Skimming gas giants

[Carlobrand]

You're forgetting ram-air effect - the faster you go, the more effective pressure you encounter.

The jet engines on the SR71 don't work with an input pressure below about 0.75 Atm... but at the speeds it flies, the problem is overpressure, not under.

Maybe I am; let's see.

We're trying to scoop fuel. A dton of hydrogen is a metric ton of liquid hydrogen. Hydrogen at 0c and 1 atmosphere's 1/1284 the density of the liquid stuff, yes? Let's take that AHL fuel shuttle, needs to take in 350 dTons in - 140 minutes? That leaves it 20 minutes for the transfer, if I understand the canon elements right. So, 2.5 dTons per minute, 0.04167 dTons per second. Compensate for it being gas - 53.5 dTons of gas per second at 1 atm and 0C. 13.5 cubic meters per second, 722.25 cubic meters per second. Adjusting for thinner atmosphere, 144,450 cubic meters per second. So, a 1 meter square vent needs to be moving at 145 kps to do the job. A vent 2.4 meters square could do the job at 60 kps. I don't know the temperature up there, but colder temps would only make it easier. The scoop might not be 100% effective in collecting atmosphere, but there's plenty of room on a fuel shuttle for larger vents. Of course, you're not scooping hydrogen the whole 140 minutes, but then there's still plenty of room for the larger vents that would compensate for that. Since the fuel shuttle's pretty close to the highest proportion of fuel to volume of craft, it only gets easier for craft with a smaller proportion of fuel tank to craft volume.

All in all, assuming I haven't made a mistake or missed some detail, it looks like I was wrong: it's possible to skim fuel at an altitude high enough to maintain that key escape velocity. Just make sure your arc is shallow enough that it carries you back out to where you want to be. Takes about an hour and a half at 2G: in, then out. Twice that for 1G craft. 3G and up, likely to be a good deal faster since they can dive deeper and pull up without needing to worry as much about the local gravity.

All of which renders that half-atmosphere mention in Secret of the Ancients rather misleading, but it's not the first time some canon element misspoke.

That was kinda fun!

 

[aramis]

As an example...

The shown scoops on the march harrier are two roughly triangular maws, 1m tall, 3m wide each. For about 3 square meters. Escape velocity on saturn is about 11.2 km/s (src).

So, it's pulling in 33600kL per second of the trace gas... and at STP (273.15°K, 1 Bar), 0.0899 kg/kL... so at 0.002 Bar and ignoring temp for the moment, that's 60kg/sec... so it needs about 827 seconds (0:13:48 H:M:S) to fill 50 tons... it will be longer, most likely, as the temp will be higher, so the density for a given pressure will be lower.

Now, looking in AHL... the plans show a central scoop 3m wide; the height isn't shown, but we can presume at least 1.5m. Roughly oval shaped, so, 0.75*1.5*pi is about 3.5 square meters. So 7/6 the area (minimum) and 7 times the fuel to be brought aboard... for roughly 4966 sec (1:12:46 H:M:S)...

And note: the fuel shuttle is 2G. It can go ahead and wallow instead of arc. So, down to 0.5 Atm, and fill at a slower rate... safely... when not in a rush. In a rush, it can go well further down than that 0.002 bar and suck in proportionally faster... balancing speed and depth to potentially even liquify by shock wave...

 

[Blue Ghost]

Someone brought up the Ship with 1g m-drive thing on normal worlds before in years past. It's why I've always said that Traveller uses Star Wars physics when it comes to ship's interacting with planets. 1g, by itself, will only get your off world if your nose is pointing straight up, because there is absolutely no lift on the supermajority of ship's in Traveller, and those, like the Type-T, that has appreciable aerodynamics, are going to need F-15 or faster like take off speeds to get airborne, and then pull back hard on the stick to get to orbit and beyond.

The Secret of the Ancients thing niggled at me a bit too, which is why again we always assumed that a ship's manauevre drive had a built in grav system, probably spread throughout the hulls undercarriage on key load points (or maybe some odd engineering where it's built into the ship's hull). Either way there's something there other than pure thrust that keeps vessels aloft on all worlds. Some field effect that works against gravity, or else players would perpetually need to use shuttles to get to and from orbital facilities to the surface while their ship is parked in orbit.

