SpaceX F9 : Starlink v0.9 : May 23, 2019 - DISCUSSION

[gongora]

Musk said that the Starlink satellites aren't tracking the sun, but is that "yet" or always?
It doesn't look like they rotate, so tracking would involve spinning and tipping the satellites from the local vertical.

That was "yet", and that time should be past now.  Elon said they do rotate on one axis.

[Comga]

Musk said that the Starlink satellites aren't tracking the sun, but is that "yet" or always?
It doesn't look like they rotate, so tracking would involve spinning and tipping the satellites from the local vertical.

That was "yet", and that time should be past now.  Elon said they do rotate on one axis.

They would have to rotate so they could operate in orbits with zero beta angle, which they all will have periodically.
Whether they rotate with respect to the spacecraft, of if the spacecraft rotates about the local vertical is another question.
That still means they can't track the sun as effectively as can those on, for example, the ISS with their dual axis rotation.
There would still be some benefit from oversizing the array, storing the excess power, and metering it to the ion engines at constant power.

[gongora]

Whether they rotate with respect to the spacecraft, of if the spacecraft rotates about the local vertical is another question.

It's not a question.  The arrays rotate around one axis with respect to the spacecraft.

[speedevil]

]

If we take a crude average, it seems 5km raising per day is approximately correct to within +-20% or so.

This is of the close order of 3m/s/day, or 33um/s^2.
If this was a continual thrust on a 1600s ion engine, the approximate power would be of the order of 400W.

May we please know how you calculate this 400W figure?
What are your assumptions for thruster ionisation power losses, power conversion efficiencies, etc?
On what model of thruster's specs did you base this calculation?

I was using a graph from a paper using 65% efficient ion engines at 1500s. (but the simplistic calculation I did agreed with it).
The solar power area is obvious with ~30m^2 and 20% efficient giving ~6kW.
I was intentionally neglecting everything but that, as it is relevant if the resultant power pops out at 3kW, but not so much at 400W.

I was assuming that the solar arrays would be sized for worst case comms draw at end of life, not thruster performance.

On batteries - it turns out that it is plausible to have a 16000 cycle life 1.5kWh battery by rather oversizing it - approaching 50kg/10kWh for a three year life.
Even this can be fully topped off in three orbits or so, with the panels fully sun-facing.

Explaining the very large solar panel rationally is difficult if there is no power consumer for it, hence the assumption of ~3kW comms load.

[Barley]

There was talk about them initiating a new burn every 90 minutes, so the increase isn't continuous. The straight lines in the chart are somewhat misleading.

Does anybody have any thoughts on boosting every 90 minutes?

To me it suggests once per orbit.  Possibly it just means they are not running right through the eclipse, but another possibility is that they are burning at perigee.  That would increase the eccentricity but I don't see any sign of that in the orbital elements.

[mark_m]

There was talk about them initiating a new burn every 90 minutes, so the increase isn't continuous. The straight lines in the chart are somewhat misleading.

Does anybody have any thoughts on boosting every 90 minutes?

To me it suggests once per orbit.  Possibly it just means they are not running right through the eclipse, but another possibility is that they are burning at perigee.  That would increase the eccentricity but I don't see any sign of that in the orbital elements.

Could it have meant starting the orbit-raising burn on a different satellite every 90 minutes? Perhaps to begin to spread them out over the plane?

[ThatOldJanxSpirit]

There was talk about them initiating a new burn every 90 minutes, so the increase isn't continuous. The straight lines in the chart are somewhat misleading.

Does anybody have any thoughts on boosting every 90 minutes?

To me it suggests once per orbit.  Possibly it just means they are not running right through the eclipse, but another possibility is that they are burning at perigee.  That would increase the eccentricity but I don't see any sign of that in the orbital elements.

My reading is that they staggered the start of orbit raising ops with one satellite initiating every 90 minutes. The spread of orbital parameters seems to support this.

