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Inspired by this question, and this article, activating self-destruct mechanism disengage coolant system for the ship's main reactor, which then increases the core temperature and triggers explosion.

However, I find it strange for such sequence to trigger an accurate 10 seconds countdown, especially because destabilizing core does not mean it will explode exactly 10 seconds after the sequence is initiated. It can be on the 8th second, or 12th second.

So, is it plausible to have a fixed (for example, 10 s) countdown for self-destruct mechanism like this? If not, why bother displaying/announcing the countdown (so people will panic)?

Note: For this question, I rule out the possibility of forced destruction, like igniting the core when the countdown reaches zero. You can pick the technology for the core, but it certainly will explode when the sequence is activated.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Tim B Sep 27 '17 at 20:57

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Yes, it is entirely plausible. But the question is not.

The concept of scuttling is not exactly new, the most (in)famous example being Cortés. Scuttling is a function that is sometimes worked into the design and the operating procedures of a ship.

Heck, we even have a scuttling function on spaceships today.

For example:

Designing the scuttling procedure and mechanism is an engineering task. And no engineer worth their degree would ever design a scuttling procedure that happens willy-nilly by being the subject to random factors. Yet for some mysterious and to me entirely unknown reason, the question and the majority of answers assumes that a future space ship with a scuttling-function would somehow be similar to a contemporary ground based nuclear power reactor without such a function. I have to say: this is a most unfounded and entirely unnecessary — not to mention unrealistic — assumption.

Scuttling would — by necessity — be:

  1. Controlled, that is to say it happens only if the ship's command structure says it will happen.
  2. Deterministic; it happens only when and how the ship's command structure says it will happen, not sooner, nor later, not more violent, not any less.

These are the principles the architects and engineers of the ship will work with. They will not deliver something that happens at random since there is no reason for it, and it would be really poor engineering. And if the normal physical processes of the ship cannot ensure that the ship will destruct at the right time, then they will build in extra mechanisms that help the process along.

So if the customer would say:

I rule out the possibility of forced destruction

...then I — as an engineer(*) — would cross my arms, look them straight in the eyes, and say: No... I will not design your scuttling function with such a silly and contrived design restraint. Either you let me do my job properly or go find yourself another engineer. Good luck with that...

So to summarize: the assumptions of the question — that scuttling would be left to random uncontrollable processes — are by any and all engineering principles faulty. Scuttling will be made controlled and deterministic, and the necessary steps to make it happen will be taken. Anything else would simply be incompetent engineering, and no one will want to go into space on a ship made by incompetent engineers. Nor would anyone allow management to interfere with the construction of the ship in such a way.

....because we know what happens when management adds silly requirements to a ship project.

(*) Yes, I am an engineer by degree and trade

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    $\begingroup$ I enjoy reading that last part. Definitely +1 from me. $\endgroup$ – Vylix Sep 26 '17 at 8:00
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    $\begingroup$ This answer is brill - I'd add that a scuttling process is almost certain to be included in any large spaceships in future - after all, if they're capable of travelling fast enough to make their use worthwhile, then they're also a huge risk to static installations like towns, cities, space stations. Scuttling charges allow for destroying the craft prior to serious damage being done should there be a major incident requiring it - better to nuke a billion dollar craft than lose a billion lives. $\endgroup$ – Miller86 Sep 26 '17 at 8:50
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    $\begingroup$ @Miller86 Ah, good point. All the more incentive to make the scuttling deterministic since a twenty minute discrepancy between the intended and actual actual moment of destruction is the difference between a Rapid Scheduled Disassembly and a planetary catastrophe with millions / billions dead. $\endgroup$ – MichaelK Sep 26 '17 at 8:55
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    $\begingroup$ I think there is a 3rd thing that scuttling should be, which is complete. Blowing up a ship to prevent a crash is only so helpful if large chunks of the ship are still going in basically the same direction, but now at a higher, uncontrollable velocity. This is also important if the ship is blown up to prevent use/recovery by an enemy force. $\endgroup$ – IllusiveBrian Sep 26 '17 at 16:17
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    $\begingroup$ @IllusiveBrian True. So "deterministic" does not only concern time but also mode of destruction, which can vary wildly between "Salvageable Derelict" to "In yo' FACE EmmEffers!!!" $\endgroup$ – MichaelK Sep 26 '17 at 16:21
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With the sorts of self-destruct mechanisms we see portrayed cinematically, there are two different sorts of countdown: time to override the self-destruct command, and time to reach a safe distance to escape the effects of the self-destruct system.

