Skimming gas giants
- Thread starter Blue Ghost
- Start date
Jupter's gravity; 24.79 m/s²
Saturn's gravity; 10.44 m/s²
1g-Earth = 9.8 m/s²
=> you need ~ 3g manuevre drive rating to surf/reuel on a world with the mas of Jupiter
=> 1g is ~ 93% of Saturn, => M rating ~ 1.067g ~1.1g to effectively "surf" in Saturn's atmosphere and refuel.
It makes me wonder what kind of flight profile you need to refuel on a gas giant. A "stall maneuvre" demonstrated by flight demonstration teams comes to mind;
http://www.youtube.com/watch?v=rHFO7Xx1NTs
Or, do you need to go in hot, at hypersonic speeds to keep your ship's momentum up, so you can easily burn out of the gas giant's orbit?
Discuss
Saturn's gravity; 10.44 m/s²
1g-Earth = 9.8 m/s²
=> you need ~ 3g manuevre drive rating to surf/reuel on a world with the mas of Jupiter
=> 1g is ~ 93% of Saturn, => M rating ~ 1.067g ~1.1g to effectively "surf" in Saturn's atmosphere and refuel.
It makes me wonder what kind of flight profile you need to refuel on a gas giant. A "stall maneuvre" demonstrated by flight demonstration teams comes to mind;
http://www.youtube.com/watch?v=rHFO7Xx1NTs
Or, do you need to go in hot, at hypersonic speeds to keep your ship's momentum up, so you can easily burn out of the gas giant's orbit?
Discuss
It makes me wonder what kind of flight profile you need to refuel on a gas giant. A "stall maneuvre" demonstrated by flight demonstration teams comes to mind;
Or, do you need to go in hot, at hypersonic speeds to keep your ship's momentum up, so you can easily burn out of the gas giant's orbit?
Discuss
The latter (see aramis' example). The former would result in possible getting caught in the grav well.
Well, there's another possibility; lift.
Something like Wayne Peter's magnificent Florian Class 110 ton scout could probably just fly right in like a normal jet air craft, and then just rocket back out like it was nothing.
A regular 100 ton scout would probably need a near stall maneuvre.
Something like a fat trader or Beowulf would need to go fast and leave a hot trail of ozone as it rocketed out of Jupiter's well.
I'm going to have to go get one of my AIAA books...equations are escaping me at the moment.
*EDIT*
Yeah, there's no lift on any Traveller ship, except maybe the Type-T, possible the two corsairs (Human and Vargr), the standard 100 ton (with some fancy flying), Wayne Peter's Florian, and maybe the Rampart fighter, but even stuff like the various trader designs would be hard pressed for normal flight. High speed parabolic course with ports wide open. One wonders if that'll burn more fuel than they can take in.
Just an FYI here
Normal Force atop wing (suction force);
(N) = Lift * cos α + Drag * sin α
Tangential Force;
(T) = -L * sin α + Drag * cos α
Where alpha (α) is the angle between the angle of attack and the air flow; i.e. at which angle relative to the chord of the wing the air molecules hit the wing's lower surface.
Throwing in a little hadwavium physics, I would say that maybe there's some kind of "lift" generator attacked to the maneuvre drive in addition to the grav drive; something that can create an electronic "wing area" over the ship's upper body. Something like a "defelctor shield" but works on air only, the resultant of which is to create force N to add life to the ship against the planet's gravity well.
Something like Wayne Peter's magnificent Florian Class 110 ton scout could probably just fly right in like a normal jet air craft, and then just rocket back out like it was nothing.
A regular 100 ton scout would probably need a near stall maneuvre.
Something like a fat trader or Beowulf would need to go fast and leave a hot trail of ozone as it rocketed out of Jupiter's well.
I'm going to have to go get one of my AIAA books...equations are escaping me at the moment.
*EDIT*
Yeah, there's no lift on any Traveller ship, except maybe the Type-T, possible the two corsairs (Human and Vargr), the standard 100 ton (with some fancy flying), Wayne Peter's Florian, and maybe the Rampart fighter, but even stuff like the various trader designs would be hard pressed for normal flight. High speed parabolic course with ports wide open. One wonders if that'll burn more fuel than they can take in.
Just an FYI here
Normal Force atop wing (suction force);
(N) = Lift * cos α + Drag * sin α
Tangential Force;
(T) = -L * sin α + Drag * cos α
Where alpha (α) is the angle between the angle of attack and the air flow; i.e. at which angle relative to the chord of the wing the air molecules hit the wing's lower surface.
Throwing in a little hadwavium physics, I would say that maybe there's some kind of "lift" generator attacked to the maneuvre drive in addition to the grav drive; something that can create an electronic "wing area" over the ship's upper body. Something like a "defelctor shield" but works on air only, the resultant of which is to create force N to add life to the ship against the planet's gravity well.
