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I understand that thousands of small rocket fragments will cause less damage when they fall than an intact rocket. But at the time Starship was detonated, I believe it was over the ocean, and not headed towards land. There was nothing to worry about damaging beneath it. Why not let it fall in one piece, so it will be more intact and thus easier to investigate as to what went wrong? This seems like it would be especially useful for a test launch.

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    $\begingroup$ In addition to answer by The Rocket Fan, the answer here notes various operating restrictions space.stackexchange.com/a/63380/26356. Failure to destruct on deviation would probably mean no future permits to fly, on basis that next time it deviates it might be towards people. $\endgroup$ Apr 21, 2023 at 6:32
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    $\begingroup$ Plus, it is a really good opportunity to test the destruct mechanism (so you will have higher confidence that it will work when you really need it). $\endgroup$
    – Rob
    Apr 21, 2023 at 13:59
  • $\begingroup$ they got to test more than what they wanted,the launchpad looks like it was hit by a nokia 3310. $\endgroup$ Apr 21, 2023 at 14:17
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    $\begingroup$ I highly suspect its impact with the ocean would be just as explosive as the detonation was in air (assuming any propellant was left, which there would be). It just becomes a matter of how far you want that explosion to be from where people and buildings may be. $\endgroup$
    – Drake P
    Apr 21, 2023 at 17:28
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    $\begingroup$ These ships might think otherwise about: there is nothing to worry about: MarineTraffic. I know there is a lot of space between the ships, but still you have the chance of sinking one... $\endgroup$
    – Arsenal
    Apr 24, 2023 at 12:40

6 Answers 6

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Why not let it fall in one piece, so it will be more intact and thus easier to investigate as to what went wrong?

There were designated areas where it was okay for Super Heavy and Starship to encounter the water per the launch license. Having them do so outside those areas endangers ship and aircraft traffic. The flight termination system is there expressly to mitigate risks to those not involved with the launch.

Neither Starship nor Super Heavy were intended to be recovered at all from this test flight, even if things went perfectly. Per the Environmental Assessment Re-evaluation Super Heavy was to be sunk after landing and Starship was intended to explode on impact from mixing of the remaining propellant. There was no way there would be a forensic recovery effort after the stack lost control.

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    $\begingroup$ Maybe a stupid question; doesn't blowing it into many smaller pieces increase the chance of debris going places it wouldn't be welcome? (as opposed to if it fell as one or two very large pieces)? $\endgroup$
    – stevec
    Apr 22, 2023 at 3:04
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    $\begingroup$ @stevec perhaps this xkcd will be illustrative as to why large pieces are less desirable than small ones, though the raindrop isn't also full of rocket fuel $\endgroup$
    – Erin Anne
    Apr 22, 2023 at 3:14
  • $\begingroup$ Ha. That was a great read. But in the case of a rocket, if I were on the ground within 100km of a rapid unscheduled disassembly, I'd feel better knowing two or three large pieces were coming down than tens of thousands of smaller pieces. $\endgroup$
    – stevec
    Apr 22, 2023 at 3:26
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    $\begingroup$ @stevec why? Dissipating all that chemical energy higher up, and mitigating the gravitational potential and the kinetic energy by scattering it across pieces with low terminal velocity and small mass, makes it much less likely to damage ANYTHING, even if some location in the debris field is more likely to be struck by something. A mostly-intact Starship + SuperHeavy stack obliterates whatever it hits and whatever is near where it hits. That isn't preferable. $\endgroup$
    – Erin Anne
    Apr 22, 2023 at 3:55
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    $\begingroup$ @ErinAnne I’ve actually had to discuss this type of thing with people before. Based on those experiences, most people who aren’t aerospace engineers (and even some aerospace engineers) tend to significantly underestimate the destructive power of the large pieces, and significantly overestimate the destructive power of the small pieces (essentially, they assume it’s going to be deadly and highly destructive either way), which usually leads to a strong fixation on the probability of being hit. $\endgroup$ Apr 24, 2023 at 1:59
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The SpaceX Starship rocket blew up because it had the self-destruct sequence activated.