In the first case, the mechanism triggers at the end of the countdown; if you override the self-destruct command in that time, nothing happens. That can, of course, be timed very precisely.

The time to reach a safe distance may be less definite.

Obviously, if you've got no way to override the self-destruct command, and there's an explosion instantly when the mechanism triggers, then there's no distinction between the two types of countdown, and it can be very precise.

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Disabling the cooling should not trigger a meltdown or explosion on any kind of reactor. No engineer would design a reactor that catastrophically fails in 10 seconds if the coolant is cut off.

So you have a typical 1 GW fission reactor, that means you generate 10 GJ in 10 seconds worth of electricity and perhaps another 20 GJ in heat, so convert it all to heat and the have 30 GJ -- The energy content of almost 5 barrels of oil. No sane design would permit such a small amount of energy to destroy a reactor. Whether fusion or fission, 1 GW reactors will have a lot of material and 5 barrels of oil worth of energy could certainly heat things up, but they won't be a self-destruct.

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  • $\begingroup$ The coolant system is obviously not the only system affected when the sequence is started, however I like that first part very much. $\endgroup$ – Vylix Sep 25 '17 at 17:54
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    $\begingroup$ You might want to talk to Japan about that. Loss of power to the damper rods and the cooling is what caused that reactor problem. $\endgroup$ – ShadoCat Sep 25 '17 at 19:27
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    $\begingroup$ @ShadoCat it was much longer than 10s, and no one expected loss of power to auxiliary generators — it was, after all, a stationary plant. And they underestimated wave height by 3m in area where everyone believed 10m is overestimation. Thus, author is right that no engineer would design such a thing. Fukushima guys designed plant that runs separate generators when main cooling power fails. Rest is just a tragic mistake caused by lack of knowledge, justified by data from previous years. $\endgroup$ – Mołot Sep 25 '17 at 20:11
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    $\begingroup$ "Disabling the cooling should not trigger a meltdown or explosion on any kind of reactor." Certain types of reactors (like the RBMK at Chernobyl) will explode if they lose coolant for too long. Modern designs will melt down relatively safely. While both will probably take longer than 10 seconds, and there are other active or passive safeties in place, cooling is critical for fission reactors to continue being reactors. $\endgroup$ – Carl Kevinson Sep 25 '17 at 20:20
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    $\begingroup$ Fukishima had problems with leaking radiation- big difference between what happened there and a massive explosion that would destroy a starship $\endgroup$ – Paul Becotte Sep 26 '17 at 12:30
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So, you need to self destruct your ship at a certain time. But you don't want to rig it with explosives and you don't have an anti-matter reactor, so you can't just turn off containment and have it go BOOM. You need to design a way for your regular ship's system to serve double duty as a reliably timed self destruct.

One option is for your reactor to have no upper bounds as to how much energy it can generate/fuel it can use. You set your self destruct timer and when it reaches zero the computer sets your rector from 100% power output to 100,000% in a nanosecond. It uses a decade worth of fuel instantly and none of the safeties come even close to being able to manage the power. KA-BOOM.

That's your best bet. But what happens if your technology doesn't allow you to dump arbitrary amounts of fuel into your reactor at a moment's notice? Maybe flooding the reaction chamber with that much fuel that quickly 'stalls' out your engine?

Then we need to examine what a (conventional) bomb is. It is a very strong container. When the bomb's trigger goes off, the powder burns up quickly creating a bunch of heat and pressure. Eventually this pressure becomes too great for the container and it ruptures, releasing all of the stored energy in a big boom. If the container is too strong it won't rupture, and no boom. If it is too weak it will rupture too early with no pressure inside, no boom. If you build the pressure slowly then the container will rupture when it just barely has too much pressure, that pressure will be released then there won't be enough pressure left over to make the big boom.

