Want to read Slashdot from your mobile device? Point it at m.slashdot.org and keep reading!


Forgot your password?
Check out the new SourceForge HTML5 internet speed test! No Flash necessary and runs on all devices. ×

Comment Re: Impressive spec (Score 1) 88

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) 65

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) 65

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) 88

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) 88

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) 88

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.

Comment Re:True and false - billion$ moving the wrong way (Score 1) 88

To be fair, we don't know that it was a COPV failure (although that would be most likely) - all we know at this point is that the failure was in the helium system and was unrelated to the strut failure in CRS-7.

While technically you can also produce other fuels off-world methane is indeed the highest throughput and efficiency, as sabatier synthesis yield by far the highest mass fraction as methane, so you don't have to do a lot of recycling of the methane to try to get your desired fraction as an output. Yet the mass is still mostly carbon, which is much easier to get than hydrogen (to the point that even a lot of in-situ production concepts have still called for bringing the needed hydrogen from Earth (reliable/sustainable fuel cell decomposition of CO2 to CO and O2 is pretty much a solved problem; reliable/sustainable permafrost brine mining and purification in Mars conditions, not so much). Methane is also cleaner burning, aka less likely to clog up your injectors and the like. Its disadvantages compared to RP-1 are its cryogenic nature and much lower density (increasing rocket bulk/cross section/mass and decreasing thrust). The lower density does however have its advantages as well - by virtue of requiring bulkier launch vehicles, you inherently increase fairing diameters, which allows for bulkier payloads.

Comment Re: Impressive spec (Score 1) 88

Chances are that even the sea-level version of Raptor will have its vacuum Isp in the 360-370 range anyway

"Chances are" in no way that a sea-level version of a vacuum-optimized rocket with an Isp of 384 will have an Isp of 360-370. Merlin-1D vacuum has a vacuum ISP of 348, but the nearly identical Merlin-1D designed for atmospheric use (same thing, just without the nozzle extension) has a sea level Isp of 282 sec.

Comment Re:Haha no. (Score 3, Insightful) 88

Indeed. Part of the reason that satellites are so expensive is how light you have to make everything; there's a huge amount of engineering that has to be done in order to achieve mass goals, as well as make use of the most expensive materials on the planet. An example is the use of things like top-end Spectrolab multi-junction solar cells, which get the highest efficiencies, but since they're basically lab-scale production hardware they cost two orders of magnitude more than what slightly less efficient panels on Earth cost (about $400/W).

Beyond the simple cost effect on engineering, there's the size effect. Look at James Webb and the massive expense they had to try to make it "origami" itself to fit into smaller launch vehicles. Or how many parts ISS had to be built out of in space, dramatically escalating both ground engineering / production costs and in-space assembly costs (the latter costing nearly $10m per man-day). There are serious expenses to trying to compensate for a lack of space or payload capacity when you really need it.

Then there's the size of the market. As launch costs have been dropping, the number of companies looking to launch payloads has skyrocketed. The skyrocketing launch demand has been reducing average payload development costs, as designs get more reuse. Both of these trends will continue as costs continue to decline. Meanwhile, new markets will continue to open up. Space tourism has always been hindered by the absurdly high launch costs, limiting it to only the wealthiest individuals. While it will remain a "small" market for the forseeable future, it can expand by orders of magnitude with reduced launch costs. Which again makes more demand..

Lastly, there's economies of scale. It's generally recognized in the rocketry world that - at least up to a point - larger rockets get a better cost per kilogram than smaller rockets. So wherein you can launch a lot at once - multi-satellite launches, large geo launches, large interplanetary probes, fuel depots for tugs/boost stages, shielding mass for manned missions, etc - you tend to save a lot of money by going big rather than using multiple smaller launches. The caveat is that just simply going big is no guarantee of a price reduction. Just like you can make an absurdly-expensive smaller craft, you can also make an absurdly expensive large craft. And even a "moderately priced" large craft isn't generally a "win" - if you can sell payload space for $5k/kg on a 10 tonne payload rocket and for the same price per kilogram on a 100 tonne payload, the vast majority of customers will choose the former. But if you're a company like SpaceX that's been delivering cost reductions on the small scale, and you can carry it over to the large scale, it gives you the potential to take the cost reductions even further.

If you can pull it off.

Comment Re:Human missions = funding (Score 1) 88

There is, of course, one situation that would change this, and it's what SpaceX is working towards: radical reductions in launch costs. Just like it's a common oversimplification of manned space exploraiton advocates to pretend like humans can fix anything and do a million times more than robots, it's often a weakness on our side to dismiss how much it actually costs to engineer long-term reliable high-throughput autonomous vehicles (probes, rovers, etc). Right now, they pretty unquestionably provide a better cost return on scientific data gathering. But this isn't some fundamental law of nature; it's only this way because launch costs are so high, so miniaturization and reduction of consumables is so critical. Change the launch cost picture dramatically, you change the optimal parameters for exploration.

That said, this isn't really the driving force. The driving force is, humanity becoming a multiplanet species would be a very good thing, both in our ability to survive catastrophic events and for our ability to bring the vast resources of space into our reach. Nobody who's being realistic about it thinks that some offplanet colony could be independent any time even remotely soon. But you have to start at some point. That "start" means bringing costs down and doing the required engineering. And I see no need to damn SpaceX for doing so.

Comment Re:Human missions = funding (Score 1) 88

How much excitement did that ever generate? The mars rovers landed in 2004 and NASA's budget (in constant dollars) has only gone down since.

Yes, nothing since to Mars since 2004! Excepting of course MRO, Curiosity, MAVEN, InSight, and the upcoming Mars 2020 Rover. Some of those being much more expensive than the MERs.

. There's no reason for the government to throw money at Musk that they wouldn't throw at NASA

Nor is there any expectation nor plan of such. SpaceX and NASA have a no-cost arrangement, where NASA provides free consultancy, access to the DSN, and sterilization assistance, while SpaceX will provide any data they collect.

and Musk certainly can't afford to do this on his own.

The opinions of experts interviewed on the subject are almost in universal disagreement with you on this, at least concerning Red Dragon. SpaceX already shoots things up to GEO; the energy required to get to a Mars Transfer Orbit is not much more. SpaceX already decelerates capsules with an aeroshell facing an entry load similar to that needed for aerocapture at Mars. And Dragon is already designed for automated vertical powered landing. Red Dragon is not some super-ambitious mission, it is quite doable without any radical advancements.

Actual colonization, boots on the ground, is of course a much bigger challenge. This will be many years in development. But SpaceX was specifically founded for this purpose. Just like how Tesla was founded when Musk found that he couldn't pay anyone to build him a clone of the tzero, SpaceX was founded when Musk found that nobody could launch him payloads to Mars for an affordable price. Ultimately getting humans to Mars is a founding principle of SpaceX. All of its investors are aware of this. SpaceX will continue to work towards this so long as they are an operating entity. And unlike NASA, they're not burdened with massive mandated costs and constantly changing mandated missions, both of which have utterly crippled it over the past decades.

Slashdot Top Deals

Top Ten Things Overheard At The ANSI C Draft Committee Meetings: (8) I'm on the committee and I *still* don't know what the hell #pragma is for.