How to cool ships, or, Why is my fuel tank so friggin huge?

[Klaus]

Hey all fascinating stuff!

I've got some related queries and speculations about drives and ship design.

I've been trying to work out exactly how much ships weigh. We design vessels using displacement tons, and this displacement is of L-hyd, therefore 14 cubic metres per ton. However, the actual components will be much denser than L-hyd; so what would be average ship density? water at 1 bar? That would mean vessels mass 14 times their displacement.

The design tools are tied to this 14m3/dton ratio: so a stateroom is 4.5x3m and there's 1.5x3m in communal area. If you change the fuel to H2O, density changes too, unless you increase the mass requirements by 14.

There's also the fact that if the M-drive is a fusion reaction drive, then its drive plume will be very large. A 100k dTon (so massing much more than 100ktons) 6G battleship's drive plume will be absolutely enormous, to the point where a spinal mount may be superfluous.

Now I don't know the physics or engineering well (no more than I gleaned from Jerry Pournelle's 'A Step Farther Out') but IMTU I work on the assumption the M-drive is an ion engine in concert with MRT, using heavy metal atoms as the source of the ion stream. This would negate the need for lots of reaction mass, and tie M-drive into the powerplant. Query: how often would the hvy metal need to be replenished. Also, does a PGMP require 'refuelling' for its plasma stream?

As far as the J-drive is concerned, the THB states somewhere that to open the jump bubble requires a crash burn of the fuel in a very inefficient reaction to achieve the vast energy requirement, and this is why all this fuel is required. Perhaps L-hyd is particularly useful in jump operations, perhaps some special quality - after all it is the primordial element. Peter F Hamilton uses He3 for his fusion reactors in his Confederation trilogy, and I assume he's got his reasons, so there has to be a particular reason for L-Hyd.

Now I get to the point!

I've been thinking about batteries to store extra power for either emergency operations or for limited use of lasers on low power vessels like the Far Trader. When discussing black globe generators the THB mentions capacitors, 1 ton holding up to 36 EP, but does not specify whether this can be used to power other systems. I've postualted that these capacitors are rapid charge/discharge but cannot stay charged for a significant length of time: otherwise ships could precharge the jump capacitors in port and therefore need less fuel. The capacitors act as a sort of buffer between the crash reaction and the J-drive. The max time a capacitor could hold this energy is the j-drive in hours (this should be enough for a ship combat to finish one way or the other...).
There are some interesting numbers this throws up. A far trader has 3 tons of capacitors, therefore can hold 108 EP. Its PP can produce 4 EP, therefore supposedly 27 rounds to power up its j-drive. If this was all that was required, it wouldn't need 40 dtons of L-hyd. This energy stored may be required just to contain the super-reaction that uses the fuel. Maybe the L-hyd is pumped into the anti-matter universe to be annihilated, and this is actually the energy required to get into jump.

This is where I start to get confused in relation to batteries. A capacitor is 1 dton, and holds 36EP. 1 dton is 14000 vlitres. A TL9 basic vehicle battery of 14000vl would hold 140000EP at the vehicle scale. So what is the relationship between vehicle EP and spaceship EP?

Now these are for the jump capacitors, which as I said before are high charge/discharge and cannot store energy for proper lengths of time. I'd assume something more like 1dton of long-term battery gives 10-20EP at up to 2-4EP/round, though this power is unavailiable for M or J drive operations (but maybe for agility).

I think this might become a problem (I know rpg rules can't and shouldn't have to stand up to scrutiny like physical laws). If you replaced that far traders main L-hyd fuel tank with TL11 batteries you'd get 14 million vehicle EP's stored. Is this enough for a 2 parsec jump?

As a final note, if we assume efficient conversion from heat into electricity or whatever, then batteries or capacitors would also make perfect heatsinks.

hope that was worth tuppence.

 

[aramis]

MDrives may or may not be a fusion torch... THAT factor is VERY MUCH edition dependant. (MT and T4 typical craft use gravitic thrusters. CT doesn't actually define in bk2 nor 2d ed Bk5, but several places imply something not too far off from HEPlaR or Fusion Torch drive systems... TNE and T4 both have HEPlaR. T20 implies gravitic thrusters ala MT, but is not explicit. I don't recall GT's version.)

In general, though, most FF&S 1/2 and MT designs work out to between 10 and 20 Mg per Td (Mg = MegaGram = Metric Tons), with commercial designs coming close to 10, and some warcraft reaching 60+ Mg per 14kl Td.

