How do jump drives really work?

[rancke]

So?

Both SOM and FFS1 specifically mention the use hydrogen as coolant in jump drives.

So? That doesn't make sense either.

If you don't use it as coolant ( and assume that game's use of 'only' hydrogen as fuel is merely a gameplay abstraction ) then you have to explain how fusing so much hydrogen gives up so little power.

It's a knotty problem all right.

It covers the bases with the least number of changes, allows for some interesting ships and it makes sense.

It does not make sense, because if you can use hydrogen for coolant, there's a strong economic incentive to use something that has the same effect while taking up less volume instead and no plausible reason not to use it.


Hans

 

[Dragoner]

Liquid hydrogen makes sense as a coolant 1) it's already super cooled 2) the cooling system is open.

Water isn't feasible, nor oxygen. Water would need a closed system and an anti-corrosive, antifreeze substnce added. Oxygen is ridiculously corrosive.

 

[rancke]

Liquid hydrogen makes sense as a coolant 1) it's already super cooled 2) the cooling system is open.

Water isn't feasible, nor oxygen. Water would need a closed system and an anti-corrosive, antifreeze substnce added. Oxygen is ridiculously corrosive.

I'm no expert on the subject, but I've been told otherwise and that the calculations included the temperature of liquid hydrogen.

Still, if you are an expert, I may have to revise my opinion. Are you?


Hans

 

[Dragoner]

An expert on Jump Drives?

Why yes, yes I am....

 

[rancke]

An expert on Jump Drives?

An expert on the use of various substances as coolants.

I think the substance of choice is liquid nitrogen rather than liquid oxygen, but as I said, I'm no expert. I've just been told that liquid hydrogen is inferior for that purpose by people who knew more about it than I do.


Hans

 

[Dragoner]

Lets see, the min temp of LH2 is −252.87 C, that is pretty cold. Since it is your fuel source already and you have that around in abundance, it would mean you would only have to vent the fuel tanks into a cooling system around the drive and not have and entirely separate system, the drive could be just in a sealed box for example with a lower inlet and a high outlet. Water, would not take off as much heat, even though it is denser, it's ambient temp is higher and would have to be in a separate sealed system, plus it would need additives. LOX is far too errosive, it's straight out. LN2 is min temp (−196 C less cold, but also will freeze solid at −210 C, so not only would you need a separate system, but it would have to be insulated from your LH2 and you would need extra tankage, etc. and have extra mechanicals as per condensers and such. I would go with the LH2 myself, while maybe not the most efficient, it seems easiest considering you already have a large supply, which would make it economical.

 

[rancke]

Lets see, the min temp of LH2 is −252.87 C, that is pretty cold [...]

So, not an expert, then?

It's not the temperature itself that's interesting, it's the coolant effect per volume. This is related to the number and weight of molecules per volume, and hydrogen is a lot less efficient than many other substances. Or so I've been told.

Maybe someone who actually knows about this subject will do us a favor and chip in and spare us further 'appeal to authority' vs. 'guesswork' debate?


Hans

 

[Dragoner]

Lets see, I am a mechanical engineer, I can build a race engine blindfolded. What are your credentials again? I understand how cooling systems work, mean temp is important because that is what the coolant will try to bring the ambient temp too. If you are such the expert, care to elucidate on what the actual materials are that the drive itself is made of? That will have a big effect on cooling rate as well. As well as to test the measured rate as to not damage the materials.

 

[rancke]

Lets see, I am a mechanical engineer, I can build a race engine blindfolded. What are your credentials again? I understand how cooling systems work, mean temp is important because that is what the coolant will try to bring the ambient temp too. If you are such the expert, care to elucidate on what the actual materials are that the drive itself is made of? That will have a big effect on cooling rate as well. As well as to test the measured rate as to not damage the materials.

Read what I wrote.


Hans

 

[Dragoner]

So in other words, you have no clue and no sources. Great.

You understand that it is all hypotheticals in fact, a jump drive doesn't exist?

 

[rancke]

So in other words, you have no clue and no sources. Great.

