SpaceX Dragon 2 Updates and Discussion - Thread 3

[envy887]

Given the amount of propellant on-board remaining, after the deorbit burn, it would require very lengthy Draco burns. Substantially more lengthy than the deorbit burn. I don't think the Dracos are qualified for that. And probably neither is the Crew Dragon primary structure.

If burning off the remaining propellant - via the Dracos - had been the easy solution than SpaceX probably would have taken that route already.
The fact that they haven't is telling.

Deorbit burn from 408 to 120km is 84m/s.
The superdracos with a max weight capsule and full tanks do around 330m/s.
Assuming only that all dracos are identical, and that they use only one axes to do that deorbit burn, you have at least six times that that total delta-v = ~500m/s.

Deorbit burn alone lasts roughly 15 minutes on Crew Dragon (going by the numbers for DM-1, as provided by SpaceX in their press kit). So if I understand you correctly Crew Dragon would have to do a single-axis Draco burn lasting 1.5 hours (6 x 15 minutes)  to burn off the excess propellant.

Crew Dragon has 16 Dracos, which each burn ~0.13 kg/s. To match the 8 SDs burning 30 kg/s for 7.5 seconds would take just over 14 minutes of Draco burns.

[WindyCity]

On the Musk/Bridenstine tweet flap:

Elon draws criticism for his overly optimistic "Elon TIme" (ET) timeine predictions and perhaps on occasion justifiably so. It's always better politically to under-promise and over-deliver. But the people who criticize ET don't grasp its probable purpose (which Gwynne Shotwell has has stated in interviews and which has also been mentioned many times in NSF), namely that Musk's aggressive timelines motivate his team to work harder and longer.

ET reflects, I believe, the man's personal zeal to make as expeditious progress as possible toward reaching his overriding goal:  the establishment of a  settlement on Mars and its steady growth into a self-sustaining and thriving extraterrestrial outpost of human civilization.

Would Bridenstine be any happier if Musk had laid out a more conservative schedule for Spx-DM2, say mid-year 2020? Unlikely. As Elon said in the CNN interview following the SS presentation, aerospace is always delay-ridden, especially when it involves ground-breaking, experimental engineering.

Bridenstine's frustration is understandable; he's a politician getting pressured from all sides to deliver yesterday. However, he surely knows that rushing to get a job done can lead to oversight and carelessness, which observation is a big duh. But it was unfair of him to single out SpaceX.

Musk handled the criticism well, saying that 95% of the company's resources focus on Falcon and Dragon. Seeing what's been accomplished in such a short time at Boca, Cocoa, and the Cape  might make that hard to fathom, which is perhaps what prompted Bridenstine to post what he did.

[woods170]

Given the amount of propellant on-board remaining, after the deorbit burn, it would require very lengthy Draco burns. Substantially more lengthy than the deorbit burn. I don't think the Dracos are qualified for that. And probably neither is the Crew Dragon primary structure.

If burning off the remaining propellant - via the Dracos - had been the easy solution than SpaceX probably would have taken that route already.
The fact that they haven't is telling.

Deorbit burn from 408 to 120km is 84m/s.
The superdracos with a max weight capsule and full tanks do around 330m/s.
Assuming only that all dracos are identical, and that they use only one axes to do that deorbit burn, you have at least six times that that total delta-v = ~500m/s.

Deorbit burn alone lasts roughly 15 minutes on Crew Dragon (going by the numbers for DM-1, as provided by SpaceX in their press kit). So if I understand you correctly Crew Dragon would have to do a single-axis Draco burn lasting 1.5 hours (6 x 15 minutes)  to burn off the excess propellant.

Crew Dragon has 16 Dracos, which each burn ~0.13 kg/s. To match the 8 SDs burning 30 kg/s for 7.5 seconds would take just over 14 minutes of Draco burns.

SuperDracos burn 30 kg/second each.
You only get to 14 minutes IF you burn ALL 16 Dracos at the same time. Which Crew Dragon has not been designed for and has not been qualified for.

[envy887]

Given the amount of propellant on-board remaining, after the deorbit burn, it would require very lengthy Draco burns. Substantially more lengthy than the deorbit burn. I don't think the Dracos are qualified for that. And probably neither is the Crew Dragon primary structure.

If burning off the remaining propellant - via the Dracos - had been the easy solution than SpaceX probably would have taken that route already.
The fact that they haven't is telling.

