The physics of avatar

[jrphilps]

By the by: does anyone have any idea how to calculate the mass of the propellant tank? The only way I know how makes me keep adding smaller and smaller amounts of propellant to the design ad infinitium.

There are a few different ways, one is to compare the propellent tanks (of which there are four) to the rest of the ship. If the venture star is 1530 meters long, then the tanks are 78.18 meters in diameter. Therefore, they each have a volume of 2.66 x 10 5 meters, or 266,000 meters. To get your final answer, you just need to know what density the deuterium is being stored at.

[jrphilps]

Okay, this has been an interesting discussion so far, but I want to steer things back to the original premise.

In this video (http://youtu.be/rWDRq16o6ic), bantokfomoki claims that the ship outputs 174 petwatts via its engines, which is equal to all the sunlight received by earth. But the sun emits this energy from a surface of 2,730,000,000 square meters, whereas the venture star emits this energy from a surface of just 300 square meters. According to him, the engines will therefore be 9,100,000 times hotter than the surface of the sun. Does anyone see the problem with this claim?

[meve12]

Okay, this has been an interesting discussion so far, but I want to steer things back to the original premise.

In this video (http://youtu.be/rWDRq16o6ic), bantokfomoki claims that the ship outputs 174 petwatts via its engines, which is equal to all the sunlight received by earth. But the sun emits this energy from a surface of 2,730,000,000 square meters, whereas the venture star emits this energy from a surface of just 300 square meters. According to him, the engines will therefore be 9,100,000 times hotter than the surface of the sun. Does anyone see the problem with this claim?

The part where the plasma doesn't actually touch the nozzle? That's what you have the unobtainium magnets for.

Heck, you don't even need to have the reaction inside the nozzle, if you're using magnetism to contain it. Just outside the nozzle works just as well, and makes most of the waste heat radiate straight into space.

And what little heat gets radiated onto the nozzles gets dumped out the radiators.

I've been trying to derive the design from scratch as a learning exercise. How would the rocket scientists calculate the tank mass?

[Bill Phil]

By finding the payload. Also, you calculate how much volume you need, and calculate the amount of material required for the tensile truss...

[AngelLestat]

There are a few different ways, one is to compare the propellent tanks (of which there are four) to the rest of the ship. If the venture star is 1530 meters long, then the tanks are 78.18 meters in diameter. Therefore, they each have a volume of 2.66 x 10 5 meters, or 266,000 meters. To get your final answer, you just need to know what density the deuterium is being stored at.

That is the wrong approach, these tanks are just draws, even if they had certain measure, you need to take into account how to keep them cold and contain with magnets, then there is the issue of density.. There is experimental evidence that a hypothetical state of deuterium would be 1 millon times more dense than liquid hydrogen.

So how we calculate volume if we dont know nothing about density, magnetic confine require, etc.

How to storage and gather antimatter are the concepts most difficult to solve in antimatter propulsion. So lets keep them out of this discussion.

Okay, this has been an interesting discussion so far, but I want to steer things back to the original premise.

In this video (http://youtu.be/rWDRq16o6ic), bantokfomoki claims that the ship outputs 174 petwatts via its engines, which is equal to all the sunlight received by earth. But the sun emits this energy from a surface of 2,730,000,000 square meters, whereas the venture star emits this energy from a surface of just 300 square meters. According to him, the engines will therefore be 9,100,000 times hotter than the surface of the sun. Does anyone see the problem with this claim?

Lets make something clear. We can not keep talking about different mass ratios, we need to choose one so we all have the same number.

From my part, I have plenty of evidence which show that the dry mass is close to 350t or 400t. And the wet mass is 1000 tons (or 600 tons if we use a magnetic sail to brake, it does not add mass)

So if someone disagree with my mass estimation, lets see who present more evidence to prove me wrong.

Now about the video.

He calculate the required energy by second to brake from the kinetic energy approach.

