The Power of a Star....(calculations of Warship engine power and heat)

[Vehrec]

Earlier today I was talking on Discord with someone who is creating her own hard-SciFi universe, and she went off on a tangent I found interesting about fusion reactors.  Her 15 kTon frigate design, about the size of a medium dropship, with an acceleration of .13 g, 2000 tons of radiators, 1000 tons of drive assembly, and 5000 tons of internal fuel, giving it about 2 days of slow burn maximum efficiency transfer.  Plus the option to bolt on external fuel tanks for in-system work.  All of this is powered by a 5 TeraWatt reactor.

Compare this to a McKenna, which can accelerate almost 8x faster, is 128x more massive, has only 32% of the fuel, and has 20 times more burn time eeked out of it.  So...it's gotta be at least a hundred PetaWatts of reactor, right?  But you'll need to contain that reactor, which means you need to tap it off for energy-lets assume you only need 10% of it's power to contain the reaction, and you have a 98% efficent way to turn the charged particles into electrical power, so only .2% of the 100,000,000,000,000 watts is turned into heat.  That's still like 200 terawatts of heat to radiate.  Those radiator fins must be white hot!

[Daryk]

Strategic thrust has always been a problem in BattleTech, usually hand waved away by linking it to KF jump physics being discovered in a fusion reactor.

[Vehrec]

Strategic thrust has always been a problem in BattleTech, usually hand waved away by linking it to KF jump physics being discovered in a fusion reactor.

  Consider, however, the implications for weapons-heat!  If flank acceleration for the McKenna is 200 terawatts of heat, and that's only what, seven heat points?  How much power is wasted by the NPPCs!

[Daryk]

Oh, plenty... best not to look too hard at THAT sun... ;D

[Pat Payne]

  Consider, however, the implications for weapons-heat!  If flank acceleration for the McKenna is 200 terawatts of heat, and that's only what, seven heat points?  How much power is wasted by the NPPCs!

It's a good thing that BattleTech has acceptible breaks from reality -- otherwise how would a WarShip radiate all that heat quickly enough to keep from flash-frying the crew?

[DevianID]

The plog art where the mckenna's many fins are very clearly radiators is one of the cooler redesigns for warships, and what I wish was a thing from the start.

As for the reactors in Btech, even a small, .5 ton reactor can power infinite lasers/flamers, if you can keep it cool... So the fusion engines in btech are practically limitless for energy, only kept in check by burgeoning heat dissipation needs that make it more efficient to just run a second power source after the first 10/20 heat.  So on warships, you can potentially be cranking them up to just silly heat levels, and somehow containing all that heat and pressure with no wastage.  It almost seems like the engines in battletech eat heat, instead of emit heat, based on how the rules work.

[Vehrec]

I'm just saying, we need a colorization of that Plog-art where the McKenna's radiator fins are glowing properly white-hot from the rejected heat and flash-burning the paint off anything that comes too close. :D

[Daryk]

Plog has very reasonable commission rates, and since that's just a re-color of something he already did it might be even more reasonable... ;)

[cray]

The estimate for a McKenna's engine power is probably orders of magnitude low.

Exhaust power equals 0.5 x exhaust velocity x exhaust force / efficiency

If you use exhaust velocity in meters per second, force in newtons, and efficiency as a unitless fraction (0.00001 to 1), you get power in watts.

It's easy enough to find a McKenna's newtons of thrust. The problem is exhaust velocity. For ships that are using grams of fuel per second in strategic mode, you can find superluminal values of exhaust velocity (if you stick with Newtonian math). The resulting power value exceeds the amount of energy you can get from nuclear fusion of hydrogen, or potentially from antimatter.

[Paul]

The problem is exhaust velocity. For ships that are using grams of fuel per second in strategic mode, you can find superluminal values of exhaust velocity (if you stick with Newtonian math)

If I recall an ancient thread correctly (with Fallguy, to date it some) there is some 'gain' in the sense that the mass of the fuel goes 'up' relative to the vessel the closer to c the fuel is made, but still nowhere in the ballpark of removing the sheer magic of it all. I can't recall if it's better or worse for DropShips; I assume there's more magic in WarShips.

[Daryk]

Yes, the Lorentz transformations from Special Relativity mean the mass of the exhaust goes up the closer to light speed it becomes.  And no, it's still not enough to make Strategic Thrust work, whether you're working with a WarShip, DropShip, or Small Craft.

