Realistic fusion fuel?

[Liam's Ghost]

Assume a clean slate, no forty years of irregularly researched lore or multiple rule rewrites getting in the way...

What would be a simple but "physically realistic" fuel efficiency for spacecraft propelled by a battletech scale fusion torch? How many fuel points per ton (or tons per fuel point as the case may be for larger craft)?

On a related note that leads from the first: operating under these parameters, how big of a ship (and how much could it carry) can be expected to be built, launched, landed, then launched and landed again safely and effectively off of a planetary surface? IE how close can you get to normal dropship operations under those fuel constraints?

[Primus203]

Its fairly high though I don't have firm figures I once calculated several years ago that a 100k ton flying wing gunship using fusion reactors at something like only 10 percent efficiency would only burn about 1 ton of hydrogen daily. And I believe that was conservatively.

And my assumed efficiency was only so low because it was multiple reactors being run below design spec which is less efficient. The reason for that was in case one failed another could be brought to higher power if they were already going.

[worktroll]

It'll take a while to try and re-run some numbers I did a decade or more ago, but the biggest change is "no small fusion plants". Whether tokomak, or inertial containment, you need a big installation, with something to get the reaction going (eg. a fission plant). With a sub-based fission reactor (with steam exchangers) coming in with change from 1000 tons, you see what I mean? That's the sparkplug.

I played with this in terms of 'space carriers'. If you can't make a self-contained fusion plant for less than multiples of 1000 tons, then how do you have space fighters (let alone battlemechs - hello, there, Ogres!). My 'solution' was that the fusion engine could be shrunk to 100 tons or so, provided the energy to start it came from external sources (it would be self-sustaining afterwards, barring accident.) So your 'space fighters' (more like space PT boats) got lit up inside the hull of the carrier, and then pushed out before thrusting. BT's magical lasers might well reduce the fusion element to 10s of tons, but you still have to get the energy from somewhere to start the reaction.

Mind you, this is also assuming deut-trit to protium fusion. If you want to stick to canon protium-protium fusion, add one or two zeros to the mass, to account for the layer of  catgirls catalysing the reaction.

[Daryk]

Tactical fuel consumption rates are entirely feasible.  Only Strategic Thrust is way out there.  My solution for some time has been to simply reduce Strategic Thrust by an order of magnitude.  If that's still too much for you, just impose tactical fuel consumption for that.

[DevianID]

Tactical fuel consumption rates are entirely feasible.  Only Strategic Thrust is way out there.  My solution for some time has been to simply reduce Strategic Thrust by an order of magnitude.  If that's still too much for you, just impose tactical fuel consumption for that.

Tactical fuel as 'Strategic' fuel works, but only if you fix the break points.  Since fuel is an afterthought, the chart for fuel points per ton doesnt scale well... heck they just stopped adding brackets before even getting out of the low end of warships.

I prefer a % system personally.  So 10% fuel load = x burn days, and work from there.  Burn days as your 'delta V' value works pretty well, and existing dropships like the leopard and union and such can all still make it from zenith to planet, it just takes longer with their tiny fuel bunkerage, as even using tactical thrust the tactical engines are that good even on a shoestring fuel point budget.  But aerospace needs a rewrite anyway, one we are not likely to ever see, so I can assume in a rewrite the Leopard/Union would get their numbers shuffled around to keep the 1g burn rate by upping the fuel% or including the ability to 'over fuel' them, so the Leopard is 1.9k tons empty instead of 1.9k tons with 137 tons of fuel.  I like the 'empty' weight model for a rewrite, as it (A) gives room to make the armor heavier without changing any listed dropship's weight and (B ) lets you fill the very large volume of the dropships with low density fusion fuel, filling the vast number of empty decks and cavernous spaces on even the tiny Leopard, without sacrificing the lore of cramped accommodations.

As for fuel/fusion, if I had my way mechs and such would be battery operated, and charged from the big fusion engines on ships, or with a backpack charger for extended operations like how fighters have drop tanks.  The material science for batteries in Btech is pretty great, and I much prefer having a .5 ton battery then a .5 ton fusion engine in a truck-sized vehicle.  Plus, 'Fusion Battery' still sounds futuristic. 

[Daryk]

In the truck, that would be 0.75 tons, but I see your point.

[Mechanis]

It'll take a while to try and re-run some numbers I did a decade or more ago, but the biggest change is "no small fusion plants". Whether tokomak, or inertial containment, you need a big installation, with something to get the reaction going (eg. a fission plant). With a sub-based fission reactor (with steam exchangers) coming in with change from 1000 tons, you see what I mean? That's the sparkplug.

I played with this in terms of 'space carriers'. If you can't make a self-contained fusion plant for less than multiples of 1000 tons, then how do you have space fighters (let alone battlemechs - hello, there, Ogres!). My 'solution' was that the fusion engine could be shrunk to 100 tons or so, provided the energy to start it came from external sources (it would be self-sustaining afterwards, barring accident.) So your 'space fighters' (more like space PT boats) got lit up inside the hull of the carrier, and then pushed out before thrusting. BT's magical lasers might well reduce the fusion element to 10s of tons, but you still have to get the energy from somewhere to start the reaction.

Mind you, this is also assuming deut-trit to protium fusion. If you want to stick to canon protium-protium fusion, add one or two zeros to the mass, to account for the layer of  catgirls catalysing the reaction.

I mean
we really don't need to worry about the critical mass problem, because the core handwave of the entire BattleTech setting was that there's Fekery™ happening in their fusion reactors that make them impossibly efficient according to "modern"/IRL physics.

[Lagrange]

Here, I did a calculation suggesting about 50 points of fuel per thousandth of vehicle mass in fuel.

So, a 1000 ton dropship would require 2 tons for a 50 minute thrust at 1g or about 58 tons for a 1g thrust for 24 hours. 

[Daryk]

That was an interesting discussion! :)