Author Topic: SpaceX Falcon 9 - Iridium NEXT Flight 1 DISCUSSION (Jan. 14 2017)  (Read 339584 times)

Offline LouScheffer

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Matt Desch (Iridium CEO) confirmed yesterday on Twitter that the launch window will be instantaneous (see below).
Not sure if it was obvious or not, so posting in the discussion thread.

Na45597459: @IridiumBoss is Sunday's launch instantaneous or does it have a window?
IridiumBoss: @Na45597459 Instantaneous.  Need to launch to a specific plane (#6) and that's when it's overhead that day.
https://twitter.com/iridiumboss/status/816018732262817793
So F9 can't do yaw steering like Atlas V when launched out-of-plane?
The hardware seems like it should be capable of doing this, and the needed performance is there for at least a few minute window.  So my guess is that the software does not support this. 

Implementing yaw steering might be a low priority for SpaceX since steering gets expensive once your time from optimum exceeds a few minutes.  And if I remember right, it takes SpaceX about 10 minutes to recycle the count.  By that time the needed yaw steering would only be practical for missions with lots of extra performance.  Also, yaw steering would throw yet another wrinkle into recovery of the first stage.  So maybe they decided to stick with instantaneous windows for now.

Offline Jim

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The hardware seems like it should be capable of doing this,

Based on what?  Do you know the avionics architecture?
« Last Edit: 01/04/2017 06:27 PM by Jim »

Online Chris Bergin

Per the Static Fire attempt. Nothing's been held back...there's just nothing to report. :-\

Online Chris Bergin

This isn't from us, but the above reference to delays with Static Fire could make this fit:

https://twitter.com/VincentLamigeon/status/816656697708187648

#SpaceX Falcon 9 return to flight finally planned on Monday Jan 9th, a little bird told me. Backup dates Jan 11 & 12th

--

Totally unconfirmed of course, but it played to my tapping of fingers against the desk waiting for word on F9 drinking RP-1. ;)

Live webcam of me right now:
« Last Edit: 01/04/2017 06:56 PM by Chris Bergin »

Offline Robotbeat

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The hardware seems like it should be capable of doing this,

Based on what?  Do you know the avionics architecture?
It seems this would be a software limitation, not a hardware one, right?
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline LouScheffer

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The hardware seems like it should be capable of doing this,

Based on what?  Do you know the avionics architecture?
Well, the first stage can clearly do closed loop steering of large magnitude, or it could not land.

The second stage can do closed loop steering, or it could not get accurate insertion, or recover from a first stage problem, which it has demonstrated.

Determining the desired trajectory is quite straightforward compared to landing.

So what do you imagine that the avionics could not do?


Offline Jim

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The second stage can do closed loop steering, or it could not get accurate insertion, or recover from a first stage problem, which it has demonstrated.


That has no bearing on the matter.  Delta can not do yaw steering or launch windows for planetary missions, yet it can provide accurate insertion or recover from a first stage problem. 



Determining the desired trajectory is quite straightforward compared to landing.


Not really.  There are sensors that help find the landing pad and the landing pad is fixed wrt the trajectory.  And it does change its relative location wrt time.
« Last Edit: 01/04/2017 07:43 PM by Jim »

Offline Robotbeat

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The second stage can do closed loop steering, or it could not get accurate insertion, or recover from a first stage problem, which it has demonstrated.


That has no bearing on the matter.  Delta can not do yaw steering or launch windows for planetary missions, yet it can provide accurate insertion or recover from a first stage problem.
But this is a software limitation, not a hardware one, correct?
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Jim

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The second stage can do closed loop steering, or it could not get accurate insertion, or recover from a first stage problem, which it has demonstrated.


That has no bearing on the matter.  Delta can not do yaw steering or launch windows for planetary missions, yet it can provide accurate insertion or recover from a first stage problem.
But this is a software limitation, not a hardware one, correct?

Don't you think they would have changed it by now if that were so?

Offline Robotbeat

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Not sure that they would have. Software is incredibly difficult when it needs to be certified and already works great. Also, ULA has Atlas V. Also, could be that there's limited memory or computing power on Delta, whereas I think Falcon uses more modern computing hardware and so would have less of an issue.

I can't imagine how it would be a literal hardware issue unless it has something to do with the ability to upload a new trajectory on the fly to the flight computer immediately before launch... But even that sounds largely like a software issue.

I honestly don't know how it could be a hardware issue, but I understand you might not be at liberty to tell us.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline pb2000

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#SpaceX Falcon 9 return to flight finally planned on Monday Jan 9th, a little bird told me. Backup dates Jan 11 & 12th

--

Totally unconfirmed of course, but it played to my tapping of fingers against the desk waiting for word on F9 drinking RP-1. ;)

I hope not. I was hoping for a leisurely Sunday drive from Nevada (and maybe even back again), but it would be an aweful shame to drive 7.5 hours, only to get stuck in traffic and miss the launch.
Launches attended: Worldview-4 (Atlas V 401), Iridium NEXT Flight 1 (Falcon 9 FT), PAZ+Starlink (Falcon 9 FT)

Offline LouScheffer

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The second stage can do closed loop steering, or it could not get accurate insertion, or recover from a first stage problem, which it has demonstrated.
That has no bearing on the matter.  Delta can not do yaw steering or launch windows for planetary missions, yet it can provide accurate insertion or recover from a first stage problem.
But this is a software limitation, not a hardware one, correct?
Don't you think they would have changed it by now if that were so?
It's entirely possible that for Delta, it's a hardware limitation.  The Delta-II RIFCA, which was in production in 1997, according to A Report on the Flight of Delta II's Redundant Inertial Flight Control Assembly (RIFCA), has 64K of RAM and 64K of program store.  The same unit is used in the Delta-IV.  The report of human rating Delta-IV called for replacing the RIFCA due to "significant limitations" in enhancing it, so it may well be near capacity.

