Able to skim, but can't land in atmosphere - how is that possible?
- Thread starter Murphy
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BlackBat242
SOC-14 1K
Video of Armstrong's ejection from the LLRV:
http://www.youtube.com/watch?v=6Qhcs6qiHLI
Test pilot Stuart Present ejects safely from crashing LLTV (NASA), 29 January 1971. he is the black blur at the left (you can see the exhaust flame from the seat's rocket pack).
http://www.youtube.com/watch?v=6Qhcs6qiHLI
Test pilot Stuart Present ejects safely from crashing LLTV (NASA), 29 January 1971. he is the black blur at the left (you can see the exhaust flame from the seat's rocket pack).
And, that was "stick & rudder" work! With computerized flight controls this is a no brainer...
Doyle Hunt
SOC-10
You don't have to look to exotic flying machines for examples of landing on thrust alone. Look no farther than the Harrier "jump jet."
As I see it, the real problem in landing an unstreamlined craft from orbital altitude to the surface of a world with atmosphere, is that you start *in* orbit, and have to shed a *lot* of orbital velocity before you can begin your descent. The ISS orbits at an altitude of 380 km, at a velocity of 27,650 km/h. In the case of an unstreamlined starship trying to land on Earth from the ISS's orbit, one would have to dump 27,350 km/h of velocity before hitting the upper atmosphere at 300 km/h using your manuever drive (anyone want to calculate how long that deceleration would take at 1G?), or else you'll come in like a meteor, and burn off all the bits that stick out.
Skimming doesn't require shedding anywhere near as much velocity. You would only have to lose enough velocity to turn a circular orbit into an ellipse that slightly enters the upper atmosphere. At that kind of velocity, you don't need a lot of atmospheric density to fill scoops. I very much doubt one would need to descend below the altitude of "trace" atmospheric density (atmosphere code 1 at that altitude) to skim hydrogen from a gas giant. Especially since, the higher one stays, the more likely one would be to skim nearly pure hydrogen, and not the methane, ammonia, helium, or any of the other gases also present in the lower gas giant atmosphere.
As an aside, I've had heartburn about the reverse case, an air/raft ascending to orbit in a number of hours equal to the planet's size code. Sure, it may achieve orbital altitude, but it will never achieve orbital velocity in that amount of time. It would remain semi-ballistic and could only remain aloft on its grav modules. Still, the air/raft rules provide a fair compromise for landing unstreamlined ships in atmosphere. I'd say that, once you've dumped your orbital velocity and you're staying aloft on thrusters only, it take a number of hours equal to the planet's size code to land your ship on the surface through an atmosphere (no different from landing an air/raft from the same altitude). With a Pilot or Ship's Boat skill check every hour, and a Determination roll to maintain concentration for that long if the GM is feeling sadistic.
Or, to account for atmospheric density and composition, the time required to land an unstreamlined ship through an atosphere could be a number of hours equal to the *greater* of the planet's Size and Atmosphere codes.
As I see it, the real problem in landing an unstreamlined craft from orbital altitude to the surface of a world with atmosphere, is that you start *in* orbit, and have to shed a *lot* of orbital velocity before you can begin your descent. The ISS orbits at an altitude of 380 km, at a velocity of 27,650 km/h. In the case of an unstreamlined starship trying to land on Earth from the ISS's orbit, one would have to dump 27,350 km/h of velocity before hitting the upper atmosphere at 300 km/h using your manuever drive (anyone want to calculate how long that deceleration would take at 1G?), or else you'll come in like a meteor, and burn off all the bits that stick out.
Skimming doesn't require shedding anywhere near as much velocity. You would only have to lose enough velocity to turn a circular orbit into an ellipse that slightly enters the upper atmosphere. At that kind of velocity, you don't need a lot of atmospheric density to fill scoops. I very much doubt one would need to descend below the altitude of "trace" atmospheric density (atmosphere code 1 at that altitude) to skim hydrogen from a gas giant. Especially since, the higher one stays, the more likely one would be to skim nearly pure hydrogen, and not the methane, ammonia, helium, or any of the other gases also present in the lower gas giant atmosphere.
