MatterBeam

The Expanse's Epstein Drive: Explained

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On 10/7/2019 at 6:57 PM, sevenperforce said:

Ah, good point. That could prove problematic. If you want to burn radial, great, but if not then you're going to have a great deal of trouble "collecting" all that hydrogen.

Of course, relative to 8.9% c, 700 km/s might as well be standing still.

The spaceship itself will only be travelling a maximum of 2000km/s

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Regarding the complaint about ship densities, I can imagine some pretty awesome composite materials using honeycomb titanium alloys and Styrofoam, with carbon fiber outsides, reinforced with something like titanium chicken wire and kevlar.  It would be expensive as hell to make right now, because we don't have any demand for such materials, but it would certainly be within possibility to manufacture such panels at high volume.  We know that the ships in The Expanse are multi-hulled, like submarines, with the outer layer being armor and main structure, and the inner hulls being pressurized crew areas.  I would fully expect all armor to be lightweight ceramics, and most of the frame might be as well, albeit reinforced with titanium and composites where necessary.  The pressure vessel would be as light as possible while still supporting the weight and pressure it needs to at design limits plus margin.  Inner walls would be just strong enough to support their expected loads at design limits, and would be akin to G tolerant paper walls with a bit of sound deadening.  Any decks supporting heavy-ish equipment like reactors, storage lockers, or crew seats, would be reinforced with cross members that tie in with the outer hull and it's structure.  The whole thing could be made incredibly durable and light to the point of the mass of the atmosphere inside increasing the density significantly.

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22 hours ago, Thor Wotansen said:

We know that the ships in The Expanse are multi-hulled, like submarines, with the outer layer being armor and main structure, and the inner hulls being pressurized crew areas.

It'd probably be smarter to make the pressure hull loadbearing.

Also, all Western submariners disagree with your statement. Silently.

Hull_LosAngeles.jpg

Hulls_ALFA.jpg

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@DDEThere is a scene in the show where a character (Amos) takes a bumpy ride in the space between external and internal hulls. Also, the projectiles hitting spaceships like the Donnager or the Rocinante go through the walls with minimal resistance or shrapnelling, suggesting they are very thin walls. 

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1 hour ago, MatterBeam said:

Also, the projectiles hitting spaceships like the Donnager or the Rocinante go through the walls with minimal resistance or shrapnelling, suggesting they are very thin walls.

They are also very fast projectiles. Then again, railguns in The Expanse have always been a little inconsistent, at least in the books. They're not mentioned at all in the first book, then in the next couple of books it's suggested they fire rounds at around five thousand meters per second - which would only make them useful at extremely close ranges, as at a modest 1000 km range it would take more than three minutes for the round to arrive at the place the target used to be. In the later books it's been suggested they fire rounds at "an appreciable fraction of the speed of light", which would make the rounds annihilate pretty much any ship they hit if they had a mininum of splintering capability. Heck, air resistance alone should be enough for them to flash-fry the rooms the rounds passed through, and they'd create pretty nasty shock waves in the process too.

But instead, railgun rounds in The Expanse seem designed to pass through the target with as little friction as possible, punching a small, clean hole through the entire ship and exiting on the other side almost without having slowed down at all. It seems like their primary purpose is exactly the type of behaviour you described - going cleanly through walls for narrative suspense. They provide the threat of a "hole puncher" that can make sudden holes appear in the ship, killing anything along the straight line between them, but sparing the guy who just happened to sit in the next chair over. If railguns had provided immediate destruction of the entire ship every time, those "dodged the bullet" moments couldn't happen.

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10 hours ago, Codraroll said:

They are also very fast projectiles. Then again, railguns in The Expanse have always been a little inconsistent, at least in the books. They're not mentioned at all in the first book, then in the next couple of books it's suggested they fire rounds at around five thousand meters per second - which would only make them useful at extremely close ranges, as at a modest 1000 km range it would take more than three minutes for the round to arrive at the place the target used to be. In the later books it's been suggested they fire rounds at "an appreciable fraction of the speed of light", which would make the rounds annihilate pretty much any ship they hit if they had a mininum of splintering capability. Heck, air resistance alone should be enough for them to flash-fry the rooms the rounds passed through, and they'd create pretty nasty shock waves in the process too.

