SpaceX Falcon 9 : AMOS-17 : August 6, 2019 : DISCUSSION

[Mike_1179]

Can anyone explain why they would choose guidance controlled vs minimum residual?

I had asked this before and the answer I got was roughly 'payload specific requirements'.

One reason is timing of on-orbit maneuvers and ground station coverage.

With a targeted shutdown, you can plan on when the sat will be over a ground station well before the launch. If you’re trying to make sure things won’t happen in the middle of the night (or on the Sabbath?) you define what you want your GTO to be instead of “get me as close to GEO as possible”

[edkyle99]

Yeah, seems kinda meh, compared to Intelsat 35e which was heavier (6761 kg), flew on a Block 4 rocket but still reached 296 x 42742 x 25.85°.

I don't find this surprising.  Block 5 introduced changes to improve reusability, which likely increased dry mass.  Also, I believe that Intelsat 35e used a Block 3 first stage and Block 4 second stage, with the first stage probably designed from the start to be expended.

 - Ed Kyle

Whatever the explanation is, it's not this.  Amos-17 separated at 9520 km/hr (2644 m/s)  and 79.6 km.  Intelsat 35e separated at 9480 km/hr (2633 m/s) and 73.6 km.  The lesser mass of Amos-17 (6500 kg vs 6770 kg) only accounts for about 3 m/s of the 11 m/s delta.  So at least in this case, the Block 5 first stage has higher performance than the Block 4. 

The payload fairing has to be heavier now since it carries recovery equipment.  The second stage is likely heavier thanks to the man-rating effort.  Etc.  Also, isn't there some uncertainty in those first stage cutoff numbers since they likely come from the webcast display?

 - Ed Kyle

[LouScheffer]

Yeah, seems kinda meh, compared to Intelsat 35e which was heavier (6761 kg), flew on a Block 4 rocket but still reached 296 x 42742 x 25.85°.

I don't find this surprising.  Block 5 introduced changes to improve reusability, which likely increased dry mass.  Also, I believe that Intelsat 35e used a Block 3 first stage and Block 4 second stage, with the first stage probably designed from the start to be expended.

 - Ed Kyle

Whatever the explanation is, it's not this.  Amos-17 separated at 9520 km/hr (2644 m/s)  and 79.6 km.  Intelsat 35e separated at 9480 km/hr (2633 m/s) and 73.6 km.  The lesser mass of Amos-17 (6500 kg vs 6770 kg) only accounts for about 3 m/s of the 11 m/s delta.  So at least in this case, the Block 5 first stage has higher performance than the Block 4. 

The payload fairing has to be heavier now since it carries recovery equipment. 

True, but the effect is quite small.  A 4500 kg capacity parafoil for the military masses 238 kg for the parachute + control equipment.  Assume that SpaceX uses 2 of these, one for each half, even though they are more than twice what they need.  Then the overall mass penalty is 500 kg.  So assume the regular fairing is 4000 kg and the recoverable one 4500 kg.  The fairing is jettisoned about 50 seconds into the second stage operation.  Since the Merlin eats about 300 kg of fuel/second, that means 15000 kg of fuel is used during the burn.   Consider the burn from second stage ignition to fairing jettison (the rest of the burn is identical). The mass of the stack at the start of the burn is roughly 106t fuel (from env. impact statement) + 4.5t stage + 6.5t payload + fairing.  At the end of the burn, it's 91t fuel + stage + payload + fairing.  Now using the rocket equation, we can find the difference in the delta-V.  It's 348*9.8*(ln((117+4)/(102+4))- ln((117+4.5)/(102+4.5))), or about 2 m/s, a tiny fraction of the 100 m/s or so discrepancy here.  The effect on the first stage is bigger, but the Block 5 first stage does better, even though pushing the heavier fairing.   So it's not the mass of the fairing recovery hardware.

The second stage is likely heavier thanks to the man-rating effort.  Etc. 

Possible, but we've seen no evidence of this, and the structural margins were originally designed for man-rated standards.   So the difference should be small, nowhere near enough to explain the discrepancy.