Deorbit burn from 408 to 120km is 84m/s.
The superdracos with a max weight capsule and full tanks do around 330m/s.
Assuming only that all dracos are identical, and that they use only one axes to do that deorbit burn, you have at least six times that that total delta-v = ~500m/s.

Deorbit burn alone lasts roughly 15 minutes on Crew Dragon (going by the numbers for DM-1, as provided by SpaceX in their press kit). So if I understand you correctly Crew Dragon would have to do a single-axis Draco burn lasting 1.5 hours (6 x 15 minutes)  to burn off the excess propellant.

Crew Dragon has 16 Dracos, which each burn ~0.13 kg/s. To match the 8 SDs burning 30 kg/s for 7.5 seconds would take just over 14 minutes of Draco burns.

SuperDracos burn 30 kg/second each.
You only get to 14 minutes IF you burn ALL 16 Dracos at the same time. Which Crew Dragon has not been designed for and has not been qualified for.

Well, it's 14 minutes of burn time per Draco. Whether it's all at the same time depends how much time you have. Lighting alternate sets of 8, it's 28 minutes. For alternate quads, it's a little under an hour. Even burning only 2 at a time it would only take a little over 1 orbit.

[su27k]

Does going back to 3 parachutes actually buy you anything significant? If I understand the parachute situation correctly, even with 3 parachutes some redesign is still required, it's just 3 parachutes will need less # of tests than 4, so you're not buying a lot with changing 4 parachutes to 3?

And if hypothetically we go back in history and somehow SpaceX chose to dump fuel instead of going to 4 parachutes, they may not have discovered this failure mode in testing, which is actually a bad thing? Seems to me this is blessing in disguise in the long term.

[whitelancer64]

If SpaceX could go back in time and redesign everything, they'd probably go with a jettisoned LAS tower. That removes all the abort system issues and they can land with 3 parachutes no problem.

[gongora]

If SpaceX could go back in time and redesign everything, they'd probably go with a jettisoned LAS tower. That removes all the abort system problems and they can land with 3 parachutes no problem.

Or trunk mounted pusher system.

[spacenut]

Why not burn the fuel of the Super Draco's during re-entry to lower wear on the heat shield.  Then you could go back to 3 parachutes. 

[Todd Martin]

Dragon 2 was designed for propulsive landing.  If the parachutes fail, software should be installed to allow for the Super-dracos to fire and save the crew lives.  SpaceX should be allowed the opportunity to show this as genuine risk reduction.  I don't understand why NASA would insist on only parachutes with no propulsive back-up.  It is tying SpaceX hands behind their back.

[MattMason]

Dragon 2 was designed for propulsive landing.  If the parachutes fail, software should be installed to allow for the Super-dracos to fire and save the crew lives.  SpaceX should be allowed the opportunity to show this as genuine risk reduction.  I don't understand why NASA would insist on only parachutes with no propulsive back-up.  It is tying SpaceX hands behind their back.

I believe this has been discussed elsewhere with the same answer, if I recall correctly (others please note the topic or clarify):

Propulsive landing was omitted because the certification and testing process may have delayed Commercial Crew further, nor did NASA ask for this method. Even if propulsive landing was not the primary means of descent and landing, the ability would still require certification as secondary means of descent, which would mean the same impact on time.

[gongora]

There is a thread for discussion of propulsive landing.  Further discussion on the subject should move to that thread.

Should Crew Dragon have ability to use SuperDracos for landing emergency?

[Comga]

Given the amount of propellant on-board remaining, after the deorbit burn, it would require very lengthy Draco burns. Substantially more lengthy than the deorbit burn. I don't think the Dracos are qualified for that. And probably neither is the Crew Dragon primary structure.

If burning off the remaining propellant - via the Dracos - had been the easy solution than SpaceX probably would have taken that route already.
The fact that they haven't is telling.

Deorbit burn from 408 to 120km is 84m/s.
The superdracos with a max weight capsule and full tanks do around 330m/s.
Assuming only that all dracos are identical, and that they use only one axes to do that deorbit burn, you have at least six times that that total delta-v = ~500m/s

It’s better AND worse than that
Draco’s, being more expanded, have higher Isp than SuperDracos so the total delta V would be greater....
If the all fired in one direction, but they don’t.
In fact some are opposed, or close to opposed, and balanced firing would result in zero delta V
And velocity can be added perpendicular to the flight path and gravity which would shift the landing point but not the reentry angle or speed.

[Comga]

Has the possibility of using the Super Dracos for a chute failure scenario been discounted? It seems like a little software could make all the difference.

