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 Post subject: Relative power of on-board reactors in AV:T
PostPosted: Thu Apr 10, 2014 3:39 pm 
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Joined: Tue Apr 08, 2014 8:10 am
Posts: 50
Greetings,

I have a question regarding the relative power of fusion torches and reactors in AV:T. For example “Wasp”, one of the ships from AV:T tutorial has a 5.08 TW torch and a pair of reactors with 125 MW combined electrical power output. I understand that the listed power of the torch is thermal power output, so to properly compare them I need to get reactor thermal output (i.e. before the heat produced by the reactor was converted into electricity). Given the efficiency of 30% that’s 416.(6) MWth, or ~8% of the torch output.

Do I understand correctly that the reason for such difference is that the torch expels most of the generated heat with the propellant, and the radiators on the magnetic bottle need to deal with only a tiny fraction? How one can calculate that fraction? And if a comparable power was used for electricity, a ship would need the ability to radiate most of it as waste heat, am I right?

The second question is about reactors themselves. I've read somewhere that in AV:T ships have fission reactors. Is this right? The presence of fusion torches imply that humans have mastered controlled nuclear fusion, so why don’t they use smaller scale fusion reactors for power generation on starships? Are there fusion reactors that produce electric power on planets, moons and asteroids?

Thank you


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 Post subject: Re: Relative power of on-board reactors in AV:T
PostPosted: Fri Apr 11, 2014 7:35 am 
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Joined: Thu Sep 08, 2005 10:00 am
Posts: 5095
Location: Milwaukee, WI
Danil Martynuk wrote:
Greetings,

I have a question regarding the relative power of fusion torches and reactors in AV:T. For example “Wasp”, one of the ships from AV:T tutorial has a 5.08 TW torch and a pair of reactors with 125 MW combined electrical power output. I understand that the listed power of the torch is thermal power output, so to properly compare them I need to get reactor thermal output (i.e. before the heat produced by the reactor was converted into electricity). Given the efficiency of 30% that’s 416.(6) MWth, or ~8% of the torch output.


That 30% conversion efficiency is under optimum conditions for extracting energy from a fusion reactor. It doesn't take too many complicating factors to drive that number down. Among them is that the exhaust of the fusion reactor is highly radioactive. This will ultimately cause brittleness and other issues with the radiators.

Quote:
Do I understand correctly that the reason for such difference is that the torch expels most of the generated heat with the propellant, and the radiators on the magnetic bottle need to deal with only a tiny fraction? How one can calculate that fraction? And if a comparable power was used for electricity, a ship would need the ability to radiate most of it as waste heat, am I right?


Right on both counts. We do have an MHD power extraction system in the game - it's explained in the rules in section B. None of the ships published use it, but it allows you to recharge your batteries each segment by an amount based on the number of fuel units you used. From the perspective of 10 Worlds naval planners, MHD systems were one of those "It looks good on paper, but it didn't work out in actual engineering" examples.

The reason for this is two-fold. The first is that having a ship regenerate power too quickly gets rid of interesting decisions. The second is that it's really hard to run a fusion reactor at lower outputs, and having a power source that isn't moving the ship is a good thing.

Quote:
The second question is about reactors themselves. I've read somewhere that in AV:T ships have fission reactors. Is this right? The presence of fusion torches imply that humans have mastered controlled nuclear fusion, so why don’t they use smaller scale fusion reactors for power generation on starships? Are there fusion reactors that produce electric power on planets, moons and asteroids?


Actually, the presence of D-T fusion torches implies imperfect mastery of fusion; the only way they can get fusion torches to work is by running a constant stream of non-recoverable coolant over the reactor; our reactors imply a fusion plasma density that's about 1/8th the density of plasma on a star, for a D-T reaction.

D-T fusion is the kind of fusion reaction we produce now with fission-fusion pulse hydrogen bombs. It produces almost 80% of its energy in high energy/high speed/very hard to harness neutrons. The easiest way to capture them and get useful work out of them is to put something in there to catch them and have that something leave the reaction area at a high level of speed and generate thrust. It turns out that a fair number of light elements can do this, each with different trade-offs...but it does mean that to make this work you've got an open-flow coolant problem and the open flow coolant is going to be highly radioactive.

D-3He plants were always firmly in the realm of science fiction in this setting - the energy density of the plasma needed to make D-3He fusion is nearly 300x what's needed for D-T fusion.

Note that these are setting assumptions based on physics. If you change the minimum plant size to generate a D-3He fusion reaction, then yes, D-3He fusion plants become viable. Since we didn't have a working fusion plant in 2002-2003 when I wrote the game...and fusion has been "20 years away" since 1950... I just went conservative.

Interestingly enough, some of the graphene batteries being posited are getting comparable to both the charge/discharge rates and energy storage per kg densities of 1st generation AV:T batteries. This is important, because once you have that kind of energy storage capability and charge/discharge rate, dispersed power generation from solar cells becomes much more viable...and, as the saying goes:

"What do you call a solar energy spill?"
"A nice day."

_________________
Ken Burnside
President
Ad Astra Games


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 Post subject: Re: Relative power of on-board reactors in AV:T
PostPosted: Sun Apr 13, 2014 1:43 am 
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Joined: Tue Apr 08, 2014 8:10 am
Posts: 50
Thank you very much for the detailed answer.

I'm reading the full book now, so I will wait with any further questions.


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