How big is optimal for a scifi starship?

[KerikBalm]

Well, in such a fictional science scenario, just install inertial dampeners, and build as big as you want!

[Spacescifi]

36 minutes ago, KerikBalm said:

Why use google, when you can use math?

1) a=v^2/r; 2) v=distance/t; distance in this case is equal to circumference, which is 3) 2pi*r. So combine 2 and 3: v=2pi*r/t, and then combine that with 1) a= (2pi*r/t)^2/r ; a= 4*pi^2*r^2/r*1/t^2 =pi*4*r/t^2

So if we set a to 9.81 m/s (1 g), and t to 60 seconds. then 9.81= 4*pi^2*r/3600; r= 9.81*3600/(4pi^2) = 894.56 meters. That is the radius. Assuming a CoM near the center, the ship would have to be nearly 1,800 meters long to produce 1 G at that RPM. At 900 meters long, given the linear nature of a to r in this equation, you'd only get about 0.5 g's. Now the relationship with time is ^2, so to get to 1 G with this diameter would take 42 seconds.

But I have to ask, what sci fi setting requires that a ship makes a turn in less than a minute? Such a time is irrelevant for interplanetary or interstellar travel. Assuming the ship uses smaller shuttles to reach the surface of a planet, it doesn't need to turn fast for landing, because it won't land.

Assuming combat takes place at distances measured in light minutes (achievable with X ray lasers and apertures with radii, in the 10's of meters, which would seem to fit on ships of this size), that turn rate would be sufficient as well.

So first you need to establish how fast the ship needs to be able to rotate in the context of your Sci-Fi setting.

Just saying 1 rpm is the limit is arbitrary and doesn't get you to an answer.

 

A setting where big spaceships actually land on planets. It does not utilize smaller shuttles because it is primarily a liquid tanker shaped like a thick saucer. It needs to carry as much fluid as possible so it can ship it to developing colonies inhospitable to life naturally.

Also it has scifi engines with the necessary power to do all of that without nuking all in it's immediate wake.

So turning in a reasonable amount of time would be reasonable during air flight.

If I wanted spaceflight to have more forced encounters, I could simply have the warp/FTL method auto-adjust the spaceship's speed and trajectory to the destination target on dropping out of warp. The rest would be course corrections and slowing for landing/intercept. Afterall, that is what popular scifi already does essentially. It is not as if they ever mention adjusting for speed or trajectory before going to warp. They just always arrive that way.

 

 

 

22 minutes ago, KerikBalm said:

Well, in such a fictional science scenario, just install inertial dampeners, and build as big as you want!

 

Yes... I realized that some form of fiction is required to make a scifi shape like a saucer work when it flies forward.

 

Instead of inertial dampeners though...

 

Antigrav pods: insulates a crew member from g-force via the spaceship's acceleration, centrifugal force, or even a planet or star. Makes them weightless in the small pod. Rest of ship is still normal. Only in the antigrav pods is it always weightless.

The bridge is also ALWAYS weightless. Because desparate maneuvers are a lot easier to make when the crew is not effected.

The only restraints now are the structural integrity of the saucer.

Edited June 24, 2019 by Spacescifi
Antigravity

[KerikBalm]

1 hour ago, Spacescifi said:

A setting where big spaceships actually land on planets. It does not utilize smaller shuttles because it is primarily a liquid tanker shaped like a thick saucer. It needs to carry as much fluid as possible so it can ship it to developing colonies inhospitable to life naturally.

Why not just harvest from icy bodies with very low gravity? The amount of liquid on rocky worlds like Earth is quite small. If you want those elements/molecule, the best thing to do is to skim off the top of a small ice giant, or take from frozen moons/farther out kuiper belt like objects. Earth's mass is just 0.05-0.02% water (the higher value is if you count hydrated materials, and subsurface water, not just out oceans, lakes, rivers, and atmosphere), for example:

https://phys.org/news/2014-12-percent-earth.html

http://sciencenordic.com/earth-has-lost-quarter-its-water

Ganymede on the other hand is 2.5% the mass of Earth, and is about 50% water ice. so lets say 1.25% water which is 25x more than on Earth.

