Quantum vacuum plasma thrusters as starship propulsion?

[Sillychris]

I like how a discussion on quantum thrusters turned into a discussion on flywheels.

So, how exactly did that happen?

[Nuke]

thread has reached train wreck status.

[KASASpace]

Let's do an analysis:

Now, with that let's use this wonderful little equation:

Ke = 1/2mv^2

Let's solve for m assuming we know that we want 300,000 Watts (it's in one second anyways, so let's do the conversion early) as (Ke), and 300 m/s as (v).

300,000 = 1/2m(90,000)

300,000 = 45,000m

300,000/45,000=m

6.67=m

Look at that!

6.67 kilograms!

Even the first rockets with tiny payloads could manage more! (Jupiter-C)

So, how much power per kilogram?

45,000 Watts.

Or, 45,000 Watts per 1,000 grams.

Assuming moving in the same direction.

However,

1/4mv^2 = E in this case

so,

1/4m(90000) = 300,000

22500m = 300,000

m= 13.34

Now, since that/s 13.34 kg, we can say that it's 13,340 g.

So, since there is more than one type of metal to build it from, the densities vary greatly between them.

And plus, with more and more improvements in structural construction techniques, better flywheels can be constructed, by using 1 millimeter of carbon nanotubes from the center of the flywheel, it can have much more than this mass, at a rotation that will put a force of 1g on it.

[N_las]

(it's in one second anyways, so let's do the conversion early)

What do you mean, it is in one second anyway? And 'Power per Kilogramm' doesn't make any sense. What is interessting is 'Energy per Kilogram'.

So the flywheel weights 13.34 kg and can store an energy of 300kJ.

If we use up all this energy for the thruster in ONE SECOND, then we get a power of 300kW.

If we assume the number of 300kW/1N for the thruster is correct, than we can have a force on your spaceship of 1N for 1 second.

We don't know how heavy the spaceship is, but is has to be at least 13.34 kg heavy (because of the flywheel).

So with a spacehipmass of ONLY 13.34 kg we get an acceleration of 0.075 m/s². But only for 1 second, because then all the energy in the flywheel is gone. So we have a dV of 0.075 m/s. Do you really think thats a big achievement? I would guess letting the astronauts spit out the rear window will be a better thruster.

And if you use a metal with propertys that allow it to store double the energy and wheigth only half, then you will have a dV of 0.3 m/s ... wow...

If you use double the amount of flywheels, you can create 2 Newton force on the ship, but you have double the mass, so we have the same acceleration and hence the same dV.

And keep in mind, I assume the ship consists ONLY of the flywheel, because that is the absolute minimal mass the ship could have in the thought experiment. A real ship that weights several tons (let us say 10t) will have a dV of 0.0001 m/s.

Let us do a comparision, a mentos in cola rocket can rise up to 10 ft. that is 3 meter. To rise 3m on earth, on need a dV of 7.7 m/s. A full cola bottle weigths 1.5 kg. An empty one weights around 50g. That gives an Isp of 0.23 seconds. If we put that bottle on a 10t spaceship, we have a dV of: 0.0003 m/s

So: A cola bottle with mentos is better thruster technologie than a quantum thruster fueld by a flywheel.

Edit: Granted, a quantum thruster fueled by batteries will be nearly as bad. And one fueled by solar cells or nuclear generators, or even fusion reactors will also by worse than a simple chemical thruster, or ion thruster with a small battery and small propellant tank.

And by the way: All your talking about flywheels are better than batteries: From your calculation, we see that a flywheel can have a energy density of 300kJ/13.34kg, so 22 kJ/kg. Lition-ion batteries have an energy density of 360–950 kJ/kg. Babbling about 'flywheel energy is in a more useful form' doesn't make any sense.

Can't you just admit that you are wrong? There is no reason to use a flywheel as energy storage on a spaceship, ever. And apart from that, a quantum thruster can't compare to ANY thruster in use today. Even a bottle cola with mentos is more useful for spaceship propulsion. You will see that admitting one is wrong isn't bad. It feels good and opens the door to new knowledge.

You always behave as if you would have the ultimate knowledge and we are just simpleminded fools. Have you ever heard about the Dunning Kruger effect? http://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect I would guess you are still in high school, so don't think I am saying you are stupid. You just have to gain much more knowledge and experience.

Edit Edit: Just for your info. Maybe you are not aware, why the people here want to use the formula E = 1/4 m v^2 for the energy of the flywheel, instead of E = 1/2 m v^2.

If the v in the formula describes the velocity at the edge of the flywheel, the mass nearer at the center of the wheel has a lower velocity, right? From that, one can derive the factor 1/4 instead of 1/2.

Edited April 19, 2014 by N_las

[KASASpace]

"weights 13.34 kilograms"

WOW. Do you realize that is what we call mass, not weight?

Okay, yeah, if you only had ONE flywheel with only THAT MASS.

But, let's be realistic here.

Let's say every 30 degrees of a 1 meter radius cylinder had ~ 13.34 kilograms.

That's 12, multiplied by 13.34.

160.08 kg

Now, we need to factor in the velocity it will be travelling.

Let's solve for it:

(Hmm, this kind of reminds me of 1/2gt^2 = d)

1080000000 = 1/4 (160.08)v^2

1080000000 = 40.02v^2

26986506.75 = v^2

5194.85 = v

Hmm, that's quite a ridiculous speed, so it probably won't be done anytime soon, but no one said you had to have one flywheel.

That was just an example, and it was a really bad one at that. You must not have read that last part, I recommend you do so, it explains how you can get much greater surface velocities by using an extremely useful thing called carbon-nanotubes. Granted they are in the laboratory setting, but Q-Thrusters haven't even entered that yet.