It is a balancing act. And as we went over, you are limited by the fact that your engine design can only build pressure slowly. So if you can only build up heat and pressure slowly, how do you get an explosion?

I'd suggest that you do it backwards. Instead of creating a strong container then building explosive levels of heat and pressure within it, create massive amounts of heat and pressure, then 'shove' it into a container with the self-destruct.

Imagine instead of turning off the cooling systems, then self destruct timer turns the cooling to max. Slowly you ramp up the reactor. After 5 minutes the reactor is in dire straits, and the cooling systems can barely keep up. Then, when the timer hits zero, All the cooling is shut off. Not just shut off, thrown into reverse, maybe. Perhaps there are explosive bolts in the cooling system that will allow it to instantly seal every system the removes heat and pressure from the reactor.

This won't allow perfect nano-second timing the way shutting off anti-matter containment would, but it would probably get fairly close.

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    $\begingroup$ "when it reaches zero the computer sets your rector from 100% power output to 100,000% in a nanosecond" That's not really how it tends to work, though; it's the same as when people ask "is it safe to use my 15-volt 3-amp power supply for this device which needs 15 volts 1 amp?" -- a properly functioning device won't suddenly start drawing more power than it needs. (A fixed current power supply is a somewhat different matter, but those would present... hurdles in a situation like this.) Also note that contemporary fission reactors are slow to adjust; hence used primarily for base power. $\endgroup$ – a CVn Sep 26 '17 at 7:17
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    $\begingroup$ @MichaelKjörling a properly functioning device won't suddenly start drawing more power than it needs Generally speaking, a 'properly functioning' device is one that is designed not to self destruct. That's not the kind of device we're talking about here :) $\endgroup$ – Shane Sep 26 '17 at 13:59
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    $\begingroup$ point of language - wrt fission reactors "critical" is the normal state of affairs, it simply means the reaction is producing the same number of neutrons it is consuming. A super-critical reactor is ramping up, a sub-critical one is backing off. $\endgroup$ – Joseph Rogers Sep 26 '17 at 16:58
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    $\begingroup$ on your point about traditional gunpowder bombs: if you have a reactor that works like conventional rocket's combustion chamber but w/ some kind of nuclear reaction instead of chemical combustion ('plausible' only w/ semi-indestructible unobtanium lining or a radically new way to use magnets, but hey..) you could have it able to constrict flow more for higher exhaust velocity at hi speed cruise, so self destruct could just cut power, pinch all the way shut, then full power. Which would be precisely timable $\endgroup$ – Nathan Smith Sep 26 '17 at 17:06
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    $\begingroup$ @Shane I felt a bit pedantic saying it tbh because of course it's commonly used and understood the way you used it, it just really grates with anyone who knows much about nuclear reactors if characters who should have an understanding of nuclear physics use it that way $\endgroup$ – Joseph Rogers Sep 26 '17 at 19:12
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It is not plausible for a self-destruct to have a count-down by purely mechanical or chemical means, especially one that could be stopped at the last second.

HOWEVER, it makes a great deal of sense to have a settable countdown for a triggered self destruct system; knowing when it will happen gives people time to prepare or escape. There are circumstances in war where intentionally giving your life or using your ship as a bomb is the strategic best option: Say to kill an opponent that otherwise is about to gain enough power to win the war, or to deny an opponent intelligence about your ship that can be used against all your other ships, or deny your opponent the chance of flying your ship as a false flag: For example, making it even more explosive and flying it directly into your headquarters.

In my career I have worked on military equipment that had built in self-destruct mechanisms, to prevent discovery and/or capture of the technology or secrets.

