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RainDreamer

What would a Kinetic Orbital Bombardment Strike looks like?

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Much easier would be asteroid redirection. Eg Oort cloud objects don't orbit very fast - 3m/s ish. You could divert a very large object into the inner solar system and it would arrive at earth at approx 42km/s or even faster if it were slingshotted around Jupiter.

Somebody check my maths on this, but a high performance ion thruster (exhaust velocity 5km/s) could halt the oort orbital velocity of a 10,000 ton object with 600kg of propellant. The sun would them do almost all the rest.

If that could be brought in at 60km/s the kinetic yield would be 4 megatons, or 2 megatons without a slingshot.

The drawback would be a painfully long deployment time, but if you brought in many objects periodically on a near miss trajectory they could be altered to strike the earth at much shorter notice.

With the very small amounts of DV required and resource extraction, one spacecraft could potentially redirect many such objects.

 

But considering the launch costs for getting out there in the first place, it would be far cheaper to just nuke it. Unless you were already in an oort civilisation.

Edited by RCgothic

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16 minutes ago, RCgothic said:

The drawback would be a painfully long deployment time, but if you brought in many objects periodically on a near miss trajectory they could be altered to strike the earth at much shorter notice.

No, the energy to modify the trajectory will increase exponantionnaly as you approach the earth, becoming soon impossible to deviate with an ion thruster.

You are going to precise as love at the very beginning and then you will have to use your thrust to try to correct your orbit to make sure you strike the right place at the right moment... or your 2 megatons will strike anywhere on earth. 

The negociation will be fun too... 

"Hey! We have a cease fire agreement" 

"Yes we do... but you are going to be strike next week anyway...."

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Next week?! Dont Oort objects take *millenia* to fall into the inner system?

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I followed up the above post with a calc on the half-period of an orbit with apoapsis at oort and periapsis at sol.

2 million years.

So that idea's out. Although imparting an initial velocity would massively reduce that, but the maths is escaping me at the moment.

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:D

Only amateurs play the short game. 

 

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Necro alert!

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2 minutes ago, KAL 9000 said:

Necro alert!

I swear to god it wasn't me this time!

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On 4/13/2015 at 6:28 AM, Darnok said:

One more thing what if we would shot from satellite orbiting around the Moon?

It should increase speed of projectile even further :)

You would get roughly an increase of a factor of five over the the delta-v it took to get it off the Moon and toward Earth.  If you really want to hammer the Earth, you need to grab some asteroids and maneuver them from there.

Yes, this always bugged me as a RAH fan in my youth.  It would work, but it would only give you a factor of 25 times over the energy used in the launcher.  I'd expect that it will always be more than 25 times easier to get energy out of a bomb than to lift spacecraft (unless we build Orion).

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Meteorites come into the Earth with 15-70 km/s speed. Still a dull explosion, no cutie landstorms.

Meteorite crater shape almost doesn't depend on the trajectory angle. It's created by a shockwave, which is spherical and appears at once near the contact point.
The impactor would either vaporize immediately on ground contact and make a usual spherical burst, or just penetrate several tens meters and lose its structural integrity. No trenches or landstorms.

P.S.
OP video shows London. Probably, they occasionally hit V's underground explosives storehouse.

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Airbursts are more damaging than ground impacts, which is why nuclear weapons are designed to airburst. Meteors often airburst, depending on what they are made of (rock or iron) and what angle they impact at.

Edited by mikegarrison

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The problem with Kinetic bombardment is air drag. It has been said that standard size rods from gods could achieve the power of a tactical nuke, but that was ignoring air drag. Is it possible to make a simple, fueless scramjet, with just a compressor, to help speed up the projectile?  

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4 hours ago, Emperor of the Titan Squid said:

The problem with Kinetic bombardment is air drag. It has been said that standard size rods from gods could achieve the power of a tactical nuke, but that was ignoring air drag. Is it possible to make a simple, fueless scramjet, with just a compressor, to help speed up the projectile?  

"Fuelless"? What good do you imagine that would do?

Edited by mikegarrison

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On 13/04/2015 at 3:15 PM, tomf said:

I guess the 10-tonne of explosive makes sense

The rod isn't gong to have more energy than the chemical energy rocket that put it into orbit.

