Rockets and Other forms of propulsion

[Theory5]

Im sure im not the only one who thinks rockets are a tremendously expensive and inefficent form of propulsion. I am reading a book called "gardisil" (fiction of course) and It got me thinking. While in the book humans have found a way to get into outerspace with an electromagnetic generator that harnesses the earths magnetic poles, one of the characters states that NASA has become fixated on 11 Kilometers/second (the escape velocity for our atmosphere), and only try to achieve that speed, rather than looking at other ways to get into outerspace. Does anyone have any insight into this? I know there are others ways, but to test and build other methods take lots of time and money. Is ballistic propulsion the only way? Each and every rocket sent up into space requires LOTs of money. Could that money be used in a better way?

[CraigD]

Im sure im not the only one who thinks rockets are a tremendously expensive and inefficent form of propulsion.

…

I know there are others ways, but to test and build other methods take lots of time and money. Is ballistic propulsion the only way? Each and every rocket sent up into space requires LOTs of money. Could that money be used in a better way?

You’re correct. Many folk have suggested alternatives to rockets, and many have been discussed here at hypography.

 

After weeding out the ones that nobody’s been able to show are even close to feasible (which include, unfortunately, ones using the Earth’s magnetic field), I think you’re left with tow basic alternatives: guns (AKA mass throwers or drivers) and elevators (AKA skyhooks), and lots of hybrid and variation designs.

 

Space guns - machines that accelerate a projectile – the spacecraft, in this case – over a short distance compared to the distance it ultimately travels – are usually energy efficient, but pose some serious disadvantages and challenges. Because they accelerate the projectile over a short distance, they do it in a brief time, so the projectile experience great forces - for example, a 100 m long gun would need a peak acceleration of at least 627200 m/s/s, about 64000 gs, far more than a human or other fragile cargo could survive. If a thick atmosphere is present, as it is at low altitudes on Earth, the projectile will lose a lot of speed due to friction, experience a lot of structural stress, and get very hot.

 

Space elevators – the kind usually discussed between a point on the surface of the Earth’s equators with its center of gravity at a geostationary altitude of about 36000000 m – are also very energy efficient, but have heights roughly equal to the distance around the Earth, and must be tremendously strong to support their own weight.

I am reading a book called "gardisil" (fiction of course) and It got me thinking.

This got me thinking that, despite being a pretty avid SF reader, I’ve never heard of this book. The usually reliable google search engine suggests it’s a misspelling of the HPV vaccine Gardasil! :) Do you have another spelling, or better yet, a link to this book, Theory5?

While in the book humans have found a way to get into outerspace with an electromagnetic generator that harnesses the earths magnetic poles, ...

While a mechanically simple idea – in essence, treating the Earth as a permanent motor magnet, requiring the spacecraft merely to perform as the armature with a self-contained electrical source – this idea suffers from the physical fact that, though the energy ideally required by such a system is as small as physically possible, because the Earth’s magnetic field is so small, the current required is beyond the practical and current theoretical capabilities of any normal or super conductor.

 

Propulsion systems of this kind, such as tether propulsion are practical for lower-force applications, such maneuvering satellites, though to date an actual spacecraft hasn’t successfully used it

... one of the characters states that NASA has become fixated on 11 Kilometers/second (the escape velocity for our atmosphere), and only try to achieve that speed, rather than looking at other ways to get into outerspace.

Though they mention this quantity in conversation quite a lot, I wouldn’t say any space engineer or knowledgeable enthusiast is fixated on it. It’s just the result you get when you calculate, using this formula

[math]v_e = \sqrt{\frac{2u}{r}}[/math]

(where [math]r[/math] if the distance from the Earth’s center, and [math]u[/math] is Earth’s standard gravitational parameter, a constant)

to calculate the speed (“escape velocity” is a misnomer, as the object’s direction doesn’t matter, only its speed – provided its direction doesn’t result in it hitting something too big) an object on the Earth’s surface must have, right there, to coast free of the Earth, ignoring the complication of air friction. It has nothing to do with the atmosphere, in fact ignoring it, so for a projectile to escape from a point within the atmosphere, its speed would have to be even higher.

