The 1000D Limit and Maneuver Drives (A Different Take)

[Terry Mixon]

There has been some robust conversation in other threads about the 1000D limit where maneuver drives fail to generate thrust because they are too distant from the gravity sources.

That got me to thinking. Is it really isolated out there? I submit to you that it is not. It might be too far away from the star(s) or planet(s) to interact, but there is another handy gravitational source: the center of the Milky Way.

Sagittarius A* masses 4.3 million suns, the area within one parsec of it holds 10 million stars, and the galactic bulge that surrounds it holds 1/5 the mass of the Milky Way. I can’t find it now, but something I read suggested the galactic bulge had 10 billion stars.

That is a lot of mass and it is relatively close to Charted Space. The bulge is about 13,000 light years in diameter and Terra is 26,000 light years from the center of the bulge.

I respectfully submit the entirety of Charted Space is well within the gravitational hold of the galactic core to count for the maneuver drive continuing to work. Heck, it might be inside the 100D jump limit.

Thoughts?

 

[Condottiere]

A black hole would indicate that actual gravitational force would be part of the equation in defining that limit.

 

[Terry Mixon]

A black hole would indicate that actual gravitational force would be part of the equation in defining that limit.

I believe that it is gravity defined, yes. It’s just simpler for the players/GMs as a game mechanic to use the diameter of the mass.

 

[Limpin Legin]

There has been some robust conversation in other threads about the 1000D limit where maneuver drives fail to generate thrust because they are too distant from the gravity sources.

Surely the velocity of the ejected fuel produces thrust on the spaceship? So if velocity of ejected fuel is known , then the change of velocity of the rocket as a result of burning all of its fuel can be calculated.

9.11: Rocket Propulsion

A rocket is an example of conservation of momentum where the mass of the system is not constant, since the rocket ejects fuel to provide thrust. The rocket equation gives us the change of velocity …

phys.libretexts.org

Also,

[In general relativity,] rather than two particles attracting each other, the particles distort spacetime via their mass, and this distortion is what is perceived and measured as a "force". In such a model one states that matter moves in certain ways in response to the curvature of spacetime,[7] and that there is either no gravitational force,[8] or that gravity is a fictitious force.[9]
Source: various sources via Wikipedia

Gravitational acceleration - Wikipedia

en.wikipedia.org

 

[Nelphine]

There has been some robust conversation in other threads about the 1000D limit where maneuver drives fail to generate thrust because they are too distant from the gravity sources.

That got me to thinking. Is it really isolated out there? I submit to you that it is not. It might be too far away from the star(s) or planet(s) to interact, but there is another handy gravitational source: the center of the Milky Way.

Sagittarius A* masses 4.3 million suns, the area within one parsec of it holds 10 million stars, and the galactic bulge that surrounds it holds 10 billion stars. I can’t find it now, but something I read suggested the galactic bulge had 10 billion stars.

That is a lot of mass and it is relatively close to Charted Space. The bulge is about 13,000 light years in diameter and holds 1/5 the mass of the Milky Way. Terra is 26,000 light years from the center of the bulge.

I respectfully submit the entirety of Charted Space is well within the gravitational hold of the galactic core to count. Heck, it might be inside the 100D jump limit.

Thoughts?

I'd say no. Unlike a star, the galactic bulge isn't 'full' of mass (not that a star is either, but even 10 million stars within one parsec isn't very full. Space is Really Big, Mind Bogglingly Big).

So the area can't be just added up and used as a whole to determine the base diameter. So next, maybe you could pretend you smash all those stars into one big object, and use that as the base diameter.

Even 10 million stars, all jammed together into one object.. doesn't go that far. 1000 times that is far far far far short of 26000 light years. It's short of 26 light years (so even the 10 billion star version won't have a 1000D that reaches charted space).

 

[MongooseMatt]

Funnily enough, Geir was talking about calculating this just last night...

 

[Nelphine]

so, assuming average star is the size of the sun (i realize that's a silly assumption, but meh, i don't feel like figuring out what the average size actually is)

volume of a sphere is (pd^3)/6 (where p = pi because i also don't feel like going and figuring out an actual special character)

the sun is ~1.4 million km diameter

a light year is ~ 9.5 trillion km

therefore, 26 light years is ~ 250 trillion km

so, we calculate the number of stars that fit into a sphere of diameter 26 light years (this would be enough stars such that if they were all smashed into a single object, their 1000D would reach charted space)

(p*250,000,000,000,000^3)/6 / (p*1,400,000^3)/6
= 160,000,000,000,000,000,000,000 stars (160 sextillion stars)

which, considering the upper bound mentioned of 10 billion stars in our galactic bulge, means we would need to have

16,000,000,000,000 galaxies worth of galactic bulges to have enough stars for the 1000D to reach charted space.


i'm sure i'm off by a factor of 1000 somewhere. possibly multiple places. it doesn't matter, there aren't remotely enough stars. they could all be black holes the size of Sagittarius A*, and we could assume that gravity took the place of diameter, and so each one was worth 4.3 million suns, and it still wouldn't be close. Space is Really Big, and stars, even galactic center black holes, are Really Small. (Which also gets in to Dark Matter and why its needed. All the mass in the galaxy simply isn't remotely enough to keep our galaxy together. Unlike planets orbiting a star due to the gravity exerted by the mass of the star, there is not a known explanation for why galaxies orbit galactic centers, even though they obviously do.)

