Science fiction inventor with physics question

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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?
 
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  • #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.
 
  • #3
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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.
 
  • #4
Ryan_m_b
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Ryan_m_b Thanks for your help
No worries.
Maybe i'll go with fusion at 10^11 joules per gallon of water for propulsion melted off of the ice ship itself
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.
but still I'm kind of deflated. I thought it was so much a better idea. Thanks.
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
 
  • #5
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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?
 
  • #6
D H
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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?
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.
 
  • #7
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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.
 
  • #8
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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
 
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  • #9
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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!
 
  • #10
D H
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What PaulS wrote about is called a Bussard ramjet.
 
  • #11
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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?
 
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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.
 
  • #13
Ryan_m_b
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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.
No problem, yeah unless you're reading very hard SF you're going to be reading about impossible/impractical scenarios.
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?
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.
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.
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.
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.
Again: no idea what you mean here.
 
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  • #14
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My question I want to know is how much punishment from kinetic kill weapons or nukes can it handle?

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.
 
  • #15
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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!
 
  • #16
Ryan_m_b
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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.
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?
 
  • #17
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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?

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.
 
  • #18
Ryan_m_b
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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.
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:
 
  • #19
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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.
 
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  • #20
Ryan_m_b
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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.
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?
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.
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!
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.
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.
 
  • #21
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Ryan_m_b don't worry I'm not disheartened. this criticism just gives me more to think about. I'm not writing a story that's why I've decided to take the title of science fiction inventor because aspiring to be a writer didn't get me anywhere. I'm terrible at character development, and my english teacher says my thoughts are not well organized while writing. So I love science fiction and the only input I can put in is science fiction inventions. also I thought of something else; when you are firing off your 17 tsar bomba bombs directing the energy toward my ice ship wouldn't all that energy create an impenetrable dense fog around the ship that furthur energy would be required to disipate,as well as make the actual ship harder to pinpoint in that dense mist? Also wouldn't 17 tsar bomba bombs entirely wreck a large steel plate battleship anyway?
 
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  • #22
Ryan_m_b
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Ryan_m_b don't worry I'm not disheartened. this criticism just gives me more to think about. I'm not writing a story that's why I've decided to take the title of science fiction inventor because aspiring to be a writer didn't get me anywhere. I'm terrible at character development, and my english teacher says my thoughts are not well organized while writing. So I love science fiction and the only input I can put in is science fiction inventions.
No problem, that's called worldbuilding and there are forums/sites that exist solely for the discussion of things like this (not saying you should leave here just that you might be interested in taking a look).
also I thought of something else; when you are firing off your 17 tsar bomba bombs directing the energy toward my ice ship wouldn't all that energy create an impenetrable dense fog around the ship that furthur energy would be required to disipate,as well as make the actual ship harder to pinpoint in that dense mist?
The directed energy of a nuclear weapon of that size could blast through most of a major city, a thinly spread cloud of water vapour isn't going to do much. Also that vapour is itself formed from an explosion of the ice heating up very quickly and would likely destroy most of the ship as well.
Also wouldn't 17 tsar bomba bombs entirely wreck a large steel plate battleship anyway?
Yup. There's not much defence against a nuclear bomb except perhaps hundreds of metres of rock (and even then the enemy could just hit the part of your asteroid-vessel where the engines/doors are and melt them leaving you floating free inside a slightly radioactive ball of molten rock).

As an overall point you might want to give some thought to the "ship" analogy. It's a common and dated trope for space vehicles and space combat to be analogous to naval combat (ships, fleets, formations etc) but that doesn't really hold up any more than proposing that aerial combat would resemble naval combat. Along those lines here are some good articles about the treatment of "ships" in SF;

http://www.projectrho.com/public_html/rocket/spacegunexotic.php/#id--Space_Fighters
http://www.rocketpunk-observatory.com/spaceguideS-Z.htm#space_fighters
http://www.rocketpunk-manifesto.com/2007/08/space-fighters-not.html
http://www.antipope.org/charlie/blog-static/2009/11/the_myth_of_the_starship.html
 
