Anatomy of a Tramp Freighter

          For those of you who missed my late post yesterday, it was a preview of the upcoming Pumpkin-Suit's Manual, which is still scheduled for release on Feb. 16th.  Be sure to check it out.

           Today, we're gonna explore an idea a proposed last Friday, about how to incorporate the classic tramp steamer trope into our space infrastructure model.  I came up with the idea of independent command modules that can connect to SSTO drive-trains for the boost from the surface and then re-fit for a lunar run while in space.  This is what I came up with:

            First, I've adjusted the dimensions of the cargo containers used in The Black Desert to cylinders measuring  nine and a half meters long and five meters in diameter, and has 62.5 tons of internal volume.  This is only slightly longer than the Conestoga rover, which makes sense for a infrastructure that grew up in part due to the Destiny Foundation's subsidized exodus in the previous century.

            This type of spacecraft, with the working title of "Liberty Bell" both for it's appearance and the connotation of "Liberty Ship".  The Bell part is an independent command module that boasts its own power, life-support, fuel supply, and thrusters.  It can support a crew of five, with up to 20 passengers bound for orbit.  It has couplers that can hold up to four of the above mentioned cargo containers.  The Liberty Bell also has four remote manipulator arms for moving said containers.  The nose cone is topped by an androgynous docking port that comes standard with an airlock/maneuvering unit attached.  The whole assembly masses about 50 tons dry, and carries 50 tons of the rocket's listed propellant.  This is what we will think of as "The Character's Spaceship" from now on; everything else is just add-ons and extensions.

            The first add-on is the L-Drive Assembly, which is typically owned and maintained by what ever spaceport the Liberty Bell is boosting from.  These are the most massive of the rocket's components, but also the least expensive:  With the exception of the turbofans and fuel pumps, the L-DA has no moving parts and is fabricated in nearly one piece.  This makes the cost of building one (at a dirt-cheap $10,000/ton) only one-and-a-quarter million dollars.  Our Trampers will rent it for about $100,000 (including fees, taxes and the use of a propulsion laser) to carry their 'Bell into orbit.

            What makes the Tramp 'Bell different from the hundreds of other tin cans in space is her cargo and destination; instead of just going up to a commercial hub with it's full capacity of 250 tons (four containers), swapping cargo and dropping back down to Terra, our Tramp is carrying two containers (125 tons), a twenty-ton Trans-Lunar rocket engine (with thrust matching a SSME), and a small, cobbled together weapons package containing two ten-ton KKVs (40 tons each fully fueled), two "caltrop" mines at ten tons apiece,  and a laser turret with its own power supply (5 more tons).   This whole weapons assembly is about 105 tons, which puts it within the mass limits of the rocket.

          Once the L-DA is jettisoned, it will wait in orbit for refueling by a tanker for the trip back to Terra.  This may be with the Tramp Command Module that it carried into space, or another craft that leased the assembly for the return journey.  Because the spacecraft do not own the launch assemblies, they can be abandoned once used with the assurance that the Tramp can always reserve another one for the trip back home.

          The Tramp then moves into a higher orbit.  This is where things get hairy; The orbits are zealously patrolled by the Space Forces of Terra's polities, any of which may at a whim decide to detain our freighter for inspection, suspicion of suspiciousness, or anything else.  The Tramp, being armed with missiles, can move through Chinese Alliance pickets without much trouble, as the Zhang Qing cutters have little armor, but will have to avoid EASA-patrolled routes, because their Hoplites have whipple shields.

       Once the Tramp 'Bell has rendezvous with the commercial hub, they start fitting out for their Trans-Lunar Injection.  The Lunar engine is moved to the nose, and the airlock/maneuvering unit is moved to the cargo area.  Like the ISS, the cargo arms on our Liberty Bell can "walk" across the hull,  which makes this kind of orbital assembly possible.  A cupola is fitted to the docking ring, and this unit will now serve as the craft's cockpit for the Lunar run.  In addition to the two containers that were carried into orbit by our 'Bell, the Liberty picks up three more that were lifted by a business partner already.  The Tramp must also lease four propellant tanks for the Lunar engine.

     Once the 'Bell has finished its assembly, it's practically a different ship entirely.  The Command Module now faces opposite the direction of thrust, with the cupola and weapons package at the fore.  The new lunar spacecraft carries three hundred tons of cargo and has the consumables to haul 20 passengers (either the same ones they hauled up, or different ones) on the five -and-half-day trip to the Moon or one of the La Grange stations, where they can catch a ride on an IPV or visiting cycler.  Sure, it takes longer to run a cargo with a Tramp than one of the regular, 1000-ton shuttles, but for a settlement on the fringe like Independence, ships like the Liberty Bells supply much needed medical supplies and other necessities.  The Tramp 'Bell can also ride an Interplanetary Vehicle, it's a carrier, and just might if the Martian market is promising enough to tempt them away from the choked shipping lanes of Terra.  

For those that like the hard numbers, here is yet another table:

Liberty Bell	Launch	Lunar Run
Structure Mass	150	75
Cargo Capacity	250	300
Dry Mass	400	375
Wet Mass	820	585
Thrust (Engine)	6.18 MN	2.2MN
Mass Ratio	1.05 (4.2)*	1.56
Acceleration	10 m/s	4.58 m/s
Mass Flow	1132.4 kg/s	1000 kg/s
Exhaust Velocity	5457 m/s	2200 m/s
Specific Impulse	556.2 s	452.3 s
Burn Duration	770 seconds	105seconds
Flight Time	≈2hrs.	5.56 days
∆-V	7842 m/s	4947 m/s

         That's all for today, RocketFans; I gotta go work on the Pumpkin-Suit's Manual some more.  Enjoy the pics, and comments are welcome.