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Comment Re: Impressive spec (Score 1) 109

Well would you look at that indeed! I argued for a loss of 348-282=66 sec in Merlin, and said that Raptor would be somewhat less of a difference but not much, as "chamber pressure has a positive but fairly weak correlation with ISP". You said 384 - "360-370" = 14-24 sec difference.

And the reality is... drumroll... the envelope please...

384-334 = 50 sec

I hope this has been a learning experience for you.

Comment Re:Wat (Score 2) 90

This is not correct. Juno is planned to do some limited observation/a> of the Galilean moons. It's a side mission, not central to it's focus (and Juno is anything but optimized for it), but it's one of those cases where, if you're there and you have the hardware...

Concerning Europa (remember that this was before the recent news):

The most significant opportunity for Juno to do Europa science would be to follow up on the plumes possibly detected by Hubble Space Telescope. Confirming Hubble's detection would be very scientifically valuable. Any information on the source location would be valuable. This science goal just may not be possible with the large distances from Juno to Europa, but we will look.

JunoCam or ASC can only detect plumes if they contain fine particles. The Hubble discovery (if real) only shows the presence of water vapor. We can predict by analogy to Enceladus that water vapor plumes will also contain particles. However, it is important to remember that the Hubble discovery was of gas, not particles. If the putative Europa plumes are Enceladus-like and do contain particles, they would not be as tall as Enceladus', because of Europa's higher gravity. Scaling for Europa’s gravity gives a maximum plume height of under 140 kilometers. To detect plumes, we need at least two pixels, so the image spatial scale would need to be better than 70 kilometers, at a relatively high phase angle where the particles would forward-scatter light to JunoCam and ASC.

To achieve resolutions better than 70 kilometers per pixel, UVS needs to be within 40,000 kilometers of Europa; JunoCam, 100,000 kilometers; and ASC, 170,000 kilometers. For the cameras, given the low expected height of the plumes, there is not much flexibility.

There are just four orbits that have Europa flybys that are closer than 300,000 km. Juno reaches the best available geometry in September 2017 as the rotation of the line of apsides brings Juno’s orbit close to Europa’s orbit:

2017-03-08 253,118 km
2017-09-19 264,043 km
2017-10-03 92,267 km
2017-10-17 204,654 km

Comment Re:So how is it supposed to communicate? (Score 1) 90

It's pretty limited what you can gather from individual grains captured at hypersonic velocities and analyzed with spacecraft-sized instruments. Certainly there was no "clear evidence of life" from Enceladus - although it showed us some very promising things about the potential habitability of its oceans.

Personally, I'm not a believer in the theory that wherever there's liquid water, there's life. First off, it'd make the Fermi paradox even worse, as water is bloody everywhere. Secondly, I think it's incredibly naive. The argument goes, wherever we find water on Earth, we find life, and whereever we don't, we don't, so we should expect that with the universe. But that says nothing about how life came about. Sure, LAWKI requires hydrogen, and water is the most convenient source of hydrogen, so obviously that's going to form the boundaries of where life has spread to. But where it's spread to says nothing about where it originated, or what it looked like when it did. We have no reason to think that the entire wet surface of Earth just spontaneously erupted into life; we certainly don't see anything resembling this in laboratory abiogenesis experiments. So what were the specific conditions that brought life about? I think it's a safe bet that they were rare. Quite likely no longer present on Earth, as Earth was a radically different place back then. And quite possibly rare in the universe as a whole. Little bursts of luck separated by great relativistic distances.

Indeed, bodies like Europa (and the many other bodies confirmed to or believed to have subsurface water in our solar system) should help answer these questions. I'm also exceedingly curious about what's gone on with alternative solvents and polymeric compounds, such as at the surface of Titan (I find the cyanide chemistry there fascinating, it seems to be extremely flexible).

Comment Re:Wow (Score 2) 158

Yeah, landing the booster right next to the refueling tanker seems little,eh... optimistic

The video is clearly stylized and not meant to be taken that literally. Unless you think the arrival of the spacecraft is supposed to make Mars spin until it develops oceans ;)

That said, while there's much to like, there's one aspect of it that's really clawing at me... the fact that they plan to make it out of composites. Including the LOX tank. We've never succeeded (and failed multiple times) at making flight-intent LOX tanks for orbital rockets. And they want to make the first time be on what's by far the largest rocket ever built? Without a lining?

Is it worth mentioning that they just had an explosion somehow related to the only major carbon fiber component in the Falcon 9 in a LOX tank?

CF becomes brittle in LOX. It leaks. And most concerningly, it's impact / shock sensitive in LOX. At atmospheric pressure it usually won't do a self-sustained burn on impact, but it chars on impact, and even that alone would be bad. But they plan to have significant pressure as well. He mentions briefly that they expect this to be one of the biggest challenges, getting stable coatings and the like. I think that's an understatement.

I just don't want to see the largest rocket ever built turn into the largest flying fireball on Earth. I don't trust composites with LOX. Composite cryogenics tanks are an active research topic, and they're making progress, but it's not a solved problem.

Comment Re: Impressive spec (Score 1) 109

Why compare apples with oranges?

