APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Comments and questions about the APOD on the main view screen.
User avatar
DavidLeodis
Perceptatron
Posts: 1169
Joined: Mon May 01, 2006 1:00 pm

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by DavidLeodis » Tue Dec 29, 2015 9:54 pm

Thanks Chris and neufer for your help. :)

Having read more of the many links I've now found this "Moving back at supersonic speeds that sent a sonic boom across Florida’s Space Coast, the first stage engines ignited to slow the booster down as it neared Landing Complex-1".

User avatar
JohnD
Tea Time, Guv! Cheerio!
Posts: 1157
Joined: Wed Feb 16, 2005 2:11 pm
Location: Lancaster, England

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by JohnD » Thu Dec 31, 2015 3:02 pm

Chris will correct me, neufer, but I don't think that the landing Falcon would have taken off again if the rocket had kept on firing.
It's thrust , and the acceleration it caused counteracted the acceleration due to gravity, but not quite.
The result was a system that behaved like a much lighter vehicle, that came down slowly.
Once it contacted Earth, the reaction of a solid surface provided a further acceleration that precisely counteracted gravity.
Continued rocket thrust (of the same amount) would have made gravity and the Earth's reaction less, but not enough to counteract gravity completely and launch the vehicle again.
John

User avatar
neufer
Vacationer at Tralfamadore
Posts: 16153
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by neufer » Thu Dec 31, 2015 3:25 pm

JohnD wrote:
I don't think that the landing Falcon would have taken off again if the rocket had kept on firing.
It's thrust , and the acceleration it caused counteracted the acceleration due to gravity, but not quite.
The result was a system that behaved like a much lighter vehicle, that came down slowly.
Assuming a constant thrust (with little change in total mass) the rocket is either:
  • 1) accelerating up (i.e., decelerating down)
    2) hovering or
    3) accelerating down (i.e., decelerating up).
If the thrust more than counteracts the acceleration due to gravity (as I suggest)
then the rocket is 1) accelerating up (i.e., decelerating down) and the rocket lands softly.

If the thrust can't quite counteract the acceleration due to gravity (as you suggest)
then the rocket is 3) accelerating down (i.e., decelerating up) and the rocket lands with a thud.
Art Neuendorffer

User avatar
BMAONE23
Commentator Model 1.23
Posts: 4076
Joined: Wed Feb 23, 2005 6:55 pm
Location: California

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by BMAONE23 » Thu Dec 31, 2015 6:22 pm

Would that be Elmer Thud?
Be Vewwy qwiet, we're hunting wetwo wockets
Click to play embedded YouTube video.

User avatar
Chris Peterson
Abominable Snowman
Posts: 14458
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by Chris Peterson » Thu Dec 31, 2015 6:49 pm

JohnD wrote:Chris will correct me, neufer, but I don't think that the landing Falcon would have taken off again if the rocket had kept on firing.
It's thrust , and the acceleration it caused counteracted the acceleration due to gravity, but not quite.
The result was a system that behaved like a much lighter vehicle, that came down slowly.
Once it contacted Earth, the reaction of a solid surface provided a further acceleration that precisely counteracted gravity.
Continued rocket thrust (of the same amount) would have made gravity and the Earth's reaction less, but not enough to counteract gravity completely and launch the vehicle again.
John
You're right. I'm right. Art's right. We're all just telling part of the story.

My original comment is correct, that landing involves controlling the thrust such that gravity is almost, but not quite compensated for. But that's just the global view of the entire landing sequence. What Art's getting at is the fact that any time you have a force, you have an acceleration. If you have a constant velocity, there is no net force.

