Student seeks information

I'm a student investigating the possibility of large electrically powered thermal airships and I require some information. 1. How hot is the air in typical hot air balloons and thermal airships? 2. What sort of materials are used for the construction of the envelope? 3. How thick is a typical envelope? I've investigated several possible temperatures, envelopes matterials and thicknesses, even replacing the air with steam to get more bouyancy. Still I'm either exceeding my power output by several times or ending up significantly overweight.

The temperature inside the envelopes of hot air balloons and thermal airships ranges between 200°F and 250°F. The envelopes are normally made of ripstop nylon coated with a urethane or silicone sealant, and some are made of polyester (dacron). I am not sure of the exact thickness of the nylon but it is about as thin as paper but many, many times stronger. Look for a ripstop nylon with a weight of 1.9 ounces per square yard - that is what most balloons are made from. I am not sure of the denier (threads per inch) of this fabric, so just keep 1.9 oz/sq.yd in mind.

Hope this helps, Jon Radowski

writes

Depends on loading. Most manufacturers lift calculations are based on equilibrium at 100C. But remember that this temperature is true only for the hottest area at the top of the balloon, the air near the mouth is not much above ambient temperature. The air in the middle being something in between.

Ripstop nylon with a urethane coating to prevent it being porous. It is this material which prevents internal air temperatures from being higher. If it gets heated to >130C it can seriously degrade the nylon reducing its life expectancy dramatically. In a worst case scenario it can make the balloon unsafe.

Thin! Measured in microns.

You appear to have missed the most important factor of all - envelope volume. Size does matter in this instance!

Typical balloon envelopes have an upper limit of 250-300F based on the material of construction (see below). In cooler weather or carrying lighter-than-capacity loads, the temperature is usually much less. On our last flight (passengers + fuel totalling about 560lb in a 69,500cu ft envelope with empty weight 500lb), the crown air temperature didn't get above 150F since it was a cold morning.

Ripstop nylon or dacron (polyester). The polyester has a higher redline temperature. Fabric weight is typically a couple of ounces per square yard. Just about anything with low porosity that is light and heat-resistant would work. Early experimental modern hot-air balloons used polyethylene, I think.

Not sure what the specs are for fabric thickness.

Volume was one of the first things I looked into, it was starting to get excessively large so I started looking for other options.