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Let's not debate whether is there any friction while traveling inside vacuum space and assume all ships can brave a perfect storm brewing inside the giant molecular cloud. OK let's get down to business and tell me why my generation ship turns into a tuna? I remembered asking for a tetrahedron to save trillions of dollars so why should I pay that much to cross the galaxy? I see it also comes with sail and antimatter reactors. (No FTL upgrade option)

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    $\begingroup$ It can also be for mechanical structure reasons. A rocket-shape ship is better suited for stress if the engine is put at the rear. $\endgroup$ – Binson Jan 13 '17 at 6:16
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    $\begingroup$ Cilindrical, maybe yes, for structural reasons as user Binson says. Streamlined, not likely. $\endgroup$ – AlexP Jan 13 '17 at 6:25
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    $\begingroup$ The common answer to this question is "The Rule of Cool": tvtropes.org/pmwiki/pmwiki.php/Main/RuleOfCool The Borg in Star Trek were developed explicitly after debates of this question arose in the 1990s among sci-fi fans of the day (pre-World Wide Web). $\endgroup$ – SRM Jan 13 '17 at 6:41
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    $\begingroup$ @Binson is right. For straight acceleration cylinder is best. For turning, sphere is best. For ship that mostly accelerate one way, but needs a turn sometimes, "tuna" is neat compromise. This, plus what nzaman answered. $\endgroup$ – Mołot Jan 13 '17 at 6:45
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    $\begingroup$ Well, we were over-budget, so we outsourced the hull design to some tuna fishermen in India... $\endgroup$ – HopelessN00b Jan 14 '17 at 4:55

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One reason that might play a larger part than you'd think is actually aesthetics.

Sure, a big ugly ball of components is the cheapest option now, but will it net you any sponsors for your next mission? Will it inspire the next generation of potential astronauts? Will people back home look at your ship and think 'Yes, that's a ship on which I want to set out to the stars!'?

If your ship is nothing but a mess of cold welded metal and ugly thruster ports then nobody will be inspired by it, like it, or want to sail (if sail is the right term for you) on it. If all you're doing is accelerating in a straight line to get from star to star and have the structural engineering wherewithal to avoid overloading the structure of your ship (which I really hope you do) then you can use any shape you like. If you make the substructure of your ship as ugly as you like and then slap an aesthetically pleasing superstructure on it (potentially with some nice holo-screens for in-orbit advertising revenue) then you can get more sponsors, recoup the costs of the nice design, and also net more recruits for your interstellar mission in the process.

So oddly one suitable answer to your question is: because it looks good.

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    $\begingroup$ Good point....wait, let me ask my wife if she wants to take a cruise to Europe on that luxury liner or on Jim Bobs Fishing Trawler! $\endgroup$ – PhasedOut Jan 13 '17 at 13:49
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    $\begingroup$ > "General officers in the Air Force were not too shy about their distaste for Boeing’s design. One two-star famously declared the Boeing option just too ugly to be in the Air Force fleet." aviationweek.com/blog/when-lockheed-martin-won-jsf-award-2001 $\endgroup$ – pkamb Jan 13 '17 at 19:00
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    $\begingroup$ There are some sound technical reasons for wanting a ship to look good. "Beauty is more important in computing than anywhere else in technology because software is so complicated. Beauty is the ultimate defence against complexity." — David Gelernter $\endgroup$ – Jeutnarg Jan 13 '17 at 20:13
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    $\begingroup$ @tox123 : Not sure evil dictatorships need to worry about attracting sponsors for their planet-killing doom stations. :-D $\endgroup$ – Joe Bloggs Jan 16 '17 at 9:55
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    $\begingroup$ @GrimmTheOpiner: It all comes down to technology available and the speed you want to be going at. At any appreciable fraction of C any impact is going to be catastrophic (It will literally make the hull of the ship act like water), so shape doesn't really matter and you may as well make it pretty. Below that streamlining and aesthetics go hand in hand, so why not make your ship pretty at the same time as streamlining it? $\endgroup$ – Joe Bloggs Jan 17 '17 at 9:36
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Space isn't a pure vacuum. There's still bits of rock and other debris floating around. Do you really want masses impacting your ship at fractions of light speed?

Streamlined designs would have anything hitting you from the front sliding off at at angle, instead of transferring all the momentum to one point on an airtight vessel in a vacuum. The ones hitting the sides must be endured, but they won't have nearly the relative momentum as a rock hitting the front while the ship is at full speed. Basically, the same reasons that ships are streamlined.

