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Set in near future Space tourism becomes a thing, the older spaceships have some issues with their nuclear reactors that occasionally suffers from positive void coefficient whereby the saturation of air bubbles in the neutron moderator chamber result in more neutron leakage. Usually many operators would rather throw out the entire reactor core before meltdown occurs hence the core ejection system is born.

Back to present day as people begun switching over to fusion. This type of reactor uses hot plasma instead so that it will hopefully be extinguished when power is interrupted, also many space agency mandates all commercial and private spaceships must switch to fusion if they haven't already.

Question: Why newer spaceships still have core ejection system despite the proliferation of fusion reactors?

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  • $\begingroup$ Why not take it for granted there was no reason to change what didn't actually need changing? If it's a re-fit, slot the new engine into the old bay with no thought of ejection systems. If it's a new-build, make only necessary changes to the original design. By the way, how does any nuclear reactor power a spaceship, please? Isn't the reactor on a world-bound sea-going ship merely a replacement for coal to fire the steam in a simple boiler? $\endgroup$
    – Robbie Goodwin
    Commented Jan 20 at 18:36
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    $\begingroup$ @RobbieGoodwin The good old propellant problem. So difficult to explain away even in fiction that The Expanse basically had to ignore it in otherwise the highly dynamic spacecraft activities required for the story be implausible. $\endgroup$
    – DKNguyen
    Commented Jan 21 at 5:03
  • $\begingroup$ @RobbieGoodwin: actually it is drawing power from fission instead of chemicals, here's 2 ways thermal and electric: heat is to convert liquid into superheated gas for propulsion and as for electric... er it's classified alright I admit I don't know because ion drive is complicated😅 $\endgroup$
    – user6760
    Commented Jan 21 at 5:52
  • $\begingroup$ @DKNguyen: agreed anyway those interested may look up space thermal nuclear propulsion and electric nuclear version on NASA websites🔍 $\endgroup$
    – user6760
    Commented Jan 21 at 5:55
  • $\begingroup$ Actually no, it's drawing energy from wherever you like but converting energy into power and then power into force are very different things. $\endgroup$
    – Robbie Goodwin
    Commented Jan 23 at 23:09

15 Answers 15

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Because paperwork

It is still specifically forbidden to wear a suit of armour in the UK Houses of Parliament, following violence that broke out in the twelfth century. Most theatres in the UK and US still have a huge metal safety curtain to separate the audience from the stage in the event of a fire that must, by regulation, be shown to be working at every performance, even though the majority of fires probably now break out front-of-house rather than rear-of-house.

Humans are often very slow to get their act together to pass safety regulations, but once established, they tend to stick around for a very long time, often way beyond the point where they actually make sense. No one wants to be the one who suggested removing the safety feature that would have saved the hundreds of charismatic vulnerable-group humans who tragically perished in the terrible-but-foreseeable disaster. If your ships have nuclear reactors, and safety regulations require that all nuclear drives have core ejectors fitted, and the cost of continuing to fit core ejectors is not significantly more than the cost of wading through the bureaucratic quagmire to get them de-mandated, and then do the detailed engineering work to design new drives without them, then they'll stick around potentially indefinitely.

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    $\begingroup$ There's a novel in the Star Wars EU I believe in which the escape pod with C-3PO and R2-D2 is noticed by an officer in the star destroyer but not shot down because the officer did not want to do the paperwork on why he shot down an escape pod. $\endgroup$
    – Seggan
    Commented Jan 16 at 17:00
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    $\begingroup$ I mean...it sounds like a silly law, but think about it: In modern times, does it seem to okay to wear a suit of armour in the House of Parliament? The answer is still no. Who's a bigger threat? That dude wielding a pocket knife? Or the guy wearing a suit of armour wielding a spoon? $\endgroup$
    – DKNguyen
    Commented Jan 16 at 19:32
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    $\begingroup$ Especially given that all your competitors have to comply, too, but they don't have to join you in getting the mandate repealed. $\endgroup$
    – Mary
    Commented Jan 16 at 22:12
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    $\begingroup$ In addition, lawmakers are not scientists. If you tried to explain to them the difference between fission and fusion, their eyes would glaze over and they'd start to drool. For them, nuclear = scary so convincing them to repeal the Nuclear Yeet Act would be an impossible task. $\endgroup$
    – bta
    Commented Jan 17 at 2:07
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    $\begingroup$ @Shivers thats a bit like giving someone a ticket for false parking when they just drove through the street. "Yes if we pinpoint the location of my car at a certain time quantum, according to heisenberg we are so unsure about the speed of the car that we have to give you a parking ticket and a speeding ticket at the same time" $\endgroup$
    – datacube
    Commented Jan 18 at 13:18
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Because there is still a small nuclear reactor attached to the core which is used to provide the "start up" energy to get the fusion reactor running innitially or after a powerdown for maintenance. 99% of the time the nuclear reactor isn't running but its mere existance is reason enough to keep the ejection system in place.

