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I understand the benefits of reusability for the rocket booster. But I don't understand why they're landing it on a barge. I've read tons of articles about this but not one has even hinted at an answer. Why is landing on a barge preferable over a launchpad or any other large flat surface that isn't moving with the waves?

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    See http://space.stackexchange.com/questions/6123/how-much-fuel-can-spacex-save-by-landing-the-falcon-9-booster-on-a-barge – Hobbes Jan 10 '15 at 21:49
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    Land tends to be covered with people and buildings and lawyers. – Dan Jan 11 '15 at 03:56
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    I don't know rocket physics or the design of SpaceX, but I'd suppose that the rocket probably require lots of local positioning equipments to be able to land on a platform, which need to be positioned in precise configuration, like Autoland system for airplanes. Thus, they probably think its cheaper to build the positioning equipments into a movable ship rather than to have to disassemble, move, reassemble, resurvey, and reprogram every time a launch/landing platform is needed. And the ship can also carry it straight to the next launch location, allowing it to be much more flexible. – Lie Ryan Jan 11 '15 at 13:56

6 Answers6

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Edit Jan. 25, 2015: Geoffcc recently provided a link to words straight from the Horse's mouth, Elon Musk. Musk says the payload hit for RTLS (Return To Launch Site) is 30% vs a 15% payload hit for landing on a downrange ocean platform. Musk also said said safety is a reason for having an ocean landing platform -- "As far as the safety aspect of the return to launch site of the first stage that's part of why we want to do it first in the ocean just to make sure that things will be fine. For any landing area that we would have, the landing ellipse, the sort of error that the stage could encounter would be an unpopulated region. So we would aim to have a landing site that's unpopulated with a radius of a couple of miles (which can be achieved in Cape Canaveral and Vandenberg)."

So it seems safety as well as delta V are reasons for the barge landing

End Edit.

To get back to the launch pad, a booster stage would need to kill it's eastward velocity and then do a westward burn. This extra delta V "boost back" expense inflicts a penalty on payload mass as others have said.

However a Jon Ross info graphic from an NBC story indicates a boost back burn to get to the barge:

enter image description here

I had imagined a near parabolic trajectory for the booster's path from launch site to barge but Ross' graphic indicates a boost back burn as well as a reentry burn and landing burn. I haven't been able to get actual numbers but the delta V savings of this path doesn't seem that great.

Further, if this sort of barge landing offers substantial delta V savings over RTLS, then why is SpaceX still negotiating with the air force for the use of SLC 13 for RTLS?

PearsonArtPhoto
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HopDavid
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    Interesting, but how accurate is it? Could be just an artist's guess... – Hobbes Jan 12 '15 at 20:24
  • The CRS-5 launch of January 2015 was a test. That may account for the unusual trajectory with partial boostback, giving them the opportunity to test the boostback principle without bringing the stage close to the Cape. The additional 15% payload hit for boostback makes it likely that downrange landings will be used occasionally in the future. – Hobbes Jan 25 '15 at 18:22
  • So what, a 30% mass hit? Then simply build a 30% larger rocket. Since it is reusable it doesn't matter how heavy it is. Fuel costs are negligible. See ocean shipping. There are many ships around which are older than most of us reading this. And the ocean is a tough environment. – LocalFluff Jan 19 '16 at 08:20
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    @LocalFluff thats not a linear effect, for adding 30% more propelant you need to add another X% more propelant to push it.. And more to push that.. – jkavalik Jan 19 '16 at 09:43
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    @jkavalik Falcon 9 takes a fifth or a tenth of the mass to orbit of what the Saturn V did. If Saturn V was reusable it would be very cheap to use it to launch cubesats to LEO. Mass of launcher is not a good proxy for launch costs. Not anymore. The paradigm is shifting. – LocalFluff Jan 19 '16 at 10:15
  • @LocalFluff I meant something different - because of the rocket equation for making the effective payload 30% bigger you end up with a much bigger rocket than only 30% (unless I understand it wrong, which is sure possible) - thats why Falcon Heavy is being developed. – jkavalik Jan 19 '16 at 13:20
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    As far as I know, they have three options: RTLS (easiest but most costly), landing on a barge with boost-back burn (harder than RTLS but allows for more payload / higher orbit), and landing on a barge without boost-back burn (hardest, but allows for even heavier payload or higher orbit). At the time of writing, they achieved 1 succesful RTLS, 1 succesful landing on sea with boost-back burn, 2 succesful landings on sea without boost-back burn, and since then one more almost succesful landing on sea without boost-back burn. – Daan Wilmer Jun 17 '16 at 09:13
  • @DaanWilmer Thanks! Very interesting. Can you point to where you got this info from? If I can include this info with cites, it'd much improve my answer. Or if you gave this info along with cites in an answer form, I would upvote it. – HopDavid Jun 18 '16 at 03:15
  • @LocalFluff even if you build the 30% bigger (or more) rocket, you still have the tradeoff between using all its +30% mass to go back or again land on a barge and deliver more mass in orbit than the smaller rocket. – BlueCoder Nov 22 '19 at 10:43
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Most people would cite danger to populations and property as the foremost reason to stay away from land. You're setting down a rocket on a tail of flame like a 50s sci-fi film, after all. But the economics of the launch physics is the overwhelming factor.