Getting back to refuelling; I think the mechanics take in drag from various atmosphere, such that 3gs at sea level is the same as 3gs in the ionosphere or above. That's how I read it. And if that's the case, then density and drag don't mean a whole lot in the game mechanics.

And yes, I'm deliberately avoiding the density/scoop arguments here until I see what else you guys conjure.

 

[HG_B]

Someone brought up the Ship with 1g m-drive thing on normal worlds before in years past. It's why I've always said that Traveller uses Star Wars physics when it comes to ship's interacting with planets. 1g, by itself, will only get your off world if your nose is pointing straight up, because there is absolutely no lift on the supermajority of ship's in Traveller, and those, like the Type-T, that has appreciable aerodynamics, are going to need F-15 or faster like take off speeds to get airborne, and then pull back hard on the stick to get to orbit and beyond.

IMTU 1G = 10/m/s/s So, a 1G Free trader can lift off from a Earth sized (8) world with no problem.

 

[SpaceBadger]

IMTU 1G = 10/m/s/s

I agree, that does simplify the math!

 

[HG_B]

I agree, that does simplify the math!

I took this page, changed the code constant to 10 m/s and saved it onto my tablet.

 

[aramis]

I agree, that does simplify the math!

It was also mechanically introduced in TNE. Two simplifications have been used since TNE:
C=300,000. km/s
G=10 m/s²

 

[Blue Ghost]

IMTU 1G = 10/m/s/s So, a 1G Free trader can lift off from a Earth sized (8) world with no problem.

How do you lift off of any world with a 1g pull or greater in a ship whose manuevre drive is rated at 1g?

 

[Carlobrand]

How do you lift off of any world with a 1g pull or greater in a ship whose manuevre drive is rated at 1g?

JETS! (heh heh) :devil:

Or some variant on that SOM overdrive idea. You don't need more than an additional 0.3G, so you could argue a small overdrive capacity that's too weak and too short in duration to show up on the scale of the space combat rules.

 

[HG_B]

How do you lift off of any world with a 1g pull or greater in a ship whose manuevre drive is rated at 1g?

Merchants don't. They use the High Port. (Starport C or better has an orbital station) That's why Scout Couriers have a 2 G drive. They might have to deliver stuff to worlds that are larger and that only have a Down port or, no port at all. The larger M-drive isn't there to just shave of an hour or 2 getting to the 100 diameter jump point. However, you are only looking at worlds of size 9 or larger.

 

[pennshome723]

I can't say I have ever seen this listed anywhere but maybe it has been. Is there a chart as to how long it actually takes to skim fuel and process it from a gas giant?

Now I know you guys have been implying that a ship will kind of fly into the gas giant clouds and then fly out to escape the gravity pull of the planet. When I consider that, I kind of have visions of SyFy SGU ancient's ship that flys into the outer corona of suns to gather energy to refuel itself.

Honestly I would just have a ship fly into low orbit of a Gas Giant and start to suck in gases and then refine process to fill their fuel holding tanks.I usually would work out how long it would take to suck in the needed amount of gasses and then refining cleaning process to purify the fuel. I always have the process take hours to complete, with a chance of an encounter. Mostly though nothing happens.

 

[rancke]

How do you lift off of any world with a 1g pull or greater in a ship whose manuevre drive is rated at 1g?

By having a lifting body.


Hans

 

[HG_B]

By having a lifting body.


Hans

I've been toying with that idea as a rule. My 1st thought was allowing it for up to 1/2G above M-drive rating & requiring very long runways given the large payloads that a typical Trader ship carries. But in my game a hull that is a lifting body costs quite a bit more than the standard hull and the Pilot requires a level in Flyer to use it properly.

 

[aramis]

I've been toying with that idea as a rule. My 1st thought was allowing it for up to 1/2G above M-drive rating & requiring very long runways given the large payloads that a typical Trader ship carries. But in my game a hull that is a lifting body costs quite a bit more than the standard hull and the Pilot requires a level in Flyer to use it properly.

In CT, there's no extra charge beyond the streamlining (note the Types T and R).

In MT, it's a boost to certain types of hull at minimal expense.

In T20, we (well, it was my idea, and Hunter adopted it) made airframe a 5% loss of space, and not too terribly expensive.

 

[HG_B]

In CT, there's no extra charge beyond the streamlining (note the Types T and R).

