For Questions That Don't Merit Their Own Thread

[Gargamel]

The number I always saw was 14, but that was a particular curated set of individuals, ~4 males ~10 Females, with specific generic markers to force diversity in later generations.     The offspring mating arrangements are planned out like 30 generations before some element of choice is allowed.   So, not a random sample.  

[kerbiloid]

36 minutes ago, Gargamel said:

~4 males ~10 Females

The males should be called Young, Addams, McCoy, and Quintal.

The female names are inessential.

Spoiler

https://en.wikipedia.org/wiki/Mutiny_on_the_Bounty

https://en.wikipedia.org/wiki/History_of_the_Pitcairn_Islands

[TheSaint]

On 5/18/2023 at 12:14 AM, SunlitZelkova said:

Alright, now I have another question. Is the minimum viable population for humans known? I have looked around for this but have gotten answers varying between 14 and 14,000.

One last thing- would it make sense to build a giant interstellar ark or a fleet of smaller ships? Would there be any pros to building a single ship?

More is always better. If you send out the minimum number of humans needed to maintain genetic viability, you're running the risk that some of them will die of peripheral causes before they have a chance to reproduce. If you send out one ship, you run the risk that it won't reach its destination at all because of mechanical failure, debris collision, etc. There's also the chance that your carefully selected destination world will wind up being a disaster zone for reasons you had no way to anticipate. (Read Legacy of Heorot or Destiny's Road by Larry Niven, et al, for some great examples of how that could happen.)

When I wrote my "Earth is dying, send out the arks" sci-fi setting for GURPS, each ship held 5,000 colonists in cold sleep and 100,000 frozen embryos for surrogacy. They sent out twelve ships, six of which resulted in successful colonies. That seems like a pretty reasonable effort/result to me.

[AckSed]

On a related question, how many humans could you cram into a generational ship of 10,000 tons? Because I heard that figure quoted for a sun-grazing solar-sail ship with a 1000+ km-wide inflatable "pillow" sail. By having an incredibly light sail, and diving to a perhelion of 0.05AU, it would reach 0.00264c in a day.

[kerbiloid]

2 hours ago, AckSed said:

how many humans could you cram into a generational ship of 10,000 tons?

10 000 t?
One. It's enough when it is

Spoiler

Zathras.

 

It will grow the next Zathras from a frozen embryo, to let the interstellar machine keep running.

 

[K^2]

6 hours ago, AckSed said:

On a related question, how many humans could you cram into a generational ship of 10,000 tons?

A modern oceanic cruise ship of about 100k tons will carry about 6k people. Working with only 10k tons, and having to do with life support, provisions, etc., not even accounting for provision, I'd say you'd be lucky to cram a few  hundred people.

[Shpaget]

On 5/20/2023 at 9:47 AM, K^2 said:

modern oceanic cruise ship

To be fair, cruise ships have a whole bunch of stuff dedicated to unnecessary luxury, while a generational ship would probably lack all the casinos and shopping malls and as a whole look a lot more like the crew quarters of the cruise ships than the passenger areas.

[TheSaint]

2 hours ago, Shpaget said:

To be fair, cruise ships have a whole bunch of stuff dedicated to unnecessary luxury, while a generational ship would probably lack all the casinos and shopping malls and as a whole look a lot more like the crew quarters of the cruise ships than the passenger areas.

Not necessarily true. If anything, a generation ship may have more square footage per capita than a cruise ship. Remember that these people have to spend their entire life on the ship. They don't get off in ports or fly home for vacations. Psychologically they're going to want open spaces, recreation, etc.

[kerbiloid]

5 hours ago, TheSaint said:

Remember that these people have to spend their entire life on the ship. They don't get off in ports or fly home for vacations.

A stereotypical generation ship will be named Hikiko Maru.

5 hours ago, TheSaint said:

Psychologically they're going to want open spaces, recreation, etc.

The first phase of the crew selection. Sort out all travellers and claustrophobes.

[sevenperforce]

On 5/20/2023 at 3:47 AM, K^2 said:

A modern oceanic cruise ship of about 100k tons will carry about 6k people. Working with only 10k tons, and having to do with life support, provisions, etc., not even accounting for provision, I'd say you'd be lucky to cram a few  hundred people.

Arguably the bulk of the tonnage of a modern oceanic cruise ship is, well, ship, and not passenger quarters or other spaces. Simply having a steady platform on the high seas takes a lot of structural mass.

A ship with artificial gravity will be heavier than something like the ISS, but still vastly lighter than an ocean liner.

[TheSaint]

2 hours ago, sevenperforce said:

Arguably the bulk of the tonnage of a modern oceanic cruise ship is, well, ship, and not passenger quarters or other spaces. Simply having a steady platform on the high seas takes a lot of structural mass.

A ship with artificial gravity will be heavier than something like the ISS, but still vastly lighter than an ocean liner.

You're also going to need shielding if you're going to be accelerating it to a fraction of the speed of light.

