Effectiveness of Nuclear Fire in Space

It's well understood that torchships are the most powerful spaceships that we know of without inventing new physics or handwavium. Getting around on the back of a nuclear flame is crazy fast. Also, attacking something with a nuclear flame is a no-brainer since that much energy in a directed fashion will melt through the thickest tungsten armor with ease.

But, what about when something is attacking the hot end of the torchship? How effective is the nuclear flame at defeating any and all kinds of attacks?

Attack types under consideration:

• Beam: All Types and wave lengths
• Projectile: Kinetic
• Explosive: Chemical, nuclear and anti-matter

Torchship Specs:

• Magnetic bottle containment
• Continuous thrust
• 4.5 Tera watt drive output
• Wet Mass: 1,000,000 kg
• Dry Mass: 500,000 kg
• Exhaust Velocity: 3,000,000 m/s
• $\Delta$V : 2,000,000 m/s
• Thrust: 290,000 N

(Ship specs shamelessly stolen from Project Rho. Credit where credit is due.)

Out of scope:

• Any attack that doesn't have to go through the drive system. I know that attacking a torchship from any other angle will render it the same as any other spaceship; I'm not interested in that.
• So I can throw some thermodynamics at the problem, what are the mass flow rate and the exhaust velocity? – Joe Kissling May 22 '17 at 14:45
• Do the attacks have to go through the exhaust? Would an attack on the engine from the side be considered in scope? – sphennings May 22 '17 at 14:57
• @RonJohn, $\Delta$V is the capacity of ship to change speed/direction. The ship may only accelerate at 3m/s^2. Just because you have 2M m/s delta-v doesn't mean you use it all at once. – Green May 22 '17 at 15:18
• @RonJohn a change in velocity is a change in velocity. Acceleration requires a time component. I'll prove it to you: if I go from 1 kilometer a second to ten kilometers a second, what is my acceleration? – Joe Kissling May 22 '17 at 15:46
• It may be a poor analogy, but in modern terms, a typical SAM could not down a commercial jetliner by attacking the hot end of an operating engine. That end is made to withstand forces on the same scale as the weapon would explode with, the exhaust stream is sufficiently powerful that the missile itself would be likely to be redirected away from the engine, and if an explosion did occur it would be forced away from the engine by the exhaust before ti could do more than incidental damage. – pojo-guy May 22 '17 at 19:06

You aren't as safe as you think

Other properties of the ship

The combustion chamber need not be made of any sort of imagined material. Conditions in today's rocket nozzles readily exceed the material properties of the nozzle, so measures are taken to protect them. Cooling is the biggest, some use the cryogenic fuels through the nozzles as a preheat, others use a stream of fuel to form a layer between the combustion and the nozzle.

The same could be done with the nuclear fire of a torch ship. The plasma in a test fusion reactor is contained in powerful magnetic fields and kept away from the walls of the container. The same could be done with the torch ship, stand off the reaction from the walls of the chamber, add some radiation shielding and a cooling system and you are all set.

Beam Weapons (To include directed energy)

A beam of charged particles could easily wreak havoc on the nuclear reaction, admittedly it would be challenging to ram through the exhaust stream, but focused enough and it could get through. Adding alpha partilces to a fusion reaction could snuff it out or cause a runaway reaction. Beta Particles could cause issues as well by changing the charge of the plasma. The effectiveness of this depends on the magnitude of the magnetic containment and its polarization. A beam of neutrons could also cause issues, interfering with the reaction and adding a ton of extra energy to the chamber as they decay. Heavier Ions will also interfere with the reaction and do physical damage to the containment chamber.

Pick your wavlength of the laser, and dump plenty of power into it. Depending on the content of the reaction some wavelengths may pass right through it and damage the walls of containment. I'm thinking gamma radion of a magnitude that exceeds the design of the torch ship. Likewise, some wavelengths may be absorbed by the reaction and exceed the design of the ship, I'd guess microwaves. Results may vary, ask your lead engineer if you have questions

Kenetic Weapons

Moving fast enough and your kinetic projectile will be unaffected by the nuclear fire. If the contact with the heat is brief then the projectile won't have time to vaporize/ablate away. I'll see if I can apply some thermodynamics to it later on. Also, even if you manage to vaporize it, now you've got a cloud of vapor with the energy of the projectile moving towards you.

Explosives

Getting them into the chamber is the main issue. Get through the exhaust stream and your payload will do its job.