But instead, railgun rounds in The Expanse seem designed to pass through the target with as little friction as possible, punching a small, clean hole through the entire ship and exiting on the other side almost without having slowed down at all. It seems like their primary purpose is exactly the type of behaviour you described - going cleanly through walls for narrative suspense. They provide the threat of a "hole puncher" that can make sudden holes appear in the ship, killing anything along the straight line between them, but sparing the guy who just happened to sit in the next chair over. If railguns had provided immediate destruction of the entire ship every time, those "dodged the bullet" moments couldn't happen.

Yeah... Not always the best writing moments IMO. The ship I posted above has no weapons, as IMO in space any weapon that hits the target is a MAD (mutually assured destruction). If you don't hit the target, then yeah, it's okish. Simple defences against debris/asteroids etc, but no attack weapons on civilian craft except some possible small fire stuff (like cruise ships currently have to put off pirates :P ).

Today most air/sea combat is pretty much get into range, obliterate anything near it? If it's in range, it's pretty much scrap?

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16 hours ago, Codraroll said:

They are also very fast projectiles. Then again, railguns in The Expanse have always been a little inconsistent, at least in the books. They're not mentioned at all in the first book, then in the next couple of books it's suggested they fire rounds at around five thousand meters per second - which would only make them useful at extremely close ranges, as at a modest 1000 km range it would take more than three minutes for the round to arrive at the place the target used to be. In the later books it's been suggested they fire rounds at "an appreciable fraction of the speed of light", which would make the rounds annihilate pretty much any ship they hit if they had a mininum of splintering capability. Heck, air resistance alone should be enough for them to flash-fry the rooms the rounds passed through, and they'd create pretty nasty shock waves in the process too.

But instead, railgun rounds in The Expanse seem designed to pass through the target with as little friction as possible, punching a small, clean hole through the entire ship and exiting on the other side almost without having slowed down at all. It seems like their primary purpose is exactly the type of behaviour you described - going cleanly through walls for narrative suspense. They provide the threat of a "hole puncher" that can make sudden holes appear in the ship, killing anything along the straight line between them, but sparing the guy who just happened to sit in the next chair over. If railguns had provided immediate destruction of the entire ship every time, those "dodged the bullet" moments couldn't happen.

The problem with railguns will always be that they punch neat holes through things.  If I were designing a spaceship for the kind of combat we see in The Expanse, I would engineer the railgun projectiles to punch through the crew compartments exactly like you see in the show.  The reason is that the railgun is meant to take out things like drive clusters and reactors, and seriously damage the structure of the vessel, and if it was built to shred the crew compartments, it wouldn't to squat to a reactor with it's heavy shielding and (presumably) armor.  I would even go so far as to say that taking out the crew compartments isn't even a priority.  Sure, the PDCs and railguns will poke holes in them, and any crew in the way, but the real targets in a fight like that are the crucial systems of the ship, not the crew.  This is why I'd have the PDCs firing 20mm armor piercing incendiary rounds, so they wreck any surface mounted equipment like thruster clusters, PDCs and launch tubes, as well as doing damage to the underlying structure of the ship.  Missiles would be either nukes or would be designed to take out the main drive systems.  The railguns are to poke holes in the reactor(s) and reaction mass tanks from farther away then PDCs can do.

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28 minutes ago, Thor Wotansen said:

The reason is that the railgun is meant to take out things like drive clusters and reactors

Nuclear plant buildings are designed to survive a passenger aircraft hit.
A railgun projectile unlikely can be faster than 2 km/s.
So, it's like a 500 kg part of that airplane.

28 minutes ago, Thor Wotansen said:

If I were designing a spaceship for the kind of combat we see in The Expanse, I would engineer the railgun projectiles to punch through the crew compartments exactly like you see in the show.

If I were designing that ship, it would have a polymer protector layer between the hulls, to close the holes.
(Say, a thin layer of sticky foam in micropellets between two hull layers.)

17 hours ago, Codraroll said:

they fire rounds at around five thousand meters per second

Don't the rounds become pancakes in the barrel due to the accelerations and waste heat?
What's that railgun barrel length?

Edited by kerbiloid

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29 minutes ago, kerbiloid said:

Don't the rounds become pancakes in the barrel due to the accelerations and waste heat?

What's that railgun barrel length?