Callisto is a similar percent water, but only 1.8% the mass of Earth... so still much much more water to be found there than on the "wettest" rocky planet that we know of.

So in any sci fi setting, it makes more sense to get your liquids from small bodies where its frozen, and deliver them to where they will melt. If its for terraforming, you can just dump them on a collision course with the planet. If its smaller supplies for a sealed colony, a smaller surface to orbit shuttle makes more sense. At most your tanker need to land on a body with less surface gravity than the moon. I still see no need for high rotational rates, particularly if you just have your engines on a gimbal.

 

[Shpaget]

5 hours ago, KerikBalm said:

Ganymede on the other hand is 2.5% the mass of Earth, and is about 50% water ice. so lets say 1.25% water which is 25x more than on Earth.

That's one of those cases where intuition breaks apart when faced with hard data of reality. Before reading your post I never considered this, and would have happily and ignorantly claimed that there was much more water on Earth than on some backwater moon (pun intended).

[KerikBalm]

I should have said its total water mass is 1.25% the mass of Earth (not 1.25% water), thankfully you understood.

This just makes alien invasion stories where they come to steal our water (most recently, Oblivion?) all the more ridiculous, when they can get orders of magnitude more water from icy moons without having to deal with sneaky humans and their nukes/computer viruses/microbes

[Spacescifi]

7 hours ago, KerikBalm said:

Why not just harvest from icy bodies with very low gravity? The amount of liquid on rocky worlds like Earth is quite small. If you want those elements/molecule, the best thing to do is to skim off the top of a small ice giant, or take from frozen moons/farther out kuiper belt like objects. Earth's mass is just 0.05-0.02% water (the higher value is if you count hydrated materials, and subsurface water, not just out oceans, lakes, rivers, and atmosphere), for example:

https://phys.org/news/2014-12-percent-earth.html

http://sciencenordic.com/earth-has-lost-quarter-its-water

Ganymede on the other hand is 2.5% the mass of Earth, and is about 50% water ice. so lets say 1.25% water which is 25x more than on Earth.

Callisto is a similar percent water, but only 1.8% the mass of Earth... so still much much more water to be found there than on the "wettest" rocky planet that we know of.

So in any sci fi setting, it makes more sense to get your liquids from small bodies where its frozen, and deliver them to where they will melt. If its for terraforming, you can just dump them on a collision course with the planet. If its smaller supplies for a sealed colony, a smaller surface to orbit shuttle makes more sense. At most your tanker need to land on a body with less surface gravity than the moon. I still see no need for high rotational rates, particularly if you just have your engines on a gimbal.

 

 

I really do not have a problem with this in a fictional setting.

Moving entire worlds? No thanks.

Takes too long and really unnecesary.

I would sooner connect a portal between world's and siphon off an ocean to terraform rather than spend years moving a planet into position to crash it into another.

Also, is not water on other worlds radiated?

Like you cannot just thaw it out and drink it righr? It must be treated or processes somehow or else you get cancer?

Sea water around nuke blasts is known to be radioactive due to salts and minerals in the water.

I guess what I am saying is that space water won't be pure watet by anymeans due to cosmic rays ionizing the minerals and salts in the water.

[Kerbart]

On 6/23/2019 at 12:05 AM, Spacescifi said:

With a 900 meter radius or length, simply pitching or yawing it at ONE revolution per minute creates 1g at the ends/perimeter.

It’s hard to conceive that the center of gravity would be at one end. Hence, a 900m radius would be half the length, making the limit (for the, as others pointed out, rather arbitrary one rev/min) 1,800m #pedantry.

It is practical, though, that there’s a limit to the rotation speed of a vessel, which scales up linear with size and with the square of the desired rate, something I never thought off.

Making behemoths does impose limits on maneuverability, unless some handwaving is done (artificial gravity?) to explain it away.

The side effect of crippling an enemy vessel’s artificial gravity generator would hence reduce its ability to maneuver, which is an interesting side effect.

[kerbiloid]

Humans should put thermonukes on coral atolls and warn the aliens that they will make the water undrinkable.

