Galileo 13-14 launch

[dino1984]

Launch is still on-going, but... Why is it losing altitude since some moment? Does it went past Apogee, or there is other reason?

Edited May 24, 2016 by dino1984

[Streetwind]

It's not unusual for RL rockets to complete orbit insertion after passing apogee, yes. The atmosphere is a bit more of an obstacle as it is in KSP, since you have to accelerate to Mach 25 instead of "only" Mach 7. So real rockets like to get the heck out of it early on, and then spend a long time burning horizontal. They will usually pass apogee and lose a few kilometers in altitude before they manage to get their perigee above the planet's surface.

EDIT: compare this graph of an Ariane 5's altitude during ascent. It falls back a little bit before apogee and perigee finally flip and it rapidly starts gaining altitude again as it achieves its geostationary transfer orbit.

Edited May 24, 2016 by Streetwind

[YNM]

If it was on board Ariane 5, then that's something to expect. Almost all Ariane 5 launch will do this.

[Theysen]

Not knowing the numbers out of my head but upper stages are usually having a TWR < 1 so once it reaches its apogee it has to pitch up for a certain amount (This effect becomes lower when you're close to orbital speed). During this phase it uses some of its propellant to counter act the falling and the rest goes into orbital energy. You can actually see this behaviour on most modern LV's with low thrust upper stages where the first stage boosts it up to a higher apoapsis which means more time for the upper stage to circularize. 

[dino1984]

Ok, thanks.

One more thing - can anyone tell me what orbits will those sats have and how they will be released? I understand that Fregat will carry both of them to desired orbit and release them. Do they have own propulsion to achieve two different orbits?

[Streetwind]

Yes, the satellites have their own propulsion. Much like most geostationary launches, the launch vehicle places them into a transfer orbit, and the satellites fire an "apogee motor" to circularize themselves when they reach the correct altitude. By using their maneuvering thrusters, they can then introduce subtle disfferences in their orbit that makes them drift apart over time until the correct separation distance has been achieved. Kind of like a reverse rendezvous maneuver, really.

The constellation will consist of 24 active satellites (plus some spares) in three groups of 8, each group in its own orbital plane offset by about 120 degrees (a third of an orbit) from the others. Altitude for all of them is about 23,000 km (medium Earth orbit), inclination is 56 degrees.

Here's an animation of how the constellation will work:

Edited May 24, 2016 by Streetwind

[epsonik]

Small question myself:

Why wait so long for the second Fregat burn? Is it just that the apogee is so high?

 

[edit] Oh yeah, never mind, didn't realize the altitude was so high, I thought it was a much lower orbit for these kind of satellites.

Edited May 24, 2016 by epsonik

[dino1984]

Is anybody able to describe all orbital manouvers after SECO, because I still don't understand how they are supposed to reach different orbits

[Streetwind]

Imagine two satellites, right next to each other, approaching their apogee. They circularize together. However, while one satellite assumes a perfecty circular 23,000 km orbit, the other one raises its perigee to only 22,995 km.

Since the second satellite's orbit is just a tiny bit lower, it will need just a tiny bit less time to circle the Earth once. And so it slowly leaves the first satellite behind. Over many, many orbits, it eventually advances a whole 45 degrees ahead. There, it fires its RCS system and raises the perigee the remaining five kilometers, making its orbit perfectly circular at 23,000 km as well. Since both satellites now occupy the same orbit, their position relative to each other stays permanently fixed at those 45 degrees separation.

This is called "orbit phasing", and you do it everytime you try to make an orbital rendezvous in KSP. Only there, you try to get closer to another object instead of further away from it. But it works the same way.

[dino1984]

Great, thanks!

I know all the theory of orbit phasing and rendezvous maneuvers, just didn't know that they do it this way.

 

[wumpus]

12 hours ago, Streetwind said:

Here's an animation of how the constellation will work:

[full color disco ball deleted]

Will it tend to direct you to the nearest disco?

[Streetwind]

It's a navigation system, so I'm pretty sure it will be able to!

Edited May 25, 2016 by Streetwind