Space Stations - What's the use of Docking Arms?

[phavoc]

I was going back through Space Stations and after some thinking, I started scratching my head in regards to the implementation of docking arms. After further review I think it's not a logical construct.

For those that have not seen Supplement 14, here's the primary description of what docking arms are, and what they do:

Docking Arms
External arms which connect to the vital feeds on a starship, docking arms do not allow for the transfer of cargo or passengers without the use of shuttle craft. They do allow for a ship to refuel and for atmosphere to be exchanged. Each ton of docking arms allows 20 tons of starship to dock.

For example, a ship with 200 tons of docking space could only dock a 100 ton starship. However it would only take 5 tons worth of docking arms to dock the same vessel.

Conceptually I have no issue with it or them. It's a space-equivalent of a cowboy tying up his horse to the rail outside a saloon that also has a water trough. Your horse (ship) isn't going anywhere and while it's just standing there it can get a drink (fuel and atmosphere). But it's still limiting, as you have to take your horse back to the livery stable to get fed. The space equivalent means you need a shuttle to get to the station (unless you plan on floating across in your ship).

So here are my specific issues:

(1) The tonnage scaling makes no real sense. One ton of docking arm allows for 20 tons of ship. The designer part of me is a big fan of percentages and scaling. So a scoutship can 'dock' at a 5ton addition and get fuel. A 5,000 ton freighter can 'dock' at a 250ton addition. But after a certain point it stops making sense. One could argue that the larger docking arms are necessary because of the added mass of a ship, and the increased need for larger pipes to pump larger quantities of hydrogen, except that doesn't quite work out. As long as the ship is stationary smaller docking arms should be more than adequate to keep the ship attached in in place. We can make cables today that can hold massive amounts of weight that also have to deal with stresses that aren't present in space. As for the size of the fuel pipe, that, too, gets to be ridiculous because fluid engineers will tell you that you don't build pipes that large to pump more fuel. You increase the pressure/flow to pump faster. Not to mention that if you had a huge diameter pipe on the station your flow is going to be limited by your internal piping - which will always be smaller due to the need to squeeze every cubic centimeter of useful space out of a spacecraft. The only logical reasoning I can find to make them so big is to physically extend the arm away from the station.

(2) The operation of a docking arm doesn't make a lot of sense either. Today, at airports we have planes that will park away from a jetway (assuming there is one). Buses with passengers will come to the aircraft. Fuel trucks will drive out to refuel, and cargo too will be loaded externally on the aircraft. So a ship, once it arrives and is on-station, could also be serviced in such a manner. And it would have to if any cargo or passengers needed to be exchanged. It would be just as easy to have the transfer lighter/shuttle that delivers or picks up people and goods to bring fuel too.

(3) The book gives a chart of docking fees. Technically a ship docked externally via a docking arm is assumed to be docked, just the same as a ship that is inside a hangar. The problem here is that the fee for docking is the same, whether you are internal or external. External docking is far and away a much less useful state to be in. Yet the pricing for docking doesn't reflect this. It would probably be cheaper to just take up station a few kilometers away and use a small craft to access the station. This would save you the docking fee and it's what you would need to do anyway if you wanted to board the station.

(4) Docking arms take up their full tonnage as part of a stations overall tonnnage. That, too, doesn't make sense really. They are protrusions away from the station and have no access points other than fuel/atmosphere connections. There is no corridor, and even if we were to assign SOME tonnage to anchoring them to the hull and providing pumps and piping, the scalar model breaks very quickly. To hold a 5,000 ton freighter in place should really take no more space than to hold say a 500 ton ship. The tensile strength of the hull materials is more than sufficient, as would be a relatively small set of pumps to transfer hydrogen. So while you could rate a docking arm for larger tonnages, there should be no need to add actual tonnage once you reach a certain plateau. Each plateau could be rated for ships UP TO a certain size, and then you'd use the next size up.

When I first heard about the concept behind docking arms I thought we were talking about space jetways, which would be extended from the station, perhaps along arms or corridors, that would allow people to leave via the ships airlock and walk onboard the station. Instead of a large hangar (with the 30% overage in required displacement) you could make a space station to operate much more like an airport, with the long arms of docking areas radiating out from a central area. Ships without large amounts of cargo would be able to dock and the crew/passengers board the station. This would still allow a station to service a larger number of vessels AND allow easy egress to/from the ship. You could even have 'passenger' and 'freight' designated arms, such that the 'freight' arm is large enough to allow the loading/unloading of bulk cargo and containers in a sealed atmosphere environment. Hangars would still be needed, but routine docking and servicing could be accomplished via this concept.

