
SpaceX Dragon Successfully Docked With The Space Station - lelf
http://www.forbes.com/sites/alexknapp/2014/04/20/spacex-dragon-successfully-docked-with-the-space-station/
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throwawaymsft
Congrats to the SpaceX team, always inspiring.

The correction made in the article (5000 tons => 5000 lbs) shows the
importance of having mental visualizations for basic numerical concepts.

5000 lbs of cargo is about an SUV. I can imagine a rocket carrying that up.

5000 tons of cargo is... a giant Wal-mart parking lot full of SUVs (2000 of
them). Stacked, they'd make a tower of metal a few miles high. (How? Figure 6
feet tall x 2000 = 12000+ feet.)

Can you imagine a rocket carrying that up and dropping it off at the space
station? Nope.

When numbers are just symbols, mistakes like this are easy to make.

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est
> Can you imagine a rocket carrying that up and dropping it off at the space
> station? Nope.

Yes!
[https://www.youtube.com/watch?v=8Ia2N4vpaEo&t=284](https://www.youtube.com/watch?v=8Ia2N4vpaEo&t=284)

~~~
grinich
I was hoping this thread would have a KSP link ;)

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rwitoff
After 3 scrubs most of our non-critical colleagues left the cape, but the view
of our payloads in the trunk after 2nd stage sep made it all worthwhile. It's
amazing what SpaceX has been able to accomplish and reinvent in a business
this risk-averse and I can't wait to see what's next.

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modeless
Any more news on the performance of the first stage recovery test? Elon Musk
tweeted that it was successful but I haven't heard anything more detailed than
that.

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greglindahl
The two things that were new this time:

* The 3-engine deorbit burn worked; previously it was screwed up by uncontrolled roll preventing the engines from getting fuel

* The 1-engine landing burn, previously separately demonstrated by the Grasshopper and F9R test vehicles, worked for a rocket coming down from orbit

The remaining items:

* 3-engine burn to take the rocket back to the Cape from downrange

* Making everything safe enough to dare landing at the Cape, without killing anyone or destroying all the expensive ground equipment that's nearby.

~~~
InclinedPlane
As for the last item, it's not as daunting as it seems. Range safety is
actually fairly comparable in either direction. The trajectory of the booster
won't naturally head over populated areas and it will have to be adjusted
until it reaches back to land. If, for whatever reason, the trajectory
adjustment goes to far and the booster would end up landing farther back on
land then the stage can be signaled to be destructed, which will screw up its
drag coefficient enough so that the debris will just fall safely into the
water. And that's a worst case scenario.

~~~
trothamel
Another reason it's less daunting is that, by the time of the landing burn,
there's very little fuel remaining in the rocket. On the way up, the rocket
has to have enough fuel to power the 9 first-stage engines for 181 seconds,
and the second stage engine for 412 seconds. (As well as fuel for the various
return and landing burns.) So it will have over 2000 engine-seconds of fuel.

In the landing burn, it's powering 1 engine for maybe 10 seconds. So that's
less than half a percent of the initial fuel mass - something that would make
a potential failure a lot less dangerous.

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Recoil42
Ten seconds seems like an incredibly liberal estimate. You need a pretty
decent "margin of error" reserve. I'd wager we're looking at something more
like 30-60s of fuel during the last stages of landing, at least. So let's call
it something like 2-3% of the fuel.

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ceejayoz
I'd imagine it's nowhere near full power for most of that, as it's an empty
shell of a craft and thus quite light.

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JshWright
The engine can only throttle down to about 70%. For the final landing, the
first stage will only use the center engine (leaving the other 8 shut down).

Even with only one engine at 70%, the thrust to weight ratio is greater than
1. That means the rocket can't hover, they just have to time the burn
perfectly so the speed reaches 0 at the moment it reaches the ground.

~~~
grinich
Why can they only throttle to 70%? Is that related to old-school turbopumps?

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Mvandenbergh
Most rocket engines don't throttle down well. The main reason isn't the
turbopumps (although that can also be an issue) but combustion instabilities
in the chamber and flow separation in the nozzle.

Nasa overview of liquid engine throttling:

[http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/2010003...](http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100033271.pdf)

~~~
grinich
Exactly the answer I was looking for-- thanks!

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Spittie
SpaceX/Nasa livestreamed the whole docking (sadly I was asleep at the
time...).

I don't know if the whole footage is up yet, but this is a nice summary:
[https://www.youtube.com/watch?v=3fDzvdEfSgc](https://www.youtube.com/watch?v=3fDzvdEfSgc)

~~~
eric_h
Thanks for the link to that. "Free drift trickery", indeed.

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bane
You know, thinking over
[https://news.ycombinator.com/item?id=7617720](https://news.ycombinator.com/item?id=7617720)
I almost wonder why Musk hasn't set an interim step to Mars that involves
space habitats. Right now SpaceX is probably the closest to making an economy
based on

[http://www.bigelowaerospace.com/](http://www.bigelowaerospace.com/)

and [http://www.planetaryresources.com/](http://www.planetaryresources.com/)

working. If they get the 1st stage reusable it brings the bootstrap costs down
significantly.

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rm445
It's not clear that it's an interim step exactly. Mars has gravity, earth for
radiation shielding and some resources to work with. I don't know exactly what
SpaceX have in mind for a Mars mission, but I'd be surprised if it wasn't a
variant on 'Mars Direct'.

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vermontdevil
They also tested the new replacement to grasshopper called Falcon 9R Dev

[http://youtu.be/0UjWqQPWmsY](http://youtu.be/0UjWqQPWmsY)

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jokoon
any video of the landing of the 1st stage, which is not a test ?

or is spacex still using expendable stages ?

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wolf550e
The Falcon 9 is still expendable. They plan to recover the first stage this
year and re-fly a stage next year.

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iotakodali
an awesome start for reusable rockets, amazing!!

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kanwisher
9.

Nf

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frade33
Just do not watch gravity movie tonight. on a serious note, it's friggi'
amazing and makes me feel proud of our race every single day.

~~~
spoiler
I'm curious actually: what's more appropriate to say in such statements:
species or race, or something else entirely? I'm guessing It's would be
species, but I'm not an expert on biology.

~~~
001sky
FYI, "human race" has idiomatic usage in english.

[https://en.wikipedia.org/wiki/Human_Race](https://en.wikipedia.org/wiki/Human_Race)

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pearjuice
I am skeptical of SpaceX its future endeavours. They proved they are on par
with government subsidized space programmes, but haven't done anything
astonishing yet. I mean, they probably bought a majority of the tech from NASA
or the Soviets and just made it work. Not that it isn't impressive, just that
an actual own in-house rocket would have been more ground breaking.

~~~
bane
Their rocket designs are in house -- unlike competitors (like Orbital or
Lockheed Martin who use refurbed Soviet Era engines). The vast majority of
SpaceX's equipment is designed, built and tested all in house.

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avmich
RD-180 was designed in 1990-s, after break-up of USSR, and you can't just saw
in half RD-170 to get RD-180. "Refurbished" usually applies to something used
for some time already - not to the rocket engines, certainly not to RD-180 and
NK-33, which apart from being fire-tested weren't used in flight.

~~~
bane
You're right about the RD-180, I think those are still produced?

I think Orbital also has used the NK-33 and NK-43, which were all built in the
60s and 70s. Refurbished can also just mean "something that was sitting around
for a long time and needed to maintenance to get up to working". Anything that
sits around for 40 years is going to need some kind of work done on it before
it's put in service.

Either way, SpaceX doesn't use them.