As an aside, I've had heartburn about the reverse case, an air/raft ascending to orbit in a number of hours equal to the planet's size code. Sure, it may achieve orbital altitude, but it will never achieve orbital velocity in that amount of time. It would remain semi-ballistic and could only remain aloft on its grav modules. Still, the air/raft rules provide a fair compromise for landing unstreamlined ships in atmosphere. I'd say that, once you've dumped your orbital velocity and you're staying aloft on thrusters only, it take a number of hours equal to the planet's size code to land your ship on the surface through an atmosphere (no different from landing an air/raft from the same altitude). With a Pilot or Ship's Boat skill check every hour, and a Determination roll to maintain concentration for that long if the GM is feeling sadistic.
Or, to account for atmospheric density and composition, the time required to land an unstreamlined ship through an atosphere could be a number of hours equal to the *greater* of the planet's Size and Atmosphere codes.
leo knight
SOC-12
I've tried to figure this one out too, if only to drive myself a little crazier. It seems to me that, if I were to start from scratch, landing and streamlining might be separate properties. So, landing with streamlining would be equivalent to the Shuttle, Spaceship One, various SSTO spacecraft, etc.
Landing, no streamlining would be equivalent to Surveyor, the LEM, Viking, etc.
Streamlining, no landing would be equivalent to various schemes for aerobraking, JP Aerospace's airship-to-orbit concept, etc. I remember seeing a painting, I think by David Hardy, of two deuterium tankers scooping from Jupiter. They looked like ramjet engines, just a sleek cylinder with an intake and aerospike cone on the front, rocket in back, no landing, just dive, scoop, repeat.
No streamlining or landing would of course be any pure space vehicle. And, as pointed out before, once we invoke anti-gravity or reactionless drives, all bets are off.
As a completely irrelevant aside, a friend of mine got indignant when Star Trek: Voyager landed the title ship on a planet with an atmosphere. He just couldn't believe it. I pointed out that they had force fields, anti- and artificial gravity, inertial dampers, structural integrity fields, honest politicians, and other marvels to achieve said feat. I also pointed out that they went from zero to C in no time flat, all without folding the ship into origami, smearing the crew all over the rear bulkheads, or even spilling the captain's coffee from her fine china cup. Dealing with mere gravity and atmosphere should be a snap!
Landing, no streamlining would be equivalent to Surveyor, the LEM, Viking, etc.
Streamlining, no landing would be equivalent to various schemes for aerobraking, JP Aerospace's airship-to-orbit concept, etc. I remember seeing a painting, I think by David Hardy, of two deuterium tankers scooping from Jupiter. They looked like ramjet engines, just a sleek cylinder with an intake and aerospike cone on the front, rocket in back, no landing, just dive, scoop, repeat.
No streamlining or landing would of course be any pure space vehicle. And, as pointed out before, once we invoke anti-gravity or reactionless drives, all bets are off.
As a completely irrelevant aside, a friend of mine got indignant when Star Trek: Voyager landed the title ship on a planet with an atmosphere. He just couldn't believe it. I pointed out that they had force fields, anti- and artificial gravity, inertial dampers, structural integrity fields, honest politicians, and other marvels to achieve said feat. I also pointed out that they went from zero to C in no time flat, all without folding the ship into origami, smearing the crew all over the rear bulkheads, or even spilling the captain's coffee from her fine china cup. Dealing with mere gravity and atmosphere should be a snap!
Enoki
SOC-14 1K
I'd think the two problems here are:
1. Re-entry velocity. Now, with Traveller, you have ships that are capable of space maneuver and have engines that are capable of slowing or stopping a ship's orbital speed, can maneuver independently of orbital speeds, and can even maintain a non-orbital position relative to a planet. Therefore, unlike current space vehicles one that is poorly streamlined could simply slow to a speed at which entry into the atmosphere is possible without having to worry about burning up. Decent rate could be controlled using the maneuver engines.
2. Operating in an atmosphere. This requires the craft to be able to take the stress of the atmospheric pressure, any stresses associated with flight and velocity, gravitational stresses, etc. To use the venacular, you can get a brick to fly with enough engine on it.
Given that ships are all required to have an armored hull, I doubt that atmospheric pressures and gravity are problems outside gas giants or going deep into a thick atmosphere (like a gas giant) or submerging the craft in liquid oceans of something.
Velocity would be limited by streamlining mostly. This would also could severely restrict maneuverability in an atmosphere. Obviously something that is aerodynamic is going to have an maneuver advantage over the proverbial brick. While grav systems would definitely help any ship, having airfoils to take advantage of the atmosphere would still be a big plus.
