DOA Scifi Tropes

[kerbiloid]

A small LEO station is dragged by air.
A large LEO station suffers from tidal forces.

And a really large station has so thick hull that doesn't need LEO.

[DDE]

Broke: icebreaker

Woke:

16 hours ago, Scotius said:

 

 

1 hour ago, kerbiloid said:

A small LEO station is dragged by air.
A large LEO station suffers from tidal forces.

And a really large station has so thick hull that doesn't need LEO.

If the station is sufficiently tall, it becomes a terrestrial building.

[kerbiloid]

A terrestrial building enough tall to touch the Moon becomes an interplanetary bridge.

[sevenperforce]

5 hours ago, KSK said:

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.

The MVac pushes 348 seconds of specific impulse; 311 is the sea level version in vacuum. But I think your math is off. At 311 seconds you'd need 3,286 tonnes of propellant for a 10 m/s reboost; at 348 seconds you'd need 2,937 tonnes.

But more to the point, a civilization capable of building a million-tonne space station would be reboosting it with something a hell of a lot more energetic than kerolox.

One VASIMR concept was originally envisioned for reboosting the ISS. VX-200 had a specific impulse of 5000 s. A million-tonne space station would have gigantic power requirements anyway so it would make sense to have scheduled brownouts for allocating power to the onboard VASIMR for reboosting. A more realistic 5 m/s boost to a million-tonne space station would require 102 tonnes of VASIMR propellant.

6 hours ago, Dragon01 said:

10 hours ago, kerbiloid said:

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

@sevenperforce, that's your cue.  

The reason metallic hydrogen gets everyone excited is that it's monatomic rather than diatomic. In other words, you can essentially burn it with itself and produce diatomic hydrogen with a huge energy release. Yay!

But there's simply no evidence that metallic hydrogen can exist in a metastable state at meaningfully low pressures. All the maths that show a workable metallic hydrogen engine simply assume that it could conceivably be. For example, the 2010 Harvard paper that got everyone excited about it said, "We assume that metallic hydrogen is a metastable solid or liquid at ambient conditions, that it is compatible with launch vehicle propulsion environments, and that it can be safely and affordably produced and handled in large quantities." That's a hell of a lot of assumptions. It's rather like saying "If we could compress plasma from the core of the sun and store it at room temperature then it would be a very good way to cook food." I mean, yes, but that's not really helping us.

In some models, metallic hydrogen can form at several hundred GPa but remains metastable to 10-20 GPa. And while that may seem like a huge jump, it doesn't help us, either. The 643 bar pressure inside Raptor's CH4 turbopump is just 0.0643 GPa. The physics of metallic hydrogen simply do not allow it to remain metastable at pressures achievable on a macroscopic scale.

[kerbiloid]

Yes, but so what?

It's already a cult of metastable metallic hydrogen in this forum, and a holywar between the opponents and proponents.

But supporting any of them you just join one side.

Edited October 8, 2020 by kerbiloid

[sevenperforce]

18 minutes ago, kerbiloid said:

Yes, but so what?

It's already a cult of metastable metallic hydrogen in this forum, and a holywar between the opponents and proponents.

But supporting any of them you just join one side.

All of this nonsense about metastable metallic hydrogen reminds me of something I once heard about electrons and protons. It was remarked to me that if one were to remove the electrons from a single potato, the Coulomb force between all of the protons in that potato would result in enough destructive energy to blow up the moon. It seemed awe-inspiring and almost unbelievable.

But how would one actually go about that? You can’t just “delete” electrons. The Coulomb force between electrons and protons is the very thing which keeps matter solid. Even if you set aside the ionization energies of all the atoms in all the molecules in the potato, the act of removing all the electrons from the potato one by one would require exactly the same amount of energy that would subsequently be released. Each electron would be harder and harder to pull out due to the growing positive electric field and the nucleon cloud left behind would become more and more difficult to contain. There is no free lunch.

Imagining that we’ll have metastable metallic hydrogen energies is like imagining that we’ll be able to levitate by “simply” pulling pockets of air underneath ourselves and hovering on them. "Yes, if you could do something you cannot do, then you could do something you cannot do." Welcome to the tautology club.

[kerbiloid]

2 minutes ago, sevenperforce said:

if one were to remove the electrons from a single potato

I.e. if heat a potato up to 10kK, it will be a continent-sized crater...

[KSK]

@sevenperforce

Thanks for checking the maths - I was posting in a hurry so I’m not surprised that I goofed with my calculation.

Your corrected figures get the point across even more effectively though. Even using my (deliberately) suboptimal MVac example, the propellant requirements for reboosting large LEO space stations are not excessive. 

In fact I would argue that getting enough kerolox to orbit (to reboost with MVac) would be challenging but just about feasible with current technology. (SpaceX’s Starship operating at a cadence of one launch per week would be able to lift 5000 tons per year assuming that the vehicle lives up to its planned performance). 

