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The Large UV Optical Infrared Surveyor (LUVOIR) is NASA's next planned flagship telescope following the James Webb Space Telescope, and a direct successor to Hubble covering the 0.1 μm (far ultraviolet) to 5 μm (mid-infrared) wavelengths. It may launch with either an 8-meter mirror or a larger 15-meter mirror. The latter would have required the Space Launch System (SLS) Block 2's large payload fairing, but now SpaceX's Big "Falcon" Rocket (BFR) is in contention:
Conceptualized to follow in the footsteps of NASA's current space telescope expertise and (hopefully) to learn from the many various mistakes made by their contractors, the LUVOIR (shorthand for Large UV/Optical/IR Surveyor) concept is currently grouped into two different categories, A and B. A is a full-scale, uncompromised telescope with an unfathomably vast 15-meter primary mirror and a sunshade with an area anywhere from 5000 to 20000 square meters (1-4 acres). B is a comparatively watered-down take on the broadband surveyor telescope, with a much smaller 8-meter primary mirror, likely accompanied by a similarly reduced sunshade (and price tag, presumably).
[...] The reason LUVOIR's conceptual design was split into two sizes is specifically tied to the question of launch, with LUVOIR B's 8m size cap dictated by the ~5 meter-diameter payload fairings prevalent and readily available in today's launch industry.
LUVOIR A's 15-meter mirror, however, would require an equally massive payload fairing. At least at the start, LUVOIR A was conceptualized with NASA's Space Launch System (SLS) Block 2 as the launch vehicle, a similarly conceptual vehicle baselined with a truly massive 8.4 or 10-meter diameter payload fairing, much larger than anything flown to this day. However, the utterly unimpressive schedule performance of the SLS Block 1 development – let alone Block 1B or 2 – has undoubtedly sown more than a little doubt over the expectation of its availability for launching LUVOIR and other huge spacecraft. As a result, NASA has reportedly funded the exploration of alternative launch vehicles for the A version of LUVOIR – SpaceX's Cargo BFR variant, in this case.
While only a maximum of 9 meters in diameter, the baselined cargo spaceship's (BFS Cargo) payload bay has been estimated to have a usable volume of approximately 1500 cubic meters, comparing favorably to SLS' 8.4 and 10-meter fairings with ~1000 to ~1700 cubic meters.
(Score: 0) by Anonymous Coward on Wednesday July 11 2018, @11:54PM (1 child)
My optical bench isn't massive and tied to the earth. It's a metal plate for random attachments backed by a composite honeycomb which is both nicely stiff and damped. It sits on air bags to isolate it from unpredictable building vibrations. Admittedly it's light by concrete and steel standards, but probably massive by flight standards.
https://www.thorlabs.com/navigation.cfm?guide_id=41 [thorlabs.com]
With regards to do I have a better idea.
Given that it's my tax money being spent, let me give it a try and you tell me.
Here's a theory for a dynamics control architecture.
First, lightweight structures can be very stiff relative to their mass, but I'm not sure stiffness is your friend because stiff raises the resonance of the structure.
Over short periods of time, mass is the natural way to keep things in place.
Stiffness works against this and so it would make any active control servo have to work faster.
On the ground, I can't use this trick because I don't know the forces from external things like the AC air handler and folks walking around.
In an unmanned space vehicle, I should be in control of everything that moves.
So the servo system should be able to predict and choose what to do with the servos.
(This seems a fundamental advantage for an instrument in space?)
The actual heirarchy I have in mind is
A predictable but somewhat springy backbone
Separate servo systems connecting each critical component to the backbone (mirrors, sensors, relay lenses, and measurement?)
Perhaps also some servos connecting various parts of the backbone together.
Measurement reflector stations on each critical component
A common optical position measurement system looking at the stations
A servo loop watching the measurements and controlling the servos at a loop bandwidth faster than unpredictabilities in the loose, springy backbone can mess things up.
I can see 2 things which are necessary for success.
Getting a good complexity balance between the structure design and the control loop. Today's availability of crazy amounts of CPU cycles help this, but there might be a temptation to go too far in this direction. (Obviously, I think the design you propose is too far in the other direction.)
Apriori knowledge that some things are built right on the ground with known error bands. IE the relationship between each critical component and it's measurement stations. Perhaps go so far as grinding the stations into the blank of each mirror and use them in the mirror surface grinding..
Given sub wavelength relative position sensors for the mirrors and sensors, and suitable servos in the right place, it seems like it should be possible to make a big gadget that self aligns in space.
(Score: 0) by Anonymous Coward on Thursday July 12 2018, @01:17AM
Still quite confident in my ignorance, let's talk about those servos.
Since old school space has torquers, for fun lets call them forcers.
A critical component like a mirror would have three between it and the vehicle backbone.
When not operating, they capture the component to the vehicle backbone.
But when operating, if you really believe that mass is your friend to keep things still, then a forcer should only apply force.
That means after releasing capture and there should be no contact between the component and vehicle.
In other words, when operating, this makes the instrument a set of random objects free floating in space with servo system forces to keep them friends.
Once said, why would you want anything else?
How to make such a forcer?
Perhaps a few permanent magnets on the component side and on the vehicle backbone side a local servo positioning coils to good positions to apply force.
How far these servos can move is the compliance range of the forcer.
The assemble in space part and vehicle dynamics only have to be good enough to stay in the compliance range.
If you have good control of the magnetic gaps, you can use induction to send power to the component.
A short free space optical path can provide data.
(Not sure how to handle cooling.)
You can't try these on the ground, but ISS could.
Seems like a great way to embrace space to make a truly great instrument.
If the compliance range is sufficient, it might work on a manned vehicle like ISS.
A telescope with free floating pieces needs a great name, that part and actually making it work is yours.
Definitely not your grandfather's telescope.