[Spaceships] Variant Fusion Reactors

[Trachmyr]

I'm posting my variants on Reactors for a TL9 Hard-science game set in the Epsilon Eridani system. The locations are primarily within an asteroid belt, where hundreds of small colonies have been founded. The ‘government’ (EFC- Eridani Futures Committee) relies and subsidizes private traders to maintain the flow of resources and police the region from numerous pirates.

One of the three terrestrial planets, Cania, is a large world with a very dense atmosphere of Helium. This has propelled advancement in Fusion power systems, lowering costs and making these the “only real choice” for colonies and starships.

Below are the basics on my variant reactors, propulsion systems obviously benefit as well. If anyone has suggestions, feedback or can spot major problems with these statistics… please let me know. I am trying to keep this bound to a “hard-science” genre, so if any of these seem implausible… please let me know.


Class I Reactors
These reactors use internal Deuterium fuel, producing Tritium as a radioactive byproduct. These reactors are designed to ‘trap’ the Tritium internally. The amount of energy required to produce fusion is substantially less than in a Class II reactor, but the liberated energy is also substantially less. These reactors can be installed in a SM+6 or larger craft, have one-half the cost of a Class II reactor but only produce 1 Power Point. Class I reactors can run for 50 years on their internal supply of Deuterium fuel.

Class II Reactors
These reactors use internal Deuterium and He3 as fuel, producing Helium (He4) as a byproduct. The amount of energy required to produce fusion is substantially more than that of a Class I reactor, requiring a facility two orders of magnitude larger. However, the liberated energy is greatly increased. These reactors can be installed in a SM+9 or larger craft, and produce 2 Power Points. Class II reactors can run for 50 years on their internal supply of Deuterium and He3 fuel.
Derated Class II reactors are available that produce only 1 Power Point, but they have half normal cost, and can run 100 years on their internal supply of Deuterium and He3 fuel.

SM . . . . . . . . +9 . . . +10 . . +11 . . +12 . . +13 . . +14 . . +15
Workspaces . . 0 . . . . 1 . . . . 3 . . . . 10 . . . 30 . . . 100 . . 300
Cost ($) . . . 20M . . . 60M . . 200M . . 600M . 2B . . . 6B . . . 20B

Also related…

Tritium Betavoltaics (TBV)
These generators use an internal supply of Tritium, and generate power by converting the electrical energy in electrons ejected by radioactive decay. Cost are low as fuel is abundant, and the device components are mass produced. They supply a constant stream of power that slowly reduces in strength as the radioactive material decays. The generator provides 1 Power Point for 12 years, after which the power levels fall below usable amounts. Tritium Betavoltaics cannot be refueled, the entire device must be replaced. The value of the core and remaining fuel supply covers the additional retrofit costs. Betavoltaics cost the same as a Fission plant one SM smaller than itself, but require no Workspaces.
Note: If using split modules and partial power points, apply the following rules. A Tritium Betavoltaic produces 1 Power Point for six years, 2/3 a power point for an additional six years, and 1/3 a power point for an additional 12 years. A “split module” small generator provides 1/3 a power point for six years, then it must be replaced. A six year old Tritium Betavoltaic can be traded in to cover retrofit costs and 50% of the cost of a new generator. A six year old generator can be purchased for 65% cost.

Capacitors (TBV)
These high capacity batteries store three power-point hours of energy, and can power up to three power point at a time. For example, capacitors can provide 1 power point for three hours, 2 power points for ninety minutes or three power points for sixty minutes. Capacitors are recharged in a similar fashion, using excess power to replace their spent energy, but cannot recharge faster than 3 power points per hour. Capacitors cost the same as a Fuel Cell of the same SM, but require no Workspaces.

Thanks for reading through this and any feedback you may have...

[SuedodeuS]

Someone with a stronger physics background will likely be along shortly (or will indeed post as I'm writing this), but nothing looks problematic to me. I really like the capacitors - I could see a ship using them to power weapons or other combat-only systems, then using the onboard reactor to recharge them once combat is finished. They could also be useful for small fighters, which would actually use their carrier's reactor to recharge. Hell, a carrier could have a hangar bay with small capacitors for the sole purpose of recharging the fighters!

For your price chart, you might want to consider using [ CODE ] tags. Using them you can more easily line things up (I'd recommend writing it out in a .txt first). Here's your chart using [ code ] tags:

SM		+9	+10	+11	+12	+13	+14	+15
Workspaces	 0	 1	 3	 10	 30	100	300
Cost ($)	20M	60M	200M	600M	 2B	 6B	20B

EDIT: I did some math. You could scale a hangar bay down 3 size categories and have 29 sub-modules as hangars and 1 as a capacitor. This would give the carrier the ability to completely recharge every single ship (assuming each had only one capacitor) within its hangar in an hour and still have a bit of juice to spare (1 such sub-capacitor is enough to recharge an entire full hanger). Since you're doing a hard-science setting, fighters might not play much of a role, but this idea could certainly be used in other settings as well.

[Trachmyr]

Thanks...

