SpaceX Falcon 9 v1.1 - Dragon - CRS-5/SpX-5 -Jan. 10, 2015 - DISCUSSION

[winkhomewinkhome]

Huh?  If the vehicle has more thrust than weight, then it follows that it WOULD be able to hover.  Just not for very long, because the available fuel, I'd imagine, is quite low.

The *minimum* thrust is still greater than the weight.  So once the stage hovers, it can only go up after that point --- until the engines are cut (or run out of fuel) and then it's gravity who's in charge.

And no, the stage can't toggle on and off rapidly.

There is a minimum thrust limit because the rocket combustion is unstable at low fuel pressures.

Some of this Thrust to Weight Ratio discussion has left me rather confused.  The Merlin engine is able to be throttled - yes.
Ratio >1, Falcon goes up - thus throttle up - Falcon up (Grasshopper did)
Ratio < 1 Falcon goes down - thus throttle back - Falcon down (Grasshopper did)
Ratio = 1 Falcon stabilizes vertical momentum +/- thus Falcon hovers (fairly certain Grasshopper did along with horizontal transitions)

Am I missing some thing here?

To say that only a +1 TWR exists would basically translate into what we have seen with Grasshopper was a figment of our imagination...

Grasshopper carried more fuel, weighed more, and thus had enough weight to allow a <1 or =1 Thrust/weight ratio.  F9 first stage on return is at the ragged edge of dry.

Then timing and consumption is everything, otherwise failure is all but certain?

[Karloss12]

Maybe SpaceX just over saw the need to adjust the landing datum up 10 metres from the surface of the ocean to the surface of the barge.

[cambrianera]

Then timing and consumption is everything, otherwise failure is all but certain?

Yes

Timing is Paramount.

[meekGee]

Huh?  If the vehicle has more thrust than weight, then it follows that it WOULD be able to hover.  Just not for very long, because the available fuel, I'd imagine, is quite low.

The *minimum* thrust is still greater than the weight.  So once the stage hovers, it can only go up after that point --- until the engines are cut (or run out of fuel) and then it's gravity who's in charge.

And no, the stage can't toggle on and off rapidly.

There is a minimum thrust limit because the rocket combustion is unstable at low fuel pressures.

Some of this Thrust to Weight Ratio discussion has left me rather confused.  The Merlin engine is able to be throttled - yes.
Ratio >1, Falcon goes up - thus throttle up - Falcon up (Grasshopper did)
Ratio < 1 Falcon goes down - thus throttle back - Falcon down (Grasshopper did)
Ratio = 1 Falcon stabilizes vertical momentum +/- thus Falcon hovers (fairly certain Grasshopper did along with horizontal transitions)

Am I missing some thing here?

To say that only a +1 TWR exists would basically translate into what we have seen with Grasshopper was a figment of our imagination...

The correct relationship is:
Ratio >1, Falcon *accelerates* up, which means that if it's in a descent, it is slowing down, then reversing, then going up.
Ratio < 1 Falcon *accelerates* down, which means that if it's in a descent, it is speeding up.
Ratio = 1 Falcon continues to move (up or down) at constant velocity.

[cscott]

The key factor is the thrust-to-weight ratio of the Falcon core *as it approaches the ground after flight*.  The possible thrust is limited by the throttling range of the Merlin: something like 40% (or 60%, there's some debate) to 100% of rated thrust.  It can't provide less thrust than that.

All grasshopper flights are weighted so that the weight of the grasshopper is more than the Merlin minimum thrust level, which allows hovering as the T/W approaches 1 (varying slightly up and down and the control system compensates for wind gusts, residual velocity, varying fuels levels, etc).  IIRC there was at least one Grasshopper flight where T/W > 1 landing was tested, but even that flight required the grasshopper to maintain T/W < 1 when it stopped ascending and began descending.

Stable hovering also requires a certain throttle response speed, but you can still bounce up and down around a target altitude even if your throttle is slow, as http://www.flutterbye.com demonstrates.  But the Grasshopper tests show that the Merlin throttle is plenty fast enough for hovering.

The returning Falcon core is so light that it *cannot* achieve T/W <= 1 during the landing burn, without shutting down all of its engines completely.  So it cannot hover.  It can arrange its deceleration carefully so that its *velocity* reaches *zero* at exactly the same time its altitude reaches zero -- and then turn its engines off.

But if it did not turn its engines off it would not hover there -- it would begin to accelerate upward again, "bouncing".

I hope that makes things clearer.

[ericspittle]

Especially on these NASA missions I have my doubts they will be allowed to scrub a launch for something that will not be a danger to the actual mission.

Allowed?  SpaceX can scrub for any reason they want to.  They can scrub because Elon isn't feeling lucky that day.

Annoying your customer isn't a good idea, though, so if they scrub too often for reasons NASA doesn't like (or even if they scrub too often for reasons NASA is fine with, like replacing valves), it will impact their likelihood of getting follow-on NASA business.

