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Science fiction inventor with physics question

  1. Jul 12, 2012 #1
    Hello, all! Thanks for all the help I have received on other threads. My latest science fiction invention is an battle ice cruiser. I have done the math for how to use energy weapons to compromise the ice shield around my ice ship but have no clue as to how to calculate how much impact ice at -200 degrees centigrade can handle before shattering. The ice ship is five miles long and 2.5 miles thick and is propelled by an Orion type stardrive. The thinest area of ice around the rocky core of the ice ship is 1000 feet. My estimate on the mass of the ice is 4.35 billion metric tons. please help. My question I want to know is how much punishment from kinetic kill weapons or nukes can it handle?
     
    Last edited: Jul 12, 2012
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  3. Jul 13, 2012 #2

    Ryan_m_b

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    If I've worked this out correctly 17.4 Tsa Bomba size nuclear weapons would be enough to boil off your entire ice shield if the energy was evenly distributed (which it wont be so you will get smaller holes easily drilled through, especially as nuclear pulse propulsion requires shaped nuclear charge technology which would be very benneficial for nuclear space warfare). This is equivalent to one of your "rammers" travelling at 430kmps. To address your question on strength of ice perhaps this resource will help.

    A final point at an aside: have you worked out how much fuel and energy it will take to move something of that mass? I'm a laymen in this area but running through the ideal rocket equation given an isp of 10,000 (which IIRC is roughly what a nuclear pulse propulsion would give you) your craft would need 2.21 billion tonnes of fuel to accelerate to and decelerate from 1kmps once. Perhaps someone could check that for me though to make sure I've not done the calculation incorrectly.
     
  4. Jul 13, 2012 #3
    Ryan_m_b Thanks for your help. I think I need to go back to the drawing board on this one. Your suggested references were helpful. Maybe i'll go with fusion at 10^11 joules per gallon of water for propulsion melted off of the ice ship itself, but still I'm kind of deflated. I thought it was so much a better idea. Thanks.
     
  5. Jul 14, 2012 #4

    Ryan_m_b

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    No worries.
    The problem with working it out that way is that those 10^11 joules of fusion energy aren't going to become 10^11 joules of kinetic energy (if you have taken this into account already I apologise for reiterating). My advice would be to look into the ideal rocket equation; essentially by plugging in exhaust velocity, payload mass and final velocity it can tell you how much fuel you need to bring along. Plugging that number back into the equation will tell you how much more fuel you need to stop as well. Like I say I may be working it wrong (I hastily plugged it into an excel sheet) but even with fusion propulsion you are looking at billions of tonnes of fuel for the same velocity I mentioned.
    Think of it in a positive way: you've not failed you've just learned one way not to do it. If you haven't used a resource like atomic rockets before I would advise it (great for those interested in writing SF). This page in particular might help
    http://www.projectrho.com/public_html/rocket/spacewarintro.php
     
  6. Jul 14, 2012 #5
    Ryan_m_d are you telling me that even if i had complete fusion down to fusing to iron nuclei I couldn't get away with pulling this off?
     
  7. Jul 15, 2012 #6

    D H

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    Yep. You have a rocket, a rocket of the very worst kind, a single stage rocket. You're screwed.

    Let's take a little walk down science fiction woo-woo lane. Suppose you want to send a colony ship to some star. The ship comprises the colonists, an environment to support the colonists, biostocks to feed them, an unobtainium hull to contain this, an unobtainium fusion drive, and ice. Lots and lots of ice. We won't use the ice as a shield against the interstellar medium. We'll use it as fuel for our unobtainium space drive.

    As far as protection against that interstellar medium, hey, we've got fusion. Just deploy a magical disruptor field in front of the spaceship that ionizes whatever of the medium isn't already ionized. A magnetic field will sweep the ionized medium around the spaceship. We don't need no stinking ice shield!

    Suppose the colonists, the support environment and equipment, the biostocks, the unobtainium hull, the unobtainium propulsion system, etc. mass to 1000 metric tons. We need that unobtainium keep the mass this low. A thousand metric tons is a bit more than twice the mass of the International Space Station.