Why show how much sea level and vacuum ISPs vary in other hydrocarbon engines? Because they vary that much in all engine, even non-hydrocarbons (same sort of difference in LOX/LH and solids). Methane is not some sort of magical exception to the rule.

The RD-0162 is the closest unit you can compare the Raptor with. It pushes all its propellant mass through the chamber. It uses the same propellant mixture. Therefore the real world vacuum performance of the sea-level version Raptor can be best guesstimated from the RD-0162 figures rather than by comparing it with dissimilar units.

No, they cannot. You have no clue whatsoever how the efficiency of the RD-0162 compares to Raptor. Not in the slightest. Which makes it a pointless comparison.

Comment Re:So how is it supposed to communicate? (Score 4, Insightful) 90

I think you're confused. Plumes means "in space". The whole benefit of plumes is that you don't need to go under the ice at all, you can do flybies to collect ice particles, or have a lander observe and sample the plumes at the surface. The key is that it means a recent connection between the depths and the surface, and that would be huge for simplifying exploration.

We're nowhere near to being able to launching an ice boring / swimming probe. If I recall correctly the last thing I read on the subject, however, the most promising means for communicating with such a probe on an affordable mass budget was.... not communicating with it. Aka, having it fully autonomous - melting its way down, sampling/observing the ocean, then re-melting its way back to the surface - then and only then transmitting. The waiting period with no data would be stressful (as if it failed you'd never know why), but it could potentially be used on almost any icy solid body regardless of the ice thickness.

It's also possible that there's liquid water much closer to the surface than the global ocean. There are some inferred lakes at a depth of only a few kilometers, which is potentially short enough for a probe to maintain a fiber connection with the surface. And after JUICE and Clipper, we may well have found locations that are even shallower.

Comment Re:Wat (Score 3, Insightful) 90

Right. Because brief observations from an already present spacecraft that could help make critical design decisions about an upcoming multibillion dollar mission are an absurdity.

Look, we know Juno wasn't designed for this sort of mission and is not well equipped or positioned for it. But if researchers determine that its observations could help pinpoint more details of the plumes, then yes, they damn well should regardless of whether "tomhath at slashdot" considers that to be "real science" (apparently some vague category that he doesn't even feel the need to expand upon -- apparently planetary scientists have been working on "fake science" all these years, who knew?).

Comment Re: Impressive spec (Score 1) 109

Look, I'm sorry but what sea-level Merlin does or doesn't achieve is totally irrelevant for Raptor - even for sea-level Raptor.

Because pointing out the typical difference between vacuum and sea level performance in hydrocarbon engines is "totally irrelevant" in a discussion about the difference in vacuum and sea level performance in hydrocarbon engines?

The fact is that the 17 MPa LCH4/LOX sea-level RD-0162 is rated for 356 s of vacuum Isp, so the 30 MPa (+76%!) LCH4/LOX sea-level Raptor is definitely going to be in the 360+ s vacuum Isp territory.

I see your argument - all engines for a given propellant mixture are identical except for only one varying parameter (pressure). Why it's so simple, why didn't I think of that? ;) *snicker*

Meanwhile, back in the real world, performance varies widely between different engine families, and there are many factors that affect them. What you're doing is equivalent to saying "Because my gasoline hybrid engine is super efficient, then your non-hybrid gasoline pickup truck engine should be too!" If you want to compare the performance of vacuum engines to sea level engines, you need to compare for the same engine.

There is nothing magical about methane that makes it somehow, unlike all other fuels, have a tiny difference between ISPs in optimal vacuum vs. optimal sea level designs.

Comment Re: Impressive spec (Score 1) 109

The sea level version of the aforementioned Merlin 1D is 311. Not "~340". We're comparing different nozzle versions of an otherwise identical engine. You don't lose a mere 15-25 sec ISP when losing your nozzle extension and operating at sea level. Period. That's just not reality. If you think for some reason that the Merlin-1D is a bad comparison, pick another engine with otherwise identical vacuum and sea level versions, and cite the vacuum ISP for the vacuum version and the sea level ISP for the sea level version. The sea level ISP will always be vastly lower, not a mere 15-25 sec. I strongly challege you to find a single engine where the difference even remotely approaches your figures.

And to be clear, Merlin 1D is already a fairly high pressure engine, 100 bar is no slouch. And it's not some sort of linear relation with pressure because a lot of the heat is from the internal energy difference between the high-pressure and low-pressure exhaust; it's not simple thermal expansion, as there's a change in the reaction equilibria and in some cases release of the latent heat of vaporization. Chamber pressure has a positive but fairly weak correlation with ISP; most people overestimate its influence.

Comment Re:coal???? (Score 1) 109

A coal-LOX hybrid rocket would work, and the ISP wouldn't be too bad (though hydrogen-rich fuels would be better). It'd be an interesting challenge.. normally with hybrids you want the fuel to melt and whip up into droplets at the surface to increase their surface area, but I imagine with a coal hybrid you'd want it to break up into a dust. So maybe fine coal dust with a paraffin or polyethylene binder.... that'd actually probably have excellent thrust performance.

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