> common question of "why not use parachutes"

My typical answer to this is to mention Mars and the very thin atmosphere. There's no point in SpaceX developing technology that they can't use on Mars.

Moon landings, too. While Musk has his sight set on Mars, I can't imagine he'd turn down the opportunity to beat NASA to a lunar return.

Yeah, he said: "Loop around the moon, possibly land on the moon - just to prove the capability" - http://www.youtube.com/watch?v=thKMqN-2E4s#t=50

> That "solid surface" will presumably be a barge. Very exciting stuff.

I get nerd chills from the level of precision needed to bring a launch vehicle from hypersonic->subsonic speeds to a target that size, and land to reuse the vehicle.

I'm impressed with the barge concept if only from the wave action the'd have to compensate for. Anyone know wave amplitude in the area they'd be landing in? That's a flat surface that's coming up to meet you or dipping away, making for a pretty heavy drop...

You could use a SWATH style ship - http://en.wikipedia.org/wiki/Small_waterplane_area_twin_hull

They get very little wave loading due to having semi-submersible hulls, much like an oil rig.

It's apparently called "Active Heave Compensation":

http://www.boschrexroth-us.com/country_units/america/united_...

It's used at sea for winches, but I'm sure platforms can compensate just as well with the proper force actuators and motion reference units.

It's actually much cheaper and easier to sink some anchors and winch the barge down a bit than to try and do active compensation. It's one thing to compensate a crane or a winch, it's another to compensate an entire vehicle. The anchor thing is one way that they handle off-shore drilling rigs.

Another is to use a semi-submersible rig. What that means is that you fill the bottom half or 2/3 of the vessel with water making it much heavier. That changes the way it rides in the waves and can dampen the effects that a wave has substantially.

Maybe use an old oil rig? Its legs would bolted to the seabed, which I would assume is sturdy enough.

Depends on the rig. There are plenty of rigs in 5000' feet of water and I assure you that they are not mile-high building. They are semi-submersible.

http://en.wikipedia.org/wiki/Semi-submersible

Oil rigs are really only for drilling, not for getting the oil out of the ground and into whatever container there is. So that means they move around all the time. Like every couple of weeks.

What you're thinking of is called a jack up rig. They go in water up to a few hundred feet deep. Nothing over 1000 feet for sure. That's because the entire truss has to come out of the water when the barge is being moved from one location to the other.

http://en.wikipedia.org/wiki/Jackup_rig

An old oil rig has the right level of Bond-villain-ness about it. Gets my vote.

The Glomar Explorer, which was custom built by the CIA to raise a sunk russian nuclear submarine (https://en.wikipedia.org/wiki/USNS_Glomar_Explorer_(T-AG-193...),used heave compensation for the entire crane/rig that lifted the submarine from the sea floor.

It's not about the rocket tipping over. The use of parachutes would require the loss of landing accruacy. They want to be able to guide the first stage to a specific location (like a launch pad), and parachutes would not allow them to do this.

Powered landings mean precise, highly controlled landings. You land exactly where you want with minimum damage to the vehicle and you go out and put the stage on a truck so you can bring it inside and begin processing it for reuse in a matter of hours.

Parachute landings mean you have a recovery zone, you need a recovery team, you need to find the rocket and bring it back. Whether landing in the ocean or land with a parachute there's a high probability of significant damage from impact and/or seawater intrusion. So now you have a huge amount of effort just to get the rocket back, then you have to clean it up. And the parachute landing likely means much lower chance of being able to fly even one additional flight, let alone many, unless you develop some sort of cushioning system like airbags, in which case you have to add that complexity, cost, and weight to the system. And all of that is aside from having to repack the parachutes for every flight.

SpaceX is planning powered landings for the same reasons that commercial airlines do powered landings. It's safer, it's more controllable, it's a smoother operational experience. It just makes sense.

There are precisely steerable parawings. They looked at recovering Saturn stages by glidinng back to Florida with a Rogallo wing.

It all factors in. SpaceX has performed grasshopper tests on windy days that would be a problem for a parachute landing. There is also the issue that parachutes sufficient to slow down the stage to a soft land-landing would be far too heavy.

> "We will attempt our next water landing on flight 13 of Falcon 9, but with a low probability of success."

I'm wondering why the next water landing has a low probability of success?

This is not confirmed by SpaceX, but has been floating around the internet as the suspected reason.

Last year, SpaceX attempted a first stage water landing, but was not successful. Their two most recent successes on this front had at least two hardware differences from that old attempt: landing legs and heavier duty RCS thrusters. However, due to some combination of manufacturing and launch scheduling, an old core without these improvements, similar to their attempt last year, will be flying on flight 13.

Flight 13 will be a flight to ISS on a core without landing legs and other modifications for landing. Apparently, they swapped cores earlier, and now they have a rocket left without legs.

I believe it is because of this:

> However, our next couple launches are for very high velocity geostationary satellite missions, which don’t allow enough residual propellant for landing.

Based on the press release they plan on using Falcon Heavy for this type of launch in the future but it sounds like that project isn't ready for primetime yet.

I took that statement to mean they can't even attempt a landing on the next couple of launches because there will not be enough residual propellant for landing.

And then the next statement, I assume, refers to the next launches where they can attempt a landing.

EDIT: my assumption is confirmed here http://en.wikipedia.org/wiki/List_of_Falcon_9_launches

Next up are flights 11 and 12, which can't be landed.

Then 13 has the "low probability of success".

Funny that you cite Wikipedia, which is citing... the link that this HN discussion points to.

Yeah, I actually made that particular edit to Wikipedia about 20 minutes before seeing it cited as a _verification_ of that article, here. Funny to see one of those infamous internet self-confirmation loops get established in near-realtime...

Might be the flight path or payload limitations.

probably because it's another water landing

The last 2 water landings have been successful.

Parachutes also hit a lot harder. Powered landing you can touch down with ~0m/s. Parachutes you'd get something like 10 m/s (~22mph.) Not necessarily nice on your equipment.

Yup. The Soyuz landing capsule uses solid fuel retro-rockets at the very last second to slow down, but even then it is apparently a rather unpleasant bump.

What's interesting here is the difference:

Soyuz uses a 'suicide burn' that releases a ton of delta-V in a short amount of time.

From the video, it seems that Falcon uses a progressive burn as it approaches. They also mention that they can restart it up to 3 times in atmosphere.

The third burn here is also a suicide burn. The first one is to change its path and the second is to prevent it from falling too fast.

Not particularly nice on your astronauts / cosmonauts / 赵里昱 either, though they have training, preparation, and (according to an interview with Cmdr. Hadfield awhile ago) specially-poured per-backside crash seats to ameliorate it somewhat.