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SpaceX BFR - Earth to Deep Space / Re: BFS Engineering Thread
« Last post by envy887 on Today at 07:25 PM »
That was in hypersonic glide... This vehicle is in ballistic flight mimicking a capsule...

I would like to see that "work" as then the canard and rear surfaces are useless
They’re effectively airbrakes, not wings. With all due respect the late lamented (or not) Steve Jobs, think different.

anything is possible I guess but if they are used as pure drag devices to maintain pitch and roll the 1) aeerodynamic forces on them will do nothing but grow as the vehicle reenters..producing enormous bending motions and 2) the heat load will be enormous..

Watch the entry sim shown (twice) in the stream last night. The vehicle does a belly-flop at extremely high AOA for the entire entry. The fins are definitely acting purely as drag devices, aka air brakes.

Yes, bending and temps will be very high.
At startup I didn't see the green flash.
Looks like they now have spark ignition working.  ;)

Here is the Raptor startup in GIF form...
Space Science Coverage / Re: Pluto-Planet debate discussions
« Last post by DRussell on Today at 07:21 PM »
So add a definition for "satellite planet" to clean this up.

Why not just continue to call them moons and if more definition is needed then add major and minor moons.
Any spherical moon that gets ejected from the solar system would automatically be reclassified as a rogue planet.
Any rogue planet that is captured into solar orbit would automatically be reclassified as a planet of the appropriate type; major/minor/dwarf.

That is another good example - a captured rogue planet.  That is really the point.  These bodies are all the same broad geophysical category.  Spherical sub-stellar bodies formed in a proto-planetary disk undergo internal differentiation and have physical and chemical alteration from the raw material state.  The idea is to create a separate category.  It could be "major moons" and "minor moons".  The advantage of "satellite planet" is that the IAU has already established a connection between the word "planet" and a spherical shape.  So they have "planets" and "dwarf planets" that are spherical bodies and there are "rogue planets".  If you call spherical moons "satellite planets" you complete the set.  The word "planet" is fully connected to the geophysically significant spherical bodies.

It probably would not be enough to satisfy Dr. Stern, but IMO by completing the taxonomic framework for sub-stellar bodies by adding the "satellite planet" category would make it easier to put the Pluto demotion into a full context. 

And when you start talking about changing dynamical circumstances it is easier to discuss.  So Triton was a dwarf planet in the past but is now a satellite planet.   If Triton was ejected from the Solar System in the future it would be a rogue planet.   The word "planet" connects all these circumstances.
Presumably the aft cargo is going to be most relevant for a moon mission as I'm pretty sure they'd need Raptor vacs to get to Mars.

In the presentation Elon said that this version can deliver 100 tons to the Martian surface. So no, there is no need for Raptor vacs for Mars. Those potential performance upgrades would be happening years down the line, once version 1 is flying regularly, and they seem to want to get to Mars ASAP.

Point being SpaceX again has correctly decided to build a good enough to get the job done rocket and fly it.  It has future upgradeability to reach the nirvana of elite specmanship.

Last decade I was doing a high tech startup company.  One startup CEO I talked to gave me some great advice.  He said, "Tone down your architectural elegance and build something along your path that can get to market ASAP and get some revenue."  Being the lead engineer/architect/VP I was in love with my concept and did not like the advice given.  But later when push came to shove I thought harder, downsized and built a less comprehensive focused product.  It took longer than I said it would and more R&D $, but it worked spectacularly, exceeding its low cost goals, beating the Five Nines reliability and meeting all performance specs.  Had I stuck with my original approach, it would have been utter disaster.

Build a Mars capable, low cost, robust, re-useable spacecraft and FLY IT.  Then, you have lived long enough to build a better one later.  That's the SpaceX way.

Yup, this is the key difference between SpaceX and Oldspace. Oldspace puts every bit of effort into squeezing the maximum amount of performance and efficiency out of their product, regardless of cost or schedule (a huge reason for this being that everything is expendable).

SpaceX is very much of the startup mindset: get your Minimum Viable Product out the door as soon as possible. Just build something that works. It doesn't have to be the best, just get it out there, start testing it, and improve on it from there. It's how Falcon 9 went from an expendable vehicle using 4,940 kN engines to lift 10,450 kg to LEO, to a fully reusable booster with 7,600 kN engines and a 22,800 kg capacity to LEO. Had they gone straight for the highest performance possible they would have spent decades in development and probably never even gotten there. BFR is already incredibly impressive and there's only room to grow to bigger and better numbers in the future.