As far as your Hanger/Fighters idea... My assumption would be that any craft that is in your hanger (or even docked in a manner that allows fuel transfer) could tap directly into your reactor to charge capacitors on board (or vice versa)... the rate of recharge/drain would be based on the capacitors in the recharging vessel.


Capacitors are also a great way to limit stardrives in a not so Hard Science setting... set energy requirements high, but allow them to 'charge up', 'spin Ftl', etc.

[SuedodeuS]

Originally Posted by Trachmyr

As far as your Hanger/Fighters idea... My assumption would be that any craft that is in your hanger (or even docked in a manner that allows fuel transfer) could tap directly into your reactor to charge capacitors on board (or vice versa)... the rate of recharge/drain would be based on the capacitors in the recharging vessel.

That's kind of what I was thinking too. The problem is that I think most ships tend to be designed such that, in combat, every powerpoint is in use. So, sacrificing a little power from the reactor to recharge fighters could mean a few of your PD turrets going offline. A dedicated sub-capacitor would let you recharge without sacrificing any of your other systems. Of course, the draw is small enough that you might be justified in considering it to be negligible (a single hangar bay module, filled to capacity with ships that each use 1 capacitor, would only burn 0.1 PP to be able to completely recharge every single ship in one hour; this would knock out 1 secondary battery gun or 3 tertiary battery guns).

[lwcamp]

Originally Posted by Trachmyr

Below are the basics on my variant reactors, propulsion systems obviously benefit as well. If anyone has suggestions, feedback or can spot major problems with these statistics… please let me know. I am trying to keep this bound to a “hard-science” genre, so if any of these seem implausible… please let me know.


Class I Reactors
These reactors use internal Deuterium fuel, producing Tritium as a radioactive byproduct. These reactors are designed to ‘trap’ the Tritium internally. The amount of energy required to produce fusion is substantially less than in a Class II reactor, but the liberated energy is also substantially less. These reactors can be installed in a SM+6 or larger craft, have one-half the cost of a Class II reactor but only produce 1 Power Point. Class I reactors can run for 50 years on their internal supply of Deuterium fuel.

When running a pure D-D fusion reactor, you will probably not end up with much, if any, tritium, since the D-T reaction is so much faster. The tritium will be burned up with the deuterium in the plasma almost as soon as it is produced.

A pure D-D reactor will require lots of radiation shielding, since the majority of its liberated energy is in the form of energetic neutrons. If these can be stopped, you can make use of that energy by using the heat to run a heat engine (such as a turbine).

Modern designs involve stopping the neutrons with a blanket of lithium, which produces tritium and generates even more energy from the neutron capture. By adjusting the amount of Li-6 to Li-7, you can get enough tritium fuel from the lithium to keep your reactor going (Li-6 turns into T + He-4 when it absorbs a neutron, and liberates energy when it does so. Li-7 turns into T + He-4 plus an extra neutron, but this takes energy. The extra neutron can go on to produce more tritium when it hits a Li-6, however, so this makes up for neutron losses). In this sense, you only need to replenish the deuterium and lithium. Note that D-T fusion is much easier to get going and sustain than D-D fusion.

Originally Posted by Trachmyr

Class II Reactors
These reactors use internal Deuterium and He3 as fuel, producing Helium (He4) as a byproduct. The amount of energy required to produce fusion is substantially more than that of a Class I reactor, requiring a facility two orders of magnitude larger. However, the liberated energy is greatly increased. These reactors can be installed in a SM+9 or larger craft, and produce 2 Power Points. Class II reactors can run for 50 years on their internal supply of Deuterium and He3 fuel.
Derated Class II reactors are available that produce only 1 Power Point, but they have half normal cost, and can run 100 years on their internal supply of Deuterium and He3 fuel.

The reaction rates and ignition temperatures and other ignition criteria of D-D and D-3He fusion are about the same. A reactor that can ignite one can likely ignite the other. Both reactions are much harder to ignite than D-T. The advantage of D-3He is that it produces very little neutrons, thus making the reactor a much lower radiation environment (neutrons are nasty difficult to stop, especially high energy neutrons, and they tend to have undesired effect like embrittling metals). Further, you may be able to directly tap the fused plasma for energy using something like an MHD generator, rather than running a turbine or similar mechanical heat engine to turn a generator. This would make the whole reactor assembly of the D-3He reactor less massive and bulky than the D-D reactor because it can dispense with the generator and turbine. This also lets you reduce the amount of entropy you absorb along with the energy, thus reducing the heat load you need to dissipate, making your reactor more efficient and significantly reducing the mass of the radiators you need to lug around to get rid of the heat your systems generate. Since it produces an order of magnitude less neutrons, you can also greatly reduce the radiation shielding. The D-3He reaction still results in about 20% of the energy going into soft x-rays which will add to your heat load - if you put the turbine and generator back in you can capture some of this energy at the expense of more mass.

