# Tag Info

25

"[T]here seems to be no mention for the need to manually slow down the reaction wheel speed and instead merely applying an external torque has the effect of reducing the wheel speed. This confuses me because I thought that the wheel speed only depended on the current, which is something we supply and hence control. How does the application of an external ...

16

Most spin-stabilized rockets are solid-fueled, so slosh-free. Solid fueled stages also tend to have heavier structure than liquid-fueled ones, because the propellant container needs to contain combustion pressure, so they are less bendy than a liquid stage of similar volume would be. The proportions of spin-stabilized solid upper stages tend to be fairly ...

12

Neither fins nor Dracos, a dedicated cold-gas system. Source: https://space.stackexchange.com/a/23654 Confirmed by the Falcon 9 User's Guide (April 2020 version) In addition, the second stage contains a cold nitrogen gas (GN2) attitude control system (ACS) for pointing and roll control. (emphasis mine)

8

First, ignore what Wikipedia claims about attitude indicators. It shows a diagram of the inside of an airplane attitude indicator. I am not inlining this image because -- although some airplanes may use such an indicator -- it is completely wrong about spacecraft attitude indicators. Let's look at what is displayed on a spacecraft attitude indicator. The ...

7

A single axis rotation (SAR) was performed from the initial to final attitude, unless they were avoiding gimbal lock, in which case it was split into two rotations. It has been shown in MIT/IL Report E-1832 that it is convenient to perform attitude maneuvers by simultaneous maneuvers in three axes. However, under certain ...

6

You are describing the Local Vertical Local Horizontal (LVLH) frame of reference. It is used to describe the orientation of the spacecraft in relation to the Earth's surface. For example, if you wished to point an instrument at the point on the Earth directly below the spacecraft, the craft would fly in a constant LVLH attitude. But its inertial attitude ...

6

During first stage both the SSME engines and SRB nozzles gimbaled to provide thrust vector control (TVC); the SRBs provided most of the control authority due to their long moment arm and high thrust. For all of your sub-questions, the TVC would move the thrust vector slightly to rotate the vehicle in the desired direction. Nose up pitch: nozzles deflect ...

6

There are 3 possibilities: SpaceX deliberately rolled it It rolled naturally, and SpaceX did not expect / want this It rolled naturally, and SpaceX did expect / want this. Looking at the video, the roll seems to intensify without any use of the rcs thrusters, so it seems likely the roll occured naturally. In this case, the torque that induced the roll ...

6

There is no 'contradiction', all are valid attitudes.1 The one you choose depends on what you want to use it for. The most commonly used of these frames in Mission Operations (for robotics, EVA, etc.) during the shuttle era was ISSACS (the second one pictured in the question). LVLH is used for matters involving the Earth (observations, etc). 1 the first ...

6

Reaction wheels are used to control attitude of the spacecraft and they work due to conservation of momentum. Let's say we have a reaction wheel on the x-axis of the space craft, aligned with the principle axis of inertia, then the total moment of inertia is $h = h_s + h_w$, where $h_s$ is the moment of inertia of the spacecraft (excluding the wheel) and $... 5 The order in which rotations are applied matters. 260 roll around REFSMMAT x followed by 90 pitch around REFSMMAT y is the same orientation as 90 pitch around REFSMMAT y followed by 260 yaw around REFSMMAT z. 5 As you rightly point out in your question, Canopus star trackers are pretty nifty instruments. I don't think it's really possible to do them justice in a single post, so I'll include a few summarizing points here and will include links to other articles, etc. for more information. 1. How do they work? NASA Technical Report 32-1559 does an excellent job of ... 4 The lux value of the Sun is ok. The albedo of Earth is not constant, it depends on cloud cover and the location above oceans or land. Dragongeek is right, lux is defined by using the standard sensitivity of the human eye. Radiance as watts per square meter does not depend on the human eye's sensitivity for different wavelength. The radiance above the ... 4 First, you will not be able to detumble without at least one sensor. For CubeSats the normal way to detumble is through the B-dot algorithm which measures the magnetic field change and turns on appropriate magnetorquers to slow down the motion. Since you have to measure the magnetic field to begin with, you've already established a rough attitude in two-... 4 This paper is a sizing exercise, made before the thrusters in question had been flown on any spacecraft. The moment of inertia can be backed out from the numbers in the article. Given the bang-bang control scheme used in the article, the relation between angular acceleration$\dot \omega$, firing time$t$, and angular change$\theta$is$\$\frac12\dot\omega\...

