The physics of avatar

[jrphilps]

This is sortof a follow up to a thread I made almost a year ago about the ISV venture star. As you might remember, a guy named Bantokfomoki made some videos about the ship purporting to show why it could not work. While I agreed with his weak claim that travelling at 70% of light velocity is impossible for a reaction engine, I didn't accept his strong claim that travel to other stars within a human life span is impossible.

We all had a good discussion about this, and after the thread got wrapped up, I moved onto other things. But a couple months ago, Bantokfomoki made another avatar video and I offered to show him the errors in his thinking. I wrote two articles on my blog as a rebuttal, and engaged in a debate with him: It seems like I came out as the winner. Even so, I can't help but wonder whether or not my statements were factual, and I wanted to get a second opinion from you guys.

Here are my articles

http://kesler12-jamesrocket.blogspot.ca/2014/10/bantokfomoki-in-space-part-1.html

http://kesler12-jamesrocket.blogspot.ca/2014/12/bantokfomoki-in-space-part-2.html

Are there any glaring errors I made with my physics and math, or any points of his I did not adequately address? Go into as much detail as you want.

Edited December 19, 2014 by jrphilps

[NERVAfan]

IIRC the VentureStar isn't a pure reaction drive ship, it's launched by lightsail powered by laser banks in the Solar System and only uses the (antimatter/fusion) rockets to decelerate... which halves the on-board delta-v requirement.

70% of lightspeed is really really fast though.

---

As for the heat problem: you don't transfer the heat into any kind of solid engine component that remains attached to the ship, you use pulse propulsion (like Project Orion). If the "engine component" (IE 'pulse unit'/bomb) only has to last a nanosecond then it can be a trillion degrees and who cares?

You run into this problem way before getting to significant fractions of lightspeed, BTW - the melting point of the reactor is what limits NERVA type NTRs to specific impulses in the 750-1000 seconds range. That's why a gas-core NTR or a fission Orion would have much better specific impulse than a NERVA type (solid core) NTR.

[Bill Phil]

Hmm...

The first thing is that the Venture Star uses unobtainium... And antimatter. Not to mention the giant radiators that take up half of the ship. I assume the engines are huge. So not only is the antimatter exhaust taking heat away ( remember, the antimatter was locked in a magnetic trap), but the coolant is taking the ( relatively) lighter heat load from the walls to the radiators, which are big for a reason.

Now, chances are that the venture star is impossible, however, a multi staged fusion drive may be possible. Problem is, it needs to be huge, like, size of Manhatten first stage.

[SargeRho]

It needs to be said that the first ISVs weren't built of unobtanium, and were 4 times as massive though.

[AngelLestat]

Ignore that guy, I was in that discussion long time ago (you can search me in the commnent section), and only one thing was clear.. He has no idea of today technology, he has no idea of estimations, and he has no vision even at 5 years into the future.

If someone try to use energy values using atomic bombs, is not serious enoght, is a clown who wants to impress some kids.

After I understand he was a loss cause. I keep discussing with another guy with some physsics knowledge. And I prove him it can be possible. The main problem of all is the mass estimation.

I saw you came to the conclusion that 1200 tons was a better estimation, but is even lower.

I even convince to Adam Crowl about my mass estimation. He also show me some papers that explain how to achieve 100% antimatter efficiency, but we are still far.

I give you all my calculations and what I find out:

First the knowing facts:

From

http://james-camerons-avatar.wikia.com/wiki/Interstellar_Vehicle_Venture_Star

http://www.pandorapedia.com/human_operations/vehicles/isv_venture_star

Mass:

If the cargo is 350 tons, then is mentally absurd to think that its mass is 100000 tons.

In the page said that is 350 tons total (all the cargo), this include the 2 shuttles, machinary resources(going)-unobtainium(back), 200 crew, 200 passengers in cryosleep, sail, etc. This exclude the antimatter fuel, craft structure and engine. These last 2 are negligible in comparison

And I found that it is pointless to carry 2 shuttles for each of the 12 ISV. Why for? you only need 2 in pandora and 2 at earth. In any case 2 extra for backup.

