Just a silly theory to illustrate my question. If we were to build a globe (or a detached runway, could be any structure really as long as it goes around earth and meets on both ends) around the earth, which itself travels at 17500 miles per hour. From there we launch space shuttles, does this mean they could go into orbit at a reduced speed and cost? As they have a starting speed of 17500 miles per hour. And does the height of the globe (in relation to earth) matter?
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$\begingroup$ Lower force? Why do you want to lower the force? $\endgroup$– Mark AdlerFeb 5, 2017 at 18:10
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2$\begingroup$ There's no such thing as a free lunch in physics $\endgroup$– GdDFeb 6, 2017 at 8:51
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$\begingroup$ @GdD: But sometimes you can swipe someone's lunch ;) Like in case of gravity assists. $\endgroup$– SF.Feb 6, 2017 at 12:27
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3 Answers
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A structure like that would reduce the amount of energy needed to get to orbit. But:
- getting a structure up to 17,500 mph would take vast amounts of energy.
- transporting the spacecraft from Earth to that structure still means you have to accelerate the spacecraft to 17,500 mph. So you're not saving any energy.
- a ring around the Earth would consume a massive amount of resources, making it impractical to build.
The most practical implementation of what you're describing is a space elevator: a vertical structure attached to Earth. If you climb this structure to 36,000 km up, your speed will be orbital speed. You can step off the elevator and be in orbit.
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$\begingroup$ But if you want a lower orbit or an inclination, a space elevator could do only a small part of the job. For all that mass that is necessary to transport in a geostationary orbit at first in order to build a space elevator, you could launch a very large number of heavy satellites using the same amount of propellants. A space elevator could not be built like a tower from base to top, it must be built from orbit down to the earth surface. But all material needed in the orbit must be lifted there using conventional rockets. $\endgroup$– UweFeb 8, 2017 at 19:51
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Your basic question seems to be whether we can build megastructures that use passive momentum and other orbital characteristics to make entering orbit simpler. We can, but the specific example isn't a very good one: it uses far too much material and has an unclear method of use.
A better example is the classic space elevator. This uses the principle that required orbital speed gets lower and lower at higher altitudes, along with the increase of speed with a fixed angular velocity, to produce an upward force at certain altitudes that matches the downward pull of gravity at lower ones. This way, a single continuous cable's weight in its lower half is supported by its "negative weight" in its upper half. And, in principle, one can simply attach a load to the cable and crank it up with solar power, beamed power from earth, or something similar, and when it gets high enough it's automatically orbiting. Unfortunately, there are many very serious problems with the basic science required to make a space elevator.
It's also possible to make skyhooks, which function somewhat similarly and have most of the same challenges, but can be made to orbit at lower altitudes and higher speeds, trading off the ease of connecting for a much cheaper infrastructure. They can also be made to counter-rotate to allow more time and less energy to connect. Skyhooks all need some way to balance the momentum lost to accelerating their payloads; this can either be decelerated masses reentering in the opposite way, or high-efficiency, low-thrust ion engines or the like, which are too low-powered to be of any use in conventional rocket launch.
Finally, there's the launch loop, which takes the basic premise that something needs to be moving faster than orbital speed and uses magnetic levitation to separate it from its container, which is the only part that needs to be attached to the planet and directly support payloads. Specifically, the launch loop idea is a very long loop of vacuum tubing magnetically guiding and supported by a chain of iron cylinders moving considerably faster than normal orbital speed. The two sides of the loop would be parallel for most of the length, which would be on the order of 3000 km, and use tethers from the ground to stay at an altitude of 100 km or so (low enough to avoid most debris, but high enough to ignore ram heating even at orbital speeds). Payloads would use magnetic induction to transfer momentum from the circulating chain to themselves as they moved along the chosen main track, and the chain would be re-accelerated in the end loops to compensate for loss of speed, and radiate the extra heat from induction all along its length. The launch loop idea has several considerable advantages:
- it's practical with current materials and knowledge
- it can largely be built on earth, then lofted just by accelerating the chain to design speeds, rather than needing rockets
However, it is undeniably still very expensive (tens of billions of dollars, very likely) and somewhat challenging to place and secure (presumably somewhere in the ocean, on the equator).
answered Feb 5, 2017 at 22:31
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To reach a given orbit, anything starting out on the surface (presumably starting at zero speed relative to the surface) has to be accelerated to the same final speed, regardless of method used. Whether it has to accelerate itself (like rockets do), or hitches a ride on something already moving that fast, that acceleration has to happen somehow. Some methods (like rockets) are most efficient when they apply a hefty kick to get the payload above the atmosphere and up to orbital velocity quickly. Whatever the method, a generally bad thing is to be traveling at or near orbital velocity within any substantial atmosphere. For one thing, it can be inefficient because of the drag produced - wasting energy. The other is heat; all that energy lost in atmospheric drag becomes heat which has to go somewhere, like into the skin of the vehicle.
Space elevators side-step some of the problems. A geostationary orbit still requires acceleration, but to a lower final speed, though it requires the input of more total energy than does a low Earth orbit. The elevator adds this energy mostly by lifting the payload up, and imparts the required acceleration by keeping the payload aligned with the cable. Most of the required orbital acceleration takes place along the portion of the cable which is above the atmosphere (which is most of it).
Rail guns on the other hand, face the atmospheric drag problem head-on. The payload is accelerated to extreme speed and as soon as it leaves the gun, becomes ballistic, except that it encounters atmospheric drag, immediately slowing it down.
Any other method would have to either deal with hypersonic drag, lift the payload out of the atmosphere before accelerating, or simultaneously accelerate the payload and raise it up out of the atmosphere while keeping it enclosed in a vacuum.