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    $\begingroup$ The problem is, no matter how much you streamline, the dust particle moving near light-speed relative to the ship will see the ship as a flat disc, due to length contraction. $\endgroup$ – sampathsris Jan 13 '17 at 11:23
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    $\begingroup$ Not really rocks. "Approximately 70% of the mass of the interstellar medium consists of lone hydrogen atoms; most of the remainder consists of helium atoms." en.wikipedia.org/wiki/Outer_space#Interstellar_space $\endgroup$ – JollyJoker Jan 13 '17 at 13:54
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    $\begingroup$ @JollyJoker it only takes one rock $\endgroup$ – wjchulme Jan 13 '17 at 14:52
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    $\begingroup$ This seems just wrong to me. At orbital velocities or above then sloping or not isn't going to matter, the velocities involved are just huge. $\endgroup$ – Tim B Jan 13 '17 at 16:57
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    $\begingroup$ @Maxander Yeah, the trouble is - at those speeds things don't so much explode as punch holes in your ship. esa.int/Our_Activities/Operations/Space_Debris/… $\endgroup$ – Tim B Jan 13 '17 at 21:59
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Radiation Hazards, or why I learned to stop worrying and build my ships like skyscrapers.

One reason among many is to shield the crew from Radiation. Not incident interstellar radiation, though - radiation from your own ship.

Since solar isn't really an option in the dark interstellar void, chances are you're using some variant of nuclear power, fusion or otherwise. Sterilizing your crew and giving them cancer is generally frowned upon, and as such we need to design around it. The most obvious solution is to just put big monolithic blocks of lead between the reactor and the crew, but lead isn't known for being lightweight, and space travel is about getting the most use out of the least mass. Huge radiation shields are technically an option, but not a good one. There's hope, though.

Radiation (Both particle and EM) falls off by the inverse square of distance. If I'm twice as far from the source, I get only one fourth the dose. Four times as far, and I get only one sixteenth the dose. It logically follows that I want to put my crew as far away from the reactor as possible.

The easiest way to do this is to make the ship long and skinny, put the reactor on one end and the crew on the other. Between them and the reactor we pack our fuel/propellant tanks to help soak up some radiation, as well as any systems that won't be bothered by it. If the radiation is still too extreme you can put some shielding there, but you won't need anywhere near as much as you would if the crew were right next to the reactor.

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    $\begingroup$ as seen in 2001 a space oddyssey. not particularly streamlined though. $\endgroup$ – Jasen Jan 14 '17 at 3:06
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    $\begingroup$ @Jasen Actually streamlined because Kubrick left out the enormous tanks of ammonia that will be needed to decelerate X-1 into Jupiter orbit. $\endgroup$ – David Tonhofer Jan 14 '17 at 18:53
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    $\begingroup$ @svick It will not create long lasting radioactive elements like fission. Still, Fusion power would likely irradiate the reactor blanket and the inevitably generated gamma radiation is able to penetrate a lot of shielding. $\endgroup$ – Adwaenyth Jan 16 '17 at 14:41
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    $\begingroup$ @UIDAlexD Yeah, even fundamentally, at some point you're moving a lot of charge - that's bound to at the very least generate some high energy photons (though there might be tricks to limit that effect). If it changes nucleons, there's a pretty good chance you'll get high energy photons directly, as well as neutrons and other dangerous or unstable particles (like pions). Some of the danger is really inherent in the amount of energy involved - it's just "that big of a 'fire'" - and especially the concentration. $\endgroup$ – Luaan Jan 17 '17 at 16:29
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    $\begingroup$ @Luaan When you think about it, Humans and our material creations exist on the extreme low-end of the temperature/energy scale of the universe. Our highest-temperature alloys will melt and vaporize at only a few thousand kelvin, while the hearts of the most common and boring stars sit comfortably in the millions. Solid matter seems to be one of the most difficult materials to maintain conditions for, existing in a range of only a few hundred to thousand kelvin. Water-based protein-bags exist in a range of only a few tens of kelvin. $\endgroup$ – UIDAlexD Jan 17 '17 at 17:07
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A streamlined shape has more advantages than just reducing drag while travelling through some medium.
The streamlined appearance is only on the outside. The outside also is the barrier between non-inhabitable space and the cosy inside.
It is safe to assume that the material of the hull is fairly expensive, so you want to minimize cost for hull material. That means you want a shape that provides maximum volume per surface area.
Geometry tells us that the perfect shape for this would be a sphere.

