Freelance
Traveller
Drop tanks can be used to give a ship the extra fuel it might need to extend its range beyond that possible with internal tankage. Like all externally mounted equipment drop tanks require a mounting hardpoint. The hardpoint provides the auxiliary services necessary for the use of the drop tanks. The Imperial Navy has standardized on a specific hardpoint design for drop tanks of a range of standard sizes.
The typical hardpoint has a retaining collar which mates up to the retaining collar on the tank. Inside the collar are the connections for the power couplings to run the cryo system that keeps the liquid hydrogen in the tanks at the proper temperature. Each tank typically includes a power cell to keep the cryo equipment running during transfer of the drop tank from tug or lighter to ship. Also in the collar, on the ship side, is the gravitic mechanism that allows the tank to be jettisoned. This is typically a low power repulsor mechanism capable of rapidly pushing the tank away from the ship.
Ships equipped to use drop tanks must also have extra fuel transfer equipment. This typically means gravitic inline pumps, motor operated valves, extra cryo capacity to cool the transfer lines and pumping equipment. Tanks usually have a set of filling connections independent of the fuel lines in the collar. These allow the tanks to be refilled from a fuel tanker while mounted, without using ship's systems.
Imperial Navy Regulations require that drop tanks be inspected each time they are recovered. Tanks that are used but not jettisoned are inspected annually. TAS's recommendation to insurance companies for commercial shipping using drop tanks is that drop tanks be inspected after each use, whether or not they have been subjected to jettison stresses. Drop tanks have not done well in commercial use; with a number of failures that have put some insurance companies off of covering ships using drop tanks entirely. TAS has officially stated that properly constructed drop tanks, that are diligently inspected and used with correctly maintained hardpoints are perfectly safe. The Insurance industry seems unconvinced.
Demountable Fuel Tanks
Not to be confused with drop tanks, demountable fuel tanks are also used to provide extra fuel to ships. DFT's are carried in cradles or installed internally in holds. Externally mounted tanks are not capable of being jettisoned, but are mounted and removed by tugs. Demountable fuel tanks generally are larger than drop tanks. They are not disposable, but are used over and over, though not always by the same ship.
Like drop tanks they require power to run the onboard cryo systems. DFT's do not usually have independent backup power. Tanks are typically moved while empty and only filled after being locked into their cradle, although there are designs that can be filled prior to transport. Some tanks have quite large capacity and include a support skid with its own pump and valve manifold, as well as cryo support equipment. Power is supplied by the carrier vessel.