Common Near-Future Sci-Fi Spacecraft Design

[Spike Spiegel]

I've noticed over the years that many sci-fi spacecraft that are meant to appear "near-future" in design or construction have a major similarity. They often have a central truss/beam that the other components are "mounted" to. For example, the Discovery from 2001/2010, the Event Horizon, the ship from Defying Gravity (Antares?), the ship from Avatar, and even the Deepstar addon for Orbiter.

I'm wondering if there's a really good engineering reason why this design is so popular. Sure it looks pretty cool, and is acceptable since the ship won't fly in an atmosphere, but why not use a different design? Why is that type of design popular?

 

[ar81]

Probably because of structural reasons, the same reasons why practical design of buildings are usually a block with details.

 

[Andy44]

Well, look at the ISS. It's a central truss with stuff bolted onto it. The reason is that it's much easier to construct a structure in orbit that way, each launch bringing up a new module.

You could build a space ship that looks like a Romulan Warbird, but why? A ship made for use in vacuum doesn't have to be pretty or aerodynamic, it just needs to stand up under thrust loads and torques. External cladding and other things have mass, and useless mass is the last thing you want aboard a spacecraft with limited delta-V.

Vehicles like those in Star Trek, by comparison, have almost magical propulsion systems with nearly unlimited delta-V, and their shapes are explained away as necessary for the warp drive to work. On top of that you can add the artificial gravity which allows the dacks to be arranged like a terrestrial ship or airliner. All whimsical, no realistic approach at all.

A real space vehicle needs to have strength, radiation protection for the crew, and easy access for maintenance by men in spacesuits. So it will wind up looking pretty much like the examples you gave, or like the ISS.

 

[Moach]

...in order words, beautifully ugly :thumbup:

 

[jedidia]

Why is that type of design popular?

a) simple to build

b) structural advantages. You could, in theory, put an engine sideways on a truss (so the length of the truss is aligned with the X or Y axis of the ship), but that would lead to balancing problem as well as integrity problems (it would be like balancing a truss with a few hundred kilos bolted to each end on a relatively small area).

c) weight. Of course you could build a fancy hull around the truss, but you'd still have to have a truss inside to actually hold the weight at acceleration. If we'd have weight to waste, certainly some designers would come up with a nice looking shell around the truss, but as long as every unnecessary gramm of spaceship is worth tons of money that won't happen.

For a sci-fi scenario, I guess it would be acceptable to have a fancy streamlined ship as kind of a luxury yacht, since it's purpose is majorly showing off. Everything that has to do some actual work will use the most efficient design.

 

[tori]

Trusses are an engineering miracle - they're very light, cheap, and as long as you keep the load vector pointed straight down their axes*, they're also very, very strong in compression. A flimsy looking truss can hold itself and tons of load. Since spaceships tend to be linear and accelerate in the direction of the longest dimension, a truss comes as a natural solution for a backbone.

* if you don't, you're inducing a torque on the truss, which is something they can't handle (e.g. that's how a radio tower behaves when you cut the guy wires)

 

[Spike Spiegel]

Excellent answers, thanks. I figured it might be something like that, but you guys are good at filling in all the details and explaining the engineering and such.

 

[Andy44]

Of course you could build a fancy hull around the truss, but you'd still have to have a truss inside to actually hold the weight at acceleration.

Not necessarily; most launch vehicles are basically structural tubes which are also used as propellant tanks. Some of them have stiffening ribs or an outer skin, but the extreme example is the Atlas I/II, which used a very thin-skinned pressurized tank as the main structure, using pressurization for stiffness, without which it would crumple like a beer can.

So parts of a vehicle can do without a truss if the propellant tanks or crew spaces can handle loads, but the parts of the vehicle which must be long for radiation protection (like 2001's Discovery) will probably be trusses.

 

[jedidia]

Not necessarily; most launch vehicles are basically structural tubes which are also used as propellant tanks. Some of them have stiffening ribs or an outer skin, but the extreme example is the Atlas I/II, which used a very thin-skinned pressurized tank as the main structure, using pressurization for stiffness, without which it would crumple like a beer can.

We're talking a bit more mass here, though (but probably also a bit less thrust...) .So yes, one could engineer a hull design that doesn't need an internal truss. I doubt it would be as efficient and lightweight as a real truss, but it could certainly be done. And if it is, it will probably be just as uggly... :lol:

 

[Ark]

We're talking a bit more mass here, though (but probably also a bit less thrust...) .So yes, one could engineer a hull design that doesn't need an internal truss. I doubt it would be as efficient and lightweight as a real truss, but it could certainly be done. And if it is, it will probably be just as uggly... :lol:

Depends on your interior space requirements. If you can support the acceleration of the ship with the outer skin, that frees up a ton of volume inside the ship. If you don't need the additional space, though, a truss is the most efficient option. It's amazing what you can get away with when you only need to handle acceleration from one direction, on one axis.

