Discussion: Heat expansion transit drives for ASFs?

[wellspring]

Small craft get a transit drive that lets them use dropship fuel economy rates in long-term flights.

What if we attempted to mount such a capability in an aerospace fighter? How would this affect the setting?

What do you think it should cost (tonnage/spaces/c-bills)?

Should it be mountable in omni-pods?

Interested in hearing everyone's thoughts.

[Prince of Darkness]

What if we attempted to mount such a capability in an aerospace fighter? How would this affect the setting?

What do you think it should cost (tonnage/spaces/c-bills)?

Should it be mountable in omni-pods?

Interested in hearing everyone's thoughts.

Mounting a heat expansion engine into aerofighters would probably take up far more space (maybe 1.5 to 2 times) and would weigh something along the same range.  C-bills wouldn't be too much more however, as my understanding of the expansion engine is that it's actually less sophisticated than an aerofighter drive.  Because of this, it cannot be mounted in omni pods and if made standard across all ships, would render ship sizes between 80-150 tons relatively obsolete.

[worktroll]

There's another issue in that ASF typically have life support rated in terms of hours, while DropShips can manage for weeks - sorry, not certain of exact figures. So there's not much point having a long-range drive with short-range breathing

In terms of effect on the setting, it probably wouldn't change a lot. At the moment, invading ASF thrust hard on launch, then brake for battle/re-entry, both at high G. ASFs with transit drives would thrust more slowly but for longer at first, and probably still brake using the combat drives.

By the time you put a transit drive, enhanced life support and more fuel into an ASF, you're really talking about a small craft. Maybe you'd see more combat small craft employed to patrol jump-points (pirate or standard), but by then you're reaching the point where it doesn't matter if the patrol is a Leopard CV with 6 ASF, or 6-8 long-range smallcraft, I suspect.

W.

[evilauthor]

In tactical terms, Dropship drives just don't compete with Fighter drives until around the 400 rating, and that's only for the very heaviest fighters. To illustrate:

100 ton fighter
Standard Engine requires 52.5 tons to get 6/9 movement profile.
Dropship engine requires only 39 tons to do the same.
But to get a 5/9 movement profile, the Standard Engine requires only 19 tons while a Dropship engine would cost 32.5 tons. That's a huge weight savings that goes into guns and armor.

At lighter tonnages, fighters get better and better acceleration profiles for less and less tonnage spent on engine.
At 65 tons with a 5/8 movement profile, the Lucifer only spends 8 tons on an engine. A Dropship engine providing the same acceleration would be just over 21 tons.
A Seydlitz at 20 tons and 11/16 movement spends 7 tons on engine. A Dropship engine would cost 14.3 tons (14.5 when rounded off).

And those values are for standard engines. With XL Engines, the 400 rated engine for the 100 ton fighter only weighs 26.5 tons, which is much less than the 39 tons a Dropship engine requires.

In short, while I see no technical reason a fighter can't mount a Dropship engine, doing so would result in a fighter that's much less capable of performing its primary job: fighting. For the extra long range endurance, you'd be giving up weapons, armor, acceleration, or all three. And that's BEFORE you do something like replace the 3 ton cockpit with a 7 ton crew facilities just so the pilot could survive the trip. What good is making a fighter go for days without resupply if it can't fight effectively when it gets to its destination?

[Tai Dai Cultist]

Another way to look at it is small craft built for combat/system patrol ARE asf's with transit drives.

They suffer the initiative penalties because the dropship-rated drives used by small craft are like truck gears.  A pure 'sports-car' engine'd ASF has more raw acceleration at the same TW thrust band than does a dropship/small craft.

[wellspring]

There's another issue in that ASF typically have life support rated in terms of hours, while DropShips can manage for weeks - sorry, not certain of exact figures. So there's not much point having a long-range drive with short-range breathing

From the TechManual errata: 96 hours endurance for a fighter. Also from TM errata: "External Consumables Pods: Fighters may add life support endurance in lieu of bombs, with each pod providing another 96 hours endurance per fighter occupant. Because of cockpit space and amenities limits, a fighter may only carry 1 pod per 25 tons (round up) of fighter mass, even though the pods only occupy 1 hard point each."