    The spacecraft needs to start from a stop with respect to the Earth, coast for a while (a long while), and finally come to a stop with respect to the target planet at the target star. Why the coast? There isn't enough mass in the universe to have spaceship accelerate all the way to the halfway point, then turn around and decelerate to finally come to rest at the target. Why come to a stop? You don't want to have your colony ship drifting through space forever, do you?

    Because you need to start from a rest, coast, and come to a stop you need to apply the rocket equation twice. Apply it once and you're already in trouble. Apply it twice and you are totally screwed. Let's play with some numbers. We have two variables at play, the exhaust velocity ve and the coast velocity Δv. The rocket equation, applied twice, says that the mass of the ice needed for fuel in terms of the mass of the ship proper mp (mass of the colonists+environment+biostocks+hull+drive) is
    [tex]m_{\text{ice}} = m_p \left(e^{2\Delta v/v_e} - 1\right)[/tex]

    Suppose we want the coast velocity to be 1/10 the speed of light and the exhaust velocity is the best produced by a VASIMR-like drive, 120 km/s. This would require 3.7×10219 metric tons of ice. That's 6×10179 times the mass of the Milky Way. Obviously VASIMR does not have the woo-woo power needed. Upping the woo a bit, let's go with 500 km/s exhaust velocity, which is a fusion drive with exhaust speed augmented by a super duper ion thruster. (Those are NASA's words, not mine. See http://www.nasa.gov/centers/glenn/technology/warp/ipspaper.html.) Now the mass of ice needed is only 2.2×1027 times the mass of the Sun. Up the woo a bit more to an exhaust velocity of 1,000 km/s and you still need 6.7 earth masses of ice.Up the woo even more (but now you are violating the laws of physics) to an exhaust velocity of 10,000 km/s and you still need 150,000 metric tons of ice.

    Things become a bit more doable if the coast speed is 1/100 the speed of light, but now you have a multi-generation spaceship. That 150,000 metric tons of ice will now work with an exhaust velocity of 1,000 km/s. A 500 km/s exhaust velocity requires a lot more ice, 59 million metric tons of it.


    The only escape is to not use a rocket. You can't carry the fuel with you.
     
  8. Jul 15, 2012 #7
    D H wow I didn't know it was so difficult, you know watching science fiction as a young'un and reading science fiction books. I guess they were taking some serious liberties with their fiction. Thanks for bearing with my ignorance of these difficulties and thanks Ryan_m_b for referring me to the ideal rocket equation. sorry if I ever doubted you! you guys really know your stuff.
     
  9. Jul 15, 2012 #8
    Since you have a fusion reactor why not ionize particles in front of the ship with a laser and direct the ionized particles into a hollow ship to a laser powered fusion reactor that vaporizes the particles and ejects them out the back at nearly the speed of light.
    that's believable and more or less possible.

    Note: you will need some mass to begin your trip and another bunch of mass to slow down when you get to your destination but you can continuously add to your cruise speed all the way and use your main engine to decellerate using electro-magnetic thrust vectoring. Once you are travelling too slow to take in enough matter to power the ship you will have to rely on the stored mass to finish the braking maneuver.
    Relativistic speeds are not attainable with this imaginary ship - due to the time required to vaporize the stuff you collect as you go.
    If you want shields redirect the ionized particles toorbit your ship. It would conceal you from others and offer some resistance to penetration by stuff shot at you. The thicker your "shield" the more you look like space stuff and the more protection you have.

    None of the above has been well thought out and there is no patent of the "machinery" involved. Use it at your own risk. :)
    Paul
     
    Last edited: Jul 15, 2012
  10. Jul 16, 2012 #9
    Thanks PaulS1950 I'll do that with an hollow ice ship and since I'm imagining it in the year 2250 I'll give the human race the benefit of the doubt that they can do those kind of things by that time. The totally hard science fiction thing is great but I think humans will be smarter in the future than we are now and again thanks all for your helpful input!
     
  11. Jul 16, 2012 #10

    D H

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    What PaulS wrote about is called a Bussard ramjet.
     