Starship's first stage didn’t go quite as planned because it was overweight and not all engines were working. It was because stage separation failed, that the rocket tumbled and soon after went off course. After that an explosion was seen.

According to SpaceX

At 8:33 a.m. CT, Starship successfully lifted off from the orbital launch pad for the first time. The vehicle cleared the pad and beach as Starship climbed to an apogee of ~39 km over the Gulf of Mexico – the highest of any Starship to-date. The vehicle experienced multiple engines out during the flight test, lost altitude, and began to tumble. The flight termination system was commanded on both the booster and ship.

It is also a requirement for rockets to have a self-destruct system onboard according to this site:

During a suborbital or orbital launch, the launch vehicle would be equipped with either a thrust termination or a destructive flight termination system, or both. In the event the vehicle varied from the planned trajectory, the applicable system would be initiated, and the vehicle would break up.

To sum it up, the Starship was commanded to self-destruct after spinning out of control and going off course.

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    $\begingroup$ I know it was self-destructed. My question is why bother when it was over the ocean; there's nothing it could crash into out there. $\endgroup$
    – Ryan_L
    Apr 21, 2023 at 4:53
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    $\begingroup$ The OP acknowledged that it wouldn't make orbit, the question they have is why destroy it at all instead of leaving it intact for easier forensics. $\endgroup$ Apr 21, 2023 at 4:57
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    $\begingroup$ I suspect the answer is something like "it was a legal requirement as part of the launch" or similar, but as-is, this answer doesn't actually answer the main body of the question, only the title. $\endgroup$ Apr 21, 2023 at 4:59
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    $\begingroup$ "Starship first stage went normally" – Citation needed. Just from what I could see with my own two eyes: it lifted off significantly after T-0, climbed a lot slower than it should have with a thrust-to-weight ration of 1.5, lost six or seven engines on ascent, encountered Max-Q much later and with much lower pressure than planned, and it looked like at least one, possibly both, hydraulic power units exploded. I'm also pretty sure stage separation should have happened much higher than 39km. $\endgroup$ Apr 21, 2023 at 12:18
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    $\begingroup$ @user2705196. I watched the launch and wrote at the top what I saw. Then I quoted SpaceX response. Afterwards there was a comment from fyrepenguin about the self-destruction things so I edited the question with his link. I do not see how AI generated things could have been involved. $\endgroup$ Apr 21, 2023 at 14:52
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Why not let it fall in one piece, so it will be more intact and thus easier to investigate as to what went wrong?

If we just focus on this part, the big reason is diminishing marginal returns (data/dollar). Crash scene reconstructions are extremely expensive, time-consuming, and complicated. I doubt SpaceX even has the staff to perform such an investigation. The only reason they are typically done is because plane (or human-manned space flight) crashes are extremely rare and human lives are involved. It's very valuable to society to know exactly what went wrong down to the smallest bolt. When the Challenger blows up, you need to know exactly what went wrong because the Space Shuttle, at that point, was supposed to be a safe, proven spaceship. When a unmanned test of a new spacecraft blows up, that's a completely expected part of rocket development.

Thus, a test flight of a new spacecraft is nothing like a problem with an established craft, particularly for SpaceX due to their iterative design process. They are already getting mountains of data coming in from the gigantic number of sensors they installed. Because of this, the minute details a recreation might provide are much less important. This is even more true for initial test flights because the problems you need to fix are often glaring and obvious. They don't need a painstaking recreation to know that they need to fix the engine / pad problem(s). When you do iterative design you are always going to attack the large problems first, because fixing those might obviate the need to deal with the smaller ones you may or may not have discovered through an extremely careful (and expensive) review. Indeed, the next Starship already had "100's" of changes and improvements they made without even them knowing the results of the first launch.

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At the point where the launch has failed, you have two options:

  1. Detonate explosives to self-destruct the spacecraft.
  2. Let it fall to the sea and break down randomly.

In case 1, you end up with metal fragments on the seafloor and combustion gases in the atmosphere. Fall area becomes more predictable due to small size => high relative drag of the fragments.

