Author Topic: Pushing the limits of the hoverslam landing  (Read 66112 times)

Offline Robotbeat

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Re: Pushing the limits of the hoverslam landing
« Reply #60 on: 03/20/2016 06:14 pm »
If you're going to be a stickler about it, the stage would never reach terminal velocity even with greater drag. Terminal velocity is something you can only approach. But it likely gets within 10-20% of terminal velocity, so close enough for practical purposes.
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Offline Okie_Steve

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Re: Pushing the limits of the hoverslam landing
« Reply #61 on: 05/06/2016 04:25 pm »
So successful 3 engine landing. Presumably they were running somewhere close to full throttle. I've been surprised that the airframe of an an almost empty stage could take the G loading, but obviously it can - at least once. It's diminishing return but I wonder if it could handle  a 'we who are about to die' 5 engine burn. Re-plumbing  for 5 vs 3 engine TEA/TEB would allow changing both the boost back and landing burns. Did we ever establish for sure that only 1 engine is used for the entry burn 'plasma shield'?

IIRC Elon commented recently that small percentage changes add up in the rocket business.

Offline Lars-J

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Re: Pushing the limits of the hoverslam landing
« Reply #62 on: 05/06/2016 05:04 pm »
So successful 3 engine landing. Presumably they were running somewhere close to full throttle. I've been surprised that the airframe of an an almost empty stage could take the G loading, but obviously it can - at least once. It's diminishing return but I wonder if it could handle  a 'we who are about to die' 5 engine burn. Re-plumbing  for 5 vs 3 engine TEA/TEB would allow changing both the boost back and landing burns. Did we ever establish for sure that only 1 engine is used for the entry burn 'plasma shield'?

No. And if it was just a single engine burn, it wouldn't be for just a 'plasma shield'. It is done to seriously brake the stage.

Offline Saabstory88

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Re: Pushing the limits of the hoverslam landing
« Reply #63 on: 05/06/2016 05:07 pm »
Now, I'm not a structural engineer, but I would imagine that a ~4G landing burn with little propellant remaining would be less stressful on the airframe than the ~3.5G at the top of the ascent with ~125 tons on top of the partially loaded vehicle.

I would imagine that the streses in the legs would be of the greatest concern in the 3 engine scenario.

Online acsawdey

Re: Pushing the limits of the hoverslam landing
« Reply #64 on: 05/06/2016 05:11 pm »
So successful 3 engine landing. Presumably they were running somewhere close to full throttle. I've been surprised that the airframe of an an almost empty stage could take the G loading, but obviously it can - at least once. It's diminishing return but I wonder if it could handle  a 'we who are about to die' 5 engine burn. Re-plumbing  for 5 vs 3 engine TEA/TEB would allow changing both the boost back and landing burns. Did we ever establish for sure that only 1 engine is used for the entry burn 'plasma shield'?

IIRC Elon commented recently that small percentage changes add up in the rocket business.

Regarding the G loading ... from the F9 user's manual v 2.0, max axial acceleration loading is 8.5g. But, from the point of view of the first stage, that is with a ~120 ton weight sitting on top of it. That's equivalent to the first stage sitting on the ground with a 1000 ton weight sitting on top of it. So, the tanks by themselves can probably take a lot more than 8.5g with nothing on top. Full throttle with 3 engines and completely empty stage would be about 10g -- 3*167000/(~50000).

On another thread somebody did fuel savings calculations for different numbers of engines for landing. By the time you get to 3 engines you have got most of the benefit and increasing the number of engines beyond that doesn't improve it by very much.

