SpaceX Falcon 9 : Intelsat 35e : July 5, 2017 : DISCUSSION

[envy887]

I was wondering if anyone could educate me on the reason for using a parking orbit and then a 2nd burn over Africa to get to GTO instead of a single direct burn? Here's what I think happened:

On the webcast the speed dropped about 500Km/h and the altitude went up to 248km from 164Km during the coast; so my guess is that once they achieved a LEO orbit, they waited to hit apogee where a perigree raising burn is most efficient and then they burnt a minute to raise that perigree to a new apogee of 43000Km, and the new perigree will be approximately where they started the 2nd burn around 250km?

Does that make sense? Thanks in advance!

No, that's not the reason. Inclination changes are most efficient over the equator, which is the 2nd burn is done there. (which raises the apogee and simultaneously changes the inclination)

Single direct burns for GTO are only practical when launching from a near equator launch site.

Also, they have to raise apogee while over the equator regardless of inclination, or the apogee will occur above or below the equatorial plane.

[burningsheep]

No, that's not the reason. Inclination changes are most efficient over the equator, which is the 2nd burn is done there. (which raises the apogee and simultaneously changes the inclination)

Single direct burns for GTO are only practical when launching from a near equator launch site.

I googled plane changes to see if I could make sense of your statement, and wikipedia states "The simplest way to perform a plane change is to perform a burn around one of the two crossing points of the initial and final planes." which to get to 0 degree inclination would be at the equator, however it also states this: "However, maximum efficiency of inclination changes are achieved at apoapsis, (or apogee), where orbital velocity is the lowest." which is closer to what I originally thought. Does the initial burn target apogee at the equator to make both of the above statements true?

[envy887]

I was wondering if anyone could educate me on the reason for using a parking orbit and then a 2nd burn over Africa to get to GTO instead of a single direct burn? Here's what I think happened:

On the webcast the speed dropped about 500Km/h and the altitude went up to 248km from 164Km during the coast; so my guess is that once they achieved a LEO orbit, they waited to hit apogee where a perigree raising burn is most efficient and then they burnt a minute to raise that perigree to a new apogee of 43000Km, and the new perigree will be approximately where they started the 2nd burn around 250km?

Does that make sense? Thanks in advance!

I'd guess that you'd want the apogee to be in a place the will put the satellite at the right geographic position when you circularize the orbit.

No, the final position is easy to adjust during the perigee raising burns later. (which is done through many orbits) It is easy for a satellite to drift slowly to its final position by a slight raising or lowering its orbit.

You may note that all of SpaceX GTO launches make the second burn as the upper stage passes the equator. This is because it is the most effective spot (the equator) to deliver the satellite to its transfer orbit. The final satellite position is irrelevant for this step.

No, that's not the reason. Inclination changes are most efficient over the equator, which is the 2nd burn is done there. (which raises the apogee and simultaneously changes the inclination)

Single direct burns for GTO are only practical when launching from a near equator launch site.

I googled plane changes to see if I could make sense of your statement, and wikipedia states "The simplest way to perform a plane change is to perform a burn around one of the two crossing points of the initial and final planes." which to get to 0 degree inclination would be at the equator, however it also states this: "However, maximum efficiency of inclination changes are achieved at apoapsis, (or apogee), where orbital velocity is the lowest." which is closer to what I originally thought. Does the initial burn target apogee at the equator to make both of the above statements true?

That is maximum efficiency for a single burn (e.g. going from an elliptical inclined transfer orbit to a circular equatorial orbit).

If doing two burns, maximum efficiency is achieved by correcting a small part of the inclination change at perigee and the rest at apogee (e.g. going from a circular, inclined orbit to an elliptical, somewhat less inclined transfer orbit, then to a circular equatorial orbit).

[Lars-J]

I googled plane changes to see if I could make sense of your statement, and wikipedia states "The simplest way to perform a plane change is to perform a burn around one of the two crossing points of the initial and final planes." which to get to 0 degree inclination would be at the equator, however it also states this: "However, maximum efficiency of inclination changes are achieved at apoapsis, (or apogee), where orbital velocity is the lowest." which is closer to what I originally thought. Does the initial burn target apogee at the equator to make both of the above statements true?

The parking orbit is essentially circular, so that's not why the burn is done there. Envy887's explanation is perhaps best, and highlights why - if the burn is not done at the equator, the resulting apogee will not properly cross the geostationary orbit plane. The apogee could otherwise end up in the far northern or southern hemisphere, which would be terrible.

[cppetrie]

I get the equator part of the second burn, but is the launch window timing intended to help approximate the orbit to the intended orbital slot (longitude)? If not, what is the reasoning behind picking a particular launch time for GEO sats. I know for ISS missions it has to do with orbit matching. But what about GEO missions?

[envy887]

I get the equator part of the second burn, but is the launch window timing intended to help approximate the orbit to the intended orbital slot (longitude)? If not, what is the reasoning behind picking a particular launch time for GEO sats. I know for ISS missions it has to do with orbit matching. But what about GEO missions?

Launch window has nothing to do with the GEO slot, it's about getting the bird into daylight at the optimal time for solar power.

[cppetrie]

Makes perfect sense. I knew there was a reason. So launch window timing would vary by geographic location of the launch site? Are they usually in the evening time locally at the launch site, which just makes them seem all different when translated to my local time?

Edit: clarified thought behind my question(s)

[envy887]

Makes perfect sense. I knew there was a reason. So launch window timing would vary by geographic location of the launch site? Are they usually in the evening time locally at the launch site, which just makes them seem all different when translated to my local time?

Edit: clarified thought behind my question(s)

Local time at the launch site is typically late afternoon or evening, in order to launch into orbital dawn shortly after payload sep. It varies somewhat due to payload requirements, I'm not sure why specifically. Someone more familiar with the differences between particular birds could probably explain it better.

