Author Topic: SpaceX Eastern Range Landing Facilities  (Read 216968 times)

Offline mvpel

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Re: SpaceX Eastern Range Landing Facilities
« Reply #320 on: 05/01/2016 02:33 AM »
Here's a video from Cocoa Beach with three booms at the very end of the LZ-1 landing.  Got to the end, it's very clear.

"Crack-a-thoom," as Stan Lee might put it, at 1:59. Sonic booms echo, and the viewers were quite a ways away, so I'd be more inclined to chalk it up to an echo or some distortion or smearing of the sound as it traveled through the layers atmosphere than multiple shockwaves.
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Offline meekGee

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Re: SpaceX Eastern Range Landing Facilities
« Reply #321 on: 05/01/2016 05:14 AM »
Here's a video from Cocoa Beach with three booms at the very end of the LZ-1 landing.  Got to the end, it's very clear.

"Crack-a-thoom," as Stan Lee might put it, at 1:59. Sonic booms echo, and the viewers were quite a ways away, so I'd be more inclined to chalk it up to an echo or some distortion or smearing of the sound as it traveled through the layers atmosphere than multiple shockwaves.

I don't think so.  That rumble afterwards is echos.  The "ratatat" sounds like three primary shocks.   You'd need a small and focused reflector to generate a sharp retort like that as an echo.  Mostly it's from the distributed environment, and that's why it's a rumble.
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Offline mvpel

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Re: SpaceX Eastern Range Landing Facilities
« Reply #322 on: 05/01/2016 02:06 PM »
I think the rumble afterward may be the landing burn, it reminded me of the CRS-8 launch rumble.
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Offline mvpel

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Re: SpaceX Eastern Range Landing Facilities
« Reply #323 on: 05/01/2016 02:38 PM »
You called it - here's a more pronounced series of three booms at 2:21:



And at 10:24:

"Ugly programs are like ugly suspension bridges: they're much more liable to collapse than pretty ones, because the way humans (especially engineer-humans) perceive beauty is intimately related to our ability to process and understand complexity. A language that makes it hard to write elegant code makes it hard to write good code." - Eric S. Raymond

Offline CorvusCorax

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Re: SpaceX Eastern Range Landing Facilities
« Reply #324 on: 05/02/2016 09:42 AM »
There have been speculations in this thread that Falcon Heavy first flight would not land all boosters due to missing landing capability at eastern range (contingency pads not yet built and not enough drone ships)

Now we have a tweet:

https://twitter.com/elonmusk/status/726301598398312448

Quote
Quote
Danny S. Parker ‏@dannysparker Apr 29

@elonmusk For 1st launch of Falcon Hvy will there be effort to simultaneously land all 3 booster stages?  #FalconHeavy

Elon Musk Verified account
‏@elonmusk

@dannysparker yes

That raises multiple questions regarding the how:

1. Extra pads being built?
2. Multiple landings on the same large enough central pad?
3. Extra drone ships?

Would they even do center core RTLS?

This is going to be fun, at least now we know no one is going to throw cores away - or any such nonsense ;)

Online timverhoeven

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Re: SpaceX Eastern Range Landing Facilities
« Reply #325 on: 05/02/2016 10:52 AM »
That raises multiple questions regarding the how:

1. Extra pads being built?
2. Multiple landings on the same large enough central pad?
3. Extra drone ships?

Would they even do center core RTLS?

I'm guessing they are going to let de 2 side cores do a RTLS. If they don't build an extra pad somewhere, my best guess would be that they would set them to land left and right of the, large, pad they have now. It is pretty big.

The central core would most likely do a barging on the drone ship since it will be a lot further downrange then the side cores.

