7
$\begingroup$

Nuclear fission rocket is becoming obsolete and is superseded by the more efficient antimatter engine, in fact there will be no pollution and the efficiency is around 100%.

I'm wondering if in a space battle that takes place within the solar system is there any place for nuclear powered spaceship or they belong in space museums?

$\endgroup$
8
$\begingroup$

In wartime, quantity is a quality of its own. You do not need ideal weapon during war time. You just need more of it than your enemy.

Given that:

  • Fission fuel is easier to obtain than antimatter as you can dig it on homeworld or other celestial bodies
  • There is no natural antimatter source, at sufficient quantity, in solar system
  • Fission fuel is easier/cheaper to store than antimatter, as it does not explode when touched nor requires fancy EM cages
  • Fission process, and its failure modes, is well known
  • Ships explode from enemy fire more often than from reactor failure
  • low energy/time cost to start up reactor, just move those rods a bit and you got juice

You should be able to build, and fuel, more fission powered ships which will give you an edge in combat.

However I'd suggest adding fusion to your list of reactors, as it should be a stepping stone between fission and antimatter annihilation.

  • multi purpose fuel. All you need is (heavy) water to provide oxygen, reactor fuel, reaction mass
  • Hydrogen is very easy to obtain (for spacefaring society) as you can get it from oceans, atmosphere, ice asteroids, gas giants, or even from solar wind
  • fuel can be shared with antimatter (mother)ships
  • clean energy, when compared to fission
  • you need water anyway
| improve this answer | |
$\endgroup$
  • 2
    $\begingroup$ Durability is a factor to bear in mind. If we can get shot in the aft and still have a functional ship, that is a big plus. $\endgroup$ – Gustavo Dec 12 '19 at 13:57
  • $\begingroup$ @Gustavo I was thinking about reactors, rather than NERV style rockets, while writing my answer. But I'd say it does not change much about antimatter tendency to annihilate when touched. $\endgroup$ – PTwr Dec 12 '19 at 14:12
4
$\begingroup$

(A couple options -- feel free to use either or both!)

Stability

Antimatter is really finicky about how it's contained. High-G combat maneuvers and delicate magnetic bottles are not friends, and even a minor bit of battle damage to the bottle is an all-but-guaranteed loss of ship with all hands. Fission is much easier to keep stable in combat, and containment wobbles or damage have much less catastrophic failure modes.

Expense

Antimatter drives are far more expensive compared to fission drives. So they're fine for something like a capital ship, carrier, or large transport, but smaller, short-range craft like tenders, shuttles, and fighters, and especially expendable craft like drones or missiles, still use much cheaper fission drives.

| improve this answer | |
$\endgroup$
4
$\begingroup$

Yes, I think that there might be no nuclear fission powered ships, but nuclear fusion ships will still be there. Or maybe there will be hybrid ships that can run on fusion or annihilation.

The reasons:

  • There are no known natural occurrences of antimatter (at least not in amounts that could be useful for spaceships), so you can't harvest antimatter, but have to produce it. The production will probably run on solar energy in a Dyson sphere or by using a fusion reactor
  • On the other hand, hydrogen that is needed for fusion reactors is the most common element in space, so you can easily siphon a star to fill up the fuel tanks
  • Antimatter is super dangerous. You might not want to transport your antimatter inside of your spaceship. You only need to have a failure in one of the magnetic traps or a shock that causes the antimatter to touch normal matter and your ship will be blown up. So the best strategy might be to have an unmanned reactor ship that contains antimatter and flies in a safe distance to your main ship. That ship somehow produces hydrogen and can send probes to the main ship, so the main ship can run on much safer fusion energy.
| improve this answer | |
$\endgroup$
  • 2
    $\begingroup$ Worth noting that natural fusion fuels are less common than you might think... deuterium makes up about 1/6500th of hydrogen in our oceans, and its cosmic abundance might be lower... somewhere between 1/5000th and 1/500000th. Helium-3 is largely locked up in very deep gas giant gravity wells, and suitable lithium and boron isotopes are about a billion times rarer than hydrogen. $\endgroup$ – Starfish Prime Dec 12 '19 at 10:04
  • $\begingroup$ That's a good point. So it depends on future technology, which type of reactor our space ships will use. Maybe future fusion reactors are able to use proton-proton reactions, so they can really use "standard hydrogenium" (protium?). Or we will be able to build probes that are able to withstand the pressure in gas giants. My suggesion of "siphoning stars" will also need better technologies. As far as I know, uranium and plutonium are also pretty rare. Especially the isotopes that are needed for nuclear reactors. $\endgroup$ – Dorian Dec 12 '19 at 10:46
  • $\begingroup$ Proton-proton fusion will almost certainly remain impractical forever (because it relies on diproton decay forming deuterium, which is a 1 in 10^28 chance per fusion, making it many, many billions of times less useful than literally any other nuclear reaction). Pulling fuel out of gas giants isn't about pressure, but about the energy demands of hauling mass out of a very deep, very strong gravity well meaning that you might not get enough energy out of the fusion reaction to make it economically sensible. $\endgroup$ – Starfish Prime Dec 12 '19 at 10:50
3
$\begingroup$