So a fighter gets several days by default, and one hardpoint increases that to more than a week of life support. Granted, it's cramped, barely livable, and pretty rank after a few days, but so is a commercial flight in coach and I do that all the time.

For small craft and dropships, it's as long as the consumables tonnage holds out, at a level of consumption (and comfort) commensurate with the quarters you use.

In terms of effect on the setting, it probably wouldn't change a lot. At the moment, invading ASF thrust hard on launch, then brake for battle/re-entry, both at high G. ASFs with transit drives would thrust more slowly but for longer at first, and probably still brake using the combat drives.

By the time you put a transit drive, enhanced life support and more fuel into an ASF, you're really talking about a small craft. Maybe you'd see more combat small craft employed to patrol jump-points (pirate or standard), but by then you're reaching the point where it doesn't matter if the patrol is a Leopard CV with 6 ASF, or 6-8 long-range smallcraft, I suspect.

That's the trick. What I'm looking for is a one-man craft that can handle more or less like an ASF but still handle long voyages. So maybe you do it by adding a "fighter control system" add-on for a small craft which reduces its crew requirements to one.

Or maybe you add a second engine (yes, I know it's not permitted, but this is the fan rules forum, right?) Evilauthor and TDC made some really good points about the engine types, responsiveness, and tonnage. What if I took a 100 ton fighter, say a Kirghiz since I'm a clan player. A 2/3 small craft drive for it would run me 12.5 tons. That's a lot, but not insane, and thanks to the mathematics of transit drives, a 1g torch drive isn't all that much worse than a 3g+ drive.

So I get my normal "sports car" 5/8 acceleration curve in combat. Or I switch to the heat-expansion drive for an out-of-combat thrust profile of 2/3. My five tons of stores (the max I could carry and still fly at 1/2), I could have life support for a four day patrol and two burn-days of fuel without even touching my combat load.

And, yeah, the tactical value of such an arrangement is dubious. To me, it's more important for strategic/rpg use rather than on-board utility.

[evilauthor]

So I get my normal "sports car" 5/8 acceleration curve in combat. Or I switch to the heat-expansion drive for an out-of-combat thrust profile of 2/3. My five tons of stores (the max I could carry and still fly at 1/2), I could have life support for a four day patrol and two burn-days of fuel without even touching my combat load.

Exactly what are you patrolling that you need 4 days' worth of flight endurance for? The three plus hours worth of flight time standard fighters get is enough for anything around a typical planet while 4 days won't even get you a one way trip to a standard jump point except around the smallest, dimmest stars.

And if you by chance do have a system where you need a standing space patrol to operate for days at a time, you're likely better off with a carrier dropship and standard fighters. That at least lets the pilots get up and stretch their legs instead of confining them to cramped cockpits.

[worktroll]

Interesting point - Gemini astronauts spent 3+ days in their capsules, basically with as much room as two people sitting in the front seats of a medium sized car. They suffered no major ill effects as a result. They were however on low-residue diets, and experienced significant fatigue when performing EVA.

The effects of weightlessness are known, but not in the context of suddenly having to pull 6G accellerations. Plus, if you had the fuel to maintain low levels of thrust, that brings its own problems - pressure sores.

If you're looking at long flights, then a small craft - where you can get out of the gee couch, wash, and eliminate - would seem to be highly preferred.

[Red Pins]

...Interesting.  But I have a few questions I haven't been able find answers for so far.

What, exactly, makes up a 'heat expansion drive'?  What are the mass/space requirements?
Are transit drives incompatible with the canon explaination for 'Overthrust'?

[evilauthor]

If you're looking at long flights, then a small craft - where you can get out of the gee couch, wash, and eliminate - would seem to be highly preferred.