Landing involves managing a specific velocity profile. Over the entire landing, the integrated force applied by the rocket is less than the integrated force of gravity, which is why you go down. Instantaneously, the thrust force may be greater or less than the force of gravity- if you're increasing your downward velocity, it will be less; if you're decreasing your downward velocity, it will be more (you'll be accelerating upward, even though your velocity is still downward). If you're maintaining a constant downward velocity, you might think that your two forces would be equal, but actually, you would have slightly less thust because if you have a non-zero velocity, you also have an upward directed drag force. And when you get very close to the ground, things probably get very complicated because now you don't have a purely Newtonian action/reaction system, but who-knows-what nonlinear ground effects.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

suicidejunkie
Ensign
Posts: 32
Joined: Wed Oct 21, 2015 6:46 pm

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by suicidejunkie » Mon Jan 04, 2016 7:14 pm

If the engine were able to throttle down to get anywhere near "just counteract gravity", then the landing would be MUCH easier, since the rocket could hover until it got things right (or ran out of fuel).

Instead what happens is the rocket comes screaming down at a high speed, and the engines light up with a lot of thrust, but timed perfectly so that it slows to a dead halt just a tiny distance above the ground. The engines are then shut down so it doesn't start moving upwards again. (AKA suicide burn)
If you start thrusting a fraction of a second too late, you come to a stop embedded 10m into the ground. If you start thrusting a fraction of a second too early, you come to a stop 10m up, and then crash from there.

IIRC, the engine on that booster can be throttled between about 70%-100% thrust, which is close to 2g acceleration on the bottom end, but reduces the timing requirements from absolutely perfect to "very small margin for error, but doable". Also, you can't just shut the engine off and then start it up quickly to fake lower throttle; that's a good way to destroy your engine.
Apparently the longer term goal for future designs is to get a minimum throttle of 40% which would allow hovering, but throttle control is definitely on the hard end of rocket science ;).

User avatar
neufer
Vacationer at Tralfamadore
Posts: 16153
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by neufer » Mon Jan 04, 2016 7:34 pm

suicidejunkie wrote:
If the engine were able to throttle down to get anywhere near "just counteract gravity", then the landing would be MUCH easier, since the rocket could hover until it got things right (or ran out of fuel).
Neal Armstrong had to do some hovering over the moon until he found a nice place to land (or run out of fuel).

The Falcon 9 knows exactly where it is & where it wants to land at all times so that hovering shouldn't be necessary.

Hover, v. i. [OE. hoveren, and hoven, prob. orig., to abide, linger, and
  • fr. AS. hof house; cf. OFries. hovia to receive into one's house. See Hovel.]

    • Macbeth Act 1, Scene 1

    Witches: Fair is foul, and foul is fair:
    • Hover through the fog and filthy air.
Art Neuendorffer

User avatar
Chris Peterson
Abominable Snowman
Posts: 14458
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by Chris Peterson » Mon Jan 04, 2016 7:42 pm

suicidejunkie wrote:If the engine were able to throttle down to get anywhere near "just counteract gravity", then the landing would be MUCH easier, since the rocket could hover until it got things right (or ran out of fuel).
We were, of course, discussing the nature of landing rockets in general. That said, I doubt hovering would ever be a desirable option, nor that it would actually make things any easier. To make this cost effective, the system needs to land on the least possible amount of fuel. I imagine that the landing profile is designed with that in mind. And all profiles should be equally easy (but not necessarily equally robust) given that it's just an elementary Newtonian dynamics problem and basic control theory. As long as the rocket actually does what it's commanded to do by the computer (which is an engineering problem), the actual landing profile is pretty arbitrary.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

suicidejunkie
Ensign
Posts: 32
Joined: Wed Oct 21, 2015 6:46 pm

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by suicidejunkie » Mon Jan 04, 2016 10:10 pm

I think it is quite fair to say the *option* to hover is quite desirable, since it allows you to correct for quite a lot of undesirable situations. (Low velocity at any altitude becomes survivable) Needing to use it, however would not be good, but that's how safety margin works and at least it doesn't cost you mass when not used. Things diverge a bit from a simple Newtonian dynamics problem when you have air and wind and many other variables you can't control, after all.

