Need help with orbital mechanics

[Pyromancer]

The situation:

A binary system consists of two nearly identical brown dwarfs with the following properties:
mass = 2.5e28 kg (each)
diameter = 1.5e8 m (each)
distance from each other = 1.5e9 m

There is a space station orbiting the centre of mass of the two brown dwarfs at a distance of 1.5e10 m (around the 100D limit).

The problem:
A spaceship starts at the space station, wants to touch the atmosphere of one of the brown dwarfs and then return to the space station. It has a Thrust 1 manoeuvre drive that provides 10 m/s² of continuous acceleration.

How long does the trip take?

Additional question:
The spaceship now wants to spend some time (for skimming fuel) in the upper atmosphere of the brown dwarf. Is this even possible, only taking into account the orbital mechanics and the propulsion system?

I only need rough estimates.

 

[Hopeless]

Wouldn't it be easier if they went to slingshot around that brown dwarf scooping up fuel as part of the course to pick up momentum for the swing around and back where they came from?

 

[Pyromancer]

Yes, that's what I want, but I have no clue how to calculate it.

 

[phavoc]

I don't have my booms with me, but if you are at the 100 limit you'd take the average time to travel 100 at 1g, then add in the amount of time it takes to refuel, roughly 4 to 6 hours. Technically you could get a potential speed bump by doing a slingshot as part of your maneuver, but that's a lot more math.

 

[Pyromancer]

The gravitational acceleration at the edge of the star's atmosphere is 34 G. With only 1 G thrust, I need some kind of slingshot manoeuvre to escape from it.

 

[Tom Kalbfus]

So your brown dwarf has 13.6 Jupiters worth of mass each, this is one of the lighter ones, still would probably be about the size of Jupiter.

I don't have my booms with me, but if you are at the 100 limit you'd take the average time to travel 100 at 1g, then add in the amount of time it takes to refuel, roughly 4 to 6 hours. Technically you could get a potential speed bump by doing a slingshot as part of your maneuver, but that's a lot more math.

Also some brown dwarfs will glow a dull red, they are probably hot enough to cook you, hotter than Venus most likely, the gravity would likely be in the range of tens to hundreds of gs, brown dwarfs are denser than most nondegenerent stars (white dwarfs, neutron stars, black holes etc.) With no fusion going on at the core, a brown dwarf packs it in quite a bit. You can have up to 80 Jupiters worth of stuff packed into the volume of one Jupiter, since Jupiter's gravity is around 2.5 gs, that means you could have an atmosphere gravity of up to 200 gs, if a spaceship slows down too much while skimming, its manuever drive won't be able to get it back into orbit again! Brown Dwarfs also have unusual weather, where it sometimes rains iron! You definitely don't want to plunge too deep into a brown dwarf's atmosphere while refueling!

 

[Pyromancer]

Those are ultra-cool brown dwarfs, spectral class Y, with a temperature of about 300-350 K, cold enough that skimming fuel is possible.

 

[Hopeless]

Would it make more sense to just shoot small to large asteroids on a collision course with something on the opposite side of the brown dwarf so rather than slow down it impacts distributing whatever material it picked up during its flight path through the outer edges of the brown dwarf gathering up materials and fuel as it does so they can safely siphon them off once they can isolate the remains of their erstwhile bucket?

Just how close do you need to go to scoop fuel?

Can you detonate some kind of warhead to instigate a solar flare shooting said material clear enough so you could scoop it up safely?

 

[phavoc]

Fuel scooping from an atmosphere requires a certain density. I don't think if you caused some gases to be ejected from the atmosphere of the brown dwarf that you'd be able to successfully scoop any fuel. Likewise hurling debris at it wouldn't be sufficient.

There's so old CT material out there that talks about gas giant atmospheres and about at what altitudes would a starship be able to successfully gather gases.

 

[Tom Kalbfus]

Fuel scooping from an atmosphere requires a certain density. I don't think if you caused some gases to be ejected from the atmosphere of the brown dwarf that you'd be able to successfully scoop any fuel. Likewise hurling debris at it wouldn't be sufficient.

There's so old CT material out there that talks about gas giant atmospheres and about at what altitudes would a starship be able to successfully gather gases.

If you could get both brown dwarfs to collide with each other however, that would definitely cause a splash!
You know what would be more interesting? Have a white dwarf and a red giant both of equal mass, the red giant is expanding outward towards the orbiting white dwarf, and the white dwarf is pulling some of the red giant's outer atmosphere onto itself and is thus adding mass. There is a certain maximum limit to the mass of a white dwarf, go beyond that and the white dwarf will collapse into a pulsar with an accompanying Type II supernova explosion. These sorts of explosions are very predictable, all the observer has to do is measure the mass of the white dwarf and the rate at which more mass is flowing into it, and he will know when this white dwarf will go supernova, it is an interstellar time bomb. Imagine the places you could go if you designed a jump drive that was powered by a supernova explosion!

 

[FallingPhoenix]

The gravitational acceleration at the edge of the star's atmosphere is 34 G. With only 1 G thrust, I need some kind of slingshot manoeuvre to escape from it.

Given that in the CRB you can have gravity levels of greater than 1G on worlds you generate, and there's no mention anywhere of 1G ships being limited as to worlds they can visit, I always assumed that the Traveller drive includes some sort of null-G part, so you get ~10 m/s^2 acceleration no matter where you are.

