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Tirno

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Apparently most jump points have recharge stations for jump ships, perhaps they refuel while using them to recharge the K-F drive too?

Recharge stations would themselves need fuel for their station-keeping drives as well. Nothing at the Zenith or Nadir of a star is actually in orbit, so there is no free ride.

The recharge stations primarily work by beaming power into the receiving jumpship's jumpsail. There is also a hardlink option to charge via a cable, but that would require precision stationkeeping under thrust, including outward thrust to keep the weight of the cable from pulling the recharge station and the jumpship together.

If the "momentum killing" thing wasn't a part of the canon, but jumpships could re-orient their velocity vector at the arrival point, the way to get a free ride would be to put the jumpship into a circular polar orbit around the star, i.e. velocity vector is normal to the gravitational acceleration vector. It takes about a week to recharge the KF drive, so they'd jump to a point along the orbit so they've pass over the Zenith or Nadir pole of the star about half-week. Picture it like jumping in at the edge of the Arctic or Antarctic circles on Earth, orbiting straight north or south (as appropriate), passing over the poles, then jumping out as you hit the circle on the other side. By keeping the momentum, they get a free ride of being in a freefall orbit, then jumping to another freefall orbit at the next star. A bonus byproduct of this kind of scheme would be to minimize the chances of jumpships jumping in or out close to each other, as there is a LOT of space around a circle with a diameter of seven days at orbital velocity.

Recharging stations are trickier due to the need to align orbital planes, but if you had a bunch of them, and had them in elliptical polar orbits, they could get closer to the star to collect a lot of power, then spend aphelion beaming it out to jumpships. (Hardlink would be Right Out.)

Dropships in the current canon already have the issue of killing their orbital velocity in the ecliptic so they can get on a trajectory to the Zenith or Nadir jumppoints, or regaining that orbital vector starting from dropping straight toward the star. Changing the canon slightly to to have them catch the jumpship in solar-polar-orbit isn't that much of a stretch, particularly since they have at least seven days warning of what their final position and vector must be to make a nice relaxed docking in freefall with the jumpship. Not really that hard at 1G acceleration. They could aim to be right over the pole before the scheduled jumpship comes in, see the incoming jump signature while they are boosting up, use that to calculate a final vector, and meet up with the jumpship right as it is passing over the pole.

Maybe a mod for Kerbal Space Program would help...
 

Adam Steiner

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There's some hardcore nerd at the center of this universe. :)
Maybe a mod for Kerbal Space Program would help...

I played that game so much a few years ago. I must've logged 200 hours in it. It had some amazing mods too.
 

Adam Steiner

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I don't even want to imagine how much deltaV all these solar polar orbits > planetary orbit and back again would cost.

On approach to the planet (or star) it's just as easy and efficient to move into a polar orbit as an near-equatorial one.
 

Timaeus

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What I don't get is why everyone/everything on the Argo and Leopard are standing on the floors and not floating about when in orbit around a planet, not under transit acceleration. The Argo only has simulated gravity in the 3 hab modules, yet the "Mech Bays, engineering, and etc all have the look of a place under normal gravity all the time. Sure, I think there are MagBoots in the fiction that could explain a persons stance, but their hair and items strapped to them would be floating free. Same goes for items on the ground/tables and everything else. The 0g enviroment would make things like moving massive 'Mechs and equipment very fast and easy (also more dangerous) but I'm sure it was really more of a "We don't have the time/budget/employees" to make a set of 0g and 1g art assets for every sim game scene and chat environment.
I'm going to vote for cost of art assets and be done with it.

One thing I would actually like to see, but haven't found, would be some sort of illustration or technical description of how a Leopard dropship gets configured for aft-directional G-forces.
I think Sarna.net even mentions that most leopards are indeed set up so the aft bulkheads functions as floors during spaceflight, but I have seen no further details on it, and I find it kind of an interesting engineering problem.
Admittedly I have never read any Battletech novel, maybe one of those touches on it.
Could be done like an RP Flip.
 

Adam Steiner

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Recharge stations would themselves need fuel for their station-keeping drives as well. Nothing at the Zenith or Nadir of a star is actually in orbit, so there is no free ride.

Could you explain that a little? I'm afraid I don't know what you're saying at all. :|
 

Tirno

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Could you explain that a little? I'm afraid I don't know what you're saying at all. :|

OK, now you're going to regret asking such things of a former meteorologist and freshman-level physics TA.

An orbit around a celestial object is basically falling forever and always missing that thing.

Let's start with the basics: An object in motion stays in motion, unless acted upon by an external force. And by motion, we mean a straight line, neither speeding up nor slowing down. Such things only happen in the abstract world of simplified physics, which is just a subset of mathematics. Real life is messy, as the Chinese Tiangong-1 space station is demonstrating today.

