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T-19 minutes to LOS (Loss Of Signal)
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SpaceX General Section / Re: Starlink : Speed Discussion
« Last post by M.E.T. on Today at 03:22 pm »
Also, ~50Mbps is pretty fast. I have that on my home fibre connection in a city, and its enough to support two TVís streaming Netflix at the same time as well as various mobile devices using the Wifi.

Thatís plenty for some guy relying on Starlink in his cabin up in the mountains, or cruising around in his RV.
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687 miles away from the lunar surface

LOS in 19 minutes.

Best views of the Moon during this mission.
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Historical Spaceflight / Re: MOL discussion
« Last post by LittleBird on Today at 03:16 pm »

I'm not expecting any deep insights into the "concerns and criticism around the MOL program," but I think there are some aspects of this that have not been uncovered yet. Many years ago I heard that a well-regarded American physicist and science/technology/intelligence advisor, Richard Garwin, had led or participated in a review of MOL that determined that the presence of astronauts would degrade the optics performance substantially. Only recently did I see a CIA document that mentioned that such a study had been performed. We don't have that study.

I vaguely remember being told that there had been two such studies--one in 1967 and another in 1968--that had cast doubt on MOL. I found this notable, because a lot of people involved in MOL said that they were totally surprised when it was canceled in summer 1969. But there had been some high-level criticism of it for several years, plus the delays and cost overruns.

I don't think I'd realised just how unkeen Garwin was on MOL !

These grabs seem the most indicative quotes I could find, first is  from an AAAS  interview by NPR's David Kastenbaum in 2006, transcript here: https://rlg.fas.org/060110-aaas.pdf while the others are from Garwin's paper, "National Security Space Policy" from the journal International Security ,  Vol. 11, No. 4 (Spring, 1987), pp. 165-173
published by The MIT Press, Stable URL: https://www.jstor.org/stable/2538846 (paywalled).

Was also intrigued by the allusion to the use of astronaut "spotters" in that.
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Q&A Section / Re: ISS Yellow Handrails
« Last post by SWGlassPit on Today at 03:14 pm »
For some more context: there are two colors of handrails used on ISS, and the color carries a specific meaning.

The ones you are thinking of are the gold-anodized ones (because yellow paint would eventually flake off, while an anodize layer is much harder to damage).  These handrails can be used for crew members to translate on.

The other handrails are clear-anodized and are silver in color.  These are handrails meant only for equipment handling and are not meant to support translation loads.
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According to the Artemis I press kit that has been provided to the general public as well as the media and the press by NASA, Orion will be 79.2 miles (127.5 km) from the surface of the Moon.
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Advanced Concepts / Re: Using shaped charges as a rocket engine
« Last post by lamontagne on Today at 03:10 pm »
TNT has an energy density of 4.184 MJ / kg. Explosives apparently exist with up to 2.38 times greater energy density than TNT (https://en.wikipedia.org/wiki/TNT_equivalent), but those explosives have not been synthesized in any quantity, probably for good reasons, so I'll use 1.90, the highest value for an explosive that seems like someone actually seriously considered using. With perfect efficiency the best possible exhaust velocity is sqrt(2 * 1.90 * 4.184 MJ / kg) = 3,987 m/s. That's not bad, but that assumes 100% efficiency, i.e. that the products all leave the rocket in the desired direction at the same speed. I don't know how efficient shaped charges can be but my hunch is you'd get less than half of this, probably much less, which makes this idea not competitive with traditional rocket propellants. Two things are hurting this idea: the energy density of explosives isn't as good as for bipropellants, and the efficiency of shaped charges is probably much worse than a nozzle's efficiency.

Something similar has been suggested using nuclear bombs: https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) .
Shaped charges typically use explosives with an energy density of 9 MJ/kg, which is comparable to methalox. But the most important thing is the gigantic pressure of hundreds of thousands of bars during the formation of a cumulative jet, which allows it to be accelerated to a speed of 10 km/s or more. For comparison, in the combustion chamber of the Raptor engine, the pressure is "only" 300 bar, and the jet velocity is 3.5 km / s, and this is an incredible achievement - it is not possible to significantly increase this value in a classic rocket engine.
A nozzle in a rocket is needed for only one task - the formation of a directed high-speed jet. In a shaped charge, this jet is formed by the charge itself, so no nozzle is needed.
But since the jet is made of the liner, isn't most of the energy lost in the expansion of the charge itself?
It's all very nice to get the inner liner moving at 90 km/s but if most of the mass is moving as an expanding gas at much lower velocity the overall thrust will not be that good. 
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Now 1,250 miles (2,012 km) from the moon.
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