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Old March 30 2014, 10:03 AM   #8
zDarby
Lieutenant
 
Location: NorCal
Re: Warp-power-scaling data point

Robert Comsol wrote: View Post
Great to learn you are still with us! And another fine article getting into the details of nuclear fusion reactants, beautiful!
Thanks for the complement. Actaully, I was away for a while on a project to create an alternative history. I'm not done with that project but I hit a wall with it and am waiting for inspiration.

Robert Comsol wrote: View Post
This always reminds me of the scene in the original "V-The Visitors" where the main protagonist goes to these huge tanks aboard the V-saucer and opens a valve to taste it. Last time I thought "What is the idiot doing, drinking deuterium?" but of course it just turned out to be water...doh!

I had been under the impression (like in "Oblivion") that aliens would harvest deuterium from our sea water but those in "V" apparently could cross vast interstellar distances but didn't have a fracking clue how to manufacture H20... (IIRC we had one Star Trek species with the same "capabilities" but I'll leave it to someone else to name it - Seriously, turned me off at first to continue watching this ST series).
Didn't see Oblivion and don't remember V. But I've had the feeling you describe over and over again. SciFi movies and TV series are so often so damned science illiterate. Water is everywhere and races that travel between stars don't need to take over our planet for water. Collecting deuterium would be far easier on Titan than on Earth and there's quite a bit more of it there. Indeed, you can collect deuterium anywhere where there's hydrogen, because it *IS* a kind of hydrogen; and there's hydrogen EVERYWHERE!!

But, on the other hand, it's hard to think of a good reason for star-traveling aliens to come to Earth and take over. Such aliens can make their own, perfect electronic or biological slaves; there's plenty of uninhabited, terraformable planets; resources of all types and kinds are cheap to anyone who can harness enough energy to cross from star to star. The only thing I can think of is xenophobia: destroy the unfamiliar. In which case, we'd be totally screwed. So, for a good story, you have to cling onto something that gives us a chance to survive.

There have been some scifi books that had some good answers: Mote in Gods Eye, Enders Game, Childhoods End, Rama, etc. But these answers are good for the cerebral people who read books, not the glandular movie crowd.

So I have some sympathy for writers and producers who create this tripe, but it so pisses me off none the less!!

Robert Comsol wrote: View Post
I'd like to believe I understood the basics. To decrease volume of ship storage it's essential to compress the reactants. Could there be a way by the 23rd Century to compress deuterium from a liquid into a more solid form or would you need the "chemical compound" (and what would / could this probably be)?
As it turns out (AFAIK) hydrogen is less dense as a solid than as a liquid, which is also true from water and pig iron. This is an effect of its chemistry (it's electrons) and so is true for both protium and deuterium. (IE light and heavy hydrogen, respectively.) You can, of course, compress liquid hydrogen to greater densities via pressure, but that requires heavier tanks.

Of course, if you apply force fields you can use as much pressure as your field projectors can stand.

There is an answer that might be possible in the 23rd century, or even the late 21st: ultradense deuterium. AFAIK the experiments have not been confirmed yet, but there was an announcement of the discovery of deuterium nuclei sharing electrons much more freely than even metals. It's called the Rydberg state. The stated density was on the order of 130 kg/cc = 130e6 kg/kL. This would be a highly efficient way of storing deuterium fuel. This means a cube a yard on a side would store 13 times the deuterium stored in a Galaxy class! (Assuming Galaxies store D2 as a cryogenic slush at 13K & 1atm, which seems to be the TNG Tech Manual's intent.)

Unfortunately for the Federation, every bit of evidence points to them using slush deuterium as a storage method. Ultra dense deuterium is not hinted at in any of the episodes I remember and none of the books I've read.

Robert Comsol wrote: View Post
The "liquid storage" is inspired by publications like the TNG Tech Manual? Would chemical storage have better advantages?
Yes. The tech manual specifically states deuterium slush stored at 13 kelvin. It does not state (to my knowledge) the internal pressure or the density, and I don't remember it stating a total mass of fuel; just the volume of the storage tank, the temperature at which the fuel was stored, the word "slush" and a figure for the mass of deuterium lost per unit time. If anyone knows something different, I'd love to be corrected.

I also do not remember any direct statement in any of the episodes. The need for deuterium has come up more then once but I don't remember any direct or implied statement as to the storage method or state of the deuterium. Again, I'd love to be wrong.

This, of course, means my calculations and conclusions are relying on assumptions that are not "cannon". However, I think they were probably the intent of the authors and directors at the time.

Storing deuterium chemically would do only two things for you: make deuterium storage easier and more dense, by volume. Easier because the liquids need not be as cryogenic: 100K instead of 13K (in the case of propane). AgainiIn the case of propane, it's 163% more energy dense per volume. However, this comes at the cost of energy density per kilogram: liquid D2 has 323% more energy per kilogram.

As the above table demonstrates, for rockets, this is an excellent compromise: with the Galaxy class tankage and mass, the worst fusion reaction with propane fuel out performs the best reaction with liquid deuterium fuel. But, apparently, this is not a good compromise for warp ships, even ones that use a fusion rocket for slower than light travel.

Robert Comsol wrote: View Post
I assume there's one or more things I missed or did not understand. Bigger mass always makes bigger power requirements inevitable to move it forward (equally at sublight or FTL), I thought.

I'd be grateful if you could elaborate. Thanks for sharing this with us.
Ok. I know about rockets, so let's discuss that first. Exhaust velocity and fuel mass is everything for a rocket. Rockets are literally separated into fuel mass and the mass of everything else. Here's why:

If the fuel mass is exactly the same as the mass of everything else in the rocket, and the exhaust velocity is, say, 4000 meters per second, then once all the fuel is burned the rocket's velocity will have changed by 4000 meters per second. If the fuel's exhaust velocity is 2 kps, then the rocket's final velocity will be the same. So higher exhaust velocity means a faster end velocity. The relationship is linear: all else being equal, half the exhaust velocity, half the final rocket velocity. (Unless, of course, you have to take into account relativistic effects.)

However, if the fuel mass is more than the mass of the rest of the rocket, the rocket's end velocity will be more. Conversely, if the ratio is less, then the velocity will be less. But in this case the differences are exponential. Half the ratio is not half the final speed but considerably less. How much less depends on what numbers you started with.

So there's a balancing act: fuels with a better energy densities by mass have higher exhaust velocities because more of their mass has been turned into energy; higher exhaust velocities are more efficient, giving you the same velocity change for less fuel burned. However, fuels of higher density make for a better fuel-to-rocket mass ratio, which can overcome the disadvantage of a lower exhaust velocity.

This is exactly the case for the deuterium/propane comparison. For the same tankage and vehicle, you get a much better mass ratio for propane than liquid deuterium and, even though it releases much less energy when it burns, you get a far superior rocket performance.

However, my argument here is that this can not be true for a warp ship or they would use chemically stored deuterium instead of storing it as a liquid. But, according to everything I think I know, this is not the case.

Of course, I'm assuming quite a bit to make this conclusion, so it's shakey at best. The "real" explanation is that the writers didn't think of it. But the in-universe explanation must be (IMHO) that the per-mass energy-density is more important than the per-volume energy-density. Which is another way of saying extra mass penalizes warp drive more than extra volume.

What, precisely, that means is up for debate.

Did I answer your questions?
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