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

[JimO]

Where's the ground observation opportunities for GTO insertion? I've developed a small list of Indian Ocean observer emails I can sent alert notices out to.

Any ISS line-of-sight opportunity for same burn?

[Garrett]

That gives me a good "in" for a few questions I was wondering.

I'll give a go at answering some of your questions from an armchair expert (*cough*) perspective:

Q1 - if the u/s falls a little short of it's intended dV (but all else is OK), I presume that would that just place the sat at a lower apogee? IE the sat would need to expend some extra prop to get itself to GSO.

Your scenario implies a successful 2nd stage relight. A lower apogee would be the least of SpaceX's and SES's worries at that point. The S/C should be in a very good position to move into GEO from a slightly lower orbit than expected.

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.

Q3 - I presume there's no value in going to a higher apogee if they have lots of margin left? (I'm certain the answer is "they'll just go for the apogee they promised", but wanted to ask just in case.)

I would presume the same

Q4 - would we expect the u/s to perform a disposal burn after s/c separation? If so, is it likely to position the stage to re-enter, or to push itself on the escape?

I would expect it to perform fuel venting and nothing more. Enough to avoid an explosion in the event of a collision with space debris.

Q5 - how long would the u/s usually wait after s/c sep before doing it's disposal manoeuvre? (It's not listed in the press kit.)

No idea. And again, I'd presume only a venting would occur.

Q6 - if no disposal manoeuvre takes place, how often would the u/s cross GSO / be a danger to other GEOsats?

My assumption would be that the U/S and S/C will separate at such an orbit and inclination such that the U/S will never cross GEO.

[Silmfeanor]

Q6 - if no disposal manoeuvre takes place, how often would the u/s cross GSO / be a danger to other GEOsats?

My assumption would be that the U/S and S/C will separate at such an orbit and inclination such that the U/S will never cross GEO.

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.

[Garrett]

Q6 - if no disposal manoeuvre takes place, how often would the u/s cross GSO / be a danger to other GEOsats?

My assumption would be that the U/S and S/C will separate at such an orbit and inclination such that the U/S will never cross GEO.

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.

Not necessarily. Take two ellipses, one "fat", one "thin", in 3D space and you'll see that you can position the thin one (the high apogee) with respect to the fat one (GEO), with an object at a common focal point (Earth), while avoiding an intersection between the two*.

* I think. It's been a while since I've manipulated ellipses and their focal points.

[deltaV]

Now you should really use the rocket equation rather than a linear approximation, but the basic conclusion seems unavoidable - the accelerations must be quite large.  Do they throttle back to keep to 5 Gs or so?  Or can the satellite take more?

Your analysis looks good to me. I guess that they will throttle back but I dunno.

What would they do if they had a light payload to Earth escape - it would not seem they could throttle back enough to hold it to only 5 Gs...

A kick stage would solve that problem. Earth escape is a bit off-topic so let's not discuss this further in this thread.

[simonbp]

And I'll just add that most escape third stages (e.g. the Star 48 on New Horizons) are integrated as part of the payload, not the launch vehicle. So, if someone wants a third stage, they can just provide it and SpaceX would treat it as part of the integrated payload.

[MP99]

Many thanks.

That gives me a good "in" for a few questions I was wondering.

I'll give a go at answering some of your questions from an armchair expert (*cough*) perspective:

Another one, eh? 

Q1 - if the u/s falls a little short of it's intended dV (but all else is OK), I presume that would that just place the sat at a lower apogee? IE the sat would need to expend some extra prop to get itself to GSO.

Your scenario implies a successful 2nd stage relight. A lower apogee would be the least of SpaceX's and SES's worries at that point. The S/C should be in a very good position to move into GEO from a slightly lower orbit than expected.

I understand that ultimate performance of a new rocket is one of those questions which can only really be characterised by confirming what dV the stages can actually impart. And, of course, F9 #006 never got the chance to show what it could do on it's second burn.

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.

Q4 - would we expect the u/s to perform a disposal burn after s/c separation? If so, is it likely to position the stage to re-enter, or to push itself on the escape?

