Recent comments in /f/space

DeanXeL t1_j9o10qo wrote

Reply to comment by TegTowelie in What are some of the major goals we hope to achieve, or discoveries that we hope to make, with the JWST? by m_and_t

How do you suggest that would happen? If they come over here to mess with it, why wouldn't they just move sliiiightly over to the side to come visit us? If they have detection capabilities to notice an insignificant speck of dust like the JWST in the vastness of space, they can also see our entire huge planet, and could've been "interacting" with us all along.

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Decronym t1_j9nvd63 wrote

Reply to DARPA is Reigniting the Nuclear Engine by Afrin_Drip

Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:

|Fewer Letters|More Letters| |-------|---------|---| |DARPA|(Defense) Advanced Research Projects Agency, DoD| |DoD|US Department of Defense| |ISRU|In-Situ Resource Utilization| |Isp|Specific impulse (as explained by Scott Manley on YouTube)| | |Internet Service Provider| |LEO|Low Earth Orbit (180-2000km)| | |Law Enforcement Officer (most often mentioned during transport operations)| |LOX|Liquid Oxygen| |NEV|Nuclear Electric Vehicle propulsion| |NTP|Nuclear Thermal Propulsion| | |Network Time Protocol| |NTR|Nuclear Thermal Rocket| |ULA|United Launch Alliance (Lockheed/Boeing joint venture)|

|Jargon|Definition| |-------|---------|---| |cryogenic|Very low temperature fluid; materials that would be gaseous at room temperature/pressure| | |(In re: rocket fuel) Often synonymous with hydrolox| |hydrolox|Portmanteau: liquid hydrogen fuel, liquid oxygen oxidizer| |methalox|Portmanteau: methane fuel, liquid oxygen oxidizer|

^(13 acronyms in this thread; )^(the most compressed thread commented on today)^( has 32 acronyms.)
^([Thread #8600 for this sub, first seen 23rd Feb 2023, 08:44]) ^[FAQ] ^([Full list]) ^[Contact] ^([Source code])

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MrTurdFace69 t1_j9nvcnk wrote

Reply to comment by i_can_has_rock in Time dilation question by [deleted]

There is no true time constant. It's all relative, as Spacetime is curved by mass/energy. So there doesn't exist a point that isn't affected by mass/gravity, that we know of.

There is no end to the gravitational field a black hole produces. In fact there is no end to the gravitational field that your body produces.

They both extend the entire universe. However, far enough away, their effects become negligible.

If there are two 1kg spheres of metal in space, and they are 1 meter apart. Then, to the human eye, it seems like they experience no gravitational effect.

Yet they will experience the same gravitational effect that 2 earth mass objects would experience when they are 6x10^25 meters apart.

I think what you're trying to understand is sort of what Einstein was trying to understand when he was figuring out how to imagine what happens if you could travel along side by side with a beam of light. Which would basically mean you experience no time.

However, he concluded it's not possible to accelerate to that speed and that the speed of light was constant for all observers, and hence came up with the special relativity theory. Which eventually led to general relativity. Which, ironically in this discussion, predicted black holes haha.

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Shrike99 t1_j9nuiwe wrote

Reply to comment by nic_haflinger in DARPA is Reigniting the Nuclear Engine by Afrin_Drip

>Starship would be better utilized to build a fast human Mars transport vehicle in LEO than being used directly as the crew transport.

I agree, but the way to do that is with NEP, not NTP. Or at least not of the solid core variety anyway; gas core NTRs might do the job, but it doesn't seem likely they'll be a thing anytime soon.

Solid core NTRs don't really get you to Mars any quicker than Starship. With an Isp of 900s and a mass ratio of 5 you're looking at 14.2km/s. Starship with it's 380s Isp and ~12 mass ratio only gets 9.2km/s - 5km/s less.

On the face of it, that seems like a 50% speed increase, which is nothing to scoff at. The problem lies in slowing down at Mars. Starship is able to aerobrake, saving it ~4km/s of delta-v as compared to propulsively braking into orbit.

So there goes most of that 5km/s lead for the NTR stage, and the remaining 1km/s has to be split in two - a 500m/s higher cruise speed also incurs 500m/s more braking requirement - a much more modest 6% speed increase.

You could fit the NTR stage to aerobrake as well, but given the massive size of the tanks and the high dry mass of the NTR, it's likely to suffer a proportionally larger performance hit than Starship paid for the same capability.

 

Now, to an airless body such as Ceres or the Jovian moons it's obviously a different story and the NTR takes a considerable lead over Starship. However, NEP's advantages over NTP grow even more pronounced over larger distances, bringing me back to my original point.

The only real exception is of course Luna, where you can't aerobrake, and it's very close proximity puts NEP at a huge disadvantage.

NTRs perform a bit better if you focus on payload capacity instead of speed, but only in terms of mass fraction. In terms of cost the pure hydrogen and enriched uranium might well cancel out any mass savings if you've got cheap LEO lift. And in terms of ISRU, pure hydrogen is an order of magnitude more energy intensive to produce than hydrolox or methalox.

I'm definitely a Starship enthusiast, but I don't think Starship 'solves all problems'. It solves many problems, and NEP solves most of the other ones, while NTP only solves a few niche ones in between, so I have to wonder if it's really worth the trouble.

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space-ModTeam t1_j9ns8km wrote

Reply to Time dilation question by [deleted]

Hello u/Siliskk, your submission "Time dilation question" has been removed from r/space because:

  • Such questions should be asked in the "All space questions" thread stickied at the top of the sub.

Please read the rules in the sidebar and check r/space for duplicate submissions before posting. If you have any questions about this removal please message the r/space moderators. Thank you.

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TheMiddleBoogerHook t1_j9ns69y wrote

Reply to Time dilation question by [deleted]

Time has great influence on perspective.

Ive always wondered if time moved slower for flies than humans. So them buzzing around dodging my hand means I move slower by their perspective as if they are able to finish a conversation before moving out of the way.

Or whales move slow because they are big. But they feel time faster.

Scientists found that adrenaline changes perception of time... how time is perceived can be different.

I think time dilation is a real mechanism depending on location but most importantly, time is perspective by the observer.

So maybe if a planet on the outer rim of a galaxy, travelling slower, might be larger in size and gravity have more effect making them shorter and smaller. Maybe what feels like 30 minutes to us feels like 25 minutes to them simply because their size difference and chemical makeup inside whatever mechanism their body uses to perceive time.

A thought ive had since I was a child.

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space-ModTeam t1_j9ns1cc wrote

Reply to Honest question, what if we accepted the assumption that God created the universe 6,000 years ago, could this explain away dark matter and galaxy rotation? by DrMilzie

Hello u/DrMilzie, your submission "Honest question, what if we accepted the assumption that God created the universe 6,000 years ago, could this explain away dark matter and galaxy rotation?" has been removed from r/space because:

  • It is not related to space.

Please read the rules in the sidebar and check r/space for duplicate submissions before posting. If you have any questions about this removal please message the r/space moderators. Thank you.

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