So, why do you end up with a tuna, or a zeppelin, or something similar?

Space is mostly fairly empty, but also really, really huge.
As a result, you need to go very, very, VERY fast to get from somewhere interesting to somewhere more interesting.

So you are travelling at mind-boggling velocities.
Now imagine an obstacle. For example a pebble. Just sitting there in space (which it won't do, it will be moving, but that is not that important).

Imagine hitting a pebble with a delta-V of something like 2000 times the speed of a sniper rifle's bullet. This thing will scratch the paintjob of your nice new ship somewhat badly.
So you also want to minimize the cross section as seen from the front, because that is where the pebble will hit. A smaller cross section simply means less pebbles. Also, the steeper the slope of the front, the easier it is to deflect this pebble.
Now you have a bullet nose. As for the tail section: This will most likely be anything that your engine requires, plus maybe some fairing, because even engineers tend to simple shapes when they can.

Voilà, here is your beautiful interstellar spaceship.

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    $\begingroup$ So what happens when your accelerating and the rocks come from the side with high velocity? Then the sphere is better then the bullet nose, so I think it is also dependent on the usage of the ship. Lower speed one would be more sphere like or a disk, depending on how good your pre warning is, the one for fast travelling of high distances would be more bullet like. $\endgroup$ – C.Fe. Jan 13 '17 at 16:04
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    $\begingroup$ @C.Fe. for a frontal impact, velocities add up. that is not true for the sides. A disc would be a very bad idea. It would leave you with one very large side. It would also work against the volume-to-surface-ratio. $\endgroup$ – Burki Jan 13 '17 at 16:07
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    $\begingroup$ Isn't a pebble sitting in space pretty much the definition of not moving: it is in free fall with no external forces acting upon it? Other things will be moving relative to the pebble, but the pebble itself is completely still. $\endgroup$ – Evil Dog Pie Jan 17 '17 at 12:27
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    $\begingroup$ @MikeofSSTif you add a force to your pebble you can see if it was sitting there or moving: will the pebble move directly along the vector of the newly applied force, or will it move in a different direction? If the latter, then it was moving. $\endgroup$ – Burki Jan 17 '17 at 14:23
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Manufacturing and maintaining a ship type in a … larger space yard

As long as your ships manufacturer is producing types or models of ships—instead of custom products—they will need to have a space yard fitting the ship. This factory would need to be either larger than the objects size, or it would have to adjust to a growing (for e.g. spherical) shape. Both difficult and expensive things to do.

         /———⚇———⚇———\                         Manufacturing area
         \———⚇———⚇———/                         ↓
               ❚                           ---------
  /☳☳☳|☳☳☳|☳☳☳|☳☳☳|☳☳☳|☳☳☳|-///       ↑
‹[ ☰☰☰|☰☰☰|☰☰☰|☰☰☰|☰☰☰|☰☰☰|-===       Ship
  \☶☶☶|☶☶☶|☶☶☶|☶☶☶|☶☶☶|☶☶☶|-\\\
               ❚                ↑ 
         /———⚇———⚇———\          section
         \———⚇———⚇———/

If you have a cigar-shaped vessel, you can produce section by section by moving along while making progress. In any case where you would manufacture sections that you can not stack on top of (ok, it's space, there is no top, so: next to) each other, you would need an additional factory, completely different in size and shape from the first one, just to mount parts together. This would result in an additional step in production. If one step delays, the other factory is stalled.

When a manufacturing yard would have to adapt (by transforming) to the size of the ship, it would loose time. Just compare the process to what Henry Ford did. Another aspect would be, that it would be unable to perform maintenance or upgrades on existing ships if the factory would have to adapt to the shape of the ship. Simplified: A cigar (or tuna) shaped ship can easily vary in size (even as upgrade), while any other shape will not allow an equally easy format for the yard.

The Reliant Robin, License: CC BY-SA 3.0 Wikipedia-User "Charles01"

One example of bad, but cost efficient design: The Reliant Robin (Wikipedia).

Shape boils down to: Money. It's simply not cost efficient in production.

Money is the reason for a lot of shapes that seem to be less practical than others, but are practical when your stakeholders want to see the most monetary output for their investment.

TopGear on YouTube

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    $\begingroup$ This is something that is overlooked consistently and that I hadn't thought of. Manufacturing always plays a huge roll in design. It is really the reason why we aren't all wearing carbon-nanotube shirts or driving electric cars with a 1000 mile range (examples are said with some amount of hyperbole but gets the point across). $\endgroup$ – theinvisibleduck Jan 16 '17 at 18:51
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Another additional reason: they need the extra surface.