 

[Kerbart]

9 hours ago, KerikBalm said:

Why use google, when you can use math?

1) a=v^2/r; 2) v=distance/t; distance in this case is equal to circumference, which is 3) 2pi*r. So combine 2 and 3: v=2pi*r/t, and then combine that with 1) a= (2pi*r/t)^2/r ; a= 4*pi^2*r^2/r*1/t^2 =pi*4*r/t^2

Well,done! Two minor remarks:

Using superscripts for squas makes things a lot easier to read. The forum supports it, so why not write x² instead of some arbitrary x^2 notation used by some programming languages?

I’ve never done kinematics with anything else than radians as the angle unit of choice. Why make things so much more complicated using degrees? You get rid of a giant bunch of 2π in your calculations that way.

[Spacescifi]

47 minutes ago, Kerbart said:

It’s hard to conceive that the center of gravity would be at one end. Hence, a 900m radius would be half the length, making the limit (for the, as others pointed out, rather arbitrary one rev/min) 1,800m #pedantry.

It is practical, though, that there’s a limit to the rotation speed of a vessel, which scales up linear with size and with the square of the desired rate, something I never thought off.

Making behemoths does impose limits on maneuverability, unless some handwaving is done (artificial gravity?) to explain it away.

The side effect of crippling an enemy vessel’s artificial gravity generator would hence reduce its ability to maneuver, which is an interesting side effect.

 

I like saucer starship desogn, including this:

 

That said, the engines are off place, and even if I put them properly in the center and behind the ship the crew would fall backwards. That does not even include the centrifugal gravity issues.

To solve the centrifugal gravity issues, I would make the inner deck walls concentric. The outer most ring deck would be a swimming pool. The next outermost deck would be the crew area, as would be the rest of the inner decks.

In the event of manevering or acceleration, the crew would run with oxygen masks to the outer ring pool, grab onto a holding beam and foothold underwater, and ride out the acceleration and manevers until they are over. The hand and footholds would keep them from flying loose, and the water would make them safer if anyone did fly loose.

[KerikBalm]

53 minutes ago, Kerbart said:

I’ve never done kinematics with anything else than radians as the angle unit of choice. Why make things so much more complicated using degrees? You get rid of a giant bunch of 2π in your calculations that way.

There's nothing about degrees or radians in those equations.

The ratio of the circumference of a circle to its diameter is pi. Not one radian. Even when you do things in radians, you use pi. Instead of saying turn 360 degrees, you say turn 2 pi radians...

There's no way around it, you need to use pi to determine the circumference, you need the circumference to determine  velocity, and you need velocity to determine acceleration

I suppose I could use superscripts, but I remember when we had to type 

Quote

blah blah

edit: ah, turns out that still works, I meant (ignore spaces): [ quote ] blah blah [/ quote ]

and such on forums to make quotes :p Its just habit

Edited June 24, 2019 by KerikBalm

[Kerbart]

9 minutes ago, KerikBalm said:

There's nothing about degrees or radians in those equations.

The ratio of the circumference of a circle to its diameter is pi. Not one radian. Even when you do things in radians, you use pi. Instead of saying turn 360 degrees, you say turn 2 pi radians...

There's no way around it, you need to use pi to determine the circumference, you need the circumference to determine  velocity, and you need velocity to determine acceleration

Let’s see. Centripetal acceleration is: a = v²/r, with velocity being v=ωr (ω being angular velocity). That makes a = ω²r²/r, or a=ω²r. Note how we’ve gotten the whole equation down to something rather simple without using π once.

At this point we can plug in one revolution per minute, being ^2π/₆₀ or approximately 0.1047. Instead of littering “pi” all over the place, so yes, it does make a difference in readability.

As to “that’s they way I’m used to doing it” as an excuse to post equations as unreadable as possible... Sure, it’s an excuse. But wouldn’t you rather have more people read what you write, than less? A little bit of effort goes a long way in communicating your ideas. Engineers often claim that people can be swayed by “pretty pictures,” but the opposite is true as well; if your message is hard to read, it won’t be read by many.

