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

[Thorny]

Crowdsourcing this.  Correct me if I'm wrong, but at 13,620 kg, this is the heaviest payload launched by SpaceX to date?

Do we know how much Zuma weighed?

[ZachF]

Crowdsourcing this.  Correct me if I'm wrong, but at 13,620 kg, this is the heaviest payload launched by SpaceX to date?

Do we know how much Zuma weighed?

Zuma was RTLS of a pre-B5 booster, so it almost certainly weighed less.

[ZachS09]

Crowdsourcing this.  Correct me if I'm wrong, but at 13,620 kg, this is the heaviest payload launched by SpaceX to date?

Do we know how much Zuma weighed?

No. The best reason I can give is because it’s Northrop Grumman’s classified business.

[ArbitraryConstant]

I think they showed how it will look like in the patch: The main body is half a square, and it will host the antennas (waves coming out of it), then a very big solar array (several times the area of the main body) is extended from one side.

Interesting!

Does the drag have to be evenly distributed to avoid applying torque?

[niwax]

I think they showed how it will look like in the patch: The main body is half a square, and it will host the antennas (waves coming out of it), then a very big solar array (several times the area of the main body) is extended from one side.

Interesting!

Does the drag have to be evenly distributed to avoid applying torque?

Considering the satellites have to permanently rotate to keep their antenna pointed towards the ground ant the solar panel towards the sun, drag-induced torque isn't much of a worry.

[Orbiter]

227kg puts total stack mass at 13,620kg.

Not counting the payload adapter. They must have something underneath the double stack to support it and transfer the loads either to the standard payload adapter or possibly directly to the 2nd stage if its fully integrated.

That's going to be a few hundred kg, too. I'd assume based on size, load distribution  and supported payload mass, even the fully integrated version would still be heavier than their standard adapter from the user guide.

Agreed, I'm betting the total payload mass is closer to 14,000kg. I'm surprised that the F9 is able to return to OCISLY at all with that much mass.

[DistantTemple]

Note on orientation control stability. The press pack says "Hall thrusters" - plural - so managing the orientation of a very non-symmetrical starling can be done by differential thrust levels on different thrusters - possible (just a guess). And as for off centre drag, I assume that rotation due to drag is so low as to be hidden by manoeuvring for pointing, and orbit raising. (5 years to de-orbit if dead).

Note on space-junk avoidance: with so many satellites in each orbit, the "AI" or other computer system managing that ring of satellites can use data from all the trackers in the plane, to build up a local map of debris, possibly down to very small,, and certainly very much smaller than is observable from the ground ( about 100, x 100mm x 10 mm ? according to the illegal unlicensed microsats - company)

The number of satellites in one orbit, and their speed appears to increase the risk of a cascading problem destroying a whole plane. Mapping debris and managing the orbits of all birds in the plane according to the map, makes more sense than obviously impossible "quick" avoidance with .... Hall Thrusters! (again a guess on my part) .

[ArbitraryConstant]

I think they showed how it will look like in the patch: The main body is half a square, and it will host the antennas (waves coming out of it), then a very big solar array (several times the area of the main body) is extended from one side.

Interesting!

Does the drag have to be evenly distributed to avoid applying torque?

Considering the satellites have to permanently rotate to keep their antenna pointed towards the ground ant the solar panel towards the sun, drag-induced torque isn't much of a worry.

I thought about that but I thought of some potential problems and I'm not entirely sure how they balance out.

Satellites have to rotate to point antenna and solar and so on, but they are able to do so using reaction wheels, without expending propellant. But if you get a net torque from drag reaction wheels wouldn't be able to cancel it. They'd be able to overcome it in the short term but eventually it would require the expenditure of propellant, that's when it would be an issue. So it seems like a satellite low enough to experience nontrivial drag would want to be mostly balanced in terms of drag and mass distribution to avoid a net torque, so the only issue they'd have to deal with would be reboosting and fine tuning their orbit.

[Greg Hullender]

I take it Krypton is a whole lot cheaper than Xenon?

About 33 cents per gram vs. $1.20 per gram.

[ZachS09]

How are the upper level winds looking?

I'm well aware that they're a separate forecast from the regular weather, so if there's anyone who knows how to access the ULW data, that would be nice.

[eeergo]

I take it Krypton is a whole lot cheaper than Xenon?

Yes, at the cost of performance, unless there has been some important basic development in the last couple of years (see for example www.sitael-hellas.com/wp-content/uploads/2016/05/3125194_ELECTRIC-PROPULSION-SYTEM_ALTERNATIVE_PROPELLANT_SELECTION_Giannetti.pdf or erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2011index/IEPC-2011-003.pdf )

Could they also be using a Kr/Xe mixture prop?

