DEFINITION OF AN AIRSHIP FOR THIS ESSAY: a rigid, fully-framed aerial vehicle that is buoyant but does not use gas pressure to maintain its shape.

So, blimps and semi-rigids are out.

The airship is probably the greatest unsung hero in avaiation history. It is one of the best ways to transport anything from one place to another, yet there are very few airships around today. This is mainly because of the Hindenburg disaster, which made many people believe airships were nothing more than flying gas bombs waiting to explode. In fact, the Hindenberg had a perfect safety record and would not have been in danger if US president Franklin Roosevelt had not refused to supply the Zeppelin Company with helium. But that's another story.

Today there are thousands of people who would like to see airships come back. Some of these are working to make it happen while others (like me) can only dream.

In dreaming, I have learned about many of the problems facing airship designers both past and present. Here I offer my solutions, free for the taking to anyone who is willing and able to build the dream.

1. WEIGHT OF LIFTING GAS REDUCES LIFT

SOLUTION: reduce the mass of the lifting gas.

I know what you're thinking: "how can the lifting gas be any lighter?" After all, isn't helium the lightest SAFE gas there is? The only other gas is hydrogen, and we don't want to use that, right?

Well, yes (sort of). But changing the gas isn't the only way to reduce it's mass. We can also use less gas.

The helium in an airship's gas cells has to be pressurized above sea level air pressure, 15psi, in order for the cells to maintain their shape. Otherwise the cells are collapsed by outside air pressure and do not displace enough air to be buoyant. What if the cells could be kept at their full size even with helium below standard air pressure?

In a balloon, blimp or semi-rigid this would be impossible. It is possible in an airship, however, because the cells are surrounded by a rigid frame. Why not attach the gas cells to the frame, thus preventing them from collapsing? This would mean they always would have the same volume, even de-pressurized. With air removed and helium in its place, the cells will lift. Once all of the air in the cells is removed, then helium can also be removed to lighten the load.

Buckminster Fuller once designed a spherical airship based on his geodesic domes. The sphere was rigid and completely evacuated, i.e. it had no gas of any kind inside. Vacuum is really the ideal lifting "gas" but is impractical except in a rigid container, and a fairly large one - in this case, the airship had to be over 1,000 feet in diameter.

Attaching the gas cells to the frame is another way to achieve a similar result. The cells are not completely evacuated, but they are at low pressure, which means less mass. They don't have to be rigid, just able to withstand the pressure from outide air. This also has other benefits (see below).

The two methods for attaching gas cells to the frame are stitching and gluing. I favor stitching, mainly because it's less messy and would not be weakened by changes in air temperature, pressure and humidity. Also, the threads would be a lot lighter than the quantity of glue needed to make this work. Third, stitches are removable and replaceable, while a glued bag would be hell to remove or replace. Practicality favors stitching.

2. WATER BALLAST IS HARD TO MOVE AROUND, THE TANKS ARE HEAVY AND YOU CAN'T REPLACE IT AFTER IT'S GONE.

SOLUTION: use air as ballast.

I am surprised this idea has never been used before. In fact it could be used in any airship with normal, inflated gas cells, but using stitched cells makes it a lot easier.

Ballast has always been thought of as something that adds weight, but in fact it is really something that reduces net lift. The way to do this with air is simple: allow the air to displace helium in the cells.

At this point you might object, "but reducing lift is BAD!" Well, carrying water on board also reduces lift: you just don't notice it because that lift is reduced all the time, lost when the water is taken aboard. With air as ballast, you only bring it aboard when you need it.

(And anyway, reducing lift isn't always bad; it's actually the only way to make an airship maneuverable on the ground, as you'll see below.)

Blimps use something similar even now to maintain pressure in their gas envelopes. A blimp's ballonets aren't really used for trim, but they could be, if they didn't have to be used for structural integrity. Airships don't have that problem.

I have seen some airship designs that use aerial condensers to gather more water for ballast. That's fine in many places, but you cannot always count on the local air having enough water. If air is used directly, it doesn't matter how dry it is - in fact, dryness can be a benefit. By cycling the ballast air through a de-humidifier, water vapor can be collected a lot more easily than if the condenser were simply open to the outside.

3. THE ENGINES WEIGH A LOT, AND THE FUEL WEIGHS MORE.

SOLUTION: use lightweight engines and even lighter fuel.

The Hindenburg carried heavy, iron marine diesel engines which weighed two tons each. Even worse, the diesel fuel for those engines weighed over SIXTY TONS. This is not good for a vessel that depends on being as light as possible.

Today of course we have jet engines, which have much better power to weight ratios. I have seen plans for moden airships with piston engines and they confound me. What are their designers thinking? Jet turbines are the most efficient way to get thrust from chemical fuels, period. After all, the very burning of the fuel itself produces thrust. In a piston engine that expansion produces no benefit. To run propellers, turboprops are far better than even the best psiton engine.

So, any sensible airship design will use turbines, whatever their final form may take. For any given horsepower or net thrust they are much, much lighter, more durable and more efficient than piston engines. What about the fuel?

That's easy: HYDROGEN.

Don't panic. Hydrogen is actually the perfect fuel for airships, and it can even be stored safely.

For decades, forward thinkers have been telling us hydrogen is the ideal fuel. Their usual line is, "hydrogen has more energy per unit of weight than any other fuel!" They wrong in two ways.

First, while hydrogen does have more energy per unit of weight than any other chemical, it isn't weight that is usually a problem, but volume. Per unit of volume, hydrogen is actually a pretty crappy fuel. A cubic centimeter of gasoline (petrol for you Brits) has much more energy than a cubic centimeter of gaseous hydrogen. To gather enough hydrogen to match the energy of a full tank of gas, the hydrogem tank must be many times bigger. Liquid H does have more energy than an equal volume any other liquid, but the cost and weight of tanks, insulation etc. and the difficulty of handling it make it nonviable.

So, in any application where volume is a problem, hydrogen is O-U-T.

Luckily, volume is N-O-T a problem on an airship. Airships have lots of volume!

What's even better, that volume does not have to be wasted. Remember the air ballast cells mentioned above? They are fitted inside the helium cells. Hydrogen can also be stored inside the helium cells, allowing a full load of fuel to be carried with *NO* loss of useful interior space.

Even beter, the hydrogen provides extra lift. An airship built this way will have to partially fill its ballast tanks (cells) before launch to offset the extra lift of the fuel. There's a problem few people will complain about!

What about safety? Well, hydrogen can be dangerous, but only if it comes in contact with air. This will be prevented by surrounding the fuel bags with helium. To prevent air ballast from leaking into the fuel cells - this would require rips in both the ballast cells and the fuel cells - the airship can be designe so no helium cell has both a fuel cell and a ballast cell. Rather, they can alternate: helium/fuel, helium/ballast, helium/fuel, etc.

Thus we arrive at some general rules for moden airship design. Such an airship will have a number of separate helium cells firmly attached to its frame. These cells will contain helium at lower-than-standard pressure, maybe 10psi or 5psi. Inside some cells will be air cells used for ballast. Inside the others will be hydrogen cells, hung so they ate completely surrounded by helium.

Oh, what was the second thing the hydrogen mavens are wrong about? They are wrong to call hydrogen a fuel. To be more accurate it is an energy storage medium. It isn't a fuel because it takes more energy to make (isolate) hydrogen than it contains. This could be a problem, because it makes hydrogen more expensive than petroleum fuels. For this application however, I think hydrogen is a winner.