Using your much more sensible VASIMR example , you would need one Starship resupply per reboost.  Limited risk of bankruptcy there, I would say.

[KSK]

And by way of a last couple of comments on the large space station trope.

In fairness, I agree that @Spacescifi’s example does not look plausible. I have no idea how much a Starbase masses but given that it rather dwarfs that very large starship alongside it, my best scientific guess is ‘a metric shedload’.  Even with Trek levels of technology, parking the Starbase in a high enough orbit that atmospheric decay becomes a non-problem, would seem more sensible than reboosting it.

On the other hand, I’m not sure it’s fair to write off a trope as DOA on the back of a single example. 

 

[magnemoe]

5 hours ago, sevenperforce said:

All of this nonsense about metastable metallic hydrogen reminds me of something I once heard about electrons and protons. It was remarked to me that if one were to remove the electrons from a single potato, the Coulomb force between all of the protons in that potato would result in enough destructive energy to blow up the moon. It seemed awe-inspiring and almost unbelievable.

But how would one actually go about that? You can’t just “delete” electrons. The Coulomb force between electrons and protons is the very thing which keeps matter solid. Even if you set aside the ionization energies of all the atoms in all the molecules in the potato, the act of removing all the electrons from the potato one by one would require exactly the same amount of energy that would subsequently be released. Each electron would be harder and harder to pull out due to the growing positive electric field and the nucleon cloud left behind would become more and more difficult to contain. There is no free lunch.

Imagining that we’ll have metastable metallic hydrogen energies is like imagining that we’ll be able to levitate by “simply” pulling pockets of air underneath ourselves and hovering on them. "Yes, if you could do something you cannot do, then you could do something you cannot do." Welcome to the tautology club.

Yes you would create plasma not an battery having nuclear energy level storage. Not sure how many electrons an potato would have left if you replaced the 3rd stage lead dummy with an potato. 
No its not an good use of an potato, eat it. 
 
And metallic hydrogen is more likely than em-drive but less likely than some sort of cold fusion, note that fission is cold in this setting, even an nerva engine. 
An good and simple fusion reactor would be an high number of magnitudes more profitable than metallic hydrogen.

On the other hand we found some very dense exoplanets as in denser than lead. 
I assume this is the degenerated matter as in (white dwarf material) cores of gas giants who dense atmosphere eroded away, planet might been inside the star as red giant. 
However the high gravity keeps the core very dense even without the thick atmosphere. 

This is not that hard: As I understand modern nuclear bombs uses an explosive charge so effective its compress the plutonium core significantly, guess 50% denser something. 
This is an serious safety improvement as none sort of accident up to setting off another nuke close to it will make it go critical. You get an dirty bomb as in an anti tank mine and 10 kg of plutonium worst case. 
 

[sevenperforce]

17 hours ago, magnemoe said:

Yes you would create plasma not an battery having nuclear energy level storage.

This is the fun bit -- the Coulomb force is MASSIVELY powerful.

Calling it "nuclear energy level storage" doesn't even come close. If you took just 4% of the electrons out of a potato, it would have a specific energy density of 2e20 J/kg, making it 25 MILLION times more energy-dense than the most powerful thermonuclear bomb ever detonated. Take all of the electrons out of a potato (or any chunk of matter) and the specific energy becomes 220,000 times more than the specific energy of PURE ANTIMATTER.

Obviously it is nonsense to imagine an energy storage mechanism which contains more energy than the mass-energy of the storage device. In fact a 180 gram potato with all the electrons taken out of it would lose about 16 milligrams of mass (from the lost electrons) but would gain about 40 tonnes of mass due to the Coulomb mass-energy it contained, making it approximately 7% as dense as a white dwarf star.

17 hours ago, magnemoe said:

its not an good use of an potato, eat it. 

I agree.

17 hours ago, magnemoe said:

On the other hand we found some very dense exoplanets as in denser than lead. 
I assume this is the degenerated matter as in (white dwarf material) cores of gas giants who dense atmosphere eroded away, planet might been inside the star as red giant. 

White dwarf material comes from dead stars -- exoplanets don't get that dense. A white dwarf is 10,000 times denser than lead.

[Codraroll]

On 10/9/2020 at 5:08 PM, sevenperforce said:

A white dwarf is 10,000 times denser than lead.

How did Futurama put it? "It's so dense that each pound of it weighs over ten thousand pounds."

Edited October 10, 2020 by Codraroll

[DDE]

17 hours ago, Codraroll said:

How did Futurama put it? "It's so dense that each pound of it weighs over ten thousand pounds."

Spoiler

 

 

[kerbiloid]

What's heavier: a kilogram of steel or a kilogram of sugar?

Spoiler

_{(The former Soviets should know the right answer.)}