[Cannonshop]

The estimate for a McKenna's engine power is probably orders of magnitude low.

Exhaust power equals 0.5 x exhaust velocity x exhaust force / efficiency

If you use exhaust velocity in meters per second, force in newtons, and efficiency as a unitless fraction (0.00001 to 1), you get power in watts.

It's easy enough to find a McKenna's newtons of thrust. The problem is exhaust velocity. For ships that are using grams of fuel per second in strategic mode, you can find superluminal values of exhaust velocity (if you stick with Newtonian math). The resulting power value exceeds the amount of energy you can get from nuclear fusion of hydrogen, or potentially from antimatter.

If I recall an ancient thread correctly (with Fallguy, to date it some) there is some 'gain' in the sense that the mass of the fuel goes 'up' relative to the vessel the closer to c the fuel is made, but still nowhere in the ballpark of removing the sheer magic of it all. I can't recall if it's better or worse for DropShips; I assume there's more magic in WarShips.

Yes, the Lorentz transformations from Special Relativity mean the mass of the exhaust goes up the closer to light speed it becomes.  And no, it's still not enough to make Strategic Thrust work, whether you're working with a WarShip, DropShip, or Small Craft.

Y'know, this kind of logic-break in a setting begs for a baloneyium in-universe explanation, something to lampshade it, or even use it as a key 'trick' in a story.

Somewhere along the lines of either a character realizing that the engines they've been using are violating the hell out of the Physics they think they know and understand (then figuring out how to exploit it) or something...

[Maingunnery]

Y'know, this kind of logic-break in a setting begs for a baloneyium in-universe explanation, something to lampshade it, or even use it as a key 'trick' in a story.

Somewhere along the lines of either a character realizing that the engines they've been using are violating the hell out of the Physics they think they know and understand (then figuring out how to exploit it) or something...

The hyperspace tap effect could be good story element in itself, but that would have to be set in the early space era. If it was a trilogy, the first part would be a story of Thomas Kearny and Takayoshi Fuchida as they made their discovery, the second part would be people trying to optimize their engines to exploit that effect (late 21st century), and a final part would be about the first hyperspace jump.

For the people of the more common eras, it would simply be a fact of life, a handy oddity of their known physics. 

[Daryk]

As I said right off the bat, the usual handwave is to link Strategic Thrust to the fact that KF Physics were discovered in a fusion reactor.

[General308]

Earlier today I was talking on Discord with someone who is creating her own hard-SciFi universe, and she went off on a tangent I found interesting about fusion reactors.  Her 15 kTon frigate design, about the size of a medium dropship, with an acceleration of .13 g, 2000 tons of radiators, 1000 tons of drive assembly, and 5000 tons of internal fuel, giving it about 2 days of slow burn maximum efficiency transfer.  Plus the option to bolt on external fuel tanks for in-system work.  All of this is powered by a 5 TeraWatt reactor.

Compare this to a McKenna, which can accelerate almost 8x faster, is 128x more massive, has only 32% of the fuel, and has 20 times more burn time eeked out of it.  So...it's gotta be at least a hundred PetaWatts of reactor, right?  But you'll need to contain that reactor, which means you need to tap it off for energy-lets assume you only need 10% of it's power to contain the reaction, and you have a 98% efficent way to turn the charged particles into electrical power, so only .2% of the 100,000,000,000,000 watts is turned into heat.  That's still like 200 terawatts of heat to radiate.  Those radiator fins must be white hot!

So the first mistake you made is considering reality and BT in the same thoughts.  Don't do that it will hurt your head.    Seriously though trying to do that with BT will drive you made.  I know on a Glance it looks like a realistic Scifi universe but that is a poorly vailed illusion.

[monbvol]

Yes, the Lorentz transformations from Special Relativity mean the mass of the exhaust goes up the closer to light speed it becomes.  And no, it's still not enough to make Strategic Thrust work, whether you're working with a WarShip, DropShip, or Small Craft.

Even tactical thrust becomes physics breaking somewhere in the Dropship range, exactly where I'd have to work out again.

[Daryk]

I don't remember that being much of a problem but could certainly be wrong (and welcome the math to prove me so)... ;)

[monbvol]

For tactical thrust:

We know one space hex is 18km and one turn is 60 seconds. This gives us a target velocity of the unit itself of 300m/s.

