CobaltSunrise
06-01-04, 10:35 PM
I read about this on the internet a few days ago. It's just so outrageous, like a space elevator to GEO or a 50km tower as the first step to LEO. Very promising, too. If an airship scheme can achieve orbital velocity, the JP Aerospace people are talking about reducing the cost to $1 a kilogram. That's a four order-of-magnitude decrease in expense, with a similar increase in safety due to the nature of airships. Now, even if these guys are wildly off, and their design only presents a single order-of-magnitude cost decrease, it's still a great idea.
Their airship plan has multiple stages, like conventional rocket-based plans: a surface-to-40km airship, a suborbital station at 40km, and a 40km-to-60km airship that uses "some sort" of electric propulsion to gain the 8km/s velocity for LEO. Links:
http://www.jpaerospace.com/
http://www.hobbyspace.com/AAdmin/archive/SpecialTopics/orbitalAirship.html
I'm not a physicist. But I'm not so undereducated I have to rely on the good word of the JP Aerospace people. After some google-assisted cramming on the physics of high-altitude balloons and airships, here's what I've got:
Problem #1: It's easy to get to 50km using balloons. A single, spherical hydrogen balloon (call me crazy) would need a diameter of 576 meters to lift 11.3 metric-tons from surface to stratopause. I calculated that, at the surface, only 10% of the balloon would be filled with hydrogen, and the other 90% would inflate as pressure decreases with altitude. But ah, it's tricky to get from 50km to 100km using balloons. We would need a balloon system with the equivalent of a single 5760 meter diameter sphere to hold 11.3 metric-tons aloft at 100km. The balloon would have to be over 3.5 miles wide --and 100km is just over half-way to LEO. So it's implausible that we get to orbital altitude using Archimedes principle. The JP Aerospace people talk about designing the airship so that the airship itself is a lifting-body. More on this later.
Problem #2: While increasing the diameter of a balloon will increase its lifting power by an order of magnitude, which is great, it also increases the surface area (and thus, the weight of the balloon's material) by two orders of magnitude. I have not been able to get a reliable weight for possible balloon surface material. However, this innate relationship between balloon lift and weight effectively puts a ceiling on the size of a balloon within a three magnitude pressure difference (1000mb at surface to 1mb at 50km, 1mb at 50km to 0.001mb at 100km). I would calculate that ceiling if I had reliable numbers, but alas, Google does not reliably provide in-depth data. However, I feel that this is a mere technical problem that can be readily solved by the materials-science people.
Problem #3: Electric propulsion *can* be used to accelerate a mass to orbital velocity. Using the same balloon (0.9 metric-tons of hydrogen), it would require ~45 days to push a 10 metric-ton payload to orbit using a standard electric (ion) engine with 20 Newtons thrust; ~4.5 days to push the same load using a magnetoplasmadynamic electric engine with 200 Newtons thrust. However, this is discounting air friction and drag. These forces are not a problem for a stationary balloon at 50km because there is only 1mb of air pressure. However, 50km is not the place to accelerate an airship to orbital velocity using electric propulsion: even the newest electric-propulsion engines (VASIMR) do not have that much thrust to loose to air-friction and drag. And as they accelerate to 8km/s, the drag increases and net thrust decreases. Even if the electric engines do not meet the point where they can't accelerate any more due to drag, drag is a problem.
Electric-engine airships-to-orbit faces two primary problems: achieving orbital altitude and orbital velocity. Lift alone will not get an airship to orbit; and atmospheric friction prevents an electric engine from being effective. My solution is to turn the problem on its head: both of those problems can be made to cancel each other out.
First, consider that the airship does not require any lift to maintain altitude at 50km. Second: if the airship is designed to be a lifting body, then any lift at all will result in a net gain in altitude. Third: this lift can be generated by the forward motion of the airship in an atmosphere, and in fact the lift increases the greater the airship velocity at a given altitude. Thus, so long as the airship is designed as a lifting body, all energy provided by the electric engine will contribute to orbital altitude and velocity.
The problem with this solution is that, as the airship gains altitude, more and more of the lift that is generated is being used to support the payload. A 10 metric-ton payload might have no weight at 50km with our balloon, but unless it expands to 1000x its volume (10x its diameter) by 100km we will either #1) have popped the balloon and gone hurtling to the Earth below; or #2) have bled 899.1kg of our original 900kg of hydrogen, resulting in our payload having an actual weight of 10 metric-tons. At that point, there must be enough lift to support the 10 metric-ton payload or the airship settles at a lower altitude.
This problem may be solved by calculating the effective (non-atmospheric) lift generated by any satellite with orbital or near-orbital velocity. As I don't have the physics degree, I haven't calculated it out. But the physics of the situation is that a satellite orbiting another body is not supported in its orbit by atmospheric lift. Its velocity relative to the gravity and diameter of the orbited body is what keeps a satellite in orbit. This is another factor to consider: the lift --and hence, orbital altitude-- gained by the increase in velocity itself. This sort of lift is a function of the velocity.
