Fuel Skimming

[Spartan159]

Book 3 page 29 says this: Gas Giant skimming: Ship's G must exceed GG size/10.

The smallest GG size is 20, therefore ships must be at least 2 if not 3 Gs to skim. wing/air frame/lifting body give +1G in atmosphere so 2 G craft with those are good up to a size 29 GG, but anything 1G or any streamlined or less configuration 2G is SOL for gas skimming.

Does not take into account lifters though.

Thoughts/Comments before I go stripping fuel scoops out of slow craft?

 

[AnotherDilbert]

T5 says a lot of things, e.g.:

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.

 

[Wol]

These two are not necessarily inconsistent. Jupiters "surface" gravity is 2.4 times that of earth at the surface. Without something to counteract gravity You need about 42km/s for a stable orbit. At one bar pressure with hydrogen that is about 36691200 N Force on a cone 1m square. Going for thinner stuff (knock off a couple of zeroes) and convert to something meaningful and that is still 36 tonnes force per sq meter. I don't want to think of the heat effects.

So you will need significant engine power to stop your decent into the vortex...

Given the refueling times I suggest that you are travelling quite slowly, relatively speaking.

regards

 

[AnotherDilbert]

These two are not necessarily inconsistent.

As far as I can see they are directly contradictory: One require 2-3 G, the other require none.

Jupiters "surface" gravity is 2.4 times that of earth at the surface. Without something to counteract gravity You need about 42km/s for a stable orbit.

Since there are no surface, the "surface" is by convention the altitude in the atmosphere where pressure is equal to standard Earth atmosphere.

We don't need a stable orbit, just a parabolic orbit passing close to, into the atmosphere of, the GG.

At one bar pressure with hydrogen that is about 36691200 N Force on a cone 1m square.

At 1 atmosphere ≈ 100 kPa pressure a force of 100 kN is applied to a 1 m² surface, just like on Earth.

Going for thinner stuff (knock off a couple of zeroes) and convert to something meaningful and that is still 36 tonnes force per sq meter. I don't want to think of the heat effects.

You don't want atmospheric pressure, you want air resistance (drag) to calculate the braking force and heat development.


The Book 2 text seems to describe what is basically a slingshot orbital manoeuvre in the upper atmosphere, something you can do without turning the drives on if you have enough speed to begin with. Roughly like a comets inner orbit around the sun:

In our case the ship follows the red orbit around the central GG. (The blue stable orbit would represent a moon.)


The greater the speed the higher up in the atmosphere (with lower density) you can skim, and still sweep the same amount of gas.

 

[kilemall]

That was always my assumption, upper gas giant atmo at a speed that gets the ship in and out of the skim without going anywhere near the surface, slingshot off.


Any kind of damage that would induce drag would be a death sentence particularly to a typical commercial ACS. It would be a race between burning up and being crushed like a sub under pressure.

 

[Spartan159]

I think both flying and slingshot approaches are viable, depends on the individual ship. Each method has pros and cons. But speaking for my self, I'd rather in in a controlled positive lift and plenty of power situation than the slingshot method. I'd think long and hard before trying to skim in a Free Trader.

 

[AnotherDilbert]

Any kind of damage that would induce drag would be a death sentence particularly to a typical commercial ACS.

Agreed, a compromised hull would be problematic.

It would be a race between burning up and being crushed like a sub under pressure.

Pressure isn't a problem, the outer atmosphere has very low pressure, no worse than a normal planet. Only below the "surface" can the pressure be a problem.

If a low-tech shuttle can deorbit by entering the atmosphere at something like 28 km/s, I believe a medium tech starship should be able to handle the perhaps 50 km/s needed to slingshot through the upper atmosphere.

 

[Wol]

As far as I can see they are directly contradictory: One require 2-3 G, the other require none.



Since there are no surface, the "surface" is by convention the altitude in the atmosphere where pressure is equal to standard Earth atmosphere.

We don't need a stable orbit, just a parabolic orbit passing close to, into the atmosphere of, the GG.



At 1 atmosphere ≈ 100 kPa pressure a force of 100 kN is applied to a 1 m² surface, just like on Earth.



You don't want atmospheric pressure, you want air resistance (drag) to calculate the braking force and heat development.


The Book 2 text seems to describe what is basically a slingshot orbital manoeuvre in the upper atmosphere, something you can do without turning the drives on if you have enough speed to begin with. Roughly like a comets inner orbit around the sun:

In our case the ship follows the red orbit around the central GG. (The blue stable orbit would represent a moon.)


The greater the speed the higher up in the atmosphere (with lower density) you can skim, and still sweep the same amount of gas.

Hmmm I was just running some numbers for drag using gas density at 1 bar (for reference) and velocity for stable orbit. Jupiter's nominal surface is defined as the point at which the pressure is 1 atm (10^5 Pa) and 70,000km radius. At 800 km beyond this pressure is only 10^-5 Pa. Not much hydrogen there about 4x10^-7 kg/m^3 and gravity is still about 2.4G.

It probably does not matter as this is all a bit of a handwave, but I suspect that you will need thrust - thus the rules - and that is to play at the very margins of the atmosphere for skimming. You would need a lot more thrust if you wanted to get in close and gently harvest.

YMMV and this is, after all just BOE stuff.

regards

 

[AnotherDilbert]

YMMV and this is, after all just BOE stuff.

Agreed.

