Position of ships in orbit

Daryl wrote:Consider the Lagrange points? Stable orbits in two bodied systems.
Ships could loiter there with minimal use of reaction mass.
How that would apply to Manticore and its complex orbital dynamics I'm unsure, but some areas must be more stable than others?
Or once you get a few million kms away it probably wouldn't matter.

The stable orbits will be used is my first thought. Intuitive even if in error. Consider the many ships in Home Fleet's OOB. The Lagrange points will act like one lane highways thinning out the fleet. If so, Imperator will be vulnerable to attack.

penny wrote:Best way to do it is to hide the entire operation within the inner workings of a legitimate business / enterprise / company. A large enterprise is expected to draw lots of power. And if it is legit, it can fund the operation.

Indeed, especially if you really meant "legitimate business." In that case, the excess of power draw that isn't converted into economic activity could pass unnoticed. In fact, they may not need to keep a full charge: if they have the necessary power connections to the grid, they may be able to charge the graser at a moment's notice. It may not be as fast as a military installation - the grid operator probably has expert algorithms running that would notice a sudden spike and cut the connection - but you could do it within, say, a quarter hour.

Were the Space Stations in orbit before they were destroyed?

Yes, and in very predictable orbits. We don't know if they were in low, medium, high or stationary orbits, but they were definitely in orbit. Given the amount of time the wedge tugs had to block debris falling on the planets and that some did fall on the city of Yawata on Sphinx, my guess is the stations were in low orbit.

In fact, they were probably not in a stable orbit in the first place. They were probably at low altitude and using the magic of the HV energy production to keep position over a point in the surface. That would explain even more why the debris fell on the planet: once the station-keeping is removed, it falls on pure gravity.

And is that Just Imperator and not the rest of the Fleet? And is that normal or was that a special case?

The whole of Eighth Fleet was kept in Trevor's Star, not just Imperator, before the Battle of Manticore. The only time they came as a whole to the MBS was during the battle, then stayed as the effective Home Fleet.

A single ship transiting is maybe an hour's inconvenience (because they wouldn't start from the transit lanes), but transiting the full fleet would add a day to any schedule and wreak havoc with the Junction. Therefore, during the war with Haven, keeping Trevor's Star as a base made a lot of sense because they'd be much closer to the targets and that's where they'd set off from. And remember that Trevor's Star was, like Manticore-B, kept mostly as a military reservation, so no enemy spy ships could observe the fleet strengths and exercises.

Where was the RMN component of the Grand Fleet based during the war with the SLN? We aren't told, but my guess is still Trevor's Star, because of the absence of prying eyes and because that would make it much easier for the RHN component to meet and exercise. The travel time for the GSN component would be roughly the same to either the MBS and Trevor's Star. As for the IAN component, for the fight with the MAlign, they'd transit from Gregor, so the time to reach the other components would be small anyway and the extra inconvenience to the Junction minimal.

If it were only travel time, the actual best place to base those fleets might be the Junction itself, though, making the RHN transit once and the IAN only once, but zero times for the RMN. And they launched through the Beowulf terminus.

Or maybe not. The GA was well aware of the streak drive by this time, so launching from the Junction, through Beowulf, to Warner-Mannerheim and thence to Galton could be observed and the enemy warned. So it's entirely possible they travelled entirely in hyper, from Trevor's Star. Does the timeline fit?

Daryl wrote:Consider the Lagrange points? Stable orbits in two bodied systems.
Ships could loiter there with minimal use of reaction mass.

I did, but as I said in my post above, the Langrange Points would be valuable real-estate for space industry, so you may not want to park a couple hundred warships nearby.

How that would apply to Manticore and its complex orbital dynamics I'm unsure, but some areas must be more stable than others?
Or once you get a few million kms away it probably wouldn't matter.

The planet Manticore has a single moon (Thorson), so it would have a normal set of Lagrange points. Sphinx with two moons may not. But all five of the Manticore-Thorson points would be valuable for other uses, especially the L4 and L5. And so would the star-planet L2 - in the Sun-Earth case, L2 is where we put the James Webb, the Euclid and where the Nancy Grace Roman is planned to go too.

The star-planet L4 and L5 would be less valuable for industry due to the distance - a 60° orbital distance. But that also means it's less interesting for the warships, because they'd be far from their charges and would make logistics of resupply, personnel transfer, etc. more challenging.

ThinksMarkedly wrote:

penny wrote:Were the Space Stations in orbit before they were destroyed?

Yes, and in very predictable orbits. We don't know if they were in low, medium, high or stationary orbits, but they were definitely in orbit. Given the amount of time the wedge tugs had to block debris falling on the planets and that some did fall on the city of Yawata on Sphinx, my guess is the stations were in low orbit.

In fact, they were probably not in a stable orbit in the first place. They were probably at low altitude and using the magic of the HV energy production to keep position over a point in the surface. That would explain even more why the debris fell on the planet: once the station-keeping is removed, it falls on pure gravity.

The velocity of the debris that hit Yawata Crossing was too high for gravity alone to have accounted for the majority of it; as they were traveling "at a paltry eight kilometers per second" [MoH] when they hit the atmosphere.