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  • #23
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D H I wonder if the math of all that rocket fuel required to propel my ice ship takes into consideration a certain bizzare thing about matter. For example in a room full of warm air out of the cold there are particles that have very extreme temperatures both high and low. I mean to say that not all the air particles are a cozy 76 degrees farenheit. So what if some of the particles in the exhaust of the rocket are very close to the speed of light. So that to consider the exhaust effectiveness you might multiply the exhausts over all relativity factor with the exhausts mass and velocity which would have to equal the rockets mass and velocity. Since the rocket has only an infinatestimal relativity factor the exhaust would prevail. It is something for the rocket to push against because I really don't believe it would take that much mass to propell something through space like that. I just think that is something we should have to consider when making these rocket equations,and it is not something I can prove mathematically because I haven't gone through my college physics class yet but it is something I can feel with my gut.
 
  • #24
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D H the chemical reaction in an internal combustion engine is no where near the power of a fusion propulsor so if it takes that much mass to propel a large hunk of ice in space where there is no resistance why doesn't it take a gazillion gallons of gas to accelerate a car up to 65 mph? I don't get it.
 
  • #25
Ryan_m_b
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D H I wonder if the math of all that rocket fuel required to propel my ice ship takes into consideration a certain bizzare thing about matter. For example in a room full of warm air out of the cold there are particles that have very extreme temperatures both high and low. I mean to say that not all the air particles are a cozy 76 degrees farenheit.
AFAIK temperature does not make a difference at all, it's the speed at which the exhaust leaves the ship (although temperature will play a part in the system overall). Think of it in terms of Newton's laws: if you throw something one way then you will start moving in the opposite direction.
So what if some of the particles in the exhaust of the rocket are very close to the speed of light.
That would be great, doing it is another thing entirely.
So that to consider the exhaust effectiveness you might multiply the exhausts over all relativity factor with the exhausts mass and velocity which would have to equal the rockets mass and velocity. Since the rocket has only an infinatestimal relativity factor the exhaust would prevail.
I have no idea what you are trying to say here, could you be clearer?
It is something for the rocket to push against because I really don't believe it would take that much mass to propell something through space like that. I just think that is something we should have to consider when making these rocket equations,and it is not something I can prove mathematically because I haven't gone through my college physics class yet but it is something I can feel with my gut.
Ask yourself: are you rejecting the science that you admittedly don't understand because you don't like the answer?
D H the chemical reaction in an internal combustion engine is no where near the power of a fusion propulsor so if it takes that much mass to propel a large hunk of ice in space where there is no resistance why doesn't it take a gazillion gallons of gas to accelerate a car up to 65 mph? I don't get it.
Cars, airplanes and even animals are subject to the same rules but because they are trying to reach a very low speed and their mass is so miniscule compared to a huge, fast rocket the numbers aren't as bad. The simplist way to explain the rocket equation is this: it takes into account that because the fuel has mass you have to bring a bit extra, and because the extra has mass that needs a little extra to etc etc.
 
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  • #26
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Ryan_m_b. Maybe you are right I don't like the answer, so I want it to be what I want. All right then.
 
  • #27
Ryan_m_b
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Ryan_m_b. Maybe you are right I don't like the answer, so I want it to be what I want. All right then.
Remember as well this isn't just abstract maths, these conclusions are based on experimentally verified physical models of the world we live in.
 
  • #28
D H
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D H I wonder if the math of all that rocket fuel required to propel my ice ship takes into consideration a certain bizzare thing about matter. For example in a room full of warm air out of the cold there are particles that have very extreme temperatures both high and low. I mean to say that not all the air particles are a cozy 76 degrees farenheit.
The answer is yes and no.

The yes part first: The term ve in the ideal rocket equation denotes the effective velocity of the exhaust. This term incorporates into a single number all kinds of things from the real world such as back pressure, variations in the velocities of molecules in the exhaust stream, etc.