I can't wait.
SpaceX BFR - Earth to Deep Space / Re: BFS Engineering Thread
« Last post by meekGee on Today at 07:15 PM »
For refence:

This is a plane that's supposed to fly aerodynamically, and has trouble recovering from a state of spin.

BFS is designed to "fly" stalled.  You can't stall it any more than it already is...  And it's not trying to recover from that state, it's trying to remain stalled.

It's an actively stabilized barn door, trying to remain largely perpendicular to the airflow.  It's not trying to glide, or achieve range, or anything like that.

They'll have issues for sure, but this won't be one of them.

Measuring off the video and assuming the handrails are a standard 42" height, the Raptor shown is 1.1 meters diameter and slightly underexpanded.

This fits with the full-size Raptor being 1.3 meters and optimally expanded or slightly overexpanded at SL, and with this being a full pressure or near full pressure test (since at lower pressure it would look overexpanded).

(added image)
Looks still slightly overexpanded in image. Look at flame dia. at mach disk, it looks smaller than nozzle dia. So looks like flight spec. nozzle on Raptor being tested.

This is of great general education interest. The simple public illustrations of over versus under expansion by visual inspection overlook this detail of complex plume features. 
How does mach disc diameter relate to nozzle exit diameter and pressure?

Maybe this question and answer belong in the questions and answers forum for better discoverability?
At startup I didn't see the green flash.
Looks like they now have spark ignition working.  ;)
Space Science Coverage / Re: Pluto-Planet debate discussions
« Last post by DRussell on Today at 07:09 PM »
Of course that leads into the messy brown dwarf/giant planet end of things.   Do you include general formation mechanism (star-like formation vs. proto-planetary disk core accretion formation) to distinguish brown dwarfs from giant planets or not?  It makes a difference because the mass regime for core accretion and star-like gas collapse mechanisms overlaps.  What do we call a spherical body orbiting a brown dwarf?  If we go by formation mechanism then brown dwarfs form like stars and bodies orbiting brown dwarfs are planets. 

And what about rogue planets?  A true rogue planet should not be a body that formed from gas collapse mechanisms.  It should be a body that formed in a proto-planetary disk around a star and was subsequently ejected from the orbit.   Many bodies currently called "free floating planets" or "rogue planets" are really just bodies formed by star-like gas collapse mechanisms that are smaller than the deuterium burning limit.

Per Schlaumann (2018), there seems to be divide at around 10 Jupiter masses between objects tending to form like stars and objects tending to form like planets. I have no problem defining a brown dwarf as an astronomical body with sufficient mass to have preferentially formed by gas collapse (>10 Jupiter masses) but insufficient mass to ever sustain hydrogen fusion in its core.

Undoubtedly there is some overlap in reality, and with that definition some objects that formed like planets may get labeled as brown dwarfs (and vice-versa). But knowing every object's evolutionary history will be challenge, so I think applying that divide in practice may be the most sensible course of action since it is based on an easily measured variable. It is also preferable to the deuterium-burning divide at around 13 Jupiter masses, since the latter is arbitrary and is not significant in an object's history.

What Schlaufman found is that there is a transitional mass range between 4 and 10 Jupiter masses where objects lower than 4 MJ form by core accretion and above 10 Jupiter mass preferentially form via gas collapse.

This is consistent with what has been found for the sigma Orionis cluster 18 years ago.  The cluster has free floating bodies down to ~5 Jupiter masses which formed like stars:

Santos et al. 2017 also find that there is a transition at 4 Jupiter masses indicating a difference in formation mechanism:

So in my opinion, if there is going to be a single mass limit dividing giant planets and brown dwarfs, then 4 Jupiter masses should be that limit. The reason I would go with 4 Jupiter masses is that bodies formed via gas collapse will not be smaller than 4 Jupiter masses so there would be no contamination of the <4 MJup population by brown dwarfs.   There will be some contamination of the > 4 Jupiter mass population by bodies formed via core accretion.  If you adopt 10 MJup as the limit then you have contamination both above and below the mass limit.  There will be some gas collapse bodies smaller than the 10 MJup limit and some core accretion bodies exceeding the 10 MJup limit as some authors have argued that core accretion could in fact form bodies exceeding the deuterium burning limit.

