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OK, here's the problem: I want to have a spaceship that is built for transporting large machines (in particular, heavy mining equipment). It does not and is not designed to leave space; transport down to a planet (if necessary) is done by other, specialized ships. It is operated by two people, and its flights can span several years (that is, the two people must be able to survive that long on the space ship). It is owned by a transport corporation which is interested in saving cost, so it will have to be built as cheaply as possible for this purpose, unless the extra expense is justified in reducing transport costs.

Note that the crew cannot reduced to less than two people (like a one-person ship or even a completely automated one) for legal issues (each ship is required by law to have a captain and an engineer).

In particular, I'm interested in the following questions:

  • What general ship design would be the most practical for this purpose, and why?
  • How much space would be reserved for crew needs (life support, food supplies, etc)? Recall that the ship owner (a big, greedy corporation) will want to reduce this to the absolute minimum, in order to maximize profits.
  • What is a realistic ratio of cargo mass to ship mass?
  • Anything else that should be considered when designing the ship?
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    $\begingroup$ @MichaelKjörling: Quote from my question, with emphasis added: "It normally does not leave space; transport down to a planet (if necessary) is done by other, specialized ships." IOW, the ship is not used to get something off the planet, but to get something that's already in orbit to an orbit around another planet. In particular there will not be any takeoff and landing systems. $\endgroup$ – celtschk Sep 26 '15 at 14:43
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    $\begingroup$ @Green: I'd prefer to minimize the violations of physics (I do need some sort of FTL drive, but as long as that one is off, I'd prefer not to bend the laws of physics unless absolutely necessary). $\endgroup$ – celtschk Sep 26 '15 at 15:36
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    $\begingroup$ Why does it have a crew? The slow/cheap propulsion and long months of coasting can be done automatically or by remote. People could rendevous with a fast craft if a problem arises that can't be handled. We've sent Cassini to Saturn (7 years) etc. Without a "captain". $\endgroup$ – JDługosz Sep 27 '15 at 1:12
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    $\begingroup$ Unless slavery, indentured servitude, or the like is de rigueur for your universe... getting two highly trained people to give up two years of their lives (and also, finding two people who can stand each other/take the risk of killing each other/psych issues) for minimal costs, or to skimp on their living environments (without them fixing it) is not something I would buy; suspension of disbelief-wise. $\endgroup$ – user3082 Sep 27 '15 at 1:42
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    $\begingroup$ @celtschk I concur about authors like to write about humans. But if you expect crackerjack engineers who can handle 'things going wrong' to live hot-bunking in a prison cell for 2 years... well, you're gonna get what you pay for (ie: I can live anywhere else, including in prison). I wouldn't entrust my X*illion-dollar spaceship to two people that I'm stiffing on wages. Wages and nice living conditions are peanuts in the total cost of transport/doing business, in this case. I also don't want them killing each other, thus making it so the ship no longer meets the legal requirements. $\endgroup$ – user3082 Sep 28 '15 at 5:05
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Getting to space is the hard part, since you are already there you only need to care about

  • accelerating
  • decelerating
  • lifesupport for two people over the course of, lets say 5 years

Shape
I guess pickaxes and shovels arent worth an interplanetary travel, so lets take a bunch of Tunnel Boring Machines and cranes, totaling 100000 tons of cargo. Since they are made of huge pieces of metal, specifically designed to withstand repeated physical abuse, there is no need to put them inside a cargo container, just mount them on a frame (Although clients can choose to pay extra for wrapping it with thick metal plates). Aerodynamics are not a problem in space and all the sensitive parts can be removed and transported separately in a protected, pressurized box with a negligible weight.
Since you need to both accelerate and decelerate the ship will either need two sets of thrusters on both side of the transport frame (expensive), be able to ride around the frame on a rail (acceptable) or have the frame around it like a donut (cheapest, but need to be very close to the center of mass)

Now for lifesupport. The crew will need Air, Water and Food, for each of those we can choose between stocking up before- or generating it during the trip.

