[The following is an excerpt from a letter to Allen Meece]

[Updated 4 September 2002]

I was recently doing some research on old derigible airships and kite blimps, those being among the few things of comparable size to the VBP platform that have ever been called upon to fly under comparable stresses, and naturally my search led me to the R-101 and the Hindenburg. These two are the most famous passenger airships ever to go up in flames. What's interesting is that in both cases, the same culprit was blamed: static electricity buildup on the lift cells. (The R101 crash also had an impressive stupidity factor, but that's a whole other letter.) Nowadays, multiple lift cell designs tend to be avoided even in helium filled airships because static discharges cause microscopic holes and accelerate deterioration of lift cell fabrics.

The old ZMC-2 design, technically classified as "non-rigid", presents an interesting solution to this problem. The ZMC-2 used a lift cell "fabric" of aluminum alloy laminate. It was heavy with relatively little cargo capacity for its size. However, being made of a conductor, it didn't have any static electricity problems.

I propose a similar design element for the VBP platform. Aluminized plastic films weigh only slightly more than uncoated ones, and several plastics (Mylar, PET, etc.) can take coatings thick enough to conduct electricity. Aluminized films rubbing against each other resist the buildup of static charge, and the conductor allows the charge to equalize quickly. Even if small patches of the metal coating gradually wear off, the conductor will prevent high voltage charges accumulating on too large a surface area. The cell fabric should be aluminized on the outside only, where it is placed in contact with the surrounding cells. (A metal layer on the inside of each cell would turn it into a Leyden jar capacitor, which we don't want.) The shroud should also be aluminized at points on its inner surface. A reflective metal coat on the outside of the shroud is useful for thermal control, but care should be taken to short circuit it to all other metalized shroud surfaces.

This requires slightly thicker and heavier cell fabric, but the difference is likely to be less than 5 to 10kg per cell. That's acceptable to extend lift cell lifetime and minimize fire hazard, but will require shaving up to 200kg weight from something else.

Speaking of fire hazard, it's important to note that the hydrogen will be well within its explosive limits over the entire altitude range of the platform. However, at stratospheric altitudes a good many of the other materials used on the platform will not be. This means that there is a fair bit of difference in the damage we can expect from a lift cell fire depending on the altitude it starts at. At high altitudes, most of the damage would be caused by radiant heat from the hydrogen flame, and a thin shield of fiberglass cloth or even aluminum foil could protect almost anything you wanted protected. Lift cells would fail by melting, not burning, and without additional fuel the fire would likley not last long enough to spread to the keel, tether, or other superstructure. A lift cell fire at lower altitude would be the more conventional, Hindenburg-type fire, there being sufficient air for convection and sufficient oxygen for other portions of the structure to ignite. As such, it would last much longer and be far more destructive. A layer of R30 fiberglass insulation or better is capable of sheilding the hab from any fire, regardless, and I recommend extending its aegis to external tankage as well.

We must anticipate that any lift cell fire will destroy the entire lift array no matter what precautions are taken. However, the hydrogen's role in any fire is mainly that of kindling. If we can prevent other materials on the platform from igniting, a fire that destroys the lift cells will not necessarily destroy the hab. Then we can preserve some capability for a coherent emergency response and survival even in the face of such an extensive disaster.

CME