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Per SpaceX, Starship test 3's landing zone for the orbiter is now in the Indian Ocean (rather than off of Hawaii):

[Starship] will also fly a new trajectory, with Starship targeted to splashdown in the Indian Ocean. This new flight path enables us to attempt new techniques like in-space engine burns while maximizing public safety.

What safety difference is produced by the trajectory change? Is there any publicly-available source that expands on what hazards were averted / risks were reduced by this change?

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    $\begingroup$ On the previous test flights the plan was to launch Starship on a suborbital trajectory that would bring the ship down in a specific location in the Pacific Ocean near Hawaii without requiring a deorbit burn. For this flight they plan to do a simulated deorbit burn by briefly igniting one the Raptors during the coast phase. It is not known for sure if it will be a retrograde or posigrade burn. Either way the landing location will depend on the success of the burn, creating a much larger range of possible splashdown locations. It is believed the Indian Ocean was seen as safer for this test. $\endgroup$
    – Steve Pemberton
    Commented Mar 14 at 0:43
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    $\begingroup$ A SpaceX commentator during today's launch webcast said: "We are intentionally flying this new steeper trajectory so that we can test things like engine relight without substantially changing where we expect to splash down". Maybe it's not that the Indian Ocean better handles a larger zone but that a trajectory which keeps the zone smaller will result in a shorter downrange distance. Meanwhile another commentator said that the planned test burn (which they wound up skipping) would have raised the perigee. Perhaps it is PR terminology to refer to a smaller zone as maximizing public safety $\endgroup$
    – Steve Pemberton
    Commented Mar 14 at 21:09
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    $\begingroup$ Close voters: I don't agree that this should be attracting opinion-based answers, though obviously it is. Any suggestions? I'll accept I misworded the question, but I think there's obviously a version of this that is backed by fact-based answers. $\endgroup$
    – Erin Anne
    Commented Mar 14 at 22:10
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    $\begingroup$ @ErinAnne I agree. Here are the rules for everyone's reference: Some subjective questions are allowed, but “subjective” does not mean “anything goes”. All subjective questions are expected to be constructive. What does that mean? Constructive subjective questions: inspire answers that explain “why” and “how”, tend to have long, not short, answers, have a constructive, fair, and impartial tone, invite sharing experiences over opinions, insist that opinion be backed up with facts and references, are more than just mindless social fun. $\endgroup$
    – phil1008
    Commented Mar 15 at 20:12
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    $\begingroup$ I feel that close votes should not be cast for this question. Reason: Rules for people who have the cast close and reopen votes privilege:. Questions should be closed by casting close votes if: the question is a duplicate ..., the question needs details/clarity, needs more focus, is primarily opinion-based, or is otherwise problematic to identifying the problem in a way that can be properly addressed by answerers, the question is sufficiently off-topic ..., or otherwise disallowed for a community-specific reason. $\endgroup$
    – phil1008
    Commented Mar 15 at 20:18

3 Answers 3

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An Indian Ocean splashdown is about halfway around the world from Boca Chica. Re-entering there after reaching near-orbital velocity requires a high, lofting trajectory and a (relatively) steep reentry angle. This also means there's plenty of margin for error: a change of a few meters per second in velocity means a change in impact point of a few tens of kilometers.

A Hawaiian splashdown is nearly a complete orbit away from Boca Chica. A re-entry there happens at a significantly shallower angle, and a slight change in velocity could easily move the impact point several hundred kilometers up or downrange.

Since the mission profile includes a possible simulated reentry burn, the increased margins of an Indian Ocean splashdown permit skipping the burn without unduly increasing the size of the hazard area. It also keeps an off-nominal burn from causing the ship to come down on land (the Hawaii trajectory passes over New Guinea) or, in the worst-case scenario, putting it into a decaying orbit that could come down anywhere within about 25 degrees of the equator.

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  • $\begingroup$ The first two paragraphs make sense to me, but the last paragraph still doesn't. Where is the burn occurring that you have more risk of landing in New Guinea this time than doing the same on IFT1 or IFT2 with an off-nominal orbital insertion? Is the difference in impact point sensitivity to delta-V really that big? I'd like to see some figures on it $\endgroup$
    – Erin Anne
    Commented Mar 15 at 4:38
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An important factor as to why it would maximize public safety is because it will fly mainly over water. If Starship crashed on land, then there would be a much higher chance of somebody getting hurt. During IFT1 and IFT2 Starship was planned to fly on a trajectory which would send it over the pacific near Hawaii. I found these 2 images on X showing the flight path of the first 2 missions.

enter image description here

enter image description here

Now during IFT3 the goal of public safety remained the same, but this time they want test an inflight engine burn and land in the Indian Ocean.

There are 2 possibilities. The first is that they will fly a very similar trajectory. So if the engine fails to fire, it will most likely crash in the pacific ocean, while if it succeeds, then it will land in the Indian ocean.

The second possibility is that Starship will fly on a suborbital trajectory with a high apogee so that no matter if the engine works, it will land in the Indian ocean.

In the image below posted on X, it does not show the trajectory change of the in flight engine burn. However, the image does depict that if Starship would fly on a similar trajectory to IFT1 and IFT2, then it will also land in water.

enter image description here

SpaceX has not published the exact trajectory of Starship neither did I find any warnings for a possibility of debris in pacific ocean meaning that the chance of the suborbital trajectory with a high apogee is more likely. During the livestream, they usually show the altitude of Starship meaning that we will know afterwards.

