Traveller as a Spaceship Modeling Hobby

[Old School]

I just assumed they used cold fusion

[Sigtrygg]

Even if cod fusion can be made to work the waste heat generated will still need to be removed.

It doesn't matter how you generate the gigawatts of power on your ship you will still need to radiate the waste heat - unless you have a handwavium gravitic heat sink

Oh, wait a minute, 'cold' fusion...

[Condottiere]

Never got around to installing a peltier cpu cooler.

With no moving parts, might be a solution.

[AnotherDilbert]

A Peltier element is a heat pump still obeying thermodynamics. It does not eliminate waste heat, it just moves it, consuming even more power (producing even more waste heat).

It is in effect nothing more exotic than any other AC or refrigerator.

[Linwood]

What about a technology that converts heat into electricity directly?

[Pyromancer]

What about a technology that converts heat into electricity directly?

A refrigerator that cools the stuff inside while generating power would be great, but is against the laws of thermodynamics.

[Old School]

What about a technology that converts heat into electricity directly?

In industrial applications today that’s usually done with a boiler and a steam turbine. Sounds problematic in space. No more so than jump drives, gravity manipulation, and jumpspace, however.

[Pyromancer]

What about a technology that converts heat into electricity directly?

In industrial applications today that’s usually done with a boiler and a steam turbine. Sounds problematic in space. No more so than jump drives, gravity manipulation, and jumpspace, however.

In industrial applications today, you don't convert heat into electricity, you convert the equalization of different heat levels into electricity (by way of motion). This is a crucial difference.

The problem with Traveller and waste heat - compared to jump drives and gravitics - is that it isn't addressed. There could be interesting ramifications, like dumping heat into jumpspace, but that only works outside the 100D limit, or some kind of energy shield that heats up and can explode when it has to absorb too much heat, like the ships in "The Mote in God's Eye". Or the need for fragile radiators. ...

Traveller does nothing with this topic.

[Sigtrygg]

You don't turn heat into electricity, you generate electricity from a heat gradient. The electricity generated than contributed more waste heat. Eventually you need a way to get the waste heat off your ship, which requires either radiators or dumping coolant overboard.

Or you can handwave a gravitic heat sink that is discovered at the same time as the gravitic technology that makes the m-drive, grav plates, acceleration compensation and grav modules all possible. Other suggestions over the years have been some sort of sub-space heat sink (Traveller has no sub-space technology) or shunting it off into jump space. I prefer the gravitic heat sink personally

[steve98052]

What about a technology that converts heat into electricity directly?

As already noted, that defies thermodynamics. What can be done is to convert a heat difference into useful energy. A fusion reaction, in the absence of technological magic, takes place slowly at solar core temperature (circa 15 million K*), generating about the same amount of energy per unit volume as a compost heap. In a thermonuclear reaction (fusion bomb, inertial laser fusion, nova** star), the reaction takes place at a much higher temperature (over 100 million K) and much more rapid pace.

Given that the Traveller technology tree has gravitics and fusion arriving together, I'm going to assume that a Traveller fusion reactor works kind of like inertial laser fusion, except with a pulse of gravitic energy collapsing the hydrogen pellet to thermonuclear temperature. It spends a lot of energy to generate the gravitic pulse, collapsing the pellet to microscopic scale and heating it to a few hundred million K, where fusion takes place, releasing an excess of heat. The fusion products (including un-fused hydrogen) expand in the reaction vessel, cooling tothe point that the combination of magnetic (and possibly gravitic) containment keep the liner from eroding, probably around 6000 K. That heat powers some sort of generator, possibly a steam turbine, maybe a magnetohydrodynamic generator, maybe a peltier generator, etc.

But to convert the heat to another form of energy, such as electricity, there has to be a differential between that 6000 K and the heat sink. The blackbody temperature of space is under 3 K, so there's plenty of temperature difference. But unlike a terrestrial heat engine (such as a similar fusion reactor in a nautical ship), the only way to exploit the 6000 K difference is radiators. It's a lot easier to exploit the slightly narrower difference between that 6000 K reaction vessel and ~300 K seawater, through a heat exchanger.

So in space, we're back to radiators -- which, by the way, work adequately for the peltier-based radioisotope thermal generators used in real-world space probes sent to the outer solar system, where solar panels generate too little power.

One possible substitute to radiators might be dumping energy into cryogenic hydrogen and dumping it into space at middle temperature. But that may consume too much hydrogen. (Then again, maybe that's why a jump drive requires so much liquid hydrogen.) The fusion energy content of hydrogen is so large that a GURPS Traveller power plant, which is refueled only during annual maintenance, is viable. But if heat is dumped to space in heated hydrogen, maybe the Mongoose and classic Traveller fuel usage rates mean that the "fuel" is really mostly heat-dump coolant.

If we assume that waste heat is dumped in vented hydrogen, how much would be needed?

This means that there could be several types of fusion power plants:

• Planetary fusion plants, cooled by sea water, river water, or giant air-cooling towers.
• Space radiator fusion plants, which dump heat through giant space radiators, usable on interplanetary ships where solar panels are inadequate.
• Coolant-carrying fusion plants, which dump waste heat into coolant they carry aboard, and vent into space, for ship that don't have space for radiators.