Also note: Steel is typically around 7 Mg (tons mass) per cubic meter. (Specific Gravity raging from 6.0-8.0 by specific iron dopant mixture.) So, given that many MT and FF&S systems have SG's AS A SYSTEM in the 2-4 range, that is a LOT of open space, both in those systems AND in the ships overall.

 

[Klaus]

I've been musing on this further...

What we know about starship design is that the jump drive uses L-hyd and ships are classified by their displacement in L-hyd. It seems what is important for jump is volume rather than mass. We don't bother calculating actual mass at all. And this volume is measured in tons of L-hyd. Now we know fusion plants can run on a variety of fuels (at least in our SF it can), so why would jump displacement be tied into L-hyd if an alternative (and perhaps more compact) fuel could be found? So this tends to support the idea that the L-hyd is used directly by the J-drive in some way. I think its been established that working as a coolant won't be the primary function. I was thinking that H is the primordial element, just a proton and an electron. Maybe its this property the J-drive uses.

Maybe it strips away the electrons and creates a forcefield out of protons to enter and transit jump-space, or vice versa. I can imagine a static charge building on the hull as the j-drive is charged, with sparks crackling around the control surfaces. Maybe scientists say the wierd feeling some feel in jump space is just a reaction to the static charge (but no one can ever know for sure). When a ship emerges from jump space it zaps its charge back into the wherever, with attendant fireworks. (That's another thing we don't talk about - spacecraft would build up massive charges in orbit and so on; after all, aircraft and the space shuttle pick up a small charge differential don't they?)

This could be why unrefined fuel increases the chance of a misjump. If atoms other than H get into the field they may cause drop outs or fluctuations, too many neutrons in the mix etc.

We could possibly go as far to suggest that the power plant may well use He3 or another alternative fusion fuel. At least the fact that power plant fuel efficiency does increase by tech level (twice as good at TL15), but j-drive fuel efficiency doesn't, implies the L-hyd is used for a different function than mere energy.

There, run out of waffle now.

 

[aramis]

The hydrogen based fusion is one of the highest energy fusion reactions, and requires some of the lowest thermokinetic regimes of known fusion reactions.

It could just be that the JDrive needs such energy that ALL that fuel is burned into He and moved through fast enough to minimize the He cycle and it's lesser energy per reaction.

(I once read that up to the Oxygen+Oxygen fusion, fusion produces energy greater than the energy needed to trigger the fusion, but past that, the energy of fusion is less than that needed to cause the atoms to fuse.)

My original (read as during high-school) view of powerplants was a gravitic "Bucket" fusion reactor; a massive bucket, lined by a massive magnetic walls, to maintain a bottle, into which fuel was trickled; a vent at the bottom let waste C, O, and Li out. I assumed that the Jdrive instead energized a jump coil by sending just-fused plasma through it at humongous rates, and that the JDrive was more a weapon style particle accelerator. By the way, said buckets were assumed to be 100+G localized artificial gravity, inducing fusion by simple pressure. (It also meant cold starts required an external power source!)

I later adopted the torrid style that my later deckplans used for PP's and JDrives, but I still assumed (until SSOM) that JDrives put the "waste" product to the jump coils... which used lanthanum and or irridium to transform the thermal energy into jump fields when coupled with the power from the zucchai crystals to alter and shape the fields.

 

[TheEngineer]

Hi !

Is there any statement in the official rules at which time what portions of jump fuel are consumed ?

Regards,

Mert

 

[Klaus]

There's some vague talk of the crash reaction requiring the capacitors mentioned in the THB, and I remember some stuff in JTAS that talks of the remainder of the fuel sustaining the jump, as people have already said, but no figures or definitions.

 

[far-trader]

Originally posted by TheEngineer:
Hi !

Is there any statement in the official rules at which time what portions of jump fuel are consumed ?

Regards,

Mert

T20 does some on pg. 354. I seem to recall something similar from earlier. Might have been T4 or a mtu rule. The basic break or 80% to go and 20% to sustain did work out right with the drop tanks for the CT Gazelle class I translated to T20 iirc, maybe.

 

[TheEngineer]

Thanks Dan.
Pretty good hint with the drop-tank usage.
AFAIK at least in MT is no specification, but as drop/disposable tanks are mentioned for usage I might assume a 100 % to go usage....

M.V.

 

[MR TEK]

Just to stick my nose in...

