Author Topic: SpaceX Reusable Falcon 9 (Grasshopper ONLY) DISCUSSION Thread (4)  (Read 360096 times)

Offline KelvinZero

  • Senior Member
  • *****
  • Posts: 4286
  • Liked: 887
  • Likes Given: 201
er.. I still dont know, because you asked a series of difficult quantitative questions rather than provide any informative quantitative answers.

You speculated about turning the stage sideways *before* the last burn. That implies RCS only. I maintain there's no RCS capability in the stage that can orient it sideways when the CoG is so much more forward than the CoP. As simple as that.
Maybe it sticks a leg out covered in sexy black nylon. I repeat, its silly to jump to detail like that before establishing what sort of gain there could be anyway, a calculation someone might be able to actually do. If it is a factor of 2 or 10 then maybe its exciting enough to apply some imagination to. You have already decided it is not that interesting which makes this conversation pretty dull for me also.

Lets just drop it, ok?

Offline hrissan

  • Full Member
  • ****
  • Posts: 411
  • Novosibirsk, Russia
  • Liked: 325
  • Likes Given: 2432
Finally analysed some of the high-quality 60fps video of Grasshopper 744m flight.

I used the shadow as an high-quality indicator of Grasshopper position. I assumed that its trajectory was straight vertical line above the pad. I used both the shadow of the tip and shadow of the leg, because there were moments when smoke made either of them invisible (but not both!) then stitched data points.

I accurately measured the last 12 seconds of flight. At the last second before landing it was impossible to discern any part of shadow, so there is a gap in the data. But the zero altitude is accurately measured using the shadow position later when the smoke drifted away.

I used the moment when the camera was horizontal at a 29.6 seconds to get camera parameters (vertical fov is 45 degrees) and approximate height of camera and the stage (around 500 meters) to check for the large mistakes.

I plotted the height and velocity, it definitely consists of 2 acceleration regions - roughly first 8 and last 4 seconds. I plotted the polynomial trend line on the mirrored plot of height to see this better.

The first 8 second looked very much like the constant-speed with speed around 32 meters/sec.

The last 4 seconds looked very much like constant-acceleration with acceleration around whopping 8.5 meters/sec/sec. This is the highest acceleration observed to date, and great source for speculations!

Then I analysed the ascent part, using several well-known shadow positions from descent part, then used mirrored plot with trend line to get the acceleration. It is around 1.4 meters/sec/sec which is similar to the previous ascents.

Edit: found a way to normalise the grasshopper height. I used the shadow azimuth (148.3 degrees) to get the sun celestial parameters on flight day, sun angle above horizon (it turned to be +47.5 degrees) allows precise normalisation of grasshopper position based on the shadow position. The test took place at 10:54 local time McGregor. :)
« Last Edit: 11/07/2013 12:55 pm by hrissan »

Offline baldusi

  • Senior Member
  • *****
  • Posts: 8356
  • Buenos Aires, Argentina
  • Liked: 2539
  • Likes Given: 8273
What's the estimation margin of error on the m/s?

Offline hrissan

  • Full Member
  • ****
  • Posts: 411
  • Novosibirsk, Russia
  • Liked: 325
  • Likes Given: 2432
What's the estimation margin of error on the m/s?
The analysis of errors of my calculations is much more complicated than the calculations themselves, that's for sure. I know how to do it properly, just afraid to spend the required time.

On previous flights I always had very good calibration of the height by the SpaceX-advertised highest point, so the overall height plot magnitude was accurate to may be ~5%, so the acceleration based on trend lines would be also around ~5% (wild guess).

The last 744m video is the only one where such calibration is very difficult, as the camera points up in the blue sky for many seconds of flight. That's why I did not plot the whole flight from ascent to descent, instead focusing on the "easy" regions.

I used 2 means of calibration - first one at the point of camera being horizontal, using remote land features. Second one being the height of the stage tank.

Let's say the acceleration errors are may be 20% for the 744m flight (wild guess again).

Offline Comga

  • Senior Member
  • *****
  • Posts: 6466
  • Liked: 4572
  • Likes Given: 5136
Nice work hrissan but..
Those cannot be the fit values.  (Despite Excel being like baby talk for engineers, I use it all the time. One just has to be careful.  In it one can use the LINEST array function to get the polynomial terms for the fit.  We could also look into the sensitivity of the results to the choice of the minimum altitude time.)

Look at the "last 3 seconds" graph.  Take out the time bias by setting 82.4 seconds to t'=0.  The graph extends 82.4-79.6=2.80 seconds on both sides.  The parabola is about 31 meters high.  2*31/2.8^2=7.91.  The quadratic term should be 7.91, not 0.156.  The units are meters per second squared, SI acceleration.
To check, the distance under constant acceleration is 0.5*a*t^2=0.5*7.91*2.80^2=31 meters.
At  7.91 m/s^2 this represents 0.81 g for a T/W ratio of 1.81:1. 
That's pretty close to 2, which is about the value that as been discussed here, IIRC.