[Pete]

I was using a graph from a paper using 65% efficient ion engines at 1500s. (but the simplistic calculation I did agreed with it).

Xenon, or Krypton ion engines, on that graph of yours?
Krypton has lower atomic mass, AND higher ionization potential. You get better ISP, but your energy-per-thrust is about doubled.

[speedevil]

I was using a graph from a paper using 65% efficient ion engines at 1500s. (but the simplistic calculation I did agreed with it).

Xenon, or Krypton ion engines, on that graph of yours?
Krypton has lower atomic mass, AND higher ionization potential. You get better ISP, but your energy-per-thrust is about doubled.

Why would you raise the ISP?

ISP is a design variable, and you pick it according to your desired propellant.

Obviously if you run an ion engine on a different propellant, ISP may go up, but the ISP depends on how much energy you put into the ions the engine is ionising and is a free choice over a moderate range per thruster design. It is not set by the propellant.

Krypton takes more energy to ionise per unit mass than Xenon, which adds a fixed cost per kilogram of propellant. The change is a fairly small addition if you keep ISP constant for a thruster designed for each propellant.

This paper compares Kr and Xe in a thruster designed for Xe.

The above graph is from that paper, and illustrates nominal flow rate (5.5mg/s)  for Xenon performance of 58mn/kW (black base-down triangles) , and for Krypton at 5.7mg/s (purple), a performance of 47mN/kW.

Or Kr, in an optimised for Xe engine, holding a constant ISP had a 15% penalty in terms of input power holding ISP constant per unit thrust.

This is far from the observed order of magnitude differences, if you were to assume maximum possible thrusting, which was the point I was raising. An optimised for Kr engine would somewhat reduce this penalty.

[ArbitraryConstant]

That still means they can't track the sun as effectively as can those on, for example, the ISS with their dual axis rotation.

hm

Given single axis tracking on the solar array plus reaction wheels in the spacecraft bus, seems like they'd be able to do dual axis tracking. Given their antennas are phased array it seems like they'd be able to compensate.

[cscott]

That still means they can't track the sun as effectively as can those on, for example, the ISS with their dual axis rotation.

hm

Given single axis tracking on the solar array plus reaction wheels in the spacecraft bus, seems like they'd be able to do dual axis tracking. Given their antennas are phased array it seems like they'd be able to compensate.

I have a feeling it's a lot cheaper to make the solar array bigger than it is to make the phased array radio bigger.  I'd think it makes more sense to keep the antenna pointed optimally and deal with a small amount of inefficiency on the solar panels.

[ArbitraryConstant]

That still means they can't track the sun as effectively as can those on, for example, the ISS with their dual axis rotation.

hm

Given single axis tracking on the solar array plus reaction wheels in the spacecraft bus, seems like they'd be able to do dual axis tracking. Given their antennas are phased array it seems like they'd be able to compensate.

I have a feeling it's a lot cheaper to make the solar array bigger than it is to make the phased array radio bigger.  I'd think it makes more sense to keep the antenna pointed optimally and deal with a small amount of inefficiency on the solar panels.

I don't think it's necessary to point the antenna sub-optimally. If the solar array can fold up and down on a one-axis hinge, and the whole satellite can rotate with the bottom pointing down, there's your two axis tracking.

Picture a swivel chair with an adjustable back. The back only has a single axis but because the whole chair can rotate that is essentially a second axis. Assuming the phased array antennas only need to be pointing down, there shouldn't be any need for sub-optimal pointing on the antennas. The satellites may not be able to point their solar arrays across the entire sphere of the sky, but they will never get sunlight coming up through the Earth so that's not an important constraint.

[mark_m]

That still means they can't track the sun as effectively as can those on, for example, the ISS with their dual axis rotation.

hm

Given single axis tracking on the solar array plus reaction wheels in the spacecraft bus, seems like they'd be able to do dual axis tracking. Given their antennas are phased array it seems like they'd be able to compensate.

