Coventional Fighter with Fusion - Why Fuel?

[V13w3r]

Hello
I have just arrive to the Inner Sphere and as i was looking to the conventional fighters with Fusion engines at my militia base i realize that they need Fuel... Why?
We have very powerful electric engines and compressors as well as high bypass or even Scramjets. We fly in a compressible fluid and got a electric & heat generator aboard this machines why do they need fuel if not for Postcombustor for the extra push or to climb to the limit and see the space

PS: i suppose it's do the game mechanics but if I'm wrong please tell me (Do LAM needs fuel once in atmo?)

[PuppyLikesLaserPointers]

It needs the fuel because it spout the jet to fly. So do LAM in fighter mode because it fly like as an ASF. It is no like the jump jet of the ground vehicle.

[Daryk]

LAMs in AeroMech mode don't use fuel, but as PPLP points out, they do in ASF mode.  Conventional Fighters fly much faster than LAMs in AeroMech mode, and thus need fuel to do it.  They get twice as many fuel points per ton as compared to ASF, though (basically for the reasons you list).

[Cannonshop]

Hello
I have just arrive to the Inner Sphere and as i was looking to the conventional fighters with Fusion engines at my militia base i realize that they need Fuel... Why?
We have very powerful electric engines and compressors as well as high bypass or even Scramjets. We fly in a compressible fluid and got a electric & heat generator aboard this machines why do they need fuel if not for Postcombustor for the extra push or to climb to the limit and see the space

PS: i suppose it's do the game mechanics but if I'm wrong please tell me (Do LAM needs fuel once in atmo?)

It's not so much FUEL, as reaction mass.  yOu can use the fusion engine to heat up a given liquid, under compression, and ride the steam.  Basically the advantage of a fusion powerplant in an airframe, is that if you have a reactive liquid (doesn't have to be jet fuel) it'll work as reaction mass. 

battletech doesn't have gravity control, something has to come out the nozzle end, or your airfame isn't going to be flying.

[V13w3r]

It's not so much FUEL, as reaction mass.  you can use the fusion engine to heat up a given liquid, under compression, and ride the steam.  Basically the advantage of a fusion powerplant in an airframe, is that if you have a reactive liquid (doesn't have to be jet fuel) it'll work as reaction mass. 

battletech doesn't have gravity control, something has to come out the nozzle end, or your airfame isn't going to be flying.

I understand this, its why the aerospace crafts have "Fuel" i think in the lore it says that it's Hydrogen because of its great specific impulse as you heat it, but you can use water too.
What i mean is that today the planes use fuel to move the turbines that ad more impulse thanks to compressing the air that keeps them cool and compress thy air for the combustion. (If not you are a Rocket) but outside of the game mechanics it's relay necessary fuel for conventional fighter that never going to leave the atmo.
We have electric motors that move planes (ok they are RC and Drones but because we dint have a way to produce the obscene energy that fusion engines generate in a compact mode) and as for the heat expansion that its produced by the combustion of the fuel you have the heat produced by the engine that can move or help the turbine to move the blades and as you reach higher speeds you can pass to ram / scram jet engines configurations using the heat/plasma of the engine... no?

[Daryk]

You can do that to a degree (which is basically what LAMs in AeroMech mode do), but beyond a certain point, as Cannonshop pointed out, you need additional reaction mass too.

[Giovanni Blasini]

Hello
I have just arrive to the Inner Sphere and as i was looking to the conventional fighters with Fusion engines at my militia base i realize that they need Fuel... Why?
We have very powerful electric engines and compressors as well as high bypass or even Scramjets. We fly in a compressible fluid and got a electric & heat generator aboard this machines why do they need fuel if not for Postcombustor for the extra push or to climb to the limit and see the space

PS: i suppose it's do the game mechanics but if I'm wrong please tell me (Do LAM needs fuel once in atmo?)

Game balance.  The game developers didn't want to open the can of worms of explaining why fusion turbines on conventional fighters didn't need reaction mass or fuel, but aerospace fighters and LAMs did.  Same reason why conventional fighters powered by fission engines still need fuel, even if TacOps lets them get 160 fuel points/ton (TacOps, pg 307).

[DevianID]

Also, as an aside, the fuel you buy in ASF isnt hydrogen with the images of fighers we have.  It must be something dense instead.  Hydrogen just takes up far far too much volume, and since the visuals of aerospace/conventional fusion fighters isnt a zeppelin/balloon shape, the fuel is likely some neutral liquid metal the fusion engine heats up to throw out the back as reaction mass, as conventional fighters have MASSIVE power to weight ratios compared to modern day fighters.  If all the fusion engine is doing is powering an electric turbine, then like you point out you shouldnt need fuel to the extent you do, instead it should be like a Vtol which also doesnt track fuel in the tactical sense when you have a fusion engine--but you also shouldnt have the massive power to weight ratio either. 