 

The speed that must be given to an escaping projectile is reduced a bit from its escape speed if you launch from near the equator, where the Earth’s surface is already traveling about 464 m/s. Things are even better if you’re already in orbit – if you work with the formula for circular orbit and escape speed, you find the neat rule that escape speed is always [imath]\sqrt{2} \dot= 1.4142[/imath] times the speed of your circular orbit, which like escape velocity decreases as the inverse square root of [math]r[/math]. So from geosynchronous orbit ([imath]r \dot= 42164000[/imath] m), you need only add about 1274 m/s. At the L2 Earth-Moon Lagrangian point ([imath]r \dot= 445899000[/imath] m), which moves about 236 m/s faster than its natural orbital speed of 945 m/s, you need only add about 156 m/s.

[Theory5]

Space guns - machines that accelerate a projectile – the spacecraft, in this case – over a short distance compared to the distance it ultimately travels – are usually energy efficient, but pose some serious disadvantages and challenges. Because they accelerate the projectile over a short distance, they do it in a brief time, so the projectile experience great forces - for example, a 100 m long gun would need a peak acceleration of at least 627200 m/s/s, about 64000 gs, far more than a human or other fragile cargo could survive. If a thick atmosphere is present, as it is at low altitudes on Earth, the projectile will lose a lot of speed due to friction, experience a lot of structural stress, and get very hot.

 

Space elevators – the kind usually discussed between a point on the surface of the Earth’s equators with its center of gravity at a geostationary altitude of about 36000000 m – are also very energy efficient, but have heights roughly equal to the distance around the Earth, and must be tremendously strong to support their own weight.

This got me thinking that, despite being a pretty avid SF reader, I’ve never heard of this book. The usually reliable google search engine suggests it’s a misspelling of the HPV vaccine Gardasil! :) Do you have another spelling, or better yet, a link to this book, Theory5?

Yup, right here http://www.amazon.com/Gradisil-Adam-Roberts/dp/1591025389. I spelled it wrong it is actually Gradisil and in the book the word is from Cyprus and their spelling of it is Yggdisil. It means something about a tree.

 

While a mechanically simple idea – in essence, treating the Earth as a permanent motor magnet, requiring the spacecraft merely to perform as the armature with a self-contained electrical source – this idea suffers from the physical fact that, though the energy ideally required by such a system is as small as physically possible, because the Earth’s magnetic field is so small, the current required is beyond the practical and current theoretical capabilities of any normal or super conductor.

I think that is how they do it in the book, with a fictional power source. But lighting works by positive and negative ions interacting, right? Even though they might need supercooled air between -15*c. Isn't the upper atmosphere filled with some sort of charged ions because they are close to the magnetic field? And by flying the opposite way along the magnetic field your would encounter these charged ions. And with a small generator that has wires along the wings and underside of the fuselage of the plane, triggering these ions will basically create an electrical storm, allowing you to have enough energy and charge to be able to interact with the magnetic field enough to lift you.

whoops I forgot to add how the ions should be charged to trigger the storm. The wires are wrapped around the wings many times, and depending on weather the ions outside are positive or negivtive the current will flow either clockwise or counter-clockwise. Im not sure of all the math and physics and stuff.

What do you think?

Though they mention this quantity in conversation quite a lot, I wouldn’t say any space engineer or knowledgeable enthusiast is fixated on it.

I think when that was mentioned in the book the character meant that they try to make vehicles that need to go that fast, rather than thinking outside of the box and finding other means of getting into orbit without needing to go that fast.

 

It’s just the result you get when you calculate, using this formula

[math]v_e = \sqrt{\frac{2u}{r}}[/math]

(where [math]r[/math] if the distance from the Earth’s center, and [math]u[/math] is Earth’s standard gravitational parameter, a constant)

to calculate the speed (“escape velocity” is a misnomer, as the object’s direction doesn’t matter, only its speed – provided its direction doesn’t result in it hitting something too big) an object on the Earth’s surface must have, right there, to coast free of the Earth, ignoring the complication of air friction. It has nothing to do with the atmosphere, in fact ignoring it, so for a projectile to escape from a point within the atmosphere, its speed would have to be even higher.

I thought you couldn't go straight up, you needed to curve your ascent to penetrate the atmosphere or else the escape vehicle would get too hot, thus the velocity part.

