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Food for thought: SpaceX has designed its new aircraft. Why doesn’t their team, develop some thing that will allow the astronauts to drive the module back to shore?

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    $\begingroup$ I can't help but think that if the spacecraft is going to be landing somewhere that a recovery ship can't get to, the problems involved in the landing are likely far more severe than the extra mass of an on-board water mobility system can fix. $\endgroup$ – notovny Aug 2 '20 at 23:20
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    $\begingroup$ How about wings and landing gear? Naw, too implausible. $\endgroup$ – Organic Marble Aug 2 '20 at 23:42
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    $\begingroup$ @ikrase demonstrably false. $\endgroup$ – Organic Marble Aug 3 '20 at 0:01
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    $\begingroup$ In all seriousness, cost-benefit ratio. That's a lot of mass & reliability issues to deal with a relatively low-risk recovery system that is what's currently used. $\endgroup$ – Carl Witthoft Aug 3 '20 at 12:17
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    $\begingroup$ @DavidHammen I prefer the term 'unrepentant shuttle hugger' :) $\endgroup$ – Organic Marble Aug 3 '20 at 15:01
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SpaceX originally proposed that its cargo and crew Dragon vehicles use propulsive landing rather than water landings. This was in part motivated by long-term desires by Elon Musk to eventually land humans on Mars.

NASA rejected these proposals as too risky, too costly, and "you're not doing that kind of research on our dime". If SpaceX wanted to do that they would have had to develop the technology at their own cost and at their own risk, and then they would have to prove to NASA that what they developed was safe and would remain reasonably priced.

The ocean recovery technology we just saw had already been developed decades before SpaceX existed. There was no convincing NASA that this was the right way to go as NASA was the organization that convinced SpaceX that this was the right way to go.

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The first thing to note is that being able to move the spacecraft around without a recovery ship doesn't really add much capability, because we'll probably want to have the recovery ship anyway, at least for the forseeable future.

As well as doing the "normal" recovery and control operations, it is also there as a rescue ship for emergencies where the capsule or crew is in trouble. For example, there has been at least one water landing where the capsule sank and the astronaut almost drowned (the second Mercury flight, way back in the 1960s), and one where the astronauts were almost poisoned during landing and required hospitalisation (Apollo-Soyuz in the 1970s). These are the sort of things where you definitely want assistance on-hand.

There's also a bit of a question mark about how much the crew could usefully do after landing. They've spent some months in microgravity, so moving around under 1g is going to be a strain, and then immediately asking them to steer a small boat for an hour or two might be a bit much.


But let's say we want to do it anyway, to make things easier after landing. From an engineering perspective, there's no dramatic technical reason there couldn't be a small motor and propellor on board - this is definitely very well-established technology, we cracked it two hundred years ago! - but it would probably be more expensive and complex than it seems at first.

Why would it be so complicated? Consider what you would need to do with your motor. You would need to develop and test it with the spacecraft, and you would need to ensure that the spacecraft is redesigned so that it has space for it and power for it (you would presumably need a substantial battery if there is no on-board motor). You would then need to make sure that the whole thing is safe and reliable after being left on-orbit for six months, and that nothing interferes with any other bits of equipment (eg if you have a propellor, where does it come out? Does it have to pass through the heatshield? Does the motor affect the balance of the spacecraft?). There are a lot of design and development costs here.

And finally, it would mean either making your spacecraft bigger and heavier, or cutting out something else.

The size of a spacecraft is primarily dictated by how much mass the launch vehicle can put into orbit, so there may be an upper limit to how much it can grow without cutting into safety margins or reducing its capabilities. (An interesting example here is the Space Shuttles - the oldest, Columbia, was also a few tons heavier than the others, which meant she was never used for space station flights - the extra weight made reaching those orbits with a useful payload more difficult.)

Alternatively, you could cut something out. But if you have 200kg to play with in your design, and the choice is between a motor for use after landing, or an extra passenger seat, there won't be much contest!

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