If a sensible sized rocket could throw an impactor that did nuclear levels of damage they wouldn't bother puttting nuclear weapons on top of them.

This is a really good insight into the feasibility (or lack thereof) of "rods of god" weapons. Of course it's circumvented if you can make the rods in space.

Anyway, as for what it might look like, well it's a hypervelocity impact and there are many small-scale tests of those.

A railgun round striking thin metal targets results in fireballs. https://www.youtube.com/watch?v=O2QqOvFMG_A

A small projectile against a thicker metal target (an empty gas cylinder), again a fireball and a crater-like hole: https://www.youtube.com/watch?v=Dqjq-LQgGDk

An impact into sand gives a spectacularly large crater, but no fireball: https://www.youtube.com/watch?v=nbkkMKkjx6k

I don't know why the third result is. I might guess it's because the potential fireball is "snuffed out" by the projectile penetrating too deep; in the previous two tests it was metal-on-metal which will mean less penetration than metal-into-sand. On the other hand I've also read that it depends on speed.

The scaling for this sort of impacts is fairly well known from simulations and experiments like these, and there's a rather nice website: http://impact.ese.ic.ac.uk/ImpactEffects/

Plugging some example numbers in: http://impact.ese.ic.ac.uk/cgi-bin/crater.cgi?dist=1&distanceUnits=1&diam=3&diameterUnits=1&pdens=&pdens_select=8000&vel=7.8&velocityUnits=1&theta=45&wdepth=&wdepthUnits=1&tdens=2500

The suggestion is that we can't get a decent fireball by just "dropping" something from LEO, it's actually not fast enough. And a fairly small object is only going to cause a fairly small crater. Basically I would imagine that sand video, but scaled up to knock cars and buildings flying. The impactor needs to pretty much bullseye it's target, and if it can do that then this is actually a relatively precise and restrained weapon. Though not covered by the calculator, I would speculate that the wider danger is of large pieces of debris - cars, chunks of buildings, body parts - creating secondary impacts.

If the speed is increased, there's another problem - a small object is likely to break up. The airblast is similar to a bomb going off. For the 3 metre iron ball example that makes it potentially a *less* effective weapon because the blast happens too high up! Although fragments may still hit the ground.

Edited by cantab

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5 hours ago, Emperor of the Titan Squid said:

The problem with Kinetic bombardment is air drag. It has been said that standard size rods from gods could achieve the power of a tactical nuke, but that was ignoring air drag. Is it possible to make a simple, fueless scramjet, with just a compressor, to help speed up the projectile?  

This is exactly why they were specifically "rods" from god, and not just boulders. The rod shape reduces drag. The tungsten telephone poles would be going around Mach 10 (3400 m/s) when they hit the ground. I wonder what would happen if they were made of depleted uranium? Anyways, the pole would be about 9 tons. Its effectiveness is not equivalent to a tactical nuke anyways (and it wouldn't be, you'd need thousands of tonnes and high speeds for that). It's more like a conventional bomb, but not delivered by a bomber, and thus impossible to shoot down. That's the part people are interested in. The ultimate form of death from above, and it could, if accurate enough, take out tanks (although that's very hard to do). It's much more expensive than conventional bombs and bombers, however, and so it would likely only see use when you can't maintain air superiority.

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59 minutes ago, cantab said:

An impact into sand gives a spectacularly large crater, but no fireball:

Camouflet

38 minutes ago, Bill Phil said:

The tungsten telephone poles would be going around Mach 10 (3400 m/s) when they hit the ground

34002/2/4.2*106 = 1.4 kg TNT/kg
I.e. upper physical limit of a tungsten rod is just like twice more heavy airbomb. So, limited with small but hard targets and requires several meters accuracy (20 times more accurate than the best ICBMs can). So, still a science fiction.

42 minutes ago, Bill Phil said:

what would happen if they were made of depleted uranium?

It would first ignite, then melt in air. Being hidden inside a tunsten shell it would melt, deform the rod and shift it CoM, make it tumble and destroy
 

 

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5 minutes ago, kerbiloid said:

Camouflet

34002/2/4.2*106 = 1.4 kg TNT/kg
I.e. upper physical limit of a tungsten rod is just like twice more heavy airbomb. So, limited with small but hard targets and requires several meters accuracy (20 times more accurate than the best ICBMs can). So, still a science fiction.

