Skimming Gas Giants

[RogerD]

I have been trying to understand the details of how gas giant refueling works, but it is a little hard to follow what the intended rules are. I'm sharing my analysis here, but these are the main questions that remain:

• Is the R=0 "Cloud" layer a viable source for skimming?
• What is the correct minimum acceleration for gas giants (if any)?

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I think my XS-AL11 Intrepid would be considered to have 2G in an atmosphere due to being a lifting body, which should make refueling possible on at least a Size 20 world, per the note "Gas Giant Skimming: Ship’s G must exceed GG Size/10." (BBB3, p.29), but in the table above it there is also a G column, which implies up to about a Size 29 GG could be skimmed. On this same page, it gives times like "To Orbit: Fast Boost: Minutes= Orbit to Range R"

There is also this from BBB2, p. 103

Gas Giant Fuel Skimming
A ship which moves to a layer of gas giant atmosphere which contains clear H2 can fill its fuel tanks at the standard rate for its scoops. A ship in a layer other than clear H2 can fill its fuel tanks at half rate.
Gas Giant Fuel Skimming is orbital in nature; the ship is not landing and restrictions based on drive acceleration do not apply.

So there seems to be a bit of a conflict there.

Also related are the atmosphere tables on BB1, p.30 which show for all gas and ice giants a clear hydrogren layer at R=6.4. Being at that level inflicts Blast-2 damage on a ship undergoing skimming at a LGG or IG and Blast-1 damage at a SGG.

Gas giants also have a listed surface gravity on BBB3, p.73. G has a note "assumes density similar to Terra" (which is inappropriate for a gas giant).

Armor is x10 vs. Blast and x100 vs. Friction/Heat.

A fuel intake can pull in 100 tons of gas per hour and the fuel purifier can handle the same rate.

Putting that all together, if the Intrepid is skimming that Size 20 GG, it could take a safe reentry in 32 hours, fill its tanks in 15 minutes and then take a safe boost back to orbit in another 32 hours with no friction concerns. It could do a slow re-entry in 6.4 hours with a heat effect of 40 or fast boost in 6.4 minutes with a heat effect of 400. The reverse trips take the same time and have half the friction concerns.

The Intrepid has Shell armor with AV=5 so it will have no trouble from friction (500 AV vs Heat) using a fast re-entry and fast boost, provided that the gas giant's gravity is not too high for it. It won't save time by just going to the R=6 layer as the minute or so of boost savings is overwhelmed by the refueling time being doubled. Likewise, the pressures are too low to affect the ship (1-2D of damage vs. 50 AV vs Pressure).

All told, it should take Intrepid about 25 minutes to top off the tanks with no real danger, provided it is possible at all.

 

[AnotherDilbert]

The important note here is:

Gas Giant Fuel Skimming is orbital in nature; the ship is not landing and restrictions based on drive acceleration do not apply.

You don't "land" on the GG to refuel, you pass through the upper atmosphere at high speed, i.e. in a low orbit.

Something like this:

Normal landing operations do not apply.

Gas Giant Skimming: Ship’s G must exceed GG Size/10.

Is presumably used instead.

+1 G for wings only apply in Atmosphere 2+ (VThin). I have no idea if you normally go deep enough into the GG to apply that bonus. See Altitude Chart, B3 p90.


The "surface" of a GG is just a arbitrary altitude in the atmosphere, IIRC where atmo pressure is Earth standard. It has nothing to do with skimming, as I don't think you dip that far into the GG.

 

[AnotherDilbert]

The Intrepid has Shell armor with AV=5 so it will have no trouble from friction ...

TL-9 shell is AV-4 (B2 p282). You round down, unless otherwise instructed.

Jump plates (as required for Experimantal drives) reduces armour by half (B2 p73), so it should have AV-2.

There is a lot of information tucked into foot-notes here and there...