Fun Fact: Carbon Nanotubes have a tensile strength that is better than steel, and titanium, and ultra high modulus graphite.

I never said I was superior in any way, I'm simply pointing out some facts that must have not crossed anyone's mind.

"ultimate knowledge"

What?

I never said that either.

Have you ever heard, that knowledge is more than being a tape recorder and knowing some equations?

No?

Than there you go.

[andrewas]

Add all the flywheels you like, at the same specific energy the available dV will work out the same. Around 0.33mm/s by the most optimistic figures I could find without straying into outright science fiction. That means your flywheel needs to be many orders of magnitude more effective than the best of today's speculative technology to store enough energy to accelerate a ship to a useful speed. And that figure was neglecting the mass of all components except the flywheel, this is the unobtainable best case scenario.

Once again, Q-thrusters are very low thrust, high energy thrusters. They're natural application is deep space mission along the lines of Dawn or Deep Space 1. Very light spacecraft relatively close to the sun so that a relatively small and light solar panel can provide enough power for useful thrust. The engine burns continuously for years. That is what q-thrusters are good for, and I don't know if they are actually the best choice, I think the power requirements are too high for them to give a lower launch mass.

You appear to be trying to force them into an RCS thruster role. That means short, relatively powerful bursts. But Q-thrusters are much less power efficient than any other thruster, so to run them as RCS thrusters you need massive energy storage, to the point where even if that storage dominates the mass of the ship the q-thruster cannot generate useful delta V.

[KASASpace]

Add all the flywheels you like, at the same specific energy the available dV will work out the same. Around 0.33mm/s by the most optimistic figures I could find without straying into outright science fiction. That means your flywheel needs to be many orders of magnitude more effective than the best of today's speculative technology to store enough energy to accelerate a ship to a useful speed. And that figure was neglecting the mass of all components except the flywheel, this is the unobtainable best case scenario.

Once again, Q-thrusters are very low thrust, high energy thrusters. They're natural application is deep space mission along the lines of Dawn or Deep Space 1. Very light spacecraft relatively close to the sun so that a relatively small and light solar panel can provide enough power for useful thrust. The engine burns continuously for years. That is what q-thrusters are good for, and I don't know if they are actually the best choice, I think the power requirements are too high for them to give a lower launch mass.

You appear to be trying to force them into an RCS thruster role. That means short, relatively powerful bursts. But Q-thrusters are much less power efficient than any other thruster, so to run them as RCS thrusters you need massive energy storage, to the point where even if that storage dominates the mass of the ship the q-thruster cannot generate useful delta V.

I'm not forcing them into an RCS thruster role, I'm saying they would be more suited as maneuvering engines, like ION engines, except they require so much power their practical applications are, to say the least, limited.

And, Flywheels are more limited by the materials than much else. Use carbon nanotubes at and around the bearing for only one millimeter, it will be able to store high amounts of energy, two millimeters, and so much energy could be stored it's ridiculous, three millimeters, and then you're just crazy, because getting to that speed is probably not going to happen.

It's just material science, after all, the tensile strength of carbon-nanotubes is rather large compared to everything else.

[TheGatesofLogic]

power is not something you can store for god's sake! i've tried to iterate this to you multiple times but you don't seem to understand. I could have a device that stored a single joule of energy and it could supply as much power as i could possibly want, albeit for only a fraction of a fraction of a second. a flywheel holds a maximum energy proportional to mass and the tensile strength of the material, so yes carbon nanotubes would have great energy densities, but even then when that math is considered its a factor of 10 less than the energy density of batteries we can build right now because nanotubes simply aren't that dense. We can't build functional macro-scale low-defect nanotube structures yet. Density is also all that matters in this situation as dV is determined by mass and energy only since the thruster is without stored exhaust.

[KASASpace]

Only a small amount of carbon-nanotubes would be used, and only at and around the center, or bearing, or joint, or whichever you call it.

It's used for supporting large masses from the center in this case, and as such a circumferential velocity that is extremely high can be used, possibly a velocity which will make the outer mass experience 1g of force, as carbon-nanotubes' tensile strength is high, you only need a few grams at most, using a much more dense material (DU, lead, molybdenum, etc) as the primary build-up.

[Rakaydos]

Only a small amount of carbon-nanotubes would be used, and only at and around the center, or bearing, or joint, or whichever you call it.

It's used for supporting large masses from the center in this case, and as such a circumferential velocity that is extremely high can be used, possibly a velocity which will make the outer mass experience 1g of force, as carbon-nanotubes' tensile strength is high, you only need a few grams at most, using a much more dense material (DU, lead, molybdenum, etc) as the primary build-up.

It still cant hold a handle to a Lithium Ion battery per Kg. It just isnt kinetically possible.

[KASASpace]

Another problem with batteries is that chemically, it most likely won't have amazing lifetimes, but then again metal fatigue will really mess up the flywheel eventually.

[KASASpace]

Hmm, let's see:

300,000 * 3600 (for one hour of full operation)

Or, if you want a full day of operation (86400*300,000)

25920000000 Joules, no matter if flywheel or battery for one day of operation.

Now, let's actually raise that mass a bit, perhaps every 15 degrees.

320.16 kg (after all, flywheels are typically massive)

25920000000 = 1/4(13.34)v^2

25920000000 = 3.335v^2

7772113943.03 = v^2

88,159.6 = v

WHEW! That is one heck of a lot of velocity. Impossible with modern materials.

So, let's do this a more "plausible" way:

25920000000 = 1/4m(300^2)

25920000000 = 22500m

1152000 = m

So, we should probably just use more velocity, as that much mass is ridiculous.

So, a decent amount of mass, so there's not too much velocity, and not to little velocity so we don't need too much mass.

In conclusion, for now, batteries are more efficient. But, in the future, who really knows what will happen. Maybe some structural breakthrough?