Although the exact trajectory of a rocket is not known before launch, could SpaceX position its landing barge even further down range of the launch pad and eliminate the need for a boostback burn during barge landings. This could allow heavy missions such as Echostar 23 to complete a very low margin barge landing.
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1$\begingroup$ They certainly could, but it has been judged to be more cost-effective to land at the launch site. $\endgroup$– SchlussteinCommented Mar 8, 2017 at 17:22
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1$\begingroup$ They could, but what would be the benefit? There are costs and risk to deploying and retrieving the ASDS. How much does that buy you over an expendable launch? $\endgroup$– TristanCommented Mar 8, 2017 at 17:25
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$\begingroup$ In addition, they probably need to make sure that no other boat is in the vicinity of the landing rocket, and that is much more difficult to plan in international waters (would it even be legal?). Do note that I suspect the barge is currently in US national waters, but I may be wrong. $\endgroup$– ChrisRCommented Mar 8, 2017 at 17:42
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3$\begingroup$ Echostar 23 will be the heaviest payload to date (well, the most demanding as CRS payloads are heavier, but they do not need as much energy because of their much lower orbit). They are sure they will not be able to bring it down safely even without a boostback burn and using 3-engine landing burn. It might have been on the line with fast propelant load but after Amos-6 mishap they changed to slower load which results in less dense fuel and LOX loads and erases some of the margins, pushing this launch over the recoverability limit. $\endgroup$– jkavalikCommented Mar 8, 2017 at 18:01
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2 Answers
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One problem is that towing speed of the ASDS is pretty slow. Around 5-8 mph. This means that to get 500 miles off shore, you need almost 100 hours of towing to get there, and 100 hours to get back.
That is a lot of time to pay staff, and ship usage. Also it reduces turnaround time if you need more time to get back and forth.
Of course, the longer at sea, the more damage the barge takes. On the way back, a short trip with a 140 foot tall structure on top of a bouncing barge is better than a longer trip. The chances of bad seas damaging something or just even big waves causing too much stress on the stage increases.
Clearly there is a tradeoff on payload, recovering at all vs expending, and so on.
SpaceX has been experimenting as discussed with many of these variables and will optimize to what makes sense, if you had all their data.
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2$\begingroup$ You may want to add, Time on the "Open Seas" is a higher risk than "Package is on land / private property" $\endgroup$ Commented Mar 8, 2017 at 19:09
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2$\begingroup$ @Philipp - I did not really want to point out that Piracy is alive and well in todays world. The 1st Stage is ... valuable. Protecting the 1st Stage from unauthorized people would be a valid risk. In one of the videos from one of the failed attempts, I swear I saw a Larger Recovery Ship take the landing zone/boat into it's "hold" via opening the back end. I have no clue if such a ship could handle a horizontal 1st stage. $\endgroup$ Commented Mar 8, 2017 at 20:54
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5$\begingroup$ @EnigmaMaitreya Valuable? Who are you going to sell it to? A burned out Falcon 9 stage is just valuable for SpaceX. For anyone else it's just worth the scrap value (unless they happen to be a space pirate. But that's not a thing... yet). $\endgroup$– PhilippCommented Mar 8, 2017 at 21:22
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7$\begingroup$ @Philipp I would think that a nation state like North Korea or iran would love to get their hands on 9 Merlin engines. This is a silly and extreme argument but if you want to go there... (You as in the OP in this topic). $\endgroup$– geoffcCommented Mar 8, 2017 at 22:09
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2$\begingroup$ By far the worst risk of having the rocket 100+ hours at sea would be salt damage. Everything at sea gets blasted with coarse, sticky salt spray constantly. $\endgroup$ Commented Mar 9, 2017 at 6:43
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The falcon 9 has 3 return modes, depending on the launch profile.
The first involves barges and has 2 burns. The reentry burn to prevent burn up in the atmosphere and the landing burn, there is no boost back burn as the rocket stage continues on its parabolic trajectory guided by the grid fins.
The second involves 3 burns and landing on land. This adds the boost back burn which reverses the direction of travel followed by the reentry burn and the landing burn.
The third also involves 3 burns but lands on a barge. This uses a boost back burn to limit how far down range the rocket travels followed by the reentry burn and the landing burn.
For heavy launches the first method could be used to allow more fuel to be used in the launch.
So in effect you are exactly right, a barge landing can be more efficient because there may not need to be a boost back burn.
But the above does not take into account the points raised by @geoffc, that a rocket at sea is at higher risk for damage after landing than a rocket on land. We can assume Spacex has considered the risks because they continue to use the less fuel efficient boost back to launch zone method when it is possible.
answered Mar 8, 2017 at 17:41
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$\begingroup$ I thought that barge landings still require a small boostback to put it on the proper trajectory to intercept the barge. On all the launches I've watched they call out a boostback on barge landings. $\endgroup$ Commented Mar 8, 2017 at 17:48
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1$\begingroup$ No - 2 burns are only used for very low margin landings, mostly on GTO launches. Barge landings which can afford it still use 3 burns as can be seen for example on the timeline of the Iridium-1 webcast. $\endgroup$– jkavalikCommented Mar 8, 2017 at 17:53
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$\begingroup$ @JakeBlocker launches like JCSAT-16 did not have the boostback burn. As a result they land more than 600km from the coast where the landings of CRS-8, Iridium-1 or Jason-3 were around 300km iirc. $\endgroup$– jkavalikCommented Mar 8, 2017 at 17:56
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$\begingroup$ I think i have corrected my answer $\endgroup$ Commented Mar 8, 2017 at 17:59
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1$\begingroup$ @jkavalik reference this image spacex.com/sites/spacex/files/16892430560_f87dff78c0_o_1.jpg Taking your JCSAT-16 is it possible that the functionality of the "boost back burn" and the "entry burn" become the same leaving one or the other "not listed" this may be consistent with what the OP was also getting at. $\endgroup$ Commented Mar 8, 2017 at 18:15