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  • $\begingroup$ Basically, scuttling the ship, in other words $\endgroup$ – Shalvenay Sep 26 '17 at 0:51
  • $\begingroup$ @Shalvenay Scuttle is one purpose, . just sink the ship: Which was done to prevent a ship from being taken and used by the enemy. I think of self-destruct as more like a computer closing a charge on a bit of C4 that destroys a secret-tech sensor, so it cannot be reverse-engineered or removed and used in another device. One could do the same for electronics, disks: A criminal hacker could rig his laptop thusly. In space, self-destruct could be a way to convert all fuel to a bomb: Not just to deny use of the ship, but to destroy other targets too, as a final blow when all is lost anyway. $\endgroup$ – Amadeus-Reinstate-Monica Sep 26 '17 at 10:23
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    $\begingroup$ Scuttling tech you don't really want the count down though. You want a 'press button, device dies' so the instant they 'break in' you can do it. $\endgroup$ – Sobrique Sep 26 '17 at 13:28
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    $\begingroup$ @Sobrique True, but if converting the ship to a bomb to accomplish both (damage and denial of the ship), a countdown for the threat may be useful. basically a restriction of options to the enemy: We both die or we both live; you have ten seconds to disengage and return control of this ship, or we all die. $\endgroup$ – Amadeus-Reinstate-Monica Sep 26 '17 at 14:12
  • $\begingroup$ Yeah, true. I was thinking laptops or computer systems. - Blow the device if they're vulnerable, but not an instant before. $\endgroup$ – Sobrique Sep 26 '17 at 14:34
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I doubt you could have complete accuracy of detonation time but a minimum time to detonation should entirely possible with a well established reactor design. Most self-destructs also require a dump of raw fuel into an overheated system so reasonable accuracy should be achievable in such a system.

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    $\begingroup$ This was my thought. To me the point of a countdown is that it gives you time to evacuate or cancel it if it's no longer required. So actually the countdown occurs before any self destruct system is activated, at which point it could be anything from milliseconds to minutes before the actual self destruct occurs (but it should also be all but impossible to stop at this point, preventing enemies from disabling it and taking the ship whole) $\endgroup$ – adaliabooks Sep 25 '17 at 17:59
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The count down is to give cinematic tension. That's all.

I can see that if the self destruct was not instant (like bombs planted throughout the ship) that the power plant destabilization might, on it's own, trigger warnings.

Also, if it is an instant destruction device, you may wish to give your crew time to evacuate.

Otherwise, it is like putting an LED timer on a bomb. Who, other than an infantile, grandstanding moron with masochistic tendencies, would let people know how long they have to defuse a bomb? If I ever had a character put a visible timer on a bomb, I'd have the bomb go off when there was still 12 minutes and 34 seconds left on the timer.

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    $\begingroup$ While I agree to a point, there are other uses for countdowns (self destruct and bomb). You put a countdown on a bomb that is intended to be diffused. The countdown is there to pressure whoever it is you are making demands of to capitulate, and also to let them know you are serious. Of course, those kind of situations mostly arise in TV shows or movies so the point may be moot. $\endgroup$ – adaliabooks Sep 25 '17 at 19:39
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    $\begingroup$ "I am a three second grenade. I will explode in three seconds. Three...two..." BOOM $\endgroup$ – Draco18s Sep 25 '17 at 21:47
  • $\begingroup$ ShadoCat and @Draco18s have both been spending too much time with the Evil Overlord List #15 and perhaps #136. :-) Warning: TV Tropes links. $\endgroup$ – a CVn Sep 26 '17 at 8:17
  • $\begingroup$ @MichaelKjörling, Ah, so you've seen my guidebook..... $\endgroup$ – ShadoCat Sep 26 '17 at 16:43
  • $\begingroup$ @pojo-guy, yep. I actually met him once. The only way to meet him at a con was to have a donor sticker from the blood drive that he made sure was there. $\endgroup$ – ShadoCat Sep 26 '17 at 16:44
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I'm looking at this part of the question :

"activating self-destruct mechanism disengage coolant system for the ship's main reactor, which then increases the core temperature and triggers explosion [by destabilising core]" is it plausible?

The kind of reactor (or other power source) isn't specified. Although fusion is suggested, it might not be fusion. Fission seems less likely. The question is generally whether ceasing cooling would ever plausibly provide a self destruct function.

I suspect the answer is "no":

Engineering and power production principles

An explosion requires uncontrolled rapid "runaway" activity. That's a "fail = unsafe" principle. Fission and raw flame can be unsafe on failure of a protective system, but generally no power source aims to be. This isn't a feature you could add to create a self destruct. It's an inherent property of an engineering/power generation method. Fission is inherently capable of runaway. Fusion (as far as we know) inherently is not - at least not on less than star-size reactors, where gravity and opacity can create runaway. Other invented systems are however you wish them.