 

[Blue Ghost]

Book 3 page 29 says this: Gas Giant skimming: Ship's G must exceed GG size/10.

The smallest GG size is 20, therefore ships must be at least 2 if not 3 Gs to skim. wing/air frame/lifting body give +1G in atmosphere so 2 G craft with those are good up to a size 29 GG, but anything 1G or any streamlined or less configuration 2G is SOL for gas skimming.

Does not take into account lifters though.

Thoughts/Comments before I go stripping fuel scoops out of slow craft?

It's a broken rule. The writeup for the Dragon class SDB specifically states that it's designed to lurk in either oceans or gas giant atmospheres for targets to attack. And I believe that Secret of the Ancients gives a specific pressure chart or guide as to how much pressure a starship's hull can take, with the implication that were an ACS to delve into a gas giant atmosphere, that that's how much pressure it can take before imploding.

 

[aramis]

Skimming is not the same as loitering nor normal ops.

the info in A12 on local gravity is specific for Komesh. Altitude of 30,000 km from center is the actual 3G point, but surface G is apparently still over 2G... and the "quantization" of Traveller drives in CT was a near absolute for playability.

The limits are not on a table, but a paragraph on the tables page.
Commercial vessels: 1000 Atm, 1000° K
Military vessels: 2000 Atm, 1500° K
SDB's Designed for deep: 3000 Atm, 2500° K
Protective suit: 400 Atm, 1000° K

Overpressure: throw 2d each 15 min. On 10+, the drives fail.

For comparison, the bottom of the Marianas Trench is 1086 Bar...
Sea level mean pressure is 1 Bar, and is 1.01325 Bar. So commercial shipping can get VERY deep in CT.

 

[Blue Ghost]

I don't see the problem. SDBs generally have high acceleration, so can move freely in high grav fields. Staying in the atmosphere of a 3-4 G gas giant with a 6 G M-drive should not be a problem.*ship*

I don't recall there being a specificity to the SDB writeup, just that it's designed to lurk in gas giants. It was a sore point of contention back in the day when we tried to apply both physics and rules to actually skim, and discove3red that unless you were at a Neptune sized world that you couldn't actually take on fuel by simply flying down into the atmosphere, opening up the ports and take on hydrogen or whatever. I guess I'm wrong, but that's what I recall from it. And then in the Gunboats and Traders write up reading about the Dragon class, it seemed to imply that if it could go down and "loiter" for targets, than any ship could. Oh well.

 

[kilemall]

Hmm, are those Kelvin hull volumes generally accepted as performance by most gearheads? This has a direct bearing on general reentry survivial and missile bombardment.

 

[aramis]

Hmm, are those Kelvin hull volumes generally accepted as performance by most gearheads? This has a direct bearing on general reentry survivial and missile bombardment.

I'd say most are completely unaware of them.

 

[Timerover51]

Skimming is not the same as loitering nor normal ops.

the info in A12 on local gravity is specific for Komesh. Altitude of 30,000 km from center is the actual 3G point, but surface G is apparently still over 2G... and the "quantization" of Traveller drives in CT was a near absolute for playability.

The limits are not on a table, but a paragraph on the tables page.
Commercial vessels: 1000 Atm, 1000° K
Military vessels: 2000 Atm, 1500° K
SDB's Designed for deep: 3000 Atm, 2500° K
Protective suit: 400 Atm, 1000° K

Overpressure: throw 2d each 15 min. On 10+, the drives fail.

For comparison, the bottom of the Marianas Trench is 1086 Bar...
Sea level mean pressure is 1 Bar, and is 1.01325 Bar. So commercial shipping can get VERY deep in CT.

Out of sheer curiosity, what material is the pressure suit supposed to be made of to handle that pressure and heat?

The heat would requires some form of insulation, which would have to be extremely efficient.

 

[kilemall]

Out of sheer curiosity, what material is the pressure suit supposed to be made of to handle that pressure and heat?

The heat would requires some form of insulation, which would have to be extremely efficient.

I've been playing that reflec is not actually reflecting laser energy (which would involve having reflect that is 'tuned' to each major laser EM wavelength) but heat insulation. Very IMTU, but makes sense to me.

 

[Spartan159]

Out of sheer curiosity, what material is the pressure suit supposed to be made of to handle that pressure and heat?

The heat would requires some form of insulation, which would have to be extremely efficient.

If I recall correctly, that suit was an ancients item. May as well call it magic.

 

[Timerover51]

If I recall correctly, that suit was an ancients item. May as well call it magic.

I would agree with calling it magic. The same would go for the resistance of the ship hulls to those extremes of pressure and heat.

 

[Spartan159]

I would agree with calling it magic. The same would go for the resistance of the ship hulls to those extremes of pressure and heat.

While I can't say for the magical heatsinks of Traveller, :CoW: :coffeesip: the armor of even "no armor" civilian ships seems to be far in excess of what modern submarines possess. I can suspend belief towards ships in gas giant atmospheres.

 

[Grav_Moped]

Out of sheer curiosity, what material is the pressure suit supposed to be made of to handle that pressure and heat?

The heat would requires some form of insulation, which would have to be extremely efficient.

One of the scenarios in Azhanti High Lightning rules booklet (Incident V, "Dead Ship," p. 39) involves personnel inside a dead starship on the fluid "surface" of a gas giant. Armor is whatever comes up in a random counter draw (Powered, Combat, or whatever). These may be up-armored versions of specialized high-pressure suits, but if so it's not called out in the text.