It said that is made with carbon nanotubes.. Right now we know about graphene.

We discover the plastic 100 years back, and now we use it in everything, the same may happen with graphene, and the movie is 140 years from now.

More if we take into account that some tennis racquets and many other products already had parts with low or high quality graphene.

About the shuttle weights is 30tons each because if the space shuttle discovery weights 80 tons. Then using graphene that is aproximate 120 times lighter than aluminum and like 500 times more lighter than other average materials like heat shield used in the discovery, we can assume that is possible to make a space shuttle that only weights 1 Tons, but lets be a little more realistic and said that is 10 tons. Of course the avatar shuttle is bigger. But we need to take into account other things.

That estimation it does not take into account how less reinforcing structure do you need to support is own weight.

Like a car. If you reduce the weight of the engine from 500kg to 50 kg, you will notice that you can use lighter screws, chassis, wheels, etc. So only reducing one thing, you generate a chain reaction in the other things related.

You can have also electric superconductor engines or other propulsion technologies that would reduce a lot more the total weight. More area it also suggest less thermal shield, more sustainability, etc.

So said that each shuttle weights 30 Tons (knowing the things we already acomplish using graphene composites) is a moderate estimation.. Almost pessimistic.

My estimation in metric tons:

shield 5 + crew 50 + habitation section 75 + both shuttles 70 + cargo 300 + structure 20 + engines 20 + radiators 10 + spheres 5, sail 20.

Here are some values for a 350 Tons ISV and 0,7ÃŽâ€V (we remove the 2 shuttles) (We can note the lorentz factor in the fuel difference)

ISP____Total mass____Fuel Mass_____Gamma ray used_____Heat Release/s___Heat Absorb/s

0,77c_____1000 t_______650 t_____________50%____________742 TW_________74 TW

0,95c______860 t_______510 t_____________88%____________139 TW_________14 TW

Engine efficiency:

We know that ISV travels at 0,7c, this mean that the antimatter engine has an efficiency greater than 70%=0,7c isp

So lets take 0,77c of ISP, in an antimatter engine means:

-77% of fue fuel is converted into propulsion in the right direction (77% of E=mc2)

-23% of the energy decay into gamaray and is not converted again to charge particles. How this energy spread in all directions we did not get any benefic or braking from this.

-the max speed of the craft it would be 0,77c

-You need 1,9kg of matter-antimatter for each kg of payload to get 0,7c of deltaV

Radiators:

-The heat released is 23% of E=mc2, then we need to take into account the amount of energy that it will absorb from all that, I would estimate watching the ship design and how penetrating the gamarays are than only a 10% from thatt 23% is absorb by the ship. In the 0,77c case it would be 74 TW which we need to dissipate.

Using P = e.A.ÃÆ’.(T14− T24) We can calculate the max heat it can be dissipated by the radiators.

If we estimate 150m x 400m then both can dissipate 24 TW using a working temperature of 4500K (graphene sublimation point is 5500k at vaccum )

We also can use the sail 200 km2 to dissipate heat, lets estimate a working temperature of 1000k (becoz is not easy transfer heat in thin structures), only 1 side and 0,1e. It give us 113 TW.

There is still room for improvement:

1) The ship can use a bussard scoop, this will help a lot in the brake by the drag generated and avoid carrying half of the fuel (I mean the hidrogen).

The bussard scoop or "magnetic sail" is just a large mesh that works like a superconductor (can be made of carbon nanotubes), first you put a current in the mesh, then this current will remain for always making a magnetic field that will slow down the ship by the drag generated meanwhile you collect hidrogen.

Zubrin explain how good can be a magnetic sail, you can push a beamed sail with a laser to 0,9c, and then you deploy the mesh and it will take only 2 years to brake to 0,0057c without any proppelent. This mesh weights less than the sail.