In MT, it's a boost to certain types of hull at minimal expense.

In T20, we (well, it was my idea, and Hunter adopted it) made airframe a 5% loss of space, and not too terribly expensive.

I just made the 5% rule (for blended lifting body, not craft winged like space shuttle). Never played T20. Maybe I'll make it a 10% hull surcharge instead of 20%.

 

[Blue Ghost]

By having a lifting body.


Hans

See my "deflector shield 'wing'" post, where a vessel generates an electronic wing above it's topside to generate lift in any atmosphere.

Aramis; wasn't there a partial streamlining rule in Mega Traveller?

 

[Thlaylie]

My Character's ships are built around a Fusion Reactor.

This reactor creates a magnetic "heartbeat" for the ship.

It also wraps the ship in a protective magnetic field similar to that produced by planets, albet on a smaller scale.

Then there's the ship's Deflector Shields as well as the fact that the Ram Fields can simply "reach" into a GG atmosphere and scoop out what's needed.

 

[Blue Ghost]

I can't say I have ever seen this listed anywhere but maybe it has been. Is there a chart as to how long it actually takes to skim fuel and process it from a gas giant?

Now I know you guys have been implying that a ship will kind of fly into the gas giant clouds and then fly out to escape the gravity pull of the planet. When I consider that, I kind of have visions of SyFy SGU ancient's ship that flys into the outer corona of suns to gather energy to refuel itself.

Honestly I would just have a ship fly into low orbit of a Gas Giant and start to suck in gases and then refine process to fill their fuel holding tanks.I usually would work out how long it would take to suck in the needed amount of gasses and then refining cleaning process to purify the fuel. I always have the process take hours to complete, with a chance of an encounter. Mostly though nothing happens.

You know I can't remember any of the such. There were some rates listed somewhere, but it's been so long I've forgotten. In all the gaming groups I've been in it's pretty much as you state; go in, gas up and thrust out of there.

I'm too lazy to look up the rules right now

 

[Blue Ghost]

My Character's ships are built around a Fusion Reactor.

This reactor creates a magnetic "heartbeat" for the ship.

It also wraps the ship in a protective magnetic field similar to that produced by planets, albet on a smaller scale.

Then there's the ship's Deflector Shields as well as the fact that the Ram Fields can simply "reach" into a GG atmosphere and scoop out what's needed.

That's pretty cool

 

[Carlobrand]

I can't say I have ever seen this listed anywhere but maybe it has been. Is there a chart as to how long it actually takes to skim fuel and process it from a gas giant?

Now I know you guys have been implying that a ship will kind of fly into the gas giant clouds and then fly out to escape the gravity pull of the planet. When I consider that, I kind of have visions of SyFy SGU ancient's ship that flys into the outer corona of suns to gather energy to refuel itself.

Honestly I would just have a ship fly into low orbit of a Gas Giant and start to suck in gases and then refine process to fill their fuel holding tanks.I usually would work out how long it would take to suck in the needed amount of gasses and then refining cleaning process to purify the fuel. I always have the process take hours to complete, with a chance of an encounter. Mostly though nothing happens.

Azhanti High Lightning offers about 3 hours per trip for a 400 dT fuel shuttle to dive down, collect 350 dT of fuel, fly up and then transfer that load to the waiting cruiser. Trillion Credit Squadron - be warned that as a quickie game it's canonicity is in dispute - borrows from that and makes it 7 turns (140 minutes) to make the dive and then 2 turns (40 minutes) to transfer fuel to another ship, assuming you want to transfer it. I don't know any reason that all ship types would take exactly the same length of time for a skim; strikes me there would be some variation based on the size of the ship's tanks, the size of its inlets, the power of its drives, and so forth.

I know of nothing in CT that speaks to a fuel purification rate.

MegaTrav Referee's Manual offers ship design including purification plants that can take either 6 or 12 hours to purify the full fuel load, depending on how much plant you get; it doesn't mention alternate rates. MegaTrav Imperial Encyclopedia, which despite the name has a wealth of rules governing interesting things, makes it a hazardous task taking - if I recall the task rules right - 1x3D6 hours, or anything from 3 to 18 hours, with an average tending toward 10 to 11 hours. So, on average about a half day for the maneuver and then another 6 to 12 hours for the purification - unless we're allowed to do that simultaneously. I don't know if that's allowed.