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  • $\begingroup$ That's what I was thinking. Say, a failure happens when you're in deep space and you have to shut down the fussion reactor. Now, how do you restart it? It's not like you can produce ultrahot plasma by rubbing two sticks very fast... you still need a really powerful power source to start your fussion reactor, hence the need for a small fission reactor as a starter. $\endgroup$
    – Rekesoft
    Commented Jan 16 at 9:00
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    $\begingroup$ Which is in fact all just an extremely well controlled version of what happens in a hydrogen bomb - fission that triggers fusion. $\endgroup$
    – manassehkatz-Moving 2 Codidact
    Commented Jan 16 at 17:10
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    $\begingroup$ @Rekesoft Deep space failure is a good point, could potentially be an emergency/backup power supply of some kind. as well. Also, I understand the point, but technically you could in fact generate ultrahot plasma by rubbing two sticks very fast, but "very fast" would have to be a significant fraction of c. $\endgroup$
    – reffu
    Commented Jan 16 at 18:04
  • $\begingroup$ This answer is the one that make most actual sense. Although it does require some more handwaving why this APU needs to be a fission reactor. Nuclear power is not really a good choice for something that is only needed seldom and then has to provide a strong burst of power. A simple hydrolox turbine or even batteries would do that job better. $\endgroup$
    – leftaroundabout
    Commented Jan 18 at 20:30
  • $\begingroup$ @leftaroundabout Well a simple answer to that would be that a fission reactor has proven itself in the past to be sufficient to power those ships in case the fusion reactor fails and cannot be repaired. They can probably not power the everything in the newest ships anymore once they started to make use of the abundand energy provided by fusion, but it should suffice for the life support and main thrust to make it to a save harbour as it did in the past. $\endgroup$
    – datacube
    Commented Jan 19 at 7:05
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The core ejection system is now used solely as a mean to separate the ship from the core during maintenance, making it easier for the maintenance crew to access and service it.

The name hasn't changed because of inertia in the conventions, and also because it was (mis)used with the same purpose on older fission core systems.

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Standardization and antitrust legislation

Basically the same reason lots of devices have USB ports for charging, despite having no use for the "serial bus" feature.

Nuclear reactors of any sort are finnicky and dangerous, only very few companies can, and are licensed to, build them. Spaceship manufacturers do not build them themselfes.

To avoid humanity becoming dependent on a monopoly, it has been mandated that all ships must have one or more standardized reactor slots which will accept any reactor (given it is of the correct size, there are a few options) of any manufacturer. Since in the past most reactors have been of the fission type, these reactor slots come with a ejection system.

The system has to be in working order, and will fail annual inspections if not, because the owner of the ship could anytime switch the reactor out for a fission one.

Bonus reason

There is, very limited, availability of antimatter reactors fitting the same slots, probably strictly for some military vessels. For these, an ejection system is a VERY good idea.

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    $\begingroup$ Yes it would cost more, and take years to have your new non-ejection system design approved by the regulators than to just stick with the existing pre-approved design $\endgroup$
    – crobar
    Commented Jan 17 at 15:55
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Retrofitting

many space agency mandates all commercial and private spaceships must switch to fusion if they haven't already.

The fusion spaceships still have core ejection systems because they used to be fission spaceships. When they upgraded, they simple ejected the core, took off the fission reactor, strapped on a fusion reactor, and reinserted the new core.

The ejection mechanism stayed in because it was cheaper to leave it than to go mucking about further.

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Redundancy and Risk.

In engineering, it is normal to have a double or even triple redundant system nowadays in order to mitigate any risks of failure.

It is often a requirement in engineering design to perform a Safety Mitigation Assessment (or similar-worded document) where all risks are identified, no matter how small, their possible impact, and their solutions designed and implemented.

If the main power system is a major component of a vessel, centrally located close to other critical infrastructure, and has the ability to affect function and life, then it would be hard to argue against implementing a pre-existing safety feature even if the frequency of occurrence is now lessened due to the nature of the failure being catastrophic if it occurs.

This is why you get a 'ratcheting' up of safety requirements over time, and it is much easier to add layers of safety and redundancy than to remove them.

If an engineer removes a previously existent safety system, and the system subsequently fails even after improvements, the engineer will be under much greater scrutiny. Most would simply retain previous safety systems because of the greater risk to the designer if they advocated for a safety feature to be removed.

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Because a fusion reactor will become quite radioactive over time and an automated dump into deep space would be a safe and efficient way to discharge the unwanted reactor containment system for replacement.

