Aircraft guidance question: Effect of mounting sensors at the CG vs on the Tail

[cessna1525]

Hi,

Can I someone please explain to me, the moment behaviors of having something fixed on CG vs on Tail in a rocket? are there any benefits of having a Sensor on CG than fixed on the tail?

Thank you

 

[Baluncore]

Welcome to PF.

Is the something, a linear accelerometer and rotational gyroscope, used for inertial navigation?

The CG will move as fuel is consumed. How will you move the something?

The tail will be where the data is needed to balance, steer, and stabilise the rocket.

 

[cessna1525]

Hi,
Thank you very much for the quick reply, actually. the case here is about an airborne Magnetometer bird, just wanted to know if there is any benefit of having the Mag sensor on CG instead of on the Tail section, The argument I heard from my colleague is Having on the CG is better because having it on the Tail section will cause amplified moments due to the arm, does this make sense? , I thought it doesnt matter like that

 

[berkeman]

the case here is about an airborne Magnetometer bird

Like this?

https://www.bgr.bund.de/EN/Themen/G...omagnetometrie/aeromagnetometrie_node_en.html

 

[cessna1525]

Yes, exactly, only difference is we are hanging it on one point, just above the CG,

 

[berkeman]

Yes, exactly, only difference is we are hanging it on one point, just above the CG,

How do you keep the sensor body horizontal if you only support it at the CG?

 

[cessna1525]

It is balanced on the CG , and CG is Infront of the Center of pressure, and it stabilized as it speeds up, usually 15 m/s forward speed.

 

[Baluncore]

are there any benefits of having a Sensor on CG than fixed on the tail?

The orientation of the magnetometer will vary in the same way, independent of its position on the bird.

However, the distance of the magnetometer from the tow vehicle, and from the ground, will vary with the changing orientation of the bird, but will be minimised if the instrument is close to the centre of mass of the bird.

 

[cessna1525]

Thank you so much for the input, is there any subject material on this which I could Study more?
what is the theory behind

 

[Baluncore]

what is the theory behind

When you stir a cup of tea, at every point in time, there is at least one point that is fixed in position. Likewise, when a magnetometer bird moves through the air, the centre of mass is probably near a virtual "fixed point" on the bird.
https://en.wikipedia.org/wiki/Brouwer_fixed-point_theorem

You have GPS, 3D accelerometers, 3D rate gyros, and a 3D magnetometer. Everything is fixed to the frame of the bird, but offset in different positions.

The orientation of the bird comes from the 3D rate gyros.
The long term track is determined by GPS. The short term track comes from the 3D accelerometers, corrected for orientation. The long and short term tracks are combined in a Kalman filter.
https://en.wikipedia.org/wiki/Kalman_filter

The position of the magnetometer is computed from the spatial track.
The orientation of the magnetic field is corrected for bird orientation.

The orientation of the bird is computed with quaternions to avoid gimbal-lock in the calculations.
https://en.wikipedia.org/wiki/Quaternion
https://en.wikipedia.org/wiki/Gimbal_lock

Transducers.
https://www.analog.com/en/technical...l-ways-to-assemble-mems-inertial-sensors.html

 

[sophiecentaur]

It is balanced on the CG , and CG is Infront of the Center of pressure, and it stabilized as it speeds up, usually 15 m/s forward speed.

Choosing the place to put it could give a least worst susceptibility to error but would it not be necessary to apply corrections all the time?
In addition, won't yaw and the vertical axis of the centre of lateral pressure also be relevant? Centres of pressure will probably move about, in any case, as the device is pulled in different directions. I'd imagine that damping and stability would also be relevant - more than just choosing the placing wrt the centre of pressure.