I have sources. They convinced me at the time. But if you tell me that you know for a fact that the per volume coolant effect of liquid hydrogen is not particularily different from the per volume coolant effect of any and all alternative substances, I'll certainly look into revising my beliefs.

You understand that it is all hypotheticals in fact, a jump drive doesn't exist?

Yes. You understand that that is completely irrelevant to a discussion about the efficiency of liquid hydrogen as a coolant?


Hans

 

[Ishmael]

It does not make sense, because if you can use hydrogen for coolant, there's a strong economic incentive to use something that has the same effect while taking up less volume instead and no plausible reason not to use it.

"Hydrogen is used as a high-performance gaseous coolant. Its thermal conductivity is higher than of all gases, it has high specific heat capacity, and low density and therefore low viscosity, which is an advantage for rotary machines susceptible to windage losses. Hydrogen-cooled turbogenerators are currently the most common electrical generators in large power plants."
http://en.wikipedia.org/wiki/Coolant

look up the heat capacity of hydrogen cp J/(g*K)

"Liquid hydrogen at -250 to -265 °C can also be used as a coolant. In the Reaction Engines Scimitar and the Reaction Engines SABRE hypersonic aircraft engines liquid hydrogen is used as a coolant in the precooler to cool down the air in the intake. At Mach 5, the intake can reach as high as 1000 °C so a precooler is needed to avoid melting of the engine parts. Liquid hydrogen is also used both as a fuel and as a coolant to cool nozzles and combustion chambers of rocket engines."
http://en.wikipedia.org/wiki/Coolant

http://www.msm.cam.ac.uk/ascg/materials/hydrogen.php

"because of the thermal conductivity of hydrogen gas this is the most common type in its field today."

so, yes.... hydrogen as coolant makes sense when used as stated in canon materials (coolant)

 

[Dragoner]

Ranke2,

I'm sorry, I explained it all in my previous post, it may seem frustrating to you due to your lack of knowledge, but I can guarantee that frustrates me as well. What can I say, if I had the actual specs for the drive before me, I could tell you more.

Regards,
Robert

 

[shadowcat20]

It just got better.

Now I know a fuel tank leak can be real bad news on a ship. Super cold jets could be every bit as bad as a steam leak. Also mixing the H2 with a oxy atmosphere can risk something going high order.

Lhyd seems to work as a coolent and a fuel. It is also VERY easy to find which is great for a scout in a empty system.

This is why I posted this question, I knew someone with more learnin than me would figger it out...

 

[Dragoner]

It just got better.

Now I know a fuel tank leak can be real bad news on a ship. Super cold jets could be every bit as bad as a steam leak. Also mixing the H2 with a oxy atmosphere can risk something going high order.

Lhyd seems to work as a coolent and a fuel. It is also VERY easy to find which is great for a scout in a empty system.

This is why I posted this question, I knew someone with more learnin than me would figger it out...

The fuel tanks are self sealing I would think, though yes, O2 and H2, are very explosive, H2 is poisonous on it's own as well as explosive. Though it does seem to say, that there would need to be a buffer of x amount of fuel for it to be used as coolant, the exact amount you could only tell if you knew how much heat the jump drive was generating.

 

[aramis]

So, not an expert, then?

It's not the temperature itself that's interesting, it's the coolant effect per volume. This is related to the number and weight of molecules per volume, and hydrogen is a lot less efficient than many other substances. Or so I've been told.

Maybe someone who actually knows about this subject will do us a favor and chip in and spare us further 'appeal to authority' vs. 'guesswork' debate?


Hans

The factors are density and specific heat.

Hydrogen is density 0.071 g/cc 9.668 J/g °C and thus 6.864 J/cc °C and remains liquid in the range -259.2 °C to -252°C. Also, 446 J/g latent heat of vaproization ~ . Thus the liquefying it from barely liquid hits 81 J/cc, whereupon it's specific heat drops by half... and it suddenly becomes a major fire and corrosion risk.

LN2 0.808 g/cc and 2.042 J/g °C, and thus 1.650 J/cc °C, and remains liquid in the range -210 to -195.8 °C, and has a vaporization latent heat of 199.1 J/g for 161 J/cc, so from barely liquid to vapor is 184 J/cc.

