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The main reason is heat rejection. NASA was asked this very question, and the answer was identified. Basically, the waste heat from the shuttle is expelled via the cargo bay doors. You don't want to ever point a radiator at the Sun, so the easiest thing is to point it at the Earth. Sometimes, if the heat was too high, they would actually point the shuttle ...


58

Yes, this is normally achieved using Reaction Wheels, they work by using conservation of angular momentum. This is also how cats right themselves mid-air while falling (Here is a great video explaining the basic physics). You could do the same thing in space by pinwheeling your arms to get yourself rotating and then stopping when you have turned sufficiently....


48

The other answers are great demonstrations of F9's capabilities, but I'll be the contrarian here and say they're all wrong and perhaps Elon oversimplified things for a tweet. This was a one engine landing burn. A single engine lacks the ability to control roll on its own, unless it has a vectoring turbopump exhaust. Merlin 1C had this feature for roll ...


36

Is any of the following true about what is needed to achieve this: continuous change in its rotation, once and for all giving it the right spin to begin with, it happens naturally. The answer is "yes" to all three questions. If a vehicle is shaped right and is given the right rotation to start with, torques that naturally occur such as ...


31

You can recreate the problem by placing a pencil point-first on your finger. Try to keep the pencil/rocket upright by moving your hand back and forth. If you managed it for more than a few seconds, congratulations! You're doing better than Proton 535-43 did. In the very early stages of flight (before aerodynamics has any major effect) the rocket can be ...


31

ISS orbits most of its time in what is called a Torque Equilibrium Attitude (TEA). Since gravitational acceleration varies depending on distance from the Earth, non-symmetric objects of any appreciable size have a net torque acting on them that is attitude-dependent, relative to the local vertical/local horizontal. For the most part, the ISS's attitude is ...


26

Most such spacecraft, including Skylab and the ISS, have their attitude maintained by reaction wheels. These wheel essentially convert the rotational energy of the entire spacecraft into a smaller reaction wheel. If the attitude failed while these wheels were spun up, then they would eventually slow down (In a timeframe of minutes). As the wheels slowed down,...


26

Rocket guidance systems generally use a fixed inertial platform based on gyroscopes to determine their orientation in space; an accelerometer solution would be useless to determine orientation (though helpful for position determination) as soon as the rocket was in motion. https://en.wikipedia.org/wiki/Inertial_platform Once operating, the platform's ...


25

Although this has indeed "worked to bits" on the Physics and other SE sites it's worth looking at, for the sake of Space Exploration, the interesting history behind the analysis of the falling cat. For the fully rigorous description of the cat's righting reflex - perfectly in keeping with conservation of angular momentum - only came about because it was ...


25

The first stage of the Soviet N-1 moon rocket (Block A) used this type of differential thrust system. It had 30 engines in 2 rings. The outer ring of 24 engines used differential thrust control to control pitch and yaw, and was set up to shut off opposing engines in case of a single engine failure. Four launches were attempted and all failed in the first ...


23

This video published on YouTube on Zero-G: "Movement in Microgravity: Skylab to Space Shuttle" 1988 NASA Weightlessness Footage, starting at 2:10 into it, shows a Skylab astronaut doing a front roll and a spiral roll in the Skylab Orbital Workshop without touching anything to push against to change his orientation. And the same video from 5:45 to 6:00 shows ...


22

Sputnik-1, and Sputnik-2, had no attitude control whatsoever, and would have tumbled freely. "[Like] its predecessor, Sputnik-2 would have no attitude control system." http://www.russianspaceweb.com/sputnik2_decision.html Sputnik technical specifications and diagram: http://www.russianspaceweb.com/sputnik_design.html


20

What are quaternions and how are they used in spacecraft dynamics? Background Newtonian mechanics says we live in a universe with three spatial dimensions, and a universal time that is the independent variable, in which we can describe translation and rotation. Relativity theory says we live in a universe with three space-like dimensions and one time-like ...


20

The attitude thrusters and TCMs are mechanically identical, all Aerojet MR-103s. From the Voyager Press Kit: The 16 thrusters on the mission module each deliver 0.89 N (0.2-lb.) thrust. Four are used to execute trajectory correction maneuvers; the others in two redundant six-thruster branches, to stabilize the spacecraft on its three axes. Only one branch ...


