For Questions That Don't Merit Their Own Thread

[Beamer]

8 hours ago, Vanamonde said:

Who knows more about astronomy than I do? If stellar collisions are supposed to be rare, how come there's so much talk about neutron stars whanging into each other? Is this the end-fate of lots of binary pairs due to that gravitational drag thing I don't really understand? 

Basically, your estimation of rarity is incorrect (or incomplete if you prefer). It is not rare for binary stars to collide. In fact you could say they (or their stellar remnants) always collide - the only alternative is they get flung apart, in which case it's not a binary system anymore (I know, that's cheating a bit, but you get the point ). Of course it takes a lot of time, there are plenty of binary system where it won't happen for a longer time than the current age of the universe.

The kind of collisions that are presumably extremely rare are the ones where 2 stars/remnants that weren't in orbit of each other collide (even in galactic mergers this is considered a rare event). This is simply because although stars are big, the empty space between them tends to be a lot bigger. The reason you've probably seen a lot about it in the news lately is because they think they detected exactly such an event recently, where 2 black holes pretty much had a high speed near head-on collision (they crashed into each other after only 1 pass, without a spiraling phase). See for example https://arstechnica.com/science/2022/11/strange-black-hole-merger-may-have-been-a-rare-random-encounter/

 

[Jacke]

10 hours ago, Vanamonde said:

Who knows more about astronomy than I do? If stellar collisions are supposed to be rare, how come there's so much talk about neutron stars whanging into each other? Is this the end-fate of lots of binary pairs due to that gravitational drag thing I don't really understand? 

It's binary pairs.  Stars are more often in binary or greater groupings than alone.  And enough of those pairs will be close binaries with both stars having enough mass to end as neutron stars rather than white dwarfs.

[sevenperforce]

On 12/8/2022 at 4:09 PM, Vanamonde said:

Who knows more about astronomy than I do? If stellar collisions are supposed to be rare, how come there's so much talk about neutron stars whanging into each other? Is this the end-fate of lots of binary pairs due to that gravitational drag thing I don't really understand? 

Neutron star collisions are (virtually) always binary pair collisions, not chance encounters. 

Classically, gravitational drag wouldn’t exist. But under relativity, it does. You know how tidal forces operate, right? And how the Moon’s tidal force impact on Earth’s oceans cause the Moon to drift away from Earth? Well, relativistic gravitational frame-dragging works basically like that, except that instead of the effect being caused by the differences in distance between the near and far sides of an object, the effect is caused by the difference in the curvature of spacetime due to extremely dense objects.

And so co-orbiting neutron stars rob each other of energy through this exchange, coming closer and closer until they merge.

The same thing happens in our solar system…just at an aggressively lower rate. Earth loses about 3 nanometers-per-second of orbital velocity each year. 

[K^2]

2 hours ago, sevenperforce said:

Earth loses about 3 nanometers-per-second of orbital velocity each year.

Most of that's still just literal drag against solar wind, though. Relativistic drag for the Earth-Sun system is absolutely miniscule.

[DDE]

Soviet Navy-pattern potassium superoxide CO2 scrubber. The plates of active ingredient are notoriously disagreeable with oil-contaminated water.

Also, that's a whole lot of asbestos lining that storage crate...

[SunlitZelkova]

What would Mars need to be like, or what would have had to happen to it, for it to have an atmosphere “roughly 10% as dense as Earth’s”?

That’s a direct quote from the Spaceflight History Blog, as I had no idea where to find the exact hypothetical numbers put forward prior to Mariner 4.

I want to write an alternate history using the winged lander designs of the 50s and early 60s. I am wondering if the characteristics of Mars could be altered to realistically have the atmosphere needed for those designs, or if I would just need to hand wave it.

[kerbiloid]

Just in case.

Googled out such nice pdf.
https://ir.ua.edu/bitstream/handle/123456789/2478/file_1.pdf

[Spacescifi]

 

What if we applied Time Lord tech to propellant tanks?

 

You know... weighs less than it should on the outside but is bigger on the inside.

 

Example: You have a 1 ton propellant tank fitted to each rocket engine on your ship.

Each tank is much bigger on the inside and can hold 100 ton of liquid hydrogen or even more of more dense propellants.

