Hello nice people!Very few vehicles have dived deep in the Ocean
, with a crew or not.http://en.wikipedia.org/wiki/Bathyscaphe
Piccard's ones reached the deepest point of 11km around 1960, a handful much later 5 or 7km, and recently 11km
The float is difficult. A hull for 114MPa water pressure sinks unless
it uses the best materials in an optimized design. http://mseas.mit.edu/publications/Theses/Alex_Vaskov_BS_Thesis_MIT2012.pdf
What has worked up to now is:
- Piccard used a liquid at outer pressure, lighter than water to get buoyancy and lift the heavy crew sphere. Hexane weighs 663kg/m3 without the hull and lifts little, only cryogenic gases would improve.
- Syntactic foam. Tiny hollow glass spheres load an epoxy to make it lighter. Not very efficient.
- Nereus used many hollow spheres of alumina. This ceramic is light, stiff, and strong against compression. The spheres weigh 233kg/m3 hence outperform the previous methods but have already failed, supposedly in a chain reaction.
Maybe silica gel or some zeolite
covered with a metal film would resist the pressure, but not lift much. Very hollow solid molecules
exist, I don't know their density nor resistance to isostatic pressure. Data welcome!
A float with titanium or steel shell is no alternative. If breaking at 1.5 times 114MPa, it lifts nothing - it's a candidate for 6km depth only. A hull of graphite fibre
composite is widely considered, but to break at 1.5 times 114MPa, it would be heavy.
I've already proposed to help a graphite tank filled with helium at about half the maximum outer pressure. Figures tell the combination is lighter, but I'm not enthusiastic about m3
of gas at 60MPa near the operators.
My preferred option presently is float elements of plain lithium
) with a liner to separate it from the water. This sounds a bit bizarre, but after all we have already metallic lithium in batteries all around us. If the float consists of many lined elements, a failure in water wouldn't propagate to the whole float, and would result in a fire if near the surface, and have little consequences at depth. I prefer this risk to pressure gas and to hollow floats.
While being lighter than hexane and syntactic foams, lithium is also stiff - much stiffer than water. From 6000m/s sound velocity, the bulk modulus is 19GPa, so 114MPa water pressure shrinks lithium by 0.6%vol or 0.2% in each dimension.
The elements can be spherical or not, but without any void: cold isostatic pressing, radiography. They must receive a perfectly conformal coating, for which metal sputtering or evaporation looks feasible. Over this first coating, the liner can be:
- Malleable, maybe niobium or tantalum. This would resist a finite but big number of cycles, easily experimented on the ground.
- Hard, with a yield strength exceeding the 0.2% dimension change. At E=200GPa, both electrodeposited nickel-cobalt and electroless phosphorus nickel have margin. Maybe evaporation or sputtering can also make the desired thickness. Hard chromium would resist the elastic strain too, but because lithium os so soft, I'd prefer the more resilient nickel or nickel-cobalt.
Marc Schaefer, aka Enthalpy