9
$\begingroup$

In my world a giant ship attempts to land on a planet, sadly it fails. Fortunately the top half is intact but the bottom is ripped up, holes all over it, basically destroyed except for some lucky spots. I want the bottom of the ship destroyed but the top intact, meaning people in the top won't die. some things to know

  • the planet has a pressure of 1.5 atmospheres and has an atmosphere similar to ours
  • the ship is 100 meters tall, 200 meters wide and 300 meters long.
  • the ship is in a egg like shape
  • the heat shield is made of similar materials to the tiles that were on the space shuttle but much bigger
  • each of the 25 floors are separated by thick steel but it is hollow for wires and other necessary things.

so what can i do to make it partially destroyed but not all. you can change the heat shield if necessary to answer.

$\endgroup$
  • 1
    $\begingroup$ you mention the entry atmosphere problem with heat shield, but not the actual landing. Is it vertical or horizontal landing? $\endgroup$ – Kepotx May 8 '20 at 13:55
  • 2
    $\begingroup$ Landing without appropriate landing gear will do nicely. Or the popular dirt-skipping. Or if you think it's too durable for that, have it touch down in lava. $\endgroup$ – Matthew May 8 '20 at 13:58
  • 1
    $\begingroup$ Something wrong with crashing into Maui? 😉 $\endgroup$ – Matthew May 8 '20 at 14:09
  • 2
    $\begingroup$ Another happy landing $\endgroup$ – Richard May 8 '20 at 23:36
  • 2
    $\begingroup$ "Just get us on the ground." "Oh, that part will happen pretty definitely!" $\endgroup$ – Harper - Reinstate Monica May 10 '20 at 12:11
13
$\begingroup$

I don't see a realism tag on this question, but realism is all I know, so I'll give it my best shot :)

What happens during re-entry? When you reenter from Earth orbit, you are going very fast, around 8500 meters every second. That is very fast. When you reenter from a higher orbit or trajectory (such as Apollo, which reentered directly from the lunar altitude, or Hayabusa or OSIRIS-REx, which have/will reenter directly from an escape trajectory (in reverse)) then you are going faster. Potentially much faster.

Worth noting, if you're reentering to a planet which is larger than earth, as your "1.5 atmospheres" indicates, then you will be reentering yet even faster.

When you are going many kilometers per second, and there is an atmosphere in the way, strange things happen. In aerodynamics there are multiple regimes--traditional aerodynamics, transonic, supersonic, and hypersonic. But Mach 26 is something else entirely. Reentry aerodynamics are considered their own regime.

The first thing is that the air in front of you is compressed, very quickly. So quickly that there is no time for heat to move. This makes it an adiabatic compression, similar to that used for ignition in a diesel engine. Remember PV=nRT? If you consider the volume in front of your reentering body as one unchanging volume, then it does not change, but the pressure does, dramatically. Your temperature will rise, dramatically. This is known as compression heating. It gets hot.

So you've got a 3000 degree C Mach 26 blowtorch pointed at your spacecraft. How do you survive it? You have to get rid of the heat so it doesn't cook/melt everything. How do you do this? There are a couple ways.

  1. Put it in tiles: If you want to survive getting very hot, you find something that can survive getting very hot, and you ride down on that. This is how the shuttle worked. Those tiles got very very hot--after all, they just absorbed the heat, but they had a very high specific heat, so they just soaked up the heat until reentry was done. This is why the tiles had to be very thick--if they weren't thick enough, the heat would travel through the tile and melt the glue on the other side.

  2. Put it inside your spacecraft: The X-15 survived its reentries by using the body of the vehicle as a heat sink. The frame was designed with a special alloy that could take the heat and still be strong, so they just let it get hot, and put an AC in the crew cabin. But Rene 41 is very expensive stuff, and most other metals tend to get a bit squishy at these temperatures, so you either need fancy metals or a lot of normal metals so the heat (energy) is distributed enough that the heat (temperature) in any one place is still fairly low.