 

Assuming we only used methalox, I reckon it could SSTO just fine. It's fuel efficiency would not be all that great, but the fact that it can carry enormous amounts of fuel without the universe factoring it in as mass until it goes through the combuston chamber would give a lot of flexibilty and allow for classic scifi SSTO adventures.

Yeah it is cheating. No radiation but cheaty bigger on the inside lightweight fuel tanks for the win.

It would explain Star Wars and other similar scifi quite well... since if those exhaust plumes were cancerous no one would fly around inhabited areas like they always do.

It would even explain laser pistols, since you can literally have an industrial powerplant inside a suitcase size laser gun, suitcase size so you can enter the industrial powerplant yourself if you wished.

 

Lots of missed opportunities that could explain all manner of safe-ish scifi tech that is still incredibly physics breaking.

Edited December 11, 2022 by Spacescifi

[JoeSchmuckatelli]

https://tenor.com/6wwq.gif

 

[kerbiloid]

Spoiler

 

 

(Fingerprints warning.)

***

On topic: why bother with tanks when you can just attach a hyperhose to a stationary fuel storage.

[sevenperforce]

10 hours ago, SunlitZelkova said:

What would Mars need to be like, or what would have had to happen to it, for it to have an atmosphere “roughly 10% as dense as Earth’s”?

That’s a direct quote from the Spaceflight History Blog, as I had no idea where to find the exact hypothetical numbers put forward prior to Mariner 4.

I want to write an alternate history using the winged lander designs of the 50s and early 60s. I am wondering if the characteristics of Mars could be altered to realistically have the atmosphere needed for those designs, or if I would just need to hand wave it.

Oh, that's certainly possible. The atmosphere of Mars was much denser in the past, but due to the planet's smaller size and lack of a magnetic field, most of the water vapor that was once in the atmosphere escaped into space.

If the polar icecaps on Mars were to melt, they would roughly double the density of the Martian atmosphere, which would cause accelerated greenhouse warming, which would cause sublimation of gases currently adsorbed by the soil, which would really start to crank up the density of the atmosphere. If you're looking at the past, then you could envision a situation where Mars was struck by a particularly large comet a few million years ago. That would probably be enough to do the trick.

5 hours ago, Spacescifi said:

What if we applied Time Lord tech to propellant tanks?

You know... weighs less than it should on the outside but is bigger on the inside.

Assuming we only used methalox, I reckon it could SSTO just fine.

Assuming magic exists and we can use magic to do magic, I reckon magic is possible.

[RyanRising]

Why do chemically ignited rocket engines (RS-27, F-1, Merlin) so commonly use TEA-TEB specifically? Why is a mixture of those two chemicals used rather than TEA or TEB on their own, or some other pyrophoric chemical?

[Jacke]

14 hours ago, RyanRising said:

Why do chemically ignited rocket engines (RS-27, F-1, Merlin) so commonly use TEA-TEB specifically? Why is a mixture of those two chemicals used rather than TEA or TEB on their own, or some other pyrophoric chemical?

It's a hypergolic starting slug.  Something that will ignite quickly on contact with the other propellant to start combustion so the non-hypergolic main propellants will be ignited.  Both TEA and TEB are used to get some performance factor to a desired value, probably ignition delay to under 50ms.

A couple of quotes from Ignition!.

EDIT: TEA/TEB aren't used as hypergolic starting slugs as they are too reactive with too many propellants.  They are injected in parallel at some point in the rocket engine to start combustion.  This allows them to be used multiple times, depending on the amount carried.

Quote

Dr. Eugen Sanger, at the University of Vienna, made a long series of firings during 1931 and 1932. His propellants were conventional enough —liquid (or sometimes gaseous) oxygen and a light fuel oil — but he introduced an ingenious chemical wrinkle to get his motor firing. He filled the part of his fuel line next to the motor with diethyl zinc, to act as what we now call a "hypergolic starting slug."  When this was injected into the motor and hit the oxygen it ignited spontaneously, so that when the fuel oil arrived the fire was already burning nicely.

