Comment by HarHarVeryFunny | Hacker News Reader

Comment by HarHarVeryFunny | Hacker News Reader

HarHarVeryFunny

I wonder if they had accelerometers recording exactly what happened as it touched down and ended up on it's back?

It was meant to use a rather odd "two-step" landing procedure where it first touches down on the lunar surface on it's rear/primary landing gear, then pivots forward/falls under gravity to also touch down on the front/secondary landing gear.

Presumably this landing procedure was simulated under lunar gravity, but it seems there are multiple potential failure modes:

1) After rear "leg" touchdown, over-rotate forwards over front leg (ending up on back), OR

2) After front "leg" touchdown, bounce/recoil backwards off front leg, flipping onto back, OR

3) Front landing gear hits rock on one side (left or right), thereby flipping it sideways onto back

The whole procedure seems to rely not only on having correctly simulated under lunar gravity (and with correctly simulated stiffness of the vehicle) to avoid scenarios 1) & 2) (which would be different under earth gravity), but also the softness of the landing site being uniform and as simulated - otherwise if landing site surface was harder (rock) or softer than expected, or uneven, then this type of pivot and bounce/not landing would not go as planned.

NASA seems to have done a better job in designing fool-proof landing mechanisms, from the bouncing ball of the first mars rover, to the sky-crane landing of the latest one (which appears over-complicated, but no doubt was chosen at least in part because it is in fact more predictable than the alternatives).

The two-step sounds precisely like how all forward-moving aircraft land: Rear touch first. The alternative (front touch first) is dangerous for reasons any BMX biker could explain: You're much more sensitive to pitching forward from forward-of-gravity friction.

I wouldn't read too much into that choice.

More likely the velocity was a tad too high.

But for a propulsive landing, why would they want to have forward velocity at all, unless by design for this type of landing ? I'd have thought the preference would be to kill forward speed and land vertically.

As an experienced KSP crasher, you never completely cancel horizontal component, so you need a way to dissipate it. And it's easier to dissipate if you know which direction your horizontal component is, which is easier to do when its value isn't so close to 0.

That being said I perfectly see how sub-ms automated command could make what I said wrong.

One of two main engines blew off at ~50m and the craft was essentially in a cross-wind landing, likely without de-crab. Having lateral velocity wasn't a part of the plan.

The engine loss was a detail in the article that I hadn't seen anywhere else in the commentary and speculation on the landing. Even now many commenters don't seem to be taking this into account. Even if there was redundancy built in, an engine loss makes things much more difficult. I can see why they're claiming 100% success despite being upside down.

> The two-step sounds precisely like how all forward-moving aircraft land: Rear touch first. The alternative (front touch first) is dangerous

That's why I was never comfortable stopping in rollerskates or ice stakes.

I never understood why those use front-brakes instead of rear-brakes like the roller blades I had as a kid. The latter I could comfortably use almost instantly, the former, I never got comfortable with.

I assume that you just drag the tip of one skate behind you, rolling on the other until you lose momentum. And having the ability to suddenly stop is akin to ice figure skates, with the pointy bits at front.

Taildraggers would like a word....

FTA:

> One of the lander's main engines lost thrust about 50 meters (54 yards) above the moon surface, causing a harder landing than planned.

When I first heard that they might have landed upside down, a part of me felt a pang of indignation and condescension, for about ten seconds. But I kept my mouth shut and realized these things are hard and something I didn't think of in the ten seconds as an armchair lander-designer might have occurred.

I found the tone of your comment pretty condescending, tbh. Hot landing is hot landing, and you're just speculating as a layman without any insight into their design and decision-making process. "Not simulating lunary gravity correctly." Like c'mon, that's just an insult to their intelligence.

Well, I did also mention stiffness which would relate to how much of a shock absorber effect there would be - preventing a bounce, so if they came in hard, that may indeed have been a factor. Maybe more to the point, it seems to be a fairly unforgiving mode of landing - maybe a NASA "bouncing ball" landing would have absorbed the harder landing without issue ?

Considering they knew these failure modes in advance, would it not have been prudent to put some sort of self-righting mechanism on the lander? Something like the mechanical arms you see in Robot Wars.

Mechanisms weigh a significant amount, long mechanisms like arms even more so. The more mass you spend on contingencies the less science you bring and the less valuable the mission is, even with a flawless touchdown.

Good point, but I wonder if some of the attitudinal jets could be employed to push it upright, assuming there's any fuel left (and power).

Neat idea, I'm not sure how much force those jets produce, maybe it could be enough? It might be risky to fire up jets after a botched landing though. If the nozzle has any material in it I'd be worried about blowing a hole in the lander!

If you can't land reliably, then a lot of the science is wasted. On the other hand, once you reliably solve the landing issue (which was one of the main objectives here, and where they made significant progress in that they successfully deployed the two mini rovers anyway), you can add on as much science as you want.

Also I've said before and will say it again, the moon is not far away. Unlike Mars and other celestial bodies where we have to time launches around orbital positions, gravitational slingshots and such, the moon is really close by, and we should be lofting stuff onto it on a monthly basis.

And it's all a moot point if you're too heavy or complicated to fly in the first place.

> "An abnormality in the main engine affected the landing attitude of the spacecraft," the Japanese space agency Jaxa said in a statement. [0] [1]

[0]: https://news.ycombinator.com/item?id=39131130

[1]: https://www.bbc.com/news/science-environment-68091389

> NASA seems to have done a better job in designing fool-proof landing mechanisms

The purpose of this landing design was explicitly to allow landing in areas that wouldn't previously be considered acceptable: slopes.

It's big vindication for the bouncing ball approach. On the other hand the proximity of the moon makes it much more practical to field crude designs on a more frequent basis, or to put parts in orbit and assemble them into something more ship-like.

>It's big vindication for the bouncing ball approach

Not really, it's only been used for a few Mars missions, and not recently. For lunar missions you don't have the benefit of aerobraking, so you have to use so much fuel to slow down already, the math doesn't work out to use airbags at the last step.

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