Sometimes I wish I had some courses in astrophysics. I really don't have the education to put all these facts together in cold, hard equations. However, the facts are there, for your judgement. As far as I'm concerned, I believe it can be done. I'd be really interested in a physicist putting it all together.
Their airship plan has multiple stages, like conventional rocket-based plans: a surface-to-40km airship, a suborbital station at 40km, and a 40km-to-60km airship that uses "some sort" of electric propulsion to gain the 8km/s velocity for LEO. Links:
http://www.jpaerospace.com/
http://www.hobbyspace.com/AAdmin/archive/SpecialTopics/orbitalAirship.html
I'm not a physicist. But I'm not so undereducated I have to rely on the good word of the JP Aerospace people. After some google-assisted cramming on the physics of high-altitude balloons and airships, here's what I've got:
Problem #1: It's easy to get to 50km using balloons. A single, spherical hydrogen balloon (call me crazy) would need a diameter of 576 meters to lift 11.3 metric-tons from surface to stratopause. I calculated that, at the surface, only 10% of the balloon would be filled with hydrogen, and the other 90% would inflate as pressure decreases with altitude. But ah, it's tricky to get from 50km to 100km using balloons. We would need a balloon system with the equivalent of a single 5760 meter diameter sphere to hold 11.3 metric-tons aloft at 100km. The balloon would have to be over 3.5 miles wide --and 100km is just over half-way to LEO. So it's implausible that we get to orbital altitude using Archimedes principle. The JP Aerospace people talk about designing the airship so that the airship itself is a lifting-body. More on this later.
Problem #2: While increasing the diameter of a balloon will increase its lifting power by an order of magnitude, which is great, it also increases the surface area (and thus, the weight of the balloon's material) by two orders of magnitude. I have not been able to get a reliable weight for possible balloon surface material. However, this innate relationship between balloon lift and weight effectively puts a ceiling on the size of a balloon within a three magnitude pressure difference (1000mb at surface to 1mb at 50km, 1mb at 50km to 0.001mb at 100km). I would calculate that ceiling if I had reliable numbers, but alas, Google does not reliably provide in-depth data. However, I feel that this is a mere technical problem that can be readily solved by the materials-science people.
Problem #3: Electric propulsion *can* be used to accelerate a mass to orbital velocity. Using the same balloon (0.9 metric-tons of hydrogen), it would require ~45 days to push a 10 metric-ton payload to orbit using a standard electric (ion) engine with 20 Newtons thrust; ~4.5 days to push the same load using a magnetoplasmadynamic electric engine with 200 Newtons thrust. However, this is discounting air friction and drag. These forces are not a problem for a stationary balloon at 50km because there is only 1mb of air pressure. However, 50km is not the place to accelerate an airship to orbital velocity using electric propulsion: even the newest electric-propulsion engines (VASIMR) do not have that much thrust to loose to air-friction and drag. And as they accelerate to 8km/s, the drag increases and net thrust decreases. Even if the electric engines do not meet the point where they can't accelerate any more due to drag, drag is a problem.
Electric-engine airships-to-orbit faces two primary problems: achieving orbital altitude and orbital velocity. Lift alone will not get an airship to orbit; and atmospheric friction prevents an electric engine from being effective. My solution is to turn the problem on its head: both of those problems can be made to cancel each other out.
First, consider that the airship does not require any lift to maintain altitude at 50km. Second: if the airship is designed to be a lifting body, then any lift at all will result in a net gain in altitude. Third: this lift can be generated by the forward motion of the airship in an atmosphere, and in fact the lift increases the greater the airship velocity at a given altitude. Thus, so long as the airship is designed as a lifting body, all energy provided by the electric engine will contribute to orbital altitude and velocity.
The problem with this solution is that, as the airship gains altitude, more and more of the lift that is generated is being used to support the payload. A 10 metric-ton payload might have no weight at 50km with our balloon, but unless it expands to 1000x its volume (10x its diameter) by 100km we will either #1) have popped the balloon and gone hurtling to the Earth below; or #2) have bled 899.1kg of our original 900kg of hydrogen, resulting in our payload having an actual weight of 10 metric-tons. At that point, there must be enough lift to support the 10 metric-ton payload or the airship settles at a lower altitude.
This problem may be solved by calculating the effective (non-atmospheric) lift generated by any satellite with orbital or near-orbital velocity. As I don't have the physics degree, I haven't calculated it out. But the physics of the situation is that a satellite orbiting another body is not supported in its orbit by atmospheric lift. Its velocity relative to the gravity and diameter of the orbited body is what keeps a satellite in orbit. This is another factor to consider: the lift --and hence, orbital altitude-- gained by the increase in velocity itself. This sort of lift is a function of the velocity.
Sometimes I wish I had some courses in astrophysics. I really don't have the education to put all these facts together in cold, hard equations. However, the facts are there, for your judgement. As far as I'm concerned, I believe it can be done. I'd be really interested in a physicist putting it all together.