Contrary to popular belief, space is not cold. Space is just empty, and that means that it has no temperature, and the ship cannot pass any surplus generated heat to the space other than radiating it.

The issue is, in any closed system, any conversion of energy (from example from fuel to kinetic energy, or electricity, or light, or from food to chemical energy in our bodies) is going to produce some unwanted heat.

And unless you get rid of that excess heat, your astronauts will arrive at their destination a little too much cooked to what is considered acceptable.

So, maybe your machinery produces enough heat that you need extra walls to heat them so they may radiate the heat outside.

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    $\begingroup$ @Craig a body producing no heat will cool to match the background temperature. A body producing heat (like a star) will cool to reach equilibrium, losing heat as fast as it's producing it, this can be arbitrarily hot. A working spaceship with a live crew produces somewhere between quite a bit, and a hell of a lot of heat. The blackbody temperature can work out to be several hundred to thousand degrees if heat production isn't taken into account. $\endgroup$ – Leliel Jan 15 '17 at 16:56
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    $\begingroup$ Why was cold a problem on the Apollo 13 mission when they turned the heater off, with those 3 bodies crammed in that tiny volume? $\endgroup$ – Craig Jan 15 '17 at 21:02
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    $\begingroup$ @Craig Square-cube law. Same reason large animals have much lower metabolism per weight than small animals. A giant spaceship will radiate more heat absolutely than the small one, but a much small fraction of its total. $\endgroup$ – Bryce Wagner Jan 16 '17 at 0:21
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    $\begingroup$ @Craig They designed the ship to lose a lot of heat and used the heaters to make up for it. $\endgroup$ – Catprog Jan 17 '17 at 3:12
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    $\begingroup$ @Craig Because they knew that heating is much easier than cooling in space. So the whole thing was designed to radiate as much heat as possible, while using heaters to keep the cabin at a comfortable temperature. This is still the best design we have today. But make no mistake, a person in a space suit will die from overheating if his life support dies. The tricky thing is that emissivity and absorption are the same thing in reverse - if you shield from direct radiation, you get poor radiative cooling and vice versa. The best we have is white paint - reflect visible, absorb (& emit) infrared. $\endgroup$ – Luaan Jan 17 '17 at 12:03
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The spaceships not only travels interstellar spaces. It might do the take off and landing in an environment that has atmosphere or friction.

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    $\begingroup$ frankly, i doubt that. lifting a really huge spaceship out of a gravity well just consumes way too much energy. park it in orbit and send shuttles instead. Most of the time you want to transport passengers and cargo. no need to carry along all your backup, life support, main engines and whatnot. $\endgroup$ – Burki Jan 13 '17 at 12:07
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    $\begingroup$ Yes. This. Remember: not all landings are planned. When an emergency happens near-ish a planet, being streamlined might make the difference between a crash landing and incineration. $\endgroup$ – Jeffiekins Jan 13 '17 at 17:22
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    $\begingroup$ @Jeffiekins If there's an emergency, you fix it or die. You can spend a lot of engineering effort making your ship streamlined for atmospheric flight, and also making it a boat, or a transforming mega-maid to suck up planetary atmospheres - all of which add cost and failure points - or you could spend time fool-proofing your design so that an emergency doesn't arise. $\endgroup$ – UIDAlexD Jan 13 '17 at 18:32
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    $\begingroup$ @UIDAlexD There is no such thing as a fool-proof design Another problem is that you may need the planet (Gas giants are perfect to refill hydrogen) and that repairing the outer hull of a spaceship on earth-like planets is much easier and safer than in space. $\endgroup$ – Thorsten S. Jan 13 '17 at 22:10
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    $\begingroup$ gravity wells are deep. if you want to re-enter you need heat shields, the apollo capsule re-enters with its least-streamlined face leading. $\endgroup$ – Jasen Jan 14 '17 at 3:10
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It's a combination of several factors, most of which have already been mentioned but I'll put them all together anyway, with some helpful analogies.

1. Aesthetics/Rule of Cool

For this I'll use the analogy of a skyscraper. The most efficient design for a skyscraper, in terms of using all the available space, is a big tall cuboid. But you won't find many modern skyscrapers that look like that because big tall cuboids are really boring to look at. The Burj al-Arab, for example, looks like a giant sail. Freedom Tower tapers neatly along its edges. The Gherkin looks like a... gherkin. Alternately, look at cars. What looks cooler, a boxy, bulky Land Rover, or a smooth, streamlined Aston Martin? Actually, on the subject of sports cars...

