SpaceX Falcon 9 : GPS III SV01 : SLC-40 : Dec. 23, 2018 - DISCUSSION

[Bubbinski]

Is the F9 with GPS-III-1 the only payload on this flight? Or is it carrying other unannounced/secret payloads that require extra performance to force the rocket to be expended?

[russianhalo117]

Is the F9 with GPS-III-1 the only payload on this flight? Or is it carrying other unannounced/secret payloads that require extra performance to force the rocket to be expended?

AFAIA single payload onboard.

[UKobserver]

My first thought on hearing that this launch was unexpectedly going expendable, was that perhaps SpaceX had offered to expend the booster in order to allow injection of the satellite into a much closer-to-final orbit than was actually contracted, both as a gesture of goodwill for the massively delayed launch, but also to help recover some of that lost time for the Air Force, who have had this satellite sitting around waiting for SpaceX to launch it for quite a while now.

I imagine that the USAF are desperate to get this into it's final orbit asap so that they can thoroughly test all the systems before they allow further satellites to be launched, the next of which they already have ready for flight.

Gaining back a few of the lost months and/or extending it's lifespan would presumably be greatly appreciated by the USAF, and maybe this was offered/negotiated as part of a wider conversation going on behind the scenes about getting a certification process in place for refurbished boosters and when the first of these might be flown.

I've been wondering if the most recent reason for STP-2 being delayed again (and moved to after Arabsat 6A) is that perhaps this is now planned to be the first USAF usage of reflown boosters, and that some additional time is needed to formulate, approve and then action the necessary processes. However much they might be keen to get all these test satellites on orbit, it would equally be a massive opportunity lost if they didn't use this low-importance launch to get used boosters established as an accepted commodity.

It would presumably be much easier for them to justify (to higher authority/external oversight) trialling re-use on what is literally labelled an experimental/test mission, rather than have to do it later on a high-value/high-priority launch of something like GPS or worse.

My $0.02

[gongora]

Most of the GPS delays have nothing to do with SpaceX.  They could have launched this on Delta IV.  The SpaceX delays seem to be with certification of the new Block 5 configuration, not lack of availability for a launcher, so there isn't really much for SpaceX to be sorry about.

It's going to be at least a couple more years before they can fully test the new features of the satellites.  The software/ground systems just aren't done yet.

[LouScheffer]

Whatever the cause for non-recovery is, it's not performance.

F9, with recovery, put the recent Telstar (> 7000 kg) into a pretty close approximation of a GPS transfer orbit, with an apogee of 18000 km.  GPS wants 20,200 km, only about 80 m/s more.  So GPS-IIII, at  less than 4000 kg, should be possible with *both* recovery and large margins.

[stcks]

Whatever the cause for non-recovery is, it's not performance.

F9, with recovery, put the recent Telstar (> 7000 kg) into a pretty close approximation of a GPS transfer orbit, with an apogee of 18000 km.  GPS wants 20,200 km, only about 80 m/s more.  So GPS-IIII, at  less than 4000 kg, should be possible with *both* recovery and large margins.

What does it look like when you give it a perigee of 1000km?

https://forum.nasaspaceflight.com/index.php?topic=30912.msg1869739#msg1869739

[LouScheffer]

Whatever the cause for non-recovery is, it's not performance.

F9, with recovery, put the recent Telstar (> 7000 kg) into a pretty close approximation of a GPS transfer orbit, with an apogee of 18000 km.  GPS wants 20,200 km, only about 80 m/s more.  So GPS-IIII, at  less than 4000 kg, should be possible with *both* recovery and large margins.

What does it look like when you give it a perigee of 1000km?

https://forum.nasaspaceflight.com/index.php?topic=30912.msg1869739#msg1869739

The F9 has enough performance to be able to put GPS-III into a 4000 km perigee and still be able to recover.  Calculations are here..  So unless the transfer orbit is really unusual, or the quoted mass is way off, it's hard to see this as a performance issue.

[UKobserver]

What is the stated final orbit for this satellite and how close can F9 deliver it if they expend S1 and also don't reserve any S2 propellant for deorbiting? I'm not suggesting that's what they'll do, but it gives us an upper boundary of performance available.