  12. Jul 16, 2012 #11
    Thanks D H I already knew that and I know that the amount of hydrogen in interstellar space is too difuse to support a ramjet. maybe I'll use zero point energy to power a drive with such an exhaust velocity that there is an newtonian action/reaction between the ships gamma and the exhausts gamma, thereby combining newton with einstein, no?
     
  13. Jul 16, 2012 #12
    O.K. so what if I do go hard science fiction how about ice sentries all around a valuable, important starsystem? Since there is so much ice out there it would be cheaper and more efficient to use the resources available, wouldn't it? If I can get an attacking fleet to waste an arsenal of nukes that can reduce a planet to slag before they even enter the system that would be beneficial. Oh, by the way I was doing some thinking and I thought what if all newtonian action reaction was gamma vs gamma anyway and only a few particles in chemical rockets actually get to the gamma point to give the reaction its power hence the best rocket would be like a particle accelerator pushing out small reactant masses with high gamma factors and we just didn't know it because nobody had thought of it. Wouldn't that be something? That's right I'm a genius and I'm just messing with you guys. Thanks bye.
     
  14. Jul 17, 2012 #13

    Ryan_m_b

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    No problem, yeah unless you're reading very hard SF you're going to be reading about impossible/impractical scenarios.
    Zero point energy can't be used to do work. Other than that I don't know what you mean by gamma or how this links Newton and Einstein.
    Why would the attackers bother? Space is so huge they could probaly just fly straight past. Supposing you did have some magic propulsion that allows near C speeds then all the attackers would need to do is drop a bunch of shrapnell before they start decelerating. Not much you can do about thousands of tiny relativistic pebbles careering towards your world.
    Again: no idea what you mean here.
     
    Last edited: Jul 17, 2012
  15. Jul 17, 2012 #14
    One way to deal with this would be to layer liquid and frozen layers of water. The liquid will spread any shock waves over a larger area of the next solid layer, and it will stop the propagation of cracks.

    Clever handling of the liquid could also allow plugging holes melted into the ice by previous attacks and facilitating other repairs to the solid structure.

    You "only" have to find a clever way of keeping the liquid liquid and the ice frozen. If you want to operate at -200 C, then the liquid is a bit difficult to maintain.

    Maybe just insert a network of heaters. Keep the ship solid in peace time and melt layers for when ready for battle. Loads of liquid sloshing around your space ship will make handling problematic.

    Another idea targeted towards kinetic impact is to use something like snow that deforms easily. Being mostly vacuum with a rather small volume fraction of ice the provides a built-in safety against the volume increase upon evaporation - no (or less) nasty shock waves.

    This will make your space sihp bigger, but not heavier.

    The weapon against an ice shield would be a focussed microwave beam - this is technology available today. Arrange several attack ships to focus and cross their beams deep inside the ice shield. The ice there will melt and eventually boil. Unless the steam is gotten rid of through venting channels the steam pressure will eventually crack off a huge piece of the ice shield.
     
  16. Jul 17, 2012 #15
    Ryan_m_b...good you didn't understand the last part because it was meant to be a joke. I'm not a genius, I can't follow the ideal rocket equation. I just wanted some one to help me develope my idea of an ice ship, or ice sentries. Thanks to M Quack for the help!
     
  17. Jul 17, 2012 #16

    Ryan_m_b

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    Wouldn't the slower speed of sound in water compared to ice mean that the water layer would actually be worse at distributing the shock waves?
     
  18. Jul 17, 2012 #17
    Good question. TBH I don't know.

    My thinking was that liquids cannot crack and do not transmit shear stress. Scattering at the interfaces might help distribute the energy of the shock wave. But then again I know nothing about the subject, I just make things up as I go.
     
  19. Jul 17, 2012 #18

    Ryan_m_b

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    I think you're right in that regard, the liquid water wont crack so when hit it will simply distribute the energy before cooling and settling again. However because it can't transmit energy fast enough and has a lower tensile strength it is worse at stopping individual shots. I'm no expert either but it seems to me that it's a bit of a trade off. Ice will be stronger but will degrade in strength over time. Water will be weaker but won't degrade.

