# How are multiple low orbit communication satellites per launch distributed along an orbital plane for global coverage?

SpaceX talks about launching 4,425 communication satellites into 83 different orbital planes at 1,100 km altitude. That's about 50 satellites per orbital plane. In terms of capacity to LEO, a Falcon 9 has about 22,800/50 = 450 kg to spend on each of them if all 50 are launched on the same rocket.

Does this mean that the 50 satellites in the same plane will be spread out 7 degrees apart all with the same orbital speed?

Would the upper stage have to accelerate and decelerate for releasing each of the satellites, or how would it be done? Does it require another kind of upper stage? Or will all satellites be released at once and reach their destinations with their individual perhaps economical low thrust propulsion? Regardless of how SpaceX will actually do this, which I suppose is still not publicly known, what would be a feasible approach? Have for example Iridium or GPS used a similar way to distribute satellites along an orbital plane?

• – uhoh
May 13, 2017 at 12:38
• @uhoh Especially the first Q (yet unanswered) is great and covers mine and more. Maybe a "kick" separation is enough to put each satellite in the right orbit, while slightly accelerating the rest of the stack until it gets 7 degrees (about 1,000 km) further away. With each kick strength optimized to give all satellites the same speed. May 13, 2017 at 13:30
• @LocalFluff Not sure what you're asking. Each satellite is going to need two "kicks": one to make it’s orbit different from the reference orbit so it can drift, and one to make it the same again to metaphorically lock it into place. Typical separation velocities are on the order of < 1 m/s, so it’s possible they could be leveraged. May 27, 2017 at 15:04
• I added a little snippet to my answer to address the specific question. Does the 22,800 figure account for the 1,100 km altitude? That's at the high end of LEO. Remember also that there will be lots of structural mass to support multiple satellites. May 27, 2017 at 15:08

Your arithmetic getting to 50 birds per plane about 7° apart seems reasonable. I would be surprised if 50 satellites could be put on a single launch, if for no other reason that I don't think they'd fit in the fairing. Regardless of how many go up on a single launch, I think we can safely assume it would be more than one, in which case the answer below is still relevant.

What you're talking about is known as phasing the orbit of a spacecraft within its orbital plane, and yes, it’s a common problem that arises when multiple spacecraft are deployed from a single launch vehicle. There are a variety of ways to solve this problem that depend on the capability of the satellite, the launch vehicle, the orbit, and the constellation concept of operations (most specifically how critical the spacing is and how much time you're willing to spend between launch and everything being in the final configuration).

# The Orbital Mechanics of Phasing

Ideally, phasing only changes a satellite's relative true anomaly with respect to the other satellites in the same plane. (I say relative because the TA changes continuously during an orbit.) A very straightforward way to do this is to temporarily change the orbital period by changing the orbit's semi-major axis. For example, consider a satellite in a reference orbit with a 90-minute period: a satellite progresses through 360° of TA in one orbit. If the orbital period was changed to 60 minutes (implausible, but we’re illustrating a concept), after 60 minutes one satellite would have a 360° change in TA, but the reference satellite would have only gone through 60/90*360 = 240°. If the 60-minute satellite was then restored to the original 90-minute orbit, the two satellites would then be permanently 120° out of phase.

In practice, such an aggressive maneuver would require a prohibitive amount of propellant. So typically the semi-major axis is changed in less dramatic fashion, and the per-orbit drift in relative TA is allowed to integrate up over several orbits. This process can take an arbitrarily long amount of time.

Luckily, the ΔV required is small if you have some time. A 180° phasing in a 450 km orbit can be done for single-digit m/s of ΔV if you have 3-months to spare. (Exercise to the reader, or perhaps a good topic for another question.)

# Sans Propellant

Many satellites don't have propulsion but still need to phase. For example, Planet's Dove cubsats do not have propulsion. As presented in this paper, they use drag to induce phasing. By controlling each's satellite's attitude they change its cross-sectional area and drag coefficient, which changes the relative drag. Drag reduces the semi-major axis of an orbit and thus it’s period. They're using the atmosphere like a low-thrust, always anti-velocity-pointing propulsion system.

# All the Fuels

Obviously the more straightforward approach is to use an on-board propulsion system. In the Seattle announcement of its communications constellation, Elon Musk indicated their satellites would have propulsion. (Note: I don't have a time-stamp, perhaps if someone watches the video and finds it they can edit this answer.) Given this, the satellites will likely be deployed by the LV into a baseline insertion orbit and then each satellite will use on-board propulsion to maneuver to a phasing orbit, wait until its relative TA is appropriately phased, and then maneuver back into its operational orbit.

A few weeks to get the phasing right should be no big deal because the satellites are likely designed to last for years — a small fraction of their operational life. Also, satellites typically go through on-orbit checkout activities just after launch that probably don't depend on operationally representative relative phasing.

# No Rocket?

Obviously the rocket upper stage can initially deploy each satellite into a phasing orbit (i.e. an orbit slightly different than what's desired for nominal operations), but the second maneuver that puts the satellite back into the baseline orbit after phasing is complete would still need to be done by the satellite because the rocket's mission would be long over.

I'd consider deploying an entire plane of satellites into a slightly elliptical phasing orbit and have each satellite circularize itself when it had reached the right relative velocity (so one ASAP, and then each subsequent satellite at a fairly regular frequency thereafter). Or if you had the ability to do at least one restart I'd use two phasing orbits: one with a longer and the other with a shorter period than nominal. That way half the plane's satellites can drift one way and half the other. This would approximately halve the time required to fully deploy a plane.