1. Re-entry velocity. Now, with Traveller, you have ships that are capable of space maneuver and have engines that are capable of slowing or stopping a ship's orbital speed, can maneuver independently of orbital speeds, and can even maintain a non-orbital position relative to a planet. Therefore, unlike current space vehicles one that is poorly streamlined could simply slow to a speed at which entry into the atmosphere is possible without having to worry about burning up. Decent rate could be controlled using the maneuver engines.
2. Operating in an atmosphere. This requires the craft to be able to take the stress of the atmospheric pressure, any stresses associated with flight and velocity, gravitational stresses, etc. To use the venacular, you can get a brick to fly with enough engine on it.
Given that ships are all required to have an armored hull, I doubt that atmospheric pressures and gravity are problems outside gas giants or going deep into a thick atmosphere (like a gas giant) or submerging the craft in liquid oceans of something.
Velocity would be limited by streamlining mostly. This would also could severely restrict maneuverability in an atmosphere. Obviously something that is aerodynamic is going to have an maneuver advantage over the proverbial brick. While grav systems would definitely help any ship, having airfoils to take advantage of the atmosphere would still be a big plus.
leo knight
SOC-12
I was having trouble wrapping my head around slowing down from, say 10 km/sec orbital velocity to near zero, or vice versa for liftoff. I was imagining it taking hours or days, and also having trouble with how the ship would "stay up" without enough speed. But, if I calculated correctly, at 1G it would only take about 1000 seconds, or roughly 17 minutes, to make such a maneuver. So I guess such a craft could slow down to match the rotation of the planet's surface, while using its drives or antigravs to "hover" over the landing site, then lower itself at a leisurely speed. If a ship is hovering at "the edge of space" 100km up, and lowers itself at say 100km/hr, the speed of a car on a highway, it would reach the surface in 1 hour. Am I getting this right?
I find it easy to imagine sheer fantasy like "Treasure Planet", since it doesn't have to make sense. I'm getting better at visualizing real physics, but still with a lot of blind spots! But imaginary or conjectural physics, that does not yet exist but must remain internally consistent, without annoying "gotchas", sometimes makes my head swim.
o:
I find it easy to imagine sheer fantasy like "Treasure Planet", since it doesn't have to make sense. I'm getting better at visualizing real physics, but still with a lot of blind spots! But imaginary or conjectural physics, that does not yet exist but must remain internally consistent, without annoying "gotchas", sometimes makes my head swim.
o:
far-trader
SOC-14 10K
I'm no better at these maths (been too long since school) but...
The thing is "hovering" at 100km altitude is still a pretty high velocity. You aren't actually motionless. If I ran the numbers right to maintain geostationary at 100km you have to be orbiting at 1695km/h. That is going to cause a lot of stress vis-a-vis the atmosphere for an unstreamlined hull. Not counting possibly contrary vector, and variable, high altitude, high velocity, winds.
I really don't get the whole issue. Unstreamlined in the rules simply means "not meant for atmospheric interface". The ship will have sharp angles, no aerodynamics, bits sticking out, materials not rated for atmosphere, etc. etc. all of which are cheaper ways and materials than making the ship Streamlined. If the world has an atmosphere your Unstreamlined ship should not land there. It will be damaged, possibly destroyed.
Perhaps a better/different nomenclature would have avoided the whole mess. Something like (simple example of US and SL):
Vacuum Hull - not rated for atmosphere, may not skim gas giants or land on worlds with atmospheres
Atmospheric Hull - fully rated for atmosphere, may skim gas giants and land on worlds with atmospheres
The thing is "hovering" at 100km altitude is still a pretty high velocity. You aren't actually motionless. If I ran the numbers right to maintain geostationary at 100km you have to be orbiting at 1695km/h. That is going to cause a lot of stress vis-a-vis the atmosphere for an unstreamlined hull. Not counting possibly contrary vector, and variable, high altitude, high velocity, winds.
I really don't get the whole issue. Unstreamlined in the rules simply means "not meant for atmospheric interface". The ship will have sharp angles, no aerodynamics, bits sticking out, materials not rated for atmosphere, etc. etc. all of which are cheaper ways and materials than making the ship Streamlined. If the world has an atmosphere your Unstreamlined ship should not land there. It will be damaged, possibly destroyed.