Like, a 50 ton mechbuster moving 5/8, without overthrust, can accelerate 5 hexes, which is 2500 meters, in 10 seconds.  That is 2.5g, so a 2.5 to 4x thrust to weight ratio with overthrust.  Seeing as 1.25 thrust to weight is class leading for modern jets, and .4 is good for airliners, I dont know what is pushing the mechbuster but its not just sucking up and moving air out a turbine.  Just imagine the sound of a conventional aircraft in battletech! 

[Wolf72]

If you're looking for an aircraft w/o fuel you can use the support rules and make a propeller driven plane with a fusion engine

[cray]

Hello
I have just arrive to the Inner Sphere and as i was looking to the conventional fighters with Fusion engines at my militia base i realize that they need Fuel... Why?
We have very powerful electric engines and compressors as well as high bypass or even Scramjets. We fly in a compressible fluid and got a electric & heat generator aboard this machines why do they need fuel if not for Postcombustor for the extra push or to climb to the limit and see the space

PS: i suppose it's do the game mechanics but if I'm wrong please tell me (Do LAM needs fuel once in atmo?)

During Aerotech 2-Revised's writing, I proposed that fusion-breathing conventional fighters (at least) should have unlimited ranges, maybe just up to their thrust rating. (Overthrust might entail dumping, say, water into the exhaust.) That got some traction with a paragraph on "airbreathing" fighter variants, but it was never carried to the logical end. BT fighters have always carried "fuel" regardless of whether they're surrounded by plenty of reaction mass in an atmosphere.

Jump jets do breath air for reaction mass, but can only work briefly because something something corrosion oxidation (see Tech Manual.) Again, that was me trying to explain why jump jets can't work continuously and why conventional fighters need hydrogen. It doesn't hold up well to scrutiny. 

Also, as an aside, the fuel you buy in ASF isnt hydrogen with the images of fighers we have.  It must be something dense instead.  Hydrogen just takes up far far too much volume, and since the visuals of aerospace/conventional fusion fighters isnt a zeppelin/balloon shape, the fuel is likely some neutral liquid metal

The fuel is explicitly liquid hydrogen for fusion-powered aerospace fighters and other spacecraft, something noted in StratOps. While artwork is canon, it is the canon of last resort after rules, novels, and sourcebook fluff. StratOps' fluff and rules say the fuel is liquid hydrogen, so it's liquid hydrogen, making the artwork incorrect.

But, then again, BT's artwork also gets aerodynamics of aerospace fighters very wrong. And spacecraft artwork plus avowed spacecraft dimensions mean that DropShips and massively armored WarShips have armor thinner than paper or a thin coat of paint. A big DropShip like a Mammoth could almost float in the air if you took out the cargo and swapped its internal atmosphere for hydrogen gas or helium. Point being: take the artwork with a grain of salt, like BT's fact checkers do.

It's not so much FUEL, as reaction mass.  yOu can use the fusion engine to heat up a given liquid, under compression, and ride the steam.  Basically the advantage of a fusion powerplant in an airframe, is that if you have a reactive liquid (doesn't have to be jet fuel) it'll work as reaction mass. 

By the time you're up to 100-ton fighters about half the hydrogen is being fused to helium, so it is both reaction mass and fuel. Strategic Operations goes into that in some depth.

[Cannonshop]

During Aerotech 2-Revised's writing, I proposed that fusion-breathing conventional fighters (at least) should have unlimited ranges, maybe just up to their thrust rating. (Overthrust might entail dumping, say, water into the exhaust.) That got some traction with a paragraph on "airbreathing" fighter variants, but it was never carried to the logical end. BT fighters have always carried "fuel" regardless of whether they're surrounded by plenty of reaction mass in an atmosphere.

Jump jets do breath air for reaction mass, but can only work briefly because something something corrosion oxidation (see Tech Manual.) Again, that was me trying to explain why jump jets can't work continuously and why conventional fighters need hydrogen. It doesn't hold up well to scrutiny. 

The fuel is explicitly liquid hydrogen for fusion-powered aerospace fighters and other spacecraft, something noted in StratOps. While artwork is canon, it is the canon of last resort after rules, novels, and sourcebook fluff. StratOps' fluff and rules say the fuel is liquid hydrogen, so it's liquid hydrogen, making the artwork incorrect.

But, then again, BT's artwork also gets aerodynamics of aerospace fighters very wrong. And spacecraft artwork plus avowed spacecraft dimensions mean that DropShips and massively armored WarShips have armor thinner than paper or a thin coat of paint. A big DropShip like a Mammoth could almost float in the air if you took out the cargo and swapped its internal atmosphere for hydrogen gas or helium. Point being: take the artwork with a grain of salt, like BT's fact checkers do.

