Re: Best choice of FTL fuel?

Helium 3 is harder to get still, but it smells of fusion which can easily
confuse the science-literate readers. Also I think helium 3 is harder to get
than what is optimal, from a world building point of view.

MacFraggin@xxxxxxxxxxxxxx wrote:
Note that the outstanding scarcity of He3 is specifically for the
concentration in the Earth atmosphere. Of course, He3 is always

All I know about He3 is that significant quantitites of it can be extracted from the lunar surface, which may be useful for fusion reactors.

scarce, but you can expect a better ratio in gas giants.
Assuming you find He3 there in a concentration of 0,0001% in a H-He
mixture, you'd have to refine 10.000t of gas for 1t of He3.

Not one million tons?

Granted, it's still a lot scarcer and more difficult to refine than
Deuterium on Earth, but it's certainly an option if you want to keep
FTL travel something special and exclusive.

That's the trick. I want to hit the middle ground. Neither make FTL travel, including "wilderness refuelling", too easy, nor too difficult.

Also, He3 might cause science-literate readers to assume that fusion is somehow involved in FTL travel, and I don't want them to make that assumption. Also, large amounts of fuel should be required for FTL, which clashes with the lots-of-energy-from-tiny-amounts-of-matter concept of fusion. So I'd much prefer to avoid readers to think along those lines.

Storage of gases: maybe you don't have to store it in liquid phase.
Hydrogen can be stored by carbon nanotubes in much higher densities
that could ever be achieved by pressure containers or liquid phase. If
my calculations are correct, you can store 2.5 tons of monatomic
Hydrogen per cubic metre. That may well scale up to 5 tons of D or 7.5
tons of T.
And maybe Helium can be stored with similar efficiency, so up to 7.5
tons of He3 per cubic metre (even if it's just a fraction of that, you
still save on costly refrigerated liquid containers)

Thanks, that's very interesting. The setting is one of diverse levels of technological sophistication, with periods of loss of technological and scientific knowledge, similar to the interregnums in Asimov's Foundation. Primitive star travel could involve cryogenic storage of hydrogen (for reaction mass) and helium (for FTL fuel), and only later on do they rediscover how to make carbon nanotubes.

What other FTL fuel choices are there, if one recognizes the fact that
compications and logistical problems make for better worldbuilding than
"push-the-button-and-it-goes" problem-free wonder technology

As it happens, I just had similar thoughts today concerning my
setting. Up to now, I had ruled that FTL drives use the same fuel as
powerful STL drives (Torchships, which are the fulcrum of my setting),

So your FTL just requires energy?

That's one of the things I want to avoid, because then you can just install a large battery (or capacitor) in your ship, and a fusion reactor (or even a fission reactor), and you're good to travel anywhere you want for the next several decades (if not centurires).

I want to avoid that. I want FTL fuel refuelling to be necessary, and frequently.

and use about 5% of the ship's mass for a 10LY jump, and that the
whole magic takes place in form of a jump rather than a long ride.

I've long disliked jumps, but a couple of years ago I sat down and thought hard on the subject, and I came to the conclusion that the dynamics of FTL travel in the Traveller RPG setting make for good worldbuidling and strategic conflicts, because you have to "land" at solar systems.

My solution is to not have jumps, per se, but rather a sort of warp travel with the requirement that the journey must both start and end close to a star-sized mass, and with FTL fuel requirements depending on the distance travelled (if your FTL fuel runs out before you reach the destination mass, you're never heard from again), and probably also on the travel speed. I'll probably want economy class drives that are very slow but use less fuel, and fast/military drives that use lots of fuel. Or probably even a more expensive fuel. Ordinary and eco drives could use He4, and military drives use He3, maybe.

Then today it occured to me that, using the same reaction that powers
the torch drive, this would mean a 3 Gigaton Boom at the instant of
FTL drive activation. So right now I'm reconsidering that model.
*cough*

What do you mean when you use the term "torch drive"?

Ideally, starships should spend quite an amount of their capacity (mass) on FTL
fuel storage, if they want much range (or speed); not entirely unlike the
Traveller RPG setting.

My explanation for FTL drives is that they need to be fed energy just
like anything else, so you basically use whatever power sources are
available in your setting.

But that means unlimited travel endurance for - at least - decades.

If your main power source is fusion, then stick with fusion. Suitable
fuels are Deuterium, Tritium, or He3, depending on the chosen
reaction. Considering the vast distances between stars, fusion as a
power source is actually only so-so - about 300TJ/kg - which means
that you can easily justify requiring many tons of fuel to activate
whatever magical FTL device you came up with.

I don't want to have energy be the "fuel" of FTL travel. I want it to be a specific substance, with a knowable abundance, so that it is possible to derive the setting from that premise.

--
Peter Knutsen
sagatafl.org
.

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