SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD

[MP99]

As I implied above, I suspect SpaceX would like to perform a burn to exhaustion (third burn after s/c sep) to confirm how much margin was really left in the stage after delivering the payload.

That is seldom done, on less there is a large amount that is expected to be left over like Cassiope or DSMP.  Venting is the standard for stage passivation.

Jim,

many thanks for that info.

You have said in the past that ultimate performance of a new rocket cannot be assumed until it has been demonstrated.

Given this would be first mission of this type for F9 (or any SpaceX rocket), I presume they will want to gather this information from this mission to enhance their performance models. Would they get good info by deriving mass/prop changes from monitoring in-tank sensors and position / acceleration changes during the GTO burn?

Thanks again, Martin

[Jim]

Would they get good info by deriving mass/prop changes from monitoring in-tank sensors and position / acceleration changes during the GTO burn?

yes

[MP99]

If the US had enough battery life - which I doubt - it could probably go straight for atmospheric disposal by a short retrofire at apogee.

No, they don't do that because they don't know how much prop will be left and don't want to risk an under burn with its consequences.

I am curious about the consequences. What would they be? The perigee would still be lowered and the orbit decays faster than without the burn. That's assuming there is enough fuel left to do any burn.

If they're not intending to dispose of the u/s, I have to assume the injection is profiled such that the u/s won't imperil sats in a lower orbit - either raising the perigee to some safe level, making sure it stays low enough to decay fairly quickly (raise apogee without raising perigee), or something of that sort.

If that's the case, a failed perigee reduction might simply move the stage back into the danger zone?

Googling: http://www.spacesafetymagazine.com/2013/07/18/compliance-rocket-upper-stage-gto-space-debris-mitigation-guidelines/

cheers, Martin

[kevin-rf]

It is worth pointing out that to perform an apogee burn the stage would have to live with propellants for some eight hours. That add's battery, thermal insulation, and LOX boil off mass to the upper stage.

It also requires an inertial guidance system that stays active/accurate/aligned for an eight hour period. We do not know how accurate the system SpaceX uses is. Is it good for eight hours? It is that or mass has to be added to keep it aligned in flight (star and earth sensors). The apogee is to high to use GPS or sensing of the earths magnetic field.

While the perigee doesn't need to be lowered by more than a few 10's of km, not much Delta V. Just a thought of why apogee de-orbit burns are not done.

[Mike_1179]

If they're not intending to dispose of the u/s, I have to assume the injection is profiled such that the u/s won't imperil sats in a lower orbit - either raising the perigee to some safe level, making sure it stays low enough to decay fairly quickly (raise apogee without raising perigee), or something of that sort.

If that's the case, a failed perigee reduction might simply move the stage back into the danger zone?


cheers, Martin

I was under the impression that, scientifically speaking, space is big, really big.  While we say that this orbit may cross other orbits, the actual chances of a collision are very remote.  If the stage were to rupture, that is a different story, so the tanks are vented.  But an intact but derelict stage isn't really a threat even if it passes through the those altitudes for some time.

Is there really a "danger zone"?  Great, now I've got that Kenny Loggins song from Top Gun stuck in my head.

[Joffan]

It is worth pointing out that to perform an apogee burn the stage would have to live with propellants for some eight hours. That add's battery, thermal insulation, and LOX boil off mass to the upper stage.

It also requires an inertial guidance system that stays active/accurate/aligned for an eight hour period. We do not know how accurate the system SpaceX uses is. Is it good for eight hours? It is that or mass has to be added to keep it aligned in flight (star and earth sensors). The apogee is to high to use GPS or sensing of the earths magnetic field.

While the perigee doesn't need to be lowered by more than a few 10's of km, not much Delta V. Just a thought of why apogee de-orbit burns are not done.

The practicalities of keeping the stage alive (electricity, oxygen, heat) are the big issue in my view. From my work with inertial guidance, I know that a commercial inertial measurement unit sitting on the bench will accurately record the Earth's rotation if given a couple of minutes of data. So I would expect it to be relatively easy to maintain the spacecraft in its attitude from the perigee burn to within a degree or two, which would mean it would be lined up for an apogee retro burn at the appropriate time.

[tigerade]

Anyone know if NASA TV will be streaming the launch?

[JimO]

That'd be kinda hard, seeing as it'll have an apogee of 80.000 km. So the upperstage will most certainly cross GEO several times, if not hundreds or thousands, before it decays.

I see there are some other replies but I want to add that unless the GTO is also equatorial, the booster will be well above or below the equatorial plane whenever it is at GEO distance. And I've never known any GTO that was equatorial -- doing the apogee circ burn while finalizing plane change is beautifully efficient and thus measurelessly attractive.

[Chris Bergin]

Anyone know if NASA TV will be streaming the launch?

No. It's not a NASA launch

I thought I had posted it in a billion places, but.....

http://www.spacex.com/webcast/

[Joffan]

That'd be kinda hard, seeing as it'll have an apogee of 80.000 km. So the upperstage will most certainly cross GEO several times, if not hundreds or thousands, before it decays.