 

[Andy44]

Here's an example of an interplanetary ship design which uses a large propellant tank as the main structure, with a backbone truss in front enclosing the expendable drop tank. This serves the dual purpose of isolating the crew from the nuclear rockets in the stern, while providing lots of propellant storage volume. This pic is from AndyMc's [nomedia="http://www.orbithangar.com/searchid.php?ID=2794"]Cargo Transfer Vehicles v0.95[/nomedia]:

 

[Izack]

I'm just postulating here, but wouldn't a pyramid also be an effective shape for a spacecraft? Say...four main engines on the four corners of the base of a rectangular pyramid? You could then have propellant mass 'inside' the shape, and attach hab modules and whatnot to the outer edges (distributed correctly for balance, of course)?
Or for a nuclear-powered craft, have one longer hab module at the 'point' of the structure?
I just came up with that on the spot...feel free to blow me out of the water. I have my life-vest ready. :thumbup:

 

[tori]

You'd end up with a bending load on the base plate of the pyramid - as the four engines in the corners push forward, the main mass pushes on the plate, inducing torques that try to fold the pyramid.

It'd be better to flip it upside down - one engine on its point and the pyramid would accelerate base-first. That way all the load would be divided between a compressive load on the side edges (going from corners to the point) and a tensile load on lines going from the corners to the center of the baseplate. Trusses for compression, cables for tension, add some tankage and what not, and off you go.

 

[Richy]

Tension is always better handled then compression, so the ISV Venture Star in Avatar is designed to pull it's cargo.
I think for interplanetary or at least interstellar space ships, the puller design is much more favorable than the pusher, as it's used today.

 

[T.Neo]

I agree- tensionary designs have many advantages.

A spacecraft is often a truss with stuff stuck to it, because this is an efficient way of doing things. Launch vehicles, for example, use their fuel tanks (roughly) as the supporting structure. But if your fuel tanks are small, or you have to be far away from the engine, or you don't have any engine, then that role is taken up by a truss.

You don't need external plating or wings or whatnot, like a Star Trek ship. You just don't need them there. Sure, if your craft is fly through an atmosphere, it's going to look more like an aircraft, but it doesn't have to (Soyuz capsules, MER aeroshells for example).

I can see cislunar "yachts" being "sleek", but I still don't see them looking like airplanes... because they won't be airplanes.

Star Trek spacecraft don't even fly like spacecraft, so one should not take design tips from them...

 

[rseferino]

I've noticed over the years that many sci-fi spacecraft that are meant to appear "near-future" in design or construction have a major similarity. They often have a central truss/beam that the other components are "mounted" to. For example, the Discovery from 2001/2010, the Event Horizon, the ship from Defying Gravity (Antares?), the ship from Avatar, and even the Deepstar addon for Orbiter.

I'm wondering if there's a really good engineering reason why this design is so popular. Sure it looks pretty cool, and is acceptable since the ship won't fly in an atmosphere, but why not use a different design? Why is that type of design popular?

Antares from Defying Gravity in turn be based on the designs for a BBC documentary called "Space Odyssey: Voyage To The Planets"

http://en.wikipedia.org/wiki/Space_Odyssey:_Voyage_To_The_Planets

was based on several ideas engineers, so that would be most acceptable, in my opinion. A two-part documentary highly recommended. In fact, be my avatar.

 

[4throck]

Disregarding capsules and the Shuttle, because they are limited by the need to reenter the atmosphere and land, we have the ISS and the Apollo Lunar Lander as good examples of current tech designs. Both are completely designed from a hardware point of view.

Realistic projects, like the LOK lunar lander and other proposals still look much the same. For example, the Russian mars trip spacecraft designs of the 1980s look much the same as the MIR modules, just assembled a different way.

As for near-future, I'd say you have to look at what materials and hardware (solar panels, tanks, docking ports, antennas, etc. ) is available today from manufacturers of satellites and probes.

For example, the GOCE satellite does look unusual:

The Grace satelites are also interesting:

 

[jedidia]

Tension is always better handled then compression, so the ISV Venture Star in Avatar is designed to pull it's cargo.

Pullers are great if your engines allow it. I can totally see pullers with Ion engines, hall effect thrusters etc.

However, in the case of the venture star (or even a ship with a much weaker, but plausible fusion drive) I would expect the crew to get thouroughly roasted when they are behind the engines (the venture star has some heat shielding on the truss, but as far as I can see none for the crew section.)

another problem with pullers is that you have to have two engines at least, which makes it an impractical arrangement if the engines used are too heavy. It goes very nice for all kinds of electrical drives like before mentioned Ion thrusters or for example VASIMRs, where the main load consists of a nuke reactor (of which you need only one, which you can drag behind), but if you need two fusion drives at 400 tons a piece (because here the reactor IS the drive, so you have to have two of them), you won't be able to make up for it (of course you get more thrust for that, but thrust is a rather subordinated concern. What we want is delta-v).

 

[T.Neo]

The engines are canted outwards from the spacecraft.

If the angle allows the actual exhaust stream to avoid the rest of the spacecraft, then that should not be a problem.

The problem then would be the light emitted by the exhaust stream as it passes the rest of the spacecraft structure. Calculations would have to be done to see if this poses any real threat.

I'm sure the characteristics of the engines have an effect, too.

 

[Richy]

That's right, additionally, both engines have to point some degrees outwards, so the exhaust jet doesn't hit the cargo linked behind.

edit: too late... ^^

May the pulling cable is designed relatively long, so the angle of the engine exhaust gives a remarkable distance between cargo and jet. And as we know, radiation decreases in square of distance.