[KerikBalm]

sure, you can do it that way, but that doesn't mean that I was using degrees. Nor was I using radians. I used an equation relating the circumference of a circle to its radius. I didn't specify that you could divide the arc of a circle into 360 units, or 2 pi units. The closest thing to a unit there is the concept of "one circle" which is neither degrees nor radians.

Also you used pi once in the equations. Each equation I gave used pi once. the difference is I repeated the equation multiple times, showing the derivation from the simpler equations.

My equations were just: "1) a=v^2/r; 2) v=distance/t;   distance in this case is equal to circumference, which is 3) 2pi*r"

Note that only 3) uses pi, and the combined equation is just:

a=π²4r/t²

all you did was replace v = d/t, with v=wr. You combined it with a=v²/r, without using π once, but I can do the same: a=(d/t)²/r. In either case, it is at that point that π needs to be injected into it.

In my case its replacing d with 2πr, in your case its replacing ω with  ^2π/_t 

In either case, it seems to me that we use pi the same amount. I just substituted pi in sooner than you did (you waited to the last step), so you see it repeated in each step.

Also, I don't find it hard to read, and aside from being used to it, that way of writing it always works to convey the idea, whereas there are many cases where for whatever reason, superscripts are not available.

Edited June 24, 2019 by KerikBalm

[Klapaucius]

2 hours ago, Spacescifi said:

 

I like saucer starship desogn, including this:

 

That said, the engines are off place, and even if I put them properly in the center and behind the ship the crew would fall backwards. That does not even include the centrifugal gravity issues.

To solve the centrifugal gravity issues, I would make the inner deck walls concentric. The outer most ring deck would be a swimming pool. The next outermost deck would be the crew area, as would be the rest of the inner decks.

In the event of manevering or acceleration, the crew would run with oxygen masks to the outer ring pool, grab onto a holding beam and foothold underwater, and ride out the acceleration and manevers until they are over. The hand and footholds would keep them from flying loose, and the water would make them safer if anyone did fly loose.

Turn in your geek cap!  Those nacelles don't generate thrust. They house the warp field generators and are placed outboard as part of the warp design.  Impulse power on Federation ships was usually located at the rear of the saucer.

 

https://memory-alpha.fandom.com/wiki/Nacelle

Edited June 24, 2019 by Klapaucius

[Spacescifi]

20 minutes ago, Klapaucius said:

Turn in your geek cap!  Those nacelles don't generate thrust. They house the warp field generators and are placed outboard as part of the warp design.  Impulse power on Federation ships was usually located at the rear of the saucer.

 

https://memory-alpha.fandom.com/wiki/Nacelle

 

I knew that. I merely mix reality and scifi see what fits and what does'nt.

Basically, with good enough internal design and a few hull modifications, I could make even this design a viable SSTO, provided I have sufficient thrust constant acceleration.

[Guest]

4 hours ago, kerbiloid said:

Humans should put thermonukes on coral atolls and warn the aliens that they will make the water undrinkable.

 

You already cant drink sea water =P

soon it will be mostly plastic anyway so we won’t need to worry >_<

Edited June 24, 2019 by Guest

[Spacescifi]

10 hours ago, Kerbart said:

It’s hard to conceive that the center of gravity would be at one end. Hence, a 900m radius would be half the length, making the limit (for the, as others pointed out, rather arbitrary one rev/min) 1,800m #pedantry.

 

 

Even so, 900 meters would mean 0.5 g's, or half of 1g. Which is more than mars and the moon's gravity.

Centrifugal force will pull the crew to the edge or ends, and 50% of Earth gravity will still pull really good. 

My point is that the bigger a scifi spaceship is, the more it is subject to centrifugal force affecting the crew.

Assuming the Behemoth vessel is also an air flying SSTO, suddenly quicker turning becomes mecessary.

 

I am at the point that I realize I have to insert fiction to make scifi starship designs that are non-optimal with known physics optimal in scifi.

Some call them inertial dampeners. Mine would be a specific fictional array of devices emitting antigravity repulsor waves in sync with the ship's roll, yaw, or forward acceleration. Thus centrifugal force due to yawing in space would be repelled so as to negate it's effect on the inside of the ship. Neither would the crew fall backward when the ship flew forward, nor fall to the sides when it rolled.