But for SpaceX, it's almost surely software.  The first stage is solving convex optimization in real time, a much harder task than yaw steering.  And I'd be very surprised if SpaceX uses a different computer for the second stage - that would go very much against their philosophy.

Offline Jim

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But for SpaceX, it's almost surely software.  The first stage is solving convex optimization in real time, a much harder task than yaw steering.

Not true.  Targeting for yaw steering is harder.  Again, the landing pad is a static target and always be in the same place no matter what time it is launched.  Launching into a specific orbital plane at anytime within a launch window is much harder.

Offline Robotbeat

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But for SpaceX, it's almost surely software.  The first stage is solving convex optimization in real time, a much harder task than yaw steering.

Not true.  Targeting for yaw steering is harder.  Again, the landing pad is a static target and always be in the same place no matter what time it is launched.  Launching into a specific orbital plane at anytime within a launch window is much harder.
Respectfully, I disagree. Landing is harder on the software. Margin for error is far, far smaller and control scheme is different, having to blend thrust and aero surfaces with real-time sensor data from radar.

Not saying yaw steering is a stroll in the park. But I cannot see it being a harder software challenge than barge landing.
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Offline sdsds

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I see how for landing GPS sensing can help a lot. I don't see how GPS sensing helps with yaw steering because the orbital track we want to target is in motion from the perspective of a GPS reference frame.
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Offline Robotbeat

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I see how for landing GPS sensing can help a lot. I don't see how GPS sensing helps with yaw steering because the orbital track we want to target is in motion from the perspective of a GPS reference frame.
Who says it helps for yaw steering?
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Offline sdsds

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Who says [GPS] helps for yaw steering?

You make my point for me! Someone implicitly asserted that since the first stage has precision descent guidance capability, then it would ipso facto "only" require software changes to give it yaw steering ascent capability.

I'm only suggesting it's unclear (to me) that the F9 ascent guidance system even has all the sensor inputs available to it that the Atlas yaw steering algorithm might be using. There's no public enumeration of the sensor inputs available to either, is there?
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Offline dglow

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What do most second stages use for position and orientation IMUs and a star tracker?

Offline Robotbeat

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Who says [GPS] helps for yaw steering?

You make my point for me! Someone implicitly asserted that since the first stage has precision descent guidance capability, then it would ipso facto "only" require software changes to give it yaw steering ascent capability.
That's called a straw man.
Quote
I'm only suggesting it's unclear (to me) that the F9 ascent guidance system even has all the sensor inputs available to it that the Atlas yaw steering algorithm might be using. There's no public enumeration of the sensor inputs available to either, is there?
What sensors are needed for yaw steering?? As far as I can tell, none extra from the typical suite needed.
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Offline LouScheffer

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I see how for landing GPS sensing can help a lot. I don't see how GPS sensing helps with yaw steering because the orbital track we want to target is in motion from the perspective of a GPS reference frame.
Transforming positions and velocities from the GPS frame to an inertial frame is not at all complex.  The Earth is rotating at a very well known and constant rate (at least to the precision we need here) speed.   Any nav with an INS system uses this form of transformation already, since the vehicle itself may itself be rotating.  The gyro and accelerometer data need to be transformed to an inertial frame before integrating.

This transformation is not computationally demanding: just sin() and cos(), then one 3x3 matrix multiply. See, for example, Rotating Reference Frames (GPS)

The Reference Frame Definitions (GPS) discusses how to integrate INS and GPS data - it's a old and well solved problem:
Quote
For example, a strap-down GPS aided INS system performing navigation relative to a fixed tangent plane frame-of-reference will typically:

1. transform acceleration and angular rate measurements to platform coordinates;

2. compensate the platform angular rate measurement for navigation frame rotation;

3. integrate the compensated platform frame angular rates to maintain an accurate vector transformation from platform to navigation coordinates;

4. transform platform frame accelerations to tangent plane using the transformation from step 3;

5. integrate the (compensated) tangent plane accelerations to calculate tangent plane velocity and position;

6. use the position estimate to predict the GPS observables;

7. make GPS measurements, compute the residual error between the predicted and measured GPS observables, and use these measurement residuals to estimate and correct errors in the sensed and calculated INS quantities;

8. transform the vehicle inertial measurements and state variables that are estimated above to frames-of-reference (e.g., body) that might be desired by other vehicle systems (e.g., control or mission planning).
In particular, note step 8:  Transform into frame desired for mission planning.

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