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Thinking about the question about Arcologies I was wondering using known science and even a little speculated science would it be possible to safely land a large space ship on a planet similar in size to Earth with a similar atmosphere?

I'm talking about city sized space ships holding 100,000 plus people maybe even a million. I'm not asking if it would make sense to land such a craft (generally I'd say no) but would one be able to land without burning up or falling apart or killing thousands on impact? This is assuming no 'anti-gravity' generators. Also the ship was designed for space travel, So it might need to reconfigure for a landing as well...

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    $\begingroup$ What would be the reason? Maybe it could make sense if we are talking about a colony ship that will land on the planet to stay there (so the ship can become the inital base of the colony), never to lift again. If we are talking about ferrying some people or cargo I doubt it. $\endgroup$ – SJuan76 Jun 16 '15 at 22:08
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    $\begingroup$ It might be neat to have the arcology park itself in orbit and have it act as the anchor to a drop-down space elevator. $\endgroup$ – GrandmasterB Jun 17 '15 at 4:16
  • $\begingroup$ I guess it would be safer to land many small parts independently (and maybe re-assemble them on the ground) $\endgroup$ – Burki Jun 18 '15 at 11:53
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    $\begingroup$ See Alistair Reynold's Redemption Ark , that's a fairly hard Sci-Fi version of exactly the same thing. $\endgroup$ – Andrew Dodds Aug 1 '17 at 9:01
  • $\begingroup$ Where is it going to land? As in, literally in what location? Even if you could set down such a craft perfectly vertically, you'd still need a huge area just to settle it down in. Unless the planet is already inhabited and the inhabitants are willing to prepare such a large area for landing, you'd probably have a hard time finding a suitable landing site anywhere. A horizontal component to the landing velocity wouldn't help. $\endgroup$ – a CVn Aug 1 '17 at 14:57

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I really really don't think so.

Without the magic of anti-gravity, a deployable-space-arcology has to fight the gravitational pull from the planet as well as burn-off its own massive kinetic energy from orbiting/interplanetary-travel.

That means it's going to need rockets. Big ones. And a load of fuel to boot. While I don't think it's plausible, I will spend the rest of this post making suggestions to make it slightly more plausible.

First, those rockets.

Lots of fuel means more weight, which means you need to more fuel to slow down that fuel. This is the classic rocket equation.

You can reduce the amount of fuel needed by blasting it out faster. This is difficult to do with chemical rockets - people have engineered them to their upper limits already.

Higher thrusts can be made from various types of nuclear and plasma thrusters.

ACME Arcology. Just add water.

Secondly, add agriculture, water, and (most importantly) people separately. Living things are fragile and squishy. Water is sturdy, but adds weight to the system. This allows the landing to be a little less gentle. Also, the heat from re-entry and your rockets won't cook everyone.

Some assembly required.

Finally, build the arcology in kit-form.

Deploying smaller sections and putting it all together at the destination is probably an easier task.

The larger the space ship, the more care you need to put into making sure the stresses and strains across the ship are balanced. This is most important when landing - otherwise it will break apart an scatter itself across the surface.

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  • $\begingroup$ and for a variety of reasons, ion thrusters are self limiting in the amount of thrust they generate. Plasma thrusters rely upon some energetic power source which is almost always nuclear in nature. What bother with the middleman - just use nuclear (fission / fusion). $\endgroup$ – Jim2B Jun 17 '15 at 4:12
  • $\begingroup$ The nuclear saltwater rocket can do it. $\endgroup$ – Joshua Nov 1 '16 at 16:21
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Space Elevator:

Another possibility is either building (or using an existing) space elevator to lower yourself down slowly. The elevator would need to be strong enough to hold the ship, and the counterweight would need to be heavier. Both are difficult to pull off, but probably easier than battling the tyranny of the rocket equation for something this big. If the ship is capable of interstellar flight a space elevator shouldn't be too unrealistic.

If it's an uninhabited planet, the ship can carry the cable(s) with it, and then drag a big asteroid into orbit for the counterweight. Then a smaller crew lands and anchors the cable. Finally, the big ship hooks itself up and slowly lowers itself down, with the counterweight keeping it from falling. Lowering the ship down in modular pieces should make all of this much easier too.