19

This video may help to answer your question. Starting at about 00:24, you can see an astronaut running around the "exercise wheel" of Skylab (an early NASA space station program, which followed the Apollo moon landings). Basically after some time, NASA told the astronauts to stop running around like this because it was causing more propellants to be used to ...


19

Background and Physics Note that there are actually two different but related types of actuators that use conservation of angular momentum1 to control a spacecraft's attitude (both of which may be lumped into "reaction wheel" by KSP): Reaction wheels (RWs, a.k.a momentum wheels) spin along a fixed axis at a variable speed. They change angular momentum by ...


18

Yes, one reason is that it is flown upside down (with the top facing towards Earth) to use the heat shield tiles to shield the astronauts from the sun, but I think a bigger reason is to shield from space debris. One reason is to protect against space debris. Here on Earth, most of the space junk (small rocks and ice, etc.) that falls towards the planet ...


18

First, let's clear the part about how far away Kwangmyongsong 4 (KMS-4, NORAD ID 41332) is. It was inserted into a 472.6 km × 508.5 km near-polar sun-synchronous orbit with inclination of 97.5°. It is not geosynchronous, so its orbit takes it over all the longitudes of the globe, and latitudes of 80.5° North to 80.5° South. That means that roughly once per ...


18

I think, image being worth a thousand words, just the picture how three engines can affect spin of a rocket explains it best: Falcon 9 engines are gimballed, and the landing was to use three. Unfortunately they are ignited only a short moment before landing, so they couldn't be used to maneuver the booster to the landing pad.


17

For the record, there's actually a third possibility, Magnetorquers. Here's why you would use one vs the other. Magnetorquers - Inexpensive, low maintenance, but don't work in all situations. Used by LEO spacecraft typically, and small in size. They work by pushing off of a magnetic field. Reaction Wheels (Or Gyros) - The trick here is that you can't take ...


17

The main issue is one of size. Each engine bell requires space and with 42 that is a lot of space. In order to gimbal there needs to be room for the bell to actually move, which means you need to space the engines further apart. But just allowing the inner ring to gimbal they are the only ones who need the extra space, which you can see in the image cited ...


17

The Ares 1-X used stock Shuttle program SRB thrust vector control (TVC) - hydrazine fueled power units drove hydraulic pumps which powered actuators that could tilt and rock the nozzle, which incorporated a flexible bearing in its design. (Pictures from "Space Shuttle", Jenkins, 1992 edition p.263 and here - p 2.13-48 of linked PDF) However, unlike the ...


16

Edit: Yes, momentum of the tape recorder (DTR) was an issue. However, I don't think it was an issue that was overlooked in the design phase. The Voyager team knew it would become an issue for the Uranus and Neptune flybys, and had a fix in place before the Uranus flyby. I've found no indication of a "last-minute fix". In 1973, NASA published a report "...


16

No, Sputnik did not have any such system. It would have added complexity and power requirements. The Soviets were trying to beat the Americans in to space, and succeeded by the launch of Sputnik, but they had little time to design it well. There were basically 3 systems in the spacecraft, radio, power, and thermal management. The power was mostly 3 ...


15

The other answers here are correct: gimbaling or other active correction measures are used. While most launchers do try and maintain a vertical flight off the pad, the Antares rocket is known for the intentional "Baumgartner Maneuver" it does on takeoff, deliberately gimbaling the engine to maneuver away from the tower in the first seconds of flight, as ...


14

The best way to keep an antenna always pointed at Earth, if you can manage it, is to stick a large weight at the tip of your antenna. The weight will receive more pull, and naturally keep the antenna pointed at that direction. Short of having something like that to help passively, the next best solution is to spin stabilize. By spinning around an axis, you ...


14

I can answer the "average" part. Here's a table from my usual go-to source for openly available Shuttle details, the Shuttle Crew Operations Manual, page 1009. In this table, "PRCS" refers to the large Primary Reaction System jets, and "YRCS" is a typo for the smaller Vernier Reaction System jets. "NORM" and "LOW" are different settings on the Digital ...


14

The most recent Falcon9 landing burns have used three engines (more engines makes for a more fuel-efficient landing, although it also requires a lot more precision). The three are in a straight line, with a center one and two outer ones that can be independently vectored. That allows them to provide torque around pitch, yaw and roll axes. As CRS-16 slowed ...


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