Question about gas giant piracy, escape windows, and playable intercept rules

[Sigtrygg]

The thing you are missing is that the ship begins in orbit not at rest.

The ship has momentum sending it in a straight line, the gravity of the world pulls it towards the centre of the world, but the momentum is such that the actual path misses the ground, you are free falling around the planet, continually being pulled towards it but never getting closer because of your momentum. This is your orbit and your orbital speed.

To skim you start off at orbital velocity - 47km/s in a low Jupiter orbit.

You fire your 1g maneuver drive to ajust your orbit to an elipse so it dips into the very outer reaches of the atmosphere. As you fall towards the planet you increase in velocity, as you enter the atmosphere you lose velocity due to drag so you offset this with your 1g maneuver drive, At periapsis you fire your maneuver drive at maximum to add velocity thanks to the Oberth affect (assuming is works with gravitic drives, I can certainly make an argument for why it wouldn't since they are reactionless and the Oberth effect requires action/reaction)

 

[MonsterX]

The thing you are missing is that the ship begins in orbit not at rest.

The ship has momentum sending it in a straight line, the gravity of the world pulls it towards the centre of the world, but the momentum is such that the actual path misses the ground, you are free falling around the planet, continually being pulled towards it but never getting closer because of your momentum. This is your orbit and your orbital speed.

To skim you start off at orbital velocity - 47km/s in a low Jupiter orbit.

You fire your 1g maneuver drive to ajust your orbit to an elipse so it dips into the very outer reaches of the atmosphere. As you fall towards the planet you increase in velocity, as you enter the atmosphere you lose velocity due to drag so you offset this with your 1g maneuver drive, At periapsis you fire your maneuver drive at maximum to add velocity thanks to the Oberth affect (assuming is works with gravitic drives, I can certainly make an argument for why it wouldn't since they are reactionless and the Oberth effect requires action/reaction)

Yeah, you definitely can do this and it makes sense to do it, and probably it is SOP for ships with 1G M drives and a need to refuel at gas giants. But it might be a little tricky. The degree to which you can dwell in a GG atmosphere instead of having to skim through the thinner layers using orbital velocity would depend on your M drive (the higher the better), and the GG's gravity (the lower the better).

(On the other hand, a big high-G gas giant would have a thicker layer of hydrogen before you start to get other stuff in there too, that goes out further from its gravitic centre and therefore affects the ship less. ) (Also, if the ship has an airframe structure, it might be able to fly in the GG, not relying on the M drive so much. But these are generally only small craft. )

If you miscalculate and end up slowing down too much you'll not make orbit again. This could happen because of winds; GGs have very strong winds. Or because of an encounter with a pirate requiring evasion.

If your M drive is higher than the GG gravity, none of this is a problem.

 

[rinku]

Yep. which is why all the pilot rolls.

Excessive thrust is brute force insurance.

 

[Condottiere]

That sinking feeling you get, when you realize that the gas pedal doesn't depress enough, as you make that planetary flyby.

 

[MonsterX]

Yep. which is why all the pilot rolls.

Excessive thrust is brute force insurance.

Brute force is a great way to make up for a lack of skill.

 

[swordtart]

The thing you are missing is that the ship begins in orbit not at rest.

The ship has momentum sending it in a straight line, the gravity of the world pulls it towards the centre of the world, but the momentum is such that the actual path misses the ground, you are free falling around the planet, continually being pulled towards it but never getting closer because of your momentum. This is your orbit and your orbital speed.

To skim you start off at orbital velocity - 47km/s in a low Jupiter orbit.

You fire your 1g maneuver drive to ajust your orbit to an elipse so it dips into the very outer reaches of the atmosphere. As you fall towards the planet you increase in velocity, as you enter the atmosphere you lose velocity due to drag so you offset this with your 1g maneuver drive, At periapsis you fire your maneuver drive at maximum to add velocity thanks to the Oberth affect (assuming is works with gravitic drives, I can certainly make an argument for why it wouldn't since they are reactionless and the Oberth effect requires action/reaction)

I'd agree with you about the Oberth effect.

I was led astry by the bogus mass of Jupiter in LBB2. I was simplifying the orbit from an ellipse to a line and ignoring the lateral motion (since we are broadly talking constant velocity other than the dip into atmo). I was also trying to see if you could do it by vector maths and threepenny bitting it round the wafer thin skin of atmosphere above the clouds. I should have just plugged it into the equation, but that's what you get for using the rules rather than Science

Apparently you can orbit 1km above Jupiter's surface at the equator if you move at 41km/s.