I have a feeling it's a lot cheaper to make the solar array bigger than it is to make the phased array radio bigger.  I'd think it makes more sense to keep the antenna pointed optimally and deal with a small amount of inefficiency on the solar panels.

I don't think it's necessary to point the antenna sub-optimally. If the solar array can fold up and down on a one-axis hinge, and the whole satellite can rotate with the bottom pointing down, there's your two axis tracking.

Picture a swivel chair with an adjustable back. The back only has a single axis but because the whole chair can rotate that is essentially a second axis. Assuming the phased array antennas only need to be pointing down, there shouldn't be any need for sub-optimal pointing on the antennas. The satellites may not be able to point their solar arrays across the entire sphere of the sky, but they will never get sunlight coming up through the Earth so that's not an important constraint.

I agree this would be two-axis tracking, but (speaking as a software guy with no solar or space résumé) it seems to me that if they have this operational capability, they would have never said they only have one-axis solar panel tracking. Is this somehow enough different to not qualify for that label? Thanks!

Edit: clarity

[Dan C]

That still means they can't track the sun as effectively as can those on, for example, the ISS with their dual axis rotation.

hm

Given single axis tracking on the solar array plus reaction wheels in the spacecraft bus, seems like they'd be able to do dual axis tracking. Given their antennas are phased array it seems like they'd be able to compensate.

I have a feeling it's a lot cheaper to make the solar array bigger than it is to make the phased array radio bigger.  I'd think it makes more sense to keep the antenna pointed optimally and deal with a small amount of inefficiency on the solar panels.

I don't think it's necessary to point the antenna sub-optimally. If the solar array can fold up and down on a one-axis hinge, and the whole satellite can rotate with the bottom pointing down, there's your two axis tracking.

Picture a swivel chair with an adjustable back. The back only has a single axis but because the whole chair can rotate that is essentially a second axis. Assuming the phased array antennas only need to be pointing down, there shouldn't be any need for sub-optimal pointing on the antennas. The satellites may not be able to point their solar arrays across the entire sphere of the sky, but they will never get sunlight coming up through the Earth so that's not an important constraint.

Might get complicated when they add the laser links between satellites.

[ArbitraryConstant]

I don't think it's necessary to point the antenna sub-optimally. If the solar array can fold up and down on a one-axis hinge, and the whole satellite can rotate with the bottom pointing down, there's your two axis tracking.

Picture a swivel chair with an adjustable back. The back only has a single axis but because the whole chair can rotate that is essentially a second axis. Assuming the phased array antennas only need to be pointing down, there shouldn't be any need for sub-optimal pointing on the antennas. The satellites may not be able to point their solar arrays across the entire sphere of the sky, but they will never get sunlight coming up through the Earth so that's not an important constraint.

Might get complicated when they add the laser links between satellites.

Agreed, but thinking about it I think that might be why they're not doing solar tracking yet hence the bright flares, they have to keep the engines pointed. It seems possible they are neither balanced in center of mass when tracking with the hinge, nor have the ability to thrust when tracking with reaction wheels.

My understanding with the laser links is that they're supposed to be able to operate between phase planes which would mean they can track across a pretty good angular size of sky. If some of the links have that tracking capability it's not a huge leap to think they all would.

[ThomasGadd]

The graph in the update thread shows the satellites continuing to raise their orbit... they're suppose to stop at 550 km right?

[gongora]

The graph in the update thread shows the satellites continuing to raise their orbit... they're suppose to stop at 550 km right?

Yes.  Some of them are getting very close.

[ThomasGadd]

Today's graphs show them past 600 km.

[su27k]

Today's graphs show them past 600 km.

I think that's just the weird font the author is using, makes 5 looks like 6. The first marker on y-axis is 450, so the next marker should be 500, not 600.

[gongora]

The highest is showing as 577x558km now, only six of them are below 500x500 (including the 3-4 misbehaving ones that might not raise their orbits).  The one lowering its orbit is down to 418x412.