On the other hand, the ship would still be subject to gravity from falling to a planet.

Interestingly, if you watch closely, the millenium falcon seems to operate in this manner.

Meaning it's forward acceleration has zero effect on the passengers, but the gravity of the planet does effect it, shown by the robot falling on the ceiling when the ship flew upside down.

 

(Fixed it)

Edited June 25, 2019 by Spacescifi
Fixed vid

[Shpaget]

Star Trek calls it inertial damper.

[Nightside]

2 hours ago, Spacescifi said:

 

Even so, 900 meters would mean 0.5 g's, or half of 1g. Which is more than mars and the moon's gravity.

Centrifugal force will pull the crew to the edge or ends, and 50% of Earth gravity will still pull really good. 

 

I don’t understand why you are worried about people withstanding 1g - I withstand 1g all day long.

Edited June 25, 2019 by Nightside

[Nightside]

2 hours ago, Spacescifi said:

Some call them inertial dampeners.

 

1 hour ago, Shpaget said:

Star Trek calls it inertial damper.

Inertial dampeners are a real thing - also known as Tuned Mass Dampeners they are installed in tall buildings to prevent amplification of periodic seismic loads, and in plenty of other mechanical applications. 

[Spacescifi]

21 hours ago, Nightside said:

I don’t understand why you are worried about people withstanding 1g - I withstand 1g all day long.

 

You would not want to fall sideways into a wall would you? Centrifugal g-force can do that.

Yet I found a low tech solution.

 

Take these seats individually and put them on bases that can rotate. Sync them with the nav computer.

Now when the ship pitches, rolls, or yaws, you will always be lying back.

Always be strapped in during acceleration or maneuvers! Especially with massive ships, as even slow rotation generates g-force.

Edited June 26, 2019 by Spacescifi

[kerbiloid]

 

Expanse, Razorback.

Spoiler

 

 

Similar (but  more primitive) seats in Interstellar, Ranger.

(Though, if the Ranger crew is tilting or not, depends on the authors wish.
When they want to show hi-tech, the seat tilt.
When they want to show how heroically the crew withstands accelerations, they tilt only heads with suffering faces.)

Edited June 26, 2019 by kerbiloid

[Spacescifi]

45 minutes ago, kerbiloid said:

 

Expanse, Razorback.

  Reveal hidden contents

 

 

Similar (but  more primitive) seats in Interstellar, Ranger.

(Though, if the Ranger crew is tilting or not, depends on the authors wish.
When they want to show hi-tech, the seat tilt.
When they want to show how heroically the crew withstands accelerations, they tilt only heads with suffering faces.)

 

There is no excuse not to use rotating seats in written scifi.

What is the alternative? Fancy inertial dampeners?

When in doubt, I suggest going with the cheaper option which does not require fiction.

If reality is good enough, I use it. If it is'nt, I make fiction for science fiction.

[wumpus]

17 hours ago, Spacescifi said:

 

There is no excuse not to use rotating seats in written scifi.

What is the alternative? Fancy inertial dampeners?

When in doubt, I suggest going with the cheaper option which does not require fiction.

If reality is good enough, I use it. If it is'nt, I make fiction for science fiction.

Er, why?  Military craft might need sudden unplanned acceleration (either intentionally or momentum transfer from enemy fire), but ordinary spacecraft will likely have the seats normal to the engine thrust (obviously anything that normally faces the other way has to be able to rotate in the "right" direction).  Exceptions might include aligning the spacecraft away from the local star for radiation shielding while burning in a different direction.

PS: I really think the answer is "as many characters as the story requires".  Multi-generational ships would have genetic reasons to have a fairly high number of passengers, but most of them could well be frozen embryos.  For long term voyages, you might have to have astronauts that can "cover all the bases", but then they look a lot like astronaut/cosmonauts with "the right stuff" (i.e. pretty much all of current astronauts where NASA can be unbelievably selective as to who can go to space), and you don't need quite so many [it also means that your characters are "really" experts in any one field, which means that not knowing something that can't be easily googled isn't a real error.  The character just didn't know that.]