Proton-boron fusion would produce even fewer neutrons than D-3He fusion. However, p-B fusion always emits much more x-rays than is produced by the fusion unless it is done under non-thermal conditions (that is no hot plasmas - keep the ions energetic but the electrons cold). Under non-thermal conditions you can greatly reduce or even eliminate the number of x-rays produced. This would allow very high efficiency conversion of fusion energy to electricity with little waste heat and no bulky turbines or generators. This may be a TL 10 reactor rather than TL 9, however.

Thermovoltaics are currently a "hot" but unproven technology that, if sufficiently developed, could replace generators and the machines needed to turn them (like turbines). It seems unlikely that they could handle the high temperatures that turbine blades could, however, and lower operating temperatures means larger and more massive heat radiators. Where you do not have to radiate excess heat (like fixed installations on planets or asteroids that can use convective heat transport to dump their entropy) these could be used to scavenge waste heat generated by other processes, but use as the primary means of turning electricity into heat on a nuclear powered spacecraft is a bit less plausible. Still, if you wanted to reduce the bulk of a reactor, you could use these as a semi-plausible way to do so.

Luke

[Trachmyr]

Luke,

Thank you very much!

I've been doing what reading I can, but that streamlined a lot of what I was looking at. In particular I was looking into Lithium as a fuel, but was unsure of it's benefits.

I'll be adjusting my descriptions (and probably some stats) to be better in line with what you have shown... in particuar my Class I will be D+Li fuel, and I'll be scrapping the D+D altogether.

Again, thank you for your time!

[Peter Knutsen]

The idea of a short-life-span powerplant, essentially similar to the ones in GURPS Vehicles (Radiothermal Generator, RTG, at old TL6 and maybe 7, and NPU from old TL 8 and up) is a good one, and I'd expect something official along those lines to show up in one of the supplements. Perhaps the one about exploration, since non-maintenance powerplants are ideal for unmanned exploration craft.


Also basing short-trip space fighters on batteries (or capacitors) instead of generators is a good idea, and one that occured to me years ago when I was actually trying to design such a vehicle manually and based on GURPS Vehicles. Monetary cost can be lower, or flight endurance can be longer, or both.

One use that a space fighter might have for its power is weapons, so a house rule needs to be devised for how that works, since it is overkill to say that a weapon module uses power all the time, when the power source is a battery with only a few hours of charge. An arithmetically simple solution might be to give the weapon a number of round-shots. Assuming the battery provides 2 power points for 1 hour, and 1 PP goes to the drive (which we should assume is running constantly, at least for this purpose), that leaves us 1 PP for 1 hour.

If combat rounds are 2 minutes long (I'm too lazy to check the PDF, but IIRC combat round length depends on the ships involved) then that's 30 round-shots, 30 rounds in which the ship may utilize one weapon module that requires 1 power points, or 15 rounds in which it may utilize two such weapon modules, and so forth.

[SuedodeuS]

I decided to give Peter Knutsen's advice for a houserule a shot.

We need to start by converting PP to output/time. The unit I ended up deciding upon is simply abbreviated U (unit), and one PP means 5400 of them each hour (90 U/minute). Below is the energy consumption per shot of each weapon type. This was made assuming that 1 PP would allow a weapon battery to fire at full RoF indefinately.

Type		Consumption per shot
Spinal
Normal		100U
RF		10U
VRF		1U
Major
Normal		30U
RF		3U
VRF		0.3U
Medium
Normal		10U
RF		1U
VRF		0.1U

Secondary
Normal		3U
RF		0.3U
VRF		0.03U

Tertiary
Normal		1U
RF		0.1U
VRF		0.01U

Note that improved guns/beams consume half as much energy per shot.

This is unlikely to be useful in most games, since a good deal of the math can't necessarily be done quickly in one's head. Determining how many shots you can get out of a given capacitor should be calculated beforehand. For example, a VRF tertiary weapon battery consumes 10 mU per shot. This means a dedicated capacitor is good for 1.62 million shots.

Personally, I think it would be best to assume that a battery consumes the same amount of energy if it makes 1 shot/round or 9000 shots/round. In that way you can just have each PP produce 60 U/hr and each module uses 1 U for every minute that it's active.

[Trachmyr]

A note on Capacitors... These are TL9 stats. I'd reccomend raising the stored pph to 6 at TL 10, but reducing it to a meager 1 at TL7-8.

Yes, the Tritium Betavoltaics are an advanced version of RTG's using some very promising current technology as their basis (efficency of RTH were less than 1%, while Tritium Betavoltaics get up to 6%)... thus they have the "umph" of a NPU, but the cost & endurance of an RTG.

With Tritium Betavoltaics and Fusion technology availablem Fission Reactors are pretty much obsolete... expecially if you raise the refueling costs of fission reactors to be more in line with the costs of Uranium Rods.

[SuedodeuS]

Originally Posted by Trachmyr

A note on Capacitors... These are TL9 stats. I'd reccomend raising the stored pph to 6 at TL 10, but reducing it to a meager 1 at TL7-8.

I was also thinking somewhere along these lines (using the Power Cell as a comparison). Do you think TL10+ capacitors would be able to discharge/recharge 6 PP in one hour, or should they be more limited? I was thinking something like 4 PP at TL10, 5 PP at TL11, and 6 PP at TL12+.