However, regarding the difficulty of identifying formation history I have two thoughts.  First, it is possible to identify formation mechanism with the right observations.  For example, core accretion will lead to metal enhancement relative to the parent star.  Orbital circumstances also have the potential to differentiate formation mechanisms.  So that leads to thought two.   There is a concept at play here.  Some sub-stellar bodies form in a proto-planetary disk and others form via star-like gas collapse mechanisms.  We don't have to know for certain which category every single object observed actually falls into to define the difference.  You could have a subset of observed bodies with classification TBD by future observations.   They might be brown dwarfs or they might be giant planets.  That is not a problem if we have the conceptual framework laid down.   

However, if we sacrifice the conceptual framework over current observational difficulties we are just kicking the can down the road and will have to deal with it again later when improved observational data allows us to make these determinations.  Why not establish the concept now with the understanding that a handful of objects will have a "might be" status.
IIRC 39A should be large enough for the current 9m dia BFR so I don't see the need for a floating platform there. I think that is most likely a reference to Boca Chica. Old de-commissioned jack up style oil platforms can be bought for $1, they are a liability to current owners. I think this would be a much better option for Boca Chica than a floating platform.
Main BFR business in the near future is Star Link satellites.  These are in SSO so they launch to the south to avoid populated areas.  They can do that with a floating platform in Boca Chica but maybe not Canaveral.

No, the StarLink constellation will have several inclinations, only the most extreme will need to be launched from a polar/SSO launch capable location. The vast majority of the satellites could be launched from Cape Canaveral.

Boca Chica is actually a bit more limited for Starlink launches, until SpaceX could convince Mexico and/or the US to allow land overflights. An ocean platform wouldn't make a lot of difference - it would have to be really far out in the middle of the gulf to allow launches in more directions.
This is much cheaper than buying a whole Falcon 9 mission anytime in the foreseeable future, and to many of these destinations, if you want to go at all, you’re unlikely to get many secondary payloads who can use your trajectory, so your main alternative would be buying a full Falcon 9.

Since all these launches would have to go to the same staging orbit anyway to reach the depot, what about using a single large rocket to carry a bunch of separate upper stage/payload stacks, and using the same method to allow several departures in quick succession? If you used F9, you'd still need the LauncherOne upper stage, and you add the expendable F9 upper stage, but eliminate the expendable LauncherOne first stage and you can probably save most of the support costs. With a LauncherOne S2 dry mass of 329 kg, and 470 kg payload, and say 200 kg for the dry mass of the largest storable stage you considered, thats basically 1 ton per stack dry, so F9 could carry like 16 of those to LEO by mass. Volumetrically, LO S2 is 1.3 meters wide and looks to be ~3 meters long. Call it 5 meters with payload. In F9's 4.6 meter internal diameter you can pack 8 of those per level. Probably can't stack 2 levels though, but the difference can be made up by fueling some of those, which also reduces the demand on the depot. LauncherOne is 10 million a flight, if we assume 1/3 of that is the upper stage (probably much less, half is generally operations/overhead) thats 27 million for 8 stages, plus the F9 at 50 million, so 77 million total, vs 80 million with equivalent number of LauncherOnes. Not a huge savings, but nonzero, and I'd say thats pretty conservative (probably-excessive cost for LO S2, no reusable fairing on F9). If you include fuel (probably just for the storable stages, you'd still want to get your semi-cryogenic fuel from the depot) thats up to 750*8 = 6000 kg extra mass carried in the same volume at no additional cost (equals 6000 kg less propellant that has to be carried separately to the depot, which if you're using Atlas V or Vulcan with extra boosters to rideshare that propellant up could be up to like 8 million dollars in extra boosters, if using GEM-63 at ~half the cost of AJ60), and it still fits in F9s reusable mass capacity. New Glenn is likely even better because this arrangement seems to be mainly volume-limited (with a 6.5 meter internal diameter fairing, you could carry 19 LauncherOne S2s per layer, and as tall as it is you may be able to fit a second layer in there), but we don't know enough about its cost and payload yet
With small LV you can fly when ready not have to wait for another dozen payloads to be ready to fill large LV. Launch windows can be limited for some BLEO which is even more reason to use small LV.

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