# Tag Info

110

With a rocket you have to carry the fuel with you. You are not just propelling the mass of the payload, but also the mass of the fuel. Installing a space elevator is a one-time event that can then be used to propel payloads indefinitely. You no longer have to carry the fuel to get to orbit.

65

In addition to not requiring fuel: A rocket has to accelerate to orbital speed. This takes a lot of energy. A space elevator can climb at a low, constant vertical speed (albeit for a very long climb), and gets its orbital speed almost for free, from Earth's rotation (see Tom Spilker's answer for far more detail on this). Because a rocket accelerates to ...

61

Here's a simple reason: Most of the rocket's fuel is used just to push the rest of fuel! It sounds strange for those unfamiliar with Rocket equation. The reality is, if we want to accelerate by exhausting something behind us - then we have a problem when the speed we need to reach (8 km/s orbital speed) is greater than exhaust speed (3-5 km/s). In this ...

38

It boils down to efficiencies of energy conversion and the cost of the technologies doing the conversions. If you have a given mass at Earth's surface that you want in geostationary orbit, you have to raise it to the geostationary radius (or altitude, if you prefer to think in those terms), and you have to accelerate it to geostationary orbit velocity. Both ...

22

Ultimately an elevator is going to be more efficient, because it doesn't have to deal with gravity losses. Let me pose a question to you. What does it take for a rocket to hover in place like Blue Origin's New Shepard? If you've watched any of their launches you know they don't shut off the engine completely, but keep them running the whole time while ...

8

Immediately before the definition section, Wikipedia references Kármán's abstract concept from his autobiography: In the final chapter of his autobiography Kármán addresses the issue of the edge of outer space: "Where space begins… can actually be determined by the speed of the space vehicle and its altitude above the earth. Consider, for instance, the ...

7

As @Evan Steinbrenner points out in his answer, a stopped space elevator needs use no energy to resist gravity. A hovering rocket must burn an enormous amount of energy just to resist gravity. The climber only requires enough energy to move up the cable, essentially slightly over 1g acceleration, and can provide that at it's leisure. A rocket must expend ...

7

Looks like the idea was tested before (https://www.nasa.gov/centers/kennedy/news/rotocapsule.html ):"The design would give a capsule the stability and control of a helicopter, but would not be powered. Instead, the wind passing over the rotors as the capsule descends would make the blades turn, a process called auto-rotation that has been proven repeatedly ...

6

Based on your initial stipulations, and the wording provided by Wikipedia, the altitude Karman was calculating was the altitude where, at orbital velocity, the lifting effect of aerodynamic forces on an aerospace frame is just sufficient to hold it aloft against gravity. Ergo, a body with sufficient lift to stay aloft at any velocity below orbital speed ...

6

For a Grid Fin, what would be the most optimal way of finding the lift to drag ratio? Because Navier-Stokes computations are involved you'll probably want to use a computer with computation fluid dynamics (CFD) software. I am thinking about comparing lift to drag ratios of square lattice Grid Fins but with different geometrical parameters (such as ...

6

As far as I know, no. In order to make a cylindrical rocket as light as possible, they are flown to minimize the side loads to the structure -- as close to a zero angle of attack as possible. If they wanted to use lift, it would increase the mass of the structure to be able to take substantial drag forces from the side. There would not be sufficient benefit ...

6

Why not? Because the people who started using the Karman line didn't see the need for a more refined definition (e.g. because nobody was going to attempt aerodynamic flight in this region). The Karman line is an approximation anyway. It depends on the lift coefficient and the state of the atmosphere, both of which are variables. Karman's calculations didn'...

6

I think there are a few misconceptions to clarify here: Rotating bodies can generate lift. This is known as the Magnus effect. Lift is a hydrodynamical phenomenon: Differences in flow velocity above and below a moving body translate into pressure differences which heave the body up. However in very rarified gases this mechanism stops working. This is ...

6

No, it would float lower if anything. To see this think about the forces on the balloon: the acceleration due to gravity is $g$ and I assume this is constant (the planet is large, the balloon isn't getting very far up: this is a good assumption); the density of the gas inside the balloon is $\rho_H$, the density of the atmosphere is $\rho_A$. If the '...

6

High-altitude ballooning is kind of a gray area as far as space exploration goes, because they can't leave the atmosphere, but they do go high enough to experience space-like conditions (e.g. the pressure is blood-boiling low, and it's darn hot on the sunlit side and cold on the shaded side). Balloon experiments measure things that you might normally ...