Water

The UN say that a human being needs 50 litres of water per day in order to prepare meals and to have enough for personal hygiene

That would be 180 tons of water for a 5-year trip if its discarded after use. Even the fanciest recycling system can be implemented in a fraction of this weight, keeping a few hundred litres in circulation

Air
The ISS life support uses two systems, one electrolyzes water to generate oxygen and another uses a chemical reaction, with one litre of Lithium perchlorate it can provide enough oxygen for one person for one day (and does not require energy input). Splitting 1l of water by electrolysis generates 16/18 kg of oxygen or 620 litres of pure O2. Almost the same efficiency as the chemical method but less dangerous.
Photosynthesis is another viable option, two people would need a small 30x40 $m^2$ forest to support them, although algae are much more compact and currently in research. Urban Algae Canopy claims to produce "as much oxygen as four hectares of woodland", although exact technical details are not available

Food
3630 kg of food are required to support a crew of three for about six months. That makes about 25 tons of food for 2 people for 5 years. Probably much simpler and cheaper to stockpile than growing food on a spaceship

Fuel
Thats the important part. If we use more fuel we can go faster and need less food, water air and salary for the crew. You need to know the weight of your cargo, how far you need to go and how fast you want to go there, at some point it might be cheaper to split the cargo and make two trips.
Since we cannot realistically use fuel in the same order of magnitude as cargo, Rocket equation $\Delta v = v_e \ln \frac{ShipMass+FuelMass}{ShipMass}$ simplifies to $\Delta v = v_e \frac{FuelMass}{ShipMass}$. Now we need to maximize the exhaust velocity. Exhaust velocities of chemical rockets range from 2 to 5 km/s, while Ion thrusters can reach 20-50 km/s but their thrust is very low and they need a very long time to accelerate. A combination of both is ideal, A chemical rocket gives an initial push and then you fire up the ion thrusters and keep them running for two years, then you coast for a year, flip the ship around and run the ion drive for another two years and finally use the second chemical rocket for braking and fine adjustments

For interstellar travel you can neither use solar sails for acceleration nor solar cells for energy so the only choice is to have a nuclear reactor in the ship

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    $\begingroup$ "Since you need to both accelerate and decelerate the ship will either need two sets of thrusters on both side of the transport frame (expensive), be able to ride around the frame on a rail (acceptable) or have the frame around it like a donut (cheapest, but need to be very close to the center of mass)". Of course the simplest option is to turn the ship around at the halfway point. No need for extra engines or complicated mechanisms for moving them around that way. $\endgroup$ – Neil Forrester Sep 27 '15 at 4:46
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    $\begingroup$ UN is full of pocky for water. 3.78 liters is needed per person, per day to survive. But you'd really like around 9-16 liters for decent living (maybe a skimpy shower a day (4-8 liters)), and assuming non-dehydrated food (but doesn't need to be wet, either). That's not getting you a flush toilet, but OTOH that can be recycled too. $\endgroup$ – user3082 Sep 27 '15 at 8:36
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    $\begingroup$ Yeah, those numbers probably include agriculture and industry, otherwise i cannot imagine how an average US citizen manages to use over 500l per day $\endgroup$ – DenDenDo Sep 27 '15 at 9:08
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    $\begingroup$ @DenDenDo I've seen some very wasteful US citizens. $\endgroup$ – Marshall Tigerus Jun 28 '16 at 21:01
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    $\begingroup$ "Splitting 1l of water by electrolysis generates 16/18 kg of oxygen". Congrats, you just created 17kg of matter using only 17*9^16 Joules or 425 billion KWh of electricity, as 1L water has a mass of 1kg. Hope you got a cheap electricity provider ;) $\endgroup$ – durandal Dec 9 '17 at 12:16
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If artificial gravity isn't available then the ship will need to sport a rotating ring or the trip durations drastically reduced. The human body does very poorly in zero gravity situations even for months-long trips in space.

Assuming a 3 year mission duration.

Considerations:

  • Food Stores
  • Crew living quarters
  • Propulsion
  • Ship design/shape
  • Defensive Systems

Food Stores

Assuming a 2000 calorie diet with macronutrient ratios at 25% Protein, 25% Fat, 50% Carbs.