2. Drag Reduction

(got ninja'd by @tom on this one)

You're right that friction isn't really a concern in space, but I'm assuming that at some point, your interstellar ship will want to land on a planet and then take off from it again. And when you're leaving or entering a planetary atmosphere - especially when you're entering - friction is a very big concern. Friction is (as far as I'm aware) what generates the massive temperatures that cause objects to burn up in our atmosphere, so unless you want your ship to disintegrate upon re-entry, streamlining - and decent heat protection - are very important.

3. Deflecting Foreign Objects

As @nzaman and @Burki have already pointed out, an interstellar ship will be travelling very, very fast. Fast enough that hitting something even the size of a pebble could cause major damage due to the relative velocity. IIRC from Wikipedia (I'll look it up later when I have time), a 6g piece of metal hitting your ship at orbital velocity will leave a 3-inch crater... and your ship will be travelling a heck of a lot faster than orbital velocity. Also, going back to point #2... space junk. If we have it, other advanced planets probably have it as well, to a greater or lesser degree, and some of it will be too small to detect and dodge.

A streamlined design will have two benefits here. First, it'll reduce your ship's frontal cross-section and thus reduce the chance of that evil space pebble hitting your ship in the first place. Second, you know how tanks have sloped armour? That's to increase the relative thickness: an object hitting a metal plate at a 45-degree angle has a lot more material to go through than if it hits it straight on. So it'll also reduce the damage caused by those tiny pieces of space debris, and increase the chance of them just deflecting off.

4. Saving Weight/Material

Interstellar spaceships are, as you hinted in the OP, expensive. Really expensive. And the materials required to build them might be hard to come by. So you'll want to cut costs. That means that you'll want your ship to have the smallest surface area possible without compromising on the interior space, not just for aerodynamic purposes, but so you can use the absolute minimum amount of material necessary. This will also save weight, which depending on your propulsion system, might make your ship more fuel-efficient and save you even more money in the long run.

So yes, there are plenty of reasons you'd want (or even need) your interstellar ship to be streamlined.

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    $\begingroup$ The smallest surface to volume ratio is a sphere. $\endgroup$ – Mindwin Jan 13 '17 at 12:41
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    $\begingroup$ Pressure is what causes re-entry heating, not friction. It's a bit of a nitpick buy hypersonic aerodynamics is completely different from regular aerodynamics. Also, I severely doubt an interstellar ship is going to be able to land. Makes more sense to have some hangars for dropships/landers than it does to turn a flying city into an aircraft. $\endgroup$ – UIDAlexD Jan 13 '17 at 18:29
  • $\begingroup$ Not even in terms of using all available space when you get tall enough. But that's not the most important thing by far - there's also wind resistance, sunlight, load balancing... The forces acting on a tall skyscraper are rather brutal. Cuboid only looks like a good to a social engineer - few people would like to live or work in such a building. The Burj Khalifa is a nice study of providing natural light to as much area as possible. It does work well enough for underground structures, though, where the constraints are very different. $\endgroup$ – Luaan Jan 17 '17 at 11:45
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Smaller Cross-section

If your ship is long in the direction of travel and narrow in other directions, it means your ship presents a smaller cross section along its direction of travel. Some advantages include:

  • Fewer collisions with dust and other debris, and thus less wear on your hull, or less energy needed from your shields or navigational deflectors
  • You present a smaller target to enemies you are approaching or fleeing

Given this design, there are also advantages to having your ship taper off at both ends: it means a greater volume of your ship is adjacent to the forward-facing or rear-facing hull. For example, this may mean you can mount a much bigger forward-facing laser cannon or deflector array or whatnot.

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Everyone seems to have forgotten a single important principle.

Round is stronger than square

https://www.youtube.com/watch?v=hUhisi2FBuw

Simple. Pay special attention to wall depth and required materials.

Now why does it need a pointy front? It doesn't. It just looks cool.

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I see at least three reasons:

1) One of the technologies being mooted at the moment for the ISS is a great big laser to blast orbital debris sufficiently to deflect it. So a smaller cross-sectional area in the direction you are traveling is an advantage for deflection.

