SpaceX Falcon 9 : CRS-11 : June 3, 2017 : DISCUSSION

[LouScheffer]

The NROL-76 reentry burn was called out as 25 seconds long. This one was significantly shorter. Are they continuing to optimize fuel usage, or is that due to different trajectories?

Quite different trajectory, see altitude plot attached. Generally shorter flight.

Here are some numbers derived from the webcast.
CRS-11:  6063 ks/hr = 1684 m/s at MECO, at 62 km
NROL-76:  1680 m/s at MECO, at 66 km
So almost exactly the same speed.   But the velocity components differ.

CRS-11:  Coasts 58 km higher.  Assuming a pure horizontal retro burn, this implies vertical velocity at cutoff of v=sqrt(2*a*s) or sqrt(2*9.8*58000) = 1066 m/s vertical, leaving 1298 m/s horizontal and angle of 39 degrees to horizon.
NROL-76:  Coasts 100 km higher, so vertical component is sqrt(2*9.8*100000) = 1400 m/s.  Horizontal speed of 928 m/s and angle of 56 degrees to horizon.  More lofted trajectory.

So CRS-11 needs a longer boostback burn, since it has to cancel out more horizontal velocity, plus needs a higher speed coming back since the coast time is shorter (lower apogee).  It burns for about 59 seconds and ends up with 1711 km/hr = 475 m/s going back towards the cape.  Total delta V = 1298+475 = 1773 m/s.  Acceleration 1773/59 = 30 m/s^2.
NROL-76:  42 second burn with 290 m/s returning.  Total delta-V 928+290 = 1218 m/s.   Acceleration 1218/42 = 29 m/s^2.  These agree within the uncertainties of estimating parameters from webcasts.

Since NROL is returning from higher, it's going faster, and takes a longer entry burn.   It starts at 65 km high and 1400 m/s.  This is consistent with a completely horizontal boostback, since that's the vertical velocity we estimated at the same altitude on the way up.   A 25 second entry burn reduces the speed to 736 m/s at 37 km up.  Braking is about 26 m/s^2
CRS-11 can wait until lower, since it's going slower and needs less braking.  It starts a 17 second entry burn at 48 km and 4539 km/hr = 1260 m/s.   It ends up at 3400 km/hr = 944 m/s at 35 km.  Braking is somewhat less at 18 m/s^2 (but a bigger fraction of the burn is single engine, which could account for some of this.)

Aerodynamic drag then brings both rockets to 4 km and 300 m/s.  Then they both do about a 30 second landing burn.

Overall, the more lofted trajectory of NROL-76 meant less fuel required for landing.  The boostback burn was 17 seconds shorter, while the higher entry speed required an 8 seconds longer burn.  Both are 3 engine burns, so the overall savings are 3 engines x 9 seconds.

EDIT: had wrong mission as more lofted in last paragraph.

[Danny452]

The NROL-76 reentry burn was called out as 25 seconds long. This one was significantly shorter. Are they continuing to optimize fuel usage, or is that due to different trajectories?

Quite different trajectory, see altitude plot attached. Generally shorter flight.

Here are some numbers derived from the webcast.
CRS-11:  6063 ks/hr = 1684 m/s at MECO, at 62 km
NROL-76:  1680 m/s at MECO, at 66 km
So almost exactly the same speed.   But the velocity components differ.

CRS-11:  Coasts 58 km higher.  Assuming a pure horizontal retro burn, this implies vertical velocity at cutoff of v=sqrt(2*a*s) or sqrt(2*9.8*58000) = 1066 m/s vertical, leaving 1298 m/s horizontal and angle of 39 degrees to horizon.
NROL-76:  Coasts 100 km higher, so vertical component is sqrt(2*9.8*100000) = 1400 m/s.  Horizontal speed of 928 m/s and angle of 56 degrees to horizon.  More lofted trajectory.

So CRS-11 needs a longer boostback burn, since it has to cancel out more horizontal velocity, plus needs a higher speed coming back since the coast time is shorter (lower apogee).  It burns for about 59 seconds and ends up with 1711 km/hr = 475 m/s going back towards the cape.  Total delta V = 1298+475 = 1773 m/s.  Acceleration 1773/59 = 30 m/s^2.
NROL-76:  42 second burn with 290 m/s returning.  Total delta-V 928+290 = 1218 m/s.   Acceleration 1218/42 = 29 m/s^2.  These agree within the uncertainties of estimating parameters from webcasts.

Since NROL is returning from higher, it's going faster, and takes a longer entry burn.   It starts at 65 km high and 1400 m/s.  This is consistent with a completely horizontal boostback, since that's the vertical velocity we estimated at the same altitude on the way up.   A 25 second entry burn reduces the speed to 736 m/s at 37 km up.  Braking is about 26 m/s^2
CRS-11 can wait until lower, since it's going slower and needs less braking.  It starts a 17 second entry burn at 48 km and 4539 km/hr = 1260 m/s.   It ends up at 3400 km/hr = 944 m/s at 35 km.  Braking is somewhat less at 18 m/s^2 (but a bigger fraction of the burn is single engine, which could account for some of this.)