If we take a engine ISP of 0,77c, we divide the fuel mass by 2, and we add the drag coefficient we get:

Without drag Brake mannuver

ISP___Total Mass:_____Fuel Mass___Gamma ray used___Heat Release/s__Heat Absorb/s

0,77c___530 t__________185 t__________50%___________ 211 TW________21 TW

With drag Brake estimated

ISP___Total Mass:_____Fuel Mass___Gamma ray used___Heat Release/s__Heat Absorb/s

0,77c___450 t__________100 t_________50%__________114 TW_________11 TW

So we find a good improvement against the 0,77c case without bussard scoop.

This not help much if we want to go back from pandora, when we need to gain speed.

2) To calculate the laser power, I took the power to push 1000 tons in the round trip, But one time you need to push 1000 t, and when it backs is only 350 t(no fuel left). So we need to calculate the difference, 650 t --> 1,2 Pw (if we take the 0,77c case without bussard scoop)

This will give us the same amount of time to accelerate and brake 1 ship, but with different time periods, Instead half year for both, it will be 3,5 month for brake and 8,5 month for accelerate.

For the last case with scoop and drag will be 350 t ---> 0,6 Pw aprox.

Edited December 20, 2014 by AngelLestat

[Nuke]

i dont think avatar had any physics, they might have taken some hard scifi pointers in the ship design, but i dont remember them talking about it in the movie at all. to me it was just oooh spaceship, now lets do pocahontas with some greenie propaganda thrown in. i didnt think it was that great of a movie.

[Bill Phil]

I think that carrying two Valkyries on your Starship is kind of dumb. You should build a station as a way point in Pandora orbit. That's where the starships should rendezvous. Then load off cargo, and use the Valkyries already at the station to deliver cargo to the surface. Also, a station elsewhere in the solar system could function as a laser to propel the starship could be built as well. Then no more antimatter is needed for the loop to and from Alpha Centauri. Of course anti-matter ships would still be needed for other star systems, but now there's less needed for Alpha Centuari. If only we could convert any kind of mass into pure energy...

Edited December 19, 2014 by Bill Phil

[jrphilps]

Ignore that guy, I was in that discussion long time ago (you can search me in the commnent section), and only one thing was clear.. He has no idea of today technology, he has no idea of estimations, and he has no vision even at 5 years into the future.

Yeah, I remember you. Bantokfomoki can be very pessimistic about the future. Its a bit like lord kelvin passing judgement on heavier than air flying machines.

If someone try to use energy values using atomic bombs, is not serious enoght, is a clown who wants to impress some kids.

After I understand he was a loss cause. I keep discussing with another guy with some physsics knowledge. And I prove him it can be possible. The main problem of all is the mass estimation.

I saw you came to the conclusion that 1200 tons was a better estimation, but is even lower.

I'm actually not sure what the venture stars dry mass is, but as for the wet mass, 100,000 tons seems reasonable enough. Getting to 70% of light velocity requires high mass fractions, even when you have AMAT (anti-matter) engines. As always, the tsiolkovsky rocket equation is a limiting factor.

I even convince to Adam Crowl about my mass estimation. He also show me some papers that explain how to achieve 100% antimatter efficiency, but we are still far.

I give you all my calculations and what I find out:

First the knowing facts:

From

http://james-camerons-avatar.wikia.com/wiki/Interstellar_Vehicle_Venture_Star

http://www.pandorapedia.com/human_operations/vehicles/isv_venture_star

Mass:

If the cargo is 350 tons, then is mentally absurd to think that its mass is 100000 tons.

In the page said that is 350 tons total (all the cargo), this include the 2 shuttles, machinary resources(going)-unobtainium(back), 200 crew, 200 passengers in cryosleep, sail, etc. This exclude the antimatter fuel, craft structure and engine. These last 2 are negligible in comparison

And I found that it is pointless to carry 2 shuttles for each of the 12 ISV. Why for? you only need 2 in pandora and 2 at earth. In any case 2 extra for backup.