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Re: SpaceX Eastern Range Landing Facilities
« Reply #326 on: 05/02/2016 09:02 PM »
Regarding the unusual 3 sonic booms for the falcon 9 landings, this would seem to make sense if you think about it a moment.  An object traveling supersonic will produce a shockwave.  Every protrusion from this object will produce its own wave.  For most objects these waves tend to align at the front/back of the object and thus produce 2 booms.  This is characterized by the phrase “N wave”.  The shape of the pressure wave has a characteristic N shape.  The sudden rise fall rise of the pressure wave produces the 2 booms that most people find startling and unpleasant.  (Recently NASA has been doing research projects to try to predict and spread out these multiple pressure waves.  The intent is to flatten out the N thus producing a more soft and acceptable rumble) So the returning falcon 9, like most supersonic objects, should produce the characteristic N shaped wave with its 2 sonic booms.  But at this point the falcon 9 does something unprecedented, it nearly instantaneously makes itself appeared to be approximately 3 times longer.  It does this by starting its engine facing into a supersonic airflow in what is now the front of the vehicle.  The outflow from this engine stretches out to between 1 and 2 times the length of the vehicle and at that this point that it interacts with the air around it to produce a new wave front.  This is why I believe you hear a boom, followed by a short pause and then almost 2 overlapping booms.  As the 2 booms at the back and front respectively of the falcon 9 are still being produced at the time of the engine start you get 3 total booms.

Offline CorvusCorax

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Re: SpaceX Eastern Range Landing Facilities
« Reply #327 on: 05/02/2016 11:11 PM »
Regarding the unusual 3 sonic booms for the falcon 9 landings, this would seem to make sense if you think about it a moment.  An object traveling supersonic will produce a shockwave.  Every protrusion from this object will produce its own wave.  For most objects these waves tend to align at the front/back of the object and thus produce 2 booms.  This is characterized by the phrase “N wave”.  The shape of the pressure wave has a characteristic N shape.  The sudden rise fall rise of the pressure wave produces the 2 booms that most people find startling and unpleasant.  (Recently NASA has been doing research projects to try to predict and spread out these multiple pressure waves.  The intent is to flatten out the N thus producing a more soft and acceptable rumble) So the returning falcon 9, like most supersonic objects, should produce the characteristic N shaped wave with its 2 sonic booms.  But at this point the falcon 9 does something unprecedented, it nearly instantaneously makes itself appeared to be approximately 3 times longer.  It does this by starting its engine facing into a supersonic airflow in what is now the front of the vehicle.  The outflow from this engine stretches out to between 1 and 2 times the length of the vehicle and at that this point that it interacts with the air around it to produce a new wave front.  This is why I believe you hear a boom, followed by a short pause and then almost 2 overlapping booms.  As the 2 booms at the back and front respectively of the falcon 9 are still being produced at the time of the engine start you get 3 total booms.

I always thought by the time the landing burn started the vehicle was already slowed down to terminal velocity and subsonic.

I am pretty sure I recall from the webcasts launch-com-net the events being called in the following order:

- reentry burn started
- reentry burn finished
- vehicle is transsonic
- landing burn started
- legs deployed
- landed

the vehicle would stop producing a sonic boom once its below the transonic region, (and it would certainly not stay in the transonic region for very long, as air drag spikes during that.

(
Drag is significantly less when fully supersonic, and also when fully subsonic - the transonic region is when the vehicle itself is technically subsonic, but the airflow being forced around the vehicle still reaches supersonic relative flow velocities, which creates massive drag (thus the term "sound barrier", as its very hard to overcome when speeding up)
)

that rules out the landing burn as a source for sonic booms. For your explanation to make any sense, the boom being heard would still have to originate from when the vehicle is in its reentry phase at >Mach 4.  Although it's possible that this shockwave eventually catches up with the vehicle after the vehicle itself slowed down below sonic speed, it would still be created incredibly high up and far away. I think by the time this pressure wave reaches the ground it has diminished a lot. Also the majority of this early, reentry sound front would be travelling perpendicular to the flight path and not end up at the landing site. (but possibly be heard by someone watching the reentry burn from further away)

also this shockwave only travels with Mach 1, so the earliest sign of a sonic boom heard at or near the landing site would be the shockwave produced just as the vehicle transients to subsonic, as this is the latest and closest part of the shock front being created (and anything produced earlier would likely be heard just as a diminishing rumble that goes on for a while, as sound produced earlier in the flight path arrives)

The simplest explanation for 3 booms is

1. bow shock at bottom of vehicle
2. grid fin shock wave produced at beginning of interstage (this is F9 specific, other rockets don't do that)
3. trailing shock wave produced by the blunt top end above interstage

of these, 2nd and 3rd would be very close together and likely hard to tell apart. the first would likely a bit ahead, as the stage is pretty long, even at 300 m/s it takes sound around .2 seconds to travel the length of the stage - that's easily discernible as 2 booms. But the double-boom from grid fins and tail end would likely merge into a seemingly single event unless you are just in the right spot in the right distance.