 

[RogerD]

+1 G for wings only apply in Atmosphere 2+ (VThin). I have no idea if you normally go deep enough into the GG to apply that bonus. See Altitude Chart, B3 p90.

The atmosphere tables say you have a pressure of 1-2 atm at the altitudes in question, so it should apply.

TL-9 shell is AV-4 (B2 p282). You round down, unless otherwise instructed.

Jump plates (as required for Experimantal drives) reduces armour by half (B2 p73), so it should have AV-2.

OK. So for this purpose, vs heat or burn you would have AV=225 and vs. pressure, you'd have 22 (which easily handles Blast-2 = 2D damage). Looks like we need anti-blast armor to negate the heat issue. Even at half value that's 450 - a little tight, but within tolerance.

I guess the issue for the Intrepid is going to be that it doesn't have high enough G to do the skim, per the more-specific rule about the G requirement. Even with the wings, it would not actually exceed Size/10. I'll retool her with rockets.

There is a lot of information tucked into foot-notes here and there...

Including links to your spreadsheets that I finally noticed. I didn't really like the FillForms or the iPad app. Maybe this will help me (though I don't have Excel on my Mac, so we'll see). Thanks for making it.

 

[whartung]

you pass through the upper atmosphere at high speed

So the suggestion is that you're using speed to pressurize the gas giant atmosphere into the tanks, rather than either pump, or simply filling the tanks to equilibrium, then pressurizing and purifying the raw material?

Skimming at speed is certainly cinematic, I just don't know if it's really a practical solution to the problem.

 

[AnotherDilbert]

So the suggestion is that you're using speed to pressurize the gas giant atmosphere into the tanks, rather than either pump, or simply filling the tanks to equilibrium, then pressurizing and purifying the raw material?

Yes, that is what this means:

B2 p103:
Gas Giant Fuel Skimming is orbital in nature; the ship is not landing and restrictions based on drive acceleration do not apply.

Skimming at speed is certainly cinematic, I just don't know if it's really a practical solution to the problem.

Why would you slow down and pump, when you can just open a scoop and let it flood in?

 

[AnotherDilbert]

The atmosphere tables say you have a pressure of 1-2 atm at the altitudes in question, so it should apply.

Is it? VThin starts at 0.1 atmosphere = 10⁴ Pa, so about 50 km over the surface of Jupiter. Do we need to go that deep into the gas giant?

At a guess the stratosphere would be deep enough, and it would contain higher proportion of H₂ as heavier gasses would be concentrated at higher pressures?

Including links to your spreadsheets that I finally noticed. I didn't really like the FillForms or the iPad app. Maybe this will help me (though I don't have Excel on my Mac, so we'll see). Thanks for making it.

It should work in LibreOffice: free and works on all major platforms, obviously including Mac.

 

[RogerD]

Is it? VThin starts at 0.1 atmosphere = 104 Pa, so about 50 km over the surface of Jupiter. Do we need to go that deep into the gas giant?

I was just going by tables 3a, 3b and 3c on BBB1, p.30 which are specifically for gas giants. Those would apply rather than BBB 3, p. 90 which are for regular planets. The cloud layer for a gas giant is at R=0 and has P=1. P is defined to be in bars (earth atmospheres at sea level).

It should work in LibreOffice: free and works on all major platforms, obviously including Mac.

Thanks - I'll try it. Numbers was pretty unhappy with it for sure!

 

[AnotherDilbert]

I was just going by tables 3a, 3b and 3c on BBB1, p.30 which are specifically for gas giants.

I missed that, I wasn't looking in B1, thanks.

The cloud layer for a gas giant is at R=0 and has P=1. P is defined to be in bars (earth atmospheres at sea level).

It has the peculiarity of starting at the "surface" and going down, i.e. it leaves out most of the atmosphere.

If we are going that deep into the atmosphere, we are leaving orbital ops and orbital speeds far behind us. Instead we are slowing down, flying around in the dense atmosphere, and then clawing ourselves back out of the gravity well.