 

Quote

And in the meantime, early in 1940, [Helmut Philip von Zborowski] and [Heinz] Mueller had made an immensely important discovery — that certain fuels (aniline and turpentine were the first they found) ignited spontaneously upon contact with nitric acid. [Dr. Wolfgang C.] Noeggerath learned of this, and joined the BMW people in their search for fuels with this interesting property. His code name for nitric acid was "Ignol" and for his fuels "Ergol," and, a fast man with a Greek root, he came up with "Hypergol" for the spontaneous igniters. "Hypergol" and its derivatives, such as the adjective "hypergolic" have become a permanent part not only of the German, but of the English language, and even, in spite of the efforts of Charles de Gaulle to keep the language "pure," of the French as well.

The discovery of hypergolicity was of major importance. Running a rocket motor is relatively easy. Shutting it down without blowing something up is harder. But starting it up without disaster is a real problem. Sometimes electrical igniters are used — sometimes pyrotechnic devices. But neither can always be trusted, and either is a nuisance, an added complication, when you already have more complications than you want. Obviously, if your combination is hypergolic, you can throw out all the ignition schemes and devices, and let the chemistry do the work. The whole business is much simpler and more reliable.

But as usual, there's a catch. If your propellants flow into the chamber and ignite immediately, you're in business. But if they flow in, collect in a puddle, and then ignite, you have an explosion which generally demolishes the engine and its immediate surroundings. The accepted euphemism for this sequence of events is a "hard start." Thus, a hypergolic combustion must be very fast, or it is worse than useless. The Germans set an upper limit of 50 milliseconds on the ignition delay that they could tolerate.

 

Edited December 11, 2022 by Jacke

[K^2]

14 hours ago, RyanRising said:

Why do chemically ignited rocket engines (RS-27, F-1, Merlin) so commonly use TEA-TEB specifically? Why is a mixture of those two chemicals used rather than TEA or TEB on their own, or some other pyrophoric chemical?

As far as I know, TEB won't ignite spontaneously (reliably? fast enough?) on contact with cryogenic oxygen. TEA will. Getting over that activation hump with cryogenics can be a bit of a pain. Looking at both, at a guess, the TEA-TEB mix used is closer in stability and handling safety to TEB, while retaining the TEA's ability to ignite on contact with cryogenic oxygen. As @Jacke  points out, ignition time might also be a factor.

[Jacke]

9 minutes ago, K^2 said:

As far as I know, TEB won't ignite spontaneously (reliably? fast enough?) on contact with cryogenic oxygen. TEA will. Getting over that activation hump with cryogenics can be a bit of a pain. Looking at both, at a guess, the TEA-TEB mix used is closer in stability and handling safety to TEB, while retaining the TEA's ability to ignite on contact with cryogenic oxygen. As @Jacke  points out, ignition time might also be a factor.

A couple of random search results that have some interesting comments on engine starting and TEA-TEB.  Note that TEA/TEB aren't used as hypergolic starting slugs as they are too reactive with too many propellants.  They are injected in parallel at some point in the rocket engine to start combustion.  This allows them to be used multiple times, depending on the amount carried.

https://space.stackexchange.com/questions/15403/why-is-tea-teb-chemical-ignition-used-instead-of-spark-ignition

Quote

Both the Saturn V and the Falcon 9 use TEA-TEB to ignite their kerosene-fueled engines. TEA-TEB is pyrophoric, igniting spontaneously on contact with air. This poses handling issues; it must be stored in nitrogen.

It's also an expendable resource which puts limits on the number of times an engine can be started in flight; the Falcon 9 only loads TEA-TEB for restarts on those engines that will be restarted on a given mission, reasonably enough, but this makes it susceptible to engine failures in the reentry and landing burns that could otherwise be recovered from by using different engines.

Some other engines, such as the hydrogen-fueled J-2 and RL-10, use electric spark ignition rather than a chemical starter. Both engines are reliable and capable of multiple restarts after launch.

https://www.astronomicalreturns.com/2020/06/get-tea-teb-kerosene-wont-light.html

Quote

Operational rockets today usually use one of two ignition methods: electric pyrotechnics or pyrophoric ignition fluid. Starting with electric pyrotechnics, these systems in rockets are conceptually no different than automobile spark plugs: a high-voltage electrical circuit makes a spark jump across a gap that's exposed to the rocket fuel, igniting it. But since rocket fuel is so energetic and under such high pressure, you can't just light the fuel at a single point, since that localized energy spike can produce uneven shock waves or cause unlit fuel to pool dangerously at the other end. Everything needs to light at once. To solve this, both the J-2 engine that powered the Saturn V upper stage and the Space Shuttle Main Engine (two famous hydrogen-powered engines) had a mechanism that swirled the hydrogen and oxygen at the moment of ignition before blasting it into the combustion chamber.