Reminding use about jump tanks reminds me that jump fuel is STRICTLY determined by volume, and mass is not even vaguely considered.

The hole point of drop tanks is you can carry more fuel and discard the tanks before jump so you move the volume of the ship without the added volume of the tanks. Same applies to fule on board, or cargo. Full holds or empty, full fuel tanks or barly what you need for jump, you always consum 10% of volume per hex jumped. (setting aside misjumps.)

It still does not answer how much is used to open the hole and how much is used in flight, but the fact that the fuel in the drop tanks converts directly to form the hole, and the fuel to maintain the field comes from the regular tanks.

I only have the t20 book and the ffe Classic books reprint. I gave everything else away years ago.

If I recall the origional sorce for drop tanks was one of the adventure books and an early edition of the JTAS. I seem to recall a limit of how much fuel could come from the drop tanks, and how much from the main tanks.

Find that limit, and you should have an answer. What is allowed to come from the drop tanks would be used to form the bubble, and what comes from the ship tanks would be used to sustain the bubble through flight.

(back to the origional question, if I recall my physics, an idea energy conversion can generate 50% useful work, and 50% waste heat, light, whatever. That is basic entropy at work. I hope I have that right, I KNOW that is the formula in electronics to transfer power from one section to the next. But I seem to recall that it applies to mechanical systems as well.)

An elctrical power supply can AT BEST in a perfect match transfer 50% of the energy it produces to the load, and 50% is waste heat. (I cannot recall now, but switching power supplys may eak out a higher effiency.)

I am way to rusty on all of this to be 100% accurate, but I am pretty sure these are right.

Again, find a pyisist that knows the theoretical limits for conversion effecebcies, and apply those to the power generated to keep the ship warm, and lights on, and the other systems hot that need to stay running, and the jump drive, and the rainder is the bulk of heat you must dissapate.

One more bit of handwaving someone mentioned above but did not follow through. Entropy is one of THE most basic physical properties. It might be possable that jump space alters the laws of thermodynamics, but if those laws change, all normal space mechanics and physics would break, including biologic processes. If any part of the ship where exposed to raw jumpspace, any active processes would destroy themseleves, likly either in a massive rupture as all reactions out run the physicallimits of the materials, or else all energy storage goes to zero, including the thermal energy in in atoms. This would drive the matirial o lose cohesion at the atomic and subatomic levels, and it instantly decays to whatever particals make up j-space. (probably nothing that normal space physics can even determine.)

So, while the interface between the jump bubble and normal space around the ship might give a little room for heat transfer, the interior of the bubble MUST not change physical laws in ANY manner whatsoever or what leaves jump space would be unusable. In fact, I would suggest that the bubble forms an absolute closed system universe, and Nothing crosses the interface of the bubble, except for the transfer of the bubble, itself. And likly, the purpose of the bubble is not to affect j-space at all, but simply bring the physical laws of normal space along for the ride, so what returns to normal space is the same as what went in.

 

[MR TEK]

The first post was getting long, so I am continuing.

This rational can explain why there is a limit to how small a vessle can enter j-space. there might be a critical limit to the formation volume of the hole, or the size of the bubble must be over a certain volume to hold the physical laws intact. as has been noted earier, there will be some pretty masive fireworks at the exit point as the waste energy that accumulated inside to bubble decays when returned to normal space.

It is hard to imagine any useful work that could be done with that waste energy, but, there is a use a military planer would rub his hands in glee over. If you could build an unmanned device, and transit it back to normal space, inside the volume of an existing object, you would probably get as near to the theoretical limit of mass to energy conversion as possable. The process would be so compleate, aand so catasrophic that I cannot fathom any way to harvest that energy for any useful purpose, but if it can be controlled accuatly enough, it would make one HELL of a weopon, probale quite litteraly, as the kind of conversion rate is going to be almost pure, raw chaos, ESPECIALLY as j-space would likely cross the threshold and interact with the fury of the normal space reaction. ( see above for conjectures about normal space matter an physics contacting j-space.

Depending on inclenation, this could either be the ultimate reach of weapons technology, or it could rip the fabric of normal space in a non repairable way. (I am sure that some misjump somewhere has landed within an other mass, and the universe still exists at the time we play, so it must not rip the fabric of space compleatly. (local effects may be pretty catasrophic though.)

