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SpaceX Successfully Completes First Mission to Geostationary Transfer Orbit

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Re: SpaceX Successfully Completes First Mission to Geostationary Transfer Orbit

#11

Can anyone explain why they placed the satellite in 295x80 000 km orbit? Is it especially hard to place satellites directly into geostationary orbit, or something else?

So, orbits are weird. As in, "burning your engines in space almost never does what you'd intuitively expect based on a lifetime on Earth with things like cars and planes" weird.

In order to get geostationary, you need to get to a circular orbit at an altitude of 35,786km above Earth's surface. Trying to do that all in one burn is, I guess, theoretically possible but is going to waste an absurd amount of fuel.

The reason for this is that changing altitude in orbit = changing your speed. Specifically, you have two points you care about: your apogee (highest altitude) and your perigee (lowest altitude). To raise your apogee in the most efficient manner, you accelerate prograde (in the direction of your orbit), at perigee. To raise perigee in the most efficient manner, you accelerate prograde at apogee. Or, more simply: what you do at a certain point in your orbit will end up affecting what happens at the point in your orbit that's precisely opposite the point where you did something.

So the most efficient way to get up there is to use a transfer orbit. First you get into a lower, "parking" orbit (which doesn't take as much fuel as going all the way up in one go). Then at perigee you burn to raise your apogee out to the altitude you want. Then you cruise to apogee, and burn again to raise your perigee, resulting in a circular orbit.

Here's a detailed article on that:

https://en.wikipedia.org/wiki/Hohmann_transfer_orbit

Except it never goes quite that simply in the real world, so you actually end up doing more than two engine burns, but under ideal theoretical circumstances, you'd do it in two.

Re: SpaceX Successfully Completes First Mission to Geostationary Transfer Orbit

#12

I am so glad this went well. Too bad it did not happen on Thanksgiving. I had my laptop out and had all the kids looking over my shoulder watching the two launch attempts. Doing my part to get the younger ones interested. Nice picture from the space-x site- http://www.spacex.com/media-gallery/detail/90781/2601

Thanks so much for posting that link. I saw the streak picture from another commenter, and wondered if there was a high-res version.

It's so beautiful that I would have it on my wall, but I doubt it would print out nearly as nicely as it appears on-screen.

Re: SpaceX Successfully Completes First Mission to Geostationary Transfer Orbit

#13
It's cool to see so many people excited about this launch. Just wanted to remind people that we have a lot of software job openings right now. People are often surprised to learn that SpaceX is hiring web developers -http://www.spacex.com/careers/position/3890. There are some interesting internal tools projects that are critical parts of each mission. We have embedded developer, PM, and test roles open as well.

Re: SpaceX Successfully Completes First Mission to Geostationary Transfer Orbit

#14

It's cool to see so many people excited about this launch. Just wanted to remind people that we have a lot of software job openings right now. People are often surprised to learn that SpaceX is hiring web developers - http://www.spacex.com/careers/position/3890 . There are some interesting internal tools projects that are critical parts of each mission. We have embedded developer, PM, and test roles open as well.

What about janitors? I'm sure I could clean up the poop!

I think I'm the reverse of (Gattaca) Ethan Hawke in this situation.

Re: SpaceX Successfully Completes First Mission to Geostationary Transfer Orbit

#15

Can anyone explain why they placed the satellite in 295x80 000 km orbit? Is it especially hard to place satellites directly into geostationary orbit, or something else?

So, orbits are weird. As in, "burning your engines in space almost never does what you'd intuitively expect based on a lifetime on Earth with things like cars and planes" weird. In order to get geostationary, you need to get to a circular orbit at an altitude of 35,786km above Earth's surface. Trying to do that all in one burn is, I guess, theoretically possible but is going to waste an absurd amount of fuel. The rea…

This is made even more complicated by the fact that SpaceX isn't launching from the equator, which means the satellite will need to change its orbital inclination. Inclination changes are less expensive the 'higher' up you are, so SES8 is in an orbit with an apogee almost twice as high as its eventual orbit. This allows it to use less fuel to correct the inclination.

I haven't read up on the burn plans, but it's possible that the mission is technically using a bi-elliptic transfer[1], rather than a straight hohmann transfer (warning: I learned my orbital mechanics from the Kerbal Space Academy).

[1] http://en.wikipedia.org/wiki/Bi-elliptic_transfer

Re: SpaceX Successfully Completes First Mission to Geostationary Transfer Orbit

#17

I am so glad this went well. Too bad it did not happen on Thanksgiving. I had my laptop out and had all the kids looking over my shoulder watching the two launch attempts. Doing my part to get the younger ones interested. Nice picture from the space-x site- http://www.spacex.com/media-gallery/detail/90781/2601

I was there on the Monday before Thanksgiving and was really disappointed that it didn't happen then.

Re: SpaceX Successfully Completes First Mission to Geostationary Transfer Orbit

#18

Can anyone explain why they placed the satellite in 295x80 000 km orbit? Is it especially hard to place satellites directly into geostationary orbit, or something else?

So, orbits are weird. As in, "burning your engines in space almost never does what you'd intuitively expect based on a lifetime on Earth with things like cars and planes" weird. In order to get geostationary, you need to get to a circular orbit at an altitude of 35,786km above Earth's surface. Trying to do that all in one burn is, I guess, theoretically possible but is going to waste an absurd amount of fuel. The rea…

Is this the same way that the Indian mars mission reached its "transfer altitude"?

Re: SpaceX Successfully Completes First Mission to Geostationary Transfer Orbit

#20

Can anyone explain why they placed the satellite in 295x80 000 km orbit? Is it especially hard to place satellites directly into geostationary orbit, or something else?

The reason they temporarily raise their apogee so high is that it reduces the delta-v cost of changing inclination. The cost of changing inclination is directly proportional to your speed at the moment when you do the inclination change. When you are in a highly eccentric orbit, your speed at apoapsis is much lower than your speed in a circular orbit.

Also, inclination burns can be combined with raising periapsis, which allows you to save some fuel thanks to the way they are at a 90 degree angle.

Another advantage is that it provides an easy and cheap way to deliver the spacecraft into the correct position in the stationary orbit, by controlling the orbital period of the eccentric orbit so that the periapsis will be where you want it to be (possibly after several orbits).

Because of this, the best possible way to launch from far away from the equator to a geostationary orbit is to launch to a highly eccentric orbit, at apoapsis when you intersect the equator simultaneously raise your periapsis to geostationary and fix your inclination, and then circularize at periapsis.

If this is interesting, and you don't already own kerbal space program, head over to the steam store right now.

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