[Janus]

 

I thought you couldn't go straight up, you needed to curve your ascent to penetrate the atmosphere or else the escape vehicle would get too hot, thus the velocity part.

 

As far as the atmosphere is concerned, straight up is better. That way you are out of the thicker part of the atmosphere before you reach a high enough speed for the friction to be a problem.

 

The reason rockets follow a curve is because they are, for the most part, putting objects into orbit and for that to happen thye have to have a high enough horizontal velocity. So rockets follow what is called a "gravity turn". They slowly level the trajectory so that wehn they reach orbital altitude they have the proper orbital velocity.

 

Gravity turns in reverse are used to land on bodies like the Moon (which has no atmosphere to speak of. The craft starts in an orbital trajectory and then turns more and more vertical as it descends, finally landing upright.

[UncleAl]

The Starflight Handbook, Eugene Mallove and Gregory Matloff. There's no way to go anywhere interesting in reasonable time unless somebody rewrites physics from the ground up. Don't get all jazzed about matter-antimatter propulsion, either - half the energy of hadron-antihadron annihalation boils off as useless neutrinos.

 

Space travel is momentum, mv. Space travel exhaust is energy, (mv^2)/2. The most efficient combination is not very useful. If you put a nuclear reactor on board for energy you pay a steep penalty in mass for shielding, even if it only protects the forward vessel.

[CraigD]

I think that is how they do it in the book, with a fictional power source. But lighting works by positive and negative ions interacting, right?

...

What do you think?

The physics of electric motors are pretty simple:

 

Force = Flux density * Current * Length of conductor

 

We worked out an example of what would be needed to make a small “flying car” – which I’m guessing resembles the crafts you’re reading about in Gradisil - a few years ago in the thread 7874, finding that it’d require a fairly easily makeable million meters of fine wire, but a prohibitively large current of about 320 amps through it. Though nobody explicitly worked out why this wouldn’t work with present-day zero-resistance superconducting wire (though I’m pretty sure it wouldn’t), we did show that for ordinary high-quality electric wire, the power requirements/waste heat would be staggering, on the order of billions of watts.

 

So, I think the idea of electrodynamicaly propelled able to fly from Earth’s surface to orbit is not unreasonable – not prohibited by the simple laws of physics - but still pure science fiction. To be made real, some unexpected breakthrough in the material science of super or very low resistance conductors would be needed.

I think when that was mentioned in the book the character meant that they try to make vehicles that need to go that fast, rather than thinking outside of the box and finding other means of getting into orbit without needing to go that fast.

It’s important to note that the flip side of the neat observation that escape speed = circular orbit speed x [imath]\sqrt2[/imath] is that a vehicle needs to go nearly as fast to achieve orbit as to escape it. Satellites move fast, while the surface of the Earth moves slow, so no matter how you do it, orbiting a vehicle involves making it go fast. A very efficient system could make this happen using a small amount of energy, however – for a perfectly mechanically efficient system, orbiting 1 kg in low earth orbit takes about 40 million Joules, in geostationary orbit, about 60 million, escaping Earth altogether, about 63 million. By comparison, it takes about 10 million Joules to operate a 120 watt lightbulb for 1 day.

[Theory5]

The physics of electric motors are pretty simple:

 

Force = Flux density * Current * Length of conductor

 

We worked out an example of what would be needed to make a small “flying car” – which I’m guessing resembles the crafts you’re reading about in Gradisil - a few years ago in the thread 7874, finding that it’d require a fairly easily makeable million meters of fine wire, but a prohibitively large current of about 320 amps through it. Though nobody explicitly worked out why this wouldn’t work with present-day zero-resistance superconducting wire (though I’m pretty sure it wouldn’t), we did show that for ordinary high-quality electric wire, the power requirements/waste heat would be staggering, on the order of billions of watts.

There is zero-resistance superconducting wire? I never knew that. WOuld you care to elaborate on why you think it wouldnt work?

 

The waste heat would probably not be a problem because the higher up you go the colder it gets... oh wait, the metal would probably warp or break or shatter wouldnt it? What about a different kind of material?

