Low Tech vs Interstellar societies...

[Enoki]

4,000kg not 4 displacement tons.

Which makes it the equivalent of the 4 ton truck.

I wasn't exactly sure, but a 4- or 5-ton capacity truck is pretty serious as a load carrier. If it's military grade you're talking a seriously big vehicle. It's something few people would need on a daily basis without specialized requirements for it.

 

[Spinward Flow]

As I have not much idea about engines and poswer plants, wouldn't fuell cells make sense where fusion plants are not efficient (as small vehicles)? Traveller seems to point they are, but I don't know in real life...

In the real world, for terrestrial applications, the most descriptive term would be Fool Cells.
The reason for this isn't the cells themselves (those are relatively straightforward tech) ... the problem is the hydrogen (and the storage thereof).

In this case, I'll simply get out of the way and let the pictures do the work of thousands of words.

The problem with hydrogen (as a power source) is that it doesn't appear as "something that can be mined" in pockets of H₂ gas "conveniently lying around, ready to be pumped" from any natural sources. Hydrogen almost ALWAYS appears in molecular form bound to other elements (H₂O, CH₄, NH₃, etc. etc. etc. etc. etc.), meaning that in order to "get at" the H₂ you have to "crack" the molecules you're wanting to source it from ... which means energy input just to make the hydrogen you want to use.

Then there's the Storage Problem™.
Hydrogen is the smallest "stuff" in the universe ... meaning is can permeate through ALMOST EVERYTHING made of other elements/molecules ... so it "leaks" out of storage REALLY EASILY.

Worse, hydrogen will react with almost all containment materials, with metals being prone to a phenomenon known as Hydrogen Embrittlement ... which does not bode well for sustainability/longevity of containment systems (that need to be pressurized, go figure). When the molecules of your "container" are being "attacked" at the atomic level ... options are ... limited ...



The problem is that a system that relies on hydrogen as an "energy storage medium" for energy transfer (short of fusion) is going to be inherently lossy and wasteful, whether you're talking combustion or electrolysis (fuel cell). You're simply better off "leaving" the electricity AS ELECTRICITY and storing it as electricity in batteries (electrochemical) and/or capacitors (electrostatic) and then using the electricity directly.

Fusion, of course, upends the table on all of that ... making hydrogen the "go to" fuel for power generation.
Once fusion becomes commonplace (TL=9+) and the hydrogen containment technologies get "perfected" (so your hydrogen fuel tanks don't "leak" constantly in self-destruct mode fashion), fuel cells can start to make a comeback for low power applications ... but even then, you're often times going to be better off (in endurance/kg and/or endurance/liter terms) just by using Batteries/Capacitors instead.

Unfortunately, Traveller of the 1970s/80s was decidedly biased against battery tech power density+pricing, so if you use CT Striker and/or LBB8 for this judgement of comparison, you'll be getting a wildly lopsided result in favor of fuel cells almost every single time. To be fair, it's only in the last 5-10 years that significant resources have been poured into battery research in the real world, lowering prices for batteries down to around Cr10 per kWh ... and in the next few years could go as low as Cr2 per kWh with some technologies currently being researched and developed for mass production.

"Predictions are hard, especially about the future."



And if all of that isn't convincing enough ... try This REALITY™ on for size ...

 

[aramis]

If you look at that gray tractor, "none" of those parts can be made locally. They all need specialty manufacturing. No one is going to pound any of those parts out at a blacksmith.

Nobody builds carburetors in their garage (not those types of garages). Can someone mate a foreign carburetor salvaged from another machine? Oh, for sure (with obvious caveats). That's a different problem. But making one out of raw materials, not so much. That requires specialty manufacturing not just for the carb body, but for the small parts, the jets, the springs. About the only thing that might be more locally available is the gaskets if you can find some suitable gasket material (but those also require specialty manufacturing -- tree bark likely won't work).

You might be able to make one with foundry that can cast the body, then machine out the holes and such, but even that it likely above a typical machinist.

Then, of course, there's the tubing...

A typical machinist can make a carburetor using standard metalworking tools in the machine shop. Hydraulics, too. And cylinders, heads...

A 1920s machine shop, in about 600 square feet, can (and did) make entire engines (see also the Wright Brothers). Not cost effectively... but there's a huge bunch between village blacksmiths and even 1880's machine shops... keeping in mind that the blacksmith was still a fixture in the 1880's, while machine shops were becoming a thing to support the industrialization... with lathes, vertical mills, belt sanders... Noting that all the tools were around by the 1830's... TL 4... 1818 for the near-modern vertical mill, thanks to Eli Whitney... His was commercially successful and allowed for standardized rifle parts...

I wasn't exactly sure, but a 4- or 5-ton capacity truck is pretty serious as a load carrier. If it's military grade you're talking a seriously big vehicle. It's something few people would need on a daily basis without specialized requirements for it.

the Deuce-and-a-half (M35 Truck, 2.5 ton capacity, road wheeled) is 48 cubic meters, masses 6.75 tons empty, and seats 3 in the cab, and upwards of 15 in the bed depending upon configuration. It has a road limit of 2.5 tons, off-road of 2 tons, cargo in bed.

 

[Condottiere]

One possibility for hydrogen is to take the infrastructure with you.

You pour in water into the tank, it's connected to a micro fuel processor, and creates just enough hydrogen to feed the engine(s).