Tungsten is a remarkable material, it is incredibly dense and is one of the toughest metals we have.  A solid slug of it won't really deform, even at thousands of Gs acceleration.  The biggest problem with railguns is getting the slug to transmit all the power without welding itself to the rails.  This is normally done by getting it going at a good clip before it reaches the rails.  The other big problem is heat.  This can be mitigated by using a high temperature superconductor like a Yttrium/Barium conductor at a temperature of around 77°K to transmit the power to the rails.  We might even be able to use the superconductor as the rails, but I don't think that's been tried yet.  Basically, with better materials and a longer barrel, as well as the fantastic thermal properties of a vacuum, it should be possible to make a railgun that fires a smallish tungsten projectile at a good 5km/s, more than enough to overcome whatever armor you'd be inclined to wrap your reactor in.  Most reactor armor will be geared toward defending against more conventional guns, like PDCs, since they'll be more common in the battle space.  If someone makes a railgun that can fire a 10kg slug at 5km/s there's really no point in trying to armor something to withstand it, especially if you want that something to move.

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54 minutes ago, Thor Wotansen said:

Tungsten is a remarkable material, it is incredibly dense and is one of the toughest metals we have.  A solid slug of it won't really deform, even at thousands of Gs acceleration.

Strength ~1000 MPa afair.
Density ~18 000 kg/m3.

Stress = Mass * Acceleration / CrossSectionArea
Velocity2 = 2 * Acceleration * Path

BarrelLength = Path = Velocity2 / (2 * Acceleration) = Velocity2 / (2 * Stress * CrossSectionArea / Mass) = Velocity2 / (2 * Strength * CrossSectionArea / (Density * CrossSectionArea * ProjectileLength)) =
= Velocity2 / (2 * Stress  / (Density * ProjectileLength)) = Velocity2 * Density * ProjectileLength / (2 * Strength);

ProjectileLength = ProjectileDiameter * ProjectileElongation;
BarrelElongation = Path / ProjectileDiameter = BarrelLength * ProjectileElongation / ProjectileLength;
ProjectileLength = BarrelLength * ProjectileElongation / BarrelElongation;

BarrelLength = Velocity2 * Density * (BarrelLength * ProjectileElongation / BarrelElongation) / (2 * Strength);
1 = Velocity2 * Density * ProjectileElongation / (2 * Strength * BarrelElongation);
2 * Strength * BarrelElongation = Velocity2 * Density * ProjectileElongation;

Velocity2  = 2 * Strength * BarrelElongation / (Density * ProjectileElongation) ;
Velocity = sqrt(2 * Strength * BarrelElongation / (Density * ProjectileElongation)) ~= sqrt(2 * 109 * (50 * 3) / (18000 * 30) ~= 750 m/s

For tank guns Velocity ~= 1500 m/s, so twice as this value. That's appropriate.

Any experiments with > 2 km/s projectiles and still stable projectiles?

Wiki says just

Quote

While explosive-powered military guns cannot readily achieve a muzzle velocity of more than ~2 km/s, railguns can readily exceed 3 km/s.

So, 5 km/s looks very optimistic.
And don't forget, these 3 km/s don't shoot bursts. While without bursts it can hardly not miss.

Kinetics is nice for 1 km headshots and for spreading shrapnel from an approaching fast spaceship for free.

Edited by kerbiloid

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4 hours ago, kerbiloid said:

Nuclear plant buildings are designed to survive a passenger aircraft hit.
A railgun projectile unlikely can be faster than 2 km/s.
So, it's like a 500 kg part of that airplane.

In the later Expanse books, railgun projectiles are said to fly at "an appreciable fraction of the speed of light", and one of the experienced engineers in the series does a double take when she looks up just how fast they go, so I think your estimate is off by a couple orders of magnitude at least. It sounds like several hundred kilometers per second is more like it. Just how the engineering of that checks out, I don't know (for a start, the guns are said to be powered by heavy-duty batteries - then again, technology will presumably evolve quite a lot in the next few hundred years), but provided you get a projectile up to that speed, there's no way you can stop it again with something you can carry on a spaceship. Nuclear plant buildings are designed to survive aircraft hits using several metre thick walls of concrete, and plating the reactor with that would be completely out of the question.