Now for the no part, and it's nasty: The ideal rocket equation describes an ideal rocket, one in which the exhaust stream is at absolute zero and is perfectly collimated (all exhaust particles are moving in the same direction). That the exhaust stream in a real rocket is not at absolute zero means the rocket is not drawing on all of the energy available from whatever reaction created the exhaust stream. Some of the energy is wasted in the form of heat. That the exhaust is not perfectly collimated means that some of the momentum, and thus even more of the available energy, is lost. A real rocket will always perform worse than an ideal one. The laws of thermodynamics get in the way. The only way to have an ideal rocket is to have an infinitely long, perfectly shaped rocket nozzle. Good luck with that, and even if you did have such a device, it would have infinite mass. It would go nowhere.

As nasty as the ideal rocket equation is, things get even nastier when the spaceship's velocity starts getting even close to that of the speed of light. The ideal rocket equation is not valid if you want to make your rocket propelled spaceship go very, very fast. You need to use the relativistic rocket equation instead, and this equation makes the ideal rocket equation look like child's play.


So what if some of the particles in the exhaust of the rocket are very close to the speed of light. So that to consider the exhaust effectiveness you might multiply the exhausts over all relativity factor with the exhausts mass and velocity which would have to equal the rockets mass and velocity.
Where does this energy come from? You cannot just wave a magic wand and have energy appear from nowhere. And sorry, zero point energy doesn't work. The energy of whatever reaction coupled with the masses of the particles that comprise the exhaust sets an upper limit on the exhaust velocity.

What if you use photons, converting all of the energy from the reaction to photons? That's the best possible exhaust velocity, right? Wrong, unless your reaction is matter/antimatter annihilation, and then good luck getting/containing any sizable amount of antimatter, and good luck collimating photons that are well into the gamma range. With anything but matter/antimatter annihilation, photons are just about the worst choice. Divide the entire world's annual energy consumption, 5×1020 joules, by the speed of light and you get 53,000 newtons. That's what photons give you: Not much. Photons have very little oomph. That thrust is about 1% of the thrust produced by SpaceX's Falcon 9, 0.16% of the thrust produced by the Saturn V's first stage. To make matters worse, you are putting all of the energy out in the form of photons, so what do you do with the byproducts of your fusion? You are faced with keeping those byproducts onboard or dumping them with the exhaust. The former leaves you with a device that is even worse than a rocket, while the latter reduces the effective exhaust velocity to much less than the speed of light.


I just think that is something we should have to consider when making these rocket equations,and it is not something I can prove mathematically because I haven't gone through my college physics class yet but it is something I can feel with my gut.
Then you ought to take some physics. Your gut feelings, fed by too much bad sci fi, are wrong.
 
  • #29
BobG
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It is something for the rocket to push against because I really don't believe it would take that much mass to propell something through space like that.

There is nothing for the rocket to push against. Rocket propulsion is simple conservation of momentum. There can be no net chang in momentum. If you throw stuff out the back, the forward momentum has to increase to keep the net momentum zero.

The rocket equation hides a bit of that because of the strange units - specific impulse in seconds and fuel rate in newtons/second, pounds per second. If you look at the units, you'll realize what you really have is mass times velocity of the fuel going out the back.

That does mean there's some hope. It takes energy to throw the stuff out the back at incredibly high rates of speed and that energy doesn't have to come from the fuel itself.

For example, ion thrusters use electrical energy to create acceleration of charged particles via a magnetic field. On satellites (which are moving through a near vacuum at relatively slow speeds compared to what you're looking for), the energy to generate that magnetic field comes from the solar arrays; not from the fuel. The specific impulse is around 3000 seconds, which is very efficient. Of course, the downside is a 25cm thruster, using 4500 Watts of power, can only accelerate very tiny amounts of fuel. Thrust is measured in milliNewtons (somewhere around 70 to 90 milliNewtons, but I don't really remember the exact number). It will take over a 10 to accelerate 1 kg at 1 meter/second squared. (And, in case you're wondering, these are used for attitude control and station keeping; not major changes in the orbit.)