 

[TheDS]

After having read SOM, I have since assumed that hydrogen is simply the best "lubricant" for passing a ship through jumpspace. You can use larger atoms, but this unrefined fuel does a poorer job of allowing the ship to slide through the wormhole. Simply, there's just way too much fuel to even burn by combustion, much less by fusion; it must be used in some other way. This has then inspired the idea that it is a cooling reserve, and upon learning that H2 is not very good at that, by going to water instead.

Yes, the ship will mass more, and thereby should theoretically have a lower thrust, but I don't think the difference will be so huge as to be unignoreable in game terms, and even if it isn't, I'd prefer something simple, rational, and believeable to something that is potentially none of the above. In game terms, water has at least as many advantages as hydrogen, and possibly more, though they are not exactly the same advantages.

I find the table on the previous page a bit startling. With only about 5 square meters of radiator, you can cool a 1 megawatt reactor. For a 1 GW reactor, we're talking a radiator that's only 100 meters by 50 meters; considering the size of the ship that would have taht kind of reactor, that's not too bad. You could possibly coat the hull with it and save yoruself extending the radiator, though this would be workable just with civilian ships.

 

[Klaus]

But that's way off canon. A displacement ton of H2O (at 1 atmosphere) is 1 cubic metre, not 14. Why would ship design be wholly predicated on the dTon relationship to L-Hyd if another fuel could be used. The only explanation is that jump and L-Hyd are intextricably linked. We don't really need to come up with 'realistic' explanations for stuff in a sci-fi game, and anyway, alot of what we know about physics now may be wrong or misleading.

I don't know if any UK peeps saw "What We Still Don't Know", on C4 on Sunday, by Martin Rees, Astronomer Royal. This show was about the atom, or matter, and essentially the nature of reality. The last half hour discussed the discovery of dark matter and dark energy, and how we could not exist without these things, but also how neither can be detected or measured, apart from the fact that dark matter makes up 85% of the mass of the universe.

The point is, there's more we don't know than what we do know, and that almost certainly, if jump-drive did actually exist, it would not fit in with our current, 21st century cosmology - it would by its very existence create a new cosmology.

Now, I'm going to contradict myself a bit. I was looking through what canon actually does have to say about jump. 168 or so hours in apparent duration, 1-6 parsecs in distance, directly related to the 10-60% of L-hyd displacement used. Then the THB says the bulk of the fuel is used getting into jumpspace and the residue used to maintain the jumpfield. Having slept on this I started to think the process resembled artillery.

Take a howitzer and point it up 70 degrees or so. Use a type 1 charge and the projectile goes up and lands, lets say 10m away. Use a type 6 charge and it flies up alot higher, but lands say 60m away. The time it takes for the type 1 projectile to land is similar to the time it takes the type 6 charge to land. OK, the analogy isn't perfect, but there is a resemblance. If the projectile required some energy field to exist in the transfer medium, it works even better.

So we could say ships are 'shot' through jumpspace like a projectile, retaining enough L-hyd to maintain its field, until it 'falls' back to the real universe 'ground'. This would explain why the bulk of the fuel is used to get into jump, and why no matter how far you travel the jump always lasts roughly the same time (misjumps not withstanding).

I don't see cooling as a problem. a) its not mentioned as a significant factor in canon, and b) a radiator could actually be very small. Its not volume but surface area thats important. Radiators could have 'fractal' surfaces, much like coastlines, which could give them an enormously larger surface area than their volume would belie. They don't have to be flat panels.

 

[mike wightman]

Why would the ship's displacement in liquid hydrogen be important if it weren't for jump fuel requirement?

Simple answer, a ship in jump space must displace it's own volume of liquid hydrogen because that's the "density" of jump space.

Any more or less and misjumps occur.

Put another way, a ship is only "neutrally buoyant" in jump space if its volume is equal to its displacement in liquid hydrogen.

Why?

I have no idea...

 

[MR TEK]

I stillm think it is more physical than that.

I still think open a hole, and keeping the hole open a set size has everything with displacement.

To answer Klause, No there is NO requirement that ANYTHING be ratonalized in a Scifi game. It is just that gearheads want to make up plasable explinations.

Look at the people that have spent years of their lives trying to understand the physics of Star Trek. They seem to enjoy their activities, but to an out sider I can not understand all that work.

At least in the game, A rational explanation give the ability to make rules that are consistant.

 

[TheEngineer]

Hi !

TeK, just a note.
Conversion efficiency could be higher than 50%.
Some real life stats:
- motor, electrical>mechanical c. , 20-95%
- generator, mechanical>electrical, 90-95%
- fuel cell, chemical>electrical, 50-80%
Refering to thermodynamics, e.g. the theoretical efficiency of a Carnot-process is 1-(sink temp/source temp) and thus could get quite efficent in a space environment.