This also avoids the problem of trying to dive into an atmosphere while traveling at escape velocity...

 

[NOLATrav]

Assuming the brown dwarf is the size of an average super Jovian, say 300,000 km diameter, you can use the transit times table.

300,000 * 100 = 30,000,000 km = 30.42 hrs one way at Thrust 1 so 60.84 hrs + 1D6 hrs to refuel for a round trip.

This would be a turn-n-burn maneuver on the way out, if you maintain constant acceleration (to defeat the BD's surface gravity?) you can cut the time in half to 15.21 hrs. I guess you would have extra velocity coming back if you did that, could probably reduce the return trip, say 25%, to 22.82 hrs so now you're down to 38.03 + 1D6 hrs.

Definitely back-of-the-envelope and some handwavium but at least it makes a bit of sense.

Note the time of 30.42 hrs is from MgT beta Sept rev Travel Times table.

Hope that helps!

 

[Pyromancer]

Assuming the brown dwarf is the size of an average super Jovian, say 300,000 km diameter, you can use the transit times table.

It's about 150,000 km diameter, actually, only a little bit bigger than Jupiter.

This would be a turn-n-burn maneuver on the way out, if you maintain constant acceleration (to defeat the BD's surface gravity?) you can cut the time in half to 15.21 hrs. I guess you would have extra velocity coming back if you did that, could probably reduce the return trip, say 25%, to 22.82 hrs so now you're down to 38.03 + 1D6 hrs.

Definitely back-of-the-envelope and some handwavium but at least it makes a bit of sense.

Just accelerating towards a brown dwarf using 1 G thrust + gravity takes a little bit under 15 hours. Handwaving some kind of double slingshot to increase the time to skim fuel, a round trip time of about 32 hours seems reasonable.

 

[Tom Kalbfus]

Those are ultra-cool brown dwarfs, spectral class Y, with a temperature of about 300-350 K, cold enough that skimming fuel is possible.

And 34 gs, that is an acceleration of 340 meters per second squared! You wouldn't want to be caught in a rain storm or a hail storm while skimming for fuel! they will come down like bullets!
Another problem is if they go walking around outside on top of the ship, and look over the edge, the weight of one's head would flip him over and send him plummeting down towards the center of the brown dwarf at a supersonic terminal velocity!

I'm not even sure how much the shi's grav plating would shield one from the planet's hideous gravity. Lets say its a scout courier and the grav plating shields him from the brown dwarf directly below his feet, if he moves towards the side of the ship, he may get pulled sideways as part of the planet below comes into view, the closer he comes to the edge of the ship, the harder the gravitational tug sideways, until he flips over the side and straight down!

 

[Pyromancer]

Those are ultra-cool brown dwarfs, spectral class Y, with a temperature of about 300-350 K, cold enough that skimming fuel is possible.

And 34 gs, that is an acceleration of 340 meters per second squared! You wouldn't want to be caught in a rain storm or a hail storm while skimming for fuel! they will come down like bullets!

I hoped to just brush the upper atmosphere, where density is thin and not much weather going on, and let my speed do the compressing, but it turns out my speed is not high, it's insane, around 18,000 km/s. Not feasible. I think I have to recalculate the trajectory and brake someplace in between.

Another problem is if they go walking around outside on top of the ship, and look over the edge, the weight of one's head would flip him over and send him plummeting down towards the center of the brown dwarf at a supersonic terminal velocity!

The whole trip is in free fall plus 1 G, so this is one of the problems I don't have. In my Traveller Universe, manoeuvre drives generate gross thrust, so I can't directly work against the gravity of the brown dwarf.

I'm not even sure how much the shi's grav plating would shield one from the planet's hideous gravity. Lets say its a scout courier and the grav plating shields him from the brown dwarf directly below his feet, if he moves towards the side of the ship, he may get pulled sideways as part of the planet below comes into view, the closer he comes to the edge of the ship, the harder the gravitational tug sideways, until he flips over the side and straight down!

Actually, calculating the tidal force was one of the first things I did. Luckily, this, too, is something I don't have to worry about, it's still negligible at this distance.

 

[Pyromancer]

If I decelerate to a circular orbit around the brown dwarf at the edge of the atmosphere, the whole trip takes around 36 hours... nice round number.

 

[NOLATrav]

Glad you figured it out. Seems you have a much better grasp of orbital mechanics and ship oops than I thought

Edit - by ship oops I meant ship ops

 

[phavoc]

Keep in mind a few things here:

1) When you are refueling you physically have open structures in your ship. So imagine the stress on your collectors, your internal pipes and even fuel tanks as they gases are coming in at very high speeds. You'd have to really engineer the strength of the materials and structures to maintain safety margins.

2) In order to collect the fuel you need the density of the atmosphere to be probably around 1 Atm or less, but probably not less than say .5 Atm. Too dense works, but then pressure builds up, not to mention your ships velocity falls as it works harder against the atmosphere. Not enough density and you'll have a very hard time collecting the gases you need.

3) You also have to think about the heat you'll generate from travelling at high speed through the atmosphere. Too steep, too fast means much more heat and you ship may take damage. This also ties into the fuel scoops and collection mechanisms. You don't want to be going so fast that you potentially will ignite the hydrogen you are trying to collect (though that depends on the oxygen content too).

There isn't a whole lot out there in regards to refueling at a gas giant other than "high guard" term. I've tried to collect snippets here and there, but I haven't yet found a reasonable collection of ideas.