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Comment by cmsmith | Hacker News Reader

I must have missed it, but can anyone explain why Orbcomm wants 11 satellites in the same orbit? The stage 2 engine isn't firing in between letting the satellites go.

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The satellites have their own thrusters and will use that to reposition. Changing your position within a single orbit is fairly cheap - even if you just go a few m/s, with enough time you can space out quite far.

Leo, they are so low they don't cover a whole lot of territory. The oribits may be at slightly different altitudes simply because of the inertia of the second stage, I really should have monitored that altitude indicator during the releases, were they identical?

Maybe there was a secondary thruster somewhere to provide some separation.

I went back and recorded the release speed (km/hr), altitude (km) for each drop:

25960 626

25954 627

25950 628

25944 630

25937 632

25935 632

A circular orbit at that height is about 29000 km around; 25 km/hour of difference between release 1 and 6 would give them a separation of 6 degrees longitude/day, or ~50 days until the faster satellites 'lap' the slowest one.

Not any sort of rocket scientist, but I gather the strategy with these kind of LEO satellites is to just throw a pile of them up there semi-randomly, and count on at least one being in the neighborhood when you want to talk to it.

edit: Last paragraph is wrong. See my comment two levels down for the answer.

The satellites themselves usually have small thrusters but I'm not sure what kind of orbital change the can effect, usually they are there for detailed maneuvering and to boost the satellite to a slightly higher orbit as it decays. When it runs out of fuel it dies so you'd try to use as little of that as possible during launch.

>A hydrazine propulsion system is installed on the spacecraft for orbit adjustment maneuvers and orbit maintenance providing a delta-v budget of more than 100 meters per second for each satellite.

>Because OG2 satellites are launched in groups of several spacecraft, they require propulsion systems to place the satellite into different nodal planes to begin operations.

There's the answer, courtesy Orbcomm.

Neat :) So they do their own placement. Nasty stuff, hydrazine. F16's use it as emergency fuel.

But you can use it to grow space potatoes! :).

After it chemically burns and then ignites you, yes.

Even a small difference in orbits adds up over time. You don't need much fuel for orbital adjustments, as long as you're staying in the same plane.

Say one satellite's orbit is a bit smaller. That means that, over time, they will naturally drift farther and farther apart, until they end up on different sides of the globe. At that point, just raise the smaller orbit slightly.

Kerbal Space Program is really good for understanding stuff like this.

You just described the way I launched my first comms network constellation[0] - basically loaded a whole bunch of satellites on a single rocket and deployed them on slightly different orbits, so that they'll spread themselves out as the time goes.

Only mid-launch I figured out I could just put three of them on Kerbisynchronous Equatorial Orbit at 120° phase difference :/.

Yes, KSP is absolutely amazing for learning this stuff :).

[0] - useful if you're playing with RemoteTech mod, which prevents you from operating unmanned spacecraft if they don't have a radio connection back to a proper command centre.

is it this one? https://kerbalspaceprogram.com/en/

Yes.

You'll notice that the speed decreases as the altitude increases. That's not maneuvering, that's just an elliptical orbit. There is no actual speed or altitude difference between the various releases, that's just the whole group coasting slightly uphill. Separation will be done by maneuvering the satellites afterwards.

Pretty sure both altitude and speed were constant during the satellite deployment phase.

Whatever the second stage's inertia was, it was more or less identical to that of the satellites' (outside of the minimal thrust to separate the payload from the second stage).

I too am confused about the utility of launching 11 satellites into virtually the same orbit.

The satellites themselves can adjust their individual phasing within the same orbit with minimal propellant.

Yes, it absolutely was firing continuously around 17,000 MPH. Doesn't take long at that speed to get some distance between sats.

The satellites are moving at that speed too....

But they're not all moving at the same speed if the second stage was still firing between satellite releases.

True but irrelevant, as the second stage engine cut off before satellite deployment.

why do you think that? the second stage engine was off during deployment.

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