I agree with you about that. Carrying 2 shuttles on the way there makes sense (since the colony on pandora uses them for alot of different jobs), but they should only be taking 1 shuttle back to earth with them. That would let the venture star carry more unobtainium in its place.

My estimation in metric tons:

shield 5 + crew 50 + habitation section 75 + both shuttles 70 + cargo 300 + structure 20 + engines 20 + radiators 10 + spheres 5, sail 20.

Here are some values for a 350 Tons ISV and 0,7ÃŽâ€V (we remove the 2 shuttles) (We can note the lorentz factor in the fuel difference)

ISP____Total mass____Fuel Mass_____Gamma ray used_____Heat Release/s___Heat Absorb/s

0,77c_____1000 t_______650 t_____________50%____________742 TW_________74 TW

0,95c______860 t_______510 t_____________88%____________139 TW_________14 TW

Engine efficiency:

We know that ISV travels at 0,7c, this mean that the antimatter engine has an efficiency greater than 70%=0,7c isp

So lets take 0,77c of ISP, in an antimatter engine means:

-77% of fue fuel is converted into propulsion in the right direction (77% of E=mc2)

-23% of the energy decay into gamaray and is not converted again to charge particles. How this energy spread in all directions we did not get any benefic or braking from this.

-the max speed of the craft it would be 0,77c

-You need 1,9kg of matter-antimatter for each kg of payload to get 0,7c of deltaV

Thats kindof a backwards way to determine the exhaust velocity. For all we know, the ship could be using a higher thrust and lower ISP engine, which would increase acceleration at the expense of payload. Remember, official literature has it moving at 1.5 gs for 6 months!

Also, aren't gamma rays the main contributor of waste heat in an AMAT engine? If so, why does the 2nd row (0.95 C) emit more gamma rays but absorb less heat than the 1st row (0.77 C)?

Radiators:

-The heat released is 23% of E=mc2, then we need to take into account the amount of energy that it will absorb from all that, I would estimate watching the ship design and how penetrating the gamarays are than only a 10% from thatt 23% is absorb by the ship. In the 0,77c case it would be 74 TW which we need to dissipate.

Using P = e.A.ÃÆ’.(T14− T24) We can calculate the max heat it can be dissipated by the radiators.

If we estimate 150m x 400m then both can dissipate 24 TW using a working temperature of 4500K (graphene sublimation point is 5500k at vaccum )

We also can use the sail 200 km2 to dissipate heat, lets estimate a working temperature of 1000k (becoz is not easy transfer heat in thin structures), only 1 side and 0,1e. It give us 113 TW.

Thats interesting. Bantokfomoki estimated each radiator as being 80 meters wide and 300 meters long, making for 75,000 square meters each. About the photon sail, do you really think it could be used to radiate waste heat? That would certainly be helpful, given that it has a diameter of 16 km!

[jrphilps]

Hmm...

The first thing is that the Venture Star uses unobtainium... And antimatter. Not to mention the giant radiators that take up half of the ship. I assume the engines are huge. So not only is the antimatter exhaust taking heat away ( remember, the antimatter was locked in a magnetic trap), but the coolant is taking the ( relatively) lighter heat load from the walls to the radiators, which are big for a reason.

Yeah, Ralathon mentioned this back in the last thread. Thing is, while you can use some of the propellant to create a cooling film along the engine chamber, it decreases the exhaust velocity by a large amount (maybe an order of magnitude).

I definitely think having unobtainium in the engines helps increase their efficiency: A room temperature superconductor would ensure that most of the reaction byproducts are safely ejected, which minimizes the waste heat. 98% or 99% efficiency is foreseeable.

Now, chances are that the venture star is impossible, however, a multi staged fusion drive may be possible. Problem is, it needs to be huge, like, size of Manhatten first stage.

A fusion powered saturn v. Now that would be badass!

