Dromedary

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The Dromedary is a heavy interorbital tanker, serving only within planetary spheres of influence. While it does not transfer propellant from its source to final destination directly, its role is essential to maintaining current interplanetary commerce.

The key to it is that its drive has low specific impulse but high thrust. While the Dromedary could manage interplanetary trips with such an arrangement, it would use up most of the propellant it's trying to transport attempting the journey. Instead, such voyages are relegated to other ships with higher impulse drives, heedless of thrust - because the journey takes so long, it stops mattering what the thrust is, the end is months away anyhow - and the general candidates are highly efficient but minimal thrust electric drives and propellantless sails. Thrust does matter within planetary spheres, where a high thrust ship can perform a burn in minutes then cruise to where it wants in days or even hours, making use of the Oberth Effect (basically a neat form of gravitational boost) to reduce expended delta-v and by extension the amount of propellant that must be used. By comparison, a low thrust ship must peddle along for months, wasting delta-v in a long outward spiral to inch out the planet's gravity well. Put into numbers, an efficient but low thrust VASMIR equipped lunar tug would take three months to get from LEO to the moon, whereas the Apollo 11 equipped with a horribly inefficient but high thrust chemical rocket managed the trip in 4 days, and the former would require 40% more delta-v.

Hence, the Dromedary serves as the middleman. In a typical work cycle, a Dromedary would take propellant from Low Saturn Orbit gas refineries up to waystations at Titan and Iapetus, reaching the moons in only a couple of days, where its cargo would be loaded onto an interplanetary tanker which will begin the months or even years-long voyage into the inner solar system. At its destination - typically Earth or Mars, at a high orbit station where the least delta-v is required to escape - another Dromedary is waiting to move propellant to stations in other orbits around the planet. Around Earth, it would move propellant from HEO to LEO, GEO, and various lunar orbits, all within a fraction of a week. In doing so, the ship has shaved months off of both ends for the delivery.

Such ships also serve as a general workhorse within planetary systems, where they easily link moons and disparate space stations. The Dromedary in particular has been crucial to development of the Saturn system, where all of the moons are only a few days apart.