Invading the Solar System

[Caiaphas]

Plus I really get the impression he's using this to get feedback for his fiction in which case IMHO it sounds like it could use some constructive criticism.

Damn. I thought I was being sneaky about this.

[Junghalli]

Damn. I thought I was being sneaky about this.

I've done it too, I know the signs.

[Star Wars 888]

Gee, are some of you really like this? Would you really order your men to shoot civilians on sight? Really?

BTW, the OP doesn't specify how powerful and advanced the ships are, how well trained your crew is, etc.

[sirocco]

One motivation for an alien invasion I personally find relatively plausible is some sort of "White Man's Burden" type deal. If we assume a borderline effective-postscarcity civilization taking over a place like Earth might cost relatively little in blood and treasure (with sufficiently advanced technology concievably it could be done by a single adventurer), and I could easily see an advanced race coming to think of itself as having a duty to help other less advanced ones, even against their will if they're too stupid to appreciate the benefits of higher civilization (or at least that's the way the aliens would think of it). This does have the advantage of nicely getting around the whole "lol drop rocks until everyone dies" answer, as aliens on such a mission would obviously probably want to avoid mass deaths of the people they're invading to "help" in the first place.

OK, it's a bit of a stretch but honestly it makes a lot more sense than aliens invading Earth for liebensraum.

And what about a race of explorers that departed their homeworld centuries ago. They weren't that good with birth control and spacecraft maintenance and now their technology is maybe no longer up to the task of sustaining them. They happened to cross our corner of the galaxy and discover that Earth is suitable for them too, well except the fact that there is already a 6-billion-people civilization on it. They think that the planet is not big enough for both of us and since we are pre-deep-space-travel race of monkeys polluting our own habitat, they should be the one to inherit Earth.

As to why they can't just build space habitats or put their population into cryo-sleep, let's say that all the smart guys were shot down by the crazy militaries that want to colonize Earth. So good enough?

[Temujin]

Which makes me wonder, if they used nuclear detonations to propel the ship (and set off bomb-pumped x-ray and gamma ray lasers), did they use directed bursts for more efficient propulsion? And if so, why not weaponize them into Casaba-Howitzers? Much simpler directed-energy weapon than an x-ray or gamma ray laser, and no need for two nuclear warheads (one to power the laser, another to set off the pump bomb since conventional explosives would detonate too slowly and allow the laser to vibrate off-target.).

Hmm. I don't know. I speculate (am not at all sure) that the authors (Pournelle in particular) couldn't tell all they knew about Casaba Howitzer at the time; it may have been classified.

But that is only speculation.

It's been years since I read the novel, but IIRC they didn't have time to weaponize them into Casaba-Howitzers; they just assembled them for use with possible minor modifications. As for the gamma ray lasers, I believe they were said to be mounted near the pusher plate and utilized some of the energy from the propulsion bombs.

That's supposed to be the design, unfortunately I couldn't find a bigger picture. But if you can read it, the gamma ray lasers are mounted in the right location near the base.

[Junghalli]

And what about a race of explorers that departed their homeworld centuries ago. They weren't that good with birth control and spacecraft maintenance and now their technology is maybe no longer up to the task of sustaining them. They happened to cross our corner of the galaxy and discover that Earth is suitable for them too, well except the fact that there is already a 6-billion-people civilization on it. They think that the planet is not big enough for both of us and since we are pre-deep-space-travel race of monkeys polluting our own habitat, they should be the one to inherit Earth.

One issue I see right off the bat is that if they can't maintain their technology enough to keep their habitat-ships going or build new space habitats how are they going to build a military to invade the Earth? I mean for an interstellar nomad civilization (what I think you're talking about) being able to maintain your habitats is pretty fundamental. Losing that ability would be like a twentieth century civilization losing the ability to build and maintain their power plants and sewer systems and factories. If their civilization is boned that badly they'd probably make a pretty lousy invasion force. They could just drop rocks on us of course, but that's less interesting.

I suppose the military tech could be stockpiled from before whatever event caused them to become boned, and is still in fairly good condition since it hasn't been really used for anything.