Edit: I just remembered a detail regarding the grid fins - they don't work in the transonic region. At subsonic speeds you have laminar flow through the grid fins. and at high enough supersonic speeds, each "leading edgelet" of the grid produces its own mini-shockwave, which widen narrowly and cancel each other out way behind the grid fin - so the grid fin has a good flow and steering authority - and likely barely any sonic boom.

That changes when the vehicle is in transonic. The air forced through the grid fin still flows at relative supersonic speeds, but the shock fronts are so wide-angled, they don't fit through a grid hole anymore - as thus the shock front "blocks" the holes in the grid and the entire fin acts as one solid "paddle" that blocks airflow.

in this configuration, just before the vehicle goes subsonic, the grid fins very likely would create a very loud 3rd sonic boom. But only during the very last part of the subsonic transition. If you get your sonic boom from earlier in the flight (because you watch from further "downrange") you wouldn't get the 3rd boom because at those speeds the grid fins hardly produce any.

« Last Edit: 05/02/2016 11:23 PM by CorvusCorax »

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Re: SpaceX Eastern Range Landing Facilities
« Reply #328 on: 05/03/2016 12:06 AM »
Transonic is generally described as the speed between .80 – 1.2 of Mack.  It is generalized this way because the more precise definition is “speed at which the airflow around the object is both supersonic and subsonic even though the overall speed of the object may be supersonic or subsonic “.  Any object that protrudes into the airflow will change both the density and the speed of this airflow.  This means that at speeds starting at transonic shockwaves will start to be produced.  After the reentry burn the falcon 9 will be traveling at high supersonic velocities.  This will also produce a high amount of drag to slow the vehicle down.  It will continue to slow down into the trans-sonic region but without a propulsive engine burn would probably not slow down to full subsonic velocities before it hits the ground.  Even the callouts you remember hearing somewhat reinforce this.  Yes the vehicle enters the transonic range but immediately after that it starts its engine burn.  Since we are talking about the high side of transonic this means the vehicle is going approximately Mack 1.2-1.1.  At these speeds it will produce sonic booms.  Even though the grid fins will lose some of their control authority they will produce a sonic boom until the vehicle is fully subsonic.  As these are so close to the aft end of the vehicle the shockwaves would be lay down so close to the shock wave produced from the aft of the vehicle that to the human observer they would be one and the same.  The same as the shockwave produced off of a supersonic fighter in the aft end of the aircraft would be perceived as one and the same.  There are multiple other shockwaves produced along the length of the Falcon 9 but they are both small and not noticeable in comparison or become combined with the other shockwaves from the front and back of the vehicle.  The Falcon 9s landing 3 sonic booms are somewhat unique as I cannot remember reading of any other like occurrences for any other single object.  The slightly longer pause between the 1st boom with the smaller spacing between the next 2 booms is what leads me to believe that the 1st boom is produced by the longer aero spike affect from the rocket engine exhaust.

Offline CorvusCorax

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Re: SpaceX Eastern Range Landing Facilities
« Reply #329 on: 05/03/2016 01:13 AM »

Let's assume the sonic booms were from the vehicle itself, travelling at 300 m/s (transonic range)

length of stage 1 is 41m (without interstage)  then comes the grid fins, then the interstage (used to be 6.5m but got lengthened, so around 7) and then the trailing edge.

if this were to produce the 3 booms, there would be 130 milliseconds between first and second boom and 23 milliseconds between second and 3rd boom. That is long enough for all 3 to be heard separately.