    Unfortunately though I don't think that gets us anywhere because in the shaped nuclear charge warfare environment the OP has proposed either method used in any combination will be about as effective as a slightly thick jumper against an artillery shell :tongue2:
     
  20. Jul 17, 2012 #19
    Ryan_m_d I see the complications but it would still be cheaper than armored cruisers or battleships. The ice sentry could be armed to the teeth and hold a fleet of nuclear armed spacefighters too small for radar to lock on to or too numerous for that matter, not to mention a really great point defense of repeating particle cannons and lasers that wave around like laser light shows striking incoming objects maybe more than once or twice before approaching the sentry. Also a spread of space mines could be laid out all around the system necessitating that the invaders take on the sentries instead of dying in a mine field. Maybe the ice could be "contaminated" with some kind of carbon fibers in the inside where the living and working areas are. They could cool their nuclear power plants with the inner layers going deeper and deeper each time and refreezing the contaminated water until they have a good thickness of carbonated fibrous plasticated ice say a hundred feet around the habitated and working areas all the way.
     
    Last edited: Jul 17, 2012
  21. Jul 18, 2012 #20

    Ryan_m_b

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    A few points you may want to consider:

    - Does a "spacefighter" make any sense? Because it's small it wont be able to carry much fuel (and if it tries to carry more it will accellerate and decellerate slowly) so it wont have a high speed relative to a bigger craft and wont be able to operate for long. Essentially they would only be capable of operating close to the ship that launched them which seems a bit pointless.
    - Does "too small for radar" make any sense? Even if it does thermal and neutrino sensors would easily be able to pick out the exhaust and nuclear engine.
    - If these ships have such great point defence then what hope do these fighters have?
    I don't understand how this would work. Explosions in space aren't very effective (no air to carry the shockwave) and the mindboggling size of the areas we are talking about mean that laying an effective field makes little sense. For instance if we propose a hugely effective 1 tonne mine capable of destroying anything within a 1km3 volume and propose a 1AU thick mine field that envelopes our solar system then you would need 20 solar masses to do it1. Even a field 1km thick would be 10x the mass of Ceres.

    Space is big...really...really big!
    Are you using this to improve the tensile strength? That would work and it would also increase the acoustic and thermoconductivity thus spreading energy faster but as I pointed out in my first post the overall hull isn't very strong (and is prohibitive in mass).

    I hope none of these comments dishearten you and you take them in the spirit in which they are intended (constructive criticism). I may be wrong but it seems to me that you're writing your story backwards which is a very common pitfall2 amongst writers; essentially you've decided that you'd like a classic military-SF "warships in space" story complete with fighters, carriers, cruisers etc and are now trying to build a realistic world around that plot to make it hard SF. The problem is the real world isn't very condusive to this scenario, mainly because space is big and getting around it quickly costs a lot of energy (which increases exponentially the bigger the mass you're trying to get around). This has the effect that when a plot device is shown not to be very viable (e.g. ice defence station) rather than going back to the drawing board and asking "how would a realistic invasion of one system by another occur and what defensive tactics/technologies would be appropriate?" you propose another plot device, in this case minefields. But as shown this isn't viable either so we're in this situation of applying leaky plasters over leaky plasters when really we need to redesign the pipe.

    I hope you find this helpful and I don't mind helping further, it maybe that you have many viable ideas that haven't been discussed yet. It's just that in my experience it's best to start a science fiction story by worldbuilding from first principles and working up (i.e. what technologies could be available and what will the social/economic/political effects be?) rather than deciding a plot and trying to build a world around it.

    1I knew the number was big but even that shocked me so if you want to check my math just in case...
    1) Radius of the system = ~30AU
    2) Volume of the system = 113040AU
    3) Volume of cubic AU in cubic km = 3.375E+24
    4) Volume of system in cubic km = 3.8151E+29
    5) Volume of sphere with 31 AU radius in cubic km = 4.20947E+29
    6) 5 minus 4 to get shell volume in cubic km (AKA number of mines) = 3.94368E+28
    7) Total mass of mines in kg = 3.94368E+31
    8) total mass of sun in kg = 1.98892E+30

    2Actually it's not always a mistake but it leaves you in a more difficult position worldbuilding wise than designing the other way.
     
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