And with that amount of energy poured into a projectile, it would be incredibly wasteful for it to be a simple hole puncher. If it doesn't lose speed traveling through its target, it means it fails to impart much of its kinetic energy onto it. Ideally, you want it to slow all the way down to zero, so the target has to deal with all that kinetic energy. The first impact of the projectile would start a cascade of shrapnel which would also travel at several kilometers per second, taking with it anything in its path and starting new cascades when the shrapnel hits something else. Essentially, a ship hit by a railgun round would turn into a giant shotgun aimed at itself. A hit at that speed would wring the ship inside out. That's why I thought that scene in Persepolis Rising was a bit bull, when...

Spoiler

...that Laconian ship took several hits from what is essentially the fortress artillery version of railgun platforms, but survived because its outer hull kept closing the holes as soon as they were created. That's fine for the outer hull, but the railgun rounds themselves weren't slowed down much, and that technology was found only on the outside. The inside of that ship would have become a shower of splinters travelling at near-relativistic velocities, going so fast through the ship's atmosphere it would be turned to plasma. The effect would have been somewhat like a lightning bolt along the entire path of the projectile through the ship, and it was hit multiple times. Laconia is shown to possess quite advanced technology, but nothing suggests they would be able to survive something like that. Surviving a nuke later on would be pretty easy in comparison, since in that instance the hyper-advanced outer hull would shield the rest of the ship.

 

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35 minutes ago, Codraroll said:

And with that amount of energy poured into a projectile, it would be incredibly wasteful for it to be a simple hole puncher. If it doesn't lose speed traveling through its target, it means it fails to impart much of its kinetic energy onto it.  Ideally, you want it to slow all the way down to zero, so the target has to deal with all that kinetic energy. The first impact of the projectile would start a cascade of shrapnel which would also travel at several kilometers per second, taking with it anything in its path and starting new cascades when the shrapnel hits something else. Essentially, a ship hit by a railgun round would turn into a giant shotgun aimed at itself. A hit at that speed would wring the ship inside out.

I you punch a neat hole in a nuclear reactor, how well does it work?  Deigning armor to defeat a high energy projectile that shatters like that is challenging, but doable, especially if you use your armored outer hull as an oversized whipple shield and armor your reactor with a good slab of high strength ceramics with a reactive surface composite over top.  It's much harder to design an armor system to defeat a solid slug that doesn't deform much on it's way through a nuclear reactor at a decent fraction of orbital velocity for LEO

I know the writers of The Expanse aren't science nerds to the level folks like us are, and therefore I forgive them for writing a railgun that fires projectiles at relativistic speeds that can be mounted to a ship or space station.  Let's be real here, it doesn't matter if you're firing a one gram projectile, if it leaves your barrel at "near relativistic" speeds, your ship/space station is being flung in the other direction with some alacrity, that is if it hasn't turned itself to plasma in the process of accelerating that slug.  5-10km/s in a vacuum environment with near-ish future tech is believable, but nothing short of a massive mass driver built into a moon is going to get anything to near relativistic speeds without some serious issues.  The Lorentz forces on the rails from the energy needed for that kind of speed would overcome any structural material we can imagine, not to mention the sheer amount of electrical energy you'd have to put through the rails to achieve those forces.  You'd be better off using a nuke as propellant for a big slab of something.  A more sensible solution would be a staged coilgun with a barrel a good 100km long accelerating a 500kg hunk of steel to 20km/s or something, maybe even 30km/s is believable for a mass driver of that size.

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1 hour ago, Codraroll said:

In the later Expanse books, railgun projectiles are said to fly at "an appreciable fraction of the speed of light", and one of the experienced engineers in the series does a double take when she looks up just how fast they go, so I think your estimate is off by a couple orders of magnitude at least

Then let me suggest 0.99 c fast railgun projectiles. They are even better.

But usually they are called "particle beam" and get launched from a particle accelerator, not railgun.

1 hour ago, Codraroll said:

It sounds like several hundred kilometers per second is more like it.

Now calculate the required acceleration in a railgun barrel of reasonable size.
Hint: it doesn't depend on power source or railgun scheme, futuristic technologies, and so on. It's elementary mechanics.

1 hour ago, Codraroll said:

And with that amount of energy poured into a projectile

both projectile and railgun will evaporate doing the first shot.

1 hour ago, Codraroll said:

The first impact of the projectile would start a cascade of shrapnel which would also travel at several kilometers per second

The first impact of "an appreciable fraction of the speed of light" projectile would cause massive flash, plasma cloud, and UV and Xray emission. Some ionization of course.

(First of all in the railgun itself, of course).