So, first problem: how to put something in an orbit that doesn't involve crashing. Top problem is gravity; it's always there. The acceleration that any object imposes on another object is equal to the universal gravitational constant (denoted usually as capital G) times the mass of that object divided by the square of the distance between those objects. In simplified physics, you calculate that from the centers of mass. In real life, every atom is attracted to every other atom in the universe, even minutely, and practically that means the atoms on the side closest to the other object are attracted more strongly than the objects on the opposite side, which means they are effectively being pulled apart (although, usually also very minutely). Let's work with a simple version for now; why the Moon is tidal-locked is an exercise for the interested student.

To make a stable, circular orbit, at all times the pull of gravity must be balanced by something else. Otherwise, the orbiting object will either start accelerating away from the planet/star/whatever, or towards it. The math to derive this get complicated, but luckily the result is simple: to pull a object in a circle, the inwards acceleration is equal to the velocity squared divided by the radius of the curve. In other words, to maintain that particular curve, you must impart a particular acceleration towards the center of the curve. Every action has an equal and opposite reaction, as Isaac Newton said. You feel this when you are in a car going around a curve: the steering wheels are imparting forces that accelerate the car toward the center of the circle the car is turning in. You, loose in the car, feel pushed towards the outside, but that is only because you are in the frame of reference of the car. Objectively, YOU are trying to go in a straight line in accordance with momentum and natural law; the seat and seatbelts (and perhaps the inside of the passenger window if I am driving) are in the way of you going straight while the car turns, and therefore impart a force on you to cause you also to turn. If those forces were not balanced, you would tear through the structure of the car and continue on your straight line until you interacted with other forces, such as the road, a tree, and the young mother on the sidewalk pushing the baby stroller.

We can balance those two equations. In a perfect circular orbit, gravitational acceleration is equal to the acceleration needed to make a circle exactly equal to the distance between the two objects. So:

Circular orbital velocity v = square root ( G * (mass of star) / (orbital radius) )

That orbital velocity isn't just a certain speed in a random direction. It is specifically a velocity normal to the pull of gravity. In 2 dimensions, "normal" means things that are 90 degrees off from each other. In three dimensions, it is all the possible directions that are exactly 90 degrees off. Point towards the center of the earth with your index finger, and stick your thumb out. Rotate around. Every possible way your thumb can point, while your index finger is still pointed to the center of the earth, is 'normal' to the gravitational vector.

The orbital velocity you're going is key: Too fast, and your inwards and outward forces won't match, and you'll accelerate away from the object you want to orbit. Too slow, and you accelerate inward, possible until you have lithobraking (i.e. rapidly slowing down by plowing through rocks).

In the context of the recharge stations, I meant 'free-ride' meaning being in orbit. You don't need to use rockets at all to be in orbit. You fall forever and always miss. But to get that free ride, you have to be going a specific speed. If you're not, you can make up for it using rockets, which means using fuel. In canon, the recharging stations and jumpships are either above or below the star, and they're just hanging out there. Gravity is still there, at about 0.1G. If they don't make 0.1G continuously with rockets, they fall toward the star. They have no orbital velocity, because in canon they need to be at a particular place above the solar poles about 30 AU out. Also, in canon, they lose all momentum when they jump, so immediately after they jump, they fall like a rock towards the star. (Also, they fall like a feather. In vacuum, rocks, feathers and jumpships drop at the same acceleration.) Any version of this where jumpships, recharging stations, etc do not need a continual supply of fresh fuel for their station-keeping nuclear rockets requires them to turn their thrust towards making an free-fall orbit. Luckily, that's achievable in canon; NASA would commit not just murder but genocide for the technology to continuously make 0.1G thrust. (Next problem: running a fusion reactor in space without cooking the crew. Getting rid of heat in a vacuum is HARD.)

This can get more complicated with non-circular orbits and thrust in different directions and complicated even more by having multiple massive objects independently asserting a significant gravitational force on our poor jumpships or dropships (try a BINARY stellar system for true screw-this-I'm-jumping-back-to-Terra times), but hopefully this answers your questions.

Also, just so you know, this was the TL;DR version. This is literal rocket science.
 

Jamey

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try a BINARY stellar system for true screw-this-I'm-jumping-back-to-Terra times
I tried that with a simulation I wrote for a physics class in college. I never did make it work, so I stuck with a simpler system for what I turned in. :)
 

Tnarien

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There's some hardcore nerd at the center of this universe. :)


I played that game so much a few years ago. I must've logged 200 hours in it. It had some amazing mods too.

Fun fact: KSP's orbital mechanics maths are so accurate that a bunch of NASA folks apparently use it to play with ideas before concept/pitch stage.
 

stjobe

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Just to put some numbers on this:

1g constant acceleration for 168 hours (7 days) makes for a turnover speed of 5,931,062 m/s (13,267,430 mph or 21,351,823 kph)

The speed of light is 299,792,458 m/s, so the speed at turnover is about 0.02 c, or two percent of light speed.

That's pretty fast, but not really fast enough for any serious time dilation - it's about half an hour per day compared to an observer at rest, so in our example the trip taking 168 hours would seem like it takes 171 hours to an observer at rest - and not even that, because you aren't starting out at 0.02c.

To get any really significant relativistic effects you need to get up towards 0.9c or more.