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79 votes
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Why were the Space Shuttle's main engines placed on the orbiter?

The quick answer--so they could get the engines back--has already been provided, but I'd like to offer some more historical context to that design decision. Note that the shuttle as originally ...
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70 votes
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Why wasn't an RTG used on the Juno spacecraft?

That is precisely it. Plutonium-238, which is used in the creation of radioisotope thermoelectric generators (RTGs) is very difficult to come by. There are plenty of news articles on this, from ...
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69 votes
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How (the heck) did Lunokhod 2 drive, navigate and survive a ~40 kilometer drive over four months on the Moon using 1970's technology?

A few interesting videos about the lunokhod rovers: Tank on the Moon The Lunokhod Rovers - It Happened in Space #7 Secret Soviet Lunar Rovers and Extra Terrestrial Cars - Lunokhod, Mars Rovers The ...
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67 votes

Why can't they just drop a solar winch down from a shuttle and have planes fly up and clip things on?

Because space isn't about going high; it's about going fast! For example, in a 400 km orbit (like ISS) you need a speed of about 27,500 km/h or 7.66 km per second. So if you would extend a ...
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60 votes
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Is the mass of paint relevant in rocket design?

When the shuttle External Tank stopped being painted white, the weight savings was ~600 lbs (~270 kg). This is not a tremendous amount from a vehicle standpoint, but the tank was carried almost to ...
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55 votes
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Falcon 9 design: Why is the cargo fairing so much wider than the lower parts?

The diameter of the stages is the largest size that can be transported by road without extensive "outsize load" issues (permits, having to move traffic lights and signs out of the way etc.). This ...
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55 votes

Why aren't rockets built with truss structures inside their fuel & oxidizer tanks to increase structural strength?

There's almost nothing to be gained by a truss. The load being applied is along the axis of the tank. A simple hoop of material is very strong in this orientation. (Try it with a piece of paper, ...
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52 votes
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Why do the Russians use these fence-like interstage fairings?

It's all to do with ullage in the fuel tanks. Newton's laws of motion mean that when a rocket is no longer firing and no force is being applied, the rocket receives no acceleration. It continues at ...
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50 votes
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Why were Space Shuttle astronauts able to walk off the orbiter?

Look at the STS-122 video. How many astronauts do you see? I see six. Seven astronauts landed with STS-122. The six you see were the crew of STS-122 who spent twelve days in space. They could walk ...
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46 votes

Why were the Space Shuttle's main engines placed on the orbiter?

So that the reusable engines could be reused. If they were mounted on the expendable external tank, they would have been thrown away each mission.
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44 votes
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Why does the Falcon 9 have 9 engines?

SpaceX initially was looking to buy an engine but could not find one on the market that would allow them to meet their goals. Once they decided to develop their own, they had to consider their goals: ...
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44 votes
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Why are Rocket engines at the base of the rocket?

..what reason is there for having the engine at the bottom, .. For the fundamental logical flaw in the desire to move the engines, please see this answer of Russell Borogove - it comes down to The ...
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44 votes

Why are Rocket engines at the base of the rocket?

Putting the rocket nozzles nearer the top wouldn't make the rocket any more stable; this is the well-known pendulum rocket fallacy. In fact, some rockets have used a tractor (engine on top) ...
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44 votes
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When did the concept of "stages" enter rocketry?

The first multistage rocket is much older then one might think. Its from the 14th century CE. Huolongchushui or fire dragon issuing from the water (Chinese: 火龙出水; pinyin: huolóngchushui; literally: ...
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39 votes
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Why did SpaceX retire Falcon 1?

The Falcon 1 was less profitable to maintain, and it didn't have the customer base to support using it. A Falcon 1 launch cost around $10 million, of which about 10% was profit. They also considered a ...
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37 votes

Why wasn't an RTG used on the Juno spacecraft?

Another interesting note is that this mission more than any other mission to the outer solar system can use solar power. Why? Juno is in a polar orbit, and will continually be in the sun. Solar panels ...
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35 votes
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Can you really use Arduino for a small spacecraft?

Can you have a spacecraft based on an Arduino? Sure you can! ArduSat was two kickstarter funded cubesats that were eventually launched from the International Space Station in November 2013. When you ...
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35 votes
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Do spacecraft ever release unneeded gases into space?

You bet. Not only gases, but astronaut pee as well! Which could result in spectacular light shows. This happens for several reasons: Spacecraft and the tanks inside them are built to be as light as ...
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34 votes
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Why are rockets cylindrical?

Rockets are cylindrical for the same reason maize silos are cylindrical: A circle has the largest area vs perimeter of any shape and also provides maximum strength from internal pressure. This means ...
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34 votes

Is it common and good engineering for a pair of cables to be easily plugged into each other's connectors in modern spacecraft

Off the top of my head I can think of several failures caused by miscabling. On Apollo 6 the signal to shutdown a malfunctioning second stage engine was cabled to a different engine, resulting in two ...
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33 votes
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Why does the ISS cupola have Interior Protective Sliding Windows

That's a "bump shield" used for filming the IMAX movie A Beautiful Planet. According to IMAX, their desire to film from aboard the Cupola forced them to design an "exclusive bump shield made of a ...
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33 votes
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How is thrust transmitted from strap on boosters to the central core?

It’s a sense of scale issue. As much as the struts might look like flimsy bits of drainpipe, those rockets are around 15 meters wide, and the struts are more like the heavy steel beams used to hold up ...
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32 votes

Why aren't rockets built with truss structures inside their fuel & oxidizer tanks to increase structural strength?

Because they don't need to be. Clearly the current design of rockets can be successful. So adding truss structures to the current design would add weight for no reason and take away from the payload ...
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32 votes
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Why was the Space Shuttle Orbiter's in-orbit time limited?

I found the answer in the EGIL Console Handbook (not online). This was the flight control position responsible for the Orbiter's electrical system. The Station to Shuttle Power Transfer System (SSPTS)...
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31 votes
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What is the "emergency crush core"?

The landing legs have a honey-comb filler that can compress to absorb significant energy of a hot landing. (Source: @SpaceX)
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30 votes
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Why is Falcon 9's shape so tall and skinny?

SpaceX manufactures their booster in Hawthorne, CA. They then truck it on the highways to McGregor, TX for test firing with all 9 engines. Then it is back on a truck for the drive to Florida to LC-40 ...
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29 votes
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What is the most aerodynamic Satellite?

The only satellite I know of that was shaped to have low drag was GOCE, which orbited at 250 km. Since it was vital to ensure that the measurements taken are of true gravity and not influenced by any ...
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28 votes
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Why weren't Saturn V and the Soviet N-1 Moon rockets made larger in order to simplify Lunar missions?

In order to use the direct ascent method of landing on the moon, which is where the entire vehicle descends and leaves the moon, you would need a rocket an order of magnitude bigger than the Saturn V, ...
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28 votes

Has SpaceX chosen to drop the Transpiration cooling of Starship? If so, why?

Despite SpaceX's high-tech reputation, SpaceX aren't where they are today because they made massive technological leaps or pushed the envelope of science--after all, just doing science doesn't earn ...
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27 votes

Why are RTGs different colors?

The big difference between the two darker RTG fins (Black and Grey) and the white RTG fins, is that the white fins were destined for use in an atmosphere (Mars). The presence of an atmosphere, even ...
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