The "flying over populated areas" question is pretty much solved by their options to launch, currently coastal areas. By the time of the first stage MECO (Main Engine Cut Off) and separation, the Falcon 1st stage is significantly northeast of Canaveral, with nothing but water underneath.

Elon Musk did pitch trying to fly back to the launch pad, but the tyranny of the rocket equation makes the price exacted on payload unacceptable. Even if some of the booster cores from the Falcon Heavy make it back to shore (and this remains to be seen), they still plan to land the center core on a barge.

Jerard Puckett
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  • "But the economics of the launch physics is the overwhelming factor." Can you expound on that? Are you saying it's easier to land on water because that's already where the rocket is? –  Jan 11 '15 at 03:29
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    @JustinY the first stage is considerably downrange from the launch site & traveling at a considerable rate of speed. It is enough to have to kill all of the forward momentum; the additional propellant required to get back to the launch site will mean that much less available capacity to launch payload. Much cheaper to provide a landing site closer to where the stage is overhead when you start the recovery burn. – Jerard Puckett Jan 11 '15 at 06:09
  • "But the economics of the launch physics is the overwhelming factor." That was my opinion, that a barge landing eliminated boost back expense. Now I'm not so sure (see my answer). If you could provide cite(s) backing up your statement, I'd be grateful. – HopDavid Jan 12 '15 at 19:57
  • SpaceX is still planning on returning the F9 first stage core to a landing pad, in fact, they have two under construction at this very moment. – marked-down Jan 12 '15 at 20:17
  • @JerardPuckett Another point is that trying to aerobrake is very difficult, given that the rocket is in effect an oversized egg-shell hurtling at several times the speed of sound. Aerobraking will likely result in broken eggs. – Aron Jan 19 '16 at 06:11
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Now that we've seen 4 landing attempts, it's time to revisit this question. We've seen both options (landing on a barge and on land) being used, albeit not yet successfully for the barge landing.

SpaceX wants both options.

  • They prefer to land the stage on land, if that's possible. This can only happen if there is sufficient margin to account for the extra fuel needed by the boostback burn. The Falcon 9 has some flexibility: the first stage cutoff point can be at speeds between 6000-9000 km/h (1670-2500 m/s). The speed chosen depends on the mission: missions to a high-energy orbit (GEO, heavy payloads) need a higher staging speed.
  • At the high end of that speed range, there's not enough fuel left for a return to the launch site, so the second option is to land the stage on the barge.

Quoting Elon Musk:

Ship landings are needed for high velocity missions.
Ship landings are not needed for flexibility or to save fuel costs.
Just not physically possible to return to launch site
If speed at stage separation (higher than) ~6000 km/hr. With a ship, no need to zero out lateral velocity, so can stage at up to ~9000 km/h.

They also used the first two barge landings to test the landing process and build confidence they can land the stage accurately. This may have been a requirement imposed by Cape Canaveral, but I haven't seen evidence either way.
The first landing on land (Falcon 9 Flight 20, Orbcomm-2) showed SpaceX can provide a safety margin for landings on land: after the reentry burn, the stage is on a trajectory towards a point just off the coast. If the landing burn fails (engine doesn't start), the stage will splash down harmlessly.

Hobbes
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    Musk had a series of tweets Sunday night that included some figures with speed at staging that limits the return to launch site - would make a nice addition to your answer. Sadly twitter seems to be down at the moment... – James Thorpe Jan 19 '16 at 09:00
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    For convenience: 6000 km/h ~ 1670 m/s; 9000 km/h = 2500 m/s. – user Jan 19 '16 at 12:07
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As this question has resurfaced following the Jan-2016 launch of Jason-3 it is worth flagging this news article that says, precisely, why a barge landing was selected in this case.

It states

Koenigsmann said SpaceX is doing the ship landing on this mission because it was not able to secure environmental permissions in time to permit a landing back at Vandenberg. “We could land back on land” were it not for the paperwork issue, he said. “We have enough energy on this mission to come back to land. It’s not anything technical.”

The source of the quote, Hans Koenigsmann, is vice president of mission assurance at SpaceX.

Puffin
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The only reason SpaceX is landing on the barge is to demonstrate to the range safety community (and themselves) that they can safely and reliably land the first stage. There is a very little propulsive advantage to landing on the barge where SpaceX is positioning it vs. returning to the launch site.

A good video showing SpaceX's aspirations in this area is shown in this video.