  3. Put it in water: The phase change of water from liquid to vapor requires a massive amount of energy. Dynasoar was a very serious proposal that would have also used Rene 41, but to help take the heat, it would have had a jacket of water surrounding all the essential melty bits. As the body of the vehicle heated up, the water would boil, and it would vent the steam as it descended. But water is heavy stuff, so for real-life purposes it's not the best idea. Maybe with scifi rocket engines it wouldn't matter so much.

  4. Let it burn you up (but not all of you): Also known as ablation, this is how most heat shields work. Essentially, you find something that takes a lot of energy to burn/vaporize/whatever, and then it absorbs the heat by burning away. This doesn't even have to be particularly high-tech. Some Chinese spy sats used wood soaked in resin to do the trick of getting their film back to the ground safely. We don't tend to do that because it's heavy (seeing a theme here?) but it works.

So how do heat shields fail? Well, generally they don't. Most people are familiar with the Columbia disaster, but fewer are aware that shortly after Challenger, Atlantis lost a tile before reentry too. So why wasn't Atlantis destroyed? Because the missing tile was luckily one that was right below a steel mounting plate, so the steel just absorbed the heat. See method #2, above. Were it anywhere else, the heat would have been sufficient to melt the aluminium frame, and once that started, the frame of the vehicle would begin to distort, and, improperly oriented, the aerodynamic forces (and heat) would have torn the orbiter apart. See Columbia.

So how would your situation occur? Well, frankly, if your entire vehicle is a "giant ship made of steel", you might not even need a heat shield. While aluminium, which most space craft are made of, melts at around 700 degrees and boils well below the 3000 of reentry (source, note that space-grade Al alloys are rather different critters), steel melts at around 1500 degrees C and has a much higher specific heat. (But do note: melting isn't required for loss of structural integrity, see "jet fuel and steel beams".)

Anyways, the very massive vehicle frame might be able to just take the heat and make it down in one piece, for the same reason that metal meteors tend to get further than stony meteors do.

But let's look at that a bit further. Meteors tend to blow up (becoming many many meteorites) because where there are irregularities in the surface of a reentering body, it concentrates airflow, increasing the pressure even more, and making it VERY VERY hot in the one spot. This tends to melt/ablate it further, making the pit larger, and increasing the effect. This eventually generates a hole with so much pressure in it that it blows the whole meteor up. Sidenote: this is why heat shields need to be very very smooth, and why damaged tiles were a grave danger to the shuttle.

Now we have a possibility.

Your heat shield was severely damaged for whatever reason, and during reentry, those pits in existing tiles and missing tiles concentrated heat and pressure, burning through to the ALUMINIUM frame. The frame melted, deforming the heat shield and eroding it further. Jets of molten and boiled metal lance through the vehicle, setting fire to the interior. Luckily, this happened sufficiently late in your reentry that the entire vehicle wasn't melted by the hot plasma entering from below before it slowed down, and what fire suppression systems you had were able to mitigate the effect of the heat from suffocating your whole crew/melting propellant tanks/lighting goddamn everything on fire. Well, at least in the upper/mid decks.

The pressure differential was enough to kill most people in the lower decks, but your vehicle is huge, so the entire frame wasn't melted. Also, your magical scifi reaction systems were strong enough to maintain attitude during reentry, so it wasn't torn apart by going sideways-side-down instead of down-side-down through the atmosphere.

If it was made of steel, you'd probably actually be in fairly good shape. Such a scenario is actually rather plausible for very large, very strong structures. In fact, most spent rocket stages could conceivably survive suborbital reentry (and many, mostly stronger Russian ones, do), but they're designed for efficiency, and while they're strong going up, once there's no more pressure in tanks and they're going sideways, it's a lot harder to stay in one piece.

So there you go. Your vehicle is large enough to endure the heat load caused by heat shield failure without totally melting and (most importantly) its reaction control is strong enough that it doesn't lose attitude and disintegrate from aerodynamic forces.