[luinil]

I was wondering if anyone could educate me on the reason for using a parking orbit and then a 2nd burn over Africa to get to GTO instead of a single direct burn? Here's what I think happened:

On the webcast the speed dropped about 500Km/h and the altitude went up to 248km from 164Km during the coast; so my guess is that once they achieved a LEO orbit, they waited to hit apogee where a perigree raising burn is most efficient and then they burnt a minute to raise that perigree to a new apogee of 43000Km, and the new perigree will be approximately where they started the 2nd burn around 250km?

Does that make sense? Thanks in advance!

I'd guess that you'd want the apogee to be in a place the will put the satellite at the right geographic position when you circularize the orbit.

No, the final position is easy to adjust during the perigee raising burns later. (which is done through many orbits) It is easy for a satellite to drift slowly to its final position by a slight raising or lowering its orbit.

You may note that all of SpaceX GTO launches make the second burn as the upper stage passes the equator. This is because it is the most effective spot (the equator) to deliver the satellite to its transfer orbit. The final satellite position is irrelevant for this step.

Thank you (and to others) I forgot about the inclination that needs to be corrected too.
I know the satellite can adjust easily it's position but I'd guessed that it would take time and so adjusting the position from the start would have been better but the inclination has the priority.
Does geographical position determines which node they use for the second burn?

[OneSpeed]

The parking orbit is essentially circular ...

Essentially, yes. For SpaceX GTO launches the parking orbit is typically 160kms x 480kms. But it should be possible to match perigee with the crossing of the equator, giving maximum velocity at the start of the GTO burn, and yet it isn't done that way. Does anyone know why not?

[Pete]

The parking orbit is essentially circular ...

Essentially, yes. For SpaceX GTO launches the parking orbit is typically 160kms x 480kms. But it should be possible to match perigee with the crossing of the equator, giving maximum velocity at the start of the GTO burn, and yet it isn't done that way. Does anyone know why not?

At least one of the concerns would be ground station coverage.
In very low orbit (160km), you are going to experience long stretches where you are below the horizon to all the ground stations. It is obviously a very, very good idea to have solid ground coverage when executing your GTO burn, spacecraft sep and initial craft startup.
With a launch from florida, this stuff happens over central Africa and the western Indian ocean, places not known for their high-tech infrastructure.

[guckyfan]

Any idea already what the perigee is for deorbit of the second stage? Or do we have to wait for data observed to be released?

I am always somewhat concerned about second stage deorbit. The sooner the better.

[vandersons]

This was a burn to depletion. That would imply no deorbit burn for the second stage. I don't think that there has ever been a 2nd stage deorbit burn on a Falcon 9 GTO mission (would be happy to know of the contrary).

[Kaputnik]

Challenger's ill-fated STS-51L flight was the first to launch from Pad 39B in January 1986.  So all 9 shuttle launches in 1985 would have been from 39A.

We tried, really hard, to launch more Shuttles that year (1985), but there were scrubs and rollbacks and delays, etc.  SpaceX is doing this year what NASA wanted Shuttle to do back then, except for the crewed launches. 

At 10 launches so far this year, Falcon 9 becomes the first U.S. launch vehicle to fly 10 times in a calendar year successfully since Delta 2 did it in 1999, 18 years ago.  With three more launches, Falcon 9 will have us digging into the 1970s launch lists for comparison.  One or two more after that, and we'll be looking at 1960s numbers.

Of course none of those frequent flyers in those days could lift 6.761 tonnes to GTO.

 - Ed Kyle

How would these numbers look if you went by payload mass rather than number of flights? Counting the Orbiter as part of the LV for this purpose. Are SpaceX beating the payload to orbit achieved by the shuttle and ELVs?

[Rocket Science]

Makes perfect sense. I knew there was a reason. So launch window timing would vary by geographic location of the launch site? Are they usually in the evening time locally at the launch site, which just makes them seem all different when translated to my local time?

Edit: clarified thought behind my question(s)

Local time at the launch site is typically late afternoon or evening, in order to launch into orbital dawn shortly after payload sep. It varies somewhat due to payload requirements, I'm not sure why specifically. Someone more familiar with the differences between particular birds could probably explain it better.

If I understand your question correctly, I would say in general for spacecraft power generation from solar arrays/panels...

[Sam Ho]

This was a burn to depletion. That would imply no deorbit burn for the second stage. I don't think that there has ever been a 2nd stage deorbit burn on a Falcon 9 GTO mission (would be happy to know of the contrary).

Orbital lifetime in GTO is generally under 25 years, so no deorbit burn is required. Lunar gravity usually drops the perigee into the atmosphere after a few years even without a burn.

[SDSmith]

How do settle the propellants for the second firing?

[ugordan]

How do settle the propellants for the second firing?

Cold gas nitrogen thrusters which provide attitude control during coast phases and presumably roll control during powered flight.

[Norm38]

Was ground software that was the issue, not the rocket.

So my friend missed his chance to view a launch due to a software glitch? Bummer.
I doubt we'll ever get details on this, but it's a bit worrisome to have software problems crop up at this point, where they're demonstrating system maturity. As a hardware guy, I hate software problems.

[cscott]

Ground-side hardware/software isn't at the same level of maturity as the rocket.  Remember LC39A came online less than a year ago, and is about to undergo another big reno for FH.  Plus, although they've been working on improving this, historically a large amount of pad wiring and such is burnt away at launch and needs to be reinstalled.  And I'm betting that F9s are more alike off the assembly line than LC39A is like LC40 or Vandenberg. The pads are still bespoke.

My opinion only, I don't have any special knowledge.