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  • $\begingroup$ Very cool answer $\endgroup$ – bowlturner Aug 1 '17 at 0:47
  • $\begingroup$ Nice idea. Even better: The spaceship consists of two parts + a space elevator cable. The spaceship insert into geostationary orbit, and then starts expanding its cable. Once the parts start brushing the planet's atmosphere, they experience drag that transfers angular momentum to the spaceship/-elevator, so that it starts rotating with the planet. (Might be tricky to ensure that the right side ends up pointing up/down in planetary coords.) Once rotation and orientation are right, continue lowering the lower half onto the planet's surface. $\endgroup$ – cmaster Feb 27 at 17:10
  • $\begingroup$ @cmaster, unfortunately that's not really how orbital mechanics works. It's weird and complicated, and I didn't understand it till I played ~50 hours of Kerbal Space Program. But basically when you're "in orbit" of a planet you're moving around it super fast. Therefore, dropping the cable into the atmosphere for too long will not speed you up. $\endgroup$ – Bert Haddad Feb 27 at 18:13
  • $\begingroup$ @BertHaddad You start at geostationary orbit, i.e. the center of mass remains over the same location on the planets surface while the parts of the ship separate. Assume symmetric parts. The two parts won't be rotating nearly fast enough to have any relevant rotation when the tether is fully extended. Instead, at noon part A might brush the very upper layers of the atmosphere while part B brushes the same upper layers at midnight. This will decelerate each brushing part in turn, supplying the tether with angular momentum until the tether rotates at the same speed as the planet. $\endgroup$ – cmaster Feb 27 at 21:57
  • $\begingroup$ The difference to normal orbital mechanics is really the tether that allows the two parts to exchange a lot of force. This force keeps the lower part from falling down, even though it's way too slow for its height; and it keeps the upper part from flying away, even though it's way too fast for its height. The orbit of the individual parts won't be anything like the orbits that you see in Kerbal Space Program. The orbit of the center of mass of the entire tether, however, does obey normal orbital mechanics, and will thus remain in geostationary orbit. $\endgroup$ – cmaster Feb 27 at 22:02
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you added the tag reality check, so I will be honnest, you don't want to consider that option. Large space ships shouldn't be meant to land, that would be as dumb as it can go. The power of the engines and the amount of energy necessary to move such a ship near a planet'surface (let alone take off from it !) is astonishing. Moreover, you would also need to have these ships designed to resist both their entry into the atmosphere and their own weight.

To avoid these great hindrances, a more logical way is to build space ships in orbital shipyards - bonus points if the yard orbits a moon - so that you have almost no gravity to deal with. The ships are then used in space and brang in planet orbits from which people travel to and from the planets with transport modules, space elevators, whatever.

The key here is not that it wouldn't be possible to your civilization, it's that it wouldn't be an excuse to be that bad at engineering.

The same way, colonization ships shouldn't land either. The settlers in it should leave the ship with landing modules. This allow these modules to also be colonization modules fitting various usages like housing, oxygen and water production, defense, whatever... while the ship orbits the planet and maybe handles the teraformation business if needed.

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  • $\begingroup$ "... it wouldn't be an excuse to be that bad at engineering" I love this. $\endgroup$ – Morris The Cat Feb 27 at 18:58
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A modular approach may provide at least a partial solution.

It should be possible to break-down a significant portion of the ship into manageable sized chunks that could be parachuted or otherwise flown down to the planets surface where they could be reassembled. Life support systems could be easily included in the necessary modules.

Those components of the ship which could not be landed safely (e.g., the ship's engines) could then remain in orbit. Modular components could include the parts necessary to build power supplies for the terrestrial construction.

For example, imagine a disc shaped ship where modular components are assembled into the outer rings of the disc, while non-modular components are located at the disc's core.

