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Scene Description in my Book:

(You can skip this part if you want, I will put the details below)

You are an admiral for "army Red". You have just gained space superiority over the planet you plan to invade by wiping out the "army Green" space fleet. Both sides suffered heavy losses and the space around is littered with the debris of ships from the battle. You decide it’s time to launch your ground forces onto the planet, but the first wave is cut to ribbons by heavy orbital guns belonging to a Green army fortress lying just 5km away from the Green's capital. A Green army traitor gives you the blueprints for the Green's fortress, only for you to discover that taking it would be a more difficult task than you first thought. The fortress is fifteen stories high and eight stories deep in the centre of the complex; the entire compound itself is a two km in diameter. The fortress is built on a hill with 150m straight incline and only one road up. It's also providing overwatch on an air force base 400m away from the hill. The Red army’s admiral has more problems as well. Your fleet may have won, but a large EMP has taken out your long distance travel for several months. With no reinforcements and with the Green army building up man power by the minute, you can’t take or besiege the fortress in time. You have to destroy it, but how? Your ships can’t bomb it, the orbital guns would shoot them down in low orbit, and you can’t use WMDs. Your air force also suffered too many losses to be of use, not that it would matter. The vast network of AA-guns and missiles would cut them down, not to mention the air base. The Admiral is at a loss. There's no way to win this. He walks to the window looking down on the planet with his fist clenched. His view is disrupted by a floating piece of a vessel and the idea comes to him to use destroyed ships as weapons.

Important details: The planet is earth-like in nature but with 0.5 times more gravity. The orbital guns wouldn't be able to shoot down something that fast. There are plenty of different sized ships to choose from. WMDs are not allowed to be used, but dropping a battle ship is. The EMP has taken out all space monitoring devices the sole exception being the fortress itself, so no one else will see it coming (or report it); some of the ships are in low orbit around the planet so accidents happen (if you know what I mean). The fortress is fifteen stories high and eight stories deep in the centre of the complex; the entire compound itself is two km in diameter. The fortress is built on a hill with a 150m straight incline and only one road up. It's also providing overwatch on an air force base 400m away from the hill.

Rules of Combat:

  • The Blast Radius and how Powerful would the Blast be.
  • The Fortress must be ether Destroyed or Damaged Beyond Repair (the airport is a minor concern).
  • The Blast Radius must NOT harm the Capital. The Sonic Boom would be big so Minor damage is fine (broken windows, missing roofs etc).
  • Can only use one ship for the attack (multiple strikes against one target would raise suspicion).

The Question is this: What Size Ship Should I Launch at the Fortress to be the Most Effective?

You can use the names/types of real world vessels to answer this question. For any more details just ask.

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    $\begingroup$ This is related $\endgroup$ – Joe Bloggs Apr 27 '18 at 6:10
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    $\begingroup$ I'd would go to use a lot of small debris, it's much less suspicious a rain of small debris that a single big ship wreck. And the small debris can respect your combat rules $\endgroup$ – Gianluca Apr 27 '18 at 6:24
  • $\begingroup$ @Gianluca in the book that's what he did. he Launched lots of small debris around the planet to make look like it just was a wave of ships coming into orbit big ones and small ones. but he needed to take out the fortress in one go. the dust cloud was cover for his drop ships as well $\endgroup$ – Creed Arcon Apr 27 '18 at 6:30
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    $\begingroup$ Impact calculator $\endgroup$ – AlexP Apr 27 '18 at 6:50
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    $\begingroup$ @CreedArcon one go as "one hit" or as "one operation" ? Because "on hit" is probably violating your rules (specifically the third). $\endgroup$ – Gianluca Apr 27 '18 at 6:52
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tl;dr

Smart bombs (dense, hard, streamlined) are much more effective than ships (big, hollow, bulky) at penetrating enemy armor, since they have a much higher terminal velocity.

https://en.wikipedia.org/wiki/Kinetic_bombardment

A kinetic bombardment or a kinetic orbital strike is the hypothetical act of attacking a planetary surface with an inert projectile, where the destructive force comes from the kinetic energy of the projectile impacting at very high velocities.

Kinetic bombardment has the advantage of being able to deliver projectiles from a very high angle at a very high speed, making them extremely difficult to defend against. In addition, projectiles would not require explosive warheads, and—in the simplest designs—would consist entirely of solid metal rods, giving rise to the common nickname "Rods from God".

During the Vietnam War, there was limited use of the Lazy Dog bomb, a steel projectile shaped like a conventional bomb but only about 25.4 mm (1") long and 9.525 mm (3/8") diameter. A piece of sheet metal was folded to make the fins and welded to the rear of the projectile. These were dumped from aircraft onto enemy troops and had the same effect as a machine gun fired vertically.[3][4] Observers visiting a battlefield after an attack said it looked like the ground had been 'tenderized' using a gigantic fork.