In that context this makes sense:

B3, p29:
Gas Giant Skimming: Ship’s G must exceed GG Size/10.

Jupiters "surface" gravity is ~2.6 G so we need 3 G to escape the gravity well.

But this makes no sense:

B2, p103:
Gas Giant Fuel Skimming is orbital in nature; the ship is not landing and restrictions based on drive acceleration do not apply.

It seems there are two scenarios for skimming implied: Orbital hi-speed pass through the extremely thin outer atmosphere or slowing down to go deep into the atmosphere.


As CT didn't even require full streamlining (or high-G drives) to skim, I have always assumed the orbital pass through the outer atmosphere.

The need to go deep into the atmosphere, below the "surface", would mean that common ships such as the Free Trader couldn't skim, which I think would break canon?

 

[G. Kashkanun Anderson]

It seems there are two scenarios for skimming implied: Orbital hi-speed pass through the extremely thin outer atmosphere or slowing down to go deep into the atmosphere.

Both scenarios make sense if the rules described gas giants as they actually behaved in our universe. The first would apply to ice giants like Uranus and Neptune, as well as close orbit, thermally juiced up 'Hot Jupiters'; the second would apply to anything roughly Jovian mass and above. Saturn, which is in kind of a gray zone between the two, could likely go either way (although scenario 2 might be faster).

Middle and outer system gas giants do not get much larger in visible size than Jupiter. They max out at roughly 140-150,000km in diameter; after that, they simply grow denser, as well as, ultimately, slightly smaller for a time, before the sheer addition of mass causes them to grow back up to the 140,000+ range again.

Incidentally, this rule also applies to brown dwarfs, which are typically only 10-15% larger in diameter than Jupiter, despite being up to 80 times more massive.

The effect is that any usable gas is going to be much deeper in a giant's gravity well for anything much above a Jupiter in mass, as well as 'compressed' into a thinner envelope. In the case of superjovians (anything above about 3-4 Jupiters in mass) the G forces one would have to withstand in order to reach that hydrogen would be so extreme that I'm not sure a typical Beowulf or Empress Marava could pull it off without incurring modest structural damage (or worse), though I'll grant a case could be made for a Sulieman being designed to pull it off.

And, for the record, this is why skimming from a brown dwarf is right out, at least for any starship built at Imperial tech levels. A drone or probe, sure. But I've never seen anything in any of the ship design rules of any edition that allows for withstanding the G stresses going on at the 'surface' of even the smallest brown dwarfs.

 

[Condottiere]

There're easier ways to gas up.

Apparently, enough propulsion to achieve escape velocity, and a pressure hull, heavily reliant on material science and structural reinforcement.

Unless there's a way to invert the inertial compensation field to support the hull.

Maybe repulsors.

 

[kilemall]

The game Imperium has that special tanker unit for Sirius. I’m under the impression normal ice, water or gas giant opportunities are not available so it goes harvesting stellar gas. Else it hauls the stuff in from adjoining systems and just has a really big fuel tank.

 

[mike wightman]

Imperium states:

Tankers are capable of manufacturing fuel directly from stellar atmospheres. When such a ship moves to a tertiary system, it is immediately capable of refueling friendly ships, enabling them to leave the hex using hyperspace jumps.
A tanker automatically provides itself fuel to leave the hex.

Dark Nebula states exactly the same rule:

Tankers are capable of manufacturing fuel directly from stellar atmospheres; when such a ship moves to a tertiary system, it is immediately capable of refueling friendly ships, enabling them to leave the hex using hyperspace jumps.
A tanker automatically provides itself fuel to leave the hex.

 

[kilemall]

Imperium states:

Dark Nebula states exactly the same rule:

So, how is that done?

I’m thinking especially armored and rad shielded scoopers.