1 hour ago, Codraroll said:

That's fine for the outer hull, but the railgun rounds themselves weren't slowed down much

The first layer of the armor shouldn't slow down the projectile.
It should make its motion unstable and cause precession, to make it hit the next layer with its side instead of tip.
So, the first layer should be placed at some distance.

37 minutes ago, Thor Wotansen said:

I you punch a neat hole in a nuclear reactor, how well does it work?

First try to do that. It's protected by bioprotection (~6 m of concrete or corresponding amount of other materials.) and by the hull armor.

Edited by kerbiloid

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1 hour ago, Thor Wotansen said:

I you punch a neat hole in a nuclear reactor, how well does it work?  Deigning armor to defeat a high energy projectile that shatters like that is challenging, but doable, especially if you use your armored outer hull as an oversized whipple shield and armor your reactor with a good slab of high strength ceramics with a reactive surface composite over top.  It's much harder to design an armor system to defeat a solid slug that doesn't deform much on it's way through a nuclear reactor at a decent fraction of orbital velocity for LEO

I know the writers of The Expanse aren't science nerds to the level folks like us are, and therefore I forgive them for writing a railgun that fires projectiles at relativistic speeds that can be mounted to a ship or space station.  Let's be real here, it doesn't matter if you're firing a one gram projectile, if it leaves your barrel at "near relativistic" speeds, your ship/space station is being flung in the other direction with some alacrity, that is if it hasn't turned itself to plasma in the process of accelerating that slug.  5-10km/s in a vacuum environment with near-ish future tech is believable, but nothing short of a massive mass driver built into a moon is going to get anything to near relativistic speeds without some serious issues.  The Lorentz forces on the rails from the energy needed for that kind of speed would overcome any structural material we can imagine, not to mention the sheer amount of electrical energy you'd have to put through the rails to achieve those forces.  You'd be better off using a nuke as propellant for a big slab of something.  A more sensible solution would be a staged coilgun with a barrel a good 100km long accelerating a 500kg hunk of steel to 20km/s or something, maybe even 30km/s is believable for a mass driver of that size.

 

Well if the expanse folks were dealing in antimatter they could pull it off, although not with railguns.

 

They could instead make a mini-pusher plate antimatter bomb propelled missile.

The only issue would be launch, since your vessel would want to back plenty far away before it does the Orion thing.

 

If you thought a nuke orion was something, wait till you get a load of this!

Edited by Spacescifi

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1 hour ago, kerbiloid said:

First try to do that. It's protected by bioprotection (~6 m of concrete or corresponding amount of other materials.) and by the hull armor.

I have a very hard time imagining a serious spaceship design that includes 6 meter thick concrete anywhere, while still being something that can accelerate at multiple Gs.  The reason we use concrete as radiation containment for reactors here on Earth is that it has a fairly high water content (hence hydrogen), it is a structural material that is easy to build with, it can be mixed with all sorts of things like pieces of lead, and it's very inexpensive compared to other alternatives.  We also tend to worry about contamination of the surroundings of the reactor, something that's just not a concern in space.  A spaceship with a fission or fusion reactor will have minimal radiation shielding compared to a ground based power plant, and it will not use 6 meters of concrete for any of it.  There are plenty of materials and techniques for shielding reactors that are significantly lighter and more compact than concrete, not to mention easier to produce in places without limestone.  A 10kg tungsten projectile traveling at 5km/s is remarkably good at poking holes in all sorts of things, since 10kg of tungsten is only 520 cubic centimeters, or a little more than half a liter (~1 quart, 22lbs).

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1 hour ago, Thor Wotansen said:

I have a very hard time imagining a serious spaceship design that includes 6 meter thick concrete anywhere, while still being something that can accelerate at multiple Gs. 

Hail Orion.  Orion shall Rise!

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5 hours ago, Thor Wotansen said:

I have a very hard time imagining a serious spaceship design that includes 6 meter thick concrete anywhere

When a ship is 20 m in diameter, easily, but the concrete is not a requirement, it's just to illustrate how much strong material should surround a reactor to keep it safe for humans.
So, you can imagine a battleship-thick steel capsule instead.

4 hours ago, wumpus said:

Hail Orion.  Orion shall Rise!

If make an Orion-class probe, it will be The Probe.

All hail-hail Orion the Probe!

Edited by kerbiloid

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