# Tag Info

9

Does it have three unequal principal moments of inertia... Does the ISS have three distinct principal moments of inertia? The answer is definitely "yes". Any object has a mass matrix, $M$. In three dimensions, this is a $3x3$ symmetric matrix. Because $M$ is symmetric, such a matrix can be diagonalised, so that $P^{-1} \cdot M \cdot P = \bar{M}$ is a ...

7

I'm interpreting this question as "Did adjustments have to be made to the shuttle Digital Autopilot (DAP) while it was controlling the mated stack (shuttle + ISS) - i.e. was a maneuver tried, the results of the trial evaluated, and the DAP settings changed based on the results of the trial during the same shuttle mission?" If that interpretation is correct, ...

6

And what is the big black rectangular "bay window"? That is one of Hubble's radial instrument bays. It currently holds the Wide Field Camera 3. This DOUG rendering shows the camera "popped" out of the bay. why do the two lower ones point nearly in the same direction? As this answer says, the three orifices below the "bay window" are the openings for ...

5

It boils down to: how much spacecraft resource is required by the attitude control method you propose using? And sometimes the mission's pointing requirements play a significant role. Thrusters use propellant. Reaction wheels (and momentum wheels) use electric power. Spin stabilization uses neither, as long as you don't need to repoint the spacecraft — but ...

5

The three "fixed head star trackers" don't point in the "nearly the same direction", they're nearly* orthogonal. They provide attitude detection to around 60 arcseconds to point the observatory in pretty much the right direction. As to why this arrangement is used, it provides sufficient accuracy while working well for the spacecraft design. (I suspect the ...

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 ...

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 ...

3

Short answer: you stop nutation the way you stop any oscillation, by giving it an appropriate opposite impulse as it crosses zero, the oscillations central, zero energy point. Longer answer: The terminology in this area can be messy. So let's start with a simple model. The classic motions of a spinning top or (Navy) gyroscope are "spin", "precession", and "...

2

M-Cubed is (was?) a cubesat build at the University of Michigan that sort-of qualifies: The satellite uses a small permanent magnet and orthogonal mu-metal dampening strips as passive attitude control to slowly orient its Z axis to the Earth's polar magnetic field lines. (It carries no other orienting hardware) Why just "sort-of"? The Michigan ...

2

There are potentially many forms of oscillation, all of which would require some form of damping. Any offset from straight is a higher energy state, and the system won't die down without some damping. There is however already a natural form of damping. The extension and contracting of the tether will damp some of that energy. Its worth noting that that won't ...

2

The cases you've mentioned are applied to three-axis attitude control systems. In the past (and oddly even today), magnetic torquers were used to replenish the angular momentum of spin-stabilized spacecraft. The simplest example that comes to mind is SCD 1 and 2: The ACS is responsible to stabilize and control the spacecraft orientation with respect to ...

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

The language here tends to get in the way a bit. Generally “reaction wheels” are used for pointing: you turn them, then stop turning them once the satellite is pointing as desired. They’re not meant to spin rapidly to soak up a bunch of angular momentum. If you just need to control pointing, a reaction wheel system isn’t going to be able to absorb the ...

2

Not a real answer, but some thoughts that are too long for a comment I doubt there was a concern about optimizing the directions for easier calculations. Positions are fixed and known, so all equations can be "hand-crafted" to allow for rather quick but still precise calculations in the on-board computers. The orientation of the 4 reaction wheels seems ...

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

Okay, so I've asked around and I received an answer: Yes, $K$ has no general formula. I do not have the exact steps, but it goes as follow: the local demagnetizing factor must be integrated with respect to the volume. This factor depends on the position of each element of the core reported to the winding, therefore there is no simple analytical answer. For ...

1

The ELFIN cubesats use magnetotorquers for attitude control.

1

To give a recent reference: According to this paper quoted below, a set of 4 GPS receivers connected to indepenent antennas can be used to determine attitude and: "To the authors’ knowledge, GAP also represents the first practical demonstration of dual-frequency-based attitude determination in space.[...] attitude solutions with 0.1–0.3° precision can be ...

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