One "_" or "X"  for 10 milliseconds each, the tripple-boom would sound like this:

________________X___________X_X______

with the first and last X around 150 ms apart.

It's subjective of course, but I think this is what is heard in the videos


If the first boom was by an aero-spike effect, this would change. Judging by the CRS-6 video:

https://www.youtube.com/watch?v=BhMSzC1crr0&feature=youtu.be&t=6

The "aerospike" shockwave of the flame is longer than the visible flame. Its reach can be seen hitting the water as the stage throttles in the final manouver, at a distance of roughly one stage size (including interstage) give or take a few meters.


Assuming 150ms delay for the full stage length, that would be another 150 ms "ahead" so like this:

________________X_____________X______________X___________

where the first boom would be the leading shock of the aerospike, the second would be by the actual engine, and the third by combined grid fins and trailing shock

however I don't think  the aerospike would even reach that far when the vehicle is still transonic and the engine is just powering up, so with a shorter flame - it would more likely look like this:

________________X______X______________X___________


That is however not the boom pattern audible in the posted videos. The "claps" are not spaced 300 or more milliseconds apart, but in rapid succession, and the long gap is after the first, matching the "leading edge + grid fins + trailing edge" model.

Offline CorvusCorax

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Re: SpaceX Eastern Range Landing Facilities
« Reply #330 on: 05/03/2016 01:47 AM »
I checked the audio in a sound sample editor.

The first sonic boom and the second sonic boom are 184 milliseconds apart.
The second sonic boom and the third sonic boom are 105 milliseconds apart.

so like

__________X________________X________X___________

Assuming a vehicle traveling at 340 m/s (Mach 1) and identical sound propagation paths and speeds for all three booms, that would correspond to a distance of 62 meters between first and second boom and 35 meters between second and third boom

that matches neither model ;)

Online S.Paulissen

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Re: SpaceX Eastern Range Landing Facilities
« Reply #331 on: 05/03/2016 02:02 AM »
Consider the encoding of the audio.  How often does it refresh?  Does this capture rate change the resolution of your time measurements. 

Also, the trailing edge sonic boom of an N-wave is not strictly at the location of the top of the interstage.  It takes time/distance travelled for the air to collapse in the wake of the stage.
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Offline AnalogMan

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Re: SpaceX Eastern Range Landing Facilities
« Reply #332 on: 05/03/2016 02:29 AM »
I had a also had a look with an audio editor for both video clip soundtracks with similar results:

1st to 2nd boom:  183.1 ms & 182.6 ms
2nd to 3rd boom:  104.0 ms & 102.4 ms

Both tracks were recorded at 44.10 ksps, giving intervals between samples of ~22.7 microseconds.

If the audio response of the cameras was good to 16 kHz, then the recorded rise-time of a fast pressure step or edge would be around 22 microseconds (10% to 90% levels).
« Last Edit: 05/03/2016 02:56 AM by AnalogMan »

Offline CameronD

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Re: SpaceX Eastern Range Landing Facilities
« Reply #333 on: 05/03/2016 04:36 AM »
I checked the audio in a sound sample editor.

The first sonic boom and the second sonic boom are 184 milliseconds apart.
The second sonic boom and the third sonic boom are 105 milliseconds apart.

so like

__________X________________X________X___________

Assuming a vehicle traveling at 340 m/s (Mach 1) and identical sound propagation paths and speeds for all three booms, that would correspond to a distance of 62 meters between first and second boom and 35 meters between second and third boom

that matches neither model ;)

I'm curious:  why would you assume identical sound propagation paths given the stage is descending at high speed through the atmosphere (ie. towards the microphone) with possibly different air densities - maybe boundary layers even?
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Offline Kabloona

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Re: SpaceX Eastern Range Landing Facilities
« Reply #334 on: 05/03/2016 10:08 AM »
Quote
I'm curious:  why would you assume identical sound propagation paths given the stage is descending at high speed through the atmosphere (ie. towards the microphone) with possibly different air densities - maybe boundary layers even?

