Starship Asterisk*

HEAPOW: Ground-breaking Mirror in Space (2019 Sep 30)

The many scientific discoveries made by the Chandra X-ray Observatory since its launch more than 20 years ago would not have been possible without its advanced X-ray optical system, the High Resolution Mirror Assembly. The High Resolution Mirror Assembly, or HRMA (prounced "her-ma") is the most advanced X-ray mirror system that has ever been built or flown on a spacecraft. X-ray imaging is much more difficult than imaging visible light because the energy of even the lowest energy X-rays is hundreds of times larger than the energy of visible photons. Because of their high energy, the only way that X-ray photons can be reflected efficiently from a mirror is if the X-ray hits the mirror at a very shallow angle, effectively skipping off the mirror surface like a stone skips off the surface of a lake. X-ray mirror systems like HRMA are thus shaped like a large barrel, with additional concentric smaller barrels nested within. The HRMA consists of four pairs of these concentric, barrel-shaped mirrors so that X-rays passing down the axis of the barrel can be deflected and brought to a focus some ten meters behind the HRMA at the exquisite imaging detectors that are part of the Science Instrument Module. Chandra's X-ray mirrors can create the most beautiful X-ray images possible because they are exceptionally large and exceptionally smooth. Chandra's mirrors were first shaped to an extreme precision, then were polished so that there are no bumps in the mirrors larger than a few atoms - as if the earth was smoothed so that the tallest mountain was no more than a few feet high. Chandra's mirrors are also excepionally clean: at launch there was no more than a single speck of dust on an area as large as the typical computer screen. The mirrors making up the HRMA are the most accurate, smoothest and cleanest ever made, and because of this, Chandra produces for us the sharpest and clearest images of the X-ray Universe.

https://en.wikipedia.org/wiki/Stone_skipping#Scientific_explanation wrote:

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<<An [18th century] explanation of the physics of stone-skipping (formerly attributed to the elasticity of water) was provided by Lazzaro Spallanzani.

The stone generates lift in the same manner as a flying disc, by pushing water down as it moves across the water at an angle. Surface tension has very little to do with it. (The stone's rotation acts to stabilize it against the torque of lift being applied to the back.)

Research undertaken by a team led by French physicist Lydéric Bocquet discovered that an angle of about 20° between the stone and the water's surface is optimal. Bocquet and his colleagues were surprised to discover that changes in speed and rotation did not change this fact, it just allowed the stone to be in balance and to continue with a straight and uniform movement, due to gyroscopic effect. Work by Hewitt, Balmforth and McElwaine has shown that if the horizontal speed can be maintained skipping can continue indefinitely. Earlier research reported by Bocquet calculated that the world record of 38 rebounds set by Coleman-McGhee, unchallenged for many years, required a speed of 12 m/s, with a rotation of 14 revolutions per second.>>

JohnD wrote: ↑Sat May 22, 2021 9:37 am
Every British schoolboy is, or used to be, familiar with the story of the Dam Busters, 617 Squadron of WW2, that used Prof.Barnes Wallace[sic]'s work to 'skip' bombs over torpedo nets and onto the inner wall of several dams, catastrophically breaching them. The "Bouncing Bombs" were cylinders and needed to be spun in a retrograde direction to their motion over the water.

So a flat disc is not the only shape that can be 'skipped'.

https://en.wikipedia.org/wiki/Bouncing_bomb wrote:

Prof. Barnes Wallace & friend

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<<A distinctive feature of the [bouncing bomb], added in the course of development, was back-spin, which improved the height and stability of its flight and its ability to bounce, and helped the weapon to remain in contact with, or at least close proximity to, its target on arrival. Back-spin is a normal feature in the flight of golf balls, owing to the manner in which they are struck by the club, and it is perhaps for this reason that all forms of the weapon which were developed were known generically as "Golf mines", and some of the spherical prototypes featured dimples.

It was decided in November 1942 to devise a larger version of Wallis's weapon for use against dams, and a smaller one for use against ships: these were code-named "Upkeep" and "Highball" respectively. Though each version derived from what was originally envisaged as a spherical bomb, early prototypes for both Upkeep and Highball consisted of a cylindrical bomb within a spherical casing. Development, testing and use of Upkeep and Highball were to be undertaken simultaneously, since it was important to retain the element of surprise: if one were to be used against a target independently, it was feared that German defences for similar targets would be strengthened, rendering the other useless. However, Upkeep was developed against a deadline, since its maximum effectiveness depended on target dams being as full as possible from seasonal rainfall, and the latest date for this was set at 26 May 1943. In the event, as this date approached, Highball remained in development, whereas development of Upkeep had completed, and the decision was taken to deploy Upkeep independently.>>