GURPS Spaceships for THS conversions?

[Crakkerjakk]

wabishtar, I have noticed you seem to become embroiled in snarkiness with some regularity. As for why, I'm not really sure, although effectively calling people ten year olds after they are snarky to you certainly doesn't help defuse the situation.

In this particular situation, I know Agemegos has his degree in economics, and can sometimes be easily annoyed by people, in his opinion, misusing economics to justify what he feels is faulty reasoning.

As to the main thrust of your argument, I agree that nuclear pellets will likely be much more common in TS, and jet fuel much more rare. I still see no reasonable way that jet fuel will cost double the price of those nuclear fuel pellets, even considering taxes, restricted supply, and public repugnance towards polluting. The process to manufacture the jet fuel is far too simple and requires very common elements, while nuclear fuel pellets use some relatively rare materials. My suspension of disbelief can't take the price differential, unless an entire moon composed out of heavy elements was discovered.

[wabishtar]

I found something else that might shed some light on this. According to Deep Beyond's entry on nuclear pellet factory, it requires 5,000 tons of rock and 5 POUNDS of H-3 to manufacture 84 tons of nuclear pellet. That's a 1 to 2,000,000 ratio. If one ton of nuclear pellets is 0.001 pounds of H-3 and the rest is just slag and casing used to produce a shock wave, then when combined with political and economic concerns, these prices make perfect sense to me. In fact, I'm not a physicist, but I even start to question the notion that a ton of nuclear pellets has more energy than a ton of jet fuel. H-3 no doubt has more energy, but does 0.001 pounds of H-3 have more energy than 2,000 pounds of jet fuel?

[Crakkerjakk]

Energy Density of aviation fuel is 43-48MJ/kg(Call it 48.) Hence, burning 1 ton of aviation fuel gets you:

48 MJ/kg * 1kg/2.2 lbs * 2000 lbs/1ton = 43,636 MJ/ton

pg 190 of TS 3E states that refueling a new fusion reactor requires .00059 ton He3 per MW per year, hence:

1 MW/Yr = 1 MJ/sec * Yr/1 * 365.25 Days/Yr * 24 Hours/Day * 3600 sec/hour = 31,536,000 MJ

Thus:

.00059 tonHe3/31,536,000 MJ or 31,536,000/.00059 MJ/tonHe3 * 1/2000 tonHe3/lbsHe3 = 26725424 MJ/lbsHe3.

However, you said 5 lbs of He-3 per 84 tons of nuclear pellets. There are 4910 tons of waste products, so its not five pounds per 5000 tons, its five pounds per tons pellet produced(or close enough, 90 tons instead of 84, if you want to include the missing 6 tons). Also, we know the robofac draws power, so it's not using the He-3 for anything besides production.

5/84 lbsHE3/tonPellet = 0.0595 lbsHE3/tonPellet

Finally:

26725424 MJ/lbsHE3 *.0595 lbsHE3/tonPellet = 1,590,799 MJ/tonPellet

So it's 1.6 million MJ for a ton of nuclear pellets, versus 40 thousand MJ for a ton of jet fuel, unless you assume they're throwing away 98% of their He-3 during the manufacturing process, and since it costs a million dollars per ton, I'm guessing they're not doing that.

[thtraveller]

Originally Posted by Crakkerjakk

pg 190 of TS 3E states that refueling a new fusion reactor requires .00059 ton He3 per MW per year, hence:

1 MW/Yr = 1 MJ/sec * Yr/1 * 365.25 Days/Yr * 24 Hours/Day * 3600 sec/hour = 31,536,000 MJ

And that is assuming a 100% conversion rate, which is unlikely. So probably significantly more than that. Double?

Note as another data point that Vx1 says NP drives produce "small nuclear explosions (equivalent to about a ton of TNT)" and that pellets weigh "a few ounces each".

Oh, and the price of the pellet factory in Deep Beyond was commented on during playtest to try and make the capital costs bear some resemblance to the production rate and the canonical cost of NP.

[Crakkerjakk]

I don't think that this assumes a 100% conversion rate, unless you're talking about some amount of He3 that remains unfused in a D-He3 fusion reaction. As I understand it, the advantage of a D-He3 reaction is that there are no high energy particles left over to irradiate anything. This becomes a moot point if you have He3 left over, I believe. The amount of He3 required is for a fusion reactor, so this is the effective energy density, since there's no real way(that I know of) aside from fusion to generate usable (read non-explosive) power using He3.

Unless you were talking about something else. 100% conversion rate where?

[mindstalk]

Simpler calculation: hydrocarbons tend to 4e7 J/kg. Fission's 1e14 (actually 0.8e14), doable fusion's around 3e14, 4P->He4 fusion is 7e14, antimatter is 1e17 (or 9e16).

So, the fusion fuel has about 1e7 (10 million) times more energy per mass as the jet fuel. The numbers for comparison were 0.001 pounds (gah) of fusion and 2000 pounds of jet fuel, a difference of only 2 million. Fusion pellet has more energy.

Actually the jet fuel number is cheating: it's assuming free oxygen. If you have to bring your own oxidiser, e.g. we're talking *rocket* fuel, then the energy/mass drops by a factor of 4 or 5. CH2 -> CO2 + H2O, so you need 3 O per CH2, mass ratio of that is 48/14, so you end up with 4.4 time as much mass. Hydrogen looks hot at about 1e8 J/kg, but when you include the oxygen the energy density drops to about 1e7 J/kg.

Not sure if those fuel pellets include the D you need for fusion, but that's just a factor of 2.

[thtraveller]

Originally Posted by Crakkerjakk

100% conversion rate where?

The reactor wont be 100% efficient at turning the heat from fusion into electricity.

But there again a jet engine wont be 100% efficient either, so it probably balances out.

[Crakkerjakk]

Well, yes, but I assumed the MW listing of the reactors is in power output, not heat produced, and thats what the calculations are derived from. As for the energy density of the jet fuel, you're probably right though.

[Allister MacLeod]

Originally Posted by David L Pulver

Ships that use "Fusion Torch" in THS are best repped by TL9 fusion rockets, sometimes with high-thrust. In Fact, the TL9 fusion rocket with/without high thrust is an almost exact match.

Does this mean that the delta-v provided by each fuel tank is at the TL9 level rather than the TL10 level?

[tshiggins]

Originally Posted by laserdog

I've gotten stuck on exactly which of the fusion reaction drives best represent the He3 powered engines.

My guess was "Fusion Rockets" for the space-only designs.

But have been at a loss for what to use to get out of a gravity well.

You use a separate interface craft (scramjet shuttle) or the beanstalk. Spaceships do NOT land. They stay parked in orbit, or docked with an orbital station or other base.