2) For rotational artificial gravity, rotational symmetry matters, at least in terms of mass, as does having bulkheads that are parallel to the rotation so they work as floors. So in a rotationally-symmetric ship, at any point there'll be a circular cross-section, or similar. (Admittedly, you could instead have a large mass on one side, counterbalancing a large arm further out but with higher velocity, orbiting around the center of mass. The center of mass would remain stationary, which is important when you want the thrust to go through that center of mass. This design is way more of a pain in the ass and more unstable, though)

3) For any kind of static anti-radiation shielding around the ship, smooth lines with no spikey bits make the field easier to calculate and keep constant.

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    $\begingroup$ +1 if we've developed generation ships, there's a good chance we've also developed technology to actively deflect incoming debris. $\endgroup$ – adelphus Jan 15 '17 at 15:41
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  • The design wants to separate two components (e.g. power plant and passenger quarters) as far from each other as it is feasible. This gives a cylindrical hull, or perhaps a dumbbell.
  • The design uses a technobabble drive with limitations on the geometry.
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For the same reason your eye is a ball. Vacuum.

  1. Spaceships may travel in space but the space is filled with gravitational fields of different magnitude.
  2. Also there are stars that constantly fill the space with many types of radiation that curved surface have larger probability of bouncing of.
  3. The streamline design put less tensions on the joints and welds of the spaceship itself. It's also take better the movement of the material due to different temperatures (expanding and contraction)
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    $\begingroup$ My eye is a ball because of vacuums? $\endgroup$ – Lightness Races in Orbit Jan 13 '17 at 11:05
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    $\begingroup$ No it didn't, but... They are balls. And, in theory, if you pull ball in every direction with equal force the ball won't be affected. Your eyeball (or actually eyeball of a living creature that have pressure inside) evolved to survive short periods of vacuum force pulling it out (out the eye socked, because the vacuum could be inside your head) $\endgroup$ – SZCZERZO KŁY Jan 13 '17 at 12:35
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    $\begingroup$ I can't imagine what sort of circumstances led to sight unaffected after exposure to vacuum being an evolutionary advantage, but okay. $\endgroup$ – Lightness Races in Orbit Jan 13 '17 at 12:36
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    $\begingroup$ I'm guessing the language barrier is causing "reduced pressure" to be mistranslated. $\endgroup$ – WGroleau Jan 13 '17 at 12:47
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    $\begingroup$ the vacuum could be inside your head LOL! $\endgroup$ – Jasen Jan 14 '17 at 3:14
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Landing

When you get to your destination, you don't know how much fuel you'll be able to get. So you have to carry landing fuel with you. Rather than carrying enough fuel for multiple trips, just land the whole thing. Once on the ground, you can easily go to and from the ship. You can walk. No fuel needed. Use solar panels, etc. and charge up the batteries on wheeled or tracked vehicles.

Refueling

I remember one of the old RPGs (Traveller?) had a streamlined option. They used it to fly down into a gas giant or ocean and refuel the fusion reactor. Without that, you had to buy fuel at a space station, which doesn't work so well for a generation ship.

The point here is that if you get to the target star and can't find a suitable planet, this could help you move to the next star system.

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Nobody has yet suggested the shape which needs dictate. The needs are to minimise the chance of colliding with anything of significant size, and to allow the people to experience gravity. The former, because at a fraction of light speed, encountering a grain of sand is akin to being successfully targeted by a nuclear missile. Not survivable. On the other hand, if it whizzes past a mere six inches away from your hull, it is harmless.

Gravity seems to be a physiological necessity for human beings, and this is a generation ship implying that there is no way to cheat and store everyone in suspended animation. It is plausibly speculated that gravity is even more vital for a baby developing in its mother's womb, than for adults, to avoid developmental abnormalities.

So the design will be a cylindrical central core containing reaction mass and main engines, and three or more long thin hulls attached to it with cables. The whole assemblage will rotate around the axis of the central core, creating pseudo-gravity in the outer hulls. Inside those hulls the experience will be similar to living in a ship if the water-borne sort, but without the waves. Lots of length, not so much width.

Radiation shielding (against the radiation caused by impact with mere molecules!) dictates that most of the inhabited hulls be filled with reaction mass, and that this mass will be the last to be used during arrival. The inhabited bit will be at the back.

The geometry is the easy bit, compared to everything else!

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If your ship uses a light sail as you described then they want it streamlined to minimize its shadow. The more stuff you have poking out the more photons they block.