Edited December 20, 2014 by jrphilps

[SargeRho]

That seemed to be the idea behind Project Daedalus. A 2-stage interstellar rocket that would accelerate to I think 12% of the speed of light, and fly through the target system, beaming back the data.

http://fc07.deviantart.net/fs27/f/2008/063/d/b/3d_prutser_by_BlikjeBier.jpg

[AngelLestat]

i dont think avatar had any physics, they might have taken some hard scifi pointers in the ship design, but i dont remember them talking about it in the movie at all. to me it was just oooh spaceship, now lets do pocahontas with some greenie propaganda thrown in. i didnt think it was that great of a movie.

Its like Interstellar, they dont explain nothing in the movie, but they have a strong science background.

To understand that you need to read the science books which came with the movies.

I'm actually not sure what the venture stars dry mass is, but as for the wet mass, 100,000 tons seems reasonable enough. Getting to 70% of light velocity requires high mass fractions, even when you have AMAT (anti-matter) engines. As always, the tsiolkovsky rocket equation is a limiting factor.

I dont follow you, if you have a 0,77c of ISP (something possible for an antimatter engine) you just need 1,9kg of matter-antimatter for each kg of payload to get 0,7c of deltaV

Also it would be silly if you have a wet mass of 1000000 tons to carry 300 tons. It will be totally nosense go to that planet to get that material no matter how much is cost. Even gathering only 250 tons on antimatter as I said, is hard to believe that something can be more value than antimatter. Unless that something help you to gather antimatter.

Also taken into account how much it cost accelerate each kg to 0,7c, then you will do the impossible to reduce all the mass you can.

And graphene and carbon nanotubes are the answer. As I explain with my shuttle example, you can have huge saves in mass.

Besides the ship is just a thin radiator, with some tensor truss, some thin shields and 2 light engines.. (why they needs to be heavy?)

The ISV is big just to avoid radiation.

I agree with you about that. Carrying 2 shuttles on the way there makes sense (since the colony on pandora uses them for alot of different jobs), but they should only be taking 1 shuttle back to earth with them. That would let the venture star carry more unobtainium in its place.

They can carry the unobtanium in any kind of compartment. Shuttles are light, but more light can be just a box from the same composite material.

Thats kindof a backwards way to determine the exhaust velocity. For all we know, the ship could be using a higher thrust and lower ISP engine, which would increase acceleration at the expense of payload. Remember, official literature has it moving at 1.5 gs for 6 months!

That is important to calculate the laser-sail, but the only clue to know the efficiency (which is important to know how much it will be your wet mass) is the maximun speed.

In antimatter engines, the calculation is easy. Because exhaust velocity is equal to the max speed = isp.

So as I said before, if we know that the speed is 0.7c, then the ISP needs to be greater than 0,7c. This gives you that if your ISP is 0,77c then you need 1.9 kg of antihydrogen +hydrogen for each kg of payload to reach 0,7c.

Also, aren't gamma rays the main contributor of waste heat in an AMAT engine? If so, why does the 2nd row (0.95 C) emit more gamma rays but absorb less heat than the 1st row (0.77 C)?

In the annihilation of hydrogen and anti-hydrogen, there is an amount of particles (I dont remember the %) lets said 60% of charged pions and 40% of gamma rays. Pions after a time they decay into gamma rays. So there are all photons at the end.

From this we can think that is not possible to get extra ISP than 60%, because you can only redirect the charged pions with the magnetic nozzle.

So the photons which can´t be redirected and had opposite direction it will reduce your efficiency, but in fact you can redirect these photons, is called pair production. When these photons hit a nucleos, then all the energy is coverted to mass again for E=mc2. And these particles can be redirected by the magnetic nozzle.

So when I said 88% gama rays, I mean that you are redirecting the 88% of all photons release in the annihilation, but the efficiency is 77% because some of these photons push backward. Seeing this from this perspective helps to understand how much waste heat we have. Then we need to calculate how much the ship absorb.

Thats interesting. Bantokfomoki estimated each radiator as being 80 meters wide and 300 meters long, making for 75,000 square meters each. About the photon sail, do you really think it could be used to radiate waste heat? That would certainly be helpful, given that it has a diameter of 16 km!