Offline Okie_Steve

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Re: Pushing the limits of the hoverslam landing
« Reply #65 on: 05/06/2016 11:21 pm »
Simplistic model in table form
   Engines      T/W      Accel      t      h      g-loss      ratio      fuel   
               (m/s^2)      (sec)      (m)      (m/s)               
   1      1.75      7.5      20      1500      200      2.33      100%   
   2      3.5      25.0      6      450      60      1.40      60%   
   3      5.3      42.5      3.5      265      35      1.24      53%   
   4      7.0      60.0      2.5      188      25      1.17      50%   
   5      8.8      77.5      1.9      145      19      1.13      48%   
   6      10.5      95.0      1.6      118      16      1.11      47%   
   7      12.3      112.5      1.3      100      13      1.09      47%   
   8      14.0      130.0      1.2      87      12      1.08      46%   
   9      15.8      147.5      1.0      76      10      1.07      46%   

This is up thread. *IF* it could be made to work 5 vs 3 would save a bit more fuel and little bits add up with rocket. But, if it's already pulling ~10Gs with 3 engines then 5 would push that up to ~16 which is likely beyond the structural limit. Although, the published G limit may be more about the payload than the rocket. I guess if they ever try 5 we can surmise that it's higher than we thought.  :)

As I understand it, if the the stage was made 'stationary' relative to the surface the terminal velocity it would achieve would be a relatively benign event. The figure ' about 15 seconds' was mentioned in the web cast for the entry burn. I'm sure that produces significant slowing of the stage but comes nowhere close to 'stationary' so the rest of the kinetic energy has to be bled off as drag and the resulting heat. The 'plasma shield' I was referring to is the engine(s) exhaust diverting much of that heating around the stage and keeping it from cooking and ablating the exterior surface. That being the case there might be an advantage of using a single engine since you could run it longer to create an increased protection duration. If that's not true, then there would be no benefit to waiting to initiate the burn once you were out of proximity  to S2 and could effectively combine the boost back and entry burns.

Offline savuporo

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Re: Pushing the limits of the hoverslam landing
« Reply #66 on: 05/06/2016 11:43 pm »
But, if it's already pulling ~10Gs with 3 engines then 5 would push that up to ~16 which is likely beyond the structural limit.

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Offline CJ

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Re: Pushing the limits of the hoverslam landing
« Reply #67 on: 05/07/2016 01:11 am »
I have to admit that when I first saw this thread, I dismissed it as  hyperbole, and unworkable. I mean, the very idea that SpaceX would try, especially during it's early landings, something as preposterous and absurdly difficult as a 3-engine landing burn - it was just too absurd...

Well, rather clearly, I was wrong (and I'm quite glad I kept my mouth shut on the issue until now).

Thank you, Prefer to Lurk, for starting this thread and crunching the numbers.

I hope better video of yesterday's 3 engine landing is released at some point - I'd love to see if they shut down the outer engines a second or two before the center engine.

Offline 2552

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Re: Pushing the limits of the hoverslam landing
« Reply #68 on: 05/07/2016 01:25 am »
Simplistic model in table form
   Engines      T/W      Accel      t      h      g-loss      ratio      fuel   
               (m/s^2)      (sec)      (m)      (m/s)               
   1      1.75      7.5      20      1500      200      2.33      100%   
   2      3.5      25.0      6      450      60      1.40      60%   
   3      5.3      42.5      3.5      265      35      1.24      53%   
   4      7.0      60.0      2.5      188      25      1.17      50%   
   5      8.8      77.5      1.9      145      19      1.13      48%   
   6      10.5      95.0      1.6      118      16      1.11      47%   
   7      12.3      112.5      1.3      100      13      1.09      47%   
   8      14.0      130.0      1.2      87      12      1.08      46%   
   9      15.8      147.5      1.0      76      10      1.07      46%   

This is up thread. *IF* it could be made to work 5 vs 3 would save a bit more fuel and little bits add up with rocket. But, if it's already pulling ~10Gs with 3 engines then 5 would push that up to ~16 which is likely beyond the structural limit. Although, the published G limit may be more about the payload than the rocket. I guess if they ever try 5 we can surmise that it's higher than we thought.  :)

As I understand it, if the the stage was made 'stationary' relative to the surface the terminal velocity it would achieve would be a relatively benign event. The figure ' about 15 seconds' was mentioned in the web cast for the entry burn. I'm sure that produces significant slowing of the stage but comes nowhere close to 'stationary' so the rest of the kinetic energy has to be bled off as drag and the resulting heat. The 'plasma shield' I was referring to is the engine(s) exhaust diverting much of that heating around the stage and keeping it from cooking and ablating the exterior surface. That being the case there might be an advantage of using a single engine since you could run it longer to create an increased protection duration. If that's not true, then there would be no benefit to waiting to initiate the burn once you were out of proximity  to S2 and could effectively combine the boost back and entry burns.