But the coolant system can only change a runaway-capable system from running as inhibited to uninhibited modes; it can't create a runaway capable system if none exists.

Containment issues

Another problem is that most explosions need time to build up, however short. Even current nuclear weapons with an explosive effect (rather than just dirty material dispersion) need a very tightly fitted and insanely precise engineered thick steel case, just to prevent the components simply melting or flying apart in the first fraction of a second. Think of a gun barrel or bullet case, used to contain the propellant as it turns to gas - same idea.

A power creating reactor doesn't typically have the kind of case needed and it probably wouldn't be compatible if desired. (Because even a small space would probably be enough to adversely affect the runaway force and make maintenance impractical, although this is obviously a bit speculative and could be handwaved away).

Power production vs. power storage

To explode, you usually need to release some kind of stored energy, and this stored energy must be capable of being released very fast.

If the power system doesn't contain a store of power, or at least the ability to produce power fast enough, this makes it a bit less plausible. But if it can produce power quickly, then this still doesn't imply a store that can be released all at once. A bullet, rocket, or fission reactor contains stored energy that is designed or inherently capable of rapid release, but not all systems have this. Without much detail, this would probably be an issue to consider as well, in assessing plausibility.

Failsafe designing

On principle, engineers tend to develop power systems that, on failure of control, are inherently safe not unsafe. You wouldn't want to fly a bomb. So your car systems have several hundred detectors (including code checks) that watch for risky engine status indicators and restrict rpm if so. (What if a switch stuck, or a detector was faulty?) Ditto all other power systems. Even your toaster's power circuit has a fuse somewhere.

But in addition they design for safety even if active systems fail. If your car battery fails and the electronics lose power, you dont want the engine to be left in a full-power state mechanically with no way to turn it off because of electronic control issues.

Coolant is a subsystem. You probably won't get engineers designing it to fail, because its so critical that the cooling system can never by accident (or due to some unlikely combination of faults), fail.

Designing a bypassable safety system of this kind, with probable catastrophic effects, would be a bit like designing a lift ('elevator' in the US) for an office block where people work daily, and building in all safety features - plus an anti-terrorism feature that could raise it to the top floor at high speed and withdraw all safety controls (over cable drum rotation, lift car safety etc) and drop it at maximum speed to impact at the ground floor, just in case of a terrorist attack where the terrorists are in the lift.

In other words, however desireable a defence it might be, you just wouldn't design a safety critical system and a path that completely undermined it. You'd do it some different way.

Possible alternative solutions

If the ship had some system that accumulated immense power, that power could be released in a specific manner. That's not a coolant failure but it would meet the needs of the question.

For example, suppose the ship has weapons, and these weapons were reliant on huge power delivery accumulated over a few seconds and released in a brief but immensely powerful burst. Then the power accumulators could be used as a source of self destruct, by providing a path that would release that power against the ship itself.

You'd have to think around the issues of containment (how do you stop the power transmission system just melting instead?) and impact (how would this lead to destruction rather than damage?)

Alternatively if the ship contained something that already had explosive effect, it would only be necessary to initiate whatever explodes it. Again that's not a coolant failure. For example if it carried some kind of torpedo weapons, or mining explosives, or a store of chemical fuel needed by some kind of pods which are too small for a reactor, or was powered by immensely dense matter in some kind of futuristic force-field, then you'd just have to set off the explosives, provide a (fail-safe!) detonator for the fuel store, or release the force-field.

But as stated, and looking at cooling systems only - no.

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There are generally two types of countdowns:

One is a timer counting down to a trigger action. This is the kind that can usually be aborted before it reaches zero, although there may be reasons to make the destruct irrevocable. When the timer reaches zero, detonation is essentially immediate.

The other type of countdown is a prediction. It is an estimated time at which the power source will go critical, containment will collapse, or whatever makes the thing go boom. Where this comes up, the surrounding science and technology always seems to be good enough to make such predictions down to the second.

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Yes. Given standardized parts, ship designs, mechanisms, flow rates, temperature tolerances, and so on, predicting the reaction and the time it takes to kick off should be possible. In the case of military ships it would likely be tested for consistency.

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