I would have hoped to see flights with higher terminal decelerations, but Grasshopper has been declared retired.
You can do a better job with your original values.  (Can we have your spreadsheet with the values?)
Again, nice work deriving the values.
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Offline cambrianera

  • Full Member
  • ****
  • Posts: 1438
  • Liked: 318
  • Likes Given: 261
Thanks hrissan, the values of the ascent part copes well with the simulation I tried after the flight:
http://forum.nasaspaceflight.com/index.php?topic=32718.msg1108494#msg1108494
what's really interesting is the final deceleration, really high for GH1, due to the reduced max thrust of his engine and the high empty mass.
No way to obtain it in my simulation unless using values of engine thrust greatly outside the 100-70% range.
Oh to be young again. . .

Offline hrissan

  • Full Member
  • ****
  • Posts: 411
  • Novosibirsk, Russia
  • Liked: 325
  • Likes Given: 2432
Nice work hrissan but..
Those cannot be the fit values.  (Despite Excel being like baby talk for engineers, I use it all the time. One just has to be careful.  In it one can use the LINEST array function to get the polynomial terms for the fit.  We could also look into the sensitivity of the results to the choice of the minimum altitude time.)

Look at the "last 3 seconds" graph.  Take out the time bias by setting 82.4 seconds to t'=0.  The graph extends 82.4-79.6=2.80 seconds on both sides.  The parabola is about 31 meters high.  2*31/2.8^2=7.91.  The quadratic term should be 7.91, not 0.156.  The units are meters per second squared, SI acceleration.
To check, the distance under constant acceleration is 0.5*a*t^2=0.5*7.91*2.80^2=31 meters.
At  7.91 m/s^2 this represents 0.81 g for a T/W ratio of 1.81:1. 
That's pretty close to 2, which is about the value that as been discussed here, IIRC.

I would have hoped to see flights with higher terminal decelerations, but Grasshopper has been declared retired.
You can do a better job with your original values.  (Can we have your spreadsheet with the values?)
Again, nice work deriving the values.
This is Apple Numbers, and its "equation" on trend line is as stupid as it can be. "x" there is not time, but cell row number!!! I have time resolution of 0.2 seconds, so to get acceleration, the 0.156 should be multiplied by 5*5 and then by 2. If you look at my post, you find that I calculated acceleration very close to your value. :)

Offline hrissan

  • Full Member
  • ****
  • Posts: 411
  • Novosibirsk, Russia
  • Liked: 325
  • Likes Given: 2432
Found a way to normalise the grasshopper height. I used the shadow azimuth (148.3 degrees) to get the sun celestial parameters on flight day, sun angle above horizon (it turned to be +47.5 degrees) allows precise normalisation of grasshopper position based on the shadow position. The test took place at 10:54 local time McGregor. :)

Offline hrissan

  • Full Member
  • ****
  • Posts: 411
  • Novosibirsk, Russia
  • Liked: 325
  • Likes Given: 2432
Thanks hrissan, the values of the ascent part copes well with the simulation I tried after the flight:
http://forum.nasaspaceflight.com/index.php?topic=32718.msg1108494#msg1108494
what's really interesting is the final deceleration, really high for GH1, due to the reduced max thrust of his engine and the high empty mass.
No way to obtain it in my simulation unless using values of engine thrust greatly outside the 100-70% range.
I compiled and attached the plumes of engine from the flights...

It seems there were never such an intensive thick and long flame as on the last flight. Either SpaceX decided to test Merlin prototype outside its nominal thrust levels or they swapped the engine to production Merlin 1D.

They already tested multiple in-flight restarts of Merlin 1D on the real flight of Falcon 9 1.1, before they could test it on Grasshopper flight.

Offline Lars_J

  • Senior Member
  • *****
  • Posts: 6160
  • California
  • Liked: 677
  • Likes Given: 195
An interesting comparison, but one note - the last flight was pretty close to the hexacopter, so the wide angle would make the flame look shorter than it is.

Online ohlongjohnson

  • Member
  • Posts: 46
  • Liked: 25
  • Likes Given: 21
Gwynne Shotwell spoke at ISPCS and mentioned a few interesting things I haven't found in this thread.

- 25 People work on grasshopper project (23:32)
- F9R in NM will be a full falcon9 first stage, it will have 9 engines retractable landing gear (24:10)
- First Stage was INTACT until 3 meters above the water surface (24:40)
- First flight from NM hopefully in December (25:43ff)

Did I miss something or is the confirmation of 9 engines and the december launch date for GH2 new?
Is it news, that the stage was intact until it hit the ocean?

Offline Lars_J

  • Senior Member
  • *****
  • Posts: 6160
  • California
  • Liked: 677
  • Likes Given: 195
Is it news, that the stage was intact until it hit the ocean?