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Much of course depends on your technology (e.g. the propulsion system and so on). For example if you travel through handwavium distortions, and the distortion is spherical, it makes sense to build a ship as a sphere extending to the limit of the available distorted volume. You could even place the most expendable materials in the periphery, so that in a pinch you can generate a smaller distortion and scamper away with your ship's core, leaving the outer layers of HMS Onion to distract an enemy or to fight to the last against whatever astronomical phenomenon you're fleeing.

In some novels by Mark L. Van Name space Portals are circular in shape, so that you want a cylindrical ship as large as possible to use the whole Portal, and as long as possible to get the most material through. The need for some structural flexibility makes you end up with spaghetti ships (or ships that can fold into spaghetti for traveling).

Warshawki sail gravitic propulsion dictates a double hammerhead shape and there's little that can be done about it.

And collapsed matter gravity generators force you to travel in the stem of large mushrooms.


If we make without handwavium propulsions and collapsed matter, and we only introduce relativistic speeds, then cylindrical shapes become almost a necessity.

  • when traveling at relativistic speeds in real space, without "energy shields", you continue to receive light from the outside on the whole ship surface, but your frontal cross-section receives the light with a strong Doppler effect compression. At not-so-low speeds this translates into a blue shifting of the light coming from the front. At relativistic speeds, the light of ordinary stars, but what's more, even the cosmic microwave background, will be shifted towards X rays and hard gamma, so much that it will not only irradiate the prow, it will start photodisintegrating its surface. Individual particles cannot travel at relativistic speeds for too long because they too incur this effect, leading to what's known as the Greisen–Zatsepin–Kuzmin limit (which also holds for starships, so they can only move so far between repaints - or ablative shield replacement). It makes sense to have as little cross section as possible. At low speeds, drag considerations lead to prefer a frontal cone or bullet shape to a blunt cylinder. Since the prow will need to be armored and made of radiation-resistant material (for lots of reasons that will probably be where the main water tank will be housed), a cylinder allows limiting the mass that needs to be lifted.
  • The ship will presumably need very powerful engines and they're likely to require lots of cooling, as well as to emit lots of radiation; they'll not be Orion propulsors, but they might be something close. Both reasons suggest to deploy a long "tail" with radiating fins, and possibly place a shield between the body of the ship and the power plant / engine section.
  • the cylindrical passenger and cargo section allows maximizing the surface that, once spun (and counterspun) to produce pseudo-gravity, experiences the same acceleration. Possibly, the cylinder could be made up of a series of "donuts" linked to a central spindle.
  • having a cylindrical "payload" allows for a modular construction, simply lengthening the spindle and adding more donuts on it. In Slow train to Arcturus the various donuts are actually separated worldlets capable of limited independent navigation. The donuts themselves could be further made up of cylindrical sections capable of limited rotation along their axes; this way the donut rotation and the ship's thrust both accelerating and decelerating could be compounded to give the desired gravity acceleration, and have the cylindrical sections rotated in such a way that the acceleration is perpendicular to the floor (a variation of this mechanism is presented in J. P. Hogan's Endgame Enigma).

In the end, you get something not unlike this:

        <||=######====|====<<<<

which could be (charitably) defined as fish-shaped.

Some reasons against cylindrical shapes which (while true) I still don't buy

  • "spherical shapes better redistribute pressure". True, but cylindrical shapes aren't so far behind; high pressure gas bottles are actually cylindrical, so their better packing factor must be enough to offset any structural advantages the spheres may have.
  • "ease of construction": a surface plate for a spherical shape can go anywhere on the surface. True, but so can a surface plate for a cylinder, except for the top. Actually, there are reasons why the top is better constructed of a different shape, different alloys and so on; having a spherical shape means that all the surface needs to be able to withstand the onslaught in the direction of the motion. And if a part of the sphere is internally different, this kills all arguments for symmetry. As someone else already observed, cylinders are actually easier and cheaper to construct on a volume basis, since they can be extruded from the shipyard. And as I myself observed, cylinders are easily modularized. Spherical wedges would be much more awkward.
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Only to impress investors who judge everything based on art and know nothing about the design elements of performance engineering. I don't think it really matters though what shape a ship is before it impacts with an object the size of a baseball or passes through a cloud of ice crystals as small as grains of table salt while traveling at a significant fraction of the speed of light.

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I don't imagine an interstellar ship would bother much with a streamlined body design. I believe there are multiple reasons, but the one I'll muse about here is mass.

The more massive the ship, the more energy it would take to accelerate it anywhere near relativistic speeds. What would be the point of any real trip between stars if you couldn't reach relativistic speeds?