I would expect it to perform fuel venting and nothing more. Enough to avoid an explosion in the event of a collision with space debris.

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.

cheers, Martin

[Jim]

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.

[Roy_H]

I would like to know what drives the launch window of 66 minutes. Since the target orbit is GEO, it seems to me that the rocket and satellite have to go through the exact same maneuvers regardless of the time of day (or time of year for that matter). Should be able to launch anytime. Why do they have to wait until 5:37 pm?

 

[Jim]

I would like to know what drives the launch window of 66 minutes. Since the target orbit is GEO, it seems to me that the rocket and satellite have to go through the exact same maneuvers regardless of the time of day (or time of year for that matter). Should be able to launch anytime. Why do they have to wait until 5:37 pm?

To put the spacecraft near the desired orbital slot and there are solar/thermal considerations during the transfer orbits

[deltaV]

The SES GTO presumably has nodes (w.r.t. the equatorial plane) at apogee and perigee. Neither apsis is at GEO altitude so the SES GTO does not intersect GEO. As the second stage's orbit decays however it will intersect GEO at some point. This follows from topology: the GTO and GEO orbital ellipses are interlocking whereas LEO (which the GTO will decay to) and GEO are not interlocking.

- A non-expert

[Prober]

The BBC has an article that appears to sum up what's involved and the importance of this flight.

http://www.bbc.co.uk/news/science-environment-25087861

Now this I found to be good info:

"Right now, we're at about a vehicle per month production rate. We'll be at 18 per year in the next couple of quarters, and by the end of next year we'll be at a rate of 24 a year, or two a month."

[Joffan]

The SES GTO presumably has nodes (w.r.t. the equatorial plane) at apogee and perigee. Neither apsis is at GEO altitude so the SES GTO does not intersect GEO. As the second stage's orbit decays however it will intersect GEO at some point. This follows from topology: the GTO and GEO orbital ellipses are interlocking whereas LEO (which the GTO will decay to) and GEO are not interlocking.

- A non-expert

Yep, my thought exactly. Unlike the payload satellite, the upper stage will keep a low perigee which will mean its apogee will fairly rapidly decay back through GSO. So there will probably  be somewhere between one and a dozen  times when the upper stage runs through the thin doughnut of space that would be described as GSO.

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

[Jim]

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.

[guckyfan]

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.

[starsilk]

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.

liability? if you 'did something' and screwed up (under burn), perhaps you're more liable than if you did nothing and it accidentally hit someone/something.

[Joffan]

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.

Retrofire at apogee should be a one-way perigee lowering. Underburn would just mean that the perigee was lowered less, which is not catastrophic - it would still speed up orbit decay.

... unless there some other consequences I haven't thought of, of course. But perigee will not be very high in the first place, so I doubt it would be bringing it into any kind of conflict at the point in the orbit, and for the rest of the orbit, "space is big".

I would put the risk-benefit of apogee retrofire firmly on the "benefit" side, if the US can endure out to that point in the orbit (which I think would be the real deal-breaker).

[Jim]

... unless there some other consequences I haven't thought of, of course. But perigee will not be very high in the first place, so I doubt it would be bringing it into any kind of conflict at the point in the orbit, and for the rest of the orbit, "space is big".

I would put the risk-benefit of apogee retrofire firmly on the "benefit" side, if the US can endure out to that point in the orbit (which I think would be the real deal-breaker).

There are many that you haven't thought of* and hence you can't declare its on the "benefit" side.  It is anchored (more than 'firmly put") on the "negative" side by people who have to deal with the risk and why it hasn't been done in the past on missions with unknown margins at the end of the primary mission.

* that exercise is left up for you to do.

[malu5531]

With my optimist "SpaceX fanatic" calculations, I predict 1.42% residual fuel in s2 after placing the satellite in it's intended orbit. (505 m/s residual delta-v for emtpy stage - not enough for C3=13 earth departure, but C3=10)

These are razor thin margins!

s1 burn time: 178.7s
s2 burn time: 369s (total time)
Acceleration at final MECO: 11.58

SpX SES Calculations

[smoliarm]

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.

RUD may be?