SpaceX F9 / Crew Dragon : Crew-2 : 22 April 2021 - DISCUSSION

[kdhilliard]

Attached are screenshots of the parachute deployment.
Links to the webcast:
  01:36:32  Drogues
  01:37:11  Drogue Separation and Main Chute Delpoy
  01:37:37  Callout: "Dragon, visual on four healthy mains.  Descent rate nominal."
  01:38:35  Fourth main chute finally starts its full inflation.
The fourth parachute completed its full inflation about 60 to 70 seconds after the other three.

[Jimmy10]

Wonder how decent speed changed over that sequence?

[darkenfast]

I see SpaceX has redone their main Mission Control in Hawthorne (or at least, this is the first time I noticed it!). Anyway, I hope the slow inflation of the chute doesn't cause any delays to Crew-3's launch. Welcome Home!

[Hyperborealis]

Liked how in the old setup you would see Gwynne and sometimes Elon watching front and center. Now the place where they would sit seems to be gone. Too bad. Gave a sense they were all one scrappy team.

[Vettedrmr]

Eric Burger's tweet in post #359 says a delay is possible.  OTOH, Kathy Leuders said that they've seen this behavior before and the deceleration rates were nominal, so hopefully not.

[kdhilliard]

Wonder how decent speed changed over that sequence?

I understand that three mains are sufficient for nominal splashdown velocity, but yes, it would be interesting to hear what effect the final main had.  (Imagine the graphs we'd have if SpaceX shared their all their telemetry with OneSpeed!  )  Per the webcast, the drogues slowed the capsule from 350 mph (563 km/h, 156 m/s) to 120 mph (193 km/h, 54 m/s), with the mains bringing it down to about 15 mph (24 km/h, 6.7 m/s) for splashdown.

I recall someone (woods170?) writing of the effects of having only one or two working mains.  Given new attention to the parachutes, could they please repeat that here?

[zubenelgenubi]

Would NASA and SpaceX even be in this situation if ASAP had not insisted on a fourth main parachute for Dragon v2?

From my viewpoint as a "not a spacecraft engineer," I perceive the fourth chute as an over-complexification.

[Eric Berger tweet]

The team will be going off and looking at how the loading was on the chute and understanding that behavior.  It is behavior that we have seen multiple times in other tests,  and it usually happens when the lines kind of bunch up together until the aero forces kind of open up and spread the chutes.  The thing that makes me feel a little bit more confident is that the loading and deceleration of the spacecraft all looked nominal.

[TrevorMonty]

Would NASA and SpaceX even be in this situation if ASAP had not insisted on a fourth main parachute for Dragon v2?

From my viewpoint as a "not a spacecraft engineer," I perceive the fourth chute as an over-complexification.

[Eric Berger tweet]

The team will be going off and looking at how the loading was on the chute and understanding that behavior.  It is behavior that we have seen multiple times in other tests,  and it usually happens when the lines kind of bunch up together until the aero forces kind of open up and spread the chutes.  The thing that makes me feel a little bit more confident is that the loading and deceleration of the spacecraft all looked nominal.

The Orion, Starliner and New Shepard all use 3 chutes. NS has demonstrated a safe test landing under two. Not sure about Orion or Starliner but would've hope they have.

Sent from my SM-G570Y using Tapatalk

[Oersted]

over-complexification.

I saw what you did there    ;-)

[kdhilliard]

Would NASA and SpaceX even be in this situation if ASAP had not insisted on a fourth main parachute for Dragon v2?

From my viewpoint as a "not a spacecraft engineer," I perceive the fourth chute as an over-complexification.

Interesting.  So you're suggesting that this type of malfunction may be less likely to occur (or more likely to quickly clear) for one of three parachutes than one of four, either because it is induced by the bunching of the chutes (though I interpreted Lueders' "lines kind of bunch up together" as being the lines of a single cute) or that the greater descent rate under two full and one partial chute would induce the greater aero forces necessary to open up and spread the chute?

