The James Corey books : Or do fusion torch-drives pass a pencil test?

[SomeGuy12]

In the James Corey books, the plot demands an engine that gives some science fiction level performance. This is so that space warships are actually practical and common, so they have to be able to perform a variety of maneuvers and burn for days at at least 1/3 G without 99% of the ship being propellant tanks. (gotta have room for guns and armor and missiles, right?)

Anyways, it's sci fi. And, for those who have read the books, aliens show the ability to bypass hard physics with insanely advanced technology, but the author tries to portray human civilization as being far more realistic. So I began wondering if such fusion "torch" drives even pass a basic pencil test of theoretical feasibility.

They are described in the books as creating a plume of plasma behind the ship vastly larger and brighter than the ship itself, which is described as a tiny spec behind this massive drive flare. Thinking about it, at this level of performance, what you would have to do is somehow contain the ball of plasma right after a modern day fusion bomb explodes using some really big magnets and electric fields and hyper-advanced mirrors to reflect all the light and heat away so your engine doesn't melt. If you could basically contain what's a nuclear explosion inside your engine and only let a stream of the plasma at millions of degrees kelvin out the back, you'd have the described performance.

Well, I glanced at Atomic Rocket's section on this. Glancing at the chart of available fusion reactions, proton-boron sounds like the one to use because it produces less neutrons. Neutrons ruin the party because you cannot contain them with electric or magnetic forces or mirrors and they will impinge on your engine apparatus and heat it up.

So if you had a 10,000 ton space battlewagon (the ship flown by the protagonist in the book is described as being armored and having several decks with a large mass driver gun, etc), and no more than 10% of the mass is fuel, and it's all boron and hydrogen. The average velocity of the fusion products leaking out the back is 1% of the speed of light. That would give you 300k ISP.

In order to accelerate the whole contraption at 1/3 G (let's ignore the fact that in the books they can "burn" at 10 Gs), F=ma, F = (10k tons * 1000 kg/ton) (3.3333 m/s), F = ISP * mass flow rate * g, mass flow rate = 11.1 kg/second. If 10% of the ship is fuel, you can burn for 90k seconds, or 25 hours. You have a dV of 310 kps.

1/2 mv^2, so you have an exhaust energy of..100 terawatts. Wow. If just 10% of that energy is soaked up as waste heat that needs to be rejected, you would need to emit 10 terawatts via droplet radiators. You'd need a square kilometer of droplet area via this calculator with 1600 kelvin coolant http://www.5596.org/cgi-bin/dropletradiator.php. Those several hundred meter long radiator booms would be a pretty big target. Not sure what you'd use to have a low enough vapor pressure at 1600 kelvin so you don't boil all your coolant.

That's not as impressive as I thought it might be. The fusion products from a p-B fusion reaction leave at 4% of the speed of light at best, and I was assuming some losses and products leaving along tracks not aligned with the center of mass of your ship.

I mean, it's still pretty fast - you'd be able to reach Mars in a bit over a week. No way you'd be doing 10 G burns, though - the 1/3 G numbers I assumed are far too high as well. And you wouldn't be under thrust the whole time, either - even with this god-tier engine, you only can burn for a day total, so it would be a 12 hour burn at the start of a trip to Mars, and a 12 hour burn at the end. Same story for a trip to Jupiter.

[Aghanim]

One question: Why proton-boron instead of helium3-helium3 fusion?

First proton-boron fusion still produces neutron, which carries 0.2% of power produced. Then helium3-helium3 fusion is more energetic, with helium3-helium3 fusion generates 207.5 TJ/kg with exhaust velocity of 6.8%c instead of proton-boron 69.97 TJ/kg with exhaust velocity of 4.5%c.

Then what I think the most important advantage of helium3-helium3 fusion is that we can mine that in gas giants, but mining boron in outer space is harder, as in I cannot find any source that states that we can find boron in outer space. Feel free to correct me though.

[Beowolf]

Torchships are a specific type of drive. It was a staple of good 50s sf, Heinlein in-particular. A torchship's drive is a magical widget that turns matter directly into energy. I'm not familiar with Corey's ships, but my guess is his torch only produces thrust while a fusion reactor makes electricity to run the torch and the rest of the ship.

The ridiculous performance he cites is right in-line with a classic torch drive, too. These ships pull stunts like flying from Earth to Mars at 1G all the way, flipping over halfway to decelerate. One of Heinlein's stories had a torchship making an emergency medical run to Pluto at 10Gs. The trip was maybe 10 days and the story was mostly about how that trip ruined the volunteer pilot.

Does Corey talk much about the exhaust? Within a few hundred km, a torch battleship's most powerful weapon could be its exhaust. It's just photons, but tens of megatons per second of them.

Same site, different page: Atomic Rockets has everything.

[Iskierka]

Torchships drives aren't magical unknown widgets - torchships are just a generic term for very-high-power ships. Heinlein's original use of the term meant a handwavium drive that did direct conversion into energy, but this wouldn't be an engine that even creates exhaust, and isn't how anyone else has used the term. From the very page you linked, modern rule of thumb is that a torchship is simply any ship with a specific drive power of 1 MW/kg or greater. For reference, the Saturn V at full power was around 30 kW/kg.

The description of a drive with only a photon exhaust would also be pretty terrible, too - photons have awful mass, and so you'll get no usefully high acceleration out of something with a photon exhaust no matter how powerful. Modern torch drives are either fusion drives, antimatter-boosted engines that absorb the energy into something much heavier, or a mix of the two. Occasionally insane ideas like the NSWR pop up, or less insane but still quite crazy ideas like Orion, but it's primarily the domain of large fusion devices.