Another point to take into account: It's a generation ship, the implication being that the folks who board it will be having children and dying before the ship reaches its destination. The folks who reach the planet at the other end of its journey will never have known any life other than what they have lived on board the ship. If they have never known any life other than living on board a tiny tin can, you may not be able to get them off to live under big wide open skies. Agoraphobia could wind up being as much a problem as claustrophobia.

Personally, when I envision a generation ship, I think of one of the O'Neill cylinder  or Bernal sphere concepts from the 1970s. Make it self-sustaining for a couple hundred years, with a non-solar power source and a slow interstellar drive.

[AtomicTech]

13 minutes ago, TheSaint said:

You're also going to need shielding if you're going to be accelerating it to a fraction of the speed of light.

Shielding would be nice; I'd appreciate not being powderized

[SunlitZelkova]

Would it be possible to launch a nuclear fusion rocket propelled interstellar ark using nuclear pulse propulsion from a ground site in the far north? I.e. stage one is an Orion, stage two is a Daedalus.

Beyond the already existing issues with developing nuclear fusion and nuclear pulse propulsion, is there anything that would cause an issue?

In the story I am writing, a nuclear war occurred nearly a century ago, but climate change is destroying humanity and the Soviets and Americans team up to evacuate a portion of their nations to a nearby star system. Because nuclear weapons and rockets were banned following the war, the project must be carried out in secret in Alaska.

[Terwin]

Nuclear pulse propulsion works in space because the pusher-plate works as a shield.

In atmosphere, the heat an radiation can bounce off of air in addition to conduction and other methods of heat propagation that are not an issue in space.

While a ground-launch Orion is more realistic than a star-trek shuttle, you would need a fairing that probably weighs more than the rest of the ship put together to have anything survive to orbit, and even that would be highly questionable.

[sevenperforce]

1 hour ago, Terwin said:

8 hours ago, SunlitZelkova said:

Would it be possible to launch a nuclear fusion rocket propelled interstellar ark using nuclear pulse propulsion from a ground site in the far north? I.e. stage one is an Orion, stage two is a Daedalus.

Beyond the already existing issues with developing nuclear fusion and nuclear pulse propulsion, is there anything that would cause an issue?

In the story I am writing, a nuclear war occurred nearly a century ago, but climate change is destroying humanity and the Soviets and Americans team up to evacuate a portion of their nations to a nearby star system. Because nuclear weapons and rockets were banned following the war, the project must be carried out in secret in Alaska.

Nuclear pulse propulsion works in space because the pusher-plate works as a shield.

In atmosphere, the heat an radiation can bounce off of air in addition to conduction and other methods of heat propagation that are not an issue in space.

While a ground-launch Orion is more realistic than a star-trek shuttle, you would need a fairing that probably weighs more than the rest of the ship put together to have anything survive to orbit, and even that would be highly questionable.

In-atmo-launched nuclear pulse propulsion would actually be slightly MORE efficient because the air provides an additional reaction mass. However, getting off the ground initially is...challenging. Most designs would use solid boosters or similar tech to get moving before the pulses start dropping.

The pusher-plate would still act as a reasonably good shield in-atmo although the standoff distance might need to increase. Alternatively, it might be better to decrease the standoff distance and use a smaller nuke in-atmo, thus trapping more of the radiation behind the shield and relying on the increased impulse from aerodynamic remass to make up for the smaller nuke size.

Average acceleration isn't significantly different than an ordinary vehicle so the fairing is not a big deal.

[Terwin]

1 hour ago, sevenperforce said:

 

Average acceleration isn't significantly different than an ordinary vehicle so the fairing is not a big deal.

The fairing would not be for aerodynamics, but to protect the vessel from the heat and radiation.  This is not an issue in space because the pusher-plate acts as an umbrella and there is nothing else to redirect it.

In atmosphere, the pressure-wave becomes an issue that the pusher plate cannot stop as it will engulf the entire vessel.  The pressure wave is also bringing lots of vessel melting heat and any radiation that misses the pusher plate can bounce off air molecules and radiate the vessel.

The 'fairing' needs to resist the shock-wave, heat and radiation of many near-by nuclear blasts and likely needs to be even thicker than the pusher plate to do that.

[sevenperforce]

2 hours ago, Terwin said:

The fairing would not be for aerodynamics, but to protect the vessel from the heat and radiation.  This is not an issue in space because the pusher-plate acts as an umbrella and there is nothing else to redirect it.

In atmosphere, the pressure-wave becomes an issue that the pusher plate cannot stop as it will engulf the entire vessel.  The pressure wave is also bringing lots of vessel melting heat and any radiation that misses the pusher plate can bounce off air molecules and radiate the vessel.

Ah, I see where you're coming from.

Do you know of any studies reviewing the effects of atmospheric Orion?

With the radiation problem, I suspect that high enough standoff would make it work. The plasma envelope is opaque to x-rays so as long as the standoff distance is greater than the initial fireball radius there shouldn't be problems there. The flash will certainly transfer quite a bit of energy to the surrounding air, but air is famously quite transparent and so the amount of energy that is absorbed and re-radiated before the ship is out of range of the blast should be low.

