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41
SpaceX Reusability / Re: F9 Second Stage Reusability
« Last post by speedevil on Today at 01:52 PM »
Remember that BFS is designed for maximum-payload EDL for Mars, not Earth. Two very different regimes, design changes to optimise BFS for Mars entry from interplanetary velocities may not be optimal (or even counterproductive) for Earth EDL from orbital velocities.
It really wasn't.
Unless you choose to entirely ignore IAC2017 and P2P, which in concert with the later tweet about 'adding an engine for safe engine out landing' pretty much implies landing nearly maximum weight BFS on earth, repeatedly.

It is also an interesting coincidence that reentry from GTO, and reentry from interplanetary transit from Mars are comparable, as are reentry from LEO and interplanetary entry to Mars.
(at least at near minimum energies).
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Quote
"We could send people to Mars, and decades ago. I mean, the technology that took us to the moon back when I was just a kid, that technology can take us to Mars but it would be at significant risk," he said. "The majority of the astronauts that we send on those missions wouldn't make it. They'd die. Because the technology is still quite primitive."

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"We're sort of like those early sailing ships, in that we don't even know what we don't know yet," he said, referring to the historic voyages of Columbus, Magellan, and Cook. "I think we need some more improvements in technology before we'll cross the oceans that are between us and Mars in any sort of practical way."

Well, the fastest way to mature technology and learn what we don't know that we don't know, is to have a short-term programme in the first place. You're never going to know what you don't know, or mature systems before you've actually done the thing, no matter how many magic engines materialize without a clear need for them. If it turns out they're really necessary or beneficial for continued human presence on Mars, the new technology will materialize much faster.

I totally agree there are a lot of technologies that still require a lot of development before humans can survive on Mars and get back safely. However, propulsion technology by itself is not high on that list. And there are ways of maturing all of the subsystems without sending humans to Mars as guinea pigs. Not more than astronauts have always been guinea pigs, that is.

Like going to the Moon for instance?
An immediate and productive effort to establish a base on the Moon would completely negate his argument.
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The ISS has one.  Lightweight, it is not.  Here's why.

You calculate above the theoretical weight of a linear system, or rather estimate it by using "1000x lighter" type multipliers.

Even if that calculation was true, you have to distribute heat over an area.  The lateral conductivity of thin films is very low. So you need to drag your heat distribution tubing all over the place to cover almost literally every square inch (or, use a thicker and conductive material for the radiator).

Either way, a problem.  If you have thin tubing, pumping requires higher pressurres. MMOD becomes an issue.



The ISS radiator systems have a deployed full system areal density of 8.8 kg/m^2, so a 1000 m^2 MW class array would be 8800 kg, which is not entirely impractical.

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This is getting too technical and OT.

The root cause is that you need to build terrestrial-scale infrastructure, and there isn't a way to make that happen gradually and organically - not even with a Trillion dollars.

Industry needs to grow while being profitable. That's what differentiates a business plan from a lofty goal.

I would love for Bezos to show his plan. There is no reason to keep it a secret, as SpaceX has shown. I suspect however that there isn't one.

He'll go for tourism if that proves profitable, and maybe the aforementioned oneWeb play once New Glen flies.

Agreed. I don't see a killer app for industry in LEO or cislunar space, yet.

But I think tourism and comms can lay the infrastructure foundations, and then perhaps industry will follow.
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Why do we call schedule delays Elon time? SLS and Orion are behind schedule. Happens in nearly every industry. This isn't unique to Elon.

If BFS initial testing is like Grasshopper testing, SpaceX will need a big concrete pad and support equipment. Won't take too long to build.
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SpaceX Reusability / Re: F9 Second Stage Reusability
« Last post by envy887 on Today at 01:34 PM »
Good point, but that was also the case with all first stage rocket nozzles until SpaceX tried it with their S1. Admittedly the S2 vac nozzle is much longer and I would guess less good at withstanding those stresses flying backwards, but who knows until they try it..

Why is there such minimal heat-shielding on S2? Is radiative heating less of an issue than hot gas recirculation? What base heat shielding is there generally on the bottom of a rocket booster that isn't designed to return tail first?

Boosters have atmospheric hot gas recirculation, and generally have much greater areal thrust density because they have more mass to lift atop the same cross-section area. Both of those get the base quite hot; if you look at pictures of the base of Atlas V, Delta IV, or the engine section of the Shuttle Orbiters, you can see the thermal protection systems required, including blankets around the gimbaling engine nozzles.

Compare those to pictures of the tail end of Centuar, Falcon upper stage, Delta upper stages, S-IVB, and S-II, and you will see a distinct difference in thermal protection.
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The ISS has one.  Lightweight, it is not.  Here's why.

You calculate above the theoretical weight of a linear system, or rather estimate it by using "1000x lighter" type multipliers.

