SpaceX Falcon 9 FT - ORBCOMM-2 - Dec. 21, 2015 (Return To Flight) DISCUSSION

[Rocket Science]

Looking at the landing limits status window posted over on the update thread. "Landing Winds Below 160 feet (<50 mph)". A 50 mph wind limit? That sounds a bit... 'sporting'.

That raised my eyebrows a bit as well since its "gale force winds"...

[jimbowman]

Looking at the landing limits status window posted over on the update thread. "Landing Winds Below 160 feet (<50 mph)". A 50 mph wind limit? That sounds a bit... 'sporting'.

That raised my eyebrows a bit as well since its "gale force winds"...

#BeatBezos

[Lars-J]

I saw this infographic (not from SpaceX) : http://forum.nasaspaceflight.com/index.php?topic=38148.msg1461581#msg1461581

I know this has been discussed before with different opinion, but I very much doubt that there is a final divert manuever. If the engine fails to light, it will crash on the pad. There is no better spot for it to happen, this is where all pieces can be recovered for investigation should it be necessary. Environmental cleanup is also much easier there.

There is no such thing as falling "harmlessly into the ocean", if it is just by the beach. So I don't see them aiming there, but I've certainly been wrong before.   

[abaddon]

I don't understand? Because that's exactly what the graphic does show happens. It's on a course to fall into the ocean if the final burn doesn't happen. If it does light, it vectors over to the pad.

Because that graphic is not from SpaceX.

[rcoppola]

I don't understand? Because that's exactly what the graphic does show happens. It's on a course to fall into the ocean if the final burn doesn't happen. If it does light, it vectors over to the pad.

Because that graphic is not from SpaceX.

That's why I deleted my post.

Edit: But from a pad damage  POV (including possible brush fires), I'd much prefer hitting the ocean.

[Hankelow8]

I saw this infographic (not from SpaceX) : http://forum.nasaspaceflight.com/index.php?topic=38148.msg1461581#msg1461581

I know this has been discussed before with different opinion, but I very much doubt that there is a final divert manuever. If the engine fails to light, it will crash on the pad. There is no better spot for it to happen, this is where all pieces can be recovered for investigation should it be necessary. Environmental cleanup is also much easier there.

There is no such thing as falling "harmlessly into the ocean", if it is just by the beach. So I don't see them aiming there, but I've certainly been wrong before.   

The landing graphic is pure conjecture, although it does make sense to have safety in mind for the first land landing.  If the stage did hit "hard" i guess more of the stage would survive in a sea landing to enable Space X to recover the wreckage for analysis.

[Bynaus]

00:02:35  2nd stage engine starts
00:03       Fairing deployment
00:04       1st stage boostback burn

Any idea why there's a roughly 2 minute delay between second stage engine start and the boostback burn? That'll put them a few hundred km further down-range than if they boosted back immediately. Maybe they have plenty of margin and want to make sure that even if the first stage's control system messes up and points its engines at the second stage, or the stage explodes, the second stage will be safe?

If you think about it, it doesn't really matter when they do the boost-back burn: they don't need more fuel if the booster has flown a bit further down its ballistic trajectory: boost-back is all about cancelling horizontal velocity and reversing it. If the core has flown a little while longer, the coast phase after the boost back burn will have to be a bit longer as well, but everything else - in particular fuel consumption - stays essentially the same (minus some minor losses due to additional atmospheric friction during the longer path after boost back, even at that height).

[deltaV]

Maybe that is to allow the 1st stage to do its end-over flip.

If they use cold gas thrusters to get both ends of the first stage moving in opposite directions at say 5 m/s it'll be done with its flip in around (50 m / 2) * pi / (5 m/s) = 16 seconds. I haven't run the numbers but I bet that's a trivial amount of delta vee compared to the increase in the boost-back delta vee from being a few hundred extra km downrange and with a much less favorable vertical speed. So the flip can't explain the delay before boost-back unless there's something other than thruster propellant that limits how fast they can flip.

If you think about it, it doesn't really matter when they do the boost-back burn: they don't need more fuel if the booster has flown a bit further down its ballistic trajectory: boost-back is all about cancelling horizontal velocity and reversing it. If the core has flown a little while longer, the coast phase after the boost back burn will have to be a bit longer as well, but everything else - in particular fuel consumption - stays essentially the same (minus some minor losses due to additional atmospheric friction during the longer path after boost back, even at that height).

Nope. Think about it a little harder. The reversed horizontal velocity is not set in stone but is a function of how far you have to boost back. If you have less distance to travel to get home you can boost back slower. There's also vertical speed to worry about.

[deltaV]

Where are people getting those screenshots of launch commit criteria? There doesn't seem to be anything at http://www.spacex.com/webcast/ yet.

[Kabloona]

Maybe that is to allow the 1st stage to do its end-over flip.

If they use cold gas thrusters to get both ends of the first stage moving in opposite directions at say 5 m/s it'll be done with its flip in around (50 m / 2) * pi / (5 m/s) = 16 seconds. I haven't run the numbers but I bet that's a trivial amount of delta vee compared to the increase in the boost-back delta vee from being a few hundred extra km downrange and with a much less favorable vertical speed. So the flip can't explain the delay before boost-back unless there's something other than thruster propellant that limits how fast they can flip.

As Lars-J pointed out above, the delay is governed not only by rotation rate but also the time needed to settle the propellants using relatively small RCS thrusters.

[just-nick]

Has anyone noticed the significant difference in 1st stage burn times between the Falcon 9 user's guide (170 seconds) and the launch day press release (2:20 or 140 seconds)? See attachments...

That seems very significant...more than I'd expect from, e.g., a different throttle profile and, on a light vehicle without a 2nd stage, a LOT of delta-v.

The payload is super light, so could they be doing a short-fill? Or deliberately bringing excess fuel along to allow them to land heavy and therefore have a less critical situation with the minimum thrust of the Merlin vs. the vehicle weight)?

Cheers,

  --Nick

[docmordrid]

All the non-NASA coverages mirror Livestream.

IT HAS STARTED!!

[deltaV]

Does anyone know which livestream is better, the YouTube one or the one linked from spacex.com?

[TJL]

What is the azimuth for tonights launch?
Thank you.

[jimbowman]

Are the song credits a new thing?

[docmordrid]

Does anyone know which livestream is better, the YouTube one or the one linked from spacex.com?

They all mirror Livestream, even SpaceX's page, but YouTube usually has more bandwidth (in my experience)

[rcoppola]

Does anyone know which livestream is better, the YouTube one or the one linked from spacex.com?

I'm Air-playing from Youtube off my Macbook to the TV. Youtube Stream is perfect right now.

[SpunkyEnigma]

Does anyone know which livestream is better, the YouTube one or the one linked from spacex.com?

I'm Air-playing from Youtube off my Macbook to the TV. Youtube Stream is perfect right now.

Youtube is ahead of Spacex.com right now by about almost minute judging on when the video started

[Eer]

Date on the opening of the video coverage still said December 20, 2015, instead of Dec 21.

[abaddon]

Seems like they are spending a lot more effort on this webcast than usual!