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Jupter's gravity; 24.79 m/s²
Saturn's gravity; 10.44 m/s²
1g-Earth = 9.8 m/s²
=> you need ~ 3g manuevre drive rating to surf/reuel on a world with the mas of Jupiter
=> 1g is ~ 93% of Saturn, => M rating ~ 1.067g ~1.1g to effectively "surf" in Saturn's atmosphere and refuel.
It makes me wonder what kind of flight profile you need to refuel on a gas giant. A "stall maneuvre" demonstrated by flight demonstration teams comes to mind;
http://www.youtube.com/watch?v=rHFO7Xx1NTs
Or, do you need to go in hot, at hypersonic speeds to keep your ship's momentum up, so you can easily burn out of the gas giant's orbit?
Discuss
Going in hot to preserve escape velocity is not an option. Drag increases by the square of velocity but only linearly with pressure. In other words, friction with the atmosphere will counter your drives so that you quickly slow down to a maximum velocity pretty much in line with those stated in MegaTraveller Referee Sourcebook - which is to say way, way, way below the needed escape velocity. You also leave a very pretty meteor trail, but you probably aren't going to care much about that because bigger problems are in store: unless your drives - or the combination of your drives and the lift produced by your flight through atmosphere - are sufficient to counter local gravity, you're basically going to continue gliding downward until pressure crushes you. Presumably, what happens after that is pretty much irrelevant.
If you manage to avoid that fate but your drives do not exceed local gravity, then you're pretty well stuck flying in the atmosphere unless you have some other source of thrust at hand.
You can counter that by staying higher in thinner atmosphere, but as I said, that part of the equation is linear: you've got to stay very high in very thin atmosphere to avoid dopping below the needed velocity, and you won't be scooping much that way, at least not in the time frames that Traveller suggests. I'm getting something in the neighborhood of 1/200th atmosphere to maintain the needed velocity, pretty darn thin.
I suggested earlier applying waste heat to the problem, taking in atmosphere and heating it up to use in a jet. Only works in atmosphere, diminishes in power as the atmosphere thins, but then drag also diminishes as the atmosphere thins, so by the time you get high enough, your speed should be high enough to manage the escape. Others have pointed out that there are too many unknowns to be certain of that - and we don't in fact know how much usable waste heat the ship produces verses how much energy is lost in ways that we can't use - but if the game says we can skim a Jovian atmosphere, then something is making it possible. So far, nobody has put down math showing the idea is impossible, so it's at least a possibility.
Starship Operator's Manual suggested that the maneuver drives could be run in overdrive by up to 40% for extended periods or up to 400% briefly. Assuming I understand that right, the typical 1G ship could run from 1.4Gs for "days" to 5Gs for "under 5 minutes;" presumably, somewhere in between is enough thrust for enough time to escape a Jovian. Unmentioned is where the power for this effort is coming from. SOM doesn't mention any ability to overdrive the power plant for the necessary power, but it's a logical deduction, possibly in combination with a small battery reserve*.
(*MT implies at least enough battery power to start the fusion reactor up from cold, but we really don't know precisely how much power is consumed in starting up a power plant. And, batteries are bulky and expensive: 1 dTon of TL15 batteries - 13500 liters - would provide only 94.5 megawatts for an hour, and that at a cost of MCr1.35. Unless one assumes a large fraction of that power plant is batteries, they won't play much of a role in the available power calculation. As an aside, 1 dTon of batteries is only enough to keep basic life support - and only basic life support - going for a couple of days on the typical free trader.)
If your "thrust" is less than the grav, you MUST maintain sufficient speed to maintain escape velocity or you fall. If it is greater, no problem. Inescapable physics.
A stall maneuver has nothing to do with this scenario. A regular Scout couldn't even do a "stall" anyway.
It is either this.
Or, this. There are no other "canonical" ways to do it. I use the 1st one in MTU as I don't allow for overclocking".
But by your reasoning here a 100 ton type-S can merely go in with the maneuvre drive in "5th" gear, suck up what it needed, then just normally fly out as if it were nothing.
40% more thrust from a 1g drive is more than enough for a body like Saturn. I think barreling in at hypersonic is a non issue. You can do the reentry thing until you hit 1atm, slow down, let your ship inhale and liquify NH3 until it's tanks were fuel, then blast out of there just as if you were flying normally.
I have a problem with the jet augmentation though. How are you going to get that hydrogen to react? It need's something to react with, doesn't it? Otherwise you're just applying heat to a molecule. I mean, why don't modern jets use Mister Clean or Pine Sol to fuel up?
You need some kind of oxidizer.
40% more thrust from a 1g drive is more than enough for a body like Saturn. I think barreling in at hypersonic is a non issue. You can do the reentry thing until you hit 1atm, slow down, let your ship inhale and liquify NH3 until it's tanks were fuel, then blast out of there just as if you were flying normally.
