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27 votes
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How come we don’t put a huge windmill-like machine in space?

As far as I understand you want the "windmill" to drive a dynamo. Have you ever tried turning a dynamo? It takes some force to do so, and that force is then (partially) turned into electricity. So the ...
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25 votes
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Does it take more energy to get to Venus or to Mars?

To flyby or impact Venus varies from 3.45 to 3.6 km/s from LEO for the optimal time every 19 months. Mars varies from 3.55 to 3.9 km/s for the optimal time every 26 months. So on average, getting to ...
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22 votes
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How much of the energy of a rocket ends up as a noise?

The maximum percentage wasted to noise is approximately 1%. The acoustic efficiency, defined as the ratio of the sound power to the rocket exhaust's mechanical power, for the majority of these data ...
19 votes
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Splitting water quickly to turn a water rocket into a H/O rocket at launch: could it work?

You'll need a nuclear reactor for that because the process of dissociation of water into constituent hydrogen and oxygen consumes vast amounts of energy and for an ascent stage you'll also need a very ...
  • 75.7k
16 votes
Accepted

How do you slow down a solar sail?

If you're asking about interstellar travel, then the answer is pretty simple; For diffuse, unfocused sources of light like the one emitted by stars, photon flux density decreases with the inverse ...
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16 votes
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Do any realistic rocket systems use different substances for propellant and fuel?

To beam the power to the rocket, especially when needing high acceleration, is not what I will expect to show up in the near future. However, separating the energy source from the propellant is the ...
15 votes
Accepted

Would it be possible to generate electricty in space with a satellite in orbit

No, it wouldn't work. In order to drive the treadmill, momentum would have to be transferred from the satellites to the treadmill. The sats would slow down and eventually stop. The "highway" itself ...
15 votes

Why are Ion Thrusters so energy hungry?

A conventional thruster with two liquid propellants requires energy too. But it is chemical energy stored in the propellants. Ion thrusters use no chemical energy at all, all the energy of the ion ...
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15 votes

Icy Hot Astronauts

Common wisdom says that space is cold. I wouldn't go so far as to call that a misconception, but it's certainly a bit of a misunderstood fact. See, space is not cold in the way your fridge or a block ...
14 votes
Accepted

How efficient would it be for an outpost to use human-powered vehicles?

Human powered vehicles used within domed or cavern cities would seem to be extremely plausible. One way to look at it is a simple bicycle is less complex than a powered vehicle. The power source (aka ...
  • 4,173
14 votes

Does it take more energy to get to Venus or to Mars?

The second table here essentially answers your question. Venus transfer from Low Earth Orbit is 3.5 km/s, Mars transfer is 3.6. This will allow you to impact either body (on Venus you will need to ...
13 votes

Never reducing propellant for space propulsion

NO As a rule of thumb: if you think you invented either a way to generate infinite energy or infinite propellant: you are wrong. But [...] No. Always. Your craft will gain a velocity ...
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13 votes
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Why are RTGs so inefficient?

Here's a brief answer since some people are trying to close the question and prevent answers: As @ikrase points out answers to the Physics SE question Why is the Peltier / Seebeck Effect's efficiency ...
  • 148k
12 votes
Accepted

Could the Hubble mirror be used to create a laser to propel solar sails?

The answer is a clear no because Hubble Space Telescope (HST) uses a fixed focal length of 57.6 m. The only thing you'd achieve by re-purposing it as a solar concentrator would be to melt its focal ...
  • 75.7k
12 votes

Could thermal energy be collected by dragging a thermoconductive device against the outer atmosphere

No: The heat produced by atmospheric reentry isn't a happy side effect of returning to the earth, it's a byproduct of the fact that your satellite/orbiter has enough kinetic energy to be circling the ...
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12 votes
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Solar panels on Mars?

Well, we don't really know how much energy a Mars base would need, but we can make some rough estimates. Bases like McMurdo and Mawson have power capacities of several hundred to several thousand ...
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12 votes
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Why are Ion Thrusters so energy hungry?

The thing to notice about this type of drive is that the ions encounter the positively charged accelerating grid first. The ions that provide the thrust to the rocket are positively charged as well, ...
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12 votes
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What is the distribution of ∆V amongst different stages of rocket flight?

Considering fuel consumption or energy expenditure may be misleading, because of the huge change in mass over the flight as fuel is expended. 2/3 of the fuel is expended by the first stage, which only ...
12 votes

How come we don’t put a huge windmill-like machine in space?

Consider the windmill as a system. If there is no wind blowing on the windmill, there is no energy being input into the system. If you pull power out of the windmill, energy is being output from the ...
11 votes
Accepted

Could steam be used as a efficient power source for a space craft?

Very simple answer: You would need some kind of heating panels through which to let the water flow while it gets heated. Solar panels are thin slices of silicon. You will not be able to build the ...
10 votes

Could steam be used as a efficient power source for a space craft?

Very simple answer. You are looking at the wrong metric for efficiency. The most expensive thing on a space station is not area, you don't pay per foot of "land" (and its associated sunlight) in ...
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10 votes

Is wind-power a viable backup power source for Mars One?

It's not that wind is not viable, it's just that it's less viable than other sources of energy. With such a small part of Earth atmosphere pressure, even the high wind speeds it's insufficient. ...
  • 52.9k
10 votes

Dissipating large amounts of energy fast, in space

Don't convert it into heat in the first place, and then don't dissipate it fast. MSL used an electromagnetic brake to slow the descent of the rover from the skycrane. It dissipated the energy into a ...
  • 57.9k
10 votes

In the absolute vacuum of Space, will an electric dynamo still work?

Unless the lubrication system fails in the dynamo that is in vacuum, electrical generation will work. Magnetic fields are unaffected by air or the lack of, at least at the level under consideration. ...
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9 votes

Why are Ion Thrusters so energy hungry?

There is a sense in which the wonderfully high ISP of ion thrusters is the cause of their energy hunger, independently of the detail of their operation. It works like this: we need a certain amount ...
9 votes
Accepted

What is the relation between the rocket equation and work, energy and power?

If you're in vacuum, the power represented by the kinetic energy of the thrust is the thrust force times the exhaust velocity (g times Isp) over two: $$P={g I_{sp}T\over 2}$$ One SSME in vacuum ...
  • 57.9k
9 votes

How much of the energy of a rocket ends up as a noise?

Of the the 0.2% to 1% of energy that is generated as sound, most of the sound is generated well after the exhaust gases have fulfilled their purpose of thrust: https://en.m.wikipedia.org/wiki/...
8 votes

Is wind-power a viable backup power source for Mars One?

Yes, to an extent. A few key facts: While the atmosphere has 1% of the pressure of Earth's, it consists primarily of carbon dioxide (MW = 44) much more than nitrogen (MW = 28). Thus the effective ...
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8 votes
Accepted

How far from the Sun can solar power be used as a reliable energy source?

Presumably this is because solar power isn't feasible at large distances from the Sun. There is a possibility to use solar energy as long as the arrays receive a quantity of energy greater than the ...
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