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Old 12-18-2019, 10:42 AM
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Originally Posted by TriPolar View Post
Wouldn't the low gravity also be the cause of the lack of atmosphere?
Not really. The relationship between surface gravity and atmospheric density is not direct. Titan's surface gravity is lower than the moon's but it's atmosphere pressure at the surface is higher than the Earth's. Venus' surface gravity is slightly lower than the Earth's but it's atmospheric pressure is much higher than the Earth's.
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Old 12-18-2019, 10:49 AM
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Not really. The relationship between surface gravity and atmospheric density is not direct. Titan's surface gravity is lower than the moon's but it's atmosphere pressure at the surface is higher than the Earth's. Venus' surface gravity is slightly lower than the Earth's but it's atmospheric pressure is much higher than the Earth's.
Thanks, interesting. This gravity stuff is tricky.
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Old 12-18-2019, 10:52 AM
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Originally Posted by Andy L View Post
Not really. The relationship between surface gravity and atmospheric density is not direct. Titan's surface gravity is lower than the moon's but it's atmosphere pressure at the surface is higher than the Earth's. Venus' surface gravity is slightly lower than the Earth's but it's atmospheric pressure is much higher than the Earth's.
Sigh. "Its" not "It's"
  #54  
Old 12-18-2019, 02:16 PM
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Or to be more precise, gravity is one factor that goes into determining atmosphere, but it's not the only one, and other factors are far more significant.
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Old 12-18-2019, 05:51 PM
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I get all this, what I don't get is how an aerospike engine uses the exhaust gas to optimize the ideal nozzle shape for a given velocity. I've tried a couple of Youtube vids but I'm not seeing how this works in reality. Can someone explain this like I'm 8?
As I understand the aerospike engine, it's just the inversion of a cone. The spike is internal, in the center of combustion, but shaped with a similar angle (starting thick then tapering to a spike, at the rate that a cone would flare out), while the cone is external to the combustion. It's a different solution to the same problem. It has it's set of advantages and disadvantages along with every other rocket exhaust nozzle. This one IIRC trades lower overall efficiency for a wider range 'not that bad' efficiency, and has it's issues like the spike overheats.

Last edited by kanicbird; 12-18-2019 at 05:52 PM.
  #56  
Old 12-18-2019, 11:27 PM
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Originally Posted by Dr. Strangelove View Post
Everyday Astronaut has a pretty good page/video on the subject.

You can get a pretty good hint as to what's going on with just this image, though. See how the ambient pressure presses the exhaust flow to the surface of the spike? That's how you achieve no flow separation at arbitrary pressure. At lower pressure, the exhaust stream starts to widen as it reaches the end of the spike, but it remains in contact. It widens just as much as it needs to in order to expand to ambient pressure.
I read through the article, thank you for posting it. I get it now!
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