I am not sure about the sail, graphene is a perfect thermal conductor, but we are talking about very thin structure, so heat transfer is also related to section.

Maybe if you manage to distribute the heat by some kind of supercritical fluid trying at different sections of the sail at the same time, then it can be done.

Edited December 20, 2014 by AngelLestat

[meve12]

I decided to derive the design of the Venture Star on a whim just now.

The required DeltaV for the reaction drive is 1.4c.

Assuming two AM Beam Core engines(Exhaust Velocity of .3c) and only carrying 350 metric tons of cargo and two 10 metric ton engines (no structural mass, no tank mass, or anything else),

I calculated that the total mass of the Venture Star would be 67,084.36 metric tons. 93% of which is propellant.

So 100,000 tons is in the right ballpark, assuming they go for the most efficient antimatter engine I can find.(Photon drives are another matter.

EDIT: And now you know why everything in the human colonies of Pandora is locally produced.

EDIT EDIT:Incidentally, the acceleration for this version of the Venture Star starts at 20 gravities and works it's way up. There is clearly not enough mass.

I'd never thought I'd have to say that.

Edited December 20, 2014 by meve12

[magnemoe]

i dont think avatar had any physics, they might have taken some hard scifi pointers in the ship design, but i dont remember them talking about it in the movie at all. to me it was just oooh spaceship, now lets do pocahontas with some greenie propaganda thrown in. i didnt think it was that great of a movie.

It had catgirls, catgirls in fur bikinis riding dragons and fighting gunships. That makes it an great movie

As other says the spaceship was designed to be plausible.

Two other things was not very plausible: the magnetic field strong enough to levitate mountains and the avataers themselves, how could you get the bandwidth then other communications was problematic.

Some fridge logic also kills the background setting. The energy requirements for launching venturestar at 0.7c is 139 TW in best case in this tread.

Current US energy production is 25000 TWh or 7TW continuous effect, yes the engine uses far more energy than the earth produces, creating the antimatter will be far more energy consuming. They could have doubled the travel time and cut the energy use to an faction.

In short they don't have an energy problem outside of waste heat, few other environmental problems either none for energy production as it either has to be fusion or solar farms in mercury orbit, most waste had been practical to just turn into plasma and separate as energy is pretty much free.

[Aghanim]

And what makes it much more scarier is that according to http://www.projectrho.com/public_html/rocket/realdesigns.php , the light sail accelerates at 1.5 g. And light doesn't produce much force at all, so to get more force, you need more light, so they use giant laser cannon array aimed to the light sail. Imagine that much of power accidentally aimed at Earth

[AngelLestat]

Assuming two AM Beam Core engines(Exhaust Velocity of .3c) and only carrying 350 metric tons of cargo and two 10 metric ton engines (no structural mass, no tank mass,

or anything else)

Your math is wrong I guess. WIth your isp I get a wet mass of 5500 tons instead of 100000 tons.

Also if you use a AM engine with just 0.3c of isp it will be trash.

Here is explain how can you obtain almost 100% of efficiency.

http://www.centauri-dreams.org/?p=22358

http://vixra.org/pdf/1201.0026v1.pdf

Also Zhang & Keane with the new particle simulator from the cern find a way to achieve 65% of efficiency with antimatter. And if you concentrate the flux even more, you get pair production, you convert energy into charge particles again and your isp increase even more. You can also absorb the heat and then emit that heat in a collimatted way, which it will increase your isp.

The problem with matter and antimatter is not how to achieve high efficiency, the bigger problem is how do you storage and make it.

Besides 10 tons for each engine?? Is a magnetic nozzle which use ubnumbtanium to get the tesla needed.. So why it has so much mass??

My estimation of 450 tons the whole ship without the 2 shuttles (70 tons less) its taking into account a composite graphene material that it is 40 times less strong than real graphene.

So the ISV mass can be lower than that.