By the time you're up to 100-ton fighters about half the hydrogen is being fused to helium, so it is both reaction mass and fuel. Strategic Operations goes into that in some depth.

The question was (*iirc) about Conventional craft.  as in "Why have fuel at all" type questions.  The hydrogen may be (is) fusing to Helium, but the primary purpose is to provide reaction mass since Battletech doesn't have gravity control or reactionless thrust.    (just so we're on roughly the same chapter, if not page here.)

[cray]

The question was (*iirc) about Conventional craft.  as in "Why have fuel at all" type questions.  The hydrogen may be (is) fusing to Helium, but the primary purpose is to provide reaction mass

Reaction mass won't do much without energy from fuel. ;)

since Battletech doesn't have gravity control or reactionless thrust.    (just so we're on roughly the same chapter, if not page here.)

Right. And with a sufficiently abundant energy source you could turn a convenient fluid (like air) into reaction mass.

[Wolf72]

Reaction mass won't do much without energy from fuel. ;)

Right. And with a sufficiently abundant energy source you could turn a convenient fluid (like air) into reaction mass.

Don't mean to get off topic much, but sometimes I hear Jayne talking about Vera when you reply to stuff.  And then everyone at Serenity's table just stares at him, even River.  It's the hat, I think. (that and I can barely follow you all when you get off on your physics rants).

Not trying to get into house rule territory, but would that ability to turn air into fuel be more like an emergency reserve, can still fly but at much reduced ability? ... Are our BT engines good enough to do that?

[Daryk]

I, at least, appreciate the physics rants! :D

[Giovanni Blasini]

During Aerotech 2-Revised's writing, I proposed that fusion-breathing conventional fighters (at least) should have unlimited ranges, maybe just up to their thrust rating. (Overthrust might entail dumping, say, water into the exhaust.) That got some traction with a paragraph on "airbreathing" fighter variants, but it was never carried to the logical end. BT fighters have always carried "fuel" regardless of whether they're surrounded by plenty of reaction mass in an atmosphere.

Jump jets do breath air for reaction mass, but can only work briefly because something something corrosion oxidation (see Tech Manual.) Again, that was me trying to explain why jump jets can't work continuously and why conventional fighters need hydrogen. It doesn't hold up well to scrutiny. 

I still wish that Improved Jump Jets had given an option to allow 'Mechs to actually fly in some fashion, albeit less efficiently than a LAM (for game balance purposes), to greater allow them to self-deploy.  It would have made a great modern era coda to LAMs, ie. "Well, LAMs may be better at pretending to be aerospace fighters, but IJJ-equipped 'Mechs were good enough to fulfill most of their roles, and the ones they couldn't were better done by dedicated ASFs anyway..." kind of thing.

[DevianID]

I still wish that Improved Jump Jets had given an option to allow 'Mechs to actually fly in some fashion

So for mechs in space, 3 JJ = .5g net acceleration.  So I always imagined something like a Phoenix hawk able to move like flappy birds, if it starts on a high building.  You thrust up 36 meters, down, up down.  If you time it perfectly you won't really gain height (unless you have 9+ jump for 1.5g net thrust), but you can bounce along in the air for a while.  Very unstable, with no real flight control, but with enough net thrust to overcome gravity for a little bit.

[Cannonshop]

Reaction mass won't do much without energy from fuel. ;)

Right. And with a sufficiently abundant energy source you could turn a convenient fluid (like air) into reaction mass.

1st: yeah, engine, requires energy.  fusion will create Helium if you start with Hydrogen.  Point is, something has to come out the back to move the aircraft forward.

2nd; isn't that edging into magical tech like Bussard Ramjets though?  (admittedly, there have been people playing with ion engines...)

https://www.youtube.com/watch?v=mnCmvxt2jn8

[Daryk]

I don't disagree Gio, but that probably belongs in fan rules...

[Cannonshop]

I don't disagree Gio, but that probably belongs in fan rules...

Point of order, Daryk, Gio wrote the original rules.  well, not entirely the original, but the ruleset most of us have become familiar with from AT2 onward.

[Giovanni Blasini]

Point of order, Daryk, Gio wrote the original rules.  well, not entirely the original, but the ruleset most of us have become familiar with from AT2 onward.

Nyet.  I helped edit StratOps and worked on LAM rules, but AT2R was before my time.  I do remember Cray trying to squeeze the fusion conventional fighter change into StratOps though.

And I wasn’t thinking fan rules, more lamenting a lost opportunity with the official improved jump jet rules.

[Maingunnery]

1st: yeah, engine, requires energy.  fusion will create Helium if you start with Hydrogen.  Point is, something has to come out the back to move the aircraft forward.

