DOA Scifi Tropes

[KerikBalm]

On 9/27/2020 at 12:46 AM, Dragon01 said:

Sure, sometimes such a company goes out of business, and then a dangerous cargo might end up with unclear legal status, but this is a relatively uncommon event.

*cough* Beirut port explosion *cough* but yea, it is an uncommon event

On 9/26/2020 at 7:18 PM, Spacescifi said:

Antigravity Kills Orbital Spacestations: If you have a way to fall up the need for large orbiting spacestations disappears. Orbiting satelites with sensors are always useful, but big orbiting stations no longer matter as much, since space would no longer require a massive presence, as getting up and down a planet would be easy.

Umm, no... antigravity is just a terrible idea for lazy SF writers who don't want to think through things.

What does the AG do? neutralize the effects of gravity? Great, you don't fall towards the Earth, but you also don't fall towards the sun. Your station flies off into interstellar space, and ceases to be a station about the planet.

If you then say that somehow it neutralizes Earth's gravity but not the Sun's (or whatever planet/star combination you wish), then ask yourself, what happens when you need to shut it down for mainenence? what if there is a malfunction? or sabotage? 

A station holding place at 300km would take about 245 seconds to fall and hit the surface of the Earth if it was "stationary". That's just over 4 minutes. Even if you got it fixed in 4 minutes, then you'd need one heck of an acceleration to stop it from smashing into the Earth.

A massive station that would be destroyed if a piece of equipment stops working for a few minutes is a terrible idea. If you have "antigravity", you can get your big station up, and push it into orbit, then shut your antigravity device off - much safer. Plus if you stipulate that the antigravity tech needs some expensive/exotic material/tech, then you don't need to have it on every vessel, just some sort of reusable cargo lifter.

[Guest]

18 minutes ago, KerikBalm said:

*cough* Beirut port explosion *cough* but yea, it is an uncommon event

If this was a common even, there'd be more ports blowing up.  The point is, a lot of things need to go wrong for that to happen. This particular case was a failure of many systems that ordinarily prevent this exact sort of thing.

[Spacescifi]

 

Another DOA trope is many of the things the Captain says in Star Trek.

One notable one is "Divert auxilliary power to engines."

In the shows and the games, doing this usually gives a speed boost.

The term is probably borrowed from Roddenberry's navy days, like so much else in ST. While diverting auxillary power to engines with virtually unlimited reaction mass (the ocean) on a navy ocean vessel can improve max speed, in space things are a bit different even though it can still improve overall speed.

Reaction mass is limited, and how much power you divert to anything is limited by waste heat loading. Too much and you burn.

And it is not as if simply diverting more 'power' can improve thrust, although it can improve delta v top speed.

In other words, diverting power to engines is not even an emergency term like ST would have you think, since extra power only improves delta rocket efficiency up to a point anyway. That point being whatever power/heat that won't melt the engines.

For speed boost during an emergency what they would say (if they wanted realism) is "Light up the auxilliary AM (Antimatter) thermal boosters! Get us out of range and warp immediately!"

Basically high thrust gets you up to the top speed of whatever engine you're utilizing at the moment. The more 'power' thermal energy pumped into this process the higher your top speed will be in the end (when you run out of propellant oh no), but that WON'T necessarily improve how much thrust your vessel is putting out in an emergency.

How much thrust you get depends on the mass flow of propellant. The dense ones give higher thrust, but lower top speeds (when we run out of propellant). Less dense ones require a lot more volume per tank for the equivalent top speed a smaller tank with denser propellant could reach.

So ideally, a spaceship captain would want a rocket engine with an ability to swap between various stored propellant tanks depending on the needs of the current episode.

This is actually an engineering challenge but can be done, although it is more complex and thus more expensive than standard rocketry engines.

Edited October 3, 2020 by Spacescifi

[kerbiloid]

If auxilliary system consume power comparable to the main engine needs, something went very wrong during the ship development...