Aerodynamic drag then brings both rockets to 4 km and 300 m/s.  Then they both do about a 30 second landing burn.

Overall, the more lofted trajectory of CRS-11 meant less fuel required for landing.  The boostback burn was 17 seconds shorter, while the higher entry speed required an 8 seconds longer burn.  Both are 3 engine burns, so the overall savings are 3 engines x 9 seconds.

Thank you.  I had no idea that there were such large differences in RTLS.  In the last paragraph you wrote CRS-11 but meant NROL-76.

[Req]

Are these 4x8(or 12) sheets of plywood or something? 

[Damon Hill]

Note last-second thrust vectoring; very dynamic all the way to the ground.  No wonder the barge landings look so sporty, having to correct for cross-wind and just catching the barge itself.

[FutureSpaceTourist]

Another incredible 4K video of landing:

Looks like a smoother, gentler landing and cleaner engine shut-off too. So the new paint job does seem to have done its job. I wonder how soon this booster might be re-used ...

[cppetrie]

Another incredible 4K video of landing:

Looks like a smoother, gentler landing and cleaner engine shut-off too. So the new paint job does seem to have done its job.

I was thinking the same thing. Almost thought it was playing in slow motion but then I noticed the waves on the beach seemed to be rolling in at expected speed and the thruster bursts seemed realtime as well.

I wonder how soon this booster might be re-used ...

August maybe?

[Jdeshetler]

Converted and close up to 1080P from 4k format for those who don't have 4k monitor. Awesome shot by SpaceX..

They may have enabled HDR (High Dynamic Range) on their drone camera which makes it a bit cartoon-like however it does bring out cool details under poor lighting conditions.

[dodo]

What was that plank flying out to the border of the circle? (00:06 in the video.)

[cartman]

Thanks for the video, the closeup reveals many interesting details. The colors you see are a typical result of shooting in a cloudy day.

Converted and close up at 1080P in 4k format for those who don't have 4k monitor. Awesome shot by SpaceX..

They may have enabled HDR (High Dynamic Range) on their drone camera which makes it a bit cartoon-like but it does bring out cool details under poor lighting conditions.

[kevin-rf]

What was that plank flying out to the border of the circle? (00:06 in the video.)

Could it have been some sort of protection for the concrete? Like the black circle is not really black paint but panels to take the punishment for the concrete? And we just saw a panel fly loose? Just seams odd, you really don't want FOD lying around on your landing zone... So it must be something in the landing zone that was mentioned to stay out.

[gospacex]

That planks were lying just outside of the circle.

[mvpel]

An interesting difference I noticed is that the post-landing GN₂ blowdown was directed downward along the body of the booster. In CRS-8, it was directed outward on two opposing nozzles.

[old_sellsword]

An interesting difference I noticed is that the post-landing GN₂ blowdown was directed downward along the body of the booster. In CRS-8, it was directed outward on two opposing nozzles.

Two different systems. The ones seen going outward on opposite sides are the cold gas ACS, the ones seen pointed down are the RP-1 and LOX tank vents.

[Dante80]

That planks were lying just outside of the circle.

Those looked like panels. The color is the same as the circle (but their position was just outside of it).

Also, apparently some birds in the distance were recorded having a heart attack. XD

[Jdeshetler]

Looks like the caliper comes out from one of the payloads inside the trunk. Not a pleasant sight.

[Jim]

Looks like the caliper comes out from one of the payloads inside the trunk. Not a pleasant sight.

Tool control process failed more than once for this to happen.
That is embarrassing

[punder]

If I'm interpreting reddit correctly, the "calipers" are now being called a chunk of ice. The original post has been removed. Of course I could be mistaken.

https://www.reddit.com/r/spacex/comments/6g20b8/pair_of_calipers_unintentionally_left_in_crs11/

[Jim]

If I'm interpreting reddit correctly, the "calipers" are now being called a chunk of ice. The original post has been removed. Of course I could be mistaken.

That is spin.  It is too deep in the trunk for that.  But if so, then there is something just as troubling, they have a humidity and temperature problem for payloads in the trunk.

[whitelancer64]

Looks like the caliper comes out from one of the payloads inside the trunk. Not a pleasant sight.

The thread on Reddit and on the SpaceX fan page on Facebook about this have been removed. Statement from mods that it was ice buildup, not calipers.

https://www.reddit.com/r/spacex/comments/6g20b8/pair_of_calipers_unintentionally_left_in_crs11/

[jimvela]

The other Jim is right here.

If that's ice, then there's a temp/humidity problem. 
That sure looks like a tool to me that comes sailing out, and if so that's a major process failure at multiple points.

I'll be interested to see how this evolves...