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Old 09-08-2017, 08:47 PM
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blackshire blackshire is offline
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Join Date: Jan 2009
Location: Fairbanks, Alaska
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Quote:
Originally Posted by Jerry Irvine
I have seen powered landings in person and up close. it is astounding. (Lunar Lander, 2 entrants)
It's ironic that it's the failed attempts of those that get more media attention, but many if most of those who cover the prize attempts may not appreciate just how much is going on during the brief descents and successful landings, and:

If any of the landers can't hover (like the Falcon 9 first stage, whose thrust exceeds its weight when its tanks are so nearly empty), they have only one shot at "sticking" a successful landing, by utilizing their downward momentum against thrust in order to reach zero altitude and velocity ^gently^. But even if they *can* hover, it's still not easy, especially if there's any appreciable wind (New Shepard's booster can hover before touchdown, and Blue Origin had to write some sophisticated guidance software to enable it to compensate for winds aloft -and- surface winds). Also:

Even Surveyor 1, launched on Memorial Day, 1966, was given only a 1-in-10 chance of succeeding (NASA would have been satisfied if its radar had simply detected the Moon and triggered the spacecraft's descent sequence). This small hope diminished even more when one of its two low-gain antenna booms failed to extend after launch, as some engineers feared that its off-center mass might prevent the lander's control system from keeping the spacecraft stable during powered descent. JPL was very pleasantly surprised when the descent and landing not only succeeded, but went exactly as planned, and after touchdown even the stuck antenna boom was found to have finally deployed. (Many years ago, someone built a model rocket motor [motors] powered Surveyor scale model that made several successful rocket-braked landings until--like Surveyor 2--one of its three vernier retro-rockets failed to ignite, causing it to tumble and crash.) In addition:

The entrant landers in the various lunar lander prize competitions (see: http://www.google.com/search?source...1.A n5MmenajxE ) that can compensate for winds would be "over-qualified"--which is a *good* thing!--for lunar landings, where that factor isn't present.
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