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In this answer to Why does JWST have such a big Blind Spot? I mention that it moves as a rigid body; in order to change the direction the telescope is pointing the whole spacecraft slews, including the sunshade.

At 20:25 in the Launch Pad Astronomy video 4 Future Space Telescopes NASA wants to build linked below the narrator says of the proposed Large Ultraviolet Optical Infrared Surveyor telescope or LUVOIR:

Both architectures keep the sunshade facing the Sun while the telescope is free to point in any direction on the leeward side.

It shows a rock-solid sunshade connected to an articulating and likely heavy telescope.

Conservation laws dictate that there must be some other mass of some kind moving somewhere that's not shown in the video, or some more exotic way of exchanging angular momentum.

How do they do it?

How can the proposed LUVOIR space telescope slew to different directions while keeping the sunshade in a fixed orientation? What compensates?

Question: How can the proposed LUVOIR space telescope slew to different directions while keeping the sunshade in a fixed orientation? What compensates?

video cued at 20:25

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  • $\begingroup$ @ uhoh Changing the conformation of LUVIOR does not change its angular momentum (still zero) so thruster use or net change in reaction wheel momentum is not needed. Consider the spacecraft to be made of two components (telescope and sunshade modules), hinged at a pivot. Each component has its own rotational moment of inertia around the combined center of mass. $\endgroup$
    – Woody
    Dec 29, 2021 at 17:35
  • $\begingroup$ @ uhoh If the conformation is changed, the position of each component will rotate (in opposite directions) in inverse proportion to its moment of inertia. The sunshade will now be out of position, but this can be corrected by the reaction wheel system acting on the entire spacecraft in its new conformation. $\endgroup$
    – Woody
    Dec 29, 2021 at 17:35
  • $\begingroup$ @ uhoh In practice, conformational change and reaction wheel actions would be performed at the same time. Conformational change to assist in pointing the telescope requires less reaction wheel effect than pointing the entire spacecraft (as for JWST). $\endgroup$
    – Woody
    Dec 29, 2021 at 17:35
  • $\begingroup$ @Woody I'm asking how it actually does this, not how it might do it. Have you calculated how big of a reaction wheel would be necessary to do this? Since there is friction, there will be regular required angular momentum unloading, if not thrusters how will that be done? $\endgroup$
    – uhoh
    Dec 29, 2021 at 22:11
  • $\begingroup$ Conformation change does not change the resting angular momentum so it puts no additional load on the reaction wheels. In fact, conformation change reduces needed transient reaction wheel capacity. If both the telescope component and the sunshield component have the same moment of inertia, the reaction system could be designed with half the capacity. Further explanation will require graphics (a formal "answer" or email). I have found no design details of the mount system beyond the NASA video you linked. But the design details don't change the physics. $\endgroup$
    – Woody
    Dec 29, 2021 at 22:34

2 Answers 2

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Question: How can the proposed LUVOIR space telescope slew to different directions while keeping the sunshade in a fixed orientation? What compensates?

Also:

I mention that it moves as a rigid body; in order to change the direction the telescope is pointing the whole spacecraft slews, including the sunshade.

In layman terms i think you are asking what is the big difference between JWST and LUVOIR, in terms of how each slews to observe its target.

As you say, JWST moves as a rigid body, ie. as one. LUVOIR does not.

LUVOIR is composed of two elements - the spacecraft and the payload:

payload is the telescope and the part that needs to be vibration free and it is this that slews around.

spacecraft is the support structure that houses avionics, fuel, sun shield, and the 4 CMG's, and everything else that generates vibration as well as providing attitude control of the entire system. The payload element determines the spacecrafts attitude.

enter image description here

The key element of this concept is VIPPS, which allows it to have a disturbance free payload (telescope):

The highest possible degree of isolation of a sensitive payload structure from spacecraft disturbances is realized through no physical contact between the two bodies.

Lockheed Martin Space Disturbance Free Payload technology, in development since 1999, has resulted in the basis for the non-contact Vibration Isolation and Precision Pointing System (VIPPS) for LUVOIR.

VIPPS uses voice coil actuators, which do not contain any moving mechanical parts, where an axial force is generated between a permanent-magnet field assembly (mounted on the telescope payload side of the VIPPS interface) and a coil-wound bobbin (mounted on the spacecraft side of the VIPPS interface).

Non-contact sensors at the VIPPS interface provide a real-time measurement of the interface relative translation and rotation; this measurement is used in the VIPPS control system to maintain stroke and gap at the interface.

Technology readiness level 6, demonstrating this technology, is proposed for a CubeSat launched sometime before 2025.

Disturbance Free Payload allows payload and spacecraft to fly in close proximity without physical contact, using custom-designed, large-gap non-contact actuators.

A DFP-configured system is actually two spacecraft flying in close formation.

dfptestTRL4 This was patented in 2002.

To control its attitude, the telescope pushes against the supporting spacecraft using a set of six noncontact linear-motion, electro-mechanical Lorentz force actuators.

Attitude of the telescope is determined using a Fine Guidance Sensor or other LOS sensor on the payload, and the error signal garnered from six non-contact position sensors is used to drive reaction wheels and thrusters on the supporting spacecraft.

The Vibration Isolation and Precision Pointing System (VIPPS) enables the telescope to achieve extreme pointing and image stability while still meeting the line-of-sight agility requirements consistent with its astronomical Surveyor goals.

The spacecraft controls its inertial attitude such that interface stroke and gap are maintained. Since the telescope is physically separated, the disturbances and structural excitation of the spacecraft and sunshield do not propagate to the telescope.

enter image description here

enter image description here

enter image description here

In these examples, the lateral position of the telescope center-of-gravity remains constant. This constraint requires both ends of the boom be articulated as illustrated above. The vertical distance between the observatory and spacecraft centers-of-gravity do change because the VIIPS boom is of fixed length.

For large-angle slewing, the 4 CMGs located on the supporting spacecraft body are used.

The mechanical interface between the telescope and the supporting spacecraft houses 6 vcm and associated sensors located between the gimbal and backplane support frame.

The payload section itself has vcms and associated sensors for the telescope itself.

http://surveygizmoresponseuploads.s3.amazonaws.com/fileuploads/623127/5043187/119-806ddbfd3b26fd17af7803ce18b5cf5f_NordtAlisonA.pdf

http://www.mrbolcar.com/uploads/1/0/6/7/106798055/103980b.pdf

https://www.hou.usra.edu/meetings/landscape2019/presentations/Nordt.pdf

enter image description here

The gimbal and boom can be seen on the back here: enter image description here

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11115/2528190/Dynamic-wavefront-error-and-line-of-sight-performance-predictions-for/10.1117/12.2528190.short

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    $\begingroup$ So to tilt, the telescope pushes against the bus, and the bus "absorbs" the rotation in its CMGs so that the bus+sunshield maintain attitude? And this is done with linear (non-contact) voice coil actuators? Sounds like an engineering work of art, love it! $\endgroup$
    – uhoh
    Jan 9, 2022 at 22:10
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Both LUVOIR-A and LUVOIR-B designs are intended to be supported at the end of an articulated arm as shown schematically in the figures below. enter image description here

enter image description here

By locating the upper pivot point close to the center of mass of the telescope reduces momentum issues. Details on the PAS (Payload Articulation System) can be found starting on page 8-50 of the LUVOIR Final Report (1).

1: https://asd.gsfc.nasa.gov/luvoir/reports/

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