3

Note that Explorer 1's spin stabilization worked fine for getting it into orbit. Dissipation during the few minutes of thrust was not enough to cause trouble.

3

The propellants are gaseous methane and oxygen propellants, they provide their own pressure just as the nitrogen used in the cold gas thrusters does. No diaphragms, bellows, or bladders involved. A thruster of the same size will give around 5x the thrust for the same propellant flow rate as the nitrogen thrusters they'll start with, so there's really no need ...

3

This is how it worked for the Space Shuttle, it was fairly autonomous. To perform insertion, orbit, and deorbit maneuvers, there must be accurate data for the orbiter's location and velocity with respect to the Earth for use by guidance and control. This is the task of the shuttle navigation software. The navigation software maintains an accurate estimate ...

3

This is not a high level answer but I think I understand what it is that you are asking, and if I'm right it doesn't need much to clear up. My understanding is that I can't just turn off or reduce the speed of the wheel because the momentum would merely transfer to the spacecraft. We could counter this torque... if we apply a counter torque using another ...

3

Think of a large telescope mounted on the surface of the Earth. To see a certain star, you need to rotate the telescope around the vertical axis (azimuth) and to lift it above the horizon (altitude). If both angles are correct, you see that star in the center of the telescope. So if the star tracker locates a certain star, two angels of the satellites ...

2

Knudsen pumps are not used in space exploration because they do not function at the altitudes required by nearly all satellites, and do not function at all when completely outside an atmosphere. As for why not, it's hard to parse the intended conclusion from the summary beyond "it may be possible at certain altitudes", but consider also some remarks about ...

2

I did some rough calculation. E(Engine Force). = 800 N m(Mass of reaction wheel) = 10 kg r(radius of reaction wheel) = 0.2 m w(max spin rate of wheel) = 6000rpm * 2pi / 60s cgo(C.G offset) = 0.01m I = m * r * r / 2 = 0.2 kgm^2 WheelCapacity = I * w = 125.6 Nms Torque experienced during engine firing due ...

2

A dynamically unstable spinner can be operated with Active Nutation Damping. This "simply" means using a control loop to conduct small manoeuvres to reduce precession. The direction of the thrust is in the body spin-axis and is at a finite radius from the spin vector (the further out the more effect per pulse). Obviously it needs quite precise ...

2

With modern computers and software models of spacecraft, there is no reason not to take the "shortest path" approach when adjusting a spacecraft's attitude outside of certain edge scenarios. This is because, for a computer, adjusting all three axis simultaneously and handling the potentially complex interplay/second order effects resulting from ...

1

The order in which the rotations is performed always important in an Euler rotation sequence. You can show this to yourself by picking up a book and applying a roll / pitch sequence versus a pitch / roll sequence. This is very unlike translation, where first going 1 km north and then 1 km east brings you to the same point as does first going 1 km east and ...

1

This needs to be assessed on a case-by-case basis, with simulations, and with all hardware and algorithms properly modeled. But to give an ideia of what is concerned and what is usual: Image blur on star trackers is a thing indeed, you may expect a star tracker to stop providing measurements while thrusting occurs. Some satellites such as geostationary ...

1

There are a few different ways, but basically they are fixed sensors in a linear array. There are usually two of them, pointed roughly 90 degrees apart. These sensors will have some pointed certainly at space, others pointed at Earth, and one or more pixels at the horizon. The brightness of the pixel on the edge, as well as its location, allows one to tell ...

1

Ok so to answer my own question (turns out I just needed to do a little more reading up): The reference plane for the Azimuth is based on True North with its 0 degree starting at the Meridian line and increasing clockwise 360 degrees until once again reaching the meridian. It is also possible to use True South to orientate, but then the Azimuth increases ...

1

Typical A, B problem answer, but: I think it would be better not to depend on a fixed hard coded value of a property of the earth. What you are interested in is the detected value. You could calculate and update this value as you go. For example, do a rough pass to calculate the relative position of the sun (vector add/integrate all light detected), then ...

1

No. No object can exert a torque on itself via only internal interactions. That would violate conservation of angular momentum. Spacecraft are constructed to have controlled, usually zero, magnetic fields to reduce torque from external fields they encounter.

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