Two other things was not very plausible: the magnetic field strong enough to levitate mountains and the avataers themselves, how could you get the bandwidth then other

communications was problematic.

talk about the bandwidth is just speculation, who knows what exact bandwidth they really need. We also dont know what advances we can get in comunications from now to 150 years into the future.

You just need to sent all electrical pulses that you receive from all your nerves.

About the montains it does not depend much on how strong is the field, it depends on how much unobtanium the mountains has.

The magnetic flux goes from pandora to the gas giant, pandora is tidal lock to the gas giant, so the flux remains in the same place.

Pandora has a very strong magnetic field due unobtanium. Also any gas giants has big magnetic fields too.

Some fridge logic also kills the background setting. The energy requirements for launching venturestar at 0.7c is 139 TW in best case in this tread.

If you have a fixed laser power, you will have variable acceleration which the avarega will be 1,5g, To accelerate at 0,7c a mass of 1000 tons (fuel included) and brake 350tons back (it does not have fuel) you need 1200 TW laser. The acceleration process takes 8,5 month and the brake 3,5 month. Which it gives you 12 ship in 6 year (6 going 6 returning, fleet of 12 ISV as is mention in the wiki).

Current US energy production is 25000 TWh or 7TW continuous effect, yes the engine uses far more energy than the earth produces, creating the antimatter will be far more energy consuming. They could have doubled the travel time and cut the energy use to an faction.

With our current technology yes, we spent a lot of energy to make just few atoms of antimatter.

In short they don't have an energy problem outside of waste heat, few other environmental problems either none for energy production as it either has to be fusion or solar farms in mercury orbit, most waste had been practical to just turn into plasma and separate as energy is pretty much free.

You can be a lot more close if you have graphene.

If you have a solar cell of 70% efficiency (something that right now is know that can be achieve with graphene nano antenna structure) you can achieve the power required (1200 TW) with a collector of similar area than the sail at 0,2 Au of distance from the sun (half the distance than mercury-sun)

And what makes it much more scarier is that according to http://www.projectrho.com/public_html/rocket/realdesigns.php , the light sail accelerates at 1.5 g. And light doesn't produce much force at all, so to get more force, you need more light, so they use giant laser cannon array aimed to the light sail. Imagine that much of power accidentally aimed at Earth

There is not problem with that, you place the laser behind the sun with respect the earth. So you never can point to earth. Also if you have a way to focus the laser at big distances as it may be beyond neptune orbit, then you can not use it to focus at earth, and if you do, the watt/m2 you achieve it would be negligible.

The only problem is antimatter. Because you have a lot of energy, very hard to contain in the same place and time.

You can use those 650 tons of matter-antimatter to make a big boom in the earth. (it would not be so destructive as a normal nuclear bomb, the amount of energy relased is absorb in a biger area)

Edited December 20, 2014 by AngelLestat

[NERVAfan]

And what makes it much more scarier is that according to http://www.projectrho.com/public_html/rocket/realdesigns.php , the light sail accelerates at 1.5 g. And light doesn't produce much force at all, so to get more force, you need more light, so they use giant laser cannon array aimed to the light sail. Imagine that much of power accidentally aimed at Earth

If the ship really masses 100,000 metric tons (10^8 kg), then to accelerate it at 15 m/s^2 is 1.5 x 10^9 N (1000 Mainsails worth of thrust!)

The force from radiation pressure (assuming a perfectly reflective light sail) is 150 megawatts per newton.

So 1.5 x 10^9 N * 1.5 x 10^8 watts/N = 2.25 * 10^17 watts, 225 petawatts, somewhat more than the entire power reaching the Earth from the Sun (174 petawatts). It's still miniscule compared to the Sun's total power output though.

If this laser actually hit the Earth... well, a megaton of TNT is 4.184 x 10^15 joules. So this is the equivalent of 53.8 megatons of TNT per second (the largest nuclear test ever was 51 megatons of TNT equivalent).