[Star Wars 888]

How about a space ship from the future crash landing on Earth due to some failed time experiment? The USA or some other country recovers and and tries to reverse engineer it. A capital ship from the future is also sent back in time and demands that they get their space ship back, but the country refuses due to claiming that it's their's by right of salvage or something like that.

[Junghalli]

How about a space ship from the future crash landing on Earth due to some failed time experiment? The USA or some other country recovers and and tries to reverse engineer it. A capital ship from the future is also sent back in time and demands that they get their space ship back, but the country refuses due to claiming that it's their's by right of salvage or something like that.

That could work. You could do the same plot but with aliens and eliminate the need for magic (time travel).

[sirocco]

And what about a race of explorers that departed their homeworld centuries ago. They weren't that good with birth control and spacecraft maintenance and now their technology is maybe no longer up to the task of sustaining them. They happened to cross our corner of the galaxy and discover that Earth is suitable for them too, well except the fact that there is already a 6-billion-people civilization on it. They think that the planet is not big enough for both of us and since we are pre-deep-space-travel race of monkeys polluting our own habitat, they should be the one to inherit Earth.

One issue I see right off the bat is that if they can't maintain their technology enough to keep their habitat-ships going or build new space habitats how are they going to build a military to invade the Earth? I mean for an interstellar nomad civilization (what I think you're talking about) being able to maintain your habitats is pretty fundamental. Losing that ability would be like a twentieth century civilization losing the ability to build and maintain their power plants and sewer systems and factories. If their civilization is boned that badly they'd probably make a pretty lousy invasion force. They could just drop rocks on us of course, but that's less interesting.

I suppose the military tech could be stockpiled from before whatever event caused them to become boned, and is still in fairly good condition since it hasn't been really used for anything.

Well I never explored that part of the story. Initially it was centered on the people of the farther colonies retreating to Mars after their home was destroyed by the nomads. And since Mars was still being terraformed, there was a strong resistance by the natives who consider that the planet couldn't sustain that additional population and that they would ruin all the efforts made (and the money invested). And all this would have led to a Martian world war until Earth government negotiated a truce with the aliens : they were going to help them with their space habitats in exchange of FTL technology.

And I stopped there because it sounded stupid even for an alien mind to give something as valuable as FTL tech. And the other alternative was an actual spacewar which is something that doesn't even make sense to me. So I quited.

And I found it interesting how you are going to make that happen.

[Swindle1984]

Which makes me wonder, if they used nuclear detonations to propel the ship (and set off bomb-pumped x-ray and gamma ray lasers), did they use directed bursts for more efficient propulsion? And if so, why not weaponize them into Casaba-Howitzers? Much simpler directed-energy weapon than an x-ray or gamma ray laser, and no need for two nuclear warheads (one to power the laser, another to set off the pump bomb since conventional explosives would detonate too slowly and allow the laser to vibrate off-target.).

Hmm. I don't know. I speculate (am not at all sure) that the authors (Pournelle in particular) couldn't tell all they knew about Casaba Howitzer at the time; it may have been classified.

But that is only speculation.

It's been years since I read the novel, but IIRC they didn't have time to weaponize them into Casaba-Howitzers; they just assembled them for use with possible minor modifications. As for the gamma ray lasers, I believe they were said to be mounted near the pusher plate and utilized some of the energy from the propulsion bombs.

That's supposed to be the design, unfortunately I couldn't find a bigger picture. But if you can read it, the gamma ray lasers are mounted in the right location near the base.

Yup. It's an elegantly simple solution to the problem.

An x-ray or gamma ray laser needs a nuclear detonation to power it. But conventional explosives used to set off the nuke don't detonate fast enough and would therefore vibrate the laser, moving it off-center as it fired. Which, with the sorts of distances we're talking about in space combat, could lead to you literally missing by a mile.

Solution? Drop the bomb-laser behind you and aim it at the target, then set it off using a nuke you're detonating to accelerate your ship. Two birds with one stone, as it were.

But if they were using directed-discharge nukes to propel the Orion spacecraft, they should have been able to make Casaba-Howitzers. The point of "they might not have been able to divulge too much information about them at the time" is a good one, and explains out-of-universe why they didn't do it. How would we explain it in-universe? Some deficiency in the weapon, perhaps?