[LouScheffer]

What is the stated final orbit for this satellite and how close can F9 deliver it if they expend S1 and also don't reserve any S2 propellant for deorbiting? I'm not suggesting that's what they'll do, but it gives us an upper boundary of performance available.

The final orbit is 20200 x 20200 at 55^o.

An expendable F9 should be able to get fairly close to this orbit - perhaps 13000 x 20200 at 55^o.  Here's the thinking:

A recoverable F9 can put 5500 kg into a GEO-apogee GTO.  That's LEO +2450 m/s.   Reducing the payload mass from 5500 kg to 3900 kg will add about 550 m/s from the second stage.  It also adds about 23 m/s to the first stage, but that will go (roughly) to the more inclined orbit.  Also, when they don't recover, they gain about 350 m/s (2640 m/s at staging, instead of 2290 m/s).  So there is a total of LEO + 2450 + 550 + 350, or about 3350 m/s to use.

A little experimenting shows this will get to a 13000 x 20200 orbit.  First go to 250 km circular (7759 m/s).  Then boost to a 250 x 13000 (9472 m/s at perigee, so needs 1713 m/s).  From the top of this orbit, add 1641 m/s, to go to a 13000 x 20200 orbit.  Total delta V is then 1713 + 1641 = 3354 m/s, as desired.  In practice you could do a little better since there is no need to pause at a parking orbit - instead go straight to an orbit with apogee 13000 km, then at apogee do the remaining burn.

13000 x 20200 would be a superb transfer orbit, needing only 316 m/s to circularize.   Compare this to 1427 needed to circularize from a 250 x 20200 transfer orbit.  That's a savings of 1100 m/s which the satellite can use.  I doubt this makes sense, since GPS satellites don't do much maneuvering, so it's not clear what they would use all that extra fuel for.

[ajmarco]

What is the stated final orbit for this satellite and how close can F9 deliver it if they expend S1 and also don't reserve any S2 propellant for deorbiting? I'm not suggesting that's what they'll do, but it gives us an upper boundary of performance available.

The final orbit is 20200 x 20200 at 55^o.

An expendable F9 should be able to get fairly close to this orbit - perhaps 13000 x 20200 at 55^o.  Here's the thinking:

A recoverable F9 can put 5500 kg into a GEO-apogee GTO.  That's LEO +2450 m/s.   Reducing the payload mass from 5500 kg to 3900 kg will add about 550 m/s from the second stage.  It also adds about 23 m/s to the first stage, but that will go (roughly) to the more inclined orbit.  Also, when they don't recover, they gain about 350 m/s (2640 m/s at staging, instead of 2290 m/s).  So there is a total of LEO + 2450 + 550 + 350, or about 3350 m/s to use.

A little experimenting shows this will get to a 13000 x 20200 orbit.  First go to 250 km circular (7759 m/s).  Then boost to a 250 x 13000 (9472 m/s at perigee, so needs 1713 m/s).  From the top of this orbit, add 1641 m/s, to go to a 13000 x 20200 orbit.  Total delta V is then 1713 + 1641 = 3354 m/s, as desired.  In practice you could do a little better since there is no need to pause at a parking orbit - instead go straight to an orbit with apogee 13000 km, then at apogee do the remaining burn.

13000 x 20200 would be a superb transfer orbit, needing only 316 m/s to circularize.   Compare this to 1427 needed to circularize from a 250 x 20200 transfer orbit.  That's a savings of 1100 m/s which the satellite can use.  I doubt this makes sense, since GPS satellites don't do much maneuvering, so it's not clear what they would use all that extra fuel for.

What would be the difference in time take to reach final orbit from both of those?

They might not be looking at fuel saving but shortening the time to bring the Sat into service.

[LouScheffer]

What is the stated final orbit for this satellite and how close can F9 deliver it if they expend S1 and also don't reserve any S2 propellant for deorbiting? I'm not suggesting that's what they'll do, but it gives us an upper boundary of performance available.

The final orbit is 20200 x 20200 at 55^o.

