esampson
SOC-13
So according to pg. 437 a ship has to have a G rating equal to or greater than the size of the gas giant/4 in order to skim. Since gas giants start at L that's a bit of a problem since it would mean that only 6G ships could skim any but the smallest gas giants.
However, rather than trying to correct it in errata I think it should probably be completely removed as the logic behind it falls apart in most cases given the machinery of T5.
First off there are the lifters which most ships seem to have which negate gravity, at least to within a reasonable amount. That alone should probably allow a ship of any configuration to descend low enough to skim gas and then reascend.
Beyond that, however, we get into issues of orbital mechanics. Basically any object that is in a stable orbit (one in which periapsis remains above any significant portion of the atmosphere) can leave orbit with even the smallest amounts of thrust. It may take a while for that small amount of thrust to build up to the point where the object has enough energy that it leaves orbit but it will happen. This mechanism is currently used in the real world in the form of the Dawn spacecraft which has the blistering acceleration of 0 to 60 mph in 4 days.
When an object drops down into a significant portion of the atmosphere the rules do change a bit. At that point what you have to have is enough thrust so that the point of equilibrium between thrust and aerodynamic drag is greater than orbital velocity. Above this point a ship can simply increase its thrust which will increase its orbit, eventually carrying it away just like our earlier example of a ship outside the atmosphere. It is possible for a ship to dip below that and to escape but you have to do tricky things with elliptical orbits and I don't want to figure out how low a ship could reasonably go. However, as long as you remain above the point where the atmospheric drag forces the ship to slower than orbital velocity then you can always escape simply by flying out.
I took Jupiter as a model and used a Scout/Courier for my ship. Assuming a Coefficiency of Drag of about .04, mass of 980 tons (from MT) and a cross section of 108m (24mx9.5m) this point turns out to be at about .01 atm of pressure which occurs about 173km above the "radius" of Jupiter (for gas giants the radius is the point where atmospheric pressure equals 10 atm) with the engine exerting 1G of thrust.
Yes, I realize that that high up there's a little over 1 gram of hydrogen per cubic meter but the ship is travelling at roughly 42.5 km/sec. If the scoop's intake is only 1.5mx1.5m then in each second it will scoop up over 95,000 cubic meters of gas. This works out to 96 kg of hydrogen per second or over 345 metric tons per hour. Since fuel scoops are only rated at 200 tons an hour it would seem that we are within enough atmosphere for what we want to do (and in fact this would seem to imply that the ship can be at an even higher altitude which increases safety margin).
This is for a 100 ton ship at 1G. As counter-intuitive as it may seem larger ships will actually have higher top speeds (the mass, and thus the engine thrust, increases at a cubic rate while the cross section only increases at a square). This is also assuming a fairly small cross section for the scoops which could definitely be increased.
Of course there are things that could throw off the formulas. Ships with higher coefficiencies of drag would go slower at a rate of basically 1/(Cd/.04).5 (this means a ship with a Cd four times higher will have a maximum speed of 1/2). It is also possible that while Newtonian mechanics say that a 980 ton ship capable of accelerating at 1G produces 9.6 MN of thrust the gravatic drives of Traveller ships produce less than that because they operate through some unknown process. However in even the worst cases a ship could simply make higher passes and take longer to refill its tanks.
However, rather than trying to correct it in errata I think it should probably be completely removed as the logic behind it falls apart in most cases given the machinery of T5.
First off there are the lifters which most ships seem to have which negate gravity, at least to within a reasonable amount. That alone should probably allow a ship of any configuration to descend low enough to skim gas and then reascend.
Beyond that, however, we get into issues of orbital mechanics. Basically any object that is in a stable orbit (one in which periapsis remains above any significant portion of the atmosphere) can leave orbit with even the smallest amounts of thrust. It may take a while for that small amount of thrust to build up to the point where the object has enough energy that it leaves orbit but it will happen. This mechanism is currently used in the real world in the form of the Dawn spacecraft which has the blistering acceleration of 0 to 60 mph in 4 days.
When an object drops down into a significant portion of the atmosphere the rules do change a bit. At that point what you have to have is enough thrust so that the point of equilibrium between thrust and aerodynamic drag is greater than orbital velocity. Above this point a ship can simply increase its thrust which will increase its orbit, eventually carrying it away just like our earlier example of a ship outside the atmosphere. It is possible for a ship to dip below that and to escape but you have to do tricky things with elliptical orbits and I don't want to figure out how low a ship could reasonably go. However, as long as you remain above the point where the atmospheric drag forces the ship to slower than orbital velocity then you can always escape simply by flying out.
I took Jupiter as a model and used a Scout/Courier for my ship. Assuming a Coefficiency of Drag of about .04, mass of 980 tons (from MT) and a cross section of 108m (24mx9.5m) this point turns out to be at about .01 atm of pressure which occurs about 173km above the "radius" of Jupiter (for gas giants the radius is the point where atmospheric pressure equals 10 atm) with the engine exerting 1G of thrust.
Yes, I realize that that high up there's a little over 1 gram of hydrogen per cubic meter but the ship is travelling at roughly 42.5 km/sec. If the scoop's intake is only 1.5mx1.5m then in each second it will scoop up over 95,000 cubic meters of gas. This works out to 96 kg of hydrogen per second or over 345 metric tons per hour. Since fuel scoops are only rated at 200 tons an hour it would seem that we are within enough atmosphere for what we want to do (and in fact this would seem to imply that the ship can be at an even higher altitude which increases safety margin).
This is for a 100 ton ship at 1G. As counter-intuitive as it may seem larger ships will actually have higher top speeds (the mass, and thus the engine thrust, increases at a cubic rate while the cross section only increases at a square). This is also assuming a fairly small cross section for the scoops which could definitely be increased.
Of course there are things that could throw off the formulas. Ships with higher coefficiencies of drag would go slower at a rate of basically 1/(Cd/.04).5 (this means a ship with a Cd four times higher will have a maximum speed of 1/2). It is also possible that while Newtonian mechanics say that a 980 ton ship capable of accelerating at 1G produces 9.6 MN of thrust the gravatic drives of Traveller ships produce less than that because they operate through some unknown process. However in even the worst cases a ship could simply make higher passes and take longer to refill its tanks.