No, that was live-tweeted and commented on here when she made that speech on Oct 16.

Offline hrissan

  • Full Member
  • ****
  • Posts: 411
  • Novosibirsk, Russia
  • Liked: 325
  • Likes Given: 2432
An interesting comparison, but one note - the last flight was pretty close to the hexacopter, so the wide angle would make the flame look shorter than it is.
Not much. All flights from helicopter were captured using the same 45' vertical fov lense. And the distances on all my screenshots are very close - I resized the frames no more than +-20% to make the tank the same height.

Offline AJW

  • Full Member
  • ****
  • Posts: 807
  • Liked: 1315
  • Likes Given: 136
It seems there were never such an intensive thick and long flame as on the last flight. Either SpaceX decided to test Merlin prototype outside its nominal thrust levels or they swapped the engine to production Merlin 1D.

Shotwell commented that on the most recent flight GH1 traveled above the hexacopter camera frame because it exceeded the pre-programmed altitude.  If the engine was upgraded and/or the thrust levels were higher, this might explain the overshot.
We are all interested in the future, for that is where you and I are going to spend the rest of our lives.

Offline Lars_J

  • Senior Member
  • *****
  • Posts: 6160
  • California
  • Liked: 677
  • Likes Given: 195
Flame length depends on thrust level, so if we don't know that for the specific frame in question, comparisons aren't very useful.

Offline Kabloona

  • Senior Member
  • *****
  • Posts: 4846
  • Velocitas Eradico
  • Fortress of Solitude
  • Liked: 3429
  • Likes Given: 741
Flame length depends on thrust level, so if we don't know that for the specific frame in question, comparisons aren't very useful.

I'm wondering if that flame length could also be due to a different mixture ratio. Would a more fuel-rich exhaust  have more afterburning and therefore a longer apparent flame? If so, perhaps they were varying mixture ratios in-flight.
« Last Edit: 11/08/2013 10:05 pm by Kabloona »

Offline hrissan

  • Full Member
  • ****
  • Posts: 411
  • Novosibirsk, Russia
  • Liked: 325
  • Likes Given: 2432
Flame length depends on thrust level, so if we don't know that for the specific frame in question, comparisons aren't very useful.
They may be useful. I plotted longest flames from previous flights, and both the longest and shortest flames from the 744m flight. There were always solution for the GH trajectory using thrust levels of 70%-100% of prototype one-of-a-kind Merlin 1D used on GH1, but on the 744m flight there is no solution. It is apparent, to get constant-velocity down you need thrust equal to weight. To get 8m/s/s deceleration you need thrust almost twice as weight.

The flight profile up is very clear - large thrust for half way up, then switch to small thrust (I plotted both flame sizes).

Then the question is - large thrust larger or small thrust smaller?

The excess baseline thrust of Merlin can be compensated by adding more ballast. Let's say you fly with Merlin prototype with max thrust 50000 kg, minimal thrust 35000 kg, then replace it with production Merlin (or change engine parameters) with max thrust 75000 kg, minimal thrust 52500 kg, but add 25000 kg of ballast.

Your "effective thrust minus added ballast" then has the same maximum value (50000 kg), but reduced minimum value (27500 kg).

Gwynne told that there was unexpected "overshot", it is very strange for a vehicle which flew many times and has no unknowns, so it appears the engine parameters were modified and tested not on the test stand, but in flight.
« Last Edit: 11/09/2013 11:23 am by hrissan »

Offline cambrianera

  • Full Member
  • ****
  • Posts: 1438
  • Liked: 318
  • Likes Given: 261
Expanding a little on differences between Merlin 1D prototype and production.
Prototype was clearly:
sandwich nozzle (not brazed pipes as 1C).
TPA similar to 1C.

Switching to sandwich nozzle you have smaller pressure drop compared to 1C, then increase of chamber pressure still mantaining old 1C TPA.
This explains the increased thrust of prototype compared to 1C (but lower compared to production 1D).
If SpaceX managed to upgrade the engine installing new 1D TPA,  they got a full thrust 1D production engine.
Oh to be young again. . .

Offline Avron

  • Canadian Member
  • Senior Member
  • *****
  • Posts: 4930
  • Liked: 156
  • Likes Given: 160
Anyone have a HI res of this pic from NM that Gwynn presented?
« Last Edit: 11/09/2013 04:23 pm by Avron »

Offline Avron

  • Canadian Member
  • Senior Member
  • *****
  • Posts: 4930
  • Liked: 156
  • Likes Given: 160

found..

presentation at http://www.ispcs.com/files/ww/files/ISPCS%20FINAL%20Oct%202013%20Shotwell.pdf


image attached.. if that a very old image?

Tags:
 

Advertisement NovaTech
Advertisement Northrop Grumman
Advertisement
Advertisement Margaritaville Beach Resort South Padre Island
Advertisement Brady Kenniston
Advertisement NextSpaceflight
Advertisement Nathan Barker Photography
0