$\endgroup$
  • $\begingroup$ I just want to note: as far as "massive scifi ships made of magic scifi alloys falling out of the sky" goes, that one scene from SW Ep. III isn't too far off. $\endgroup$ – neph May 10 '20 at 1:33
  • 1
    $\begingroup$ wow, this answer is amazing, you must have put so much effort into this, and it seems perfect. although its actually half the size of earth and inspired by titan so you got one thing wrong. also its been orbiting the planet (its actually a moon, also like titan but planet is simpler) for a while so its not going that fast $\endgroup$ – Topcode May 10 '20 at 2:21
  • $\begingroup$ Thank you. :) I wrote it between a bath & drinking another beer $\endgroup$ – neph May 10 '20 at 2:47
  • 2
    $\begingroup$ "But water is heavy stuff, so for real-life purposes it's not the best idea." - human habitats require water anyways, so perhaps they could move it to the front on reentry. Or are we talking about much more water for ablation than a human habitat needs to recycle? What if the ship used water reservoirs as a solar wind shield, is it enough water then? $\endgroup$ – John Dvorak May 10 '20 at 14:51
  • 1
    $\begingroup$ @Topcode I apologize, I wrongly believed that titan's atmospheric pressure was caused by heavier gasses - but it's mostly nitrogen. Seems like it really just has so much gas volume that its atmosphere is very thick. I rescind my objection and thank you for the opportunity to learn :) $\endgroup$ – Syndic May 12 '20 at 13:21
20
$\begingroup$

Crumple zone.

crumple zone

source

Your pilot realizes they are coming down too fast. They are thru the atmosphere thanks to the heat shield but they are in free fall and they are going to hit hard. She cannot slow down but maneuvering thrusters are ok; she pivots so they come in tail first. She uses the rear of the ship as a crumple zone to slow them down.

$\endgroup$
  • $\begingroup$ I am given to understand that this is the main purpose of struts. $\endgroup$ – chrylis -cautiouslyoptimistic- May 9 '20 at 2:05
  • 2
    $\begingroup$ Doesn't work out too well. A bit of math: The top will decelerate to zero over s=300 meters (that's the ship's length); let's assume a maximum deceleration of a=10g (which is barely survivable. Speed is v in a=v²/(2*s), so your maximum survivable speed is sqrt(2*a*s) = sqrt(2 * 300m * 10 m/s²) = ca. 77m/s. So... to make the crumple zone idea work, you have to assume that of the 8500 km/s of atmospheric re-entry, your normal deceleration mechanisms work fine for the first 84,999,923 m/s and fail on the last 77 m/s. You'll have a pretty fine ride down and a sudden crash at the last moment... $\endgroup$ – toolforger May 10 '20 at 8:14
  • 2
    $\begingroup$ What is that source lmao $\endgroup$ – neph May 10 '20 at 18:06
11
$\begingroup$

How about a textbook perfect landing, but on top of something that couldn't hold it's weight? A sinkhole under only part of the ship causes it to fall over, leading to all manner of damage wherever the author finds it most convenient.

Perhaps some megafauna attacks the landed ship, but can only reach the areas you want destroyed.

Some random schmuck was too busy getting lucky with another member of the crew to properly attend to their maintenance duties, and one of the landing struts fails to deploy. Alternatively: sabotage with the same result. Depends on what kind of story you want to tell. "Gilligan's Planet" or "The Astronaught Who Loved Me"

The ship smacked into something in the upper atmosphere shattering one of the heat shield tiles. Some fraction of the crew was able to make it to the life raft/top section of the ship before being incinerated. What did they hit? A flock of gasbag creatures? A stealth satellite? Enemy fire? Again, it depends on what kind of story you want to tell.

$\endgroup$
  • $\begingroup$ this has some good ideas, i might use one of them $\endgroup$ – Topcode May 8 '20 at 14:17
8
$\begingroup$

Heat shield failure

This happened to me A LOT in KSP before heat shields were introduced and before I got the gist of it. I would often try an insertion in Eve or Laythe and lose the bottom part of my landers, which was specially infuriating when it meant whatever was left didn't have all the components I wanted on ground, or when the vessel landed in an awkward angle which would prevent deploying solar panels or an antenna without breaking them.


Whatever part of your vessel is leading during a reentry is the heat shield for everything behind it. What we usually call a heat shield is just a mass of stuff that is built specifically for this purpose.

