Highport for a Tide-Locked World

[Garnfellow]

I've generally assumed that highports are normally placed in geosynchronous orbit above the downport.

But what do you do with a tide-locked world? Would you put the highport in orbit and just deal with a perpetually moving target, or maybe forgo a downport altogether? Would you place it out at one of the Lagrange points?

 

[AnotherDilbert]

I guess it does not matter. The time and effort to go from the downport to any point in a specific orbit is pretty much the same. When we are taking off and accelerating to orbital speed we are spiralling away from the world anyway.

 

[McPerth]

I'd say with the advent of the gravitics (when you can have thrust just from power, that can be obtained with solar pannels, so not needing reacion mass), station keeping thrusters make quite easier a geosynchronous placement of any orbital object, as it will no longer depend on orbital mevchanics, but may correct its positions as needed.

 

[sudnadja]

I've generally assumed that highports are normally placed in geosynchronous orbit above the downport.

But what do you do with a tide-locked world? Would you put the highport in orbit and just deal with a perpetually moving target, or maybe forgo a downport altogether? Would you place it out at one of the Lagrange points?

I think given the Traveller version of spaceflight, what we think of as spaceflight just won't happen, and a station can be placed at any point without concern for station keeping costs.

 

[willtron3030]

A tide-locked world... My first assumption is that is *close* to it's primary... slightly-outside-the-Roche-limit close.

If the planet is in a close orbit and is settled in the day/night boundary, put it at L2, using the planet for shelter from the primary. If it's further out, put it at L1, which is closer but might create heat management issues.

L4 and L5 are technically stationary relative to a point on the day/night boundary of a tide-locked planet, but are much farther away.

 

[whulorigan]

I'd say with the advent of the gravitics (when you can have thrust just from power, that can be obtained with solar pannels, so not needing reacion mass), station keeping thrusters make quite easier a geosynchronous placement of any orbital object, as it will no longer depend on orbital mevchanics, but may correct its positions as needed.

I think given the Traveller version of spaceflight, what we think of as spaceflight just won't happen, and a station can be placed at any point without concern for station keeping costs.

A tide-locked world... My first assumption is that is *close* to it's primary... slightly-outside-the-Roche-limit close.

If the planet is in a close orbit and is settled in the day/night boundary, put it at L2, using the planet for shelter from the primary. If it's further out, put it at L1, which is closer but might create heat management issues.

L4 and L5 are technically stationary relative to a point on the day/night boundary of a tide-locked planet, but are much farther away.

Just remember that for L1-L3, you will need some type of active thrust agent (gravitic, et.al.) to keep the station within the constantly moving Lagrange point; whereas L4-L5 (while farther away) are naturally moving along the orbital path of the planet and should be stable as stationary points (relative to the planet) long-term.

 

[AnotherDilbert]

If we are really close to the star, then L2 would make sense, but it can be inconveniently far away. For the Earth I believe it is about 1.5 million km, so about 6 h away at 1 g acceleration.

 

[AnotherDilbert]

I'd say with the advent of the gravitics (when you can have thrust just from power, that can be obtained with solar pannels, so not needing reacion mass), station keeping thrusters make quite easier a geosynchronous placement of any orbital object, as it will no longer depend on orbital mevchanics, but may correct its positions as needed.

Just because we have M-drives does not mean that they are free or trivially cheap. A M-1 drive and power system would increase the cost of the basic structure of the station by perhaps 50%. The M-drive would have to be increased anytime we added to the station.

And since we would absolutely rely on the M-drive to function at all times to maintain position, we really need a backup M-drive and power system, to cover malfunctions and maintenance.


Putting a space station in orbit is simpler, cheaper, and safer. I assume that is standard practice.

 

[willtron3030]

Just remember that for L1-L3, you will need some type of active thrust agent (gravitic, et.al.) to keep the station within the constantly moving Lagrange point; whereas L4-L5 (while farther away) are naturally moving along the orbital path of the planet and should be stable as stationary points (relative to the planet) long-term.

Valid point. If the planet is close to the primary (and that's a brazen assumption on my part based on the planet being tide-locked), the choice between L2 and L4/5 is economic: thrust for station-keeping, or extra heat management.

L2 lies in the shady umbra or antumbra of the planet, and is more or less equidistant from all points on the unmoving solar terminator.

IDEA: At a die-back planet, the PCs get to explore the hostile environs of a derelict station: either (1) the star-baked shell, still in orbit at L4/5, or (2) the icy L2 station wreckage, just an ATV ride from the terminator to the crater on the dark side of the planet.

 

[agorski]

Assuming solar flux is not a problem, why not put it in a polar orbit so it's always over somewhere in the twilight zone? Could be useful for other reasons.

 

[whulorigan]

Just because we have M-drives does not mean that they are free or trivially cheap. A M-1 drive and power system would increase the cost of the basic structure of the station by perhaps 50%. The M-drive would have to be increased anytime we added to the station.

And since we would absolutely rely on the M-drive to function at all times to maintain position, we really need a backup M-drive and power system, to cover malfunctions and maintenance.

Putting a space station in orbit is simpler, cheaper, and safer. I assume that is standard practice.

You would not necessarily need an engine that puts out 1.0g. An M-1 Drive is designed for long distance fast maneuver. For station keeping and positional correction, you might only need a drive that can put out 0.1g or 0.01g (or even less).