But I dont know.. something is wrong with that.

First he calculated for 1250 tons, but if that is the wet mass, then he forget to substrate all the mass that ISV loss meanwhile the fuel burns.

Second, I calculated the energy required by the laser to accelerate 1000 tons (we dont substrate the mass here).

This give us 2 Pw by second, how is possible that I have a similar value when the laser needs to give 150000000 times more energy than the required energy to push the ship with rockets?

Sail:

(F*c)/2=p

6.7N by Gw

a=F/m

KE=1/2*m*v2

In addition, its heat calculations are wrong. I explain already the amount of energy that the ship absorb. And is magic how the radiators area match my heat waste requirements.

[Nibb31]

Why is this technobabble even in the Science Labs forum? It's fiction, guys. They make up the pseudoscience to fit the plot, not the other way round. Unobtanium is just another word for dilithium, warp drive, or magic.

[AngelLestat]

so a superconducor material at room temperature can not exist?

Antimatter propulsion can not exist?

A laser sail can not exist?

we are using the newton and einstein formules to compare the numbers, we need to do that in a different forum section?

[Nibb31]

so a superconducor material at room temperature can not exist?

Antimatter propulsion can not exist?

A laser sail can not exist?

we are using the newton and einstein formules to compare the numbers, we need to do that in a different forum section?

None of that stuff exists. In the context of Hollywood movies, they are buzzwords than can easily be replaced with dilithium, kryptonite, warp drive, or magic. Do you really think James Cameron was interested with equations and actual physics when he ripped off the story from Pocahontas? Even the word "unobtanium" was ripped off from an engineering joke.

Edited December 23, 2014 by Nibb31

[meve12]

Hardly.

Antimatter propulsion is a well known possibility in physics studied by many respected scientists, and can be modeled even if we cannot make antimatter yet. Unlike magic.

The laser sail is an extension of light sails, which have actually sailed on two probes thus far. Unlike Warp drive.

Room temperature superconductors may not exist right now, but they are a possibility with properties based on those of actual superconductors. Unlike dilithium.

James Cameron may not have been interested in equations, but he was definitely willing to base the design off of what we know of actual physics. Unlike Star Trek, which is what you seem to be trying to associate all of the above with.

And failing.

[Nibb31]

James Cameron picked up buzzwords. He is a storyteller, not a scientist. There is no science behind it, other than what he picked to make the story work.

[Bill Phil]

Antimatter is the most energy-dense material in the universe. It's the best way to travel at near light speeds. Unless you want to use staged fusion rockets. Plus, there is plenty of materials we don't know of. Like Unbihexium, it hasn't even been synthesized yet!

[magnemoe]

Hardly.

Antimatter propulsion is a well known possibility in physics studied by many respected scientists, and can be modeled even if we cannot make antimatter yet. Unlike magic.

The laser sail is an extension of light sails, which have actually sailed on two probes thus far. Unlike Warp drive.

Room temperature superconductors may not exist right now, but they are a possibility with properties based on those of actual superconductors. Unlike dilithium.

James Cameron may not have been interested in equations, but he was definitely willing to base the design off of what we know of actual physics. Unlike Star Trek, which is what you seem to be trying to associate all of the above with.

And failing.

Yes, the starship tries to be realistic, its based on existing realistic starship ideas, main objective is the high speed.

Unobtanium is a joke, name has been used on materials who need to developed to solve some issues, was used for some material during the moon program.

And yes its purpose is simple to be something so valuable you want to go to another star to mine it, while still being plausible.

The avatars themselves are a obvious plot device for guy meet girl. As I see its its also the most questionable part everything from bandwidth too the purpose of using them at all however its needed for the plot.

[AngelLestat]

Do you really think James Cameron was interested with equations and actual physics when he ripped off the story from Pocahontas? Even the word "unobtanium" was ripped off from an engineering joke.