A biproduct of fission reactions is neutrons, and neutron radiation is the only common radiation process that actually makes other objects radioactive. As the fusion system works, producing waste that is far, far less hazardous than a typical fission system (likely helium-4, so totally nonradioactive) it is also going to produce significant quantities of neutrons. They will slowly but surely make the containment system for the reactor more and more radioactive, as well as degrade its mechanical properties over time by transmuting the elements it contains (basically turning it into element soup instead of whatever alloy it was originally made of).

If the core dump includes not just the contents of the core but the walls that contain those contents, then deep space disposal of a highly radioactive fusion reactor containment system via dumping would be easier than having to remove it using waldoes or someone in a heavily shielded suit. That combined with the fact that the tech already exists in your world for fission to make it work - seems like a no brainer for when the reactor walls are getting fragile due to neutron absorption.

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Time

Fission reactors have been around for centuries. Fusion things are new.

Take a page from the real world. In the US there are strict regulations about what kinda engine you can put cars. Ever since the mid or late 90's every new car in California has had an electronic injection system. You can still find plenty of cars that have carburators instead - they are no longer made, but enough were made last century that there are still plenty around. They even become collector items here and there.

Pure luddism

Roombas are a thing in 2024, and they are expletive cheap. People still use brooms. That's just how humans are.

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    $\begingroup$ But brooms can double as a weapon. $\endgroup$
    – DKNguyen
    Commented Jan 18 at 2:22
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    $\begingroup$ @DKNguyen A competent discus thrower could inflict plenty of damage with a Roomba :) $\endgroup$
    – Kuba hasn't forgotten Monica
    Commented Jan 18 at 19:02
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    $\begingroup$ Roombas simply aren't sturdy enough to replace the traditional uses of a broom. When my upstairs neighbour was noisy, I bashed my Roomba against the ceiling to get him to be more quiet, and it never worked right after that. $\endgroup$
    – cjs
    Commented Jan 19 at 4:17
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    $\begingroup$ @Kubahasn'tforgottenMonica Impractical to pursue a mouse in such a matter. High skill and high potential of collateral damage. $\endgroup$
    – DKNguyen
    Commented Jan 19 at 22:46
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Maintenance

On top of safety concerns with the unexpected radiation, there's the consideration of when the ship takes damage.

Whether it be from micrometeorites, or cosmic rays, or the classical weapon firing that happens in combat in space...you're going to need to take it in for repairs eventually.

When you dock it in the maintenance bay, you generally don't want to accidentally spark the fusion reactor while working on the ship - especially if it ends up turning out to be a nuclear/fission reactor instead. So to prevent causing damage to it, or to be able to replace it if the fusion reactor is the issue, your maintenance tech simply asks you to engage the brakes and to "Pop the reactor out. ".

Essentially,this becomes pretty second nature, sort of like "popping the hood" of a car these days to look at the internals - the main difference is that the ship maintenance crew may just take the reactor entirely out and place it safely on the ground as they into the inside and internals of the ship to check for issues.

Once they're done, they just lift it back up and put it back in, and then tell you to start up the ship to check that everything is back in place as expected.

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Fusion isn't necessarily safe if something goes wrong. The contents of a fusion reactor chamber are under immense pressures. A containment failure doesn't necessarily mean a radioactive leak, but it can mean a big honking explosion, or your critically-hot plasma spraying out uncontrollably. Launching that thing away from the ship before it goes completely pear-shaped is still the easiest way to minimize the risk that somebody gets hurt or that the rest of your facility gets damaged. Also, it minimizes the amount of work needed to get your ship back up and running - just plug in a new core, no need to rebuild the equipment that was melted by hot plasma.

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    $\begingroup$ That wouldn't work for near-future hard SciFi. While it is true that the plasma itself is under high pressure, all torus designs currently conceivable hold very little plasma in magnetic fields in an otherwise vacuumed chamber. Losing magnetic containment wouldn't result in "a big honking explosion", it would merely result in the reaction stopping instantaneously and the plasma dispersing in the chamber. (Ref. JET achieving 60 seconds of plasma fusion for visuals of plasma in the chamber.) $\endgroup$
    – DevSolar
    Commented Jan 17 at 12:07
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    $\begingroup$ The answer could still be salvaged. A perfectly reasonable way to dump that hot plasma is to eject the whole core, let that core dump its plasma into deep space, and then retrieve the core. You already have the core ejector, why redesign the whole ship to create emergency plasma exhaust vents? $\endgroup$
    – MSalters
    Commented Jan 18 at 16:32
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    $\begingroup$ It's not a hard-science question so don't limit your scope to just that. Also, don't assume that your chamber will be intact in the event of a failure. A chamber leak fills the vacuum with air, which gets superheated by the hot plasma and rapidly expands, turning what's left of the chamber into flying shrapnel. We don't know the design or size of the reactor, so the precise failure modes are unknown. The only safe way to handle any potential failure mode is to get it as far away as possible. $\endgroup$
    – bta
    Commented Jan 18 at 17:58
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Coil Quench

Magnetically contained fusion reactors use superconducting magnets to get the needed field strength. If the magnet accidentally goes over the critical temperature it has a runaway feedback of resistance and heat. This converts all the energy in it to heat, rapidly boiling off the cryogenic coolant. With enough coolant this looks like boiler explosion. Being able to eject the core and coolant if quench is about to occur lets that not happen in your engine room.

