An acceptable alternative is to dissolve the raw rubber and mix it with gelatin, and spread it on cotton fabric. This was the material used for the gas bags on the Hindenburg and many other airships. The weight of this material was about 180 gsm, compared with a standard goldbeater's skin gas cell, which weighed about 145 gsm. Simply spreading rubber over fabric produced gas tight cells that weighed 240 gsm [Cooper].
Gold beater's skin is considered "tight". That means the diffusion of hydrogen through the layers is slow-only a few liters per m^2 per 24 hours. On the Sao Martinho, with gas bag surface area of 19,116 m^2, that's a loss of 38-134 kg of lift per day. The latex coated cotton is good for less than 9 liters per m^2 per 24 hours [Woodhouse, p.211], or no more than 172 kg per day. Generally, lift freed by burning fuel will be enough to counter this loss on a voyage, but hydrogen will need to be replenished on a regular basis.
Through his relationship with the Duke of Braganza, it is hoped the Duke of Medina Sidonia can obtain natural rubber from the Amazon for his airship.
Fuel
What fuel is to be used? Well, the final design for the Sao Martinho has assumed we'd use hot-bulb engines that can burn nearly any flammable liquid. However, no matter what propulsion system is used, the fuel will be in liquid form. This is a simple matter of energy density and ease of handling.
The steam engine variant is the only propulsion system that might consider solid fuel, but that involves using a much less efficient boiler system-one that needs constant stoking, and clearing away of the ashes. There are also problems with the fuel. Solid fuel has to be manually moved, or you use heavy automated systems. It is also heavier for its energy content. Coal is about half the energy per pound of petrol, and as for wood, that's about a third the energy per pound. A cord of wood (3.6 m^3) has about as much energy as a 450 kg of petrol, and it weighs about 1360kg. Using solid fuel doubles or triples the mass of fuel required for a steam propulsion system. Worse yet, moving the mass of fuel around the airship will cause significant trim management problems.
One thing to remember with liquid fuels aboard an airship is that, as the airship climbs, the ambient air temperature drops. That means fuel will thicken. Air temperature hits freezing at about 8,000ft. Heaters, or something may have to be added to allow the fuel to flow if the airship is to regularly fly at higher altitudes.
Engines:
We have stated that the Sao Martinho will use hot-bulb engines. These are heavy and not very economical compared with petrol or diesel engines. However, they are being made down-time in the desired power range (at least 40 HP) as early as 1634 ("The Boat" By Kerryn Offord, GG#30).
We could use petrol spark-ignition or diesel engines, but the Spanish are unlikely to be able to purchase up-time built engines. There are currently no new diesel engines being built, which leaves new build petrol fueled spark-ignition engines. The best bet would be new-build variations on the radial engines in "The Spark of Inspiration" by Gorg Huff and Paula Goodlett (GG#13), or "The Boat" by Kerryn Offord (GG#30). These are nominally 125 hp engines, and they will tend to be less economical than the water cooled inline Maybach engines we've been basing our petrol engine calculations on. However, they have significantly better power to weight ratios than the Hot-bulb engines. Two such engines could easily provide all the propulsion the Sao Martinho needs, releasing the weight of 4 gondolas and engines (1,510 kg) and removing the drag of four gondolas-something to look forward to when the Hot-bulb engines are upgraded to petrol spark-ignition engines sometime in the future.
Operating Ceiling
When you research airships, you might see a value called "static ceiling". This is the altitude at which an airship's gas capacity is at 100%, and it is only lifting the deadweight.
For the Sao Martinho, that happens where air density is 60% of sea level. From tables we can find that this happens at about 14,000 ft. Note that this does not mean that the Sao Martinho can actually climb to 14,000 ft in normal operations (because there should always be some disposable load on board).
Something else to consider is the reduction in engine efficiency when the air density reduces with altitude. For example, when air density is at 50% of sea level density, engine performance is also down 50%, so to maintain the same delivered HP you had at sea level will take something like twice the fuel.
The normal operating altitude of the Sao Martinho will be about in the range 100-200 m, as this offers significant fuel economies over higher altitudes. The Hindenburg was usually operated at about 650 ft (198.25 m), so "we are not alone". However, the Sao Martinho has an "altitude and air temperature" allowance of 10% of gross lift. That means the Sao Martinho can fly, fully laden, in conditions at sea level of 15 degrees C, 76mmHg, to an altitude with an air density of 0.90-about 915 m (3,000 ft)-without having to vent hydrogen. For every hour of flight at cruise power the Sao Martinho will gain about 40 m^3 of buoyancy due to fuel being consumed, and if nothing is done to prevent it, she will naturally gain altitude. At about 915 m the gas bags will reach 100% inflation, and as more fuel is burned, the Sao Martinho will want to climb higher. To prevent the gas bags rupturing, safety valves will automatically vent hydrogen. At the static ceiling, gas volume will be 100%, but we will have vented almost 40% of the hydrogen we started with.
Ground operations
You don't absolutely HAVE to have a hangar to store an airship. However, it is nice to have somewhere safe to put your airship, especially in bad weather. This is especially so for timber-framed rigid airships. Wood is naturally hydroscopic (will absorb water). Irrespective of what water might do to the glue holding the airship together, there is the added weight of absorbed water. That is one reason why all wood surfaces have to be waterproofed with paint or varnish. However, paint scratches-enough said.
For short periods (days), there is no real problem in leaving an airship outdoors attached to a mooring mast. Just as long as it is a low one, as the airship virtually needs to be flown (trim etc maintained) at all times while moored to a high mast [Brooks, p.146]. Certainly, on the South America run, the Zeppelins didn't have a hangar in South America until the Brazilians built one at Rio de Janeiro in late 1935, and they never built one at the Recife stopover, where they just used a low mooring mast.
However, you do HAVE to have a hangar to build your airship. These are enormous, and thus expensive structures, as witnessed by the willingness of the Germans to limit the size of the Graf Zeppelin (LZ-127) to the dimensions of the available hangar, rather than build a larger hangar. Even though they knew the resulting airship would be a sub-optimal design.
The Sao Martinho is based on the maximum diameter of the Schutte-Lanz S.L.20 class, and they were built in hangars between 26 m – 38 m wide, and 25 m – 35 m high, and up to 240 m long. The larger hangar was in Berlin (was 38 m wide, 35 m high, and 240 m long), and it took 6 months during war-time to build.
It'd be a tight fit putting a 22.96 m diameter airship into something 26 m wide and 25 m high, but a 30 m by 30m opening might be a more comfortable fit. It also has to be over 170 m long. To give an example of how big this is, St. Paul's Cathedral in London has a nave 37m wide and 30m high, and the cathedral is about 175 m long. The airship hangar for the Sao Martinho is about the volume of the main building (excluding the dome and the transepts).