The system most often described is "an orbiting tungsten telephone pole with small fins and a computer in the back for guidance".[citation needed] The system described in the 2003 United States Air Force report[citation needed] was that of 20-foot-long (6.1 m), 1-foot-diameter (0.30 m) tungsten rods, that are satellite controlled, and have global strike capability, with impact speeds of Mach 10.

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There is precedent for this approach: the Fire ship. Get a crappy old ship that still floats, fill it with flammables, set it on fire and get it going towards the enemy.

https://en.wikipedia.org/wiki/Fire_ship

enter image description here

Possibly the oldest account of the military use of a fire ship is recorded by the Greek historian Thucydides on the occasion of the failed Athenian Sicilian Expedition (415–413 BC). In the episode, the Athenian expeditionary force successfully repels an attack by the Syracusans:

The rest [of the Athenian force] the enemy tried to burn by means of an old merchantman which they filled with faggots and pine-wood, set on fire and let drift down the wind which blew full on the Athenians. The Athenians, however, alarmed for their ships, contrived means for stopping it and putting it out, and checking the flames and the nearer approach of the merchantman, thus escaped the danger.

The Fire ship tactic has been done a lot over history. Cool reading. Might I suggest you dig in and read up on this. You will find one that resonates. Maybe you might explicitly copy what was done - some strategist who knows military history comes up with the idea.

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given your combat rules, I think that a single wrecked ship is not feasible since the impact will damage what is around.
Everything you can drop from orbit will generate some sort of collateral damage if too big and will do nothing if too small.

So you have only one option: de-orbit a wrecked ship and once it is on course destroy it, this will create a cloud of debris where you can hide some guided missiles. Since this will make make very hard for the enemy AA to shoot down all of them, your missiles have a good change to enter the atmosphere and then you can activate them to make a targeted attack when is to late to intercept them. And also if they manage to shoot down some the missiles, you always have the debris as backup.

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Raising the questions that need to be asked, not the one you actually asked. Apparently there are fighters and battleships, and the equivalent of anti-fighter Flak and anti-battleship coastal batteries.

Q: If a mostly intact battleship cannot go over the fortress and bombard it, how can a wreck survive long enough to hit it?
A: The mostly intact battleship has a crew that wants to live and fight another day. The wreck is expendable.

Q: How can you make a wreck (too damaged to maneuver and fight) hit the exact spot of the fortress, and not the capital next door?
A: Let's put the capital further away from the fortress. As a side effect, you can attack the fortress with more overkill.

Q: A fortress on a hill, with a steep slope for defense?
A: Mountaintop castles are so 15th century. By the start of WWI, forts were dug in, but often still concentrated. By the start of WWII, forts were dug in and dispersed.

So instead of one fortified hilltop, 2 km wide and 5 km from the capital, have the capital ringed by various emplacements at a distance of several dozen km from the capital. The job of the attacker is to disable most of the "coastal artillery" installations without gross damage to the city center.

It is not necessary to destroy deeply buried command centers and shelters, it is enough to destroy all the (armored ?) gun emplacements on the surface. Or possibly just all of those which can bear in a specific direction, if they don't have 360° fields of fire. Look at the attack on Fort Eben-Emael in WWII.

This might be done with a series of wreckage drops, perhaps over hours or days. If the fragment misses because somebody had mis-estimated how it would tumble, try again.

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  • $\begingroup$ hmm i like the way you said it. the only reason the capital is so close is its a plot point early on and the dust cloud was cover for his drop ships when he attacks, i could change that. thanks for the effort $\endgroup$ – Creed Arcon Apr 27 '18 at 15:20
  • $\begingroup$ Modern forts are dug in and distributed to withstand artillery fire from the side. This would be a minor concern for interplanetary war. On the other hand, the anti-orbital battery may needs to cover as much sky as possible, and thus be elevated. And it may be concentrated because it requires an autonomous energy source (a reactor or such). So I think OP's premises are justifiable. $\endgroup$ – Dallaylaen Apr 28 '18 at 14:18
  • $\begingroup$ @Dallaylaen, I think the separation of firing positions, command posts, and logistics areas still makes sense, if only by a couple of miles of tunnels. Forts were dug in and distributed to better withstand attack from all directions, and to prevent destruction like that of Fort Loncin. You are right regarding good fields of fire, but that depends on geography. A cupola on flat ground without obstructions is not much worse than one a couple hundred feet up in hilly terrain. A single hill rising from a plateau would be very good, of cource.. $\endgroup$ – o.m. Apr 28 '18 at 15:09
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So, you want to nuke it from orbit?

I will assume the fortress is no bigger than 0,7 miles in diameter. I assume that because we know that to create crater of such size you need to drop 160 feet wide iron meteorite.

I name that vessel you need to drop Two bedroom house w/o porch.