So, I think the idea of electrodynamicaly propelled able to fly from Earth’s surface to orbit is not unreasonable – not prohibited by the simple laws of physics - but still pure science fiction. To be made real, some unexpected breakthrough in the material science of super or very low resistance conductors would be needed.

 

It’s important to note that the flip side of the neat observation that escape speed = circular orbit speed x [imath]\sqrt2[/imath] is that a vehicle needs to go nearly as fast to achieve orbit as to escape it. Satellites move fast, while the surface of the Earth moves slow, so no matter how you do it, orbiting a vehicle involves making it go fast. A very efficient system could make this happen using a small amount of energy, however – for a perfectly mechanically efficient system, orbiting 1 kg in low earth orbit takes about 40 million Joules, in geostationary orbit, about 60 million, escaping Earth altogether, about 63 million. By comparison, it takes about 10 million Joules to operate a 120 watt lightbulb for 1 day.

What do you mean by perfectly mechanically efficent system? No resistance? No waste output?

[CraigD]

There is zero-resistance superconducting wire? I never knew that.

There is. It’s been available commercially for a couple of decades, and has been used in a variety of commercial and scientific applications, CERN’s new LHC perhaps the most famous, the Holbrook Superconductor Project, a Long Island, NY USA powerline, less well known. Anyone who’s had an MRI scan, has been close to superconductors, as MRI scanners require them to generate their strong magnetic fields.

 

Though superconductors have zero resistance, and are thus perfectly efficient at transmitting electricity, all current ones must be cooled, typically with liquid helium or nitrogen, or they lose their superconducting qualities. These refrigeration systems require power and/or expendable coolant.

WOuld you care to elaborate on why you think it wouldnt work?

External magnetic fields cause superconductors to have small non-zero resistance. Fields above a particular superconductors critical magnetic field strength causes them to stop superconducting altogether (an catastrophic event if they’re carrying a large current when this happens).

 

Because a current in a moving of curved conductor induces a magnetic field, there’s a limit to how much current a superconducting circuit can carry before the magnetic field it creates exceeds this critical value. It’s complicated beyond my ability to calculate, but I suspect that an arrangement like the flying car described in 7874, or the spacecraft in Gradisil, would exceed this limitation using any present-day superconductor. Whether a superconducting material that could overcome this limitation, or some clever conductor design, could overcome this problem without violating an inviolable law of physics, I don’t know – “is it in principle possible” questions can be very difficult material science questions.

 

(Source: wikipedia article "superconductor")

The waste heat would probably not be a problem because the higher up you go the colder it gets... oh wait, the metal would probably warp or break or shatter wouldnt it? What about a different kind of material?

Metal, even very thin pieces, aren’t especially fragile in space. Waste heat, however, is a big problem, as in near vacuum, there’s little to carry it away the way water and air can on Earth. Spacecraft must have radiator panels that glow in at least the infrared range, and the rate at which they can radiate – the cooling system’s power – is a function of the maximum temperature and surface area of the radiators.

 

Cooling is an often-overlooked design requirement in SF spacecraft. On real world, present day spacecraft, radiators are usually prominent features. In the ISS, they’re a pair of roughly 35 by 75 foot wing-like panels on the innermost (S1 and P1) trusses, and four 11 by 65 foot panel near the large solar panels on the outermost (P/S 3-4) trusses. On the Space Shuttle orbiter, they’re on the inside of the payload bay doors, which is why the shuttle must fly in space with its doors open. (sources: Heat Rejection Radiators | Lockheed Martin; wikipedia article “Space Shuttle orbiter”)

What do you mean by perfectly mechanically efficent system? No resistance? No waste output?

Yes - zero mechanical or atmospheric friction, less than 100% efficient motors, etc.

 

In practice, nothing can be perfectly efficient, but it’s still useful to calculate the work/energy requirements of hypothetical perfectly efficient systems, because it tells us what the absolute minimum requirements of any real system will be.

 

A super-efficient system, such as a space elevator, could in principle approach these minimum energy costs. Thus, assuming a 1% efficient system (super-efficient as spaceflight systems go) placing a 100 kg person in geostationary orbit would take about 6 billion Joules. At a typical electric utility cost of US$0.15 / kilowatt hour, then, this would cost $250.