Basic weights per macronutrient:

  • 4 calories per gram of carb.
  • 4 calories per gram of protein.
  • 9 calories per gram of fat.

500 calories of protein = 125 grams of protein 500 calories of fat = 56 grams of fat 1000 calories of carbs = 250 grams of carbs

Each crew member needs: 431 grams of food per day and 3 kg of drinking water per day.

$0.431 \,\text{kg} \cdot 365\,\text{days} \cdot 3\,\text{years} = 471.945 \,\text{kg}$ or 1 short ton of food for two people for three years. Some reserves will be helpful in the event of an emergency or a bad batch of food.

$1000 \,\text{liters} = 1 \,\text{meter}^3$

$3 \,\text{liters} \cdot 365\,\text{days} \cdot 3\,\text{years} = 6570 \,\text{liters}$ or $6.5 \,\text{meters}^3$

Crew living quarters

SuperMax prison cells in the United States offer everything a lonely captain and engineer will need (in terms of living space). Each cell measures 4 meters long by 2 meters wide by 3 meters high. This compressed space contains a toilet, sink, shower, bed, desk and chair. With hallways connecting these quarters to the bridge and engineering sections, the crew should be able to keep up on any cardiovascular fitness they may have.

If artificial gravity isn't available then a spinning crew quarters will be required to mimic gravity. Failure to provide some form of gravity will result in lawsuits by the captains and crew that the company failed to meet minimum safety standards.

Waste management and atmospheric conditions will need to be controlled too. Equipment areas for these functions will need to be provided for.

Propulsion

Whatever shape the FTL drive requires. Since FTL drives aren't strictly , there's a lot of leeway in how large the drive needs to be along with any fuel supplies required for the drive.

Ship Shape

Since this ship never needs to worry about entering an atmosphere, the shipwrights will want to maximize the ratio of volume to surface area. The most efficient shape for maximizing this ratio is a sphere. However, a continuously curved shape is more expensive than welding flat plates together. An icosahedron is roughly spherical and can be welded from flat hull plates. The shipping company can find a sweet spot between internal volume and hull plate size.

Defensive Systems

If these are required and assuming a spherical hull, point defenses would be placed at the "north" and "south" poles and at 0°, 90°, 180°, and 270° longitude around the equator. This provides consistent firing arcs across all approaches and large zones of overlapping fire.

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    $\begingroup$ You probably don't need to have a hull around the whole ship... it will be cheaper and more lightweight to have a pressurized crew section in the center, and just tie all the cargo around it on the outside. There's a major advantage if those thousands of tons of mining equipment will be ok in vacuum, and they would make a fine radiation and micrometeorite shield for the crew, too. $\endgroup$ – Innovine Nov 10 '16 at 22:24
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Modular design, cargo carried outside, ship operates non-stop

First off, not needing to land means that you have more freedom to design the ship to improve the economic profits.

Those ships would be expensive. As with a current airline plane or cargo ship, you want them to stop as little as possible. In space this is doubly critical; not only you waste a lot of time decelearating/accelerating when you arrive at destination and depart, you lose a lot of energy (in a planet surface you lose that energy anyway due to friction).

So, you want a basic design with a central hull with motors, fuel load, crew cabin, etc. Cargo is stored in containers, which are attached to the sides of the ship.

When the ship reaches its destination, it points towards the Sun to use it as a gravitational slingshot. It launches the cargo toward local "tugboats", which attach to those containers and put them into a parking orbit.

It first receives some cargo containers that deliver fuel and supplies for the ship itself (maybe including a replacement crew), while the ship approaches the Sun the fuel and supplies are transshipped to the main body.

Once the Sun is being orbitted, the ship accelerates in order to get into the orbit that will make it leave them system in the appropiate direction. Once it is done, the supply cargo containers are jettisoned too (with the leaving crew in it) and the ship body begins receiving the containers that hold the cargo; which would be at nearly the same velocity than the ship thanks to the "tugboats" effort.

This way, the ship may use the Sun's gravity well to change its destination without lossing any significan speed and with a minimum waste of energy.