Thank you for the response, I had figured that it would be in the contracts that missions could not be scrubbed for things that wouldn't affect the actual mission. Clearly I was wrong, thank you for the correction.

[Orbiter]

I didn't catch it during the ascent yesterday, what was Dragon's initial orbital insertion apogee/perigee at S/C sep?

[ugordan]

I didn't catch it during the ascent yesterday, what was Dragon's initial orbital insertion apogee/perigee at S/C sep?

It was never revealed. Wasn't specified in the press kit, either.

Go, commercial!

[salpun]

There was some talk of a single day approach if launched on Saturday. That did not happen. The fact that the the target changes day to day would be why its not in the PDF.

[ugordan]

Only phasing would change, not target orbit, IMHO.

[Mongo62]

There was some talk of a single day approach if launched on Saturday. That did not happen. The fact that the the target changes day to day would be why its not in the PDF.

I thought that single day approach was if it launched on Frday. A Saturday launch had a two day approach with a Monday berthing.

[edkyle99]

I didn't catch it during the ascent yesterday, what was Dragon's initial orbital insertion apogee/perigee at S/C sep?

It was never revealed. Wasn't specified in the press kit, either.

Space Track shows three objects from the launch, as follows:

Object A:  206 x 353 km x 51.64 deg
Object B:  206 x 352 km x 51.65 deg
Object C:  207 x 347 km x 51.66 deg

I would have expected the second stage to be deorbited, leaving only Dragon and debris.

 - Ed Kyle

[NovaSilisko]

Space Track shows three objects from the launch, as follows:

Object A:  206 x 353 km x 51.64 deg
Object B:  206 x 352 km x 51.65 deg
Object C:  207 x 347 km x 51.66 deg

I would have expected the second stage to be deorbited, leaving only Dragon and debris.

 - Ed Kyle

Presumably A and B are the solar panel covers and C is Dragon itself, since it's been maneuvering and the covers (hopefully) have not.

[CJ]

Sorry is this is an inept question, but I can't seem to find an answer anywhere;

During the countdown, it was mentioned (SpaceX webcast) that the launch had been delayed from the initial target time by either 3 seconds or 30 seconds (I couldn't hear it well enough to be sure) due to avoiding orbital debris.

If the above is true, could that explain the longer rendezvous times we're seeing (as opposed to earlier plans for a same-day berthing?). I think it'd have to have an impact.   

[ChrisWilson68]

Sorry is this is an inept question, but I can't seem to find an answer anywhere;

During the countdown, it was mentioned (SpaceX webcast) that the launch had been delayed from the initial target time by either 3 seconds or 30 seconds (I couldn't hear it well enough to be sure) due to avoiding orbital debris.

If the above is true, could that explain the longer rendezvous times we're seeing (as opposed to earlier plans for a same-day berthing?). I think it'd have to have an impact.

There's no need to explain the longer rendezvous times.  They're already understood.

Twice a day the plane of the ISS orbit passes through KSC.  One of the times it's pointing south east and the other it's pointing north east.  The south-east path is not good for launch because of what land is under that path.  So only the north east path is allowed, so once a day there is a launch opportunity.

The ISS might happen to be anywhere in that plane when the opportunity happens.  If it's close to where Dragon ends up when launched, there's a short time before rendezvous.  If it's on the other side of the Earth, there's a longer time before rendezvous.

[Antilope7724]

Elisabeth III which should be towing barge will enter Jacksonville approximately in 2.5 hours, I hope we will get at least some view of barge via webcams in port.

http://www.vesselfinder.com/?mmsi=367017460

From Jacksonville, FL - Fox TV 30 - 6 photos of damaged SpaceX barge docking on Jacksonville FL St Johns River:

You can see dents and scorch marks on equipment on the barge deck:

"Gallery SpaceX barge docking St Johns River". 6 photos.
http://www.fox30jax.com/gallery/news/local/gallery-spacex-barge-docking-st-johns-river/gCQSB/

http://www.actionnewsjax.com/news/news/local/spacex-barge-docking-shore-st-johns-river/njmCJ/

[Oersted]

Antilope, many thanks for the links to those photos. I am very surprised to see so many containers and other stuff topside on the barge. I thought it would be completely clean to provide as big a flat surface as possible.

[Galactic Penguin SST]

I didn't catch it during the ascent yesterday, what was Dragon's initial orbital insertion apogee/perigee at S/C sep?

It was never revealed. Wasn't specified in the press kit, either.

Go, commercial!

http://spaceflightnow.com/2015/01/05/spacex-5-mission-status-center/ mentioned that the targeted orbit is 199.2 x 365.5 km x 51.6 deg.

[darkenfast]

Question: does SpaceX Hawthorne ever talk directly to the ISS, or does everything go though the Communicator at Houston?

[Jarnis]

Question: does SpaceX Hawthorne ever talk directly to the ISS, or does everything go though the Communicator at Houston?

No. Only NASA CAPCOM talks to ISS.

NASA Controllers do talk to SpaceX controllers naturally but anything SpaceX would want to tell to the astronauts would go via CAPCOM.