As far as we know, most or all of that energy--in the form of some kind of fuel--would need to be transported on the ship. That's bound to take up some room, and of course the mass of the fuel itself must also be accelerated.

Pushing an object to the actual speed of light would require an infinite amount of energy. Pushing an object to, say, half the speed of light would merely require a gigantic, massive, currently unfathomable amount of energy. So we're talking about a problem where a vessel carrying a finite amount of fuel needs to produce an effectively infinite amount of energy (grossly over-simplifying).

All told, the mass of the ship is going to be an absolutely fundamental engineering problem requiring incredible technological breakthroughs. Adding a fancy streamlined extra outer hull would add mass that the ship simply cannot afford.

On the other hand, who's to say that humanity won't come up with some kind of infinite energy drive. Maybe we'll be able to pull power from subspace, or use a matter/antimatter reaction involving di-lithium crystals, or harness power from a black hole or magnetar through some kind of subspace conduit. Or since the subspace conduit will probably be a wormhole, which may well be a black hole, maybe we'll be able to channel mind-blowing amounts of energy from magnetars or gamma ray bursters via blackholes.

Or we might develop a warp drive that compresses and expands space around the ship so it skips across the universe faster than light could travel conventionally, but without phasing out of sync in time, and making the mass of the ship effectively irrelevant. That would be cool, and the ship could look cool just for the sake of looking cool.

And, who's to say that super-streamlined outer hull couldn't be made out of a feather-lite yet surprisingly flexible and strong, one-atom-thick carbon lattice of some kind?

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The interstellar space is a vacuum, but not complete (100%) vacuum. There is a super rarefied gas with density > 0. So the faster the space ship travels the denser will become the contrary gas flow and the windage (incl. friction) will increase. Although the most of the gas is plasma (ions) and it is good to have a shield against it.

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The sections with air pressure would be most economically shaped like a balloon

Also halving the cross section would half the chance of hitting interstellar debries. So if everywhere onboard is pressurized, and we reduce the cross-section: we get hotdog spaceships

Though, if we assume the engine puts out a large amount of force at a single point (as other people mentioned), then this will mould our shape in much the same way as the carriage of a hot air balloon moulds the balloon into a tear drop

... And a tear drop is kind of streamlined, at least back-to-front

But since we dont have galaxy travelling spaceships, we can bet they will to some extent be based on technology/physics that we also dont have . Maybe they will travel on gravitational waves, and thus most optimally be shaped like surf boards?

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I think that it's because they look good but also it's science fiction. When it becomes real it will look as unusual as a solar sail or something similarly unique.

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Filling the envelope defined by some field

Supposing that even in the future no hull material is itself strong enough to endure high speed collisions/friction implied by the scale of space and space travel; it might be that some kind of protective field must be used instead. A protective and/or time-and-space-bending field, in fact.

The apparent streamlining of the ship may be the result of making the most efficient use of the space within the envelope defined by such a field; or it may be the shape from which it is most economical to project such a field.

This would be slightly different from the pulp sci-fi generic "shields" as used in combat. Those would probably be blunt; having to provide adequate protection in every direction.

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Scientifically reasoned, all of the above could be true...a long streamlined ship with crew at one end and ?power source? at other could protect crew, dislodge space rocks, and be aesthetically pleasing. Gas atmospheres would slow anything but a streamlined ship...there are also vast clouds and pockets of gases in space. Structurally, a slim, triangular shape is physically the strongest shape, and more able to nudge aside space debris. Strength is needed, and a universal geometric shape, or a needle-like design seems best. Form and function exist in geometric designs we know, and a disc/triangle makes universal sense (even a Borg-cube would suck in our atmosphere without a special field eliminating mass, inertia, drag, or resistance).

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It wouldn't.

Obligatory link: http://www.projectrho.com/public_html/rocket/basicdesign.php

Your interstellar space ship would likely not look like anything you see on earth. Not a ship, not a plane. The closest resemblance would be an oil rig. There would be various parts connected by pure structural beams.

If you insist on having it somehow streamlined, you need to invent a reason. One could be some kind of force field that protects or works as a warp bubble or whatever. This could, by physical laws, be a sphere or oblong sphere and enforce a restriction that leads to a specific shape.

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It was designed by an intelligence incapable of conceptualizing Euclidean geometry

Extra-dimensional visitors, or maybe an AI that lost (or never had) the ability to calculate simple geometries.

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