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Old 09-18-2018, 01:32 PM   #16
jedidia
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So, even if this works, could they ever afford to blow up one of these and still continue?
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Old 09-18-2018, 01:32 PM   #17
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I got a good vibe from the presentation but the reentry looks odd.

But it's all very much wishful thinking.
The renders are still generic, no interior detail, no mention of crew safety systems. Do they eject if something goes wrong?
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Old 09-18-2018, 02:04 PM   #18
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 I got a good vibe from the presentation but the reentry looks odd.
Yeah - especially it looks like the whole wings are supposed to move for attitude control additionally to the canards, so a very huge surface with a pretty high dynamic pressure, that moves a lot = heavy bearings and hydraulics.

And canards on a spacecraft during lunar reentry should be a hot event even when taking in account that half of the spacecraft should be empty fuel tanks.
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Old 09-18-2018, 02:27 PM   #19
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 And canards on a spacecraft during lunar reentry should be a hot event
I expect them to estabish a circular orbit around earth first before re-entering so that the speed will be as low as possible. Or will they indeed re-enter directly from the moon?
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Old 09-18-2018, 02:41 PM   #20
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 I expect them to estabish a circular orbit around earth first before re-entering so that the speed will be as low as possible. Or will they indeed re-enter directly from the moon?
Circular Orbit around Earth coming from the moon costs as much fuel as leaving circular orbit towards the moon. Both DV together is about 2/3 of the DV for getting from ground to LEO. A lot of fuel together.
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Old 09-18-2018, 02:57 PM   #21
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 ... A lot of fuel together.
Aerobraking perhaps? I wonder how they will deal with the heat otherwise.
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Old 09-18-2018, 03:06 PM   #22
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 Aerobraking perhaps? I wonder how they will deal with the heat otherwise.
Maybe, but that does not mean that the peak heating is much lower, you just spend less time at a peak.

Even a mild propulsive maneuver to slow down the reentry would require a refueling of the BFS BTW, something that will unlikely happen during the first mission.
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Old 09-18-2018, 03:54 PM   #23
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One spacecraft that does everything... That was the idea at the start of the Apollo program, but then they "saw the light" and went with 2. No point in taking parachutes and propellant to the surface, plus all the associated dry mass.
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Old 09-18-2018, 03:55 PM   #24
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I wonder about reentry heat, heatshield(?) and reusability.

The key must be on the exterior material.
I don't see any tiles on the "wings" but I'm sure they have a technical solution for that.

Perhaps some part of the Falcon already experiences similar heat and they only have to scale up?

Last edited by 4throck; 09-18-2018 at 04:02 PM.
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Old 09-18-2018, 05:18 PM   #25
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Are they going to have to base jump to get out of it ?
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Old 09-18-2018, 05:21 PM   #26
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 Are they going to have to base jump to get out of it ?

Maybe they can drive a Tesla down the hull, as shown in the drawing.
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Old 09-18-2018, 08:14 PM   #27
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The reentry looks even more scary than I imagined! The high AoA looked like 70 - 90° in the video. I would imagine a cylinder has a lot of degrees of freedom in this configuration, especially to roll around its longitudinal axis. The fold-back fins are not ideally placed to prevent this, as they start pretty far down the curvature of the belly. Actually they might even increase a roll tendency, if not actively controlled.

On the longitudinal stability I would assume the canards need to have nearly the same drag (lift?) as the main wings. They might even be grid fins, as these produce a higher drag than a smooth wing.

So the reentry would be in a (what the aviation guys call) "relaxed stability mode", that means the vessel is not self-rightening but dependent on active control by the onboard computers and aerodynamic surfaces. I think, SpaceX is taking their "flying the iPhone" control philosophy to a new level.
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Old 09-19-2018, 05:19 AM   #28
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Quote:
So the reentry would be in a (what the aviation guys call) "relaxed stability mode", that means the vessel is not self-rightening but dependent on active control by the onboard computers and aerodynamic surfaces. I think, SpaceX is taking their "flying the iPhone" control philosophy to a new level.
Well, that's how the Space Shuttle worked (the Shuttle is not a yaw-stable vehicle during entry, yaw-jets or adverse yaw from the elevons are used to constantly manage sideslip during entry) - so the concept isn't exactly new.
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Old 09-19-2018, 07:36 AM   #29
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Also the Shuttle was not always stable in the pitch axis - its prefered flight altitude at high Mach numbers would have been engine first, it just becomes a bit more stable at lower Mach numbers.
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Old 09-19-2018, 09:54 PM   #30
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Yes the entry looks pretty if 'y but they have flown much of the profile already. There is a lot of work yet to do. There was talk around the shop about SFS which would look and fly pretty much the same path but be sized to launch on a Falcon 9.

I too wonder about the abort and crew safety and have seen some cut outs of the inside and out side heat shields but this is all 1st generation stuff and will change over time. We are starting to receive things here in TX that are labeled BFR but just storing them now. Its testing equipment.
Can't see a manned flight in 5 years but we'll see. Space X does some unusual things.
Maybe Buck Rodgers was just ahead of his time, this thing looks like his ship
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