• XKCD is not relevant. A lightning bolt has no mass. But when your projectile hits something with real mass (or more importantly, impulse) it will be slowed down. Similarly, the laser won't work. The torch flame is highly ionized, i.e. a plasma. Plasma's are exceptionally good at absorbing light. You wouldn't think that, since they don't look black, but with all the free and weakly-bound electrons, there's always an option for a photon to lose its energy. You'd just heat the plasma, which means I can turn the reactor down a bit. You're just saving me some fuel. – MSalters May 23 '17 at 8:13
• @MSalters Of course a lightning bolt has mass, the ionized air has mass. I know well the projectile will be slowed down some, but so are bullets and they do just fine. Plasma is good at absorbing light, but you can always find a wavelength that passes through it, it's the same way that telescopes can see into a nebula. Adding heat to a carefully controlled nuclear reaction is probably not that great for it at all, especially if done asymmetrically. – Joe Kissling May 23 '17 at 13:15

There's a monumental difference between an uncontrolled nuclear explosion and harnessing one for long term propulsion. Controlling such fires is tricky. It's even trickier when some sick fool decided to take an even faster torchship and ram it up into your exhaust!

The fool will certainly not survive the process, but now you have a complicated situation. Your beautifully controlled carefully balance propulsion system now has another nuclear propulsion system crammed up inside it. And, as you melt it with your nuclear fire, it starts to behave less and less like a torchship, and more and more like a nuclear bomb wedged in your tailpipe. Anything that was balanced is now decidedly unbalanced. You really want that part balanced.

And as Munroe put it regarding the exhaust end of your spacecraft, "This end should point toward the ground if you want to go to space. If it starts pointing towards space, you are having a bad problem and you will not go to space today."

• I love that book/poster/comic so much. – Green May 22 '17 at 15:34

Any drive system is about control. A torchship flies on a continuous controlled nuclear reaction.

Probably the simplest way to destroy it is to cut through that control, fire a significant load of fissile material up the pipe contained in such a way as to explode within the constrained area. Pour too much fuel on the fire.

As for how to protect your missile on the way in, use same method as the drive itself is contained within the ship. Anything that can protect the ship against its own drive can protect an incoming missile against the drive. Part two is about balancing the missile drive against the returning force of the ship drive to get it inside but that's a solvable problem.

• I really like this idea but it really depends on being able to protect the fissile material while it's flying up through the high energy output of the engine. We're talking superscience here but I'm not sure such shielding could be downscaled effectively. – sphennings May 22 '17 at 15:57
• I wonder if you even need to get up into the pipe for this to work. I think that throwing a large amount of fissile material close to the exhaust might be sufficient. The exhaust should be hot enough to trigger a nuclear reaction. The nuclear explosion would create heat and pressure at the point of exhaust for the torchship. Given that the exhaust is also the primary "control rod" for the reaction propelling the ship, a sufficient amount of pressure at the exhaust should backup the whole system. 1/2 – A Bailey May 22 '17 at 17:22
• Additionally, any pressure close to the exhaust is going to give the ship something to "push off" of as well. If the pressure is consistent, that would just jolt the ship ahead (at a potentially fatal rate). If the pressure at the exhaust is uneven, then the ship's trajectory would be unpredictable (and in space, that tends to mean dead). 2/2 – A Bailey May 22 '17 at 17:29
• Heat alone will not trigger a fission reaction. Fission bombs are triggered by mechanisms carefully designed to move or compress fissile material into a critical mass. "Carefully designed" because sloppiness will separate the material and prevent the reaction. – WGroleau May 22 '17 at 20:58
• @MSalters, put a bigger engine on your missile, fire it in at a higher speed, as I said, it's a solvable problem. – Separatrix May 23 '17 at 11:27

You're talking about an engine that is outputting 4.5 Terawatts, in a controlled fashion, continuously. That's a 14th of the energy of the atomic bomb dropped over Hiroshima every second. A lightning bolt peaks at around 1 Terawatt, but only lasts for 30 microseconds.

I'm pretty sure anything you throw at the hot part of that won't even register when compared what it has been designed to contain.

In the case of a projectile or explosive it's going to be destroyed long before it gets within range to become dangerous. If it's a laser unless you're using a terawatt laser it's not going to make a difference. That's going to require about a thousand nuclear power plants running continuously to power it.

• What about energy beams? You haven't considered all the attack types the question mentions. – DonyorM May 22 '17 at 15:18
• @DonyorM Unless you're using a terawatt laser it's not going to make a difference. That's going to require about a thousand nuclear power plants running continuously to power it BTW. – sphennings May 22 '17 at 15:48
• Great. I would include a brief blurb in the answer. Some of us don't really know how a laser compares to other types of energy. – DonyorM May 22 '17 at 15:51
• @DonyorM the question assumes a 4.5 Terawatt drive. A 1TW laser is much less outlandish in that scenario. – fectin May 22 '17 at 18:56
• @fectin: No, it's still outlandish. The 4.5 TW drive is horribly inefficient. It's 4.5 TW thermal. But nuclear energy is incredibly energy-dense, so the conversion inefficiency isn't that bad. To make it efficient, you need control over the reaction, and that brings you back to GW-class nuclear power plants to power your laser. In space, even, and that means you have a major cooling problem. 1 GW of nuclear power also means a GW of thermal energy fed through a steam turbine - how are you going to condense all that steam? There's no convenient lake or river at hand. – MSalters May 23 '17 at 8:24

I here propose two exotic type attacks which could go through the nuclear torch and hurt the torch ship. These are suitable only for concept-driven science fiction.