Also, isn't there some uncertainty in those first stage cutoff numbers since they likely come from the webcast display?

Yes, these are from the webcast.  However the numbers are quite stable for the few seconds between MECO and second stage ignition.  This should minimize the effect of time quantization, running filtering, and other artifacts in the displayed numbers.

[Wolfram66]

Something literally does not add up about this trajectory:

I've attached the AMOS-17 telemetry below. Assuming the parking orbit was 166 x 232, and the burn started at 199 kms, the instantaneous plane change ΔV requirement would be 325 m/s. However, as the burn progresses, the apogee raises. At the end of the burn, the instantaneous plane change requirement reduces to only 67 m/s. The integrated plane change ΔV requirement would be somewhere between these values, depending on how constant the yaw angle is.

Simply raising the apogee from 232 to 35750 at 199 kms requires 2,470 m/s, so perhaps John meant that the plane change cost a total of 130 m/s?

What event occurred at 1463? As seen in telemetry plot

[pb2000]

I think the simple answer is they didn't need the extra performance. AMOS-6 was sold as a recovery mission, so the free launch of AMOS-17 would presumably be the same, meaning they would be expecting an apogee closer to 20k km.

Initial lifespan was announced at >15 years in 2006(F9 FT is flying), refined to 19 years following CDR in 2017 (Block 4 flying, but B5 announced and slated for mission) and the launch press kit said >20 years (mission changed to expendable). Solar panels and batteries degrade in a fairly predictable manner, so that may be the limiting factor.

[marsbase]

I think the simple answer is they didn't need the extra performance.

I don't see how that answers the question about why the booster was expended. Are you saying there was reserve and unneeded capacity but not enough for an ASDS landing?

[pb2000]

I think the simple answer is they didn't need the extra performance.

I don't see how that answers the question about why the booster was expended. Are you saying there was reserve and unneeded capacity but not enough for an ASDS landing?

Recovery with apogee at 35k is ~5.5Ton, so they couldn't do both.  As to why they expended instead of going subsync gto - unless Elon tweets the answer, I doubt we'll ever know. My money is on making the customer happy after blowing up AMOS-6.

[OneSpeed]

What event occurred at 1463? As seen in telemetry plot

Because of an expected loss of signal, the live telemetry updates paused at about 700 seconds, when the displayed velocity was 26,725 km/h, and the altitude was 165 kms. When the updates resumed at 1464 seconds, there was a step change to 26,611 km/h, and 191 kms altitude. The step change in velocity produced the negative spike in acceleration you see on the plot. So, it's just an artifact that can safely be ignored.

[Wolfram66]

What event occurred at 1463? As seen in telemetry plot

Because of an expected loss of signal, the live telemetry updates paused at about 700 seconds, when the displayed velocity was 26,725 km/h, and the altitude was 165 kms. When the updates resumed at 1464 seconds, there was a step change to 26,611 km/h, and 191 kms altitude. The step change in velocity produced the negative spike in acceleration you see on the plot. So, it's just an artifact that can safely be ignored.

Makes sense. Wasn’t sure if there was a stealth secondary deployment going on

[matthewkantar]

Is there any info regarding the disposition of the second stage? Has it reentered? Is there a TLE for it? I am wondering because it seems possible the first stage was expended tp leave propellant in the second stage for some sort of experiment. Long duration coast and relight?

[gongora]

Is there any info regarding the disposition of the second stage? Has it reentered? Is there a TLE for it? I am wondering because it seems possible the first stage was expended tp leave propellant in the second stage for some sort of experiment. Long duration coast and relight?

Second stage is in same orbit as AMOS-17, same as every other GTO launch.

[gongora]

https://twitter.com/AMOSSpacecom/status/1197431831194406912

The AMOS-17 communication satellite has successfully completed its In Orbit Testing and has reached its 17°E orbital position. AMOS-17 is scheduled to begin commercial operations within a few days.
Nick Zilmer, Boeing, presenting Moshe Golani, AMOS-Spacecom, with the AMOS-17 key.