Nuclear fission rocket is becoming obsolete and is superseded by the more efficient antimatter engine, in fact there will be no pollution and the efficiency is around 100%.

Unfortunately no.

Firstly, you can you create your antimatter. The pesky law of conservation of baryon number means that in most cases your maximum efficiency of antimatter creation will be 50%, because for every antiparticle your energy-to-mass system coughs up, you'll also get a corresponding normal particle. In practice, even a 1% efficiency would be astonishingly high. You'll need a vast and expensive infrastructure for antimatter synthesis.

Secondly, a 100% efficient conversion of energy release from annihilation into kinetic energy of exhaust products is basically impractical, and not just becasue thermodynamics hates you. About 33% of the annihilation energy will be in the form of neutral pions which will almost immediately decay into highly penetrating gamma rays. It will be Quite Difficult in most designs of antimatter engine to absorb those gamma rays into your reaction mass and have a high thrust engine, and in any design it will be impossible to both absorb all the gamma rays and have a high specific impulse (roughly: fuel economy). You'll get at most ~80% efficiency for a solid core antimatter engine, which will have a performance and Isp equivalent to a solid core nuclear rocket.

Thirdly, whilst your antimatter engine is running it will be kicking out, at a minimum, a lot of gamma rays. More complex designs with higher exhaust velocity will also spit out various kinds of interesting and unstable light particles which will themselves cause some issues but also eventually decay to gamma rays. Once the engine is turned off there will be a lot less residual radiation, but there will be some. A single antiparticle hitting a large nucleus (such as the tungsten core of a solid core antimatter rocket) will not annihilate the entire atom, but will transmute it and maybe even fission it. The resulting nuclides are not guaranteed to be stable!

It is at least a lot more plausible (and an awful lot safer) to use a solid-core antimatter rocket to lift off from Earth than to use a nuclear rocket of any kind. You need so little antimatter for the job that even a worse-case catastrophic accident will be quite non-destructive and there will be no fallout to be concerned about.

I'm wondering if in a space battle that takes place within the solar system is there any place for nuclear powered spaceship or they belong in space museums?

Nuclear rockets are a lot less versatile... not only are they more hazardous to maintain, but they can't be trivially turned on and off at will. This is fine for civilian purposes (where you will do be doing a couple of orbital injection burns at the start and end of your journey), or if you are making a torpedo, but it is slightly more inconvenient for a warship.

Their fuel on the other hand is almost certainly cheaper to obtain and simpler to store and a lot safer to be around once high-power weaponry starts going off in the neighbourhood.

The efficiency of antimatter is slightly less than you might think. It is up to you to determine the relative availability of antimatter vs fission fuel, which will ultimately tip the balance in favour of whichever outcome is best for your story, and we can't determine that for you!

| improve this answer | |
$\endgroup$
1
$\begingroup$

Well, someone during WW2 used bow and a claymore

John Malcolm Thorpe Fleming Churchill, DSO & Bar, MC & Bar (16 September 1906 – 8 March 1996), was a British Army officer who fought in the Second World War with a longbow, bagpipes, and a Scottish broadsword. Nicknamed "Fighting Jack Churchill" and "Mad Jack", he was known for the motto: "Any officer who goes into action without his sword is improperly dressed."

In May 1940, Churchill and some of his men ambushed a German patrol near L'Épinette (near Richebourg, Pas-de-Calais). Churchill gave the signal to attack by raising his claymore.

Though not a large scale usage, someone can be nut brave enough to stitch to the good old way of fighting just for the sake of preserving traditions.

| improve this answer | |
$\endgroup$
1
$\begingroup$

Check this link for a fusion-powered spaceship that could be ready in the next decade. The Direct Fusion Drive (DFD) engine could take flight for the first time in 2028 or so. The minivan-size DFD could get a 10,000 kilogram robotic spacecraft to Saturn in just two years, or all the way out to Pluto within five years of launch.

