Drop tanks are externally mounted fuel tanks which are drained of fuel to charge the jump drive, then ‘dropped’ just prior to actually jumping. This conserves internal space in the ship for other uses, and increases payload dramatically. A ship must be built with proper fittings to employ drop tanks. Retrofitting them is a significant and comparatively expensive undertaking, and should be performed in an A or B yard.
Understanding why jump takes so much fuel is important to the student of ship design. A brief detour into ship engineering is therefore beneficial to the gentle reader.
Jump drive requires massive investments of power in order to accomplish jump, vastly more than any powerplant one could fit into a ship could be expected to deliver in normal operation. It is consumed by the drive in a very short and extremely intense pulse, creating the ‘event horizon’ surrounding the ship which drops it out of normal space into Jumpspace.
Engineering systems are optimized for their intended purpose. A fusion reactor, to run a city, is designed for relatively slowly changing loads, high fuel efficiency and operational economy, never is operated beyond its installed capacity- at least not for long- and never twice. Plants for ships are designed with rapid and large changes of load in mind, plus the need for short duration loads vastly outside its 100% duty cycle rating. Therefore ship plants are designed with the expectation of rapidly changing loads, and an overload mode; this allows them to produce energy output vastly beyond that which could be maintained as a constant indefinite period load… this is ‘jump mode’.
The ship’s fusion reactor can operate at a greatly enhanced output level for short periods of time, the price is considerably reduced efficiency. Greater amounts of fuel would then be required to meet this demand. A large amount of fuel is actually used simply to chill the plant while preparing to jump so it does not fail, or melt. Also, given the reduced efficiency of the reactor while in overload mode, it simply is consuming much more fuel as fuel. It would not be practical to place a plant in a ship large enough to carry such a load at normal operational efficiency and conserve fuel, even at the best technology offered in the Imperium. The net result is a need for considerably more fuel to make a jump. If mounted internally, this rather severely curtails the space available for mission equipment of payload. It also places limits on how far one can go before refueling.
Both military and merchant interests gave considerable thought to these issues; the result was the concept of mounting the fuel exterior to the ship. Jump drive uses supercapacitors to store the energy generated, as the entire energy must be delivered in one huge pulse;think of a flash lamp for a camera as an example of a commonly seen device which operates in this manner. The action of the jumpdrive is to create the jumpfield that drops you out of normal space into Jump space. The energy is prepared in advance, stored up, then used, so why not take a few seconds and kick away the tanks before using it? The result is droptankage, external fuel tanks.
A ship employing drop tanks drains fuel from them to supply the voracious fuel appetite of the reactor to charge the drive, then releases the tanks just prior to jumping. This makes considerably more space available for mission purposes, at the price of not having jump fuel onboard if it is needed upon arrival;such as in case of a misjump. Some ship designs retain enough fuel onboard for a minimal jump as a safety feature, but use droptanks for normal requirements. Naval interests mount internal tanks as well, and use droptanks so as to arrive with full jumptanks in case they have to leave before securing local supplies. Merchants use them to be able to bridge long gaps and make it to other service areas, or as a safety measure; allowing internal tankage to support a single jump to safety in the event of a misjump.
Drop tanks are intended to be reused. They are released with small explosive bolts, and propelled from the ship with short period solid fuel engines (little more than several of those used in hobbiest model rocketry). Ships mounting droptank fittings include the mountpoints and electrical attachments to control the bolts;the same circuit also fires the rocket engines. When used and released properly, the tanks are undamaged and can be prepped and used again for another ship. Class A and B ports always offer droptank services, and usually one can find them at C ports which are on major jump routes. At D or Lower ports, or off the major route, droptank services are very uncertain of supply and their availability cannot be counted upon. Territory knowledge is required if one is considering a dependance on droptank services for a ship under design.
As always, there is a downside. Most ships do not have sufficent jump field reserve to carry tanks with them if they fail to release; if retained, such a jump field does not include them entirely within it. This results in matter being rendered apart at the boundry, with a large energy release from it’s disintegration. This not only destroys the tank and results in misjump, it also normally does damage to the ship, sometimes destroying it altogether; this means is strongly discouraged. As jump capacitors can only withstand the trememdous stress of containing that much energy for a limited time (they are rated for 30 minutes at full charge) a ship experiencing dropfailure must act quickly if it is to manually kick the tanks free before the capacitors fail, resulting in a massive internal explosion, normally shattering the ship. in times of war, sabotague of droptanks is a major target and is guarded against, as for the price of a man in a suit with a few handtools, an enemy can cost the nation an entire cruiser or dreadnaught.
When available, the tanks cost out as fuel price, plus Cr100 per ton. The tank is delivered and attached by a tender just short of the jump point (to keep the point clear for arriving and departing ships) and attached, then filled. The ship proceeds on to the point while charging the drive, drops the tank, then jumps. Small short duration single shot solid fuel engines give them a boost to clear the zone of the jumpfield of the departing ship, so the tank is not damaged- or the customer. The tanks are released by detonating explosive bolts, which also ignites the rocket engines that propell them away from the ship. Such circuits are tested using low current/voltage meters to insure against accidentally firing the devices on it. In emergencies, 2 standard flashlight cells are enough to usually fire the circuit, and 4 of them, or a lantern battery, is always sufficient. Not all rocket science is rocket science…
Large drop tank operations employ 1 or more groups of 3 ships for the duty—a tender optimized for carrying and readying empty tanks, a dedicated fuel tanker to fill them once attached, and a ‘catcher’ tug to recover and return empty used tanks. Small operations use single ships to perform all three functions. These are sometimes surplus tenders late of the navy, sometimes they are large merchants or tankers converted over to the duty.
In backwaters, some local independant operators have undertaken to offer drop services, using old and highly retrofitted ships. Such are usually operating in concert with local authority to encourage more efficient and larger transports to the world in question. Drop tanks are usually manufactured in factories or in B or A yards.