Perhaps.  But if such a malfunctions is just as likely to occur with a 3 chute configuration, this might be seen as vindication of that decision.  That fourth chute took 70 seconds longer than expected to fully open, and it did so only 90 seconds before splashdown.  Had it been one of three and happened to take twice as long to clear its malfunction, it would have made for a hard landing.

It's a shame that the camera was out of focus for the few seconds before 01:37:26 in the webcast -- the photo (posted above) at that timestamp is the first clearly showing the malfunction -- but it appears to have started with the initial unreefing five seconds earlier.  What looks like vertical slots in the canopy of the two front chutes in that photo are folds in the canopy, presumably held in by the reefing.  (I assume that was normal behavior.)  For the misbehaving chute, it look as if at initial unreefing those folds continued inward instead of opening outward.

[ZachS09]

Would NASA and SpaceX even be in this situation if ASAP had not insisted on a fourth main parachute for Dragon v2?

From my viewpoint as a "not a spacecraft engineer," I perceive the fourth chute as an over-complexification.

[Eric Berger tweet]

The team will be going off and looking at how the loading was on the chute and understanding that behavior.  It is behavior that we have seen multiple times in other tests,  and it usually happens when the lines kind of bunch up together until the aero forces kind of open up and spread the chutes.  The thing that makes me feel a little bit more confident is that the loading and deceleration of the spacecraft all looked nominal.

The Orion, Starliner and New Shepard all use 3 chutes. NS has demonstrated a safe test landing under two. Not sure about Orion or Starliner but would've hope they have.

Sent from my SM-G570Y using Tapatalk

During the Starliner Pad Abort Test, the spacecraft test article landed on two chutes because the third one didn't open. The pin in the third chute's pilot chute apparently wasn't positioned right, which I see as human error.

Boeing still called it a success.

[zubenelgenubi]

My italics added to KDH's post:

Interesting.  So you're suggesting that this type of malfunction may be less likely to occur (or more likely to quickly clear) for one of three parachutes than one of four, [Option 1] either because it is induced by the bunching of the chutes (though I interpreted Lueders' "lines kind of bunch up together" as being the lines of a single cute) or [Option 2] the greater descent rate under two full and one partial chute would induce the greater aero forces necessary to open up and spread the chute?

Yes, particularly to "option 1."

[intelati]

Interesting.  So you're suggesting that this type of malfunction may be less likely to occur (or more likely to quickly clear) for one of three parachutes than one of four, either because it is induced by the bunching of the chutes (though I interpreted Lueders' "lines kind of bunch up together" as being the lines of a single cute) or that the greater descent rate under two full and one partial chute would induce the greater aero forces necessary to open up and spread the chute?

Yes.

I tend to agree with you. Right now, I don't nearly have enough data to determine "How worried" I should be. I'm sure they will be able to figure out *why* the chute took longer to open. And IIRC, Dragon 2 was initially designed with three chutes for the redundancy already built in, no? That just screams the forces are different and makes these situations much more common. And something like 1.5 redundancy (2 chutes under emergency for a hard landing....) seems overkill. But, ASAP was created to be conservative...

[LouScheffer]

Note that the last chute to open has MUCH less aerodynamic force on it, trying to push it open.

Assuming the nominal descent speed is 6.7 m/s with all 4 chutes, and force goes like v^2, and they open sequentially:

The first chute to open sees an airspeed of 54 m/s, and slows the capsule to 13.4 m/s.
The second chute sees an opening airflow of 13.4 m/s, and slows the capsule to 9.5 m/s.
The third chute sees an airflow of 9.5 m/s, and slows the capsule to 7.7 m/s.
The fourth chute sees an airflow of 7.7 m/s, and slows the capsule to 6.7 m/s.

So the ratios of the opening forces, from the fourth to  the first, are 1:1.5:3:49 .

So the last chute has way less forces than numbers 1 and 2, and significantly less forces than number 3.