But since mass is as important a factor, hydrogen is total of 515.6 j/g, and N2 is 228.1 j/g... But hydrogen also then becomes highly reactive (read also DANGEROUS), while N2 is generally pretty stable, so N2 is safe to use for further cooling even as a gas, while H2 generally isn't.

A nitrogen spill isn't a hazmat fire risk; a hydrogen spill is.

http://www.uigi.com/nitrogen.html
http://www.uigi.com/hydrogen.html

I'm not an expert, but I at least paid attention when this was taught...back in 5th grade science! If you avoid crossing the liquid-gaseous barrier, you restrict yourself a little, but nitrogen then shines a lot better. (At least, the battle damage won't deplete the engineering room O2...)

 

[shadowcat20]

Ok, I will bite.

Even though Nitrogen is better at cooling can it ALSO be used as a fuel? How easy is it to skim compared to Hydrogen?

I think multi tasking and avalibility may be a factor in choice also.

EDIT: Maybe the large quanities used could be the system dumping the hydrogen after the cooling process rather than bringing it back on board. It is hot gas or plasma at that point and too dangerous to deal with. Or heat it while cooling things then burning the heated gas for power. Get duel use out of it.

 

[rancke]

The factors are density and specific heat.

Hydrogen is density 0.071 g/cc 9.668 J/g °C and thus 6.864 J/cc °C and remains liquid in the range -259.2 °C to -252°C. Also, 446 J/g latent heat of vaproization ~ . Thus the liquefying it from barely liquid hits 81 J/cc, whereupon it's specific heat drops by half... and it suddenly becomes a major fire and corrosion risk.

LN2 0.808 g/cc and 2.042 J/g °C, and thus 1.650 J/cc °C, and remains liquid in the range -210 to -195.8 °C, and has a vaporization latent heat of 199.1 J/g for 161 J/cc, so from barely liquid to vapor is 184 J/cc.

Thanks for jumping in, Wil. I asked about it over on the SJG boards and got this reply:

Ah. Well if it's just a matter of how much heat you can soak up, hydrogen is pretty bad. Commonplace, but bad. Water's common too, and can store a lot more heat.

E.g. vapourizing 1 litre of water at its boiling point absorbs about 2300kJ. Heating it from its melting point to its boiling point takes another 400kJ.

By contrast, boiling a liter of liquid hydrogen requires only 30kJ, plus another 4kJ if you first heat it from its melting point. Even if you continue to heat the gas up to 373K (the boiling point of water, above) that will only suck up another 350kJ or so.

So unless there's another catch ("must be readily found in interstellar space", "must be liquid at 30K", etc.) hydrogen is not your best choice.

TeV

So water is at least seven times as efficient as hydrogen as a coolant.


Hans

 

[rancke]

Even though Nitrogen is better at cooling can it ALSO be used as a fuel? How easy is it to skim compared to Hydrogen?

I'm fairly sure the answers are no and much less.

I think multi tasking and avalibility may be a factor in choice also.

Most ships don't need to perform wilderness refuelling. If you're a freighter or passenger liner engaged in regular traffic, you jump from one well-established starport to another. A jump-2 ship would increase it's payload from around 50% to around 67%. That's a boost in earning capacity of 33%, just from using water instead of hydrogen!

The saving would be less if a significant part of the hydrogen is used to generate power, but then you run up against the original problem (that of efficiency of the jump drive) pretty quickly, and in starship economics, even a few percentage points can have a big effect. Especially when it comes to higher-performance jumps.

EDIT: Maybe the large quanities used could be the system dumping the hydrogen after the cooling process rather than bringing it back on board. It is hot gas or plasma at that point and too dangerous to deal with. Or heat it while cooling things then burning the heated gas for power. Get dual use out of it.

The way you use coolant aboard a starship is to heat it up and get rid of it. You don't use it twice, because there's no way of getting rid of the heat other than getting rid of the coolant.


Hans

 

[McPerth]

Wow, one stays off line for less than two days and this tread is 4 pages longer...

CT - jump drives need no power plant, all fuel is used to make a jump, solar collector based jump drives are an alternative to hydrogen fueled jump drives, jump drives are bigger than maneuver drives.