Expanding the range of current speeds and altitudes from which you can safely land to me says "easier". "Robust landing profiles" is a good term to replace "easy" with, for when you're not anthropomorphizing the rocket systems. :D

... landing involves controlling the thrust such that gravity is almost, but not quite compensated for."
^ That's the bit I really worried about. Almost compensating for gravity means not accelerating much, but that is not all that relevant; you're not fighting gravity so much as you're fighting your own momentum over these small time scales. It is the velocity which is very important; you need to fall (0 thrust to be efficient) and then stop precisely (max thrust = quickest to be efficient, but also dangerous!)
Which leads back to the suicide burn stops as seen in the video.

User avatar
geckzilla
Ocular Digitator
Posts: 8973
Joined: Wed Sep 12, 2007 12:42 pm
Location: Modesto, CA

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by geckzilla » Mon Jan 04, 2016 11:43 pm

I don't think hovering is an option at all given the mass problem. I get what you mean by it being desirable, though. Then again, if the rocket fails it'll probably fail with or without a few seconds of hovering. Computers don't need moments in time to correct things like humans do.
Just call me "geck" because "zilla" is like a last name.

User avatar
Chris Peterson
Abominable Snowman
Posts: 14458
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by Chris Peterson » Tue Jan 05, 2016 12:44 am

suicidejunkie wrote:I think it is quite fair to say the *option* to hover is quite desirable, since it allows you to correct for quite a lot of undesirable situations. (Low velocity at any altitude becomes survivable) Needing to use it, however would not be good, but that's how safety margin works and at least it doesn't cost you mass when not used. Things diverge a bit from a simple Newtonian dynamics problem when you have air and wind and many other variables you can't control, after all.
The dynamics may not all be strictly Newtonian, but they are simple.

I don't see landing system like this being used for humans anytime soon, so it's just a question of recovering a booster. That will probably be configured with a near-zero safety margin, given the cost of carrying all that landing fuel up in the first place (assuming this system proves economical at all compared with what we do right now, which I think is still quite uncertain).
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
alter-ego
Serendipitous Sleuthhound
Posts: 846
Joined: Mon Apr 21, 2008 4:51 am
Location: Redmond, WA

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by alter-ego » Tue Jan 05, 2016 7:52 am

Chris Peterson wrote:...To make this cost effective, the system needs to land on the least possible amount of fuel. I imagine that the landing profile is designed with that in mind.
In this SpaceX update article, it's stated that a landing at 2g's is 5.5x more fuel efficient than landing at 1.1g's. Knowing the 1st stage length = 70 meters, I fairly accurately determined the decent profile by analyzing the video in 1-sec increments. The plot shows a simple (de)acceleration profile at 0.9g, or a nominal thrust of ~1.9g which is consistent with the article's higher-g, more efficient algorithm and the previously mentioned engine specs. In the video, the rocket is descending at ~65m/s when the rocket base first becomes visible (at 2 sec), and the simple acceleration model closely predicts zero velocity at 7.5 seconds elapsed time and -2.7m altitude. Given the simplistic analysis, I think the fit surprisingly good (for this discussion anyway) supporting the Newtonian model and successful higher-g controlled descent.
 
Falcon 9.1 Descent Profile_2.JPG
You do not have the required permissions to view the files attached to this post.
A pessimist is nothing more than an experienced optimist

User avatar
Nitpicker
Inverse Square
Posts: 2689
Joined: Fri Sep 20, 2013 2:39 am
Location: S27 E153

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by Nitpicker » Tue Jan 05, 2016 10:25 pm

Nice work (once again), alter-ego.

User avatar
alter-ego
Serendipitous Sleuthhound
Posts: 846
Joined: Mon Apr 21, 2008 4:51 am
Location: Redmond, WA

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by alter-ego » Wed Jan 06, 2016 4:12 am

Why thank you, Nit! I do occasionally get into these things :D
I was thinking of responding to the Earthset from the LRO APOD but then I said, nah, Nit will get it.
A pessimist is nothing more than an experienced optimist

User avatar
neufer
Vacationer at Tralfamadore
Posts: 16153
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Falcon 9 First Stage Landing (2015 Dec 28)

Post by neufer » Sat Apr 09, 2016 1:00 pm

.

Click to play embedded YouTube video.
Art Neuendorffer