Structural integrity of massive space vessels and mobile megastructures (>1.5km)

Question

I'm working on a worldbuilding project, and so far I've been putting scrutiny into my larger vessel designs, notably a 1.7km long spaceliner capable of landing at specific launch facilities. Someone else associated with the project has been guiding me on ways to reinforce the launch facilities and the vessel itself, but I want to be really sure something like this is physically possible. I'm worried that the vessel might simply be too big to survive landing on the platform, or output 2.5g from the vertical thrusters without snapping. I'll provide images of the structures - vessel in light blue, vertical thrusters in a deeper shade of blue, and the launch platform in red. Feedback appreciated.

Edit 1:

@bobflux

Acceleration shouldn't be a problem for the passengers. The ship doesn't spend much time accelerating, and when it does it cruises closer to 0.01-0.05g, enough for interplanetary travel. Floors could be tilted for that kind of weak acceleration? Interiors would be built for 0g but useable under lots of downwards acceleration, and there would be cabins around the vessel (like with a modern aircraft) in which passengers are strapped in for much higher acceleration burns. Could that work?

The ship is stiff enough for 2.5g forward acceleration, I'd think, but I'm more concerned about how the ground itself would handle that kind of weight. Now the Burj Khalifa isn't sinking, but I'm thinking this vessel would be denser than the Burj Khalifa, in addition to being much longer. Building might need to have a much wider base, like a cone that gets wider as you go down, that might take up a lot of space though, one of the reasons I opted for a flat landing. Any way to subvert that?

As for the weight of the fuel, bussard ramjets collect fuel from the interplanetary medium. That's what the things at the sides are.

As for the engines, I don't suppose allowing a lot of fluid of some sort to flow down the sides of the structure could help lessen the impact from the engines - could that work? Might erosion be a problem long term?

Thoughts?

Answer 1

Assuming no handwavium anti-gravity.

Problem 1

Your ship looks like a boat, there's even a bridge on top of a tower. So, intuitively, the tower points "up", as it does once landed. People prefer horizontal floors, which means the internal floorplan will have floors along the length of the ship.

However, the main engines are at the rear. If the engines have to push it hard enough to accelerate through space at a usable rate, then everyone and everything is going to slide along those "horizontal" floors and smash against the walls.

Therefore, when the engines are running, "up" is towards the nose, and "down" is towards the engines. The floorplan should be updated accordingly, just like in The Expanse.

Now, if the engines can push it to 2.5G, then the structure of the ship should be stiff enough that it doesn't fold like an accordion when that happens. If this is the case, then it can land on a planet as-is, vertically, and the floors will be in the proper direction.

Burj Khalifa tower is 828m high. It's not sinking into the ground nor falling apart, so with a bit of sci-fi materials, that shouldn't be a problem.

This is not very practical though, because it'll want to tip over. To fix this, it should look like a tetrahedron or a pyramid, instead of flat. If it's a cruise liner, you can put plenty of windows on that by hollowing out the sides.

However, the engines will burn everything on the ground, which may be a problem.

Problem 2

If it is very long, then its internal structure will have to be much stronger and therefore heavier than a shorter shape with the same internal volume. This extra weight will result in extra fuel use, which may put your cruising company at a disadvantage.

Answer 2

Space opera rule of cool: Go ahead.

If this story is a space opera / fantasy. Don't sweat the details. The beached whale of a ship can gracefully belly flop onto the spiked platform with no problems.

Hard science.

I believe you are underestimating the energies and engineering problems involved. A single stage ship design to travel from planetary surface A to orbit about A to orbit about B to planetary surface B is a very hard problem. At any size. Hard problems imply expensive solutions. Which implies that even if it is constructed in the far flung future, it will be much more expensive then the alternatives.

Flaws with proposed design.

If landing on its belly that implies rockets on its belly. Which requires a lot of weight and support on its belly and balance issues. Could be mitigated by using the belly engines as primary engines always. This would make fore, aft distinction decorative.

If the aft engines are primary then it could come down tail first and then belly flop onto the platform, but that would cause lots of pain for passenger orientation.

If it is designed as a lifting body then it needs a really big runway. Which would negate the need for the whole landing platform.

Commercial aircraft maintenance is not dominated by flight-hours, but by cycle times. Larger aircraft require more maintenance hours then small. There will be significant maintenance costs for this vessel each cycle if it is landing on surface.

Cheapest design approximation:

If trying to maintain a vessel that can go from planetary surface to planetary surface. The cheapest solution would be a larger SpaceX style rocket. Steel hull, nuclear powered engines. Upright landing and takeoff like the SpaceX rockets. Floors oriented to be nose up, engine down.

More practical solution.

