The Stack Overflow podcast is back! Listen to an interview with our new CEO.

# Tag Info

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 ...

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.

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 ...

10

There is enough hydrazine to last beyond the end of the mission, about 25% of total tank volume is still available. From Descanso volume 4, you can see enough hydrazine for attitude control is available to last until 2040/2048: As in all communications around the end of life for Voyager operations, this lifetime estimate considers the mission to end when ...

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

The User-Guide for the Falcon 9 says that pitch, yaw and roll can be controlled by the first stage's gimballed engines, see Table 2.1 on page 11. Why rockets use gimballing? Any rocket that is built to land has to have full thrust vectoring, since the grid fins or any other aerodynamic control elements only work when the rocket is moving through the air ...

7

It can be seen in ground camera footage that only one engine was burning throughout the landing. One engine cannot control roll in any meaningful way, though there might be some subtle second-order effect I don't know of which the gimbal rotation can use to apply a torque to the system: A large proportion of the spin seems to vanish when the landing legs ...

7

This paper states The reactive momentum of rotor rotation influences the control of trajectory. Therefore this parameter should be accounted for during the rocket engine layout determination. ....... For the small rockets that do not have active stabilization controls,the value of gyroscopic momentum has a ...

6

tl;dr During ISS assembly the station maneuvered more than it does these days. You are correct that the docked Shuttle / ISS stack maneuvered during the STS-115 mission. The planned attitudes for the last several Shuttle missions are available in the Attitude Timeline sections of their flightplans, published at the JSC FDF page. The STS-115 flight plan is ...

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

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 ...

5

If multiple accelerometers are spread around the vehicle, their readings can be combined to determine angular speed (from centripetal acceleration) and angular acceleration somewhat easily. There would probably need to be at least 4 or 5 to cover all the degrees of freedom, with one at the CG to cancel out linear acceleration. To calculate orientation from ...

5

This is called a magnetotorquer A magnetorquer or magnetic torquer (also known as torque rod) is a satellite system for attitude control, detumbling, and stabilization built from electromagnetic coils. The magnetorquer creates a magnetic dipole that interfaces with an ambient magnetic field, usually Earth's, so that the counter-forces produced provide ...

5

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 ...

4

This answer and the excellent article Gravity Gradient Stabilization of Earth Satellites explain that you need to use a damper of some kind, where friction or other lossy processes slowly absorb rotational energy, like a lossy pendulum slowing down. One caveat is that in the the final state, the spacecraft is actually rotating once every time it revolves ...

4

Forward is toward the US segment, +X in the ISS coordinate system. Aft is toward the Russian segment, -X in the ISS coordinate system. Starboard is toward the AMS, +Y in the ISS coordinate system. Port is toward the Cupola, -Y in the ISS coordinate system. Down or Deck is toward the Nadir, +Z in the ISS coordinate system. Up or Overhead is toward the ...

4

For passive magnetic stabilization in CubeSats two things are required. First a permanent magnet must be used to align the spacecraft with a magnetic field. Second, a soft magnetic material must be used to damp out rotational energy. This becomes quite a bit more complicated than it initially looks though because you have to worry about various perturbation ...

3

To create the oscillation around the Z axis, they'd have to run around once. The oscillation in the X axis would be created with each footfall. Skylab was about 1000 times as massive as an astronaut. Estimating the astronauts centre of mass to be at about three quarters of the circumferance, and the distribution of Skylab's mass to be skewed to around ...

3

There is no UEKF. There is UKF. Yes, it performs better than EKF/MEKF. And yes it's computationally expensive for that matter. And yes, people are looking into SPUKF/ESPUKF to reduce that computational overhead. But again, follow the golden rules: "If it ain't broke, don't fix it". Typically these filters alone are an illogical point of metrics from a ...

3

For some small sats, the unloading is done automatically by the attitude control system when the wheel gets to a particular speed. So that would depend on how much wheel momentum you accumulate over time, and that would depend on the ratio of the moment of inertia of the spacecraft compared to the moment of inertia of the wheel. Say you've got your ...

3

There is a 3 way relationship between "Where am I?", "What can I see?", and "What time is it?". From a navigation point the "What can I see" is known, the problem was knowing the time, which involved talking to earth. But when you have the time, you can deduce where you are. Having the clock on board means you don't need to do the talk-to-earth step to ...

3

It depends a bit on what technology you’re referring to. The original inertial navigation systems used rotating gyroscopes. Those were and are expensive. Modern MEMS inertial navigation systems (example) don’t use rotating gyroscopes. Instead, they get both linear and angular acceleration (and angular rate) information from their MEMS accelerometer ...

3

The explanation by Mefitico is along the right lines. The trick here is not to assume $u=0$. The thruster configuration can be chosen such that the kernal has all positive components. Then $u$ is selected to guarentee a positive solution, see SanchezPena,R.S.and Alonso,R.andAnigstein,P.A.: Robust Optimal Solution to the Attitude/Force Control Problem, IEEE ...

3

According to Marcel Sidi's book you can indeed control attitude in three axis with 4 thrusters, each of them oriented in the spaceccraft frame along a direction $v_i$, and supplying a torque of $\tau_i$. The total torque applied on the spacecraft is given then by:  T = \begin{bmatrix} v_1^T\\ v_2^T\\ v_3^T\\ v_4^T\\ \end{bmatrix} . \begin{...

3

Astra is currently developing a small sat rocket with differential thrust, and they even "launched" it. Though it's not clear how far the launch procedure has gotten. Check out The part about differential throttling starts at about 5:00, but I recommend watching the whole thing. I know this question is about past ...

2

Reaction wheels are a borderline technology, one that in certain situations might require an export license. What I suspect is the case is that they will be able to sell you the reaction wheels, but will be unable to give you all of the technical data that might make your job easier. Still, I would contact the supplier, ultimately they will be responsible ...

2

If you have linear and rotational accelerometers you can easily tell. During quiescent periods (crew sleep) the structure of the vehicle doesn't fluctuate as drastically as when the crew is moving inside and bouncing off structure. Thus, by looking at vehicle rates, ADCO can easily figure out if the crew is awake or asleep.

2

There are a couple of great open-source tools available for that including NASA's Trick library. I did an internship at NASA where Trick was heavily utilized for GN&C software-in-the-loop and hardware-in-the-loop scenarios. There is also Cesium for 3D visualization of your model and physics.

2

All the motion can be returned to the cylinder or rocket but only linear momentum can be transferred from small to large objects; this means that all the motion (linear momentum) of the cylinder is given to the spheres. A convenient place to release the weight is when the tether is at 90° to a tangent line to the circle of the satellite. A tangent line ...

1

The direction cosine matrix relating the orbit frame and an inertial frame is as followed. Essentially it is a 3-1-3 body-two rotation sequence. I talk about the DCM a little more in detail here (Calculate Argument of periapsis of orbit given focus and two points on ellipse). You will need to know all your orbital parameters in order to calculate this DCM ...

Only top voted, non community-wiki answers of a minimum length are eligible