Still begs the question of what exactly you need such long flights for and why you can't use a Dropship to deliver your fighters.

...Interesting.  But I have a few questions I haven't been able find answers for so far.

What, exactly, makes up a 'heat expansion drive'?  What are the mass/space requirements?
Are transit drives incompatible with the canon explaination for 'Overthrust'?

Dropship type drives have a second mode where they get ridiculously fuel efficient. The 5 tons of fuel that will last for ~3 hours of constant acceleration on a fighter will last a Dropship in strategic mode for DAYS.

This comes at a cost of course. A Dropship transit drive will cost 13% of the ship's mass for every 1 gravity the Dropship can cruise at. And to get off a planet, a Dropship needs a minimum 1.5 gravities of "cruising acceleration", or 19.5% of their mass. But 1.5 gravities is pretty paltry acceleration for combat operations compared to the kind of accel fighters can pull.

Fighter engines don't get strategic mode, which allows them to take smaller engines for a given acceleration profile, and thus devote more tonnage to things like weapons and armor.

Now as to what mechanical bits are different in Dropship engines than fighter engines? I don't think it's ever been specified in any sourcebook aside from the mass and performance differences.

[wellspring]

...Interesting.  But I have a few questions I haven't been able find answers for so far.

What, exactly, makes up a 'heat expansion drive'?  What are the mass/space requirements?
Are transit drives incompatible with the canon explaination for 'Overthrust'?

The heat expansion drive is a way for the fusion drive to achieve extremely high specific impulses, but at the cost of a reaction that takes a very long time to start or stop, so it's only usable for long-term transit. See the Aerospace chapter in TechManual for a detailed in-game explanation. From a metagame perspective, it's a mechanic that allows you to have a fuel constraint in tactical combat while still having week- or month-long fuel endurance for strategic movement purposes.

The mass/space requirements are zero for small craft and dropships (they're built in) and they're unavailable for fighters. My original question was asking if we were to allow this as an option for fighters what it would cost, and the consensus seems to be that it shouldn't be allowed at all.

[evilauthor]

The mass/space requirements are zero for small craft and dropships (they're built in) and they're unavailable for fighters. My original question was asking if we were to allow this as an option for fighters what it would cost, and the consensus seems to be that it shouldn't be allowed at all.

Probably not.

Hmm... 

But if it were allowed, go with a dual engine system where you'd be able to use both simultaneously.  The Dropship engine is only used for long cruises, so only provides AT MOST a 2 MP cruise speed for 13% of the fighter's mass. The fighter engine is only used in combat and used to augment the Dropship Engine; the cruise MP it provides adds to the Dropship engines cruise MP.

So on a 100 ton fighter, a 13 ton Dropship engine provides 2/3 movement. For an extra 8.5 tons, it also carries a 200 rated regular fighter engine that provides an additional 4/6 movement. End result is a 100 ton fighter capable of cruising at a steady 1G in Strategic Mode while also capable of 6/9 movement in Tactical Mode... all for 21.5 tons, which is 5 tons LESS than a 400 XL Engine that would have been required for the same movement profile using the fighter engine alone.

You can see why this could be a problem.

[wellspring]

Probably not.

Hmm... 

But if it were allowed, go with a dual engine system where you'd be able to use both simultaneously.  The Dropship engine is only used for long cruises, so only provides AT MOST a 2 MP cruise speed for 13% of the fighter's mass. The fighter engine is only used in combat and used to augment the Dropship Engine; the cruise MP it provides adds to the Dropship engines cruise MP.

So on a 100 ton fighter, a 13 ton Dropship engine provides 2/3 movement. For an extra 8.5 tons, it also carries a 200 rated regular fighter engine that provides an additional 4/6 movement. End result is a 100 ton fighter capable of cruising at a steady 1G in Strategic Mode while also capable of 6/9 movement in Tactical Mode... all for 21.5 tons, which is 5 tons LESS than a 400 XL Engine that would have been required for the same movement profile using the fighter engine alone.