An expendable F9 should be able to get fairly close to this orbit - perhaps 13000 x 20200 at 55^o.  Here's the thinking:   [...]

13000 x 20200 would be a superb transfer orbit, needing only 316 m/s to circularize.   Compare this to 1427 needed to circularize from a 250 x 20200 transfer orbit.  That's a savings of 1100 m/s which the satellite can use.  I doubt this makes sense, since GPS satellites don't do much maneuvering, so it's not clear what they would use all that extra fuel for.

What would be the difference in time take to reach final orbit from both of those?

They might not be looking at fuel saving but shortening the time to bring the Sat into service.

The difference in in-service times would only be a few hours, or at most a few days.  GPS-III has a chemical engine for apogee maneuvers, so orbit raising takes only a few discrete maneuvers, at most.  It's not a long, drawn-out slog like using electric propulsion for this task.

[ZachS09]

LouScheffer,

What I was thinking about was for the initial parking orbit to be 200 by 1000 km before a second burn changes the orbit to 1000 by 20200 km and a third burn uses the rest of the prop in Stage 2 to significantly raise the perigee.

This is based off your assumption of a MECO velocity between 2600 and 2700 m/s.

Is this profile a waste of delta-v?

[LouScheffer]

You are right that raising the apogee first is better than the sequence I proposed.  In general, you want to make burns when your speed is highest, so your delta-v has the most effect on energy, so i thought a perigee burn would be wasteful.  But it's more than made up for by the larger apogee raising burn, which is at a faster speed.

However, even better is transferring through a 200 x 20200 km orbit.  This does the perigee raise at a higher speed.  Here are the 4 possibilities with 2 burns, ending up in a 3000 x 20200 orbit:

from 200 x 1000  to  1000 x 20200
dv=  2061.8 m/s
from 1000 x 20200  to  3000 x 20200
dv=   244.3 m/s
total= 2306.2 m/s

from 200 x 1000  to  200 x 20200
dv=  1853.6 m/s
from 200 x 20200  to  3000 x 20200
dv=   357.9 m/s
total= 2211.5 m/s

from 200 x 1000  to  200 x 3000
dv=   436.0 m/s
from 200 x 3000  to  3000 x 20200
dv=  2008.8 m/s
total= 2444.8 m/s

from 200 x 1000  to  1000 x 3000
dv=   640.9 m/s
from 1000 x 3000  to  3000 x 20200
dv=  1810.4 m/s
total= 2451.3 m/s

[Semmel]

What if they want it to be a 1 burn profile? Is that possible?

[LouScheffer]

What if they want it to be a 1 burn profile? Is that possible?

Yes (or at least only one burn in addition to the normal second stage burn).  Just put the payload into an initial orbit with a 20200 km apogee, then coast to the top, then circularize as much as delta-V permits.

And this apogee first should be able to give an even better transfer orbit.  From the argument below, a non-recovery F9 should give about LEO+3350.   From a 200 km parking orbit, you can get to a 16000 x 20200 transfer orbit:

from 200 x 200  to  200 x 20200
dv=  2073.9 m/s
from 200 x 20200  to  16000 x 20200
dv=  1264.3 m/s
total= 3338.2 m/s

And going directly to the 20,200 km apogee should be (slightly) better than using a parking orbit, though you then do not get to pick *where* the apogee occurs on the orbit.

A recoverable F9 can put 5500 kg into a GEO-apogee GTO.  That's LEO +2450 m/s.   Reducing the payload mass from 5500 kg to 3900 kg will add about 550 m/s from the second stage.  It also adds about 23 m/s to the first stage, but that will go (roughly) to the more inclined orbit.  Also, when they don't recover, they gain about 350 m/s (2640 m/s at staging, instead of 2290 m/s).  So there is a total of LEO + 2450 + 550 + 350, or about 3350 m/s to use.

[Semmel]

What if they want it to be a 1 burn profile? Is that possible?

Yes (or at least only one burn in addition to the normal second stage burn).  Just put the payload into an initial orbit with a 20200 km apogee, then coast to the top, then circularize as much as delta-V permits.