Mind you, this is probably a good thing, so that we don't have to worry about things like the 100D jump shadow of the galactic center screwing things up.

 

[Sigtrygg]

The fact remains that all the way out at the orbit of Sol the effect of the gravity of the milky way is to hold things in orbit unless you have an escape velocity of 537km/s. If gravity is still strong enough to keep you bound to the galaxy you should be able to couple with that using gravitics. If you get up to 41km/s you can escape the solar system and yet you are still bound to the galaxy.

 

[Nelphine]

Yup, it's just not due to the galactic center per se.

So trying to make rules based around gravity we don't fully understand seems difficult.

If we went by escape velocity, we'll, it's a mere 11 km/s for earth. But being within 100D of Earth means you can't jump at all.

So that should imply, by escape velocity, there's too much gravity in the galaxy to ever jump, anywhere. Therefore, since we can jump, it appears we can't base it on escape velocity.

 

[phavoc]

If you are saying the drive is non-functional at 1000D, how much more effecient is it at 1D from the star?

 

[Terry Mixon]

Funnily enough, Geir was talking about calculating this just last night...

I mentioned it in another thread back then and might have sparked the urge.

 

[Terry Mixon]

I'd say no. Unlike a star, the galactic bulge isn't 'full' of mass (not that a star is either, but even 10 million stars within one parsec isn't very full. Space is Really Big, Mind Bogglingly Big).

So the area can't be just added up and used as a whole to determine the base diameter. So next, maybe you could pretend you smash all those stars into one big object, and use that as the base diameter.

Even 10 million stars, all jammed together into one object.. doesn't go that far. 1000 times that is far far far far short of 26000 light years. It's short of 26 light years (so even the 10 billion star version won't have a 1000D that reaches charted space).

I agree. It’s the mass that matters, not the strict space it occupies. 1/5 of the mass of the Milky Way is there and it exerts more force on us here at Sol than our own star, seeing as the escape velocity from the star is less than 1/10th that of from the Milky Way.

 

[Terry Mixon]

Yup, it's just not due to the galactic center per se.

So trying to make rules based around gravity we don't fully understand seems difficult.

If we went by escape velocity, we'll, it's a mere 11 km/s for earth. But being within 100D of Earth means you can't jump at all.

So that should imply, by escape velocity, there's too much gravity in the galaxy to ever jump, anywhere. Therefore, since we can jump, it appears we can't base it on escape velocity.

That was one of the implications that occurred to me as well. Escape velocity is a variable calculated based on gravitational pull, so it is an indicator that if gravity is a metric in maneuver drives and jump drives, that might very well break the game mechanic. I wasn’t looking that close when the idea occurred to me, and we of course won’t break the game, but as a thought exercise, I just had to ask the question.

 

[MasterGwydion]

I just did the math using google. The galactic core in our galaxy has a mass of approximately 4 million times the mass of our sun. The sun is about 1.4 million kilometers in diameter. Google Math tells me that something 400,000,000% bigger than our sun will have its diameter increased by about 320 times for a final 100D and 1,000D limit of 46 billion kms and 460 billion kilometers. Or just under 310AUs and 3,100AUs. Each is way less than a parsec.

 

[Terry Mixon]

If you are saying the drive is non-functional at 1000D, how much more effecient is it at 1D from the star?

I don’t believe they’ve ever indicated it gets more effective the stronger the gravity.

 

[MasterGwydion]

I agree. It’s the mass that matters, not the strict space it occupies. 1/5 of the mass of the Milky Way is there and it exerts more force on us here at Sol than our own star, seeing as the escape velocity from the star is less than 1/10th that of from the Milky Way.

Mass is irrelevant. This is Mongoose Traveller.

 

[Nelphine]

I agree. It’s the mass that matters, not the strict space it occupies. 1/5 of the mass of the Milky Way is there and it exerts more force on us here at Sol than our own star, seeing as the escape velocity from the star is less than 1/10th that of from the Milky Way.

nah, most of that escape velocity requirement is due to dark matter, not the mass of the galactic center. the mass in the galactic center simply isn't enough to impact us.

 

[Terry Mixon]

I just did the math using google. The galactic core in our galaxy has a mass of approximately 4 million times the mass of our sun. The sun is about 1.4 million kilometers in diameter. Google Math tells me that something 400,000,000% bigger than our sun will have its diameter increased by about 320 times for a final 100D and 1,000D limit of 46 billion kms and 460 billion kilometers. Or just under 310AUs and 3,100AUs. Each is way less than a parsec.

The galactic core and the galactic bulge are two separate things. Sagittarius A* alone has a mass of 4 million suns. The very innermost part of the core adds 10 million more stars, but the bulge itself has 1/5 the mass of the entire Milky Way.

 

[Condottiere]

Sudden fall off, not cliff.

 

[MasterGwydion]

The galactic core and the galactic bulge are two separate things. Sagittarius A* alone has a mass of 4 million suns. The very innermost part of the core adds 10 million more stars, but the bulge itself has 1/5 the mass of the entire Milky Way.

Right, but Traveller doesn't use mass anymore so you will have to do it by equivalent volume.