[SargeRho]

58 megatons, not 51

But yes, if that laser array was aimed at Earth, it would depopulate it rather quickly.

[AngelLestat]

nobody read my post explaning why the laser is not the problem and the real power is 1200 TW.

Also the avatar ship is inspired in the project valkyrie design.

And it said that its mass is 100 tons without fuel, this design is like ten times longer than the movie design.

http://en.wikipedia.org/wiki/Project_Valkyrie

[meve12]

Your math is wrong I guess. WIth your isp I get a wet mass of 5500 tons instead of 100000 tons.

That's actually correct if you're using the regular rocket equation.

But remember, we are accelerating to .7c. Relativistic physics come into play.

So the regular rocket equation: dV=EV*LN(M0/M1) is inaccurate.

Instead I used one that accounts for relativistic effects:

dV=c*TANH((EV/c)*LN(M0/M1))

Also if you use a AM engine with just 0.3c of isp it will be trash.

Here is explain how can you obtain almost 100% of efficiency.

http://www.centauri-dreams.org/?p=22358

http://vixra.org/pdf/1201.0026v1.pdf

Interesting. An Antimatter fueled photon drive?

That would (theoretically) have an Exhaust Velocity equal to c.

Calculating from there, assuming the same payload mass,

I get a mass of just 2094.15 metric tons, of which 'only' 70% of which is propellant.

Besides 10 tons for each engine?? Is a magnetic nozzle which use ubnumbtanium to get the tesla needed.. So why it has so much mass??

I'm mostly plugging in the numbers for the AM Beam Core engine from project rho. Cheaty, I know. As far as engines go, it's actually pretty light; gaseous core nuclear thermal rocket designs get up to 56 tons, and Orion drives are a minimum of 200.

My estimation of 450 tons the whole ship without the 2 shuttles (70 tons less) its taking into account a composite graphene material that it is 40 times less strong than real graphene.

So the ISV mass can be lower than that.

Actually, I've been assuming for the calculations that only the payload plus the two engines contribute to mass, on account of no good data otherwise. SO my calculations only assume a mass of 370 tons.

In any case, .7c is stupidly fast. Can't they try for a more sane velocity, like .25c or something?

[meve12]

nobody read my post explaning why the laser is not the problem and the real power is 1200 TW.

Also the avatar ship is inspired in the project valkyrie design.

And it said that its mass is 100 tons without fuel, this design is like ten times longer than the movie design.

http://en.wikipedia.org/wiki/Project_Valkyrie

Is that including payload?

And it calls for 2100 tons of fuel for it for a .92c trip. Which is about the same mass ratio as the AM beam core design I was calculating.

[AngelLestat]

In any case, .7c is stupidly fast. Can't they try for a more sane velocity, like .25c or something?

but .7c is the speed that is mention in the movie.

It takes 6 years to reach pandora, a little less for the people traveling.

Is that including payload?

And it calls for 2100 tons of fuel for it for a .92c trip. Which is about the same mass ratio as the AM beam core design I was calculating.

I guess, but is not a cargo ship, is just a exploration ship.

But the extra amount of fuel is because this ship reach .92c, which is a lot more than .7c.

[meve12]

Agreed, on both points.

I still think .7c cruising speed is overambitious for a cargo ship, though. It's not like unobtainium would rot in the cargo hold, right?

[magnemoe]

Agreed, on both points.

I still think .7c cruising speed is overambitious for a cargo ship, though. It's not like unobtainium would rot in the cargo hold, right?

I agree, its wastefully fast, still if the energy was free, you had spare antimatter and a light load why not.

The hero in the movie was not very smart, no an single pit mine on a planet will not destroy it. No an world with "free" energy would not be an crapsack world, yes it could still be an terrible place political but not economical.

Outside the scientists who had an a great time it looked like the other ones was mostly promoted 4 lightyear away for good reason, decent chances many was also wanted by the law.

However the catgirls made up for it

[Bill Phil]

0.5c seems like a much better cruising speed to me.

[meve12]

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.