Realistically, you won't get interstellar travel because you invent new technology. To get interstellar travel, you have to discover some new physics.
 
  • #30
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O.K. so what if I have a magical mass to energy converter converting all fuel mass to energy and could apply 100% of the energy to a photon drive. What's the best I could hope for?
 
  • #31
Ryan_m_b
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O.K. so what if I have a magical mass to energy converter converting all fuel mass to energy and could apply 100% of the energy to a photon drive. What's the best I could hope for?
That's what D H was describing with the antimatter photon rocket above.
 
  • #32
BobG
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It is science fiction, so you just do what most authors do. Just give the engine/thruster some cool sounding name and just don't explain how it's able to obtain relativistic speeds.

Or how a star fighter is able to behave like a fighter jet in outerspace.

Etc.

If you look at the most popular science fiction books, TV shows, movies; having a good story line and good characters is more important than the science. You could take the best stories out of the science fiction realm and put them into the time of Alexander the Great, the Roman Empire, or World War II and still have a great story.

For example: The movie that best compares to the original Star Trek TV series? Master and Commander! Even though the story is set during the Napoleonic Wars, the interplay between the Captain and the Doctor feels very much like Captain Kirk's relationship with Dr McCoy and Spock.

Another example: Asimov's Foundation Series. It's based on the Fall of the Roman Empire! But with a "what if" twist to find a way to reestablish the Empire at some future date - at least until the series took on a life of its own and Asimov just took it wherever (it was originally a serial for a magazine and the most important thing was to ensure readers wanted another installment so Asimov could make another paycheck, seeing as how he wasn't a famous, successful author yet).
 
  • #33
D H
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It is science fiction, so you just do what most authors do. Just give the engine/thruster some cool sounding name and just don't explain how it's able to obtain relativistic speeds.
Bingo. The MacGuffin Drive, or something like that. Even PhD physicists who write science fiction on the side do this. In fact, they know they have to do this because the only way to make science fiction compatible with known science is to write about spaceships that take generations to get from star A to star B, or write about races to whom a thousand years is but a blink of the eye, or just stop writing science fiction altogether. Amateurs who try to make their science fiction realistic typically end up with egg all over their faces.



Addendum:
If you look at the most popular science fiction books, TV shows, movies; having a good story line and good characters is more important than the science.
Exactly. The best science fiction is about people, not machines. The thingy that lets the people in the story quickly flit from place to place is just a plot device, aka a MacGuffin. Hence my name the MacGuffin Drive. Plot devices help the writer write, and help keep the reader engaged. Done right and plot devices don't need a lot of motivation or justification. In fact, an author who feels a need to provide that motivation or justification should see that as indicating that the device is being used incorrectly.
 
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  • #34
Ryan_m_b
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Exactly. The best science fiction is about people, not machines. The thingy that lets the people in the story quickly flit from place to place is just a plot device, aka a MacGuffin. Hence my name the MacGuffin Drive. Plot devices help the writer write, and help keep the reader engaged. Done right and plot devices don't need a lot of motivation or justification. In fact, an author who feels a need to provide that motivation or justification should see that as indicating that the device is being used incorrectly.
It's a matter of taste but personally I think this is an example of the worst kind of science fiction. That doesn't mean that it's bad by any stretch, just that it's a plot made possible by science fantasy rather than an exploration of the consequences of various scientificesque plot devices.

This really falls back on how hard you like your SF boiled but personally the type of SF that proposes no or very few overly-speculative plot devices and explores the social consequences of those propositions is what I consider good SF; not a variation of the Napoleonic wars in space.
 
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  • #35
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See that's where I wanted to go with this. All I really was looking for is the possibility of using ice as a starship hull. I didn't want to have to justify how the ice ship moved through space. For that I was going to use simple physics numbers to say it has a superfusion engine with spacetime compressors that allow the ship to go through more densities of spacetime than it normally would in any given moment in uncompressed spacetime. That I already had worked out.
 

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