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Space freighters

  1. Jan 19, 2012 #1
    I've been thinking about hypothetical cargo spacecraft, for transporting materials from planet to planet. Say we're talking container-ship scale. I was wondering if it would be more efficient to, rather than have the whole massive ship take off or land on a planet, to have the cargo carried in containers that can each take off and land themselves.

    I was thinking that possibly it would be more efficient because containers would only need enough fuel to reach orbit and dock with the cargo vessel, whereas a launching cargo vessel would need to take off with enough fuel to reach orbit and then to get to its destination. On the other hand, each container would have the added weight of engines that would be absent if the whole craft lands and launches.

    Maybe if the cargo vessel is not designed to land or launch (ie, constructed in space), it could be made larger/lighter/cheaper than one massive vessel that must survive re-entry and launch?

    Any thoughts? Would this design be viable?
     
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  3. Jan 19, 2012 #2

    Ryan_m_b

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    Welcome to PF!

    I think the question is largely moot unless you can give an example of what kind of propulsion you think these freighters could use. What you are essentially talking about is making a cargo container into a SSTO and the engineering difficulties to make one of them have thwarted us for over 60 years.

    Also if you did have the ability to make SSTO's why not just use them to shuttle up "cargo containers" from the vehicle they are unloading from?

    Another thing you might want to think about is fuel and weight. The larger the mass of a vehicle the more fuel it needs, but that fuel also has mass to you have to top it up with a bit more to take that into account.
     
  4. Jan 19, 2012 #3
    Hi there.

    Fuel and weight is exactly what I'm pondering. Sorry if I didn't make that clear.

    I don't really see how the type of propulsion makes much difference, that's not the issue I'm trying to resolve, but let's say we're talking about multi-stage rockets. The containers are put in orbit using multi-stage rockets and dock with the freighter. The freighter does not take off or land, so say it uses one big liquid fuel rocket engine to get around, and gets refuelled from containers.

    My question is whether this "container" model would actually have any advantages over using one single, massive spacecraft.
     
  5. Jan 19, 2012 #4
    Just my $.02, "space freighting" will never be viable with liquid propulsion. There just isn't enough of it, and it is too expensive to make such a thing commercially viable.

    This is largely why Ryan asked you about the type of propulsion. With a liquid, multi-stage rocket, most of your mass will have to be fuel. This limits the amount of cargo you can carry per rocket, and greatly increases the cost.

    Also, your alternative, the "single, massive spacecraft" would most likely not be able to take off from a planet with liquid fueled rockets. It's size and mass would be prohibitive, and you would likely not find a solution that would allow you to have enough fuel on board after getting into orbit to reach back home. Plus, consider how such a craft would land! Imagine, you would need to carry enough fuel for takeoff on Earth, then enough fuel to accellerate to cruising speed, then decellerate to approach speed, and then enough fuel for another takeoff and another return trip. Most of your cargo would be fuel...(which is not safe while you are entering an atmosphere, to say the least)

    If we are talking about as-yet unused propulsion systems, or future technologies, then it is hard to say. But it seems that there is a general consensus that if you can have a craft in orbit already and use that as a shuttle, you'll get the most bang for your buck.
     
  6. Jan 19, 2012 #5
    if you just have big containers of "stuff" couldn't you just shoot it up into space with a magnetic accelerator launch rail thingermabob? Then you wouldn't even have to worry about making them practical as space ships, more like satellites.

    I have no idea if these things are as of yet practical with our current ideas about engineering and what not, but I've heard about "railguns" being developed by the US navy and aren't those the same idea? If you could just "shoot" the big container up into space, perhaps like a space shuttle, which would then have its own thrusters to guide itself into orbit...
     
  7. Jan 19, 2012 #6
    Using numerous vessels separately would almost universally be less efficient; and the larger the single vessel the more efficient the larger it is---as there is negligible drag in space (there would be a maximum efficiency size for atmospheres, etc).

    The reason for having separate vessels (e.g. building the ISS in pieces) is the brute force ability---I mean, a single vessel might not be possible (e.g. for structural reasons, financing, etc). Additionally, in the case of something like atmospheric drag, there could be a non-linear size scaling making very large objects less efficient.

    There are certainly situations in which many small ships would be better than a single large one, and other situations for the reverse. It depends on the details, and overall as @ryan said, the question is moot.
     
  8. Jan 19, 2012 #7

    Drakkith

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    Maybe. But such technology is not currently available.
     
  9. Jan 20, 2012 #8

    Chronos

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    The volume of a sphere increases by the cube of its radius, whereas its surface area increases by the square of its radius. The vessel to cargo mass ratio is therefore smaller as its size increases, so, its an economy of scale thing. A higher fraction of fuel energy is contributed to cargo transport than vessel transport with larger vessels.
     