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If you want to not burn up you'll need to come into the atmosphere slowly, thus you'd orbit the planet, then slowly decrease your orbital velocity. If you want to not crash on impact and obliterate the planet, then you'll need to land slowly. The two ways we currently have of solving this are: gliding in onto a runway like a plane, and drifting down with a parachute.

So I think this question boils down to: Is it possible to have an airplane the size of a city? and/or is it possible to have a parachute that could slow down a city-sized object? Assuming we have some futuristic/exotic/light-weight materials, I think either thing would be possible.

Edit: A third option would be a giant, sturdy space elevator. The ship would need to connect to the elevator and then very slowly lower itself down the elevator. The downside to this is that it requires a lot of infrastructure on the planet already and the ship would likely only be able to land in that one location.

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    $\begingroup$ The Space X Falcon 9 rocket doesn't use either one of those methods for landing. $\endgroup$ – Samuel Jun 16 '15 at 22:05
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    $\begingroup$ @Samuel But the Space X Falcon 9 rocket isn't quite the size of a city, so I would assume different design and aerodynamics would apply. $\endgroup$ – Michael Lai Jun 16 '15 at 23:04
  • $\begingroup$ @MichaelLai Ha, and no ship quite the size of a city has glided or parachuted either. I think rockets are a far more likely candidate than either of those. But this answer is actually a question, so we still don't know. $\endgroup$ – Samuel Jun 16 '15 at 23:09
  • $\begingroup$ @Samuel My knowledge about Star Trek/Star Wars is limited, but how do you think they would have launched the Death Star into space, or was it build in space? $\endgroup$ – Michael Lai Jun 16 '15 at 23:15
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    $\begingroup$ The Death Stars, from Star Wars, were the size of a small moon. All were built in space. $\endgroup$ – Samuel Jun 16 '15 at 23:19
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Closest analog for your question would be, what would it take to safely deorbit an asteroid large enough to cause a continental extinction?

To do so will require reducing your arcology's speed from 20k mph to 0mph and your altitude from LEO to 0 AGL. As we have seen with SpaceX's Falcon 9, this is a difficult process to pull off. If you equipped Project Orion-style propulsion on your ship, that could give you the delta v required to skip the firey doom of reentry and just drop out of orbit. (Added benefit of this approach is that PO craft actually get better the bigger they are.) You would need a different method of actually landing because PO propulsion, if used for landing, would make the landing area into a nuclear wasteland. Not so good. Use a lifting body design while landing. Or, sacrifice the landing zone, use the pusher plate as your landing gear then move the arcology to a new area.

If for some reason using nuclear propulsion isn't allowed, conventional reentry is going to be the biggest problem. Controlling your descent profile, preventing tumbling, heat dissipation, transition to flight, landing, properly stowing cargo to prevent damage... There's so much that can go wrong. The mathematics and physics knowledge required to pull this off just boggles my mind.

You don't have to use fueled rockets to land your arcology, do what the space shuttle does and use wings to convert to an air steerable craft.

Alternative to landing one giant arcology: Build lots of smaller ones that can be assembled once they land.

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In principle, I see no reason why a reentry with a heat shield and a landing using rockets would not scale up. Most answers suggest unusual and exotic ways of landing the thing, but it's not necessarily needed. At some point you would run into issues of plausible material strength, but I suspect you're not quite there yet with a city-sized ship.

That said...

While you could theoretically land a very large spacecraft, it would likely have to be designed for it from the start. It's going to need a massive heat shield, and even more significantly its structure is going to need to take the forces of a multi-g deceleration, which is likely not a standard feature of a spacegoing cityship. And though active control could help to a degree, it'd have to be designed to at least not be aggressively unstable in atmosphere.

Do take care when landing. Those thrusters are going to be big enough to vaporize anything nearby.

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This question was faced for real in the 1960's, with man's first landing on another celestial body. The original thinking, known as earth orbit rendezvous, had a complete spaceship landing on the moon, and returning directly to earth. Such a ship turned out to be quite large.

However, an engineer, Tom Dolan, realized that it was wasteful to take the fully equipped spacecraft to the surface with all sorts of things it didn't need to land on the moon: the return fuel, heat shield for reentry, etc... instead, he proposed a smaller, lighter lander with just the bare essentials to land on the moon. This approach was called lunar orbit rendezvous, because the small spacecraft would have to rendezvous with the main spacecraft on the moon.