I have a problem with the jet augmentation though. How are you going to get that hydrogen to react? It need's something to react with, doesn't it? Otherwise you're just applying heat to a molecule. I mean, why don't modern jets use Mister Clean or Pine Sol to fuel up?
You need some kind of oxidizer.
Okay, well not a stall maneuvre, but keep the nose up enough to get enough sine-force to work with your tangential acceleration to keep your total vector longer than that of the gas giant's pull.
I'm not see the problem here.
All you do with "nose up" is greatly increase drag and thus, lower your speed. It does nothing constructive in that situation.
Another possibility would be to port something over from TNE/FF&S. TNE's gravitics rested on a slightly different presupposition: that Contragravity (CG) merely "screened-out" 99% of the gravitational attraction (and in normal atmosphere simple buoyancy would be enough to keep a CG-based vehicle "afloat". CG produced no thrust in TNE's Traveller Universe.
Now while I am not suggesting that you go over to the TNE conception of not having G-based drives, you could perhaps have your G-Drive/M-Drive utilize both concepts (different "principles" derived from the same "Hypergravity" theoretical model (to coin a term)). Gravitic based thrust systems might make use of CG to negate external gravity, while at the same time utilizing the "space-time curvature" to produce the actual rated thrust. The decision you would have to make would be whether or not the screening process of CG likewise negates the ability of the G-Drive/M-Drive to "react against" the external G-Field.
Perhaps CG can be presumed to be "tunable" to selectively allow the G-field to "bleed through" at whatever percentage the Pilot desires. This would also be useful for normal-space navigation and course plotting as well.
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I have a problem with the jet augmentation though. How are you going to get that hydrogen to react? It need's something to react with, doesn't it? Otherwise you're just applying heat to a molecule. I mean, why don't modern jets use Mister Clean or Pine Sol to fuel up?
Not necessarily. You could set up an ETR (Electro-Thermal Rocket). Electrical power could be used to heat intake hydrogen (or any fluid for that matter) in a chamber producing very high thermal-pressure. The reaction mass could then be expelled out an exhaust nozzle as a very high pressure exhaust.
EDIT: Alternately, you could add a reaction chamber around the Fusion power plant that utilizes its heat to heat the hydrogen to a plasma state, and expel it as a high-velocity exhaust. This is the basis for an NTR (Nuclear-Thermal Rocket) or VASIMR Rocket, or TNE's HEPlaR Propulsion system.
"This type of motor is much more fuel efficient than a chemical rocket, and is ideal for long duration interplanetary flights where fuel efficiency is paramount. Their main disadvantage is that they can only produce small amounts of thrust, for example a few pounds of force for a large motor array, whereas chemical rockets can produce millions of pounds of force."
That is true. However, I believe some of the current approaches (such as VASIMR) can be throttled up and down (although I don't know if sufficient thrust would be generated even in high-thrust mode). But then, perhaps at Traveller's TL they have overcome some of those engineering difficulties. Essentially, that is what HEPlaR is: a high-efficiency Plasma Rocket.
You'd have to add a whole other "drive system". It isn't going to fit in "spare" engineering space. So, you can forget about it being part of a standard M-Drive tonnage allocation..
I think we're getting out signals crossed. They way I understand the rules is that they're already set to STP. When you take off from Earth/Sol, your 1g manuvre drive is already working against 1 atmosphere of pressure, and will accelerate you into orbit because it has a slightly greater push than Earth's pull.
Similarly, if you're in Saturn's upper atmosphere (ionosphere or stratosphere) that 1g on 40% overdrive will push against that 10.44m/s^2 at all altitudes.
I've been literally over twenty years since I read the SOM, and I can't even recall what it said about skimming, but the basic CT rules seem to state that your manuvre drive is a do-all device. You apply thrust and it pushes you from point A to point B.
The drag may increase against the surface area slowing your velocity, but you apply more thrust and nose up more to gain a larger Y-component so you're overall acceleration vector is still greater than the pull of Saturn.
Similarly, if you're in Saturn's upper atmosphere (ionosphere or stratosphere) that 1g on 40% overdrive will push against that 10.44m/s^2 at all altitudes.
I've been literally over twenty years since I read the SOM, and I can't even recall what it said about skimming, but the basic CT rules seem to state that your manuvre drive is a do-all device. You apply thrust and it pushes you from point A to point B.
The drag may increase against the surface area slowing your velocity, but you apply more thrust and nose up more to gain a larger Y-component so you're overall acceleration vector is still greater than the pull of Saturn.
You didn't mention overclocking in that post. But yes, it should work as you say.
No.
SOM offers the only detailed canon explanation. Atmosphere at 1/200th pressure is too thin to draw the quantities required in the time described in canon, so that is not an option.