James Cameron picked up buzzwords. He is a storyteller, not a scientist. There is no science behind it, other than what he picked to make the story work.

Lol OMG, why are you so negative against this movie? something really bad happened while you was seeing the movie at the cinema?

About unobtanium, If that is the right name that engineers use to describe a wonder material, then if you find such material with wonder properties, you can called unobtanium. Maybe graphene could be called that way.

There is a a huge difference between fantasy and science fiction.

Avatar is science fiction.

Antimatter was already created, it was already contain, they also measure the energy release when it touch normal matter.

Every year scientists rise the superconductor state some degress more. And they dont understand yet the physsics behind superconductors and superfluidity. So they are just guessing in each try.

Laser sails were already tested in laboratory.

Sorry, but all that has no relation at all with pseudoscience.

I wonder if the scientists who designed the apollo mission, they had to gather in the fantasy room to plan the mission?

Now name me a movie with a interstellar ship more plausible than the Venture Star.

Note: James Cameron didn´t design the ship, it was Charles Pellegrino and Jim Powell, two physicist.

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Now back to the topic, someone knows why the kinetic energy requirements to brake differ so much from the one you get with a=F/m?

[meve12]

--------------------------

Now back to the topic, someone knows why the kinetic energy requirements to brake differ so much from the one you get with a=F/m?

EDIT: Please clarify the question, it's not entirely clear what you're asking.

Edited December 24, 2014 by meve12

[NERVAfan]

Yeah the ship design is probably the most realistic interstellar ship ever seen in a major movie.

[AngelLestat]

EDIT: Please clarify the question, it's not entirely clear what you're asking.

Read the second answer of this post and you will understand:

http://forum.kerbalspaceprogram.com/threads/103899-The-physics-of-avatar?p=1622122&viewfull=1#post1622122

"Second, I calculated the energy required by the laser to accelerate 1000 tons (we dont substrate the mass here).

This give us 2 Pw by second, how is possible that I have a similar value when the laser needs to give 150000000 times more energy than the required energy to push the ship with rockets?"

against the video energy requirements using rockets on the 1250tons case.

It has no sense.

Edited December 24, 2014 by AngelLestat

[jrphilps]

Yeah the ship design is probably the most realistic interstellar ship ever seen in a major movie.

Agreed. Conventional spacecraft are unacceptable for interstellar space travel, because they have the engines on the bottom and the rest of the ship built on top like a sky scraper. This results in a very long, very heavy, and very wasteful ship (see robert frisbee). By mounting the crew habitat behind the engines, designers can use flimsier materials and skimp on radiation shielding.

But there are certain risks that might be encountered by using a tensile truss. For instance, the venture star relys upon r-squared attenuation to protect the crew habitat from the engines, which are treated as a point radiation source. But what about the twin exhaust streams that run within 100 meters of the truss on both sides? They may still subject the crew habitat to dangerous levels of radiant heat and ionizing radiation.

[meve12]

I suppose that would depend on what the exhaust is composed of. If it's just hydrogen or bare protons and electrons, maybe there won't be secondary radiation?

If it was mixed with antimatter, thought, there is definitely going to be a problem.

[AngelLestat]

I guess there is some logic about the position and tilt of the engines.

If we take into account pair production effect as source to increase efficiency, then this will mostly happens at the same flux direction, because gama rays has more chance to find particles to hit and became particles them self, which can be focused by the nozzle.

So if a gama ray travel in opposite direction to the exhaust, then there is less chance to became particle again.

In this case most of the radiation (which can not be focused) escape to the front of the ship.

Then we can ask.. how much of that radiation absorb the ship and the crew.

The crew just a bit, because for the effect that I already mention and the square rule.

The ship not so much either, because gamma rays are very difficult to stop, you need materials with high atomic numbers and thicker.

So if the ship is base on graphene (carbon) composite, very thin, strong and light. Then mostly all the radiation escape to the space without increase heat.