And my best guestimates put Sphinx gravity's contribution at around 2 km/s; so ~1/4 of that peak velocity.
(Guesstimate 200 seconds from weapons hit until the debris hit the atmosphere; and a starting altitude of around 1800 km -- see below for why -- that'd give Sphinx's 1.35g pull an average acceleration of around 1.07g due to how it falls off with distance; so over 1.07g over 200 seconds ~= 2 km/s)



We also know that there was over 103 seconds [Quay’s time in the debris stream] + 20 seconds [that it took the debris to punch through the atmosphere] between weapons fire and debris impact. So we have an absolute minimum altitude of over 950 km. But, since it took Quay non-zero time to react and get into the debris stream, the station altitude is likely 2-3x that.
(For comparison the ISS orbits at around 400 km -- so these stations aren't a very low orbit by current 21st century manned flight standards)


But they clearly aren't out in geostationary orbits either (35,794 km for Earth; 45,256 km for Sphinx) -- otherwise there would have been plenty of time for other ships to respond and for Alison's aircar to actually land; rather than having to eject the VIP passengers midair.


However, I can't recall if the stations maintain a fixed point over the ground. But if they did then they'd obviously have to be using power to maintain a pseudo-orbit to achieve that while being way closer that GEO.


(Final caveat -- we only have info on Vulcan around Sphix. I' made the assumption that all three stations were using similar orbital altitudes around their respective planet; but that isn't necessarily true)

Jonathan_S wrote:The velocity of the debris that hit Yawata Crossing was too high for gravity alone to have accounted for the majority of it; as they were traveling "at a paltry eight kilometers per second" [MoH] when they hit the atmosphere.

And my best guestimates put Sphinx gravity's contribution at around 2 km/s; so ~1/4 of that peak velocity.
(Guesstimate 200 seconds from weapons hit until the debris hit the atmosphere; and a starting altitude of around 1800 km -- see below for why -- that'd give Sphinx's 1.35g pull an average acceleration of around 1.07g due to how it falls off with distance; so over 1.07g over 200 seconds ~= 2 km/s)

Hmm... the escape velocity from Earth is 11.2 km/s, which is higher than the 8 km/s. In fact, that's close to the ISS's orbital speed, of 7.67 km/s. However, if it were holding position over something, it would actually have a smaller tangential velocity. So, was the 8 km/s of the debris tangential (which could be the orbital velocity) or was it radial?

(For comparison the ISS orbits at around 400 km -- so these stations aren't a very low orbit by current 21st century manned flight standards)

Sphinx has higher gravity, so its equivalent to LEO could also be higher. On the other hand, the Sphinxian atmosphere would be more compressed and thus lower orbits would be available without too much drag. They'd just zip past everything on the planet very quickly.

ThinksMarkedly wrote:Hmm... the escape velocity from Earth is 11.2 km/s, which is higher than the 8 km/s. In fact, that's close to the ISS's orbital speed, of 7.67 km/s. However, if it were holding position over something, it would actually have a smaller tangential velocity. So, was the 8 km/s of the debris tangential (which could be the orbital velocity) or was it radial?

(For comparison the ISS orbits at around 400 km -- so these stations aren't a very low orbit by current 21st century manned flight standards)

Sphinx has higher gravity, so its equivalent to LEO could also be higher. On the other hand, the Sphinxian atmosphere would be more compressed and thus lower orbits would be available without too much drag. They'd just zip past everything on the planet very quickly.

I'd assumed that that stated velocity was directly towards the planet -- a combination of velocity imparted by the destruction of the station and imparted by gravity -- and didn't include tangential speed. But that might have been a faulty assumption.

That said, the text explicitly states it took 20 seconds from when the debris touched atmosphere until they impacted the ground. You have a good point that Sphinx's atmosphere might be more compressed than Earth; so the 100 km value I used might be lower.

Googling around it seems that it's be difficult to work out the height of Sphinx's Karman Line would be. I was able to calculate its approximate atmospheric scale height (Earth's is about 8.4 km; Sphinx's is about 6.2 km [1]); the height over which the density drops about 63%. The problem is, to then use that to calculate the Karman Line I'd also need to know Sphinx's sea-level air density. Still, even if it's as much as 2x or 3x Earth's that pressure would drop below that found at Earth's Karman Line by 13x its atmospheric scale height -- meaning that (unless I got totally confused) Sphinx's Karman Line should be a little below 80 km vs Earth's 100 km.[2]
(For reference Earth's Karman Line is basically dead on 12x its atmospheric scale height; so even up to triple the starting air density only pushes the line up by one more atmospheric scale height)


So having gone back and calculated that following is using that 80 km thick atmosphere ballpark number. To cover 80 km in the 20 seconds we're told the debris took, requires an average velocity of 4 km/s. (And since it would slow some from punching through the atmosphere it would necessarily have had to be somewhat faster than that when it hit the top of the atmosphere) So it would seem that a majority, probably a significant majority of that 8 km/s has to be downward velocity; just so the number's we've given match.

Allowing for some tangential velocity might bring the absolute minimum station altitude down some from my initial calculations; but needing most of the velocity to be in the downward vector means it can't lower it all that much.


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Also as an aside it took a bit of work to figure out what geostationary orbit around Sphinx would be. (Even after I gave up partway through and found an online calculator: https://rechneronline.de/g-acceleration ... -orbit.php)

But to use it I needed Sphinx's mass, which HoS doesn't provide. Fortunately it does provide its surface gravity and its radius; and you can calculate its mass from those.

Then the final factor needed for GEO is its period of rotation; which helpfully HoS lists (as sidereal day - which is a bit shorter than the solar day its occupants would experience)

But it doing all that I had totally overlooked that the higher gravity likely changes the height of the Karman Line


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[1] With the simplifying assuming that its atmospheric composition (and thus average molecular mass) is equivalent to Earth's (seems fairly reasonable) and that its average surface temperature is too [288 K] (less reasonable)

[2] The fun things we learn and calculate for a sci-fi forum