Erik
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    That seems unlikely to be correct. They have stated repeatadly they take a payload hit, reserving propellant for stage recovery. As high as 15-30%. Being able to land downrange on a barge, instead of trying to find an island that would only work for one orbital inclination, or some other land base. So it seems there is a great deal of propulsive advantage to landing down range (on a barge or land) vs returning to launch site. – geoffc Jan 10 '15 at 22:58
  • That 15-30% number is for first stage flyback -- not the difference between first stage flyback to a barge vs. the Cape. – Erik Jan 10 '15 at 22:59
  • The difference in a boostback burn and a reentry burn to the barge is not that different from the burns required to get back to the Cape. – Erik Jan 10 '15 at 23:01
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    The 15-30% number tells you that there is a major cost. Boostback to the cape, for a Falcon Heavy center core, which is much higher, and much faster is definitely a bigger deal. – geoffc Jan 10 '15 at 23:15
  • It would be a big deal if the barge was positioned way out in the Atlantic much farther east -- but it is not. That boostback burn is 15-30% because it negates the north-eastward component of the first stage's velocity. The additional impulse needed to give the stage enough south-westward velocity to get to the Cape is small in comparison. This is because the majority of ascent lofting occurs during the first stage. (Lofting is the difference in an ascent's trajectory from an ideal gravity turn. It is used to get out of the thick atmosphere quicker.) – Erik Jan 10 '15 at 23:18
  • And evidently, they can't. – raptortech97 Jan 11 '15 at 04:31
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    @raptortech97 - yet. – Erik Jan 11 '15 at 06:29
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    I'd like to see some numbers to support your argument that RTLS only has a minor cost in extra fuel over a barge landing. – Hobbes Jan 11 '15 at 12:09
  • @geoffc the path to the barge seems to include a boost back burn that not only kills eastward velocity but boosts altitude and gives it a little westward velocity. See my answer. I'm giving Erik's answer a plus 1. – HopDavid Jan 12 '15 at 19:38
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    @HopDavid Do not disagree with you. But the magnitude of that burn differs between RTLS and down range barge. That is the key disagreement here, the difference between the two. I dispute the "There is very little propulsive advantage". Clearly there IS a difference, perhaps we only quibble about the magnitude of 'little'. – geoffc Jan 12 '15 at 19:42
  • I've been trying to get numbers for awhile. See my question http://space.stackexchange.com/questions/5186/numbers-for-falcon-9-booster-meco-scenarios . If you could provide me numbers along with cites, I'd be grateful. So far as I can see most the answers (including mine) are speculation. – HopDavid Jan 12 '15 at 19:52
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    The original question was why the barge instead of land. If the barge was to minimize prop requirements, the barge would be much farther from the launch site and would allow the first stage to follow a mostly unpowered ballistic trajectory with some correction. However, it doesn't -- it has a boostback burn. This is because ultimately SpaceX wants to RTLS. Thus, they want to test a full, reversing boostback burn -- but can't for safety reasons. The difference between this partial reversing burn and a full burn are, relative to the entire first stage prop budget, small. – Erik Jan 12 '15 at 20:00
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    @Hobbes I agree that Erik could improve his answer by cites and numbers. But same criticism applies to Jerard Puckett's answer. At this point there seem to be two assertions: 1) This barge landing gives minor dV savings over RTLS. 2) This barge landing gives major dV savings over RTLS. So far as I can see both assertions are equally citeless. – HopDavid Jan 12 '15 at 20:03
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    @HopDavid -- I totally agree on the numerical citations -- I have none (hard data is sparse). But the original question asked about motivations. SpaceX is landing on the barge because they have to demonstrate safe landings -- not for performance. All of SpaceX's documentation for their reusable booster has the first stage landing at the Cape. For example: https://www.youtube.com/watch?v=sSF81yjVbJE. – Erik Jan 12 '15 at 20:08
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    This is an interesting video of the first stage's post-sep flight: http://youtu.be/_UFjK_CFKgA – Erik Jan 19 '15 at 21:42
  • @Erik: I would think a barge landing would be in every way superior to landing on any terrain that wasn't between the next place the rocket would be needed and the nearest waterway. Is the interest in RTLS to allow launches that are far away from water? – supercat Apr 22 '15 at 17:23
  • @geoffc This must have been resolved by now....? – SusanW Nov 24 '20 at 15:50
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    @SusanW Starlink is the good counter example. They are at basically max volume in the fairing and pretty dense, so 15,000 Kilos. They have no margin to return to launch site. They actually move the ASDS further downrange because it is so heavy. – geoffc Nov 24 '20 at 16:07
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I see nobody seems to have mentioned the simplest reason why they're not landing on land: there isn't any.

They launch east from Florida, and there aren't any islands in the Atlantic out there. Without a major course change from the first stage, which as other people point out would use a prohibitive amount of fuel, then it's basically going to come down in vast stretches of empty ocean. Hence the barge.

David Given
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