In contrast to hydrogen, RP-1 is surprisingly hard to ignite despite being a petroleum-derived product, and electric pyrotechnics proved unreliable, so pyrophoric ignition fluids are typically used for RP-1 burning engines. The term pyrophoric describes any substance that spontaneously combusts with air, no ignition source needed. For engine ignition, rocket scientists found that a ~15/85 mixture of triethylaluminum-triethylborane (TEA-TEB) was so pyrophoric, it could spontaneously ignite cryogenic oxygen so that by the time it reached the combustion chamber, the already-ignited oxygen would burn nicely with the RP-1.

 

[magnemoe]

On 12/10/2022 at 11:21 AM, DDE said:

Soviet Navy-pattern potassium superoxide CO2 scrubber. The plates of active ingredient are notoriously disagreeable with oil-contaminated water.

Also, that's a whole lot of asbestos lining that storage crate...

This is bad, water inside an damaged submarine is likely to have oil in it. , however mounting them in the roof build so water spray could not enter should work. Once water level hit the roof it don't matter as long as explosion is not strong enough to break bulkheads. 
And its better designs, you can buy scuba air rebreathers who I assume don't explode if moist as they have an backup air supply if the rebreather fails. 
Who is pointless if your blown up, now this is commercial but expensive but still pretty popular  among power users so safe. 

[K^2]

20 minutes ago, Jacke said:

A couple of random search results that have some interesting comments on engine starting and TEA-TEB.  Note that TEA/TEB aren't used as hypergolic starting slugs as they are too reactive with too many propellants.  They are injected in parallel at some point in the rocket engine to start combustion.  This allows them to be used multiple times, depending on the amount carried.

Yeah, that's why it has to ignite on direct contact with cryogenic oxygen. I wasn't able to find if it's used for gas generator ignition as well, or just the main chamber, but it would make perfect sense to inject TEA-TEB into LOX stream just before it hits the gas generator to start that going, and you really don't have much to work with there, so you really need something not just pyrogolic, but pyrogolic at cryo temperatures. I'm sure one can come up with workarounds, but this sounds like the simplest system for multiple restarts if you're prepared to eat the complexity of storage and handling. Since you need a relatively small amount, a dedicated TEA-TEB tank filled with inert gas sounds like a reasonably practical solution. Just don't let it leak. 

[sevenperforce]

2 hours ago, K^2 said:

I wasn't able to find if it's used for gas generator ignition as well, or just the main chamber, but it would make perfect sense to inject TEA-TEB into LOX stream just before it hits the gas generator to start that going, and you really don't have much to work with there, so you really need something not just pyrogolic, but pyrogolic at cryo temperatures.

For the Merlin 1D, helium is used to spin up the turbopumps first, followed by TEA-TEB ignition of the combustion chamber. It's possible that they simply use a bleed valve from the combustion chamber to the gas generator to ignite it.

[ARS]

So I recently saw a review on Youtube about a movie "The Wandering Earth". In case you don't know, it's a science fiction movie with 'science' part being so bad, it makes "The Core" science looks like "The Expanse". Let's get with the premise in a nutshell: The sun is expanding, we need to save humanity, we built thousands of city- sized rocket engines on Earth's surface and fly the damn planet like a spaceship straight to Proxima Centauri by using Jupiter to do gravity assist, which then goes haywire when one of the engine shuts down, leaving the planet with insufficient thrust and now it's being pulled into Jupiter, and the only way to save the humanity is by igniting Jupiter's atmosphere so the shockwave would throw the Earth out of Jupiter's gravity well
But that's not what I'm gonna ask here. Understandably, the movie is being mocked to death, mainly from being ignorant with so many laws of basic physics, astronomy and orbital mechanics. Most of the critics tend to point out about how insane the idea of putting rocket engines on Earth's surface is (and how it would mess up with Earth's geology and atmosphere), but one comment struck my interest:

Quote

I think they really missed a great "real science in fiction" opportunity. They completely glossed over the moon and the fact that it literally pulls at the Earth gravitationally. Most of the problems surrounding building giant rocket engines into the Earth can be addressed by building giant rocket engines into the moon instead and using it's gravity to pull the Earth. This is called a gravity tractor and was concieved as a way for a spacecraft to change the orbit of a much larger asteroid by simply orbiting and doing calculated engine burns to cause the spacecraft to add or take away the momentum of the asteroid. Moving a planet by pulling it along with it's large gravitationally bound moon is a little bit more feasible. The moon is airless, no atmosphere to interfere with efficient engine operation. It's solid and geologically dead. No popping every volcano on the planet simultaneously when you fire up the engines. You don't have to despin the Earth. It's covered in helium 3 fuel for fusion. As you tractor the planet out of the solar system you could slowly translate the moons motion from orbiting the Earth to leading it, acting as a planetary shield against impacts in transit from asteroids and interstellar dust. On the back side of the trip you can move the moon to following the Earth to brake entering the new system, and revert back to orbiting as it is steered into orbit around the new star.

So it's basically using Moon's gravity interacting with Earth, pulling at each other, and use that as a means of manipulating Earth's trajectory. Now I know the scale involved here (especially in terms of engineering) is so mind-bogglingly large, but assuming the moon's orbit could be manipulated (and you really don't care about it's effects on Earth's inhabitants), is this idea makes sense (In terms of physics)? I don't ask "is this possible", I ask "is this (in theory) possible?"

Edited December 12, 2022 by ARS

[SunlitZelkova]

2 hours ago, ARS said:

So I recently saw a review on Youtube about a movie "The Wandering Earth". In case you don't know, it's a science fiction movie with 'science' part being so bad, it makes "The Core" science looks like "The Expanse". Let's get with the premise in a nutshell: The sun is expanding, we need to save humanity, we built thousands of city- sized rocket engines on Earth's surface and fly the damn planet like a spaceship straight to Proxima Centauri by using Jupiter to do gravity assist, which then goes haywire when one of the engine shuts down, leaving the planet with insufficient thrust and now it's being pulled into Jupiter, and the only way to save the humanity is by igniting Jupiter's atmosphere so the shockwave would throw the Earth out of Jupiter's gravity well
But that's not what I'm gonna ask here. Understandably, the movie is being mocked to death, mainly from being ignorant with so many laws of basic physics, astronomy and orbital mechanics. Most of the critics tend to point out about how insane the idea of putting rocket engines on Earth's surface is (and how it would mess up with Earth's geology and atmosphere), but one comment struck my interest:

So it's basically using Moon's gravity interacting with Earth, pulling at each other, and use that as a means of manipulating Earth's trajectory. Now I know the scale involved here (especially in terms of engineering) is so mind-bogglingly large, but assuming the moon's orbit could be manipulated (and you really don't care about it's effects on Earth's inhabitants), is this idea makes sense (In terms of physics)? I don't ask "is this possible", I ask "is this (in theory) possible?"

Yes.

He goes over other options too (realistic alternatives exist to putting engines on the Moon).

EDIT- This only for moving the Earth away from the Sun though. It would not be possible to move it to another solar system without killing everything on it and making it uninhabitable, at which point it might have made more sense to build ships and travel to a star system with a rocky planet (still living in domes, but less effort than moving the planet).

Edited December 12, 2022 by SunlitZelkova

[JoeSchmuckatelli]

Maths question:

If I have a 1% chance of winning a lottery (1/100), and I buy 80 lottery tickets... what's my over-all chance of getting a 1?   (win).

(Any win = a win.  i.e. any time I get a 1, I win)

So I've googled this and just keep getting confused.  Mainly because they all want to talk about 6-sided dice rolls, and I don't have the background to convert what the websites are telling me about a 6-sided die into 'how many rolls of a 100 sided die are required to get a 1').

 

TIA.

[kerbiloid]

1-(1-1/100)⁸⁰ ~= 0.55

[K^2]

4 hours ago, JoeSchmuckatelli said:

Maths question:

If I have a 1% chance of winning a lottery (1/100), and I buy 80 lottery tickets... what's my over-all chance of getting a 1?   (win).