As for the drives being described in the books, there's actually little need to avoid the neutron radiation - they can be controlled, though are more difficult to, and the heat can be cooled off. You'd need very large radiators to do so, yes, but any sane future spaceship would have these anyway, and most of the neutron energy would be directed into heating the fusion fuel, or another expendable that's used to reduce Isp, as you actually don't always want the most excessively high Isp possible once you're beyond the limits of chemical propellants. Lower Isp would increase thrust possible, making it desirable for high-g manoeuvring ships. As a result, the most desirable option is D-D fusion, due to its large abundance - with He3-He3 being a secondary option if some particular design requirement means you do actually have to avoid neutrons, and you can actually create enough to supply your fuel.

[Beowolf]

Yeah, Rick Robinson, came up with a revisionist definition that bizarrely includes a bunch of boring realistic engines as "torchships". But Rick's just a random blogger. I don't know why Nyrath devotes space to him on Atomic Rockets. If you Google for other citations you'll see that nobody else has ever paid that claim any attention.

Reading Robinson's original blog post on the subject, in the first paragraphs he bluntly states he's redefining the term because he wants to play with the idea of torch drives without invoking handwavium. And then he fails completely at it. Torch drives have traditional characteristics that not a single one of Robinson's proposals can even approach. Hell, his designs can't even get within two orders of magnitude! No nuclear salt water rockets, no fusion drives, not even any hypothetical antimatter drives, have anything remotely like the performance of a torchship. There's a great excuse for this: You can't build a torchship that works like a regular reaction drive because every known form of matter would instantly vaporize under that absurd load.

I appear to have the same credentials as Rick Robinson, so I hearby define the term "torchship" by the thing that makes them special: A torchship can make interplanetary runs under constant acceleration high enough to feel like gravity to those inside. Because of that, they always fly brachistochrone paths rather than Hohmann orbits.

So, Heinlein's total-conversion starships: Torchships. Varley's Red Thunder "bubble drive": Torchship with a different name. The realistic constant-acceleration nuclear ion drive from The Martian: Not a torchship, as acceleration isn't enough for astronauts to walk around.

While 50s authors besides Heinlein used the term to mean a total-conversion drive (I found a Poul Anderson before even getting past our "A" shelves), after some reflection I'll grant you total-conversion isn't a necessary part of the definition. It's the way most torch drives I can think of worked. Varley, for example, specifically says that when his bubbles get squeezed hard that's what happens inside. But Spider Robinson's "guru meditation" drive in Variable Star meets my criteria, and it used vacuum energy plus Buddhism.

Corey's Leviathan Wakes is $9.99 for Kindle. The longer this thread goes, the more likely I am to buy a copy. It's sounding fun.

[Aghanim]

The book is really fun though, you should buy it

Torchships are a specific type of drive. It was a staple of good 50s sf, Heinlein in-particular. A torchship's drive is a magical widget that turns matter directly into energy. I'm not familiar with Corey's ships, but my guess is his torch only produces thrust while a fusion reactor makes electricity to run the torch and the rest of the ship.

The ridiculous performance he cites is right in-line with a classic torch drive, too. These ships pull stunts like flying from Earth to Mars at 1G all the way, flipping over halfway to decelerate. One of Heinlein's stories had a torchship making an emergency medical run to Pluto at 10Gs. The trip was maybe 10 days and the story was mostly about how that trip ruined the volunteer pilot.

Does Corey talk much about the exhaust? Within a few hundred km, a torch battleship's most powerful weapon could be its exhaust. It's just photons, but tens of megatons per second of them.

Same site, different page: Atomic Rockets has everything.

Yes, he does talk about the exhaust, and it is considered in the ship design. For short trip they doesn't use their torch drive because of that, they use steam rocket instead

Edited June 28, 2015 by Aghanim

[SomeGuy12]

One question: Why proton-boron instead of helium3-helium3 fusion?

First proton-boron fusion still produces neutron, which carries 0.2% of power produced. Then helium3-helium3 fusion is more energetic, with helium3-helium3 fusion generates 207.5 TJ/kg with exhaust velocity of 6.8%c instead of proton-boron 69.97 TJ/kg with exhaust velocity of 4.5%c.

Then what I think the most important advantage of helium3-helium3 fusion is that we can mine that in gas giants, but mining boron in outer space is harder, as in I cannot find any source that states that we can find boron in outer space. Feel free to correct me though.

My thought was that reducing the neutrons would be a major goal, and I read that proton-boron produces less neutrons. Also, boron is common and cheaply available on Earth, while helium 3 is really hard to get your hands on.

The basic concept for the engine is of course a containment chamber that uses electric fields, magnetic fields, and perfectly reflective surfaces to force all the particles produced by the reaction as well as the light produced back into the center of the fusion reaction. You might have to use mirrors that actually gas or plasma, contained by yet more sets of magnetic of electric fields, since it would be really hard to contain things otherwise. Neutrons cannot be contained this way - you can't reflect them as efficiently as you can reflect light, and magnetic and electric fields do nothing.

The main product of proton-boron fusion are charged particles that you can sap energy from directly with electric fields, directly converting the kinetic energy in the particles to electric current. So your engine would be self-powering and it would also produce enough power to run the rest of your ship and energy weaponry.

[Kibble]

Yeah, Rick Robinson, came up with a revisionist definition that bizarrely includes a bunch of boring realistic engines as "torchships". But Rick's just a random blogger. I don't know why Nyrath devotes space to him on Atomic Rockets.

Rick Robinson is brilliant, no need for the hate.