Perhaps a better/different nomenclature would have avoided the whole mess. Something like (simple example of US and SL):
Vacuum Hull - not rated for atmosphere, may not skim gas giants or land on worlds with atmospheres
Atmospheric Hull - fully rated for atmosphere, may skim gas giants and land on worlds with atmospheres
The issue arises because of that Broadsword adventure bit where it "grounds" on a planet despite being unstreamlined (or partially streamlined, under later rules, which amounts to the same thing as far as a terrestrial-type planet is concerned.) This triggered several questions, starting with why your basic basketball-with-legs can't negotiate a terrestrial atmosphere when it can negotiate a jovian atmosphere, continuing through what "grounding" is and how it differs from a streamlined ship's landing, and ending with whether other nonstreamlined ships might be able to manage that same "grounding" feat - and if so, which ones, and if not, why not.
It's basically a case of someone at GDW breaking their own rule and leaving us wondering if and how we might be able to break the rule too.
Correct me if I'm wrong on this but, if you've got the drives to hover at 100 km altitude, don't you also have the drives to match speed with the prevailing winds at that altitude? It doesn't solve all of the problem - you've got wind shifts to worry about - but it does solve most of them.
I see this as a problem of physics and engineering. To "ground" a ship on its drives requires being able to keep its axis of thrust in line with the planet's gravity, basically to keep its drives under it - and to be able to support its own weight once landed of course, and to be able to maintain that axis if the ground settles unevenly as you land on it, no small issues. Broadsword is short and squat: knocking it off its "legs" would require one heck of a lot of lateral force. Ad Astra, a subsidized liner, is tall and thin - without very powerful attitude control, it'd be like a tree with no roots when a good wind hit it as it dropped through atmosphere, and it wouldn't take much uneven ground settlement to see it pitch over like a tower in an earthquake unless it grounded on a firm landing pad. Clearly Broadsword can ground - they said so. Most likely Ad Astra can't - you might manage to set it down with luck and very good piloting skill, but unless there's a stout pad beneath it, and maybe even with a stout pad if the wind hits right, it's going down and it isn't ever getting up again. For others, the GM would have to rule on a case-by-case basis.
Icosahedron
SOC-14 1K
Your calculations are correct, Leo. Assuming the 1000km/s is correct and relative to the surface, at 1G it would take 1000 seconds to slow to zero and, assuming a height of 100km or so, somewhat longer to descend safely to the surface.
Personally, I think all that time for the air-raft to reach orbit must include accelerating to full orbital speed at fractional G.
FT, your comment about rotational motion whilst stationary above a point on the Earth's surface is true but irrelevant, since what matters is the speed relative to the surface.
Personally, I think most of the ships of the OTU should be able to enter an atmosphere. What I wouldn't like to land is something like Kirk's Enterprise. It's quite aerodynamically streamlined, but those rear booms take very little persuasion to shear off in a bit of crosswind, gravity, or contact with a surface - ask my Airfix model...
Personally, I think all that time for the air-raft to reach orbit must include accelerating to full orbital speed at fractional G.
FT, your comment about rotational motion whilst stationary above a point on the Earth's surface is true but irrelevant, since what matters is the speed relative to the surface.
Personally, I think most of the ships of the OTU should be able to enter an atmosphere. What I wouldn't like to land is something like Kirk's Enterprise. It's quite aerodynamically streamlined, but those rear booms take very little persuasion to shear off in a bit of crosswind, gravity, or contact with a surface - ask my Airfix model...
Amber Chancer
SOC-12
I'm with FT on this one.
And if the Broadsword issue is bugging you, then redistribute its fuel tanks a little, call it a flattened sphere, and Bob's your auntie ...
Here's the rub. Take an air raft. If you are slowly ascending straight up, you also, slowly match the air speed naturally. The exact same way that very high altitude weather balloons (~45 km) get there. They don't get ripped apart and stay in place several days. The same routine when descending slowly. You slowly get match the air speed. It is even easier coming down as you start in vacuum where you can match whatever lateral velocity you require...
Obviously a spaceship hull can handle more stress and wind than a balloon under the same circumstances. Therefore, barring some type of spindly dispersed structure, any ship that possess more Grav G than the planet, can land.
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I've always had the same basic concept of Traveller streamlining as Far-Trader mentioned - i.e. it relates to how fragile the ship is and what it was designed to handle more so than its aerodynamic shape (with dispersed shapes and the like automatically not qualifying, of course).
Not only could the original definitions have been better, but later writings brought various author's and editor's conflicting ideas.
Descending on gravitics through the denser than vacuum atmo - assuming one matched the velocity within a few hundred kph at an interface of significant density vs effect - would basically boil down to aerodynamics re significant wind shear, weather differentials and jet streams...