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The stated rationale doesn't make sense. There are numerous reactor designs that have negative void coefficients or for which the concept of void coefficient is inapplicable (gas cooled reactors, for example). The types of reactors that would be subject to positive void coefficients are unlikely to be used in spacecraft anyway (being very heavy, for one) and there are numerous reactor designs that simply can't melt down.

I would suggest something else...for example, the power section is separable (reversibly) to allow safe docking, loading/unloading, and maintenance of the rest of the ship, due to the reactor itself using shadow shields that don't provide any protection in directions away from the ship. This is how real space nuclear systems are expected to be designed, since it would take too much mass to fully enclose a reactor in shielding.

If a ship runs into trouble, they don't want any rescuers to have to work around a dangerously radioactive power section (which would make it dangerous just to approach for docking), so they jettison it while they can and subsist on other power sources. Fusion reactors produce less radiation, but they still produce some, and stations and shipyards are already equipped to deal with the reactor sections, so they keep being built that way. This additionally allows the ship to be put back in service quickly with a new power section while the old one is refueled or scrapped.

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Taking some inspiration from Joshua Dalzelle's The Black Fleet Saga, specifically Jackson Wolfe's perspective as a fleet captain in an age of military & political complacency:

The lawyers and politicians in charge of legislation and insurance don't want to rock the boat and change the dogma that's "worked" for centuries, they may not understand or care about the technical differences between old and new ship designs.

Meanwhile, the ship captains and engineers with the actual know-how aren't in a position to change anything (and they may not care either). Every profession has dead-weight employees who try to do as little as possible to maintain their employment: if there's no incentive for a minimum wage space-tourism-ship-engineer to save corporate dollars, then why would they?

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Because it works and it's easier than dismantling the ship to get to the fusion reactor from the outside whenever a major reactor overhaul/replacement is required.

At some point a ship is going to have to dock for a mid-life fusion core replacement or service. When that time comes you can;

A) Waste time and money having the dockside engineers dismantle a section of your hull and then work 'inwards' to get to fusion reactor itself; or

B) Once docked and hooked up to the repair stations power and life support systems (and once the engineers are ready initiate a reactor core 'ejection') all interlinks between the ship proper and its fusion reactor are disconnected internally and then the core is 'ejected'. Not as violently or quickly as the old fission models were ejected mind you because that is not required.

This is not a high speed, life or death emergency like an 'old school' fission core meltdown would be. Instead in this case 'ejection' simply means an internal mag rail system gently pushes the core rearwards out of the ship, like a chicken laying an egg - strait into the mechanical arms of the docking port's waiting capture cradle. (Reverse process to install refurbished/new reactor.)

Long story short? Its still called 'ejection' because that's what happens but it's not the same procedure and not conducted for the same reasons as a fission core ejection was or is. Spacers being spacers however just stick with the terminologies they know.

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Because the external requirements of the two power systems are close enough that ships are designed to accommodate both. Even though you wouldn't strictly need the ejection system for a fusion power plant, you can also use a fission power plant, where it's a necessary support feature. Spaceships aren't something designed to be replaced every half a year like US cars - they're expected to last for decades, with hard-working industrial ships easily lasting a century.

And of course, there are actually reasons why ejecting the fusion core would make just as much sense. Sure, you don't get a runaway fusion reaction - maintaining fusion is hard as is; but that doesn't mean nothing happens when problems arise. For example, depending on the way you harness the power of the fusion reaction, you are probably generating huge amounts of waste heat. If your coolant system fails, it doesn't matter that the fusion reactor itself isn't going to continue producing heat - the heat is already there and it can't be properly disposed of, endangering the whole ship. The same way, you're probably using very powerful magnetic fields to contain the fusion reaction and possibly capture the energy from it - that system going haywire can do large amounts of damage to the ship or crew. Nuclear fusion still likely causes a lot of residual radiation (e.g. from neutron activation); if the containment is in danger of failing (e.g. the cooling failed and now the barriers are melting or on fire), you're putting the ship, the payload and the crew in danger. Dump it to get it far from the ship.

For bonus points, these ships aren't disposable, and require considerable maintenance. You probably already need to have a system to "easily" replace the reactor anyway - for regular maintenance as well as future proofing (you want that shiny new efficient reactor in there when you can afford it). Many of the mechanisms for a core ejection are probably already there.

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