That is an interesting computation but not what I was looking for. I really meant that F9 second stage performs only one burn. The flight profile would be ridiculus, something like going basically straight up and little sideways, to achieve an apogee of 1000km, then burn S2 as slow as possible 90 degrees to achieve a 1000x20200 orbit when arriving at 1000 km. I dont know if the timing for this is possible though. It is mighty inefficient, but we are looking for ways to waste performance here, since we all agree that expending F9 doesnt make sense when looking at the performance alone.

We had a similar launch a while back.  Formosat-5 I believe. That one had a single S2 burn profile as well to arrive at a ~700 x ~700 km orbit. (Hope I am not wrong on this) but Formosat-5 was very light too.

[ZachS09]

What if they want it to be a 1 burn profile? Is that possible?

Yes (or at least only one burn in addition to the normal second stage burn).  Just put the payload into an initial orbit with a 20200 km apogee, then coast to the top, then circularize as much as delta-V permits.

That is an interesting computation but not what I was looking for. I really meant that F9 second stage performs only one burn. The flight profile would be ridiculus, something like going basically straight up and little sideways, to achieve an apogee of 1000km, then burn S2 as slow as possible 90 degrees to achieve a 1000x20200 orbit when arriving at 1000 km. I dont know if the timing for this is possible though. It is mighty inefficient, but we are looking for ways to waste performance here, since we all agree that expending F9 doesnt make sense when looking at the performance alone.

We had a similar launch a while back.  Formosat-5 I believe. That one had a single S2 burn profile as well to arrive at a ~700 x ~700 km orbit. (Hope I am not wrong on this) but Formosat-5 was very light too.

You are not wrong, Semmel. FORMOSAT 5, weighing 475 kilograms, was directly inserted into a 717 by 730 kilometer orbit inclined 98.29 degrees. And because SpaceX was not approved to do RTLS landings at Vandenberg, the first stage had to do a 2-burn EDL to the drone ship.

[Lars-J]

What if they want it to be a 1 burn profile? Is that possible?

Anything's possible I guess. But I doubt the stage 2 could throttle down enough to reach the GPS altitude in one burn.

One burn profiles are always more inefficient than multi-burn profiles.

[groundbound]

What if they want it to be a 1 burn profile? Is that possible?

Anything's possible I guess. But I doubt the stage 2 could throttle down enough to reach the GPS altitude in one burn.

One burn profiles are always more inefficient than multi-burn profiles.

And there is pretty good empirical evidence that F9 S2 restarts are reliable.

[LouScheffer]

What if they want it to be a 1 burn profile? Is that possible?

Yes (or at least only one burn in addition to the normal second stage burn).  Just put the payload into an initial orbit with a 20200 km apogee, then coast to the top, then circularize as much as delta-V permits.

That is an interesting computation but not what I was looking for. I really meant that F9 second stage performs only one burn.

It's certainly possible, but gives a much worse transfer orbit.

To see that it's possible, imagine a 2 burn profile with a parking orbit and then an apogee raise (like a normal GTO mission).  Since there is no inclination change here, you can use any delay you want between arriving in parking orbit and the next burn.  In particular, you can use a delay of zero, starting your apogee-raise as soon as your arrive in parking orbit, making it one long burn.   You'll end up with an orbit that is 'parking' x 20200. 

So now the question is how high you can raise the parking orbit and still have enough gas in the tank to get to an apogee of 20,200 km.  It's not obvious to me how to calculate this exactly but it can't be very high.  The second stage has something like 8 minutes of fuel.  It needs 6 minutes to get into orbit, and about 1 minute for the apogee raising.  So there is not much fuel to play with.  Even if you throttle down, it's not more than a few minutes until you reach the minimum fuel needed for the apogee burn.  You can't get very high during that time, and the point where you make the apogee burn will determine the perigee of the parking orbit.   Note that FORMOSAT had the throttle way back to get to a 700 km orbit.  If it needed to reserve another minute of fuel, it could not even get that high.

Now SpaceX could gain a little from the expendable booster, but it won's save a minute of second stage burn.  So I'd guess 700 km or less for the transfer orbit perigee if you insist on one burn.