Prior to that, there may be the issue of all the rest of the natural and not so natural stuff one must navigate through for tens of thousands of km (earth geosync ~36,000 km) - notably being significantly slower than the stuff in low planetary orbit (800 km for earth sized) whizzing around at over 10,000 kph (17,000 typically for LEO).
I've never done the numbers - but an order of magnitude of delta v means some limited time to maneuver to avoid debris otherwise obscured for part of its orbit (and perhaps independently move-able to boot) - and 1 to 6 G instantaneous maneuver may or may not be enough time. No actual idea... but this relates to whether a ship normally orbits and then descends - i.e. coming up to a planet at a tangent - or, simply heads 'straight' in matching the atmo shear component of velocity (curving with the atmo pressure in the last few hundred km or whatever based on world size/atmo density).
The solution to this, at least for established space faring systems, is debris monitoring and removal. Lot of interesting RW ideas today about that sort of things (EDDE, 'harpoon', thermal mirror heating systems, Boeing's gas clouds, etc.).
Another thing to note, is the planet is moving in its own orbit at a relative velocity falling in to the system center or mass (or dynamic center in the event of certain multi-star systems) which the ship needs to negate by matching regardless of whether it de-orbits or simply attempts to 'descend' through an atmo.
Not only could the original definitions have been better, but later writings brought various author's and editor's conflicting ideas.
Descending on gravitics through the denser than vacuum atmo - assuming one matched the velocity within a few hundred kph at an interface of significant density vs effect - would basically boil down to aerodynamics re significant wind shear, weather differentials and jet streams...
Prior to that, there may be the issue of all the rest of the natural and not so natural stuff one must navigate through for tens of thousands of km (earth geosync ~36,000 km) - notably being significantly slower than the stuff in low planetary orbit (800 km for earth sized) whizzing around at over 10,000 kph (17,000 typically for LEO).
I've never done the numbers - but an order of magnitude of delta v means some limited time to maneuver to avoid debris otherwise obscured for part of its orbit (and perhaps independently move-able to boot) - and 1 to 6 G instantaneous maneuver may or may not be enough time. No actual idea... but this relates to whether a ship normally orbits and then descends - i.e. coming up to a planet at a tangent - or, simply heads 'straight' in matching the atmo shear component of velocity (curving with the atmo pressure in the last few hundred km or whatever based on world size/atmo density).
The solution to this, at least for established space faring systems, is debris monitoring and removal. Lot of interesting RW ideas today about that sort of things (EDDE, 'harpoon', thermal mirror heating systems, Boeing's gas clouds, etc.).
Another thing to note, is the planet is moving in its own orbit at a relative velocity falling in to the system center or mass (or dynamic center in the event of certain multi-star systems) which the ship needs to negate by matching regardless of whether it de-orbits or simply attempts to 'descend' through an atmo.
So ... maybe possibly one can hover down some of those otherwise nonstreamlined ships, if it can tolerate the stress of its own weight once grounded - except that there's that possibility you could have an unpleasant meeting with a 25,000 kph wrench lost by some crewman on EVA last week. Ouchie!
A non-streamlined ship - i.e. one not made for atmo operations - would still have a problem with wind pressure shearing off important bits... (is that my gorram primary buffer panel that just flew off?)
Stability control might still be an issue as well - but internal gravitic compensation (ref HG) should take care of that.
As to structural strength against 'weight' gravitics again addresses that.
(IMTU gravitics provide thrust-less acceleration - i.e. there is no directional exhaust - so no need to 'land' on one's tail. I really didn't see any mention of such in CT... but maybe its there. <shrug>)
As to running into that high v wrench - could happen anytime and regardless of approach, though lingering a long time in the danger zone brings up a question of odds...
Stability control might still be an issue as well - but internal gravitic compensation (ref HG) should take care of that.
As to structural strength against 'weight' gravitics again addresses that.
(IMTU gravitics provide thrust-less acceleration - i.e. there is no directional exhaust - so no need to 'land' on one's tail. I really didn't see any mention of such in CT... but maybe its there. <shrug>)
As to running into that high v wrench - could happen anytime and regardless of approach, though lingering a long time in the danger zone brings up a question of odds...
Or simply recognize what everyone who ever saw a cannon or musket fired knows ... SPHERES ARE STREAMLINED!
Well... kind of. Not very much.