[magnemoe]

On 10/3/2020 at 7:34 AM, kerbiloid said:

If auxilliary system consume power comparable to the main engine needs, something went very wrong during the ship development...

My thought too more so even for ocean ships, was on an tour of the engine room on an small cruse ship / large ferry,  4 engines size of an full size cargo container but quite a bit higher. 
Two auxiliary engines who was not as large as the engines in an US freight train locomotives. No they would not help much even if you could use them to power the ship efficiently. 

Now some warships uses an diesel engine as main power plant but has an gas turbine for combat speed but this would be flank speed engines like afterburners on an jet. 
 

[KSK]

I can think of a few rationalisations for the ‘divert auxiliary power to engines’ trope. 

The most obvious one is that it’s a shorthand command for emergency situations, when brevity is important.

Alternatively, if the Enterprise is actually using rocket engines, it could be using some kind of plasma drive where the exhaust is accelerated by magnetic fields. Not completely unreasonable given that canonically, the Enterprise’s impulse drive is powered by nuclear fusion. Diverting auxiliary power to engines activates an additional set of accelerator magnets, increasing exhaust velocity and therefore thrust. Of course, this also generates additional heat so it can’t be sustained for too long, which is why the extra magnets aren’t routinely used.

Finally, from what I remember from Laurence Krauss’s The Physics of Star Trek all the numbers for the Enterprise’s Impulse drive are out of whack anyway and there’s no way that it can actually be a rocket engine. Therefore the ‘Impulse Drive’ is simply the warp engines operating  in sublight mode. This kind of helps in that diverting power to the warp drive can be handwaved away as making some kind of sense but it also makes things more complicated since an operational warp drive would just let you warp out of whatever trouble you were in anyway, so the command to use auxiliary power becomes superfluous.

The Physics of Star Trek is pretty good by the way. It doesn’t attempt to ‘explain’ Treknology but it’s quite a nice introduction to which bits of it are theoretically possible and how close we can get to them with present day technology.  @Spacescifi - you might find it interesting or even useful, although it’s almost entirely qualitative with almost no hard numbers.

 

[DDE]

13 hours ago, magnemoe said:

Now some warships uses an diesel engine as main power plant but has an gas turbine for combat speed but this would be flank speed engines like afterburners on an jet. 

Something like this is becoming an option on ships with "integrated" powerplants like the Zumwalt-class or many cruise liners: all engine power is converted to electricity and fed into the ship's power grid.

[Guest]

I think this could well be the case for a nuclear-electric warship (though nuclear electric isn't that good at getting high thrust in general). You could make a ship designed to have more engine, weapons and perhaps plasma shielding than the reactor can power at once. So, tactical shifting of electricity production could be a thing. You're either shooting, thrusting or defending, and if you're doing both at the same time, then not at max output.

Of course, it'd run at max power to engines at all times during normal cruise, only transferring power to other systems in battle. 

[sevenperforce]

On 10/2/2020 at 8:01 PM, Spacescifi said:

Basically high thrust gets you up to the top speed of whatever engine you're utilizing at the moment. The more 'power' thermal energy pumped into this process the higher your top speed will be in the end (when you run out of propellant oh no), but that WON'T necessarily improve how much thrust your vessel is putting out in an emergency.

No. For a constant amount of reaction mass, adding more energy DOES add more thrust.

9 hours ago, KSK said:

I can think of a few rationalisations for the ‘divert auxiliary power to engines’ trope. 

How about a trimodal nuclear-thermal LOX-afterburning rocket engine?

1 hour ago, Dragon01 said:

I think this could well be the case for a nuclear-electric warship (though nuclear electric isn't that good at getting high thrust in general). You could make a ship designed to have more engine, weapons and perhaps plasma shielding than the reactor can power at once. So, tactical shifting of electricity production could be a thing. You're either shooting, thrusting or defending, and if you're doing both at the same time, then not at max output.

Of course, it'd run at max power to engines at all times during normal cruise, only transferring power to other systems in battle. 

