science fiction inventor with physics question

schonovic

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?

Ryan_m_b

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.

schonovic

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.

Ryan_m_b

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_htm...cewarintro.php

schonovic

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?

D H

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 v_e 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 m_p (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/te.../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.

schonovic

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.

PaulS1950

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

schonovic

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!

D H

What PaulS wrote about is called a Bussard ramjet.

schonovic

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?

schonovic

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.

Ryan_m_b

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.

M Quack

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.

schonovic

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!

Ryan_m_b

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?

M Quack

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.