Bombing things from space IRL Meteor Crater

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  • $\begingroup$ Of course. It is, after all, the only way to be sure. $\endgroup$ – dgnuff Jun 20 '18 at 7:17
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In some SF circles there is an informal unit of energy known as a "Rick" (after Rick Robinson, who came up with the concept). Essentially it states that an object moving at 3 Km/sec has the equivalent kinetic energy to its weight in TNT. Given that orbital velocity around the Earth is 7 Km/sec, you can see getting sufficient "Ricks" to do severe damage is rather trivial.

Your main issue is an unmodified wreak is going to disintegrate in the upper atmosphere and all those lovely "Ricks" of energy will end up being dispersed over a wide area as small pieces of debris hit randomly over a wide area. While there may be reasons to do so, this isn't going to do much for your putative fortress destruction, and of course a real space defence fortress will resemble the command bunker at Cheyenne Mountain or the nuclear centrifuge complex in Fordow.

enter image description here

The Fordow site from orbit

In order to successfully penetrate the layers of rock, armour and even dead spaces (to disrupt incoming weapons effects), you will either have to spend a lot of time smelting wreaked spacecraft down and using the densest metals to create telephone pole sized kinetic energy penetrators (sometimes known as "Rods from God"), or perhaps more reasonably, turning your nuclear HEAT or EFP warheads against the target.

A 1 megaton warhead could propel a 2.7 ton projectile to 800km/s, while massing only about 3 tons. This projectile crosses the laser's effective range in about 30 seconds, gouges out a crater nearly a 100 meters deep and/or cracks the target in half with 2160 kN.m of momentum concentrated on a spot less than a meter wide.

http://toughsf.blogspot.com/2017/05/nuclear-efp-and-heat.html

A stream of molten metal moving at @ 3% of c will likely not even notice small obstructions like a mountain when spearing through the heart of the complex.

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What size explosion do we want?

Let's assume you can drop your ship precisely on target.

Your target has a radius of 1km, but your capital is only 5km from ground zero. We'll have to be fairly precise.

Let's use a nuke simulator (https://nuclearsecrecy.com/nukemap/), set to a surface detonation. We can ignore the radiation component. We'll ignore the thermal stuff, as it ends up not really affecting what we want. According to that, a 10kT will give:

  • Air blast radius (1000psi): 100m – that's going to totally annihilate your enemy fortress
  • Air blast radius (20 psi): 470 m – heavy buildings demolished.
  • Air blast radius (5 psi): 0.99 km – residential buildings demolished, widespread casualties
  • Air blast radius (1 psi): 2.53 km – windows broken (thus lots of minor injuries)

So this should be sufficient to take out your fortress, disable most of the compound, whilst doing minimal damage to the capital – windows shouldn't be broken.

So we're looking at the equivalent of a 10kT bomb (approx 4x10^10 J).


Impact speed

Using this site (https://www.omnicalculator.com/physics/escape-velocity), and plugging in 1.5x earth mass, the First Cosmic Velocity is 9.6km/s. I believe that's approx the speed something will impact from orbit.


Ship size So what size ship is equivalent to a 10kT bomb (approx 4x10^10 J)?

Plugging all this into KE=1/2 Mv2, we get a mass of 800kg.

You want to drop 800kg at approx 10km/s.

We'll need to drop a bit more than that from orbit to cover what burns up on re-entry, but it'll not be much more.


Issues with de-orbiting a 'wrecked ship'

From your post: "Can only use one ship for the attack (multiple strikes against one target would raise suspicion)."

You need to de-orbit fairly precisely. Your dead ship will be a fairly random-shaped bit of wreckage, so if it hits the atmosphere at a shallow angle, it'll bounce off (as any ship would), but in a fairly unpredictable manner. That's not acceptable as you need to target your city fairly accurately.

So you need to use a large delta-v to de-orbit into a fairly steep decent, so that you'll have a change of hitting accurately.

De-orbiting any dead ship like this would almost certainly raise instant suspicion. It'll look like a planned de-orbit, not a degrading orbit.


But...

But we're not de-orbiting a ship, just 800kg of metal. Compared to a ship, that's tiny, even for today's rockets. We'll pick a nice solid metal rod – a 200mm diameter 3.2m steel rod will be about right. We'll probably want to weld some fins to it for stability.

We'll then pop a rocket or similar on the back to de-orbit it to the desired trajectory.

But this burn will still be fairly large, and could easily be noticed.

So you'll have to cover it with an explosion – make it look like an ammo storage went up or something – and create a bunch of other debris. Some of that will also go towards the planet, and hopefully burn up or hit places we don't care about. We might add a few extra smaller bits on controlled trajectories to hit near the enemy base. Perhaps drop a bigger bit 50km away, so it looks like the enemy base got off lightly. We've got plenty of scope to cover this up.

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  • $\begingroup$ that is very useful info and a good why to de-orbit a ship without much suspicion, i will keep that in mind when writing that part thank you $\endgroup$ – Creed Arcon Feb 28 at 23:18

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