 

[whartung]

I don't have the physics, but, intuitively, pressurizing gasses through a high velocity pass into an empty hold just doesn't make that much sense to me. It certainly doesn't sound very efficient, at least not with the elongated orbits pictured above.

The task, it seems to me, is to take the raw product, pressurize and filter it.

There's roughly 35 moles of hydrogen in 1 liter of liquid hydrogen. A mole of hydrogen at 1 atmosphere of pressure is roughly 22 liters in volume. That means that you need 770 liters of hydrogen gas to get 1 liter of liquid hydrogen. 770:1 ratio. So, if you had a 1000 dton space filled with pure hydrogen gas, that will compress down to 1.3 dtons of liquid hyrdogen. Lets say that through high velocity you get that to 10 atmospheres, now you're up to 13 dtons of LH. Man, that's going to take a lot of passes to fill up, well, anything.

Well, it turns out that when compressed to 12 atmospheres, hydrogen turns liquid. I don't think you're going to get that via high speed skimming or scooping. I would think at a particular pressure, the hydrogen simply won't enter the intake. I can't speak to what that value might be.

That suggest to me that the processor needs to just "park", or cruise in the upper atmosphere and start pumping and processing.

 

[AnotherDilbert]

Well, it turns out that when compressed to 12 atmospheres, hydrogen turns liquid. I don't think you're going to get that via high speed skimming or scooping.

You would have to compress the collected gasses either way. There is no need to wait for the scoop to be done to start compressing.

I would think at a particular pressure, the hydrogen simply won't enter the intake. I can't speak to what that value might be.

Why would physics stop working suddenly?

A SR-71 can apparently fly at ~25 km altitude with a ~2% atmospheric pressure. Can it suck in enough air to burn the fuel in the engine at that altitude/pressure? Apparently so.

The Russians say they have flown MIG-25s at ~35 km with <1% atmospheric pressure. Apparently the air intakes could supply the engine with air.

You will get less gasses at lower pressure, but you will get more with higher speed. As long as there is some atmo pressure, you will get some gas into the scoop.

 

[AnotherDilbert]

A current experimental demonstration is the scramjet engine, working at high speeds in very thin atmosphere (~35 km, <1% atmo) where a narrowing inlet compresses the very thin air without moving parts.

https://en.wikipedia.org/wiki/Scramjet

Compress the hydrogen enough, replace the fuel injection and nozzle with a liquid separator and we have a LHyd fuel scoop? Give or take a few thousand years of technical detail, of course...

 

[AnotherDilbert]

So, how is that done?

I’m thinking especially armored and rad shielded scoopers.

How about something like a Bussard Ramjet in a low retrograde (i.e. fast) orbit around the star?
https://en.wikipedia.org/wiki/Bussard_ramjet

Might not work in extremely low density interstellar medium, but in just very low density solar wind near a star?

 

[Ulsyus]

Well, it turns out that when compressed to 12 atmospheres, hydrogen turns liquid. I don't think you're going to get that via high speed skimming or scooping. I would think at a particular pressure, the hydrogen simply won't enter the intake. I can't speak to what that value might be.

That suggest to me that the processor needs to just "park", or cruise in the upper atmosphere and start pumping and processing.

Is the gas scooped from a GG going to do that before refining? If grav technology can be used to further increase the pressure of incoming gases, then the refining could take place as AnotherDilbert's post above.

 

[Commander Truestar]

"Is the gas scooped from a GG going to do that before refining?"

This question may relate to an earlier part of this thread.....specifically, the section where the use of pumps were questioned.
The apparent 'preferred answer' was that simply opening the scoops during a high speed pass would fill the tanks and pumps would not be needed.

But, what if the valves were on surfaces not facing forward?
What if pumps were specifically called for exactly because they "can" compress the gasses to 12+ atmospheres....which would let the purifiers work on a liquid base?

Should the pumps work effectively enough, this could very easily reduce the amount of time required to fill the refined fuel tanks.