There is a whole thread for that question

[SeaRaven]

Anyone know if Ripley will be aboard IFA?

[jarmumd]

On the Musk/Bridenstine tweet flap:

Elon draws criticism for his overly optimistic "Elon TIme" (ET) timeine predictions and perhaps on occasion justifiably so. It's always better politically to under-promise and over-deliver. But the people who criticize ET don't grasp its probable purpose (which Gwynne Shotwell has has stated in interviews and which has also been mentioned many times in NSF), namely that Musk's aggressive timelines motivate his team to work harder and longer.

ET reflects, I believe, the man's personal zeal to make as expeditious progress as possible toward reaching his overriding goal:  the establishment of a  settlement on Mars and its steady growth into a self-sustaining and thriving extraterrestrial outpost of human civilization.

Would Bridenstine be any happier if Musk had laid out a more conservative schedule for Spx-DM2, say mid-year 2020? Unlikely. As Elon said in the CNN interview following the SS presentation, aerospace is always delay-ridden, especially when it involves ground-breaking, experimental engineering.

Bridenstine's frustration is understandable; he's a politician getting pressured from all sides to deliver yesterday. However, he surely knows that rushing to get a job done can lead to oversight and carelessness, which observation is a big duh. But it was unfair of him to single out SpaceX.

Musk handled the criticism well, saying that 95% of the company's resources focus on Falcon and Dragon. Seeing what's been accomplished in such a short time at Boca, Cocoa, and the Cape  might make that hard to fathom, which is perhaps what prompted Bridenstine to post what he did.

I see the administrators comment in this view: yes, 95% of the resources are being applied to Falcon and Dragon, but those resources were cut at the beginning of the year with layoffs, which I see the savings as contributing to starship development.  So it's 95% of a smaller portion or resources / personnel.  Dragon crew might be further along had those people not been let go.

[ncb1397]

On the Musk/Bridenstine tweet flap:

Elon draws criticism for his overly optimistic "Elon TIme" (ET) timeine predictions and perhaps on occasion justifiably so. It's always better politically to under-promise and over-deliver. But the people who criticize ET don't grasp its probable purpose (which Gwynne Shotwell has has stated in interviews and which has also been mentioned many times in NSF), namely that Musk's aggressive timelines motivate his team to work harder and longer.

ET reflects, I believe, the man's personal zeal to make as expeditious progress as possible toward reaching his overriding goal:  the establishment of a  settlement on Mars and its steady growth into a self-sustaining and thriving extraterrestrial outpost of human civilization.

Would Bridenstine be any happier if Musk had laid out a more conservative schedule for Spx-DM2, say mid-year 2020? Unlikely. As Elon said in the CNN interview following the SS presentation, aerospace is always delay-ridden, especially when it involves ground-breaking, experimental engineering.

Bridenstine's frustration is understandable; he's a politician getting pressured from all sides to deliver yesterday. However, he surely knows that rushing to get a job done can lead to oversight and carelessness, which observation is a big duh. But it was unfair of him to single out SpaceX.

Musk handled the criticism well, saying that 95% of the company's resources focus on Falcon and Dragon. Seeing what's been accomplished in such a short time at Boca, Cocoa, and the Cape  might make that hard to fathom, which is perhaps what prompted Bridenstine to post what he did.

I see the administrators comment in this view: yes, 95% of the resources are being applied to Falcon and Dragon, but those resources were cut at the beginning of the year with layoffs, which I see the savings as contributing to starship development.  So it's 95% of a smaller portion or resources / personnel.  Dragon crew might be further along had those people not been let go.

Here is a SpaceX statement:

"To continue delivering for our customers and to succeed in developing interplanetary spacecraft and a global space-based Internet, SpaceX must become a leaner company. Either of these developments, even when attempted separately, have bankrupted other organizations. This means we must part ways with some talented and hardworking members of our team. We are grateful for everything they have accomplished and their commitment to SpaceX's mission. This action is taken only due to the extraordinarily difficult challenges ahead and would not otherwise be necessary."

https://www.businessinsider.com/spacex-layoffs-jobs-terminated-list-california-headquarters-2019-1

This seems to suggest that these projects are having more of an affect company wide than Elon wants to portray at this moment.

Remember that the previous layoffs in 2014 seemed to correlate with a string of accidents afterwards in 2015 and 2016 similar to the dragon explosion. I practically predicted that such issues might reoccur as a result of the latest round...