Even if that calculation was true, you have to distribute heat over an area.  The lateral conductivity of thin films is very low. So you need to drag your heat distribution tubing all over the place to cover almost literally every square inch (or, use a thicker and conductive material for the radiator).

Either way, a problem.  If you have thin tubing, pumping requires higher pressurres. MMOD becomes an issue.

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This is getting too technical and OT.

The root cause is that you need to build terrestrial-scale infrastructure, and there isn't a way to make that happen gradually and organically - not even with a Trillion dollars.

Industry needs to grow while being profitable. That's what differentiates a business plan from a lofty goal.

I would love for Bezos to show his plan. There is no reason to keep it a secret, as SpaceX has shown. I suspect however that there isn't one.

He'll go for tourism if that proves profitable, and maybe the aforementioned oneWeb play once New Glen flies.
Not so fast...  When your array is that big, its easy (relatively) to conduct electricity inwards...  But much harder to pump heat outwards.

It feels like you are playing "gotcha!" here after I just finished saying that I thought that was the more significant problem.  To repeat myself, I think the heat distribution is the more significant concern.  The reason I talked about the heat exchange was because that was what you were talking about.

In regards to the distribution, I am even less fluent in thermal conduction then in radiation so it's more difficult for me to say.  However I will note again that I think datacenters would be in LEO, where they would need considerably larger arrays for the necessary power due to the shadow.  This means that the equilibrium temperature for the radiation would be even lower.  This would allow for an even larger gradient of temperature between a computer chip kept at temperatures above 0 Celsius and the radiator panels.  It seems to me that if there is a temperature gradient of around 30 degrees or so, it should be possible to design the system to achieve the task with passive thermal conduction.  It may even be possible to take advantage of the heat gradient for a small amount of energy reclamation.  If you are apt with thermal conduction calculations and would like to explain things better I would welcome the insight.  But if it's just a gut thing, 100 meters of distance with a 30 degree difference or so seems pretty reasonable to me.  Maybe the centers would be hotter then earth, maybe colder, but it feels like it would be in the ballpark of earth.

A copper conductor to move 1 MW of power 100 meters under 30 dT will need 0.833 m^2 area and mass 749,700 kg.

Even flowing water is 100x less area and 1000x less mass than copper conductors, so a pumped water system would be more like 1000 kg, plus the mass of the pump.

And in microgravity it's possible to build phase change heat transfer systems that are far better than passive conduction or water flow, both per area and per mass. I'm not sure exactly how much better, but IIRC it's in the 1,000s to 10,000s of times, which would put a 1 MW system in the 100s of kg. Both phase change and water flow mean messing with fluids though, which could be a pain.

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ABCD: Always Be Counting Down

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Are you not aware of Elon time?  This is Rocket Science.  It's hard.  This far out, all timetables will almost certainly slip.
I'll be happy if they only get the launch pad concrete work done before the end of the year.
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SpaceX BFR - Earth to Deep Space / Re: Elon The Boring Company
« Last post by jebbo on Today at 01:25 PM »
Now it's built, the guided bus is great ... stops 3 minutes walk from my office :-)
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Anyone have any reliable sources that are recent as to whether SpaceX still plans on constructing this site on Boca Chica?

They will be testing BFS there as early as H1 2019...

We should avoid making unconfirmed statements that sound like facts.
What is unconfirmed?

https://mobile.twitter.com/SciGuySpace/status/994655712239407105

https://spaceflightnow.com/2018/03/13/musk-atmospheric-tests-of-interplanetary-spaceship-could-happen-next-year/
Are you not aware of Elon time?  This is Rocket Science.  It's hard.  This far out, all timetables will almost certainly slip. 
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If BFS costs $200M, that sets an obvious floor on a space station at $250K/m^3, which seems to be moderately close to prices that have been implied for the BA-330. And - well - if you want, you can deorbit it at any time. (in LEO at least).

But, if you don't care about mass, or inflatability, or anything fancy, you can get shielded 300m^3 or so 6m internal diameter modules up for not much more than 3* launch cost of BFS, even without any on-orbit assembly.

Buy 6m aluminium inch thick tank, don't tell them it's for aerospace, pressurise to 150PSI a few times to test it, add 1m of plastic water tanks to the outside (empty), glue on aluminium foil, fill in orbit, and you've got a pressurised shielded volume that you can outfit at your leisure.

Inflatables still allow much larger volumes, though. And BFR only makes them cheaper.

For example, a single BFR launch could lift a 140 tonne, 100 meter deflated Kevlar sphere with 3 mm thick walls (5x safety factor) and a 10 tonne docking port/service/propulsion module. It would take 5 BFR flights of liquid air tanks to pressurize it to 1 atmosphere, but then you have a volume equal to 635 BFSes or 1600 BA-330s. Figuring out how to manufacture that sounds like a Bigelow specialty.

It would take some outfitting to make that volume useful though, unless all you wanted was an orbital bouncy castle :)
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