Fusion powered rockets

[Ray McCarthy]

More ambitious than Project Orion which was envisaged as fission powered:

More Science fiction than Science fact.
We May Need Fusion-Powered Rockets To Stop Comets From Destroying Earth

 

[Ray McCarthy]

In my "Talents" SF, a trip from fast moving starship, that thus can't orbit a planet but can drop off / pickup, takes about 4 months to get to edge of the Soiar system (past the Kuiper belt about 50AU).
Jupiter is 5.2 AU. Two weeks travel to Jupiter (in We May Need Fusion-Powered Rockets) is really fast,
Neptune about 30 AU, so about 7.5 times the closest distance between Earth and Jupiter (varies from about 4AU to 6AU due to orbits) which would be about 4 months. So my advanced civilisation doing about 50AU in 4 months in Fusion powered "Torch" ship sounds plausible. Journey time is noticeably less for passengers & crew compared to Earth observers.

I don't think we are building one any time soon. Four months is also about the practical limit for acceleration with 4 months deceleration with Fusion* as longer time needs more hydrogen / reaction mass (and/or water) which is more mass, thus needs more fuel ... The rocket equation even in space is a pain!

So visiting any stars you STILL need a "Jump Drive" / "Hyperspace" / "Warp drive" (Insert fictional method of choice) because even coasting at the massive velocity a fusion powered ship reaches would otherwise need a Generation ship (just about possible) or hibernation/suspended animation (a: We don't know how, b: probably not viable for 100s to 1000s of years needed).

Why is Space Travel SO HARD!

[* Assuming regular hydrogen and we only know how to make fusion work with heavy hydrogen / lithium cycle which isn't as mass / energy efficient.]

 

[Serendipity]

I think a closer look needs to taken in the proton - Boron nuclear fusion capability as this a aneutronic i.e. does not produce radiation.

 

[Ray McCarthy]

Boron nuclear fusion capability as this a aneutronic

Horribly less efficient in terms of energy from mass.

Neutrons are a problem. That's why for terrestrial use it's a good idea to have a fission power station co-sited with fusion model. The waste from fission is used as shielding on Fusion Tokamak and it then gets hot and becomes lower radioactive waste, easy to dump and makes steam for Electric.

In my fictional universe, the humans only shield habitat and put it at opposite end to fusion Tokamaks. The Aliens have a rather different way of making a fusion power supply, and observe that you don't go alongside an Earth starship unless you want "fried".

There is a lot of neutrons from the "best" fusion reactions. Solution is secondary generation, easy on ground as described (it's been tested and makes EXISTING Fusion test rigs have a net output) but not so easy in a spaceship unless it's very big and has a fission reactor too.

A nuclear sub has all the tech for a spaceship. Can be totally self contained for a year. Oxygen, power etc. Replace props with Ion drive ...

 

[Mirannan]

Horribly less efficient in terms of energy from mass.

Neutrons are a problem. That's why for terrestrial use it's a good idea to have a fission power station co-sited with fusion model. The waste from fission is used as shielding on Fusion Tokamak and it then gets hot and becomes lower radioactive waste, easy to dump and makes steam for Electric.

In my fictional universe, the humans only shield habitat and put it at opposite end to fusion Tokamaks. The Aliens have a rather different way of making a fusion power supply, and observe that you don't go alongside an Earth starship unless you want "fried".

There is a lot of neutrons from the "best" fusion reactions. Solution is secondary generation, easy on ground as described (it's been tested and makes EXISTING Fusion test rigs have a net output) but not so easy in a spaceship unless it's very big and has a fission reactor too.

A nuclear sub has all the tech for a spaceship. Can be totally self contained for a year. Oxygen, power etc. Replace props with Ion drive ...

Another point is that the best shielding against neutrons is shielding with a lot of protons in it. Put the reactor behind the fuel tank! Also, it would probably be necessary to pay attention to the materials one makes the power plant out of. Neutrons have a strong tendency to make previously safe elements highly radioactive; it isn't all that good an idea to have a power plant that you can't do maintenance or repairs on without getting killed.

 

[Serendipity]

proton-Boron fusion may produce less energy compared to other types of fusion, but at least you can get to fly with such a power plant! Safety laws, at least in the UK, would not permit anything with deuterium in it.

Also I read recently that they are closer to getting it working. They've demonstrated p-B fusion with the aid of two synchronised pulse lasers - to get the temperature up so that fusion can place.

Should be able to have cloud cities at the rate things are going and how are they going to legislate for those with all that entails?

 

[Mirannan]

Regarding various different fusion reactions used for propulsion:

It's probably true that p-B11 fusion is less efficient than proton-proton fusion (or far more achievable, D/T, D/D or D/He3) in terms of reactant mass. However, there is another issue. That is the issue of "dead weight" or parasitic mass.

p/B11 is, as already stated, aneutronic or nearly so thus reducing the need for shielding mass. In addition, all the reaction products are charged - so getting power out of the reaction is easier than for other forms of fusion. And regarding use in rocketry, some of the likely forms of fusion reactor using this method (notably focus fusion) could have the reaction geometry tweaked to fire the reaction products out of the back directly - at very high velocity because they are already at fusion temperatures, and removing the need for a huge apparatus for heating propellant.

Lastly, the reactor fuel for p/B11 fusion (likely one of the boranes) is much denser than liquid hydrogen and also liquid at reasonable temperatures - thus reducing the mass for tankage and also greatly reducing boil-off.

If there ends up being a choice between various different fusion methods and fuels, the decision is not going to be all that clear-cut.

 

[Ray McCarthy]

Also if the ship is stick like with reactor at one end and habitat at other, you only shield habitat facing power unit. The habitat is shielding problem anyway (Cosmic radiation further out, solar radiation esp. flares closer to Earth), water can be used as additional shielding and if you figured out how to to H H fusion rather than heavy hydrogen / lithium outside a star, the water is "fuel". Excess oxygen and reaction product helium is used as reaction mass in the linear accelerator (you need to accelerate the electrons separately).

The only feasible use of neutrons now is heating fission waste. So you'd want a fission power station too.

No matter how you do it, powering with fusion isn't simple*. Solar powered Xenon Ion drive has to be simplest scheme (though pathetic thrust, but at least it really exists).


[* Hence in SF, best to not be specific at all as to how the "magic" works!]