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SpaceX Dragon Successfully Docked With The Space Station (forbes.com/sites/alexknapp)
445 points by lelf on April 20, 2014 | hide | past | favorite | 71 comments

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.

> 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

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

I tutored physics for a while in college, and I always tried to emphasize this sort of sanity check. It was always amusing when a student would crank through the calculations, make a mistake somewhere, and come up with the answer that their hypothetical frictionless roller coaster was fourteen light years tall, and not even realize that the number that came out was clearly wrong.

I stress this, too, when I privately tutor. One interesting problem (albeit not directly relevant problem) that came up was "How would you determine the average radius of a hair given the number of hairs on your head?". This actually wasn't obvious to me, but my student figured it out right away! Put your hair in a pony tail, and measure the diameter of the ponytail, figure out that area, and divide by the number of hairs. Brilliant!

Strictly speaking, you'd be off by ~10%, because circles don't tile the plane, and you'd be counting the area between the hairs.

Good answer, but how do you know the number of hairs? If you can estimate that within a factor of 2x, you're golden.

Another, far more picky, correction in the headline... Dragon is berthed, it does not dock (the article itself uses the correct term, but the headline is wrong).

> Dragon is berthed, it does not dock

Strictly, any man-made vessel that can maneuver independently can be docked, one only needs a larger vessel or facility that can receive it. That any man-made vessel can be docked follows from the fact that it is man-made: some sort of contraption was used to hold it during manufacture, and then it was removed. So long as I can make a vessel big enough to reproduce that environment, I can dock the smaller vessel.

The need for the ability to independently maneuver comes from the legal nature of the issue. At some point, legal responsibility for the vessel transfers from the captain or pilot to the dockmaster. In conventional shipping, that point is when the bow crosses the plane of the dock's gate.

When a vessel is berthed, some mechanism bridges the gap between the facility and the vessel, like ropes, or, in the case of the Dragon Cargo, a robotic arm manipulated from the station.

That's not the definition used in space...

Docking = A coupling device that actively 'grabs' the other vessel and allows passage between them.

Berthing = A passive process where the vessel is maneuvered into place and bolted onto the other vessel.

The advantage of berthing is that it allows for a larger opening between the two vessels (since the port doesn't need to have all the mechanics necessary to do the 'grabbing' bit).

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.

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.

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.

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

My impression was that with the CASSIOPE attempt, the 3-engine burn was successful but the following 1-engine burn was cut short by the roll.


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.

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.

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.

SpaceX claims that Falcon-9 first stage has a pretty good mass ratio - meaning that the fuel weights many times more than the tanks and engines (and everything else).

However, even if we assume 30 as the ratio of loaded vs. dry mass, which is better, for example, than Titan II first stage, which has pretty good mass ratio, we need to add the mass of landing legs - something unique for the Falcon. Suppose legs halve the mass ratio, making it 15.

9 engines are capable of lifting the dry first stage, the fuel in it, and the second stage plus payload. That means 9 engines can lift much more than 15 times the dry mass of the first stage. Dividing the thrust by 9 makes the thrust much more than necessary to support the dry stage.

In other words, even the thrust of a single Merlin engine is more than enough to brake the dry weight of the first stage. That means when first stage lands the engine is throttled - spending less mass of fuel per second than it does when Falcon lifts off. The better mass ratio of the first stage, the smaller flow of fuel is needed to decelerate it - so less fuel is needed to brake. It's not enough to count seconds of thrust - how big the thrust is should also be taken into account.

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.

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.

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

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:


Exactly the answer I was looking for-- thanks!

That's a very readable reference, thanks.

Landing is also a way less serious range safety concern than taking off because the rocket is essentially empty of fuel, rather than full of fuel.

According to Elon it landed:

"Data upload from tracking plane shows landing in Atlantic was good! Several boats enroute through heavy seas." -- https://twitter.com/elonmusk/statuses/457307742495993856

Which is pretty amazing. Apparently the telemetry continued for 6 seconds as it fell over.

"Flight computers continued transmitting for 8 seconds after reaching the water. Stopped when booster went horizontal." -- https://twitter.com/elonmusk/statuses/457311780943822848

I wasn't really sure why SpaceX decided to launch when they knew that recovering the first stage would be almost impossible when the waves in the Atlantic were 20' high. I guess recovery of the stage was the tertiary reason for the launch (after the payload for the ISS and for the cubesats), however was it critical to launch at that point or could they have waited a few more days for the weather to improve?

One thought which had occurred to me was maybe they needed to get people working on the upcoming Orbcomm OG2 launch which is currently scheduled for May. When you think about all the time and energy which goes into fabricating the rocket along with all nine Merlin engines, it really hits home how significant of an achievement making reusable rockets will be when SpaceX pulls it off. You have to think that at some point in the not-too-distant future it would be absolutely crazy to launch a rocket where you knew you were going to lose the first stage.

Because recovering the first stage was not what they were being paid for. They are paid to move stuff to the Space Station.

That said, the ability to 'tag along' additional testing without jeopardizing the main mission is a huge win for SpaceX. The cost of the launch (about $100M according to their web site) is paid for by the contract. Getting flight test data as well is pretty priceless.

Finally, even if the seas were calm it would be unlikely they would put a ship in the area as it would put people at risk of having a chunk of rocket and fuel crashing into them. They might have gotten better video though. If I were there I would have tried to figure out how to borrow a predator drone to have it loitering right there to film the arrival.

SpaceX is getting free testing out of commercial payloads. That's already a huge boon - asking the commercial flights to delay their stuff so SpaceX can test better is pushing it a bit.