Anyway I really like the descriptions of jump bubbles purpose during jump and TeKs particle decay at the jump space borderline inspired me, too. So heres just another theory

...

What if the jump field is able able to control the flow of matter/energy from the normal space area to jump space. The j-space borderline reaction could create the momentum, which keeps the field itself in shape and also moves/directs the ship in jump-space environment. In order to provide a nearly perfect matter distribution in the jump bubble one atomic hydrogen is the best (as it fellows kinetical gas theory and distributes pretty well).
That makes the starting moment (leap) of the jump procedure a criticial thing, as the jump field has to be created with great energetic afford, additionally at first without the stablilising effect of the hydrogen bubble. Even in the initialisation phase a large amount of hydrogen reacts at the field borderline until the correct jump space level is reached.
After the stable phase is reached, actual travel starts. The field is altered in the sensible way the navigation computers calculated it, allowing the ship to fellow its navigation path in j-space. Here hydrogen usage is fairly low and just used to keep j-level and performing navigation.
For jump exit the borderline reaction capability is decreased, both by shutting down flow capability and reduced hydrogen bubble concentration. The rest of the bubble hydrogen is needed for preventing a spontanous collapse of the field. Finally the procedure causes the ship to exit j-space, surrounded by a faint hydrogen cloud.

Well, thats just another thought, but it somehow fits to my assumptions, that the jump fuel is "consumed" (maybe fits better to an energy requirement for a parsec long travel, too) and that jumps can be actively navigated (they can be "curved").

I will try to visualize that in the "speculative trader" topic

Regards,

Mert

 

[MR TEK]

Cool! I like it.

Add that rather then being navogated in flight, it is preset in the formation of the initial hole. I normal space many elementary track folow other than straigh paths that are driven not by normal physics but by the nature of their creation.

 

[TheEngineer]

Agreed

Path is predefined by initial jump field patterns.
I do not really understand the last sentence, but I guess its ok

 

[aramis]

I have always assumed that ALL the LHyd is used in a very inefficient rapid fusion cycle; the jump field is maintained by either the coils or the grids for the rest of the week until it drops below a specific energy level, whereupon it becomes inert WRT J-space, the field collapses, and out one pops.

Since several sources indicate J-space trajectories are parabolas, this fits nicely with the drive Kicks you in with final velocity. It also explains why courses can't be changed: JDrives kick off N-space, not J-space, so once no longer in N-space, they can't kick J-space, so they continue to follow the predetermined parabola.

 

[TheDS]

The problem with assuming it's an inefficient reaction is that it would be on the order of 0.0000001, whereas a regular reactor can get about 0.50 (or 50%).

That's worse than chemical reactions. (Not responsible for inaccuracies of above figures, but I did the math once.) Hence, my repeated claim that even burning the hydrogen wouldn't be that inefficient. It would take like several days to burn it in a controlled manner, or years to fuse it, given the typical size for a ship's reactor.

As to the concern about displacement: Well, once the doublespeak Sigg presented wears off,... Bear with me a moment. If refueling from gas giants still mattered, I'd say it might be an important factor in the amount of bow-shock a ship might suffer when in its atmosphere. Since displacements are usually important only when you're talking about something you want to float in, displacement of hydrogen presumes making a ship to float in it, rather than being fueled by it.

But a ship will not float in a GG atmosphere (anymore than a N2O2 atmosphere), and may not even float on an ocean.

In real water ships as well as real spacecraft, mass is the important feature. Traveller's odd use of volume is because of Jump drive. The amount of energy required for jump is directly proportionate to the volume of a craft.

One COULD argue that the energy requirements have discreet steps (quanta), that the minimum value is 100 units, and that the energy required is always in whole units, and these are simply called displacement tons, for lack of a better term. By coincidence, they are very close to the value of a hydrogen displacement ton, and that may be where the name came from. Does this tie that up for you?

 

[TheEngineer]

Hi !

Originally posted by Aramis:
Since several sources indicate J-space trajectories are parabolas, this fits nicely with the drive Kicks you in with final velocity. It also explains why courses can't be changed: JDrives kick off N-space, not J-space, so once no longer in N-space, they can't kick J-space, so they continue to follow the predetermined parabola.

Just to be curious...
Where does this parabola thing come from ?

Regrads,

Mert