Source of the images:

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    $\begingroup$ @ErinAnne I think you don’t understand. All 3 flights will be suborbital. The thing about IFT3 is that it will also do an inflight burn making it renter sooner. To make sure that it does land on water, the reentry burn will happen earlier so that starship will land in that Indian Ocean. If it fails, then it will coast to the pacific. In both scenarios, it will land on water and not on land which maximises public safety $\endgroup$
    – The Rocket fan
    Commented Mar 14 at 6:26
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    $\begingroup$ I don't believe there's any evidence that SpaceX is using the Pacific as a backup landing site in case their burn fails. Feel free to cite an actual document. $\endgroup$
    – Erin Anne
    Commented Mar 14 at 6:30
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    $\begingroup$ @ErinAnne Yes that is why I mentioned there are 2 possibilities in the answer. Since SpaceX didn’t publicly show any documents about a pacific landing, the other options seems more likely. $\endgroup$
    – The Rocket fan
    Commented Mar 14 at 6:48
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    $\begingroup$ I think your answer would get more votes if you improved the first sentence. "The main reason why it would maximize public safety is because it will fly over water and not land." This is a weak statement as both flight paths are over both water and land. It sets up the reader to dislike the rest of your answer. $\endgroup$
    – phil1008
    Commented Mar 15 at 20:01
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    $\begingroup$ this answer might get more votes if it focused on the difference on the safety affects of the changed landing zones and didn't contain completely incorrect speculation like "if the engine fails to fire, it will most likely crash in the pacific ocean." The engine DID fail to fire, and it landed in the Indian Ocean, which was the obvious intent the whole time. $\endgroup$
    – Erin Anne
    Commented Mar 15 at 23:01
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In a sense it does. Public safety is more of an absolute requirement as opposed to something that you attempt to maximize. When comparing a flight plan that reenters over the Indian Ocean to a flight plan that renters over the Pacific Ocean, its is quite possible that the Indian Ocean flight plan had acceptable risk and the Pacific Ocean flight plan did not.

For the Pacific flight plan to qualify, they would need to establish that: a) The rocket had enough delta-v to reach that sub-orbital trajectory (assuming that all of its systems performed nominally), and b) That there was a plan to safely abort if at any point the mission became off-nominal. Aborting might involve, for example, initiating the flight termination system at a point in time when the rocket's trajectory would ensure that the debris would reenter and fall into an ocean, such as the Indian Ocean, as opposed to on land.

With the Pacific flight plan, once the ship has accelerated to the point where its sub-orbital trajectory ends in the Indian Ocean, the systems on board would need to make a choice - either continue the burn to completion to move the trajectory's termination point further downrange and over the Pacific Ocean, or terminate the burn while the trajectory's termination point still ends somewhere over the Indian Ocean. An in-between termination point would not be acceptable as it might lead to debris coming down over populated land.

For the Pacific Ocean flight plan to have unacceptable risk, the probability of the ship being able to continue its burn to completion would need to be high. There are a few things that could still go wrong during the final moments of that burn. The ship could simply run out of propellant or the ship could break up due to the ever-increasing acceleration. Note that the ship is lightest when its tanks are near empty; therefore, its acceleration is at its highest. Pressures at the bases of header pipes would be peaking at this point as well as stresses within some parts of the ship's structure.

If you'd like to learn more about how the Expected Casualty ($E_c$) rates for commercial space launch and reentry missions are evaluated, there is an FAA advisory on this topic. But also keep in mind that even if it could be shown mathematically that the probability of debris coming down over land and physically harming someone is extremely low, allowing any debris to come down anywhere near land could be damaging from a public relations point of view. SpaceX needs to consider not only the mathematically calculated public safety but also the public's perception of public safety.

Update

@TheRocketFan included images (in their now-deleted answer) that Jonathan McDowell posted on X. I think that they are useful and so I've added them here. The first two show the estimated flight path proposed for the first 2 missions and the last shows the estimated flight path proposed for IFT3.

Proposed IFT1 and IFT2 Start

Proposed IFT1 and IFT2 End

Proposed IFT3 End

Source of the images:

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  • $\begingroup$ If that is the case that would mean the original plan for IFT1 and IFT2 was unsafe, and the change to a shorter flight for IFT3 made it safe. However neither the FAA or SpaceX has made any public comments about the previous profiles being unsafe (whether or not it was). Meanwhile SpaceX says the new profile change was because of the mid-flight burn they would be performing. The public statements (or lack thereof) doesn't disprove your theory, but it implies a bit of obfuscation by SpaceX, which of course is not impossible but harder to prove without evidence. $\endgroup$
    – Steve Pemberton
    Commented Mar 14 at 19:48
  • $\begingroup$ I don't understand why two people think that the previous Pacific hazard zone northwest of Hawaii is backstopped by land instead of the whole rest of the Pacific ocean to the east. $\endgroup$
    – Erin Anne
    Commented Mar 14 at 22:06
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    $\begingroup$ You're right; I misread the concerns about landing on land. I still don't think they're valid--ascending to an orbital trajectory involves your IIP moving over a great circle in front of you, and that same risk was present before with the Pacific re-entry zone. But I still badly misread what you'd written and that's on me $\endgroup$
    – Erin Anne
    Commented Mar 15 at 4:28
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    $\begingroup$ I undeleted my answer since I think that it would be better if there is a combination of both your answer and my answer $\endgroup$
    – The Rocket fan
    Commented Mar 15 at 10:56
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    $\begingroup$ Good! I'll give it an upvote! $\endgroup$
    – phil1008
    Commented Mar 15 at 18:47

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