Dark matter fusion poison. I take "torchship" to mean a fusion powered rocket: mass is converted to energy. Dark matter is unaffected by electromagnetic radiation and so a clump of dark matter should be able to traverse the energetic output of the fusion engine.

Does dark matter pack a wallop? Would I feel a lump of dark matter dropped on my toe? I cannot find the answer. Maybe dark matter moves right through normal matter. I read that as far as is known, dark matter cares about gravity and the weak force.

But the cool thing for the torchship (and a scifi endeavor) would be a wad of dark matter interfering with the fusion engine. The weak force mediates fusion reactions. A cannonball of dark particles traversing normal matter and then interfering with fusion via the weak force could poison the reaction. This would be cool in that the result of a successful hit would be that the engine would turn off. Or the people who can shoot the cannonball might not be sure what it will do, and they shoot it anyway. Other outcomes are possible.

How to fire a cannonball made of dark matter? Gravity gun?

Gravity waves. These too should be impervious to the output of the engine and just about everything else. The prospect of generating on demand gravity waves seems daunting but if you can make them (maybe converting one type of electromagnetic radiation into another?) they obey the rules governing other types of EMR. They have energy and can do work, which means to me that their energy should be attenuated when they do work. Could you concentrate them with a lens? In any case: gravity waves can travel up the exhaust. So what if they do? I could imagine that a wave which strongly reduced the distance between particles would accelerate interactions between those particles. Just like flammable vapors under pressure explode more violently, I could imagine that a fusible substrate suddenly compressed would undergo fusion more violently. Conversely, interacting particles suddenly at a distance from each other might reduce their interaction or cease to interact.

Gravity waves this strong might also have local effects on time. Again this would be a fun thing to explore in the writing: the people who can fire gravity waves as a weapon might hope to blow up the torch ship but other things might happen as well.

• While dark matter and gravity waves are real things, the amount of energy required to produce enough of either to have any kind of destructive effect would probably destroy everything else in the same solar system. It's like trying to kill some one with neutrinos, only harder. what-if.xkcd.com/73 – Shufflepants May 22 '17 at 21:03

A magnetic field can mess up the torch containment and cause the engine to wreck itself. So an explosion that’s designed to produce a momentary, incredibly strong, magnetic field could be a weapon.

Free neutrons are very penetrating because they don’t “feel” the electrons as solid barriers like other atoms would, and the nuclei are very tiny targets. A beam of neutrons fired up into the rocket will pass through the plume with high efficiency. Then those that gets through will bombard the rocket mechanism, transmuting elements and damaging it. Or, just the energy released as some of them decay in proximity of the engine parts will cause damage.

Consider a tight beam or dense pellet of antimatter. The rocket exhaust is tenuous, not solid, so only a small part of the antimatter will erode from the passage.

• I am puzzled. Neutrons not interacting with electrons. Neutrons can collide with exhaust particles which can include protons, electrons. neutrons *yes them too) and radiant energy (photons). Either protons or electrons will interact with each other, so perhaps your beam might be a proton or an electron beam instead of a neutron beam RE its interactions with exhaust electrons. Other than that, a neutron beam could be used against a torchship engine. – a4android May 23 '17 at 3:53
• The reason atoms are hard and solid is the pauli exclusion interaction on the bound electrons. Neutrons will feel magnetism, but will otherwise ignore electrons. Hitting a nucleus is a very small target. – JDługosz May 23 '17 at 4:45
• Ah! The magnetic scattering of neutrons makes sense. Calling "Hitting a nucleus is a very small target" is a colossal overestimate. It makes a long line of very's to come close. I was putting out feelers about particle collisions to find out what you might have been thinking about. Your edit improved your answer. – a4android May 23 '17 at 6:05

Since torch drives are made of unobtanium, a sufficient number of unobtanium KE penetrators of sufficient size and velocity, aimed at the drive's thrust chamber, could damage the drive causing explosion.

EDIT: unobtanium has the amazing property of being as strong and temperature resistant as I want it to be. Thus, since it's easier to break things than to build them, unobtanium KE penetrators would naturally be able to break torchships.