Does anyone know how much coolant would be necessary to dump heat into space by venting coolant? Or, for that matter, how large radiators would need to be?

* Degrees Kelvin don't use the ° symbol, unlike °C and °F.
** A nova star is a thermonuclear explosion that affects the outer layers of the star; a supernova goes all the way to the core.

[AnotherDilbert]

Radiators:

The IIS has a system with 7 radiators of about 6.5 m² each ≈ 45 m². It can radiate about 14 kW, at a guess depending on the temperature of the coolant.

A Scout has a fusion reactor producing 500 MW electrical power, or perhaps 1000 MW heat (according to CT). To radiate all of the produced heat in the same way as the IIS would require 1000000 / 14 × 45 ≈ 3 200 000 m² = 3.2 km² radiators (≈ 450 football fields). Presumably we could make them more efficient with higher tech and higher temperature difference, but we are still talking square kilometres for small ships...


Heat sink:

Hydrogen gas has a specific heat of about 14 kJ/kgK. To heat 1 kg hydrogen, say, 6000 K would take 14 × 6000 = 84000 kJ = 84 MJ = 84 MWs.

That would cool the Scout above for about 84 MWs / 1000 MW = 0.084 seconds. The entire fuel load of a MgT2 Scout, about 21 tons, would last 21000 × 0.084 s ≈ 1800 s ≈ half an hour.


Neither real world radiators nor ejecting heated coolant seems practical...

[AnotherDilbert]

* Degrees Kelvin don't use the ° symbol, unlike °C and °F.

If we are to be precise it's not degrees kelvin, it's just kelvin, hence no degree symbol.

[locarno24]

If we assume that waste heat is dumped in vented hydrogen, how much would be needed?

Well, a dTon is a literal Tonne (1000kg) of Liquid Hydrogen.
Hydrogen has a specific heat capacity of 14-and-a-bit kJ per kg per Kelvin temperature increase

Properly liquid hydrogen needs to be at about 20K (it's liquid at 33k but at 20k it stays liquid without pressurisation)

For the sake of argument, let's throw in a 100k-and-round-down safety margin, so start at 20K and throw out hot gas at 620K. That's a 600K temperature increase, so takes about 14kJ x 600K x 1000kg = 8,400 mJ of energy for each dTon of liquid hydrogen thrown out the back as hot gas. That's pretty safe, but also not very good - if you've got a reactor developing a mW of power (not unreasonable and potentially underrating it if you're a starship with ship-to-ship lasers) you could be looking at throwing a dTon of fuel away as coolant in a couple of hours.

Alternatively, you can basically vent reactor plasma at more like 15,000,000K - although how you cool stuff with that, since you need to use some method to make the heat exchanger you're cooling hotter than the coolant, god only knows! - but if you can, you can eject 210,000 mJ of heat energy with every dTon you throw overboard, which means that same dTon of coolant will last you a couple of days instead.

[AnotherDilbert]

... 14kJ x 600K x 1000kg = 8,400 mJ of energy for each dTon of liquid hydrogen ...

I believe you mean MJ, not mJ.

mJ means millijoule = ¹/₁₀₀₀ joule.
MJ means megajoule = 1 000 000 joule.

There is a slight difference...

[Condottiere]

Substitute heat sink for reactor, and manual flush for control rods.

[Sigtrygg]

Radiators:

The IIS has a system with 7 radiators of about 6.5 m² each ≈ 45 m². It can radiate about 14 kW, at a guess depending on the temperature of the coolant.

The current heat management system on the ISS can cope with 70kW of waste heat, 14kW was the original version that was upgraded a while ago.

So you only need 90 footbal pitches for a scout

[Sigtrygg]

Substitute heat sink for reactor, and manual flush for control rods.

In your posted image the cooling towers and reservoir are the heat sink.

[Old School]

Radiators:

The IIS has a system with 7 radiators of about 6.5 m² each ≈ 45 m². It can radiate about 14 kW, at a guess depending on the temperature of the coolant.

The current heat management system on the ISS can cope with 70kW of waste heat, 14kW was the original version that was upgraded a while ago.

So you only need 90 footbal pitches for a scout

So a few more orders of magnitude with the minaturization, and you’ll be able to cool a Tigress class ship with a radiator the size of a toaster oven. Problem solved.

[AnotherDilbert]

The current heat management system on the ISS can cope with 70kW of waste heat, 14kW was the original version that was upgraded a while ago.

OK, I went with the first number I could find: https://www.nasa.gov/pdf/473486main_iss ... erview.pdf

14 kW seems to be for each radiator of the old EEATCS.


The newer 70 kW EATCS seems to use larger radiators, something like 2 wings of 3 ORUs each measuring 23.3 × 3.4 m for a total of about 450 m². Assuming that each wing is sufficient and the other wing is redundancy that is the same power density as the EEATCS, so would yield the same 3.2 km² (450 football fields) estimate for the Scout as before.


Either way, it's much too large to be practical...


Note that this is only cooling for a part of the ISS, the Russians seems to have a separate system.

[Pyromancer]

The radiated waste heat scales with T^4, so there is one parameter to make radiators much, much more useful without increasing the area.