It would be simple enough conceptually to also add in fueling (and even atmosphere) connections to the jetway so that the ship would be able to load and unload while taking on fuel at the same time. As they are listed, docking arms are really nothing more than fueling stations. And if they were listed as such (with the appropriately smaller charge) then they would make perfect sense.

Does anyone else see things this way? How would you treat them? Do you think that docking arms missed the conceptual boat?

 

[simonh]

The only issue I have with the docking arm rules is that I think they should include passenger and cargo transfer capabilities into the current stats.

The smallest useful docking arm is going to be 5 tons, for 100 dton ships, which on a floor plan is about 10 1.5m squares. That's huge for just a glorified fuel pipe, but if it integrates say a 6 square long, 1 square wide corridor it makes a degree of sense. The remaining tonnage would be fuel and power conduits (very little space required) and a rail-mounted cargo container transfer grapple on the outside, exposed to vacuum. The crew attaches cargo pods to the grapple and the rail carries them to a loading dock on the station.

Realistically though the smallest practical docking arm is going to be around 20 dtons so it can handle any ship from 100-400 dtons. I'm assuming that larger arms can still connect to ships smaller than their maximum limit. A 20 dton arm is 40 deck plan squares. That's huge. Half of that as a 1 sq wide corridor would be 20 squares long.

Ruling them this way gets rid of the docking fee issue. Also I don't think running shuttles back and forth between a station and ships 'parked' nearby is a cheap or easy option. Those shuttles need to be manned by highly skilled personnel compared to a ground bus. The shuttles also cost a heck of a lot more too, and the docking problem doesn't go away. It just gets delegated to a smaller shuttle, but it still has to dock somewhere. Finally, ships 'parked' in orbit near a station won't stay still. Anyone who's played KSP or has any experience with orbital mechanics knows that ships parked nearby, with very little or even no relative motion, drift around quite considerably in the course of a single orbit. A relative displacement of a few hundred meters turns into many kilometers in the course of a single orbit, especially if one object is in even a slightly higher orbit than the other.

Simon Hibbs

 

[SpaceBadger]

One note on the comparison to airports w fuel trucks for aircraft parked away from the main buildings: Traveller starships use so much fuel, any such "fuel" truck would have to be huge, or make multiple trips. I generally require ships to park in a berth equipped with fuel pipes, or else at a dedicated fuel station with pipes and tankage, if they want to refuel at a port or highport.

 

[aramis]

One note on the comparison to airports w fuel trucks for aircraft parked away from the main buildings: Traveller starships use so much fuel, any such "fuel" truck would have to be huge, or make multiple trips. I generally require ships to park in a berth equipped with fuel pipes, or else at a dedicated fuel station with pipes and tankage, if they want to refuel at a port or highport.

A typical commercial station could easily get away with a 400Td "fuel truck"... with 350 Td of onboard payload. It's only M1 & P1, but it's also SL.

Let's assume for the moment that the R2 exists , and A:A2::R:R2... and that normal operations means week in jump, week out of jump, and fuel accordingly.

Designing them under Bk2, as a worst case for small ships...
The A has 30 tons of fuel needed.
The A2 has 60 tons needed.
the R has 50 tons needed.
the R2 has 100 tons needed.
the S has 40 tons needed.

Said 400Td fuel boat can fill 3 R2's and an R, and still cover itself.

Now, redoing the numbers under mongoose...
the A needs 24 tons of fuel
the A2 needs 48 tons
the R needs 44 tons
the R2 needs 88 tons
the S needs 24 tons.

It can now get just shy of 4 R2s ... but we can also push another several tons of fuel on under MGT, so... it can get 4 R2s, or 8 R's, or 7 A2's, or 15 A's & S's...

It can also do a refuel on a gazelle.
Most smaller destroyers (10kTd) need 5000Td of fuel... so it's 15 trips. Assuming 4 hour sorties, pretty doable. A single such "truck" can fill a 10kTd destroyer in 3 days.

So, it's not really all that impractical to have a fuel truck.