In case 2, you end up with undetonated explosives on the seafloor, plus the same metal fragments, and a mix of less CO2, but more CH4 in the atmosphere (a worse greenhouse gas, though it's negligible). Fall area also becomes less predictable, increasing the risks to shipping.

If recovery and investigation of the remains is the plan, it would've been prudent to use a parachute and limit damage to the vehicle. If there's no plan to recover and investigate, detonating the explosives at least removes them from the equation.

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    $\begingroup$ While CO₂ is a greenhouse gas, methane is a much, much, MUCH worse greenhouse gas. Better to burn it than pump 300t of it into the atmosphere. $\endgroup$ Apr 21, 2023 at 17:11
  • $\begingroup$ @JörgWMittag Agreed. I'd expect it to be a secondary consideration, but on that front too, burning it is better. $\endgroup$
    – Therac
    Apr 21, 2023 at 20:01
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    $\begingroup$ Although methane is a trace gas, at something like 1 or 2 kg/km^3 on average, there's something like 5 tera metric tons of it in the atmosphere overall if I recall. The amount of methane in one Starship doesn't amount to much in that context. $\endgroup$ Apr 21, 2023 at 21:12
  • $\begingroup$ Put Fall area also becomes less predictable, increasing the risks to shipping in bold. And not just shipping, as uncontrolled flight could bring the stack back over land before crashing. $\endgroup$
    – wistlo
    Apr 23, 2023 at 14:09
  • $\begingroup$ @wistlo: No, definitely not. It was flying at approximately 2000 km/hr away from land, in a zone from which ships were excluded. Any light-weight parts light enough to be blown away by the wind are harmless for the same reason. $\endgroup$
    – MSalters
    Apr 24, 2023 at 8:00
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There are other means of gathering information about a spaceship failure that won't be affected by the spaceship hitting the water

When the spaceship is being launched, there were already diagnostic tools available to the team - as shown in the clip here, using the timestamp in there of @2:26 (T+00:02:15), we are able to see a camera looking at what I understand is intended to be the separation step as it is not separating. They jump back to the footage again @2:49 (T+00:02:38).

I'm citing that video because it's a recording of a CBC stream of a SpaceX stream; it's entirely possible that we don't get all of the diagnostic information that SpaceX's Mission Control gets regarding the telemetry of the ship. If they were in a situation where the telemetry wasn't working as intended, they would have been better off finding a way to force SCE to AUX, although they couldn't do that from within the starship itself, so admittedly that could have caused further problems.

However, all of that telemetry is almost certainly getting logged at Mission Control, and would be more useful than trying to diagnose mechanical components after they've been subjected to an unrelated collision with water, and the potential resulting corroding of materials being suspended in water until recovery.

As a result, there's significantly less reason to try and find a workaround of the legal issues @ErinAnne mentioned regarding the license, nor risk further possibility of the rocket going out of control as @TheRocketMan mentioned, and if the self destruct mechanism did fail (Which would in itself be a bad thing to find out mid-flight test), the undetonated explosives mentioned by @Therac pose a risk to recovery at that point regardless, since they could cause issue during the recovery step at an indeterminate point of time.

TL:DR; They have data about the launch itself, and would have to extrapolate the data from the components to find issues that aren't related to the separate crashing into the water.

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There could also be national security reasons to self-destruct the rocket rather than let it fall intact. ITAR (International Traffic in Arms) regulations may come into play with some of the technology of spacecraft that severely limit how that technology can be handled to keep it out of adversaries' possession, and possibly self-destructing a rocket with likely ITAR material would make recovery of anything meaningful much less possible by an adversary.

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    $\begingroup$ Wouldn't work. The controlled stuff is generally things like engine injectors or avionics software, and a shaped charge that peels open the fuel tanks isn't going to touch those. $\endgroup$
    – Mark
    Apr 22, 2023 at 3:30

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