
An Analysis of SpaceX’s Falcon 9 Crash Landing - jonah
http://www.wired.com/2015/04/analysis-falcon-9-crash-landing/
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Animats
A key piece of information we don't have, but Space-X does, is how much fuel
was left at landing. Landing has to be done with minimum fuel, since it comes
out of the fuel budget for putting payload into orbit. They have thrust
vectoring on the main engines, and have attitude jets near the nose. They have
to land vertical, at near zero velocity, with near zero angular rate, with an
underactuated system. Underactuated means there are more degrees of freedom to
control than there are controllable parameters. They have 5 controllable
degrees of freedom - 2 axes of engine vectoring, main engine throttle, and 2
axes of relatively weak nose thrusters. They have to hit a point with three
translation constraints, three linear velocity constraints, two angular
constraints, and two angular rate constraints. So they have 5 DOF in and 10
DOF out. That, mathematically, is what "rocket science" is.

This under-actuated maneuver has to be solved as a two-point boundary problem,
or, in practice, re-solved continuously to cope with problems such as wind
gusts. You can see what happened; they had too much angular rate at landing,
requiring more torque than the nose thrusters could impart to keep the rocket
upright once the main engines could no longer help. It looks like they made a
big correction shortly before landing to hit the target, and lost angular
stability doing so.

If they'd had a bigger landing area, so they didn't have to apply big
corrections to hit the tiny target, this would have worked.

~~~
pixie_
Elon - "Looks like the issue was stiction in the biprop throttle valve,
resulting in control system phase lag. Should be easy to fix."

AKA lost constrol of the engines. It could of landed in a field and it would
of still crashed. Man there are so many armchair rocket scientists.

~~~
Animats
Aviation Leak says that message was "withdrawn."[1]

[1] [http://aviationweek.com/space/spacex-checks-throttle-
valve-a...](http://aviationweek.com/space/spacex-checks-throttle-valve-after-
flawed-falcon-9-recovery-attempt)

~~~
pixie_
[2] ITAR.

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paulsutter
Guys please.

The sideways oscillation of the rocket is an obvious control loop problem.
When it's working correctly, the booster will remain rock solid vertical and
land gracefully. Elon's (now deleted?) tweet said as much.

Let the real rocket scientists fix their rocket. They know how. It's really
unbelievably close to working. Amazing really.

~~~
tsotha
The fact that he deleted it implies they decided that wasn't the case after
all, or at least wasn't the whole story.

They didn't handle media very well this launch, what with the fumbled video
release and then a retracted tweet.

~~~
saraid216
My read on the deletion is that they didn't want bad press going into the next
attempted landing, which is going to be on land.

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tsotha
Is it? Do you know which flight?

~~~
saraid216
This is all I know:
[https://news.ycombinator.com/item?id=9387929](https://news.ycombinator.com/item?id=9387929)

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ridgeguy
It's interesting to think about a case where Just Read The Instructions (the
barge) might cooperate in the landing. Perhaps it could have an XY stage that
could help compensate for residual X,Y or X,Y roll rates at touchdown.

Looking at the video, if JRTI could have slewed a platform quickly to the left
after contact, it might have been able to return the tilting booster's center
of mass to a stable position.

That's probably not much of an incremental engineering effort over what SpaceX
has done already in this work. I'm completely awed by what they've
accomplished so far.

(edited because I can't do word->initial strings late at night...)

~~~
grecy
I keep hearing lots of great ideas about how something other than the rocket
could help the rocket land (catch it, move the platform, use a net, etc. etc.)

I honestly think SpaceX have no intention of learning how to do any of those
(even though some might work great) because they want to learn how to land it
in a place with no "smart" landing pad already setup.

i.e. Mars

~~~
robszumski
Can a Falcon Heavy get the full center core of the rocket, including the first
stage into orbit? Maybe with no payload? If so, this would mean transporting
and landing a stage on Mars, for use as a return launcher, would be possible.
Is this a possible long term plan?