The shockwave propagation around the pusher plate might be a bigger problem.

[kerbiloid]

blast-ship distance: 100 m
pusher plate diameter: 10 m

cone volume = (1/3) * 100 * pi * 10² / 4 ~= 2 620 m³.

air density: 1.225 kg/m³

air mass in the cone; 2 620 * 1.225 ~= 3 200 kg

***

tungsten membrane diameter: ~30 cm

tungsten membrane thickness: ~ 1 cm (as the liner is ~several mm)

tungsten density: 19 300 kg/m³

membrane mass: 0.01 * pi * 0.3² / 4 * 19 300 ~= 13 kg.

***

medium:plasma jet mass ratio ~= 3 200 / 13 ~= 246 times.

***

A blurry fountain of tungsten plasma will turn the air between the ship and the blast into an irregular fire cloud, and instead of precise hit, it will just turn into an explosion, partially focused in the ship direction.

The shockwave (absent in space) will move around the ship and smash it from sides.

The heat radiation of the overheated air cloud (absent in vacuum) , wider than the pusher plate,  will heat and partially irradiate the ship from sides.

Ol' boom-boom goes boom.

Nice for partially directed runway killer, but poor for motion.

[mikegarrison]

Launching an Orion-style ship from the ground has always been one of those "probably possible" but "almost certainly a bad idea for many reasons" things.

[StrandedonEarth]

… unless you need to launch a battleship to fight the alien invaders who hold the orbital high ground…

The first boom isn’t the problem. It’s getting the second boom to fire at the right time…

[SunlitZelkova]

Is there a general rule of thumb for how long it takes to slow down a ship with a magnetic sail?

Like “if your flight time without stopping would be 50 years, it will take +30% of that to brake”.

Just to be clear, I am asking in the context of interstellar travel.

[K^2]

1 hour ago, SunlitZelkova said:

Is there a general rule of thumb for how long it takes to slow down a ship with a magnetic sail?

No. For starters, your origin and destination stars might have entirely different characteristics, meaning your acceleration and deceleration rates may vary. But also, with a mag sail, most of the interstellar distance you're just coasting, so a given duration doesn't really tell you anything about how fast you're traversing that distance, meaning it's not correlated at all with how rapidly you can slow down.

To have a good chance of estimating the deceleration distance, you're going to need to know a few parameters about the destination star - ideally, the distribution of solar wind density and velocity as a function of the distance from the star, but you can probably get at least an estimate for these from temperature, luminosity, and the distance to heliopause. If you have to start braking early, you also want to know the velocity and density of the interstellar medium beyond the heliopause. Finally, you need some parameters for the ship. Mass, effective cross section of the sail, and how fast you'll be approaching the star would do. The math to compute deceleration from that is going to be pretty trivial, but you'll have to solve an integral equation over that acceleration to get your stopping point at the desired point in the system.

Now, I'm sure you can take all of that and come up with an empirical model that will get you into a ballpark, but you'll still need at least some of the above parameters for that model. I don't think you'll find significant shortcuts.

[Lisias]

I was vindicated! NASA, indeed, uses KSP for prototyping ideas!!  Nope, got screwed again!  

https://scifi.radio/2023/01/06/nasa-using-kerbal-space-program-for-virtual-experiments/

 

Edited May 28, 2023 by Lisias
nope. got screwed again!

[razark]

12 minutes ago, Lisias said:

https://scifi.radio/2023/01/06/nasa-using-kerbal-space-program-for-virtual-experiments/

Seems like a totally accurate source.

"The developer of Kerbal Space Program, Felipe Falanghe, has announced and has begun work on its successor, Kerbal Space Program 2..."

[Lisias]

57 minutes ago, razark said:

Seems like a totally accurate source.

"The developer of Kerbal Space Program, Felipe Falanghe, has announced and has begun work on its successor, Kerbal Space Program 2..."

Oh, well.. Got screwed again!  

The unhappy author had linked his source:

Quote

Since the day it was released, less than three years ago, KSP has grown to be so much more. Beneath its childish surface lies a complex physics system churning through mathematical calculations so expertly, real rocket scientists would blush to see it. KSP has even earned the respect of NASA — many of its employees play it regularly.

These past few months the team at KSP and the team at NASA have developed a professional, although distant, relationship. And this year they will begin to work together.

Soon the Kerbals will embark on the next phase of space exploration, more than a decade before their real-life human analogues. NASA hopes to land humans on an asteroid by 2025. It's their most daring mission in a half century, and they've asked the small team of eight developers headquartered in Mexico City to help promote that mission through their game.

https://www.polygon.com/features/2014/1/27/5338438/kerbal-space-program#:~:text=Doug Ellison works,on our adventure."

From having NASA employees playing regularly (including the article writer) and late teaming up with Squad to promote a mission, yeah, prototyping missions on KSP is too much of a stretch.

Some more years and I will try again!

 

Edited May 28, 2023 by Lisias
Missed link. Missed funny image.