In the end, lunar orbit rendezvous was chosen, because a complete moon mission could be carried out with one Saturn V launch. The much larger earth orbit rendezvous ship would have to be assembled in orbit from several launches.

To get back to your actual question - it's possible to land an entire spacecraft on an earth like planet, but it would have far greater mass than pure space traveling ship to withstand the gravity, and thus need a lot more energy to move around. Just like Apollo, you'd find it far more economical to build a smaller ship for the purpose of penetrating an atmosphere and landing in gravity, and use that instead.

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In theory you can land anything if it's strong enough and you have enough distribution of force that you don't get component failures, so yeah you could land an arcology scale starship if it was built for it. You'd most likely make it from a diamond mono-crystal (sorry no you wouldn't, see here for the why). You'd probably want to land it in water because the thermal input from the landing thrusters is going to be ridiculous, even landing in the sea you're probably going to sterilise continental scale areas of land around about and possibly upset the global ecology wholesale, at least for a while. If you put it down on the ground you'd convert vast swaths of land into deep pools of glass. What you're not going to want to do is try and go outside for a the first few... I'm not sure... days certainly, probably weeks or months, possibly even years while the weather etc... settled back down.

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YES! Deorbiting a 1 million tonne vehicle from LEO generates 6 petawatts of energy. This can safely be dissipated if your spacecraft has 20 liters of water per person (100,000 people) to evaporate during landing.

Challenge #1:
It has to hold it's own weight on earths gravity and be able to "withstand deceleration". Googling 'heaviest skyscrapre' led me to the fact that we can build self-supporting structures that mass in the hundreds of thousands of tonnes. I found figures like 700,000 tonnes. Needless to say, this is on the edge of plausible for your predicted 100,000 people (7 tonnes of life support + accommodation + hull + food per person). It doesn't leave a lot of overhead though. As a side note, in the 1960's, they were throwing around ideas for spacecraft weighing in excess of a million tonnes (Project Orion). Let's assume it's engine can accelerate it at 1G, then to take off/land from the surface of earth, the whole structure has to be able to withstand 2G of acceleration. Throw in some vibration and factor of safety, and you probably want to rate it a fair bit higher. This is no small feat, as most skyscrapers are designed for relatively static loading situations. I don't think any skyscraper would do well being dropped even a few meters, so your landing thrusters had better be able to set it down really really gently.

Challenge #2:
All that kinetic energy. Your spacecraft is in orbit and has to shed velocity to land. If you have super powerful engines and can both decelerate it before hitting the atmosphere, and lower it slowly through the atmosphere then this isn't a problem. If you plan on using aerobraking you're going to need a super super massive heatshield. LEO is 2-4km per second. Given a 1 million tonne vehicle you have some odd 4 petajoules of kinetic energy. While descending it will convert another petajoule or two of gravitational energy into kinetic energy. The descent time of the space shuttle shuttle was half an hour from 'entering' the atmosphere to landing. In this timeframe the city-spaceships heatshield has to deal with an average thermal load of over about three Terawatts. (Way more than a delorean). This level of power is on the same order of magnitude as USA consumes continually (average over 2005).

To avoid needing to dissipate this energy with rockets, we can aerobrake and evaporate water. Evaporating water consumes 2Mj per kg of water evaporated, so if we pump water through vents in the front of the vehicle to absorb the heat, you will need about two million litres of water. Because you have 100,000 people on board, you will be carrying a lot of water: if you have 20 liters per person then you should have enough. Of course, designing the hydraulic system to dump 20,000,000 liters of water through the front of your vehicle is no easy feat.

If you remember the comparison to the USA energy consumption, this means you could get the USA to boil a 20,000,000 litre kettle!


