promote stability-in-air, as well as for aesthetic reasons, almost all of these airships consist of two or more parallelly arranged envelopes, underslung with a substructure1 (usually holding any facilities that rely on a good view, closeness to the ground, distance from the envelopes) - keeping the center of mass as low as possible.

As such, the weight of the central structure will have to be distributed onto the envelopes somehow. In my rather limited understanding of engineering I would imagine that a central load-bearing array of T- or Y-shaped girders should work.

Q: But is that right? Would V- or inverted-A-shapes for the girders, allowing to directly apply pull to the envelopes, be more or less efficient?

1Similar to the superstructure of any seagoing vessel

• T and Y are used because they require less material than a full triangle (A), a V just is not structurally sound. A triangle is used for ships because they already have to have that shape for the hull and ships are more concerned with usable space than weight.
– John
May 29, 2019 at 15:58
• Since you're using tension not compression, is it for aesthetic reasons that you're not using cables hung down, (then perhaps girder structures horizontally)? May 29, 2019 at 16:13
• You also need something keeping the two hulls apart, making each structural element look like a down-pointing triangle $\nabla$ when seen from the front, otherwise the weight of the gondola will tend to bring them together and crush them... May 29, 2019 at 16:13
• The OP and answerers may find cable suspension bridges a useful comparison. May 29, 2019 at 16:24
• Presenting a smaller target, speed, maneuverability, and higher max altitude seem likely to defeat heavier armor. If my smallship can get above Badguy's battlecruiser, then all his downward- and sideward-pointing guns and armor are useless. May 29, 2019 at 17:32

The physical shape of the struts (your A and V) influences the strength of the vessel but doesn't effect lift efficiency. The struts tie the LTA material to the hull and make the vessel a single rigid body.

Solid rods as struts are strongest and heaviest, tubing provides good strength with less weight. x-beams provide decent strength with least weight. If cost is a concern, cost follows weight is a decent estimate.

The critical design element for things like your LTA craft is stability. Fortunately, its a pretty simple concept when dealing with basic geometric shapes likes circles and squares. I assuming your LTA can be represented by a negative mass.

The same rules that apply to submarines and naval ships apply to your LTA vessel.

For stability, you need the center of mass to be lower than the center of buoyancy. For surface ships, they load the keel with lead to ensure that constraint is always met.

If your lift encompasses your entire vessel, (visualize a circle in a circle) then you'd have the same model as a submarine. To ensure the center of mass is lower than the center of buoyancy then the origin of the inner circle (the people place) would be offset lower than the center of the outer circle (the LTA material)

If your lift strictly resides over your people place, like a figure eight (8) then your vessel is intrinsically stable. Similarly, a T would be intrinsically stable.

In your description, if I have understood it correctly, your lift is two vertical plates with the people place in between -- sort of like the capital letter H. If the H was neutrally buoyant then the vertical bars would be equal in and opposite in mass to the horizontal bar, then your vessel would have its center of mass coincident with its center of buoyancy. The vessel would be stable at times, but when the fat cook walked up to the upper deck to smoke a stoggy, the vessel would turn turtle (capsize). If the H became more U like, then it becomes more stable.

• I spent some time trying to understand where you got the idea of plates from. Seems that I accidentally misused a word that I didn't know existed / had that specific definition. When writing 'lifting bodies' I meant a shape that lifts something (from the geometric shape > körper in German); when relayed to airships and such this would apparently be called an 'envelope'. I've corrected my usage in the question accordingly - sorry Jun 1, 2019 at 15:18
• @dot_Spot, sure, no worries. So I think the elements of my answer that discuss how a submarine is designed for stability and the figure-8 design are more appropriate to your story.
– EDL
Jun 1, 2019 at 18:02