(Any win = a win.  i.e. any time I get a 1, I win)

So I've googled this and just keep getting confused.  Mainly because they all want to talk about 6-sided dice rolls, and I don't have the background to convert what the websites are telling me about a 6-sided die into 'how many rolls of a 100 sided die are required to get a 1').

 

TIA.

The trick is to realize that while there are many different ways of winning (how many tickets are winning, which ones they are), the alternative, i.e. not winning even once, can only happen one way: every single ticket loses. And that's much easier to compute. For independent events, probabilities are multiplicative. So the probability that an event with individual probability p happens N times in a row is just p^N. So odds of all 80 tickets losing is .99^80.

[sevenperforce]

5 hours ago, K^2 said:

So the probability that an event with individual probability p happens N times in a row is just p^N. So odds of all 80 tickets losing is .99^80.

For @JoeSchmuckatelli: as @kerbiloid's equation implies above, the odds of winning is the probabilistic inverse of that.

You have a 100% chance of either winning or losing. And as @K^2 correctly notes, your odds of losing 80 times when you have a 99% chance of losing each time is going to be 0.99^80 or 45%. So if you have a 45% chance of losing every time, and you have a 100% chance of either winning or losing, then your chance of NOT losing every time (i.e., winning at least once) is 100% - 45% = 55%.

17 hours ago, SunlitZelkova said:

20 hours ago, ARS said:

So I recently saw a review on Youtube about a movie "The Wandering Earth". . . . but one comment struck my interest:

So it's basically using Moon's gravity interacting with Earth, pulling at each other, and use that as a means of manipulating Earth's trajectory. Now I know the scale involved here (especially in terms of engineering) is so mind-bogglingly large, but assuming the moon's orbit could be manipulated (and you really don't care about it's effects on Earth's inhabitants), is this idea makes sense (In terms of physics)? I don't ask "is this possible", I ask "is this (in theory) possible?"

Yes.

He goes over other options too (realistic alternatives exist to putting engines on the Moon).

EDIT- This only for moving the Earth away from the Sun though. It would not be possible to move it to another solar system without killing everything on it and making it uninhabitable, at which point it might have made more sense to build ships and travel to a star system with a rocky planet (still living in domes, but less effort than moving the planet).

If we wanted to move Earth to another solar system, it might be advisable to simply make it a moon of Jupiter and then fly Jupiter itself into another solar system, using a fusion candle.

What is a fusion candle, you ask? 

I'm glad you asked.

(Even if you didn't ask.)

A fusion candle is thusly named because it is burnt at both ends, not unlike how a candle can be. You grab a large iron asteroid, reinforce it quite thoroughly, and hollow it out. Build city-sized fusion engines at both ends and giant intakes in the center, and then carefully sink it into a gas giant. Uranus would be best for this (let's leave jokes about sinking something into Uranus aside) because it has the lowest rotation speed of any giant in our system and the lowest mass and thus requires the least effort to move around. As the asteroid sinks into Uranus, fire up the intakes to suck in the Uranian atmosphere and ignite the lower fusion engine, holding the asteroid aloft on a column of its own flame. Then fire up the other engine, balancing appropriately, and start flying. It's a slow and steady process, but it's doable.

You can then use its gravity to come in and grab Earth and haul it off to a new home solar system. And you're carrying your own mini-sun with you, which isn't the worst of all possible arrangements.

[monophonic]

2 hours ago, sevenperforce said:

fire up the intakes to suck in the Uranian atmosphere and ignite the lower fusion engine, holding the asteroid aloft on a column of its own flame. Then fire up the other engine, balancing appropriately, and start flying. It's a slow and steady process, but it's doable.

Doable until hot exhaust ingestion causes a flameout, that is. Well, I guess that is what's called "engineering details."

But how much delta-V does one Uranus give with a one Earth payload? A quick jot in a spreadsheets gave me 2938455,386 m/s/s. Lot's of inaccuracies there, not to mention probable mistakes, but WOW! Of course you'll be spending majority of that keeping the candle in place, as you'll need to offset the outward thrust and gravity of Uranus the fuel tank. Let's throw in a round divide by three or so for that and call it an even

One Million Meters per Second Squared!
Insert Dr. Evil meme

So, where can we get with that?