Phasing Without Calculation: One Weird Trick to Spacing Satellites Using Just the KSP UI.

[maltesh]

This has probably come up before; It's a fairly obvious use of the KSP UI, so I'm probably not the first one to stumble across it. But it provides a way to set up a simple, equidistant satellite array, using just the KSP User interface, without having to calculate orbital periods.

As I understand it, the typical method for, say, spacing four satellites in a circular orbit is to do the following.

 

1. Launch all four satellites on a single bus vehicle, reach the desired circular orbit, and circularize.
2. Release the first Satellite.
3. Based on the period of the original orbit, calculate the semimajor axis of a new orbit that has ¾ the period of the original orbit (5/4 also works, but we'll use ¾ for the example) and burn to put your bus in that orbit.
4. Go around that orbit once.
5. When you return to your starting point, recircularize. Assuming you used the ¾ period orbit, you will now be at a point 90° ahead of the original satellite.
6. Release the second Satellite.
7. Repeat steps 3 through 6 for the other two satellites, then head home.

 

What we'll be doing here is eliminating any of the calculation of step 3: As long as your initial orbit is circular, you can use KSP's maneuver node and close-approach UI to evenly space out your satellites, without calculating delta-V, and without calculating the details of your phasing orbit. To do this, we have to come to realize two things;

 

• KSP's Close-approach markers are /always/ shown based on your presumed orbit after the last maneuver node on your path.
• By right-clicking on a Maneuver node, it will pop up a pair of buttons marked - and + that allow you to advance or retract a node one orbit at a time.

 

What this allows us to do is the following:

 

1. Deploy First Satellite and Target it.
2. Create Node 1 to Roughly Estimate Phasing Orbit.
3. Create Node 2 at 0.0 m/s for manipulating Closest Approach.
4. Use Node 2 to Advance the Phasing Orbit.
5. Fine-Tune Node 1 to refine the close-approach.
6. Execute Node 1, Phase, and Recircularize.

 

So here's an example, beginning to set up a four-satellite constellation using the above method.

1. Deploying the first Satellite

 

Once the spacecraft reaches its target circular orbit, deploy the first satellite. It is then necessary to set the first satellite as a target in the KSP UI, so its close-approach markers can be used.

2. Create Maneuver Node 1

 

Create a maneuver node far enough in the future that you are comfortable manipulating it without catching up to it. I believe the one I've created here is about 10 minutes ahead of my spacecraft. Adjust the PRO and RETRO handles on the maneuver node to create a phasing orbit that has /approximately/ the phasing you want. It's fine if it's a rough approximation, because we're going to refine it later.

3. Create Maneuver Node 2 (0.0 m/s)

 

Now create a second maneuver node further along the path of the phasing orbit. This maneuver node will have a 0.0 m/s value, and will be used to help refine Maneuver Node 1.

4. Use Maneuver Node 2 to advance the Closest Approach

 

After placing Node 2, the Close-Approach Markers in the UI will show the closest approach after one orbit. Activate Node 2, then right-click on it to bring up the Orbit-advance buttons. (Upper-left image).

Since we're doing 90° spacing, We'll want to click on the advance button three more times (Upper Right, Lower left, and Lower right) to show the spacing after four rounds of the phasing orbit.

5. Fine-Tune Node 1 to refine the Close Approach

 

Now go back to Node 1 and adjust its Prograde and Retrograde handles to fine-tune the close approach. By doing this, we're adjusting the period of the phasing orbit to get it closer to ¾ the period of the target orbit. Once the markers match up, you're golden.

6. Execute Node 1 and Phase.

Now you're ready. You can kill Node 2 now if you so desire; it's still at 0.0 m/s. Execute node 1 to enter the phasing orbit, go around once, and re-circularize when you reach the starting position. Once done, things look like this:

 

The red and green lines solid lines are actually generated by the AntennaRange mod, here, they also serve to highlight the final positions of the Bus (where the green line crosses) and the fisrst satellite (where the red line crosses.). The bus is now 90° ahead of the target. You may now deploy Satellite 2, and repeat.

And that's basically the method there. Note that I haven't checked the periapse or apoapse altitudes of the phasing orbits, everything was just done with the maneuver node UI.

This method can similarly be used to space any number of satellites; If I'd needed to space five satellites, after placing Node 2, I'd crank it forward four more orbits in step 3. For Six, I'd need to crank Node 2 forward five more orbits, and so on.

This method will also work if the phasing orbit is /larger/ than the target orbit; If I'd done the method above with a larger phasing orbit, I'd wind up putting the second satellite 90° behind the target.

It also has its utility for Rendezvous; Make your orbit cross the target orbit. Place Node 1 where your orbit crosses the target orbit. Pull prograde or retrograde to spend as little delta-V as you feel comfortable using, and use node 2 to crank the intercept forward orbits until the close-approach markers are close enough, then go back to adjust Node 1's Prograde and Retrograde handles. Execute Node 1. On the orbit where Node 2 now exists, you'll have your close-approach.

Hopefully, this has been helpful. Any questions?

Edited December 22, 2015 by maltesh
Replacing the weird ’ caused by the forum change with the apostrophes they used to be.

[GoSlash27]

maltesh,

Would this technique still work if you don't circularize the entire bus? I like to leave my bus in the resonant orbit and kick off/ circularize the sats independently.

Best,

-Slashy

[maltesh]

maltesh,

Would this technique still work if you don't circularize the entire bus? I like to leave my bus in the resonant orbit and kick off/ circularize the sats independently.

Best,

-Slashy

I'd been working off the assumption that the satellites had little to no delta-v on their own. Yeah, it will work if you kick off the satellites when you get back to the target orbit. Come to think of it, that's even simpler, because you don't have to go through the effort of re-plotting the phasing orbit for the bus after each deployment.

[GoSlash27]

I'd been working off the assumption that the satellites had little to no delta-v on their own. Yeah, it will work if you kick off the satellites when you get back to the target orbit. Come to think of it, that's even simpler, because you don't have to go through the effort of re-plotting the phasing orbit for the bus after each deployment.

maltesh,

What I mean is if I kick off and circularize the first sat directly from the initial transfer orbit, can I plot this from the transfer orbit and then go directly into the resonant parking orbit? Or will it not plot that way? I'd expect it would, but just want to be sure.

Best,

-Slashy

[maltesh]

maltesh,

What I mean is if I kick off and circularize the first sat directly from the initial transfer orbit, can I plot this from the transfer orbit and then go directly into the resonant parking orbit? Or will it not plot that way? I'd expect it would, but just want to be sure.

Best,

-Slashy

That should probably work; At least, I don't see any obvious problems without trying it.

[GoSlash27]

That should probably work; At least, I don't see any obvious problems without trying it.

Cool! I'll have to give it a go and document how it turns out.

Best,

-Slashy

[celem]

I might have to try this one, spacing sats was always an issue for my OCD.

I used to do something like this for RT relays, though I tend to put circularise dV on the sats themselves rather than the bus.

I stick a satellite up in the desired orbit and take a look at its orbital period. I then launch a bus carrying n satellites which raises apoapsis to match the desired orbit, and then raises periapsis until its orbital period is 1/nth of the desired one. (with high values for n and lower target periods this doesnt work so well, you cant get a phasing orbit short enough) At this point the bus drops the sats and heads home and the satellites circularise themselves. You kick one satellite to circular each time the swarm passes apoapsis and they space themselves that way give or take human error.

My OCD then makes me remove the first satellite because its not equidistant, but you dont even need that if you know the target period already.

Edited October 24, 2015 by celem