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tempestn
I expect the more difficult part is getting the necessary fuel to Mars to
escape. Obviously you could do it if you took enough trips, but that would
likely be prohibitively expensive.

~~~
baq
the plan that makes most sense is to produce fuel on site. allegedly there's
water below Mars surface, so LOX/LH should be doable. maybe there's more stuff
to synthesize methane, RP-1 or other fuels.

~~~
tsotha
One plan I've seen for ISRU on the moon is to use a Al-Lox hybrid engine,
since Al2O3 is all over the moon's surface. I wonder if there's enough on Mars
to do the same thing, or if you could do something with iron oxide.

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Trombone12
The title is misleading, should say "analysis of video of F9 crash". I know
the current tittle is the title wired put in it, but the crash is not actually
analysed at all, just the video of it, and that analysis doesn't provide any
information of importance.

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petrosh
I would not call such an article "an analysis".

~~~
simonh
Oh come on. Generating graphs of the rocket's vertical position and angular
deflection from vertical over time, using a video analysis and physics
modeling package, is not analysis? Sure, having the actual telemetry data from
the rocket would be better, but suppose you were a competitor to SpaceX and
wanted to understand what their capabilities were and whether this approach is
feasible, this is exactly the sort of investigation you'd want to do.

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whoisthemachine
Overall, I'm very impressed by SpaceX's efforts. However, I'm wondering why
they haven't tested the Grasshopper out on a water barge? It may be no
different than on land, but that seems like a large assumption to make and a
crucial variable to ignore.

~~~
maxerickson
Would that save them much development effort? I would think they would have to
do a lot of refit each time they dumped it in the ocean.

~~~
whoisthemachine
Very good point. It may not in terms of pure time or money. But in terms of
eliminating "unknowns", it will certainly save them many headaches. Generally
the unknowns are what cost you in development efforts. In this case, was it
the variability of a water platform (however minute) that caused the over
correction, or was it a fault of a piece (i.e. valve or something similar) in
the rocket?

When doing something new it's much easier if you eliminate all unknown
variables except for one. So first, develop a controlled rocket that can hover
and land on land (grasshopper). Next develop one that can hover and land on
water. Now all major unknowns are eliminated except for the big one, landing a
rocket from space, on either a water platform or a land platform. However, you
will now be able to discern whether the issue is related to the water platform
or just the general difficulties of landing a rocket hurtling down from space.

~~~
maxerickson
Wouldn't saving either time or money (or both) be the entire point of using a
specialized test vehicle? They are anyway working their way through the
unknowns by trying to recover the boosters.

I think they are also pricing the launches so that at least all the direct
costs are covered, so these tests are 'free'.

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AdamTReineke
I didn't see a mention of Musk's tweet, which labeled the YouTube video as
"slow motion". That will make a difference (though likely just a scalar?) in
the calculations.

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Already__Taken
I'm interested in why they haven't built another grasshopper to primarily use
for the pad abort and inflight abort to get some bonus testing with.

Does SpaceX have a location they could fly such a test rocket? They can't go
over a km in their texas location.

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sunstone
The video shows a lot of white caps in the ocean so the wind was blowing
pretty hard. If the wind was gusting on top of that it's not hard to see why
it would be hard to keep the rocket from being blown over even if it landed
properly.

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david-given
The first stage will be empty of fuel and is basically a big empty metal
balloon with a huge lump of metal at the bottom. The centre of gravity is very
low, so while it looks really unstable it's much less bad than it looks ---
like a double decker bus. OTOH, as you say, there'll also be a huge amount of
windage from that big empty metal balloon...

SpaceX seem to be competent, so I assume they'd factor this in. I'd be
interested to see an analysis, though.

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sebastianconcpt
They were so close!

Previous landings where more elegant, this one "looks anxious" (like done in a
hurry). If they leave more fuel towards the end for error margin maneuvering
then it can aproach landing with less momentum

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motoboi
Landing attempt video:
[https://youtu.be/rAzwuEmZcmE](https://youtu.be/rAzwuEmZcmE)

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mrfusion
This doesn't seem to take into account that the video was in slow motion?