 

[simonh]

One note on the comparison to airports w fuel trucks for aircraft parked away from the main buildings: Traveller starships use so much fuel, any such "fuel" truck would have to be huge, or make multiple trips. I generally require ships to park in a berth equipped with fuel pipes, or else at a dedicated fuel station with pipes and tankage, if they want to refuel at a port or highport.

That's a good point. A fuel shuttle or even cargo shuttle that does it's job in a few trips is only going to be a multiple smaller than the ship it services, not an order of magnitude so the docking capacity problem is just somewhat smaller, not eliminated, and all that shuttling back and forth and transferring cargo to and from the shuttle is going to multiply up the time it takes to service the ship.

I think the answer is to hard-dock ships to a docking arm only as long as necessary to transfer passengers, fuel and cargo then undock and hold station nearby. That way a single docking arm can service maybe a dozen ships in a day, even if the ships remain loitering for several days. Ad-hoc transfers of a couple of crew or a handful of cargo containers here or there during that loiter time can be handled fine by small shuttles. All of this (X hours of docking arm time per day and Y shuttle trips per day) would be packaged into the docking fees.

Simon Hibbs

 

[wbyrd]

A docking arm also has to refresh the ships stores of Oxygen, water, and other fluids, as well as transfer waste and contaminated water for disposal. Simply ejecting waste products into space near the station would cause no end of trouble for the station.

Also, the tonnage of the arm is not only the arm itself but the support machinery for the arm. So the entire system may be far larger than just the visible arm.

Larger arms may include multiple arms, and umbilicals to access systems away from the main docking hatch.


On the subject of shuttles...
An alternative may be drones... Small craft that have no pilot or crew. this means they can run day and night without requiring multiple pilots, and their associated expenses, and paychecks

 

[phavoc]

IMTU - Docking Arms

I posted this over on the MGT forum, but thought I would put it here for those that don't travel back and forth.

Docking Arms Revised
Type 1 – The purpose of the Type 1 docking arm is to allow for ships to take on fuel, water, life support gases and dispose of any needed waste without having to take up a full docking port or hangar. Ships that utilize a Type 1 docking arm are also able to draw upon the stations power grid if they need to shut down their own power plant for maintenance. If the ship does not have its own small craft to transfer personnel and/or cargo it can utilize the small shuttlecraft provided by the station to do so. The primary advantage of a Type 1 docking arm is its price – ¼ the standard cost for docking at a station. This appeal's to a number of traders on a budget. They are also used by ships that are just passing through and looking only to refuel. Each Type 1 docking arm requires 10 Dton’s internal displacement and can extend as much as 100 meters from the station. Due to the need to keep the space around a station relatively uncluttered and free, a station may install a Type 1 docking arm to its external hull for every 500 tons of displacement. Cost is MCr.5.

Type 2 – A Type 2 docking arm allows for the safe transfer of passengers and cargo between stations and ships. These docking arms are intended to interface with ships of 2,000 Dtons or less. The interior is relatively roomy and is sufficiently sized to allow two-way passenger traffic as well as most containerized and oversized bulk cargos. Each docking arm includes the same umbilical connections as a Type 1 docking arm. A Type 2 docking arm is 10m long retracted and can extend up to 50m from a stations hull. They are 4.5m wide x 4m tall. The connection end is equipped with dual airlock doors for safety, approximately 1m in depth. A Type 2 docking arm requires 20 Dton’s internal displacement and costs MCr 2.0.

Type 3 – Type 3 docking arms are used primarily for allowing cargo transfers between stations and ships in excess of 2,000 Dtons. Unless it is dedicated for cargo (many, but not are), the interior is far more utilitarian than a Type 2 arm. In addition, while the Type 3 includes the same umbilical and air lock safety features as a Type 2, it also includes retractable cargo moving equipment in the floor as well as support for motorized container loading/unloading machinery. It is capable of loading/unloading 80 12 Dton containers/hr. As with starships, the deck plating’s gravity controls are adjustable. Type 2 docking arms are much larger (9m wide x 6m tall), can be retracted to a minimum length of 25m and can extend out to 100m. They require 100 Dton’s of internal displacement for machinery and cost 5MCr.