Once the proper heat shield is gone, then the next layer of material is the heat shield for everything behind it. And once that is gone, the next layer becomes the heat shield.


So in your case, what you need is a failure of the proper heat shield. A hole in it will do. And this hole may be due to a collision with a micrometeoroid or a piece of the ship that broke off. That's luck of the draw, but it can happen.

Edit: also what M Arif Rahman Winandar said in the comments:

Or maybe the spaceship just re-entered at a too steep of an angle and the heat shield got all burned up because it couldn't handle the extra heat.

A good heat shield deflects the gas away from it. With a hole in the shield, a pocket of hot gas may form that will get hotter and hotter as more compressed gas is added to it. Eventually it causes an explosion which chips away more of the shield. The resulting flak also breaks more stuff.


This is actually what caused the Space Shuttle Columbia disaster:

During the launch of STS-107, Columbia's 28th mission, a piece of foam insulation broke off from the Space Shuttle external tank and struck the left wing of the orbiter. Similar foam shedding had occurred during previous shuttle launches, causing damage that ranged from minor to nearly catastrophic, but some engineers suspected that the damage to Columbia was more serious. Before re-entry, NASA managers had limited the investigation, reasoning that the crew could not have fixed the problem if it had been confirmed. When Columbia re-entered the atmosphere of Earth, the damage allowed hot atmospheric gases to penetrate the heat shield and destroy the internal wing structure, which caused the spacecraft to become unstable and break apart.

Columbia broke completely apart, but your spaceship may have a structure that allows for the top to remain relatively intact.


The top may also be built to detach, like a LES (Launch Escape System). In case of catastrophic damage to the lower side, the top side detaches. It may have a heat shield system of its own.

$\endgroup$
  • 3
    $\begingroup$ Columbia was also pretty small compared to the OP's ship, which probably improves the likelihood of it not being destroyed completely. $\endgroup$ – Matthew May 8 '20 at 14:44
  • 1
    $\begingroup$ Or maybe the spaceship just re-entered at a too steep of an angle and the heat shield got all burned up because it couldn't handle the extra heat. $\endgroup$ – M Arif Rahman Winandar May 8 '20 at 15:11
  • 1
    $\begingroup$ KSP is not a good measure of craft destruction, the vessels can take far more than a real spacecraft. $\endgroup$ – Loren Pechtel May 9 '20 at 3:33
3
$\begingroup$

Redundant heat shields

The designers of your gigantic spacecraft knew that a "hot reentry" was a possibility and designed accordingly. Different sections of the ship have their own heat shields just in case one section of the primary heat shield fails.

There's a precedent for this style of design on vessels the size of your spaceship. Boats and ships are divided into separate watertight compartments so that flooding in one area won't sink the ship. There are watertight doors between compartments that remain closed when underway. The below diagram shows one reason the Titanic was considered "unsinkable" by the standards of the day. In the aftermath of the ship's sinking, boat builders got better at designing vessels to deal with hull ruptures. Your spaceship could similarly have Permissible Hull Breach Conditions that cover the loss of part of the heat shield.

Titanic flood design

$\endgroup$
2
$\begingroup$

Collision.

The spacecraft hit something as it was coming in. It was small enough that it did tear things up too badly but it did damage the heat shield. The craft was large enough to survive this but the fire tore at it a lot.

$\endgroup$
1
$\begingroup$

You can do it like they did in Star Wars episode III.

Your ship is in bad shape and not really maneouverable. The pilot only manages to steer the ship into a smooth kind of valley where the ship slides on the ground to decelerate. Unfortunately this ship was not designed for horizontal landings, so the lower half is pretty busted, but the top section is undamaged.

$\endgroup$
  • $\begingroup$ A valley will not be smooth and symmetric enough; any irregularity will make the ship rotate and wedge it. Landing on a smooth slope (snow fields) that happens to align almost perfectly with the ship's trajectory has made for some (extremly) lucky plane crashes in the past. $\endgroup$ – toolforger May 10 '20 at 8:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.