The flight was already delayed 10 times (April 6 2013, Sept. 30 2013, Nov. 11 2013, Dec. 9 2013, Jan. 15, Feb. 11, Feb. 22, March 1, March 16 and March 3) and scrubbed once. Given the importance of demonstrating a fundamentally new capability and potentially saving millions of dollars might be worth delaying for another couple of days. If NASA was willing to wait over a year for launch, presumably they'd be willing to wait a little bit longer. They've been tremendously helpful and supportive of SpaceX up to this point.

Even if NASA was fine with it, their other customers may not be. Don't look a gift horse in the mouth. They have managed to build a system where the customers basically pay them to test new products. That's a fantastic situation! They don't want to even hint at something that might cause the next customer to say, "We want to make sure you're focused on our mission, so kindly leave the landing legs in your factory for this one, thanks."

Another way to look at it though is the customer gets really, really cheap access to space (in this case, cheaper than anyone else including Orbital Sciences), but has to put up with some amount of tinkering. I guess there's only so much that any organization is willing to bear.

Regardless, I'm really glad that the mission went off mostly without a hitch. I can't wait to see when SpaceX glues all of the pieces together and has a truly reusable rocket platform.

I'll be more interested at the moment to see how recovered engines perform in "reused" state testing. Even the breakdown on the cost of refurbishment would be big news.

From what I understand, they had a window on Friday, a fallback window on Saturday. If they missed the Saturday launch, then the mission would go to Orbital.

Maybe they technically could have waited until Saturday, but there is no way they would scrap one of their windows for a tertiary objective.

In the post-launch press conference, NASA made it clear that ISS scheduling is tight, with a lot of traffic (Dragon, Cygnus, Soyuz) and onboard activities, and rescheduling was not going to be easy.

Yeah, this launch has been delayed and rescheduled over and over again for various reasons. Too many to count actually, the customer (NASA) will always be the primary concern and they did not even have to allow Spacex to run these tests while transporting their cargo.

Well, also according to Musk, its last reported velocity was 360 m/s. That's a pretty hard landing.

That message was sent while the vehicle was still high up and had not started its powered landing over the water. It was only its last reported velocity at that time, not its last reported velocity now, or even shortly after.

Ah, did not realize that. Thanks for clarifying.

That was when it dropped over the horizon from the Cape and they lost telemetry. It was still 8.5 km in the air at that point.

I'm willing to bet that this is what everyone is most curious about.

During the post launch news conference (https://www.youtube.com/watch?v=8G2pRVQ1JhA) Elon said he was pessimistic about a successful recovery of the stage for this launch. However, he said SpaceX is "connecting the dots" and seemed optimistic that they are making good progress (for example, they successfully canceled the roll this time). The rough timeline he mentioned was that they will probably recover a stage sometimes this year, with fully successful recoveries (i.e. capture, refuel, and ready to launch again within hours) sometime early 2015.

The main reason he was worried about recovery at that point was that the booster had splashed down into something like 20-foot waves. That's high enough to be real trouble for some of the recovery boats they wanted to have out there (which may be one reason why we haven't heard much about how it finally wound up). And it could also very well sink a rocket stage that wasn't built to handle this particular kind of buffeting, and is made to be pretty lightweight.

As to longer term plans, it's not clear they will relaunch the first booster they get back, even if seawater exposure weren't an issue. More likely, they'll disassemble it, and see how the parts handled the stress of a full launch and re-entry, looking for wear that could endanger a second launch of the same booster. If there are any unpleasant surprises like that lurking, it's a lot cheaper to find out this way than in a launch with a payload at risk.

Oh, I doubt they ever intended to relaunch this one. At that point you have to pretty much rebuild everything, so you may as well just trash it and build a brand new one.

But they would sure like to know that it behaved properly after they lost contact, and they'd like to know if anything was damaged before it hit the water.

They have telemetry until it went into the water --- see tweets quoted immediately below. The last few seconds' worth of that were delayed because the rocket was over the horizon from the launch site, but they got it from chase planes.

Oh, I didn't realize that.

But still, they really would be better off if they had it to pull apart.

Nothing detailed about the first stage yet. They certainly have telemetry showing it hovered for a while but that is all the public has seen. During the launch they mentioned having video of the first stage retrofire but we haven't seen it.

Lack of news of recovery makes me think they lost the stage in rough seas. I think we would have heard if it was recovered.

Anyone know if they had video recording on the first stage? That'd be fun to watch.

At landing? Don't believe so. I think if they had that, they would have been able to say more than they did. And I think they mentioned not great weather?

It would be cool if they did :).

I don't expect them to have drones recording the landing from the water, but I would fully expect them to have mounted some cameras to the stage itself. But I can't find confirmation of that.

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

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

You know, thinking over 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


and http://www.planetaryresources.com/

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

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'.

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


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

or is spacex still using expendable stages ?

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

an awesome start for reusable rockets, amazing!!



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.

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.

FYI, "human race" has idiomatic usage in english.


Be proud of your race when it solves corruption, poverty and inequalities... You can do something about it, be proud of it and at the same time watch rockets fly http://www.watsi.org

I'm not sure if OP was making a racist statement of meant "the human race", but completely independent of that and in response to your statement:

Any technological venture is helping to solve poverty in the long run. Even if the venture does nothing to help inequality, the new technology is almost guaranteed to raise society's collective standard of living.

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.

In all sincerity, it would probably be worth 15-20 minutes of your day to go review:



What SpaceX and their sub contractors have accomplished in the last 12 years is nothing short of amazing.

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.

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.

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.

an actual own in-house rocket would have been more ground breaking

Huh? As Wikipedia says:

"In order to control quality and costs, SpaceX designs, tests and fabricates the majority of its components in-house ..."

They just demonstrated all of the steps required to recover a first stage for near-immediate reuse. Nothing close has ever been done before. How can you say that they "haven't done anything astonishing yet"?

The designs are all quite different. They use additive manufacturing for example.

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