Out of interest what are you assuming as the atmospheric density at the point of fuelling. I was thinking around 0.01 kg per metre squared but at 47 km/s the biggest factor in the drag equation is squaring of the velocity in m/s which multiplies your result by 2 thousand million. I was working it out for a modular cutter with a refuelling module (since that would be one of the more common methods). I had to assume an actual mass of 150 metric tonnes, an area of 170 square metres (based on the deck plans) and a coefficient of 0.2 since that matches a cylinder with a rounded end. The drag deceleration was in the 100's of G's.

 

[rinku]

It's a range. If you're getting drag, you're scooping hydrogen. So the actual altitude might actually be quite high; pick a zone where your ship can cope.

 

[swordtart]

It's a range. If you're getting drag, you're scooping hydrogen. So the actual altitude might actually be quite high; pick a zone where your ship can cope.

Clearly, but the rate you can collect will also depend on the drag, by the time the drag is low enough that you can compensate with 1G thrust it is becoming so thin that it is hard to see you collecting much per pass. Whilst technically that is in atmosphere it isn't what I had assumed when I envisaged fuel scooping.

Companion gives layers and the rate of collection at those layers, but it doesn't give any speed associated with those layers or any indication of the altitude of those layers.
The wisp layer subjects you to drag but it is too thin to skim fuel. The drag is also trivial and only enough to cause orbital decay in days (by which point I am presuming you drop into the next layer). The extreme shallows are I believe corresponds to the upper stratosphere (above 50km) as that equates to limited turbulence and I am going to assume the lower stratosphere (0 to 50 km) equates to the shallows as this is where you start to get turbulence. The deeps where "normal refuelling takes place" seems to equate to the deep troposphere (0 to -150km) as this is where you start having a bumpy ride.

If my assumptions are correct based on the atmospheric conditions described and the correlation of the names of the layers then "normal" fuelling in the deeps is taking place in densities of 0.15-0.2 kg per cubic metre.

However I have not accounted for the axial rotation of the planet and given the equatorial axial rotation is above the orbital velocity cited you could choose a latitude where the you could still be moving in a stable orbital speeds but your relative speed to the atmosphere is far lower (or even in the opposite direction) and maybe you can just spend time orbiting at 40+ km/s at 1 km below the "surface" and refuel at normal rates.

I thought it would be a fun exploring this as an exercise in vector space movement, but instead it has turned into an astrophysics assignment for which I don't even get a academic credit (which is why I have not been able to face working through the WBH). All this has been wrt. Jupiter which might not even be a typical large gas giant and so irrelevant to a generic LGG in a particualr system. I think I have done as much as I am willing or able.

The whole point of this thread to was explore the window of opportunity for pirates hunting those refuelling. If you have no idea how fast the refueller is moving through the atmosphere then you have no way of knowing if pirate lurking at non-orbital speeds can catch them. As always in Traveller, unless you are prepared to do a lot of work and research (and drag up stuff you last studied decades ago) you are better off just handwaving it with whatever you think might be fun and just assume the NPC can do the math even if the referee can't.

Given the extreme magnetosphere, gravity, mass, size and temperature of Jupiter the sensor signature of a sub 1000 DTon ship hiding in its atmosphere is going to be swamped by the planet itself. If you want to hide you probably can, but you'll need to establish the insertion point of your target before it gets there as hunting it though that sensor soup is going to be impossible.

 

[phavoc]

A ship that is skimming is going to have a pretty big thermal bloom - especially at the speeds being discussed. Even a 'lurking' ship - if it's moving at similar velocities - is going to show up on thermal sensors.

40 kps is 89,000 MPH. The shuttle, as it started its descent in the atmosphere was doing a pithy 17,000mph and it was generating 3,000 degrees F heat. That was at an altitude starting about 200k feet and going down to about 120k feet. That's going from mesosphere to stratosphere.

The question is within a gas giant's atmosphere what would be the density at very high altitudes.

Delving too much into the actual science makes it difficult to justify gas giant refueling as described - especially since we know the 4 in our system have very different atmospheric compositions at similar altitudes. Just how a pirate might hide is difficult to justify, but for gaming purposes it should get some sort of explanation. Since a refueling ship could essentially blind itself if we make the assumption the physics of it are similar to a deorbiting craft in Earth's atmosphere, it's sensors and communications (radio at least) would be useless until it raised itself out of the interference it's creating by refueling. At that point it would be relatively easy for a pirate to "sneak" up on a ship refueling from a higher orbit without having the same blindness. Which would make the sweet spot of the intercept being during refueling. That would give a pirate time to accelerate (no drag at a higher orbit) and catch the ship. The natural radiation and other interference could explain how it might get missed with sensors.

It would also allow for the explanation of how an SDB might lurk in the clouds itself since most have higher-G drives, and if it can offset the gravity of the planet then it can truly 'lurk' hidden from from optical, thermal and other sensors and not be blinded by the same interference that a ship refueling would have.

Without knowing more about things and using what we know today, that seems to fill in a lot of the holes.