SpaceX Falcon 9 / Dragon 2 : SpX-DM2 - EOM/Return: August, 2020 : DISCUSSION

[Chris Bergin]

DISCUSSION Thread for SpX-DM2 (crewed demo) EOM (End Of Mission) and return, post splashdown events.

Undocking Aug 1. Splashdown Aug 2.

NSF Threads for SpX-DM2 : Pre-Launch Updates
NSF Articles for SpX-DM2 :
https://www.nasaspaceflight.com/?s=DM-2
https://www.nasaspaceflight.com/?s=Demo-2


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https://forum.nasaspaceflight.com/index.php?board=60.0

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Discussion thread, but be useful with your posts. No party chat (we have a party thread)

[Robotbeat]

I made a new thread to discuss the previous discussion here: https://forum.nasaspaceflight.com/index.php?topic=51564.0
Can’t quote context because the thread is already locked.

[mn]

In reading this https://www.nasa.gov/sites/default/files/atoms/files/ccp_splashdown.pdf

(also posted here: https://forum.nasaspaceflight.com/index.php?topic=51100.msg2111414#msg2111414).

I'm trying to decipher exactly when Dragon is committed to a particular landing site and can no longer switch to alternate sites.

I would have thought that this would be at the point of the deorbit burn, but in reading that post it seems like they already committed to a specific site during free flight prior to deorbit burn.

Crew Dragon has the capability to execute a unique series of orbit-lowering maneuvers using its Draco thrusters to line up its ground track for each primary location and maintain the capability to change to alternate sites in free-flight as weather constraints dictate

This implies that indeed it can choose a landing location during 'free-flight'.

But then...

Wave-off

If conditions remain NO-GO at the supported splashdown site, SpaceX and NASA will jointly make a decision to “wave-off.” In a wave-off scenario, Crew Dragon will remain in orbit for the next landing attempt 24-48 hours later. 

This seems to be saying that if the weather is bad at the splashdown site (singular) they will wave off, doesn't give the option of switching to an alternate site.

Edit: added first quote implying that it can change landing site during free flight.

[lrk]

Perhaps it depends on how close to deorbit they wave off?  If it would require too much delta-V to adjust the ground track in time then they would need to wait for the next opportunity? 

[mheney]

You might wave off for 24 hrs on the first attempt for deorbit / landing, because there's no rush, but after 60 hours of free-flight, you're in more of a "we're coming down SOMEWHERE today", and would take advantage of all options available to get to the best splashdown site.

[Orbiter]

This needs to be monitored. Expected to be near the splashdown zone by Sunday, August 2nd.

[freddo411]

This needs to be monitored. Expected to be near the splashdown zone by Sunday, August 2nd.

If the storm follows that track and timing, you've could well have a storm affecting all the landing sites.

I predict the return date will get moved a day or two.

[Alvian@IDN]

This needs to be monitored. Expected to be near the splashdown zone by Sunday, August 2nd.

If the storm follows that track and timing, you've could well have a storm affecting all the landing sites.

I predict the return date will get moved a day or two.

Several landing sites aren't inside that, so not all of them

[Vettedrmr]

This needs to be monitored. Expected to be near the splashdown zone by Sunday, August 2nd.

If the storm follows that track and timing, you've could well have a storm affecting all the landing sites.

I predict the return date will get moved a day or two.

Several landing sites aren't inside that, so not all of them

But that's just the area of uncertainty for track of the eye of the storm; adverse weather conditions (especially for this landing) can extend out a LOT farther than the depicted cone.

[CorvusCorax]

Perhaps it depends on how close to deorbit they wave off?  If it would require too much delta-V to adjust the ground track in time then they would need to wait for the next opportunity?

The thing is, to hit a specific splashdown site, Dragon needs to be in a specific spot in its orbit at a specific time. It most certainly won't change its inclination, but as earth rotates underneath Dragon's orbit,  if Dragon arrives at the selected point too early, it will end up too far east. If Dragon get's there too late, it will end up west of its intended splashdown site.

Dragon can chose which latitude to land at - or basically where on the groundtrack it comes down by adjusting when exactly to do the reentry burn at. ( I think typically approximately 1/3 of an orbit prior to atmospheric interface) but to make sure said groundtrack also leads over the landing spot, Dragon needs to adjust its phase angle - by adjusting its orbital period.

Since the period will only differ by a few minutes per orbit, it takes quite a number of orbits to accumulate enough phase change to significantly adjust where the ground track goes over. Fly a bit higher, Dragon gets slower and will end up further west with each consecutive orbit. Fly a bit lower and it will arrive earlier, and further east with each additional orbit.