[freddo411]

I take it Krypton is a whole lot cheaper than Xenon?

Yes, at the cost of performance, unless there has been some important basic development in the last couple of years (see for example www.sitael-hellas.com/wp-content/uploads/2016/05/3125194_ELECTRIC-PROPULSION-SYTEM_ALTERNATIVE_PROPELLANT_SELECTION_Giannetti.pdf or erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2011index/IEPC-2011-003.pdf )

Could they also be using a Kr/Xe mixture prop?

There is no reason to choose the propellent with the absolute greatest ISP.   

If an alternative propellent (Krypton, say) can perform the mission while costing 1/4 as much -- that is the metric that matters.    A krypton based thruster system may mass a bit more; but SX is paying less than $1000 per pound to orbit ... this can be added into the cost based optimization formula.

It's past time to realize that total cost of the system is the design driver; not minimized mass.

[speedevil]

If an alternative propellent (Krypton, say) can perform the mission while costing 1/4 as much -- that is the metric that matters.    A krypton based thruster system may mass a bit more; but SX is paying less than $1000 per pound to orbit ... this can be added into the cost based optimization formula.

It's past time to realize that total cost of the system is the design driver; not minimized mass.

Especially if you're aiming at launching much bigger payloads on SS in the near future.

[smoliarm]

Ok, here is my dime on question "Why Kr?"

As far as I know, Xe is a usual choice as propellant in Hall thrusters because it gives more *thrust per kWt*.
On the other hand, Kr gives better Isp.
Plus, Kr is about 3 times cheaper than Xe (approximately - prices here vary with purity grade).

So, AIUI, for heavy GEO-satellites the higher thrust is the most important (especially if they want to use ion engine for GTO-GEO transfer). Also, for a quarter-billion satellite, the propellant price does not matter.

But for light and cheap Starlink - the situation looks reverse on both sides: the propellant price may be a significant factor, and lower thrust may be not that critical.

Finally, SpaceX propulsion team may want to gain own experience with HIGH Isp Hall thrusters, which may be useful for trans-Mars applications.

[Robotbeat]

Xenon is definitely better (can always use higher voltage to get the same Isp... and for a given Isp, Xenon always has lower ionization losses and thus higher efficiency... besides being denser and thus having better tank mass fraction), but Krypton is better than 3x cheaper. Xenon goes for like $1000-3000/kg, while Krypton is $100/kg. A big difference if you’re launching like hundreds of tons of the stuff in the full constellation. Also, SpaceX would run into availability problems with Xenon.

[Robotbeat]

In fact, depending on how much propellant is loaded per satellite and how often the full constellation is replaced, SpaceX’s propellant demand could be greater than the entire world production capacity of Xenon.

12000 satellites replaced every 5 years with 40kg of propellant is 96 tons of propellant per year. 2017 world Xenon production was 88 tons. Kind of a problem, especially if you want to make the satellites any bigger.

[ArbitraryConstant]

In fact, depending on how much propellant is loaded per satellite and how often the full constellation is replaced, SpaceX’s propellant demand could be greater than the entire world production capacity of Xenon.

12000 satellites replaced every 5 years with 40kg of propellant is 96 tons of propellant per year. 2017 world Xenon production was 88 tons. Kind of a problem, especially if you want to make the satellites any bigger.

I wonder if they'll have to move up to argon. Krypton might be cheaper than Xenon but my understanding it's it's still pretty rare as gasses go.

[JH]

They shouldn't need anywhere near 40 kg of Krypton per satellite. Moving from 440 km to 550 km will take less than 100 m/s, and yearly station keeping should be less than 100 m/s as well. This gives a maximum total dv of 600 m/s. Musk stated the isp of the krypton HET's at 1500 seconds. With a wet mass of 227 kg, you end up with <10 kg of fuel use over the life of the satellite.

Given 12000 satellites replaced every 5 years, that's 24 tons of Krypton per year.

[Lee Jay]

They shouldn't need anywhere near 40 kg of Krypton per satellite. Moving from 440 km to 550 km will take less than 100 m/s, and yearly station keeping should be less than 100 m/s as well. This gives a maximum total dv of 600 m/s. Musk stated the isp of the krypton HET's at 1500 seconds. With a wet mass of 227 kg, you end up with <10 kg of fuel use over the life of the satellite.

Given 12000 satellites replaced every 5 years, that's 24 tons of Krypton per year.

Have to deorbit too, and there are three levels of satellites.

[Lars-J]

18.5 tons.    (shhhh, don't tell the people that claim that the upper stage and PAF can only handle 12 tons) 

https://twitter.com/elonmusk/status/1128834111878193155