Because regardless of vehicle speed, the shock waves are generated only 150 feet or so apart (the length of the stage), and atmospheric conditions don't vary enough across any 150-foot span to make a significant difference between bow shock and " tail" shock paths.
« Last Edit: 05/03/2016 12:35 PM by Kabloona »

Offline cscott

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Re: SpaceX Eastern Range Landing Facilities
« Reply #335 on: 05/03/2016 02:03 PM »
Remember that the speed of sound varies with altitude (and weather, for that matter).  I think the results we have are consistent with the second and third booms being from the stage, and the first being from the retropropulsion bow shock.  In fact, I suspect you can calculate the altitude of the stage at the time it goes transonic from the timing we observe and the known length of the stage.

Offline CorvusCorax

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Re: SpaceX Eastern Range Landing Facilities
« Reply #336 on: 05/03/2016 03:45 PM »
I projected the boom timings back on the stage assuming Mach 1 speed:


Considering the too-long delay between first and second boom (longer than rocket length) I took into account that landing burn ignition might indeed possibly occur while still in the transonic region.

Assuming the 2nd boom originating from the grid fins, the bow shot origin location matches almost exactly the visible flame length from the CRS-8 video.

What I don't get is how the trailing shock wave ends up so far behind. Is that realistic?
« Last Edit: 05/03/2016 03:48 PM by CorvusCorax »

Offline cscott

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Re: SpaceX Eastern Range Landing Facilities
« Reply #337 on: 05/03/2016 04:52 PM »
17s out decelerating at 20m/s/s, the stage will be 2.9km up.  The speed of sound at that altitude is 328m/s, according to http://www.fighter-planes.com/jetmach1.htm. That shrinks your distances somewhat.

Further, I'm pretty sure the "second" shock will be from the front of the stage, not from the grid fins.  It wouldn't be surprising for the "first" shock to be pretty far out, considering how exhaust expands with altitude.
« Last Edit: 05/03/2016 04:54 PM by cscott »

Offline CorvusCorax

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Re: SpaceX Eastern Range Landing Facilities
« Reply #338 on: 05/03/2016 05:19 PM »
340 vs 328 m/s is negligible - that would make the distances 4% shorter which isn't even one full diameter of the stage.

A bigger difference could be the speed of the stage. I assumed it would be at Mach 1 to still be able to produce a sonic boom. If it somehow still creates a boom at Mach 0.8, that would be 20% size reduction and as such more significant.

Your expansion with altitude wouldn't matter that much. Although the plume does expand a lot as can be seen on launch tracking - that happens at really high altitude beyond 30k feet.

At 10k ft the pressure is still more than 70% sea level pressure, as such the expansion of the exhaust would not be that significant. (Less than 30%)

Mostly this would make the plume wider, but not longer, as the stage is retrothrusting into a ~Mach 1 airstream. Under these conditions the high pressure area in front of the stage would be much shorter than when it's at an almost standstill just before touchdown.

Definitely not long enough to produce the second boom with the engine section. In fact - IF the engine is already running, I doubt there would be much of a shock front from the engines themselves at all, as the air will be pushed around them by the engine created high pressure area (the same principle that protects the stage during reentry)


Offline biosehnsucht

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Re: SpaceX Eastern Range Landing Facilities
« Reply #339 on: 05/03/2016 08:40 PM »
As I understand supersonic retropropulsion, when thrusting from a single engine through the center of the shock front, it doesn't actually push the shock front away from the engine, but instead punches through it, and causes a loss of the aerodynamic drag you'd normally gain from the shockwave. For the shock front that would be at the engines (if  they weren't running) to push out ahead of the engines they would need to run the 8 outer engines to push the shockwave ahead of the stage, not the center engine.

Would the exhaust itself on it's own generate a shock front? Since the center engine running is "punching through" the shockwave, rather than pushing it away as running engines along the periphery of the stage would do, shockwave drag should be almost entirely eliminated with only thrust from the engine providing any major reduction in speed. What would this do to the shockwave, does it somehow "collapse" or otherwise cease to exist at that area? Perhaps this is part of why nothing seems to line up with observed audio...

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