But by the time the reentry burn is getting near, Dragon can only adjust its reentry target along its current groundtrack. Unless the landing zones end up lined up perfectly on the same groundtrack, any switch would require a rephasing-burn plus - typically - waiting one more day for the groundtrack to overlap the new landing spot.

From this follows, Dragon will not be committed to any specific landing spot until the actual deorbit burn has happened AS LONG AS they have enough margin left to wait one more day and make a phase-adjustment burn.

If less than 24 hours orbital lifetime margin are left, Dragon typically can no longer afford rephasing - as that typically takes one day to get there. But there are exceptions.

- For one, if the landing sites lay on the exact same groundtrack, Dragon could simply adjust the reentry timing by a very short time  to switch. ( This might be possible between Talahassee and Tampa landing locations, when coming from northwest and reentering over the states)

- For the other, if Dragon was aiming for the easternmost landing location, it might possibly be able to reach a spot in the gulf one orbit later. (a bit under 90 minutes) For that the landing spots would have to be on ground tracks that are exactly 1 orbit apart. I believe though that Pensacola, the westernmost landing location is still too close to the Cape to reach it on the next orbit, which means Dragon would likely have to wait a day and rephase. (I think there'd have to be around ~~20° longitude difference between the spots for that to work.

Additional opportunities arise when Dragon can reach the same landing location both from a northwesterly and a southwesterly reentry corridor. That could reduce the wait time from 1 day to 4-ish hours respectively 20-ish hours. (Although 4 hours is only 2.5 orbits, that might not be enough time to adjust the phasing by much, so only possible with luck and careful planning)

[NaN]

This needs to be monitored. Expected to be near the splashdown zone by Sunday, August 2nd.

If the storm follows that track and timing, you've could well have a storm affecting all the landing sites.

I predict the return date will get moved a day or two.

Several landing sites aren't inside that, so not all of them

But that's just the area of uncertainty for track of the eye of the storm; adverse weather conditions (especially for this landing) can extend out a LOT farther than the depicted cone.

It would also impact the helicopter's ability to return the crew to KSC, even if some of the outlying sites were otherwise usable.

[darkenfast]

Remember that the winds limit for this mission is very low (just over 10mph, if I recall correctly).

[Chris Bergin]

Preps for recovery:

Join NSF's Julia Bergeron (@Julia_bergeron) for an overview of the recent SpaceX Fleet recovery operations and the preparations for the recovery of the Demo-2 crew during next week's Dragon Endeavour splashdown.

Edited by Jack Beyer (@TheJackBeyer).

[gongora]

It would also impact the helicopter's ability to return the crew to KSC, even if some of the outlying sites were otherwise usable.

The crew doesn't need to go back to KSC.  They return directly to JSC.

[rcoppola]

It would also impact the helicopter's ability to return the crew to KSC, even if some of the outlying sites were otherwise usable.

The crew doesn't need to go back to KSC.  They return directly to JSC.

According to NASA: "Once returned to shore, both crew members will immediately board a waiting NASA plane to fly back to Ellington field in Houston." That's for returning to shore either via helicopter or Ship depending upon splashdown location.

[aero]

Won't they get plucked off the ship by helicopter once the ship is within range?

[Orbiter]

Track has shifted southwest somewhat. There's a decent chance this thing just dies out over the Dominican Republic, which would be good news for recovery.

[daveglo]

Apologies if it's been discussed somewhere else, but I haven't seen anything relating to a preference for an ascending (northeast) track versus a descending (southeast) track for re-entry?

Are there any kind of failure modes during re-entry that would make one path the logical choice over the other?

[gongora]

Won't they get plucked off the ship by helicopter once the ship is within range?

There is a helicopter on the ship.  All of the landing areas should be within helicopter range of an airport.

[NaN]

It would also impact the helicopter's ability to return the crew to KSC, even if some of the outlying sites were otherwise usable.

The crew doesn't need to go back to KSC.  They return directly to JSC.

Thanks, I had misread the following statement:

https://www.nasa.gov/feature/top-10-things-to-know-for-nasa-s-spacex-demo-2-return

After initial medical checks, Behnken and Hurley will be returned to shore either by traveling on the primary recovery ship or by helicopter. Helicopter returns from the recovery ship are the baseline for all splashdown zones except for the Cape Canaveral splashdown site, with travel times ranging from approximately 10 minutes to 80 minutes. The distance from shore will be variable depending on the splashdown location, ranging from approximately 22 nautical miles to 175 nautical miles.

Once returned to shore, both crew members will immediately board a waiting NASA plane to fly back to Ellington field in Houston.

So it sounds like at most splashdown sites, the helicopter will ferry them to a nearby airport, with a subsequent airplane flight back to Houston.