Déjà vu:
https://forum.nasaspaceflight.com/index.php?topic=35254.0

And we have historical hindsight for what happened afterward. According to WSJ graphs, they went from operating in the black in 2014 to operating in the red in 2015. They had a launch failure within 12 months and 2 in 2 years. You can recover what was lost as the living organism adapts to the loss of a limb, but it will likely take time. It happens to ULA as well. They are having mighty troubles launching their latest Delta IV Heavy. It wouldn't at all surprise me if that is due to hollowing out of the Delta IV team.

If past is prologue, we may be in for a rough 2019/2020.

[Johnnyhinbos]

Given the amount of propellant on-board remaining, after the deorbit burn, it would require very lengthy Draco burns. Substantially more lengthy than the deorbit burn. I don't think the Dracos are qualified for that. And probably neither is the Crew Dragon primary structure.

If burning off the remaining propellant - via the Dracos - had been the easy solution than SpaceX probably would have taken that route already.
The fact that they haven't is telling.

Deorbit burn from 408 to 120km is 84m/s.
The superdracos with a max weight capsule and full tanks do around 330m/s.
Assuming only that all dracos are identical, and that they use only one axes to do that deorbit burn, you have at least six times that that total delta-v = ~500m/s.

Deorbit burn alone lasts roughly 15 minutes on Crew Dragon (going by the numbers for DM-1, as provided by SpaceX in their press kit). So if I understand you correctly Crew Dragon would have to do a single-axis Draco burn lasting 1.5 hours (6 x 15 minutes = 90 minutes = 1.5 hours)  to burn off the excess propellant.
If that burn was initiated as the deorbit burn the Dracos would still be burning for half-an-hour AFTER splashdown. Which makes this whole scenario moot.


If SpaceX is not allowed to use the SuperDracos to burn off the excess propellant (which is the current situation) than the propellant will have to come back down, all the way to splashdown. Which is why that fourth parachute is there.

I had asked Hans about the unused propellant on D2 many months ago. He said it returns with it.

[woods170]

Given the amount of propellant on-board remaining, after the deorbit burn, it would require very lengthy Draco burns. Substantially more lengthy than the deorbit burn. I don't think the Dracos are qualified for that. And probably neither is the Crew Dragon primary structure.

If burning off the remaining propellant - via the Dracos - had been the easy solution than SpaceX probably would have taken that route already.
The fact that they haven't is telling.

Deorbit burn from 408 to 120km is 84m/s.
The superdracos with a max weight capsule and full tanks do around 330m/s.
Assuming only that all dracos are identical, and that they use only one axes to do that deorbit burn, you have at least six times that that total delta-v = ~500m/s.

Deorbit burn alone lasts roughly 15 minutes on Crew Dragon (going by the numbers for DM-1, as provided by SpaceX in their press kit). So if I understand you correctly Crew Dragon would have to do a single-axis Draco burn lasting 1.5 hours (6 x 15 minutes = 90 minutes = 1.5 hours)  to burn off the excess propellant.
If that burn was initiated as the deorbit burn the Dracos would still be burning for half-an-hour AFTER splashdown. Which makes this whole scenario moot.


If SpaceX is not allowed to use the SuperDracos to burn off the excess propellant (which is the current situation) than the propellant will have to come back down, all the way to splashdown. Which is why that fourth parachute is there.

I had asked Hans about the unused propellant on D2 many months ago. He said it returns with it.

Yes, that is what SpaceX sources tell me as well. But some folks here don't seem to be able to accept that and keep asking the question "Well why can't SpaceX just simply burn off the excess propellant prior to EDL?"

People should NOT be asking that question. There are very valid reasons why the excess propellant is not burned off and comes all the way down to splash down. I've tried to show the logic behind some of those reasons.

[alang]

More thoughts on Elon time:
I used to have a manager who, if he wanted something to be done immediately then he would give the task to someone who was busy who would then try to get it out of the way as quickly as possible.
I don't know how to properly put this into words but a small and busy self-managed team composed of competent people is many times more productive than a disparate ability directly managed team and is more likely to be busy on productive work rather than box ticking.
Such things don't spring into existence fully formed but some things seem to be necessary prerequisites: recruiting the right people and having a sense of purpose (possible business blasphemy) that is not just about making money.

[Nomadd]

Why not burn the fuel of the Super Draco's during re-entry to lower wear on the heat shield.  Then you could go back to 3 parachutes. 

Not about heat shield wear or parachute configuration necessarily, but why don't they use available fuel and engines for a re-entry burn? Is a slower, steeper re-entry bad for some reason, or just no real benefit?