CTrevised/TTB/ST - jump drives now need a power plant - X-boat is now a broken design

HG/HG2 - jump drives are now smaller than maneuver drives

And all of the above are CT changes. Make the change to MT, TNE, T4 and something pretty major changes with each ruleset.

My first version of traveller rules is Traveller book (1982), so I cannot talk about earlier rules. In it, a starship needs a PP with number equal or greater than JD, so JD need a PP.

The only system I've seen (I've not cared about TNE, throug I have it, nor T20 nor GT, neither of which I have'nt readed) where JD was not PP dependent was MT.

In the case where you could use paint as your solar collector, yes. You can armor your ship and then paint it with a solar voltaic paint.

Best Regards,

Ewan

I don't think a paint whould suffice. Surface is not enought for any power level you need (in MT you have only 0.081Mw/sqmeter at TL 12+), and it whould not last long. Even if feasible, your armor won't protect your solar paint, so, solar pannels cannot be armored, IMO.

The only way I foresee to have solar power is with arms to support the solar pannels (see my entry on Jan 4th, page 11 this thread).

I'm thinking along the lines of the amount of energy needed in a short space of time. Capacitors that can discarge very quickly. And capacitor jumps only come in with black globe generators (& TL15). Prior to this we don't have capacitor jumps, there is no TL stated in the original Annic Nova, so if you so wished you could assume it's jump drives were TL15.

Converting a bunch of hydrogen to usable power in a very short space of time is going to be inefficent is it not?

That's why I'm thinking about the speed of energy release.

Not that IR.

Best regards,

Ewan

Capacitors exist before TL 15, as they are a necessary part of JD. They don't come with black globe, through with the black globe description they are given a specific volume and capacity.

The factors are density and specific heat.

Hydrogen is density 0.071 g/cc 9.668 J/g °C and thus 6.864 J/cc °C and remains liquid in the range -259.2 °C to -252°C. Also, 446 J/g latent heat of vaproization ~ . Thus the liquefying it from barely liquid hits 81 J/cc, whereupon it's specific heat drops by half... and it suddenly becomes a major fire and corrosion risk.

LN2 0.808 g/cc and 2.042 J/g °C, and thus 1.650 J/cc °C, and remains liquid in the range -210 to -195.8 °C, and has a vaporization latent heat of 199.1 J/g for 161 J/cc, so from barely liquid to vapor is 184 J/cc.

But since mass is as important a factor, hydrogen is total of 515.6 j/g, and N2 is 228.1 j/g... But hydrogen also then becomes highly reactive (read also DANGEROUS), while N2 is generally pretty stable, so N2 is safe to use for further cooling even as a gas, while H2 generally isn't.

A nitrogen spill isn't a hazmat fire risk; a hydrogen spill is.

http://www.uigi.com/nitrogen.html
http://www.uigi.com/hydrogen.html

I'm not an expert, but I at least paid attention when this was taught...back in 5th grade science! If you avoid crossing the liquid-gaseous barrier, you restrict yourself a little, but nitrogen then shines a lot better. (At least, the battle damage won't deplete the engineering room O2...)

I won't argue here if N2 is better coolant than H2 (probably it is), but is it economic to use it?

First of all, we must take in account that ships must have Lhyd tanks for their PP (I don't think anyone will argue H2 is the better fuel for fusion PP). So having separate N2 tanks whold need an extra effort when constructing the ship (separate scoops, purification plants and tanks).

While a nitrogen leak whould not be explosive, it's quite toxic in hight concentrations (leading to chemical pneumony and, if combined with oxigen to NO, to a funny and painless death), so equaly dangerous if released on the habitable part of the ship ( and probably harder to detect, as most of its atmosphere is already nitrogen).

Nitrogen is not as abundant on space as hydrogen is. Sure, it's quite plentyful on our atmosphere, but what about other systems? While hydrogen is plentiful even in so called vacuum (it can even be taken from ici comets or asteroids in an emergency), not everywhere is an oxi-nitro atmosphere.

Even if there is amonia in GG or comets, its quite more difficult to take N2 from amonia than H2 from water.