I see there are some other replies but I want to add that unless the GTO is also equatorial, the booster will be well above or below the equatorial plane whenever it is at GEO distance. And I've never known any GTO that was equatorial -- doing the apogee circ burn while finalizing plane change is beautifully efficient and thus measurelessly attractive.

The point was that, as the orbit decays, the equatorial crossing point outside GSO will be dragged back through GSO space. If the decay is quick, it may simply step over the vulnerable zone, but slow orbit decay may mean multiple passes through this volume.

[Adonis1]

http://new.livestream.com/spacex/SES8

[JimO]

Q2 - I presume the injection is not a burn to exhaustion for the u/s?

At most it would be a burn to an "empty" fuel level sensor limit.

Thanks, that is what I meant by the phrase. 

Good, because actually burning a biprop system until one propellant runs out [they never run out simo] is a very very bad idea. Another reason not to is that it is a really, REALLY bad idea.

Seriously, tank quantity gauging is a devilishly complex and subtle problem, even under acceleration -- and tenfold more in zero-G, as in the STS OMS/RCS tanks that I was responsible for on STS-1. Boyle's Law was our 'bible - 'PVT' was our mantra -- but even then it was 'spongy' since tank evacuation led to induced temp changes that often lagged the other parameter variations. Still gives me anxiety attacks.

[Lars_J]

http://new.livestream.com/spacex/SES8

Also up at: http://www.spacex.com/webcast/  (showing animated SpaceX logo)

[JimO]

Any help on my question re earth surface coordinates of the GTO insertion burn?

[AJA]

Any help on my question re earth surface corrdinates of the GTO insertion burn?

The second stage relight? I'm guessing it happens at 295 km over 0N and 22 W. (South of Cape Verde, off the west coast of Africa). How did I guess that? I drew a line from SLC-40 to the Equator that met the latter at 28 degrees. Don't even know how to start evaluating the accuracy.

Good, because actually burning a biprop system until one propellant runs out [they never run out simo] is a very very bad idea. Another reason not to is that it is a really, REALLY bad idea.

Why? Doesn't it just stop combusting? Ok, maybe the engine gets shot, and you have some aberrations in the final few moments of powered flight, throwing off your injection in terms of attitude and maybe velocity as well.. but what's so bad about it?


EDIT: Alright, it's bad for the above mentioned reasons if you were doing it prior to Payload sep... but AFTER that... when you don't care about recovery, or the value of the largely spent stage..?

[Galactic Penguin SST]

It is interesting to see that the first 4 GSO comsats to be launched by SpaceX will all serve the Asia-Pacific region.... this one is going to 95 degrees East.

[John Santos]

Good, because actually burning a biprop system until one propellant runs out [they never run out simo] is a very very bad idea. Another reason not to is that it is a really, REALLY bad idea.

Why? Doesn't it just stop combusting? Ok, maybe the engine gets shot, and you have some aberrations in the final few moments of powered flight, throwing off your injection in terms of attitude and maybe velocity as well.. but what's so bad about it?


EDIT: Alright, it's bad for the above mentioned reasons if you were doing it prior to Payload sep... but AFTER that... when you don't care about recovery, or the value of the largely spent stage..?

Just a guess, but either the fuel or oxidizer would run out first, and when that happened, there would be an unbalanced load on the turbopump (assuming there is a single turbine driving both pumps), which could result in RUD, and a large debris cloud.  Since this would happen at apogee, some of the debris would probably end up in orbits with greatly raised perigees, i.e. it would stick around for years or decades.  This is bad, REALLY bad.

Another question, could the cold-gas thrusters attitude control thrusters be used for a de-orbit burn at apogee?  Assume enough gas is left and the stage is still alive?  (IIUC, if they ran out of fuel, they would just stop working, no explosion possible.)  Lowering the perigee even 10 km or so would greatly reduce time to decay, and would also cause the reduction in apogee at each perigee to be much more.  No idea how to do the math, though.

[Jim]

They have very little impulse

[Misha Vargas]

Elon's brother (@Kimbal on twitter) seems to be posting a few recent photos of the Falcon in its hangar, dated yesterday. A few highlights:

[AJA]

Just a guess, but either the fuel or oxidizer would run out first, and when that happened, there would be an unbalanced load on the turbopump (assuming there is a single turbine driving both pumps), which could result in RUD, and a large debris cloud.  Since this would happen at apogee, some of the debris would probably end up in orbits with greatly raised perigees, i.e. it would stick around for years or decades.  This is bad, REALLY bad.

Yeah, but that's assuming the debris isn't caught and constrained within a control volume around each engine. Like they do for the turbofan blades in jet engines. If you have multiple engines, then you'd want to ensure that the RUD of one of the engines doesn't fling pieces that cut the lines to another engine, and cause an outage there too. So, my guess is that the design would've already mitigated this risk (of turbo-pump dry run and fragmentation).

Of course, they might not have incorporated such a catch device in most upper stages, given most upper stages have only a single engine.