Timeline for Why can't spaceships go underwater?
Current License: CC BY-SA 3.0
10 events
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| Mar 12, 2018 at 18:38 | comment | added | jvriesem | At any rate, via Archimede's Law, the buoyancy force is equal to the weight of the water displaced. It doesn't matter whether the craft is full of metal or air, or "full" of vacuum: the force of buoyancy only depends on the craft's volume. The net force would be the vector sum of the crafts' buoyancy and weight, so it's (theoretically) easy for a craft to achieve neutral buoyancy by having the right ratio of metal (in the hull) and air (in the interior or in the fuel tanks). The reason that the engines would have trouble underwater is "ram pressure", as described in my answer. | |
| Mar 12, 2018 at 18:14 | comment | added | jvriesem | Correction: It does not require a large volume of air, it requires a large mass of air. The actual farm and whatnot could be relatively small with very little air. The air could be pumped in from high-density liquid-air tanks. | |
| Mar 9, 2018 at 0:06 | comment | added | Ben | @brichins I think the claim was that 4G of thrust wouldn't be desirable in space, and therefore the thrusters equipped on a spaceship are unlikely to be capable of 4G, and without that the ship couldn't get itself underwater. I don't think James meant to say that using the thrusters to counteract buoyancy would feel like 4G of acceleration underwater. | |
| Mar 6, 2018 at 21:24 | comment | added | brichins |
As to wheterh 4G acceleration does any good over "only a few hours", 4G is a huge amount to maintain for an extended duration - adding 87.75 mph/s to your speed. Over "a few hours", let's say 4, that's 4 h * 60 min/h * 60 s/min * 87.75 mph/s = 1.26 million mph [2.03 million km/hr]. For comparison, the Apollo moon missions traveled about 2% that speed.
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| Mar 6, 2018 at 21:23 | comment | added | brichins | @JamesHollis Not sure what your point is...? I was just saying 4x thrust in water doesn't result in 4G for the occupants under your scenario, that energy goes to resisting the buoyant force of the water to just maintain position, so you'd need >4G to actually continue downward. The effect of those thrusters in space is a different issue. | |
| Mar 6, 2018 at 10:12 | history | edited | James Hollis | CC BY-SA 3.0 |
fuel comment
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| Mar 6, 2018 at 8:57 | comment | added | James Hollis | @brichins but the craft is designed for space flight, where such thrusters would achieve 4G. And because it could only sustain this thrust for a few hours, they would not make you much faster on a journey lasting months/years. Less powerful, lighter thrusters would be better. | |
| Mar 6, 2018 at 3:45 | comment | added | brichins | Nitpick - crew wouldn't experience 4G; once submerged, 4x thrust would simply be maintaining neutral buoyancy against the upwards 4x of the water. You'd need >4x thrust to accelerate downward, plus far more thrust/fuel than usual to move laterally through the water (constant resistance) instead of the vacuum of space. | |
| Mar 6, 2018 at 1:08 | comment | added | ArtisticPhoenix | I thought they were full of liquid fuel? | |
| Mar 6, 2018 at 0:30 | history | answered | James Hollis | CC BY-SA 3.0 |