The containers, of course, would be standardized. Not only they will have the same size and attachment points, they will have also interlocks so the crew and passengers will be able to move from one to the other. Usually, they will be layered, with bulk/less sensitive cargo in the outer layers and containers holding passengers in the inner layers. Also, connections offering access to the ships energy/recycling units will be available.

As for utility, this idea will be more useful the heavier the standing elements (engines, control room, etc.) are when compared to the elements that are loaded at each system (cargo modules, but also supplies). If engines are lightweight, then the cost of deccelerating and then accelerating them later is less important that if they were very heavy.

As for the inner design of the propulsor units, a few ideas:

  • Control cabin
  • Engine rooms
  • Fuel deposits
  • Engineering (mainly stores for replacements parts; a small workshop). Recycling units.
  • Communication room
  • Bedrooms
  • Kitchen / Recreation room
  • Hibernation chambers / Food stores / Hydroponics (Hydroponics may sound cool, but maybe you will need an additional crew member to manage it).
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I will suggest that due to the pressure of keeping costs for bulk transport as low as possible, we will dispense with the spaceship altogether.

Inside a solar system, the cargo will be packaged in the equivalent of an ISO container (AKA shipping container, CONEX or Sea Can) and shot on a minimum energy trajectory via mass driver or some equivalent system. The container has a beacon and some very minimalistic control reaction thrusters to make mid course corrections, and relies on a mass driver, momentum exchange tether or similar to stop it on the end of the voyage.

The cargo company has no ongoing costs for crew, ship maintenance or repairs, very minimal fuel outlay and the costs of the mass drivers or tethers are assumed by the "port authority", which charges for their use. The cargo company therefor has minimal overhead costs and only has to forecast the number of cargo containers they will need to have available for any particular transfer orbit.

If we are talking about interstellar traffic, similar logic can apply if the means of getting from star system to star system is via wormhole or other transfer gate system. Package the cargo, shoot it to the wormhole mouth via mass driver and then a complimentary system in the target star system retrieves the cargo pod. Once again, the costs to the cargo company itself is minimal.

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  • $\begingroup$ That's ignoring the question. The worldbuilder wants there to be two people on a spaceship, (likely) for dramatic purposes. Our task is to help him make it plausible as possible, unless it flatly contradicts all logic. Plenty of simple tweaks, from legal, to technological can require humans: FTL only works when sapients pilot. Legally required (same reasons airplanes have crew now (and I'd argue, look at pilot pay). $\endgroup$ – user3082 Sep 27 '15 at 8:30
  • $\begingroup$ Helping the world builder also involves showing some outside of the box ideas. The real world is driven by economics and costs, and any well designed written world should also keep this in mind. If the Worldbuilder wants to incorporate the idea and still keep people in space (for whatever reason), port staff, customs officers and a "coast guard" to monitor incoming and outgoing cargo pods might provide the venue for that. $\endgroup$ – Thucydides Oct 1 '15 at 17:47
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Directly out of production armor on the transport should be just enough to deflect space derbies every now again. The owner should have the ability to add more armor and some defensive weapon systems depending on the value of the cargo. It is a cheep model with a rather small habitable area, and the crew of 2 will spend long distance journeys in hibernation chambers while the ships automatic systems take over for the rest of the journey. robots are in charge of maintaining the craft during the journey. but must wake the biological crew if any electrical problems occur, there is no artificial gravity or "hyper drive" systems.

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  • $\begingroup$ Hibernation is a good point; I should have thought of that. It also allows for a much better solution for my story than what I currently have on how they ended up in a completely different place than where they intended to be: The navigation system malfunctioned while the crew was hibernated; since the AI systems didn't notice the malfunction, the crew was not woken up until another problem occurred. It means a rather large rewrite from what I already have, but then, some of it needs to be rewritten for unrelated reasons anyway. $\endgroup$ – celtschk May 2 '18 at 18:05
  • $\begingroup$ the only thing I can't figure out yet is how it is able to land/take off on planets, you could think of something for that or it could be orbital based and just drops cargo off at space stations. $\endgroup$ – ALEXZANDER NORONHA-HYDE - STUD May 2 '18 at 18:22
  • $\begingroup$ From the question: “It does not and is not designed to leave space; transport down to a planet (if necessary) is done by other, specialized ships.” So no need to figure out how it can land/take off; it can't. $\endgroup$ – celtschk May 2 '18 at 18:25
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To answer your bullet points: These mostly depend on the technology available at the time of building the ship but I assume that we have powerful enough thrust from say, the newly invented ion engine or relativistic engine (Alcubierre drive, http://techland.time.com/2012/09/19/nasa-actually-working-on-faster-than-light-warp-drive/)