So the landing process of your craft will be:

  1. Drop orbital velocity to dip into earths atmosphere
  2. Burn off as much energy as possible by dumping water out the front
  3. Fire up some super powerful thrusters to slow your ship down for final descent.
  4. Land
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  • $\begingroup$ If you want to sacrifice volume and simplicity for mass, cryogenic liquid hydrogen is by very far the most efficient heat sink per mass. The reason why the in-development Skylon spaceplane uses hydrogen as fuel is because it is the only thing that can absorb the heat from the engines in hypersonic flight. Also note that this much steam and/or hydrogen dumped in the atmosphere, in addition to the sheer mass of displaced air, may have unfortunate effects on local climate, which may prevent it from landing on some worlds, inhabited ones in particular. $\endgroup$ – Eth Sep 12 '17 at 12:15
  • $\begingroup$ Yeah, injecting that much water and heat into the atmosphere is bound to be pretty bad. I imagine you could get near-boiling rain for an hour or so after landing. I wonder if the friction of passing would mean that you get a supermassive thunderstorm after landing? $\endgroup$ – sdfgeoff Sep 13 '17 at 12:36
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I assume this is a 'colonise a world' scenario? You probably don't want to land it as one big ship, even if the ship can withstand it.

If you want to permanently land such a ship and 'install' it into the world, it'll almost certainly want to be reconfigured to fit the landscape of the world. You can't just put down landing gear and land it as one big ship, as you'll find it rapidly sinks into the ground due to the pressure (google to see what's happening to San Francisco's Millennium Tower due to incorrect foundations).

So – after landing an engineering crew to prep the ground – you'll want to land it in sections, and rearrange them as they land to fit the terrain and spread the weight correctly over your foundations.

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Landing a craft on a planet requires braking. Even if you start out at zero velocity relative to the planet, you have to partially counter the planets acceleration due to gravity or you will do all of your deceleration at once (a less than ideal situation).

The two main ways of decelerating while landing on a planet with an atmosphere are thrust based braking and atmospheric braking.

Thrust based braking requires enormous amounts of fuel. It isn't like a Falcon 9 coming back down after launch because you still have the payload.To launch a rocket, it takes 15 to 40 times the weight of the vehicle and payload. When coming down, there will be some atmosphere braking even of that isn't the main means of deceleration. So, lets be generous and say that it takes 10 times the weight of the vehicle (including crew and payload) to safely land the ship. That is a lot of fuel to still have left at the end of the journey. That much force will require the vehicle to be heavier to deal with that stress (adding more fuel). A narrow base like a normal rocket would require a lot of energy to be generated from a very small area (relative to the mass of the vehicle). A flatter, more spread out shape, like a disk would allow the thrust to be spread more evenly but would greatly increase the structural requirements of the vehicle. It is also likely that wherever the ship lands will be a barren, cooked, hellscape.

Aerobraking (atmosphere braking) is possible but the engineering of that (heat and stress) would be massive. Creating a set of parachutes to cover an object that massive would be daunting. For one thing the object would alter the air flow around it making groups of small parachutes difficult to design. A single chute that is massive enough to ignore the airflow around the ship would probably have to be made out of some sort of unobtanium.

If the vehicle is coming in "hot" instead of braking to "at rest" relative to the planet, it will require a heat shield to slow to a speed that the parachute can operate (and not cause the initial tug of the chute to splat everyone on board). This is even more weight that has to be added. It also means that the vehicle should be designed with some sort of symmetry or it will start to tumble from the uneven air pressure.

Another method of aerobraking is the lifting body. This will fly or glide down. The trouble is that the less aerodynamic it is, the faster it has to go and the longer flat spot you need to find to land on (I recommend water landing for this). The more aerodynamic it is, the more wing you have hanging out there to get ripped or burned off during initial aerobraking (more structure, more mass).

Some sort of combination of all of these might be the best bet. It is also almost certain that any such large vehicle that lands will not be taking off.

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See James Blish's "Cities in Flight" books.

Earth discovers a way to neutralize gravity in a spherical region. This allows whole cities to take to the stars. (It also can hold an atmosphere. Cities that are built on bedrock can be equipped to fly.

Blish plays fast and loose with the science. (Most of his fiction is more exploring topics of strange social conventions. No rivets.)

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