 

[whulorigan]

IDEA: At a die-back planet, the PCs get to explore the hostile environs of a derelict station: either (1) the star-baked shell, still in orbit at L4/5, or (2) the icy L2 station wreckage, just an ATV ride from the terminator to the crater on the dark side of the planet.

Cool adventure ideas.

 

[McPerth]

Just because we have M-drives does not mean that they are free or trivially cheap. A M-1 drive and power system would increase the cost of the basic structure of the station by perhaps 50%. The M-drive would have to be increased anytime we added to the station.

And since we would absolutely rely on the M-drive to function at all times to maintain position, we really need a backup M-drive and power system, to cover malfunctions and maintenance.


Putting a space station in orbit is simpler, cheaper, and safer. I assume that is standard practice.

I guess station keeping thrusters are quite smaller than M-1...

ITTR having read somwhere (sorry, Icannot tell you where) that something like 0.05G will sufice, and that is not so expensive, I guess.

 

[sudnadja]

Valid point. If the planet is close to the primary (and that's a brazen assumption on my part based on the planet being tide-locked), the choice between L2 and L4/5 is economic: thrust for station-keeping, or extra heat management.

L2 lies in the shady umbra or antumbra of the planet, and is more or less equidistant from all points on the unmoving solar terminator.

IDEA: At a die-back planet, the PCs get to explore the hostile environs of a derelict station: either (1) the star-baked shell, still in orbit at L4/5, or (2) the icy L2 station wreckage, just an ATV ride from the terminator to the crater on the dark side of the planet.

For the Traveller 2300 world Aurore (in Atlas of the French Arm guise) L1 is unstable enough that Aurore won't even complete a single Tithonus orbit without an object at L1 destabilizing - L2 is almost as unstable. L4 and L5 remain stable for at least 50,000 years depending on how you have the other moons of Tithonus set up.

In my variant of the 2300 setting, communications "satellites" are at L4 and L5. As other commenters here said, I think in the Traveller universe even the concept of an "orbit" (except celestial bodies around other celestial bodies) is not going to be a thing.

 

[sudnadja]

Assuming solar flux is not a problem, why not put it in a polar orbit so it's always over somewhere in the twilight zone? Could be useful for other reasons.

I don't think the orbit would remain over the twilight zone, as the tide locked body orbits its primary, the plane of the twilight zone will rotate - as "tide lock" actually means 1:1 orbit:rotation period, but the satellite or space station orbital plane will not be changing. At least without providing thrust.

 

[whulorigan]

As other commenters here said, I think in the Traveller universe even the concept of an "orbit" (except celestial bodies around other celestial bodies) is not going to be a thing.

You would probably want to distinguish between a "powered" orbit versus a "parking" (geosynchronous) or "free-fall" (Low/High/Retrograde, etc) orbit.

 

[Timerover51]

Is the world tidally locked a star or to a gas giant?

 

[nobby-w]

I've generally assumed that highports are normally placed in geosynchronous orbit above the downport.

I think it is more likely that high ports would be situated in low orbit.

Geosynchronous orbit lr a L4/L5 site is quite high, high enough to make a significant difference to surface-orbit transit times - and there's little to be achieved by doing it, even when delta-V is cheap. A high port could be placed in a lower orbit with a (say) 90 minute period. This would facilitate regular shuttle flights to various destinations on the surface, and the individual shuttle flights could be quite short - maybe only half an hour or so if the launch window is timed correctly.

Cooling gets glossed over in Traveller, but the power requirements for gravitics (from the various systems such as Striker or FFS that do delve into this) would require very large cooling radiators if you were going to try and maintain a powered orbit for a station of any significant mass.

Radiators using ammonia as a refrigerant can dissipate about 400w per square metre. Gravitics would be the largest power consumer on such a station by a large margin, so designing one that didn't need them and could just sit in a stable orbit would be a massive saving on power and radiator surface area.

 

[sudnadja]

Gravitics would be the largest power consumer on such a station by a large margin, so designing one that didn't need them and could just sit in a stable orbit would be a massive saving on power and radiator surface area.

If it can generate heat, it can also generate power (via thermocouple at very oleast). Presumably it consumes more power than it can theoretically produce?

 

[AnotherDilbert]

You would not necessarily need an engine that puts out 1.0g. An M-1 Drive is designed for long distance fast maneuver. For station keeping and positional correction, you might only need a drive that can put out 0.1g or 0.01g (or even less).

To keep a station stationary above a point on Earths surface takes a force of:

F = mrω²

where r is the radius of the forced orbit, say 1000 km plus the Earth radius of 6400 km = 7400 km, and ω is the angular velocity 2π/86164s ≈ 0.000073 s⁻¹.

So F = mrω² ≈ m × 7400000 × 0.000073² ≈ m × 0.039 m/s² which is next to nothing. The problem is that gravity provides GM/r² ≈ 7.279 m/s² at that height.

So to stay in that forced orbit we need to accelerate at 7.279 - 0.039 ≈ 7.240 m/s² ≈ 0.74 g.

(Please check my calculations, I'm doing this with the help of wiki.)

To stay wherever we want over the planet we need a large fraction of 1 g, so almost a 1 g drive. 0.1 g will not make many forced orbits possible.