  1. Design should be circular and not square for its cross section upon incidence. A flying saucer would not be great as we would probably want to reduce air drag for lift off and landing. a ring would just waste precious space for a large corporation and be very expensive, requiring more metal for its shell. So altogether I'd recommend the trusty rocket, but a giant one, for its low air resistance. The question that remains is how slim it should be? This is answered in the next sections. The warp drive concept art shows two rings around the ship, one at the top and one at the bottom, so maybe that will have to be the design. Its quite a complicated subject based on how little we know about what engine will be used.

  2. To keep the crew, I reckon we should not need too much space just like a cargo ship for oceanic transport, but I'm also considering bigger. The kind where its better to make only one or two if its that big as it uses up so much of our resources. However, I also want to answer the question if the cargo was the human race itself.

  3. Realistic ratio of ship mass to cargo mass I would briefly say half and half. but this really depends on the quality of the ship. If there is light but strong alloys in use this can be reduced.

  4. Other things to consider are:

    1. What level of earth conditions your cargo needs, if it's living then you will require to use a lot of the cargo space to provide earth like conditions. If its just some metal for building work maybe that space isn't needed. For the Earth flagship however maybe we should consider a few modules that can be added or removed to provide the extra space for cargo on the occasion. The strength and density of all metal in use is crucial to know.
    2. Previously I mentioned ship height/diameter ratio (I call it the rockets pointedness = 1/stubbiness). If it is big and heavy it will be stubbier, but will never be stubbier than a 4 sided regular pyramid of the same size, turned into a cone (I hope this transformation is understandable, ask if not) otherwise its air resistance wouldn't be great, in fact probably quite a lot less stubby than that would be minimum in order to break through air efficiently. A pointed half-oval would be best shape and its stubbiness is best worked out depending on the mass and thrust of the ship in comparison to Earth escape velocity and air resistance. This is the kind of thing an engineer would be needed to simulate.
    3. How to land the ship effectively without damage? The stubbier the easier with this one as imbalance can lead to the ship toppling over and crashing, not cool for such a large bit of equipment.
    4. Another thing to consider is cargo area/propulsion engine area ratio. It may not be very effective for transport if this is less than say 0.1 which is what I approximate rockets taking parts to the ISS would have, but I could be wrong here. Besides better propulsion should have been made by the time we are making this ship.
    5. I severely doubt that the ship would ever take the human race, rich people would buy the ability to get off the planet if a disaster was to happen, leaving everyone stranded. So for a small colony, NASA have extensively designed objects such as buildings for survival that flatpack better than Ikea ware. So maybe a cargo ship the size of Star Trek's Voyager wouldn't be so necessary after all.
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    $\begingroup$ This is mostly great but the question specifically said this was an orbit to orbit ship - no atmospheric work required. $\endgroup$ – Tim B Sep 26 '15 at 18:16
  • $\begingroup$ Yeah hmm I will rethink $\endgroup$ – Nik S Sep 26 '15 at 18:38
  • $\begingroup$ It normally does not leave space This implies the ship can land. If this is the case the requirements are significantly different to a space only vehicle. $\endgroup$ – Stevetech Sep 26 '15 at 21:42
  • $\begingroup$ This "normally" was an unfortunate formulation. It is not designed to leave space. If it leaves space unintentionally, you're well advised to leave the ship beforehand, as neither you nor the ship will survive a "landing". I'll remove the word "normally" to avoid confusion. $\endgroup$ – celtschk Sep 27 '15 at 8:24

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