5

To add to the answers above, you can also retrieve energy by sending payloads back down the elevator.

5

You said it yourself, Lift is important for guidance and control. As a matter of fact a rocket is designed in such a way, that the center of pressure is aft of the center of gravity. The distance between CG and CP is also called the caliber stability margin measured in rocket caliber. You can use the fins of a rocket to control the direction and magnitude ...

5

Angling to get lift is going to increase the atmospheric cross-section of the rocket and so increase drag. For any reasonable angle of attack, the drag force is going to be much larger than the lift force, so I believe that for powered ascent it makes the most sense to minimize drag, which means zero AoA and zero lift. This also, as Mark Adler notes, ...

5

If you want to minimize heating, you need to spend time at high altitudes (>100 km) gradually losing speed. This means you need wings to provide lift. So for now I'm going to ignore the heating issue and just look at what kind of wing you'd need. This question arrives at a wing loading of 20 kg/m2 for an aircraft that can fly at 100 km altitude at ~8 km/s. ...

5

I think I solved it... I'm not sure but I think I'm on the right path. You were very close to the right solution, and you were very correct the linked equation is useless in describing actual trajectory of a "Karman Plane" - it only works for flat Earth :) Karman's Line is an abstraction that has very little physical meaning in the real world - it's ...

5

It would move Down! By the definition of the Karman line on wikipedia, the lift force and the "centrifugal force" must be equal to the gravitational force and, therefore, each other. This gives the following equation: $\frac{1}{2}\rho v^2C_LS = \frac{v^2m}{R_e+h}$ Where $\rho$ is density, v is velocity, $C_L$ is lift coefficient, S is wing area, m is ...

5

This is an interesting question that got me thinking — and calculating, during a much-needed break from designing a rotating space station! In short, you probably could fly a kite at Mars! Probably not the old stick-and-paper kites of 50-100 years ago (low L/D and high $\beta$; see below), but maybe a parafoil-style one. If you're math-challenged, skip down ...

4

Nothing can be perfectly inflexible without causing undue stress on the hold downs. Similar to using a longer crowbar to gain leverage making any wind load transfer directly to the hold-down clamps, etc. The motion displayed in the image is likely a light breeze as the clamping mechanism lets go and wouldn't affect the launch vehicle itself. The Falcon 9 ...

4

Quite alot of the answers mention that rockets need to carry the weight of the fuel along with the usual payload, and that requires more energy to lift up. This is correct, but there is also another important thing to consider: rocket engines are thermodynamic engines, and are limited in efficiency by the Second Law of Thermodynamics. Even the most ideal ...

4

The biggest problem with this question is the scale of that diagram, to give you an idea: The earth is 63.7x the height of the atmosphere in radius. This means that leaving the earth horizontally will encounter much more atmosphere than shown. A plotting of a circular model of the earth with atmosphere/without using equations shows this: Wolfaram Alpha ...

4

Some issues with this: Drag is dependent on cross-sectional area, and so is solar radiation pressure. This would mean that different spacecraft would have a different definition of space. We might get around this by designing a "standard spacecraft", but that's exactly what defining the Karman line was for in the first place. The exosphere is complicated ...

4

Your flotation scales with the cubic of radius while surface area and weight scales with the square of radius. In other words, you need a big balloon. Surprisingly, we have become incredibly good at building arbitrarily big balloons. Notably, relatively recently, NASA Big 60 (NASA announcement here). the scientific balloon reached a peak altitude of 161,000 ...

3

There's no such thing as centrifugal force in this case. The only real forces in this problem that are vertical (normal to the local surface of the Earth) are gravity and lift. "Centrifugal force" is a fictitious force which people sometimes invoke to solve problems more quickly in in certain cases. If you write a set of equations or write a program to ...

3

Let's look at the kinetic and gravitational potential energy of a satellite sitting on the launchpad, versus in geostationary orbit. It should be intuitively obvious it has more energy in orbit, so if we can calculate the total change in energy we can establish at absolute minimum bound on the energy required to get to orbit, regardless of what method we use....

3

Another factor that's being overlooked: Rockets are extremely high energy machines. Many compromises must be made in order to get the energy density needed to make a rocket reach orbit at all. Those compromises generally come at the cost of efficiency. (Off the top of my head--LH2/LOX rockets run their engines quite rich because they actually get more ...

Only top voted, non community-wiki answers of a minimum length are eligible