Launch News SpaceX Falcon 9 v1.2 with SES-9 March 4 2016

[Urwumpe]

Oh interesting! Do you have any links to the configuration? Do they use gyros for rotational control instead of thrusters?

No idea about what they use for attitude control, but I do know that it is too expensive to point the satellite antennas away from Earth for the inevitable orbit corrections, so such satellites have the thrusters placed in direction for such manoeuvres.

 

[ADSWNJ]

I read that they need to vector the ion streams to stay away from the solar panels. So the trick is to have 2 thrusters per side such that you can drive 6 degrees of freedom. Just need to figure out how to do it.

 

[RGClark]

I don't think the problem is in the algorithm or data analysis, I think it's in data acquisition. Orbiter is easy, it gives you perfect data.

That said, it could also just be a hardware fault again. Or the miss didn't actually happen, but it just looks that way. We'll have to see when SpaceX releases the video.

I'd like to see the onboard video of the attempted landing of this flight as well as for the successful return to launch site landing. For some reason SpaceX has not released these since the first return booster demonstrating the return procedure actually "landing" on the ocean.

Bob Clark

 

[Urwumpe]

About the attitude control discussion:

http://www.iepc2013.org/get?id=070

This paper describes an all electric orbit and attitude control system including a model of the thruster groups and positions. Pretty interesting and has many similarities to XIPS.

Reaction wheels or CMGs have been rejected by this paper because they would require more Xenon mass and own dry weight than just simply using the electric thrusters and carry a bit more Xenon for them.

But the number of on/off cycles of the thrusters is enormous, some years ago, the maximum possible for such thrusters was 1/20th of what the paper calculates in 15 years.

 

[ADSWNJ]

This link: http://spaceflight101.com/spacecraft/abs-3a/ suggests that the Boeing 702HP platform (which SES 9 uses), has reaction wheels for attitude control, leaving the 8 thrusters to do up down left right translation in a fully redundant configuration.

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OK - I think this is the end of the story. I found a long presentation from Telesat Canada from the Space Ops 2004 conference, discussing their experiences with on-orbit stationkeeping with the Boeing 702 platform. Here's a link to the presentation. I pulled 5 slides from it, for the TL;DR version:

1. The thrusters are all mounted on the anti-Earth side of the satellite: http://s12.postimg.org/7cf83e1rx/XIPS_1.jpg

2. Each thruster points to the CoG of the satellite, with a nominal +-50 deg. elevation and +-13 deg azimuth: http://s12.postimg.org/t7rtacd4t/XIPS2.jpg

3. Inclination control is achieved by firing the upper thruster pair or the lower thruster pair sequentially around 90 and 270 degrees LAN: http://s12.postimg.org/x6p0t61rx/XIPS3.jpg

4. Drift and eccentricity control is achieved from varying the magnitude of all on-orbit burns to make a net in-plane dV: http://s12.postimg.org/5i2dlnerh/XIPS4.jpg

5. The burns are split into 4 single thruster firings per orbit, comprising 2 pairs (the A Burn and B Burn): http://s21.postimg.org/446qormbb/XIPS5.jpg. The A Burns control inc, ecc, and momentum dumping (from the reaction wheels). The B Burns are control inc, ecc, and drift.


So now we know that the attitude is controlled by reaction wheels, the up/down by balancing the N and S thrusts, the left-right by balancing the A and B side thrusts, and the in-out (of orbit) by varying the orbital speed by over-weighting the A vs. B thrusts as needed. Cool hey?

 

[Urwumpe]

This link: http://spaceflight101.com/spacecraft/abs-3a/ suggests that the Boeing 702HP platform (which SES 9 uses), has reaction wheels for attitude control, leaving the 8 thrusters to do up down left right translation in a fully redundant configuration. Still looking for more details.

This page describes the 702 SP not the 702 HP.

 

[ADSWNJ]

This page describes the 702 SP not the 702 HP.

Yup agreed. I cannot find anything definitive on the 702HP ACS. Have you got any sources?

 

[Urwumpe]

Yup agreed. I cannot find anything definitive on the 702HP ACS. Have you got any sources?

sadly not, Boeing themselves are not saying too much there.

 

[ADSWNJ]

sadly not, Boeing themselves are not saying too much there.

I think our default assumption then is that they have reaction wheels, as (a) we know they use them on other sizes of 702, and (b) why waste expendables (Xe), when you can do the same thing with electricity (unlimited via the solar panels) and reaction wheels only.

p.s. Latest: 5,230.6 km x 41,613.7 km, Inc 14.2°. So ... half the plane change completed so far.

 

[Urwumpe]

I would not say so - because reaction wheels are not used for larger satellites. I can so far only find hints towards a reaction wheel less ACS.

I can so far only tell that Anik F2 had an ACS glitch in 2011 and had no reaction wheels.

And about the consumables, the paper above calculated already that reaction wheels plus propellant for momentum dumping weights more than the propellant for pure xenon attitude control. But what the paper did not care much about was electrical power budget, reaction wheels consume less electricity than xenon thrusters.



---------- Post added at 01:32 PM ---------- Previous post was at 12:52 PM ----------

This here suggests that there is actually both:

http://arc.aiaa.org/doi/abs/10.2514/6.2002-4348

XIPS thrusters for attitude control and for momentum dumping of reaction wheels.

 

[ADSWNJ]

I would not say so - because reaction wheels are not used for larger satellites. I can so far only find hints towards a reaction wheel less ACS.

I can so far only tell that Anik F2 had an ACS glitch in 2011 and had no reaction wheels.

And about the consumables, the paper above calculated already that reaction wheels plus propellant for momentum dumping weights more than the propellant for pure xenon attitude control. But what the paper did not care much about was electrical power budget, reaction wheels consume less electricity than xenon thrusters.



---------- Post added at 01:32 PM ---------- Previous post was at 12:52 PM ----------

This here suggests that there is actually both:

http://arc.aiaa.org/doi/abs/10.2514/6.2002-4348

XIPS thrusters for attitude control and for momentum dumping of reaction wheels.

Thanks for searching.

I was originally trying to figure out how the thrusters would be configured if there are 2 redundant sets of just 4 thrusters, and they have to do 5 degrees of motion (i.e. assuming the in-out orbit motion is controlled by centripetal force). Maybe the thrusters are gimbaled, or the electrical field can be shaped to vector the Xe+ ions? Otherwise, I can't see how it's possible.