Docking Concourse – Some stations may require a large number of docking positions without actually needing to provide additional station tonnage, especially stations that support large numbers of itinerant craft, such as a mining station. A docking concourse allows a station to increase the number of docking points (they support both docking arms and transfer vehicle docks) without impacting the station’s internal tonnage by extending large corridors (like a modern day airport’s gate concourses) away from the station. Docking concourse are generally specialized – they either only support personnel or cargo. Some smaller stations or those that cater primarily to workers only mix the type of docking arms on a single corridor. Concourses are purchased in 100m lengths; they are 10.5m wide and 6m tall. Each segment can be individually sealed off from the station or additional segments if necessary. They cost 1MCr per concourse segment (plus the cost of each installed docking arm or transfer vehicle dock) and add 462 Dton’s to the tonnage of the station. However they should not be added to overall displacement of the station when making crew or other calculations based on final tonnage. Docking arms and transfer vehicle points may be installed per the chart below.

<500 tons – 10 per 100 m (one vessel every 20m per side)

500 – 1,000 tons – 4 per 100m (one vessel every 50m per side)

1,000 – 5,000 tons – 2 per 100m

>5,000 tons – 1 per 100m

Note – All types of docking arms may be installed at ground installations for use in vacuum, hazardous or undersea environments.

Note – As a general rule, docking arms and docking concourses are not armored and are not intended to be utilized while combat is ongoing. They (Type 1 cannot be armored) may be armored to the stations armor factor by multiplying their base price by 4x. They are considered “hull” hits on the external hit table and are considered destroyed if hit (armored docking arms will ignore 1 out of every three hull hits). A hit on a docking concourse will destroy all attached docking arms and transfer vehicle docks for that specific concourse section. Referees may also consider randomly assigning hits to docking arms and/or docking concourses each time a hull hit occurs. This reflects their relatively unarmored and delicate status.

Crew Transfer Vehicle (CTV)

A small 10 Dton automated vehicle that ships in parking orbit (up to 50km) may call from stations. The CTV will rendezvous with the requesting vehicle, automatically dock to their primary airlock and shipboard personnel can board and return to the station. For safety purposes and because of the short distances, a CTV is capable of only 1/4G acceleration, more than sufficient for it’s primary task. Internally the CTV consists (from front to rear) of a 1Dton airlock, rack space with safety netting for up to 1.5 Dton of hand-carried cargo, and seating for 24 (set in 2x2 with two rows in between). At the rear of the craft 1.5 Dtons is set aside for the powerplant, lifesupport and engines. The cost is Cr50 per one-way trip. A CTV dock may be installed on a station or docking concourse (by consuming one vehicle transfer point) for MCr 2.5

Note – Larger automated versions (some with options to be manned) are available, including dedicated cargo and passenger variants, up to 100Dtons displacement.

 

[Jacqual]

A typical commercial station could easily get away with a 400Td "fuel truck"... with 350 Td of onboard payload. It's only M1 & P1, but it's also SL.

Let's assume for the moment that the R2 exists , and A:A2::R:R2... and that normal operations means week in jump, week out of jump, and fuel accordingly.

Designing them under Bk2, as a worst case for small ships...
The A has 30 tons of fuel needed.
The A2 has 60 tons needed.
the R has 50 tons needed.
the R2 has 100 tons needed.
the S has 40 tons needed.

Said 400Td fuel boat can fill 3 R2's and an R, and still cover itself.

Now, redoing the numbers under mongoose...
the A needs 24 tons of fuel
the A2 needs 48 tons
the R needs 44 tons
the R2 needs 88 tons
the S needs 24 tons.

It can now get just shy of 4 R2s ... but we can also push another several tons of fuel on under MGT, so... it can get 4 R2s, or 8 R's, or 7 A2's, or 15 A's & S's...

It can also do a refuel on a gazelle.
Most smaller destroyers (10kTd) need 5000Td of fuel... so it's 15 trips. Assuming 4 hour sorties, pretty doable. A single such "truck" can fill a 10kTd destroyer in 3 days.

So, it's not really all that impractical to have a fuel truck.

Depending on system traffic 2 or even 3 of the big fueling ships, where they may just be going back and forth constantly refueling the station wheich fuels the visiting starships much like a monster Mobil station in space. Hopefully with better trained pump jockeys.

 

[aramis]

Depending on system traffic 2 or even 3 of the big fueling ships, where they may just be going back and forth constantly refueling the station wheich fuels the visiting starships much like a monster Mobil station in space. Hopefully with better trained pump jockeys.

That's probably 20% more dockings, and about 80% more being done by non-employees. That's a significant rise in risk.