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Efficiency of planetoids as ships?

Fans of Colin Maclntyre and the great starship Dahak should take a minute to stop in here for discussions about one of David's best-loved series.
Re: Efficiency of planetoids as ships?
Post by cralkhi   » Tue Aug 19, 2014 12:48 am

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The "twenty kilometer chasm" bit kind of bugs me. Ten thousand megatons isn't a particularly large asteroid, and we don't get 20-km-deep craters on Earth; and presumably Dahak's hull is a lot tougher than rock.

I messed around with this impact effect calculator until I got a similar energy (1.02 x 10^4 megatons) (for an asteroid 400 meters diameter, 3000 kg/m^3 density, 30 km/s velocity).

It says that this would melt or vaporize 0.269 km^3 (of crystalline rock, about half of which would stay in the crater).

Now, the effect on Dahak is described as a "chasm" and not a "crater", so it's probably excavating a fairly narrow area (presumably a shaped charge), but still - its volume should be much, much more than that.

Given that Dahak's hull is probably much, much stronger than any currently available material -- much less mere rock -- I wonder if the Achuultani missiles should really be/were meant to be 10 million megatons.

EDIT: the calculator is here - http://impact.ese.ic.ac.uk/ImpactEffects/
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Re: Efficiency of planetoids as ships?
Post by DDHv   » Thu Nov 27, 2014 10:26 pm

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niethil wrote:
The ships were designed to carry 200,000+ crewmen. The area required for berthing, storage, entertainment, care and feeding of those crewmen would be massive. You also have to add into that the space needed for hydroponics, because the ships grew their own food.


That, at least, is wrong : compared to Dahak's size, the space taken by 200,000 crewmen and their entire life support equipement is almost insignificant.
To get a better idea, 200,000 is approximately the population of the city of Paris alone around 1300. Let's be crazy and say that we need the entire French territory to support Paris at that time. We have to take into account that it was a 2.5D structure though, while Dahack is a 3D structure. If the volume of space necessary to support Paris was the surface of French territory multiplied by something like maybe 100 meters in height, it only amounts to something like 5x10^8x10^2 = 5x10^10 m³. To be compared with Dahak's volume being well in excess of 4*(10^6)^3 = 4x10^18 m³.
So the volume of space required for the crew is probably on the order of 10 billionths of Dahack's volume. And the back of my envelope is screaming "hugely biased upward" back at me.

So, no, the size of the crew is not an acceptable explanation for Dahak's size, I believe.


Don't forget the planetoids were stated in the first book to be generational explorers. All of the above may be true, but multiply for a few generations, especially with the long lives involved, and more volume will be needed. It still is an enormous difference, even with much volume/person, which may be covered by the possible technical issues, including redundancy. There may also be a need to cover innovation and technical improvements to be expected later. Remember Dahak couldn't repair the IS communicator because the mutineers had removed critical resources for that. While it was working, possible improvements could come over it, but the resources would either need to be there, or extractable from the environment. He almost couldn't repair the power plants in time :!:

I still like the 500 KM outer armor.
Douglas Hvistendahl
Retired technical nerd

Dumb mistakes are very irritating.
Smart mistakes go on forever
Unless you test your assumptions!
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Re: Efficiency of planetoids as ships?
Post by fallsfromtrees   » Thu Nov 27, 2014 11:08 pm

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niethil wrote:
The ships were designed to carry 200,000+ crewmen. The area required for berthing, storage, entertainment, care and feeding of those crewmen would be massive. You also have to add into that the space needed for hydroponics, because the ships grew their own food.


That, at least, is wrong : compared to Dahak's size, the space taken by 200,000 crewmen and their entire life support equipement is almost insignificant.
To get a better idea, 200,000 is approximately the population of the city of Paris alone around 1300. Let's be crazy and say that we need the entire French territory to support Paris at that time. We have to take into account that it was a 2.5D structure though, while Dahack is a 3D structure. If the volume of space necessary to support Paris was the surface of French territory multiplied by something like maybe 100 meters in height, it only amounts to something like 5x10^8x10^2 = 5x10^10 m³. To be compared with Dahak's volume being well in excess of 4*(10^6)^3 = 4x10^18 m³.
So the volume of space required for the crew is probably on the order of 10 billionths of Dahack's volume. And the back of my envelope is screaming "hugely biased upward" back at me.

So, no, the size of the crew is not an acceptable explanation for Dahak's size, I believe.


The area of France you described doesn't include the volume for offensive weapons, defensive weapons armor, landing boats, assault ships (including parasite battleships) and all of the ancillary spaces required to support all of these things, That is going to add up to a significant amount of volume, particularly as the outer layer of armor is 500km thick.
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Re: Efficiency of planetoids as ships?
Post by JeffEngel   » Thu Dec 18, 2014 8:00 pm

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fallsfromtrees wrote:
niethil wrote:So the volume of space required for the crew is probably on the order of 10 billionths of Dahack's volume. And the back of my envelope is screaming "hugely biased upward" back at me.

So, no, the size of the crew is not an acceptable explanation for Dahak's size, I believe.


The area of France you described doesn't include the volume for offensive weapons, defensive weapons armor, landing boats, assault ships (including parasite battleships) and all of the ancillary spaces required to support all of these things, That is going to add up to a significant amount of volume, particularly as the outer layer of armor is 500km thick.

The point being made there is that Dahak's volume isn't on account of the crew size. That area of France is just accounting for the crew with extremely generous assumptions about how much space they can possibly "require". So all the rest of it you bring up should account for the rest of the volume, though I think the Enchanach drive is the biggest kicker.
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