Or it could be a situation like running the SSMEs at 104%. Your generator provides enough power to run both the engines at full thrust as well as the shields at full power, so that you can make maneuvers while shielded, but if you cut power to the shields you can run the engines at 110% or 120% or some other number for a short period before they overheat.

[Guest]

Actually, the thing with SSMEs was pure bureaucracy, they were uprated, and it was easier to routinely go over 100% than to update what 100% meant.  This would be more like flank speed in ships, very inefficient and bad for the engine in long run, but in combat, you might need it. Of course, this is related to SSMEs in that it depends on how you define "full power", but the end result is the same. 

Note, however, that power shunting would allow you to put much more firepower, shielding and engine on your ship than you would otherwise be able to. If the nature of combat means switching between thrusting, shielding and firing is safe, then you'd want to make sure that either could make use of the reactor's full power. The power system (radiators+reactors) would typically be the most significant item in your mass breakdown.

[DDE]

17 hours ago, sevenperforce said:

How about a trimodal nuclear-thermal LOX-afterburning rocket engine?

http://toughsf.blogspot.com/2019/09/nter-nuclear-thermal-electric-rocket.html?m=1

Pinging @MatterBeam

[Spacescifi]

 

Another one is a lack of modules on starships/spaceships.

 

Granted, they often are SSTO's in scifi, but even then modular designs that can break off from the ship is arguably a wise move.

Since I cannot see any disadvantages, only advantages, especially for crewed vessels.

Since they are more roomy than a tiny shuttlecraft and most importantly... can carry more reaction mass.

Small ships in real life will have the lower top speeds, compared to larger ones if they both are designed for high speeds.

Purely because one has more reaction mass and a bigger heat sink as well.

Edited October 6, 2020 by Spacescifi

[Scotius]

Are you forgetting about infamous warp core ejections in Star Trek? It's an equivalent of modern ship violently catapulting its engines into the ocean, every time something invariably breaks in the engine room. That requires modular construction for sure LOL

[Guest]

Actually, "drop the core overboard" would probably be a feasible emergency procedure in case of a nuclear reactor meltdown on nuclear-powered ships.  Better than having a superheated, rapidly fissioning mess of uranium onboard. While I'm pretty sure modern naval reactors aren't designed to do that, we'd probably have this as a last resort meltdown protection if nuclear ships were more widespread.

[Scotius]

Scram procedures are a thing exactly for this reason: https://en.wikipedia.org/wiki/Scram

One would think that advanced civilization using this technology for hundreds of years, would develop similar safeties for their expensive and complicated starships.

[kerbiloid]

And now back to the antimatter...

[Guest]

10 hours ago, Scotius said:

Scram procedures are a thing exactly for this reason

Except you can't always rely on them. They did hit the scram switch at Chernobyl, but it only made matters worse. Modern land-based reactors are designed so that in the event of a core meltdown, it'll all flow down into a very deep pit below it, which could then be safely filled in. The reactor will be unsalvageable, unlike after a scram, but you won't get the molten down core anywhere near habitable spaces.

At sea, I suggest the current procedure is to abandon ship in case of the scram system failure, but jettisoning the core out of the bottom could, perhaps, be safer, most notably by not leaving a radioactive hunk of metal floating around the ocean.

[DDE]

3 hours ago, Dragon01 said:

At sea, I suggest the current procedure is to abandon ship in case of the scram system failure, but jettisoning the core out of the bottom could, perhaps, be safer, most notably by not leaving a radioactive hunk of metal floating around the ocean.

Reactors don't float.

We know that very reliably because, for early icebreakers, the Soviet refueling procedure was to use detcord to blow up the bottom underneath all three reactors, sending them to the bottom, and then replace them with new ones at the yard.

[Guest]

What I meant was that if you abandon a ship that's afloat, but with a melted down reactor, then it likely won't sink outright. I don't think melted corium would get through the hull, which would leave you with a wreck that's too radioactive to board, but too intact to sink on its own.