2nd; isn't that edging into magical tech like Bussard Ramjets though?  (admittedly, there have been people playing with ion engines...)

https://www.youtube.com/watch?v=mnCmvxt2jn8

I think that cray is trying to say that it is theoretically possible to take in air, rip out the hydrogen from that for fusion power, and use the rest for propulsion mass.

[Giovanni Blasini]

Exactly.  In real life, the US Air Force used a fission reactor to heat up air to drive two turbojet engines at full thrust.  Should be theoretically possible to do the same with a fusion engine, but unlimited range aircraft in atmosphere can be a potential issue.  At least with prop-driven support vees they’ve got a limit on speed.

[cray]

I think that cray is trying to say that it is theoretically possible to take in air, rip out the hydrogen from that for fusion power, and use the rest for propulsion mass.

Nope. Air would be used purely for reaction mass. Since you have abundant reaction mass in the atmosphere, you don't need ridiculously high specific impulse like fusion rockets in space, which means you don't need a lot of fuel. Such an "airbreathing" fusion engine should run for years on some kilograms of hydrogen.

[idea weenie]

Nope. Air would be used purely for reaction mass. Since you have abundant reaction mass in the atmosphere, you don't need ridiculously high specific impulse like fusion rockets in space, which means you don't need a lot of fuel. Such an "airbreathing" fusion engine should run for years on some kilograms of hydrogen.

Now I'm imagining a Dropship taking off, and the first few thousands of meters it is just sucking in local air, superheating it, and spitting it out as reaction mass.  At sufficient altitude you have MECO, but in Battletech it is 'Main Engine Change Over' as the Dropship is now shifting to using internal hydrogen as reaction mass.  Once you get far enough into space you get SECO, or '
Surface Engine Change Over', meaning the Dropship's engine nozzle has changed over to a form ideally suited for space efficiency rather than atmospheric.

Unless the equipment to use atmosphere as reaction mass would be heavier than just putting more hydrogen into the Dropship?

[DevianID]

Sucking in all that air would require separate engines all on its own.  Superheated hydrogen hitting an atmosphere would provide extra pressure to push against without needing separate engines for sucking in air, so you get some benefit just being in an atmosphere.  Its possible to put several turbines on dropships for the purpose of pushing more air, but I honestly dont know if that will do anything.  These dropship engines function at pretty high G in space without atmosphere reaction mass, so they are already pushing out lots of high energy something, to the point that any air used as reaction mass might only be there to slow down the fusion byproducts youd normally be spitting out in space to less radioactive levels on the ground.

[Burzmali]

Just don't do the math on exactly how that reaction mass is being used. If I recall, fighters aren't too bad, but dropships and up carry a ridiculously small amount of reaction mass for the length of time they are able to accelerate at 1G or higher.

[Daryk]

Correct!  Tactical thrust is doable with real (if extreme) physics... strategic thrust is a WHOLE other ball game!

[DevianID]

Yeah, you are spitting helium out at big %s of the speed of light with strategic thrust if I remember other peoples comments on the matter, and doing it with less energy then the hydrogen fusion provides on the big ships, so when that alpha radiation strikes something its gonna be a cascade of bad stuff if used on the ground.

Strategic thrust might be sucking in background particles though, since you are moving so fast after a day or so of 1g burn you move though a lot of space with particles to potentially use.  Some engine concepts IRL use hydrogen scoops, so while not explicitly called out, you do move through a lot of space and thus hydrogen when doing these strategic burns which potentially could help solve the hydrogen energy problem of strategic thrust using only onboard hydrogen as a fuel source.  Having a scoop system for deep space strategic hydrogen harvesting would potentially double as the scoop needed to suck up air for the in atmosphere burns.

[Burzmali]

Yeah, you are spitting helium out at big %s of the speed of light with strategic thrust if I remember other peoples comments on the matter, and doing it with less energy then the hydrogen fusion provides on the big ships, so when that alpha radiation strikes something its gonna be a cascade of bad stuff if used on the ground.

Strategic thrust might be sucking in background particles though, since you are moving so fast after a day or so of 1g burn you move though a lot of space with particles to potentially use.  Some engine concepts IRL use hydrogen scoops, so while not explicitly called out, you do move through a lot of space and thus hydrogen when doing these strategic burns which potentially could help solve the hydrogen energy problem of strategic thrust using only onboard hydrogen as a fuel source.  Having a scoop system for deep space strategic hydrogen harvesting would potentially double as the scoop needed to suck up air for the in atmosphere burns.

The math for exactly how wide that scoop would have to be to capture enough material to provide meaningful propellant mass makes that process a non-starter for something with a mass like a Dropship or Warship.

The best I've ever come up with is that BT ships must carry a large mass of an inert material that they inject into the reaction chamber and eject at high speed as propellant. It takes a while to heat up and only works for long distances making it useless in combat scenarios and its mass is just abstracted away in calculations.