  10. Jan 21, 2012 #9
    A "magnetic accelerator launch rail thingermabob" can be made and is a possible idea to get things off the ground and into space (just not living things). The only problem i see with the "Space Freighters" idea is moving the ship from one planet to another, it will take a lot of fuel and making it lighter is not a good idea, there is plenty of stuff in space that will want to tear it apart.
     
  11. Jan 21, 2012 #10

    DaveC426913

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    There is an obvious solution to this.

    One large ship will transport the object over interplanetary distances, and park in orbit.

    Loading and unloading would be accomplished - not by anything on the cargo craft (why carry the deadweight or machinery and fuel everywhere?) - but by surface-to-orbit tugs deployed from the landside base. The machinery and fuel then makes the shortest trip necessary to get to the cargo ship and delivery goods.
     
  12. Jan 21, 2012 #11

    DaveC426913

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    What? Reducing the mass of a ship in space is a bad idea because it will be torn apart? That's a new one.
     
  13. Jan 21, 2012 #12
    most of the time when someone tries and makes something lighter they cut corners and the strength and durability is sacrificed, lighter ship means thinner or weaker walls, which means little micro-asteroids will put more holes in it.
     
  14. Jan 22, 2012 #13

    Ryan_m_b

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    I don't agree with this unless you are talking about garden shed engineering. Multi-million and billion dollar projects do not (99.999% of the time) cut corners. Things are redesigned to work with smaller mass, new materials that are just as strong but lighter are used etc. Also the more massive something is the more fuel it requires to move it, but that fuel is more massive as well so requires some additional fuel to move it.
     
  15. Jan 22, 2012 #14
    While your argument seems correct @Ryan_m_b, both yours and @DaveC426913 responses are short-sighted. Once you have a well designed device (i.e. optimal materials, structure, etc), strength always and necessarily goes with weight. If you want a stronger pipe, i-beam, plate, whatever, you add structure which increases the weight. Necessarily.
     
  16. Jan 22, 2012 #15

    Ryan_m_b

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    If the design is optimised then why would you need more strength? My point was that in megaprojects if you need something to be lighter you don't cut corners, you redesign for better materials and better efficiency. If you've got to the limit of what you can get at the moment you work with that and if that isn't good enough you cancel the project.
     
  17. Jan 22, 2012 #16

    Drakkith

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    Not only is there a performance criteria, there is a budget as well. Many of the most advanced, strongest, lightest materials are VERY expensive. If simple aluminum is able to be used to meet your performance criteria then you will most likely use that. If it turns out that aluminum is too weak then you would turn to some other material if possible.

    This doesn't just count for the materials used in construction, but the overall design as well. A good example is the Kepler space telescope. It had to be redesigned to use a non-movable antenna in order to cut down on both complexity and cost. This did however cost them in observation time, as for about 1 day a month the telescope has to be turned away from it's observation point in order to send data to Earth.

    My point is that a "well designed device" can be costly and complex, simple and cheap, or anything in between. And I can guarantee you that any space freighters in the future WILL have to take cost into account during their construction.
     
    Last edited: Jan 22, 2012
  18. Jan 22, 2012 #17

    DaveC426913

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    Look at the philosophy you are asserting. "When it comes to better and safer, bigger is worse, smaller is better." (At least that's what I'm seeing you and Gaia saying).

    Really? So when they have mega projects on Earth, smaller is better?

    Look at oceanliners, freighters, supertankers. Do they cut corners on these vessels? No. It is more efficient and safer when we build larger. That's why we build larger. If it weren't better and safer, we'd still be putting around in millions of tiny boats, carrying a pittance of payload each.

    Which would you rather be on in the middle of an ocean that's trying to kill you? A skiff or a 1000 foot cruise liner?
    Which would you rather be on in the middle of space that's trying to kill you? A small cargo ship or a giant one?
     
  19. Jan 22, 2012 #18

    DaveC426913

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    No. Other way around.

    For a given tonnage of cargo, smaller ships have more area to get holed by microasteroids.

    27 ships holding one tonne of cargo each have a surface area of 162m3.

    One ship holding 27 tonnes of cargo has a surface area of a mere 54m3.

    Additionally, that one large ship will make far more efficient use of its resources. (eg. It doesn't need 27 pilots, 27 computers, 27 sets of landing gear and 27 toilets) therefore, pound for pound, the larger ship will mass even less for its cargo. Move more cargo with less deadweight, means less corner-cutting, not more.


    Engineers figured all this out centuries ago. See previous post.
     
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