[Lee Jay]

Note that the last chute to open has MUCH less aerodynamic force on it, trying to push it open.

Assuming the nominal descent speed is 6.7 m/s with all 4 chutes, and force goes like v^2, and they open sequentially:

The first chute to open sees an airspeed of 54 m/s, and slows the capsule to 13.4 m/s.
The second chute sees an opening airflow of 13.4 m/s, and slows the capsule to 9.5 m/s.
The third chute sees an airflow of 9.5 m/s, and slows the capsule to 7.7 m/s.
The fourth chute sees an airflow of 7.7 m/s, and slows the capsule to 6.7 m/s.

So the ratios of the opening forces, from the fourth to  the first, are 1:1.5:3:49 .

So the last chute has way less forces than numbers 1 and 2, and significantly less forces than number 3.

That's assuming they open sequentially, which they aren't supposed to.  If they open in parallel, they all see the same forces.  Unfortunately on this flight, that fourth one didn't open in parallel with the others and I strongly suspect your analysis is exactly why it took so long to eventually open - because the other three had reduced dynamic pressure by such a large amount.

[slobber91]

Real world example of the impact of a failed parachute on landing velocity:  during coverage of the Apollo 15's splashdown with a failed parachute (linked below), the NASA commentator indicates that the effect of landing with two parachutes instead of three was an increase of splashdown velocity from 28 ft/s to 32 ft/s. (46:12 in the linked video).

[ShaunML09]

Former SpaceX lead confirming this is not a failure but a "lagging" parachute which can happen on occasion

https://twitter.com/SpaceAbhi/status/1458144778692927494

https://twitter.com/SpaceAbhi/status/1458144980527042565

So looks like everybody above was on point. 

[AS_501]

SpaceX's Bill Gerstenmaier says the slow opening parachute was returned to KSC, suspended from a crane and inspected in detail; no problems were found and "we don't see anything that's off nominal;" he said the Crew Dragon can safely land with just 3 chutes

https://mobile.twitter.com/cbs_spacenews/status/1458289899778478081

In fact, Dragon's 4 chutes seems overkill.  Remember that Apollo 15 landed fine with just 2 of 3 chutes.  Same during Starliner's abort test.  Then there is Soyuz......

[eeergo]

SpaceX's Bill Gerstenmaier says the slow opening parachute was returned to KSC, suspended from a crane and inspected in detail; no problems were found and "we don't see anything that's off nominal;" he said the Crew Dragon can safely land with just 3 chutes

https://mobile.twitter.com/cbs_spacenews/status/1458289899778478081

In fact, Dragon's 4 chutes seems overkill.  Remember that Apollo 15 landed fine with just 2 of 3 chutes.  Same during Starliner's abort test.  Then there is Soyuz......

What matters is not the number of parachutes, but their canopy, aerodynamics and the load's weight. AFAIK Dragon doesn't have backup parachutes, so its redundancy with four is actually its nominally-built-in redundancy. I believe it is designed to be fail-safe with three, and survivable with just two, but it could as easily be designed so that four are essential. Comparing it with Soyuz, which for starters is lighter and has a backup parachute that nominally doesn't need to deploy, is quite fallacious.

[eeergo]

FWIW:
https://twitter.com/waynehale/status/1458452706272718861

I'm old enough to remember watching a documentary about STS-107 15-20 years ago, whose name I can't remember, where privatization (United Space Alliance) was blamed from some space officials as at least a contributing cause to the disaster (IIRC the paraphrased quote was something along the lines of "we're handing off maintenance and operations of the most complex system in the world, that has to fly crew and carry out the most demanding space missions, to the lowest bidder"), together with normalization of deviance in the analysis being a pervasive issue within the Shuttle program. I might be mistaken, but I seem to remember Gerst testimony was in there too.

Not saying this issue is necessarily at the same level of importance, especially considering they've tested the system one and two chutes out - but it's telling relevant people are being spooked by the attitude.