Such a spacecraft would be a far better alternative than a fission-powered craft, and it is less dangerous than an antimatter drive, where storing the antimatter safely is a major issue. It uses deuterium and helium-3, and while the latter is pretty rare on Earth, it is fairly abundant in space, e.g. on the surface of the Moon, and it can be produced in fission reactors. The reactor/engine is fairly compact, as the reactor chamber can't be more than ca. 1.5 m in diameter. Because of this restriction in size, you will need several engines on a single spacecraft (6 is the estimate for a manned mission to Mars).

| improve this answer | |
$\endgroup$
1
$\begingroup$

Antimatter will almost certainly not replace fusion because the fuel is simply too expensive to synthesize despite the efficiency. What is probably a better idea is antimatter induced fusion, if fusion power is too hard to make work otherwise.

While antimatter is 259 times as efficient as Detuerium-Helium-3, and while antimatter rockets have a theoretical exhaust velocity that is light speed, meaning that this is the upper limit on delta-v (rocket efficiency is mostly a function of exhaust velocity, with about double effective exhaust velocity a good practical limit for rockets when dealing with lower velocities not factoring relativity into account), they are not nearly as efficient as they seem.

Let's just assume that we can handwave over the problems and have a kind of crystalline magnetic bottle of anti-hydrogen (it will still explode if it ever loses power, so hope you have good batteries). Even with this, there are two big problems you can't get around.

The first is that 40 percent of the energy from antimatter becomes gamma rays, which are hard to harness and require heavy shielding. If you then tried to harness the antimatter reaction to create a plasma rocket, you would reduce your effective exhaust velocity down to 30% of light speed because of inefficiencies in magnetic nozzles. Your best bet with antimatter is actually to create a photon rocket, also known as a flashlight drive because it really is just a beam of light. This ups your exhaust velocity up to about 50%.

So, while it is possible to build an antimatter rocket, it will never have the cost efficiency of fusion power because it doesn't actually generate energy, it only stores it. Because there are no known natural sources of antimatter, it must be created artificially. It currently takes ten million times as much energy to create a unit of antimatter as that unit of energy is worth. Even with something like solar power stations around Mercury, the energy will almost certainly never be as cheap as the alternative of fusion.

So, if you're talking about using external energy, I'd say there is a better approach than antimatter. A better use of the energy is to create beam rider spacecraft. Either laser or magnetic/electric sails are a better option. Lasers are probably easier to make and more efficient over distance, but are also much more effective as weapons(known as the Kzinti lesson), which makes it easier to sell designing shorter ranged microwave installations for magnetic sails.

| improve this answer | |
$\endgroup$
0
$\begingroup$

A space propulsion system doesn't have an efficiency, it has a certain amount of thrust, and it can produce a certain amount of deltaV, and for a fixed power level you can trade of deltaV for more thrust.

This means even with antimatter powered spacecraft, you can still build defensive nuclear craft that may not match them in operational range, but can produce the same amount of maneuvering thrust in actual combat. Alternatively, nuclear drives may be a cheaper or more compact propulsion system for range limited expendable munitions.

Further, for any real nuclear drive (that includes fission, fusion and antimatter) the primary issue is managing the heat, so the higher energy density of the fuel may not be that much of an advantage.

| improve this answer | |
$\endgroup$
  • $\begingroup$ It might not have an efficiency, but pretty much every stage of the process from acquiring the raw materials used to make fuel or reaction mass to the nozzle that kicks the exhaust out the back will have some efficiency <1. $\endgroup$ – Starfish Prime Dec 12 '19 at 10:06
  • 1
    $\begingroup$ What meant is that you cannot characterize a drive with a single efficiency number $\endgroup$ – Whitecold Dec 12 '19 at 11:27
  • $\begingroup$ Sure you can: en.wikipedia.org/wiki/Specific_impulse $\endgroup$ – Salda007 Dec 13 '19 at 0:53
  • 1
    $\begingroup$ @Salda007 That is wrong. Specific Impulse is not an efficiency, since it is not an energy. Also, chemical drives are usually very efficient in turning all their energy into thrust, while high-specific impulse drives with their low mass flow usually cannot employ regenerative cooling, and waste a lot of energy as heat. $\endgroup$ – Whitecold Dec 13 '19 at 8:19
0
$\begingroup$

Nonsymmetrical warfare.

When nationstates go at it hammer and tongs, they use the most modern and advanced ships they can build or buy.

When the Federation battles the Pirate Loonies, however, the latter can't afford antimatter ships...but old, leaking, space-rusted fission ships are just the ticket. In fact, they refit fleets of the old monsters with autocontrol circuits, remove the shielding, and send them on suicide missions, irradiating everything they get close to before the self-destruct cycle.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.