Given the diminishing returns of 5 engines vs 3 for landing, I think it's more likely SpaceX tries a 5 (or more) engine reentry burn instead. There's a lot more fuel mass onboard (15-20 tons?) when it starts, so a 5 engine reentry burn should give more of a benefit to T/W without pulling too many Gs. The reentry burn for JCSAT-14 was about 15 seconds with 3 engines, so would a 5 engine burn cut that to under 10 seconds? What kind of fuel savings would that give?

Online Herb Schaltegger

Re: Pushing the limits of the hoverslam landing
« Reply #69 on: 05/07/2016 01:33 am »
Nice to see some confirmation of what some folks had guesstimated earlier:

Per Elon's tweet (quoted in the post below), the landing burn starts with 3 engines, ends with one. I'm sure the SpecialSauce™ is in the timing and details.

http://forum.nasaspaceflight.com/index.php?topic=39843.msg1530435#msg1530435
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Offline Okie_Steve

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Re: Pushing the limits of the hoverslam landing
« Reply #70 on: 05/07/2016 02:47 am »
Simplistic model in table form
   Engines      T/W      Accel      t      h      g-loss      ratio      fuel   
               (m/s^2)      (sec)      (m)      (m/s)               
   1      1.75      7.5      20      1500      200      2.33      100%   
   2      3.5      25.0      6      450      60      1.40      60%   
   3      5.3      42.5      3.5      265      35      1.24      53%   
   4      7.0      60.0      2.5      188      25      1.17      50%   
   5      8.8      77.5      1.9      145      19      1.13      48%   
   6      10.5      95.0      1.6      118      16      1.11      47%   
   7      12.3      112.5      1.3      100      13      1.09      47%   
   8      14.0      130.0      1.2      87      12      1.08      46%   
   9      15.8      147.5      1.0      76      10      1.07      46%   

This is up thread. *IF* it could be made to work 5 vs 3 would save a bit more fuel and little bits add up with rocket. But, if it's already pulling ~10Gs with 3 engines then 5 would push that up to ~16 which is likely beyond the structural limit. Although, the published G limit may be more about the payload than the rocket. I guess if they ever try 5 we can surmise that it's higher than we thought.  :)

As I understand it, if the the stage was made 'stationary' relative to the surface the terminal velocity it would achieve would be a relatively benign event. The figure ' about 15 seconds' was mentioned in the web cast for the entry burn. I'm sure that produces significant slowing of the stage but comes nowhere close to 'stationary' so the rest of the kinetic energy has to be bled off as drag and the resulting heat. The 'plasma shield' I was referring to is the engine(s) exhaust diverting much of that heating around the stage and keeping it from cooking and ablating the exterior surface. That being the case there might be an advantage of using a single engine since you could run it longer to create an increased protection duration. If that's not true, then there would be no benefit to waiting to initiate the burn once you were out of proximity  to S2 and could effectively combine the boost back and entry burns.

Given the diminishing returns of 5 engines vs 3 for landing, I think it's more likely SpaceX tries a 5 (or more) engine reentry burn instead. There's a lot more fuel mass onboard (15-20 tons?) when it starts, so a 5 engine reentry burn should give more of a benefit to T/W without pulling too many Gs. The reentry burn for JCSAT-14 was about 15 seconds with 3 engines, so would a 5 engine burn cut that to under 10 seconds? What kind of fuel savings would that give?
Given the tweet above I think I agree.

