Rigid, semi-rigid, non-rigid, hybrid-lift or variable-buoyancy?
Why this and not something else
All five on one basis, each calibrated on a vehicle that flew. Three are lighter than the one chosen, and each is lighter for a reason that costs something a liveaboard cannot pay.
Why this architecture and not another
Rigid, semi-rigid, non-rigid, hybrid-lift and variable-buoyancy, each calibrated on a vehicle that actually flew and each run through the same gates. The comparison is the point: three of them are lighter than the one chosen, and each is lighter for a reason that costs something a liveaboard cannot pay.
| Architecture | Structure | kg/m³ | Ballast system | Can hover | A torn cell |
|---|---|---|---|---|---|
| Rigidchosen | 9,779 kg | 0.275 | 60 kg | yes | costs one cell |
| Semi-rigid | 6,007 kg | 0.223 | 60 kg | yes | costs the ship |
| Non-rigid | 3,440 kg | 0.129 | 60 kg | yes | costs the ship |
| Hybrid-lift | 7,176 kg | 0.117 | 60 kg | no, needs 18.3 m/s | costs the ship |
| Variable buoyancy | 9,779 kg | 0.275 | 1,832 kg | yes | costs one cell |
The hull girder is sized by a gust, and the gust gets worse as the ship slows down
The static case is 0.35 MN m and the gust case is 1.26 MN m, so the gust sizes the girder. A 7.5 m/s vertical gust at 8 m/s of forward speed is 43 degrees of incidence, and the Munk moment peaks at 45. An airship's gust case gets WORSE as it slows down, which is the reverse of an aeroplane's and is why station-keeping is the structural design condition.
That number then decides whether a pressure-stabilised hull is available at all, and it turns out not to be the binding criterion: 8 mbar, set by dynamic pressure: bending needs 2.4 mbar and holding shape at 30 m/s needs 8. That is 9.8 kN/m in the fabric against 18 allowable, so pressure stabilisation still works at this size. The Zeppelin NT holds 5 mbar, which is the same criterion at a quarter the radius.
Rigid
A carbon frame carrying independent gas cells behind a weatherproof cover. The Hindenburg arrangement, in a material it did not have. Heaviest primary structure of the five, and the only one where a torn cell costs a twelfth of the lift instead of all of it.
Calibrated on LZ-129 Hindenburg, 200,000 m3, 118,000 kg empty on an ISA basis.
9779 kg of structure on 35616 m3 of gas, 0.275 kg/m3. Nothing disqualifies it.
Semi-rigid
A keel truss carrying the gondola, engines and fins into a pressure-stabilised envelope. Much lighter primary structure, at the price of a single gas volume, a blower that must never stop, and a hull size limit set by the pressure the fabric can hold.
Calibrated on Zeppelin NT, 8,255 m3, 1,000 kg truss, 5 mbar, 2,000 m3 ballonets.
6007 kg of structure on 26997 m3 of gas, 0.223 kg/m3, but a single gas volume has no damage tolerance.
Non-rigid
A blimp. No primary structure at all: every load goes through the envelope and its catenary curtains. Lightest on paper and the least suited to hanging a habitat, a workshop and four propulsors off, because there is nothing to hang them from.
Calibrated on Zeppelin NT envelope practice with the truss deleted.
3440 kg of structure on 26712 m3 of gas, 0.129 kg/m3, but a single gas volume has no damage tolerance.
Hybrid-lift
A flattened three-lobe hull that flies heavy, carrying part of its weight on the hull as a very low aspect ratio wing. It solves ground handling and it cannot hover, which for a vehicle whose figure of merit is days aloft is the wrong trade in the wrong direction.
Calibrated on Airlander 10, 38,000 m3 in 98 by 50 by 30 m, three lobes.
7176 kg of structure on 61155 m3 of gas, 0.117 kg/m3, but it must hold 18.3 m/s forever or descend; a single gas volume has no damage tolerance.
Variable buoyancy
A rigid with a gas plant: compress lifting gas into tanks to become heavy, release it to become light. It never runs out of ballast, which matters over a desert and not at all over an ocean, and it pays for that in tankage that outweighs the water it replaces by a factor of forty.
Calibrated on Aeros Aeroscraft COSH, which has no published engineering figures, so the numbers here are derived from compression thermodynamics and published COPV mass fractions.
9779 kg of structure on 35616 m3 of gas, 0.275 kg/m3, but its ballast system weighs 1832 kg.