It would first ignite, then melt in air. Being hidden inside a tunsten shell it would melt, deform the rod and shift it CoM, make it tumble and destroy
 

 

The 9 ton rods would have about 47 billion joules. That's about 11.3 tons of TNT. Far less than a tactical nuke. Which is my point.

Also, that depleted uranium thing wasn't serious.

Edited by Bill Phil

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Just now, Bill Phil said:

That's about 11.3 tons of TNT.

So, just a Damb Buster, nothing special.

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1 minute ago, kerbiloid said:

So, just a Damb Buster, nothing special.

Exactly. But it doesn't come with a bomber that could be shot down. But the same could be said of missiles.

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And as 5 m accuracy for orbital strike still looks fantastic, they should launch ten rods at once. Which means equivalent of hitting that dumb with 0.1 kiloton, spending a Saturn-5 mass of rockets to launch.

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3 hours ago, Bill Phil said:

This is exactly why they were specifically "rods" from god, and not just boulders. The rod shape reduces drag. The tungsten telephone poles would be going around Mach 10 (3400 m/s) when they hit the ground. I wonder what would happen if they were made of depleted uranium? Anyways, the pole would be about 9 tons. Its effectiveness is not equivalent to a tactical nuke anyways (and it wouldn't be, you'd need thousands of tonnes and high speeds for that). It's more like a conventional bomb, but not delivered by a bomber, and thus impossible to shoot down. That's the part people are interested in. The ultimate form of death from above, and it could, if accurate enough, take out tanks (although that's very hard to do). It's much more expensive than conventional bombs and bombers, however, and so it would likely only see use when you can't maintain air superiority.

When they came up with the idea, however, it wasn't clear just how easy it is to destroy a satellite if you really want to.

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15 hours ago, mikegarrison said:

When they came up with the idea, however, it wasn't clear just how easy it is to destroy a satellite if you really want to.

Sure. But its not that easy to take out satellites in higher orbits... It's not likely to be a cost effective weapon, and its actual usefulness in a combat situation is really unknown. Since, after all, not every nation can take down satellites.

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2 hours ago, Bill Phil said:

Sure. But its not that easy to take out satellites in higher orbits... It's not likely to be a cost effective weapon, and its actual usefulness in a combat situation is really unknown. Since, after all, not every nation can take down satellites.

It has more practical value in a science fiction setting. If you are already in orbit with a warship, it is relatively easy to bombard the planet using low-cost kinetic weapons. But in the real world, there are much cheaper ways to bomb places.

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Just now, mikegarrison said:

It has more practical value in a science fiction setting. If you are already in orbit with a warship, it is relatively easy to bombard the planet using low-cost kinetic weapons. But in the real world, there are much cheaper ways to bomb places.

Oh yes, of course. However, those methods are even easier to develop countermeasures for. Bombers? Surface-to-Air Missiles. Missiles? Harder to take out than a bomber, but technically still possible, depending on how much time you have before it hits and the type of missile. Rods from God? Nothing short of destroying the deployment satellite before it deploys (or while it's deploying), if there even is one (the rods could be satellites all to themselves, deployed in a constellation, not to mention the possibility of countering anti-satellite weapons). Destroying a satellite has varying levels of difficulty as well. There are probably some situations where rods from god would be useful in the real world. But they are still very, very, expensive.

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21 hours ago, kerbiloid said:

So, just a Damb Buster, nothing special.

Tallboy (5t) : https://en.wikipedia.org/wiki/Tallboy_(bomb) Hits the ground at mach 4.

Grandslam (10t) : https://en.wikipedia.org/wiki/Grand_Slam_(bomb) 

They were valuable weapons. You could knock out u-boat pens, railway tunnels, v-weapon sites, ships (tirpitz, lutzow) and other stuff.

Designed by Barnes Wallace....who also did the dambuster bomb : https://en.wikipedia.org/wiki/Bouncing_bomb

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Well yeah, an impact equivalent to 10 tons of TNT isn't "small". It's on the level of the largest conventional bombs in history, such as that Grand Slam and the more recent MOAB. And there aren't any proper bombers that can deliver those bombs any more, only cargo aircraft, so an operational orbital bombardment satellite would be significantly more able to strike targets.

On the other hand, it's no nuke, and it's certainly nothing like the Hollywood silliness.

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