An interesting canon reference is Secret of the Ancients, in which it is stated, "Most ships refueling from a gas giant rarely venture below the level of 0.5 atmospheres". This presents an interesting quandary. Komesh is described as a large gas giant with 3G gravity, 72,000 km diameter at the point where atmospheric pressure is 1 ATM. Ships are scooping 500km above that. Ships need to have at least 3G to hover and land at the Ancient site.
Outside of SOM, there's little or no detail on how it's done. Only two facts matter: canon says you can skim, and canon places absolutely no obstacle to having a 1G ship land on and then leave a 1G or higher world. (In Traveller Adventure, Aramis subsector includes 9 worlds with gravity of 1G or higher, including L'Oeul d'Dieu and Reacher, two of the four worlds served under March Harrier's subsidy arrangement. In Twilight's Peak, the Empress Nicholle, a 1G Far Trader, plies the Spinward Main in search of enough income to repair their jump drive, with potential stops including Kkirka, Porozlo, Rech, and others.) That 1G+ bit came to us courtesy of Book-6, with it's 1.25G inhabited worlds.
So, your choices are very, very simple: you either overdrive, or you find some other source of thrust. Period. Or, I guess you could impose a house rule restricting Free Traders, Far Traders, Subsidized Liners, and Launches from landing on worlds of size class 8 or higher, but there's not a whisper in canon to support such an interpretation. The players are pretty well closed off from worlds like Focaline, Knorbes, Forboldn, and so forth
As to adding a whole other drive: the maneuver drive is there, and by canon you are able to skim and escape larger inhabited worlds. Ergo, some feature of the existing maneuver drive permits you to skim and escape larger inhabited worlds. No need to add anything. The only need that exists, if any need exists, is to try to figure out what precisely that feature is - and, honestly, that's not a need, that's more a scratching-the-curiosity-itch thing, which I think is a worthier pursuit than trying to set limits on the players that the rules don't actually set.
We can all play quite happily without ever knowing precisely how a 1G Safari Ship and its 1G launch manage to land and take of from the larger wild worlds. The speculating's mainly for fun and background.
Yes, you're just applying heat to a molecule. Modern jets take in air, combine the oxygen with fuel, and use the heated mixture for thrust. Said heated mixture includes nitrogen - a lot of nitrogen. The nitrogen doesn't react with anything, but it absorbs heat just dandy, expands like any gas will when you heat it, and provides thrust as a result.
Back in the Cold War days, there were ideas for putting small fission plants on big bombers so they could stay up indefinitely - you ran the air over a heat exchanger, and the superheated air gave you thrust. That one came to a halt - on the drawing boards, I think - on the technical difficulty of making a fission plant small enough and light enough, yet with enough shielding to keep from killing the crew and strong enough to stay intact if the bomber crashed. Nobody wanted to deal with a fission plant crashing someplace unexpected and throwing radioactive material everywhere.
An interesting canon reference is Secret of the Ancients, in which it is stated, "Most ships refueling from a gas giant rarely venture below the level of 0.5 atmospheres". This presents an interesting quandary. Komesh is described as a large gas giant with 3G gravity, 72,000 km diameter at the point where atmospheric pressure is 1 ATM. Ships are scooping 500km above that. Ships need to have at least 3G to hover and land at the Ancient site. To escape a 3G world requires either a wee bit more than 3Gs or an escape velocity of 60 kps. Even at 2G and in half an atmosphere, top speed is only about 2% of that; you're pretty well doomed unless you've got wings or some other way of adding on another G.
A thought occurs: if you can manage to enter atmosphere at a velocity that, after the skimming maneuver, leaves you exiting atmosphere with the needed velocity, or at least enough of it that your 1 or 2 Gs can get you over the hump, then you can pull off a skim - though woe unto you if you miscalculate. I don't have the math to guess what that velocity might be, but it would be truly frightening. The skimming maneuver takes close to three hours to pull off, but that's not necessarily in one dive. Accelerating to 60 KPS at 1 or 2Gs takes a long while: over an hour and a half at 1G, 50 minutes at 2G - which was the top speed of Azhanti High Lightning's fuel shuttles, so there's not a whole lot of fime for multiple dives if accelerating beyond 60 KPS. Indeed, there's barely the time for the one dive. We may be on the verge of declaring that Imperial Navy fueling shuttles can't do their jobs on large gas giants.
Put simply, it's not really possible to keep the canon time frame without some extra juice from somewhere.
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.
Incorrect. At a high enough speed, 1/200 Atmosphere is plenty dense. In fact, it can be too dense. See "control spike" used with the Pratt & Whitney J-58 Engine on a certain high altitude, speed craft. A friend of mine holds the world absolute and world class altitude record for horizontal flight and he explained a lot of this when we used to play Trav way back in the day...