Of course, a well-placed torpedo would solve that problem, but if you use the "Soviet icebreaker refueling method" as a last-ditch emergency procedure, the ship should be more salvageable.  

Edited October 7, 2020 by Guest

[kerbiloid]

3 hours ago, DDE said:

We know that very reliably because, for early icebreakers, the Soviet refueling procedure was to use detcord to blow up the bottom underneath all three reactors, sending them to the bottom, and then replace them with new ones at the yard.

With several tens of them on bottom we could have a summer spa in Arctics...

Life is beach...

[Scotius]

3 hours ago, kerbiloid said:

With several tens of them on bottom we could have a summer spa in Arctics...

Life is beach...

And then suddenly...

[Spacescifi]

 

Spoiler

The last DOA I will consider is the giant space station in LEO for centuries. Or ANY big spacestation in low orbit for centuries.

IRL the propellant cost... either in nukes or chemical will break at least some countries financially into bankruptcy.

Now once it is in low orbit, atmospheric friction will slowly but surely try to deorbit it through the decades.

Just to prevent deorbting it will once agaim either take considerable amoubts of propellant or bombs because of station mass alone.

Unless you are using solar sails, which may mor may not work good enough before the atmosphere deorbits the station.
 
This will take decades but will happen, assuming zero propellant is spent maintaining LEO.

My point is that scifi space stations as depicted are as far as IRL is concerned resource hogs and always would be.... barring scifi tech thay transcends the limitations of rocketry and pusher plates.

The only way they could really justify a presence is above an uninhabital planet that has tons of platinum or metastable metallic hydrogen on the surface.

Edited October 8, 2020 by Spacescifi

[kerbiloid]

The station on the picture probably has glass floors instead of windows, as it's probably rotating.

The metastable metallic hydrogen is a cult thing for KSP now itself.
But tons of it on surface... Probably, the station is orbiting around a pulsar.

Edited October 8, 2020 by kerbiloid

[Guest]

6 hours ago, Spacescifi said:

The only way they could really justify a presence is above an uninhabital planet that has tons of platinum or metastable metallic hydrogen on the surface.

Or simply don't stick the station in low orbit. Instead, put it safely at a Lagrange point, where it could pretty much orbit forever without being bothered by anything but an occasional rock. Of course, then you won't have such a nice view of the planet, because Lagrange points are quite far away.

3 hours ago, kerbiloid said:

The metastable metallic hydrogen is a cult thing for KSP now itself.

@sevenperforce, that's your cue.  

[KSK]

Just to throw some numbers in here to try and put things into context.

For comparison, the mass of the ISS is 420 tons. It orbits at an average altitude of about 408 km. A typical (read, the one example I found online) ISS reboost imparts a delta-V of  about 1 m/s , increasing its altitude by about 1.5 km.

So let’s assume a million ton space station, and a reboost of 10 m/s. Lets also assume that we’re performing the reboost with a SpaceX MVac engine, vacuum ISP of 311 seconds. The latter two numbers are exaggerated for effect as I’m sure you’ll appreciate. 

Plugging all that into the rocket equation, I calculate (and if someone could check this, that would be great) that you would need approximately a million tons of propellant to perform the boost.

That’s a lot but it doesn’t seem like a task that would bankrupt countries, particularly since we’re assuming an economic infrastructure capable of building a million ton space station in the first place. 

And its comparatively easy to lower that propellant requirement very substantially. Orbiting even a couple of hundred kilometres higher will cut that atmospheric drag a lot:

(https://www.spaceacademy.net.au/watch/debris/orblife.htm

Using more efficient reboost engines will also help a lot - and it’s not difficult to find even present day engines with a better ISP than an MVac.  

Also, sci-fi can get this right when it tries. Check out this clip from 2001: A Space Odyssey. Notice the size of the Earth in the background. Now compare that with the view from the ISS (I’ll leave finding a suitable image as an exercise for the reader  ). I think it’s pretty clear that the 2001 station is not in low orbit.