You can see why this could be a problem.

Except that it's already canon that heat-expansion mode cannot be used at all in tactical combat, even by ships that mount the system.

[evilauthor]

Except that it's already canon that heat-expansion mode cannot be used at all in tactical combat, even by ships that mount the system.

Well sure. But the canon Dropship drive is ALREADY capable of dual use; it can be used in BOTH the fuel economizing strategic mode and the fuel guzzling tactical mode. So if you use the fighter engine in conjunction with a Dropship engine, then by default, you are operating in tactical mode and so would that Dropship engine.

Meanwhile, the fighter engine is turned off and dead weight when the Dropship Engine is in Strategic mode. It's not like you need more than a 1G accel in strategic mode anyway.

[wellspring]

Well sure. But the canon Dropship drive is ALREADY capable of dual use; it can be used in BOTH the fuel economizing strategic mode and the fuel guzzling tactical mode. So if you use the fighter engine in conjunction with a Dropship engine, then by default, you are operating in tactical mode and so would that Dropship engine.

Meanwhile, the fighter engine is turned off and dead weight when the Dropship Engine is in Strategic mode. It's not like you need more than a 1G accel in strategic mode anyway.

Yeah. What I'm looking for is a mechanism to give small-craft level fuel economy to an ASF fighter, without affecting its ASF-ness. That is, a pure change to the strategic rather than tactical performance of the craft. Is the heat-expansion system so intrinsic to the design that the "sports car" engine of an ASF can't be combined with it? It seems to me that the worst-case scenario is that the added mass to accomplish the mods is equivalent to adding a 1-g small craft drive, without the tactical thrust implications.

[evilauthor]

Yeah. What I'm looking for is a mechanism to give small-craft level fuel economy to an ASF fighter, without affecting its ASF-ness. That is, a pure change to the strategic rather than tactical performance of the craft. Is the heat-expansion system so intrinsic to the design that the "sports car" engine of an ASF can't be combined with it? It seems to me that the worst-case scenario is that the added mass to accomplish the mods is equivalent to adding a 1-g small craft drive, without the tactical thrust implications.

The thing is, a Small Craft Drive IS capable of tactical mode. If it couldn't, you'd think you'd be able to get tonnage breaks.

Now if I were running the system in the interest of game balance, I wouldn't allow the combining of drive types like outlined. And I can think of any number of in-universe reasons for why not. After all, if you could combine a Dropship Engine with a Fighter engine, why not combine two 200 rated engines on the same fighter instead of using a single 400 rated engine and save almost 30 tons? Whatever the answer, I can guarantee you it applies to combining fighter and dropship engines as well.

[wellspring]

The thing is, a Small Craft Drive IS capable of tactical mode. If it couldn't, you'd think you'd be able to get tonnage breaks.

Now if I were running the system in the interest of game balance, I wouldn't allow the combining of drive types like outlined. And I can think of any number of in-universe reasons for why not. After all, if you could combine a Dropship Engine with a Fighter engine, why not combine two 200 rated engines on the same fighter instead of using a single 400 rated engine and save almost 30 tons? Whatever the answer, I can guarantee you it applies to combining fighter and dropship engines as well.

I think you're missing what I'm saying here.

All I'm talking about is adding the small craft strategic thrust capability to an ASF. Not an actual engine, just the fuel-expansion system. One engine. With ASF performance. And the option of engaging strategic thrust mode. With, if you like extra limitations, a 1g limit more to preserve the distinction between small craft and fighters than anything else.

The upshot being no change in tactical performance. Whatever fluff you like to explain it.

What I'm saying is that the tonnage of the resulting engine would likely not exceed the total tonnage of a 1g small craft engine and a conventional fighter engine. Surely some components are shared, but the worst-case assumption is that none are. Purely looking at the math, not actually adding two engines, just using that as an analogy.