Decompose the problem to make it easier. Have your large mega structure dock to orbital stations at either end. Passengers would reach the orbital stations via:

Surface to Orbit options:

1. Rocket
2. Sky hook
3. Shuttle
4. Space Elevator

A large ship that will always be in space can have radically different, cheaper construction(and cheaper maintenance). It can then have dedicated structures for passenger comfort. All while maintaining ability to have sustained 2.5g burns. No landing required, just docking. Such a ship can have at least an order of magnitude less maintenance with simpler dedicated design would allow for another magnitude less maintenance. That is it wouldn't be a stretch to have maintenance 100x less per trip then the proposed ship. Not counting crew.

Possible to build

But any reasonable hard science engineer will look at the proposed design and alternatives, and say, that would be really expensive, there are much cheaper alternatives.

If however you are operating in a soft science rule of cool world. Then your design is fine. Don't sweat the details. It does the cool stuff.

Answer 3

What is your ship made out of, and what temperature range will this thing go through from re-entry to servicing to launch? On a nice day just sitting there, 1.7km of steel is going to grow or shrink more than a meter just by thermal expansion. Is that a deal breaker?

Answer 4

My advise to listen to your companion.

Just 6 supporting points for 1.7 km long thing looks a bit too low, even one each 200 meters can be problematic(16 points total, at least) even if the thing looks like 100-150 high (thick).

• 200m it is something we can imagine, not that far off of modern rocket size, and in this case you have then imagine one engine or set of engines placed in one spot, like a modern rocket designs, lifts that 200x200x200m cube apllying force in a very limited area of that cube.

But it depends on materias it is made from - if it some carbon nanotbes reinforced hull then maybe it can be good as is.

Landing pad if trusters are reactive propulsion, the thing with all those pilars and delicate pads may indeed not survive even aproach of the ship, not only the landing. Current, way more moderate size rockets they do kind of putting their pads to stress test, not talking about this 1.7x0.2x0.5km ship.

Landing the thing on reactive propulsion will be a challenge, especially this vertical take off/landing approach. Not sure if any surface is good enough, it will act like huge plasma cutter or hydroabrasive cutter, in esence it constantly happening(sequende of many times per second) huge blast descending from the sky - it this kind of forces involved here.

So yeah, you may listen to your companion, if you do not have some warehouse of handwavium prepared for the situation, forces involed here are strong in this one.

To have a better subjective feel of what this situation is, as regidity and such, imagine it being made from rubber, it will descrbe behavior more or less acurately at this scale for any metall like material. To get to that rigidity which you subjectively expect from metals, from you expirience for those on small scale, you need much stronger material than we typically use.

PS

-1 already, wow that was fast, not asking for upvote, but maybe then -1 to antigravity assumption, and comment answer as well, no? Just to be fair.

PPS

Won't comment on the whole edit to the q, but one thing worth noticing

The same orientation it lands, the same orientation it can travel accelerate it space. No tilting floors required and such. There is no air in space to scream, lol.

• "The same orientation it lands, the same orientation it can travel accelerate it space. No tilting floors required and such. There is no air in space to scream, lol." Might that expose more surface area to incoming space dust than necesarry? – Garter Kukri

It could be a concern, which should be and can be solved for varity of reasons by variety of means, meaning necessity is always there and ways to achive the resilt are more than one. And for set of reasons a channel one has to clean, its diameter is more than the size of a ship, much more. So orientation for such small size(few km) really does not matter.

But you are mention Bussard jet scooping, and you scoop that dust particles along with the hydrogen stuff, so as the scooping process creates an cloud of gas/plasma you scoope and which creates the protection layer you require.

In this situation the orientation it even more so appropriate, and it possible to argue it is a better orientation than classical one for the shape, because the big side(top) facing in the direction to the destination faces cleared space already and skde faces/smaller surface faces perlendicular to safe/cleared space and from which some thinga may come. (You cleaned stuff in front, but there is a chance something arrives from the outside of that cleared channel - in front everything is cleaned, but from the sides something may fly from there.

• it not exactly a strictly correct way to look at this situation, more strictly there is no preffered orientation, once front is cleared by collecting scooping. Which also not exatly correct, but too many variables. In your case orientation is rather matter of convinience - and convinience here powerfull engines facing doown, and interrnal floor top layout/orientation.

• there may be some arguments for 90 degree turn, but it is not the space dust, in general and in your case specifically.

All in all with a scoop that collects material for reactive mass and energy - dust in front is not something to be concerned about. And diameter of the scop has to be much much bigger than ship size for to provide noticiable flow of materials, even if speed of the ship is high, so it more or less protected from dust coming from sides. So scoping solves most of the dust problems.