Offline Kabloona

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Re: Pushing the limits of the hoverslam landing
« Reply #71 on: 05/07/2016 02:54 am »
Quote
The reentry burn for JCSAT-14 was about 15 seconds with 3 engines...

Just curious, what was your source for that? I didn't see the whole webcast. On the technical webcast, the entry burn callouts were 25 seconds apart, apparently not very accurately timed...

By comparison, the entry burn callouts for SES-9 were spaced 17 seconds apart. And this time they may have shortened the entry burn slightly in order to reserve more propellant for landing.

So 15 seconds makes sense, but I'd like to find the source.
« Last Edit: 05/07/2016 02:55 am by Kabloona »

Offline 2552

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Re: Pushing the limits of the hoverslam landing
« Reply #72 on: 05/07/2016 03:17 am »
It was said on the hosted webcast at about 27:34.
« Last Edit: 05/07/2016 04:07 am by 2552 »

Offline schaban

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Pushing the limits of the hoverslam landing
« Reply #73 on: 05/07/2016 03:47 am »
Merlin takes about 3 sec to start so whatever fuel savings gained in reduced gravity losses using 5 engines might get lost in 6 sec of wasting fuel of extra 2 engines start up


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« Last Edit: 05/07/2016 03:47 am by schaban »

Offline ChrisWilson68

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Re: Pushing the limits of the hoverslam landing
« Reply #74 on: 05/07/2016 08:38 am »
Merlin takes about 3 sec to start so whatever fuel savings gained in reduced gravity losses using 5 engines might get lost in 6 sec of wasting fuel of extra 2 engines start up

Huh?  In what way do you think fuel is "wasted" during engine start-up?

Offline rocx

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Re: Pushing the limits of the hoverslam landing
« Reply #75 on: 05/07/2016 09:02 am »

Huh?  In what way do you think fuel is "wasted" during engine start-up?


The fuel/gases that come out during spin-up are nowhere near the exit velocity of a fully running engine, so they only give little thrust for their mass.
Any day with a rocket landing is a fantastic day.

Offline ChrisWilson68

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Re: Pushing the limits of the hoverslam landing
« Reply #76 on: 05/07/2016 09:52 am »

Huh?  In what way do you think fuel is "wasted" during engine start-up?


The fuel/gases that come out during spin-up are nowhere near the exit velocity of a fully running engine, so they only give little thrust for their mass.

You cut out the context of my post, and that makes all the difference.  I wasn't asking in a general sense about whether there was any Isp loss from engine start-up.  I was asking specifically how that poster I was responding to meant it.  In particular, that poster was quoting "six seconds" of wasted engine running time and drawing the conclusion from that that adding two more engines during the landing burn couldn't have any beneficial effect.

The point of the question was to elicit whether the poster thought some significant portion of those "six seconds" was simultaneously using a significant portion of the normal engine prop flow rate and at a significantly lower-than-average Isp.

Offline gin455res

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Re: Pushing the limits of the hoverslam landing
« Reply #77 on: 05/07/2016 01:55 pm »
a) What kind of upper stages might one design for a Falcon-3R, or a Falcon-5R?

(Is a Falcon-3R with parachute light enough to be caught by a helicopter?) 
     
 
« Last Edit: 05/07/2016 01:59 pm by gin455res »

Offline Kabloona

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Re: Pushing the limits of the hoverslam landing
« Reply #78 on: 05/07/2016 01:58 pm »
a) What kind of upper stages might one design for a Falcon-3R, or a Falcon-5R?

(Is a Falcon-3R with parachute light enough to be caught be a helicopter?) 
     

That's a question for a different thread, like this one:

http://forum.nasaspaceflight.com/index.php?topic=39314.0

Offline gin455res

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Re: Pushing the limits of the hoverslam landing
« Reply #79 on: 05/08/2016 07:04 am »
A Falcon-5R would be equivalent to landing on 1.8 engines. How much fuel would that save?

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