One newton of force is sufficient to move one kilogram 1 meter per second.

So all we need to determine newtons of force is to select a mass of unit.

Battletech units are supposedly measured in kilotons, so 1000kg per kiloton.

So from a zero relative velocity frame a 1,200 ton dropship to move that target velocity must have had 360,000,000 newtons of force applied to it, which means with newton's third law 360,000,000 newtons of force is applied to one fuel point worth of mass, which is 20kg for such a Dropship according to Tech Manual, but in the opposite direction.  Even with the Lorentz Transformations(at least the ones I can find to reference) this either requires one fuel point to start with more mass or exceed the speed of light upon leaving such a Dropship from such a zero frame reference.

[Daryk]

Conservation of momentum (m₁v₁=m₂v₂) for 20kg of fuel and 1,200 tons of ship going 300 m/s yields the fuel moving 18,000,000 m/s, a little over 5% the speed of light.  In that regime, Special Relativity shouldn't be a significant factor.  Which is to say: it may be hard, but not impossible (like Strategic Thrust).

[DevianID]

Tactical thrust is broken only because the designers did it in brackets, and then stopped making brackets after like 30% of the ship max mass.  So a 2megaton warship is in the same bracket as much lighter vessels, and its tactical drives are too strong.  From what I recall of the old threads, it was physically possible for tactical thrust as long as you were not too far past the last bracket milestone.

The thing I hate currently isn't fuel use, its 'solar' sails.  Using a solar sail as a light absorber for energy, when fusion power exists, is just silly.  Further, the black sail as a radiator to cool the KF drive works so much better IMHO, and has times that are plausible regarding the sun's output.  But sails as solar collectors is just terrbile, especially cause they don't actually scale with light or anything.  A super dim star with almost no output, and a super bright star, or moving closer or farther from said star, dont really change the charge time and are not at all in scale with luminosity.  But the passive rate of a radiator would be pretty stable, and radiator sails not only make real life sense, they are even cooler then silly solar sails in how they operate.

A warship unfurling its sail to cool off, versus keeping it stowed and riding the heat to keep moving, also gives better story opportunities, and heat feels much more a core part of battletech's identity.

[Daryk]

No argument about the "feel" there, but I could have sworn there was at least minimal math behind the size of the sails.

[Cannonshop]

No argument about the "feel" there, but I could have sworn there was at least minimal math behind the size of the sails.

Minimal might be the right way to describe the math.  Remember, Battletech only has about 5% more hard science than Star-Wars the Movie (and 3% more than Spaceballs.)

[Daryk]

"3% more than Spaceballs" has to the be the most hilarious way I've seen it described! ;D

[cray]

No argument about the "feel" there, but I could have sworn there was at least minimal math behind the size of the sails.

No. The sails can replace some hundreds of tons of diesel fuel for a typical JumpShip, but they don't come close to matching the hydrogen expended during a KF drive recharge.

[Maingunnery]

No. The sails can replace some hundreds of tons of diesel fuel for a typical JumpShip, but they don't come close to matching the hydrogen expended during a KF drive recharge.

My headcanon for that is that the liquid hydrogen was mostly used as flush coolant of the core.

[Daryk]

I meant math regarding how they droop at the jump point.  Cray is 100% correct there's no comparison to the energy generated by fusing that much hydrogen.

[Sabelkatten]

Originally (MW 1st ed) jump points were at ~1AU and strategic thrust was much lower (not a rule at all IIRC, thought dropship fuel use was still ridiculously low).

A 100% efficient 1 sqkm sail at 1AU is ~1.5GW. That's a bit more competetive.

(But please ignore the AT1 map that implies that each thrust point is ~68,000 Gs...)

[Daryk]

The table on page 138 of MW1e only has class K and M stars around 1 AU (G are 6.7 AU).  I don't see any discussion of fuel consumption there.  Did you mean DropShips and JumpShips?

[Sabelkatten]

Might have been AT1... It’s noted that dropships will boost at 1G for a limited time and then coast most of the way. Which resulted in the week-long travel times despite the shorter distance.

My point being that in the original setting jumpship sails and engine power/fuel use made (a bit) more sense than they now. Moving jump points further out, introducing warships, and making dropships much larger compared to jumpships than the (implied) original ratio has combined to make less sense.

[Daryk]

I don't think they moved jump points out, just filled out the table (most stars have points more than an AU away).