What sort of range would you get with a Casaba-Howitzer at any given yield, anyway? How much would its firepower drop off with distance and the jet expand, especially in the vacuum of space?

[Junghalli]

And I stopped there because it sounded stupid even for an alien mind to give something as valuable as FTL tech.

That depends: FTL could easily be possible but so impractical it is just a gimick or has only a few limited applications. It would be funny if they gave us an FTL drive and then we found out that for it to work the ship had to first be going at < .9999 c, and it has a maximum effective speed of 1.1 c and it needs lots of energy so the ship has to carry massive generators and cooling systems, and we had actually given them something valuable in exchange for that largely worthless technology.

Alternately an advanced civilization could have lots of other stuff to potentially offer. Advanced fusion reactors, for instance, or advanced medicine, or a bunch of other technologies where they would probably be ahead of us. Heck, if their advanced spacecraft will still be operational for a while they could probably make a killing just off running a ferry service between different parts of the solar system.

[sirocco]

And I stopped there because it sounded stupid even for an alien mind to give something as valuable as FTL tech.

That depends: FTL could easily be possible but so impractical it is just a gimick or has only a few limited applications. It would be funny if they gave us an FTL drive and then we found out that for it to work the ship had to first be going at < .9999 c, and it has a maximum effective speed of 1.1 c and it needs lots of energy so the ship has to carry massive generators and cooling systems, and we had actually given them something valuable in exchange for that largely worthless technology.

Alternately an advanced civilization could have lots of other stuff to potentially offer. Advanced fusion reactors, for instance, or advanced medicine, or a bunch of other technologies where they would probably be ahead of us. Heck, if their advanced spacecraft will still be operational for a while they could probably make a killing just off running a ferry service between different parts of the solar system.

Hummm...Having them settle in our solar system while we conclude our tech exchange? I like that. At least I don't have to make up space battles. =)

Don't know if it helps Caiaphas with his story.

[Temujin]

What sort of range would you get with a Casaba-Howitzer at any given yield, anyway? How much would its firepower drop off with distance and the jet expand, especially in the vacuum of space?

Nuclear Shaped Charges

Back in the 1960's, rocket scientist came up with the infamous "Orion Drive." This was basically a firecracker under a tin can. Except the tin can is a spacecraft, and the firecracker is a nuclear warhead.

Anyway, they realized that about 90% of the nuclear energy of an unmodified nuclear device would be wasted. The blast is radiated isotrophically, only a small amount actually hits the pusher-plate and does useful work. So they tried to figure out how to channel all the blast in the desired direction. A nuclear shaped charge.

The nuclear device is encased in an shell of x-ray opaque material (uranium) with a hole in the top. This forces the x-rays to to exit from the hole. Where they run full tilt into a large mass of beryllium oxide. The beryllium transforms the nuclear fury of x-rays into a nuclear fury of heat. Perched on top of the beryllium is the propellant: a thick plate of tungsten. The blast of heat turns the tungsten plate into a star-core hot spindle shaped plume of tungsten plasma. The x-ray opaque material and the beryllium oxide also vaporize a few microseconds later, as the tungsten plasma jet hits the Orion drive pusher plate. With the reference design of nuclear pulse unit, the plume is confined to a cone of about 22.5 degrees. About 85% of the nuclear devices's energy is directed into the desired direction.

About this time the representatatives of the military (who were funding this project) noticed that if you could make the plume a little faster and with a narrower cone, it would no longer be a propulsion system component. It would be a directed energy weapon. Thus was born project Casaba-Howitzer.

Details are scarce since the project is still classified after all these years. Tungsten has an atomic number (Z) of 74. When the tungsten plate is vaporized, the resulting plasma jet has a relatively low velocity and diverges at a wide angle (22.5 degrees). Now, if you replace the tungsten with a material with a low Z, the plasma jet will instead have a high velocity at a narrow angle. The jet angle also grows narrower as the thickness of the plate is reduced. This is undesirable for a propulsion system component, but just perfect for a weapon.

That said, I guess it would depend upon the warhead yield and narrowness of the cone. Eventually the inverse square law will dissipate the plume's damage potential to nil.