Long-term Laythe - Part 44

Curing Comsats

Recently I've been concentrating on my Developing Duna mission and a new Exiting Eve mission, so I've been assuming that the guys on Laythe have been keeping themselves busy. But Kerbal Alarm Clock reminded me that the ship for fixing the Laythe Comsats was approaching Jool and needed to be handled first...so I'll do a quickie report of that mission.

As you may recall, all of the comsats in the Jool system were broken when the 0.90 MeV Beta Particle Storm hit, destroying the reaction wheel capabilities of the OKTO 2 probe cores. This mission is to refurbish the Laythe probes and deliver some replacement comsats that could potentially be moved to the other Joolian moons as needed.

Below, the Standard Nuclear Tug carrying the comsat payload performs a plane shift burn, which happened to be very close to Jool's sphere of influence. Also built into that maneuver (planned shortly after the ship left Kerbin's SOI) were some small prograde and radial components to target the ship on a tangential intercept with Laythe that would result in aerocapture by Laythe. Note MechJeb's Landing Guidence display that shows the expected orbit after aerocapture (this was tweaked using RCS after the ship passed into Jool's SOI to give a Laythe apoapsis of 3,000 km).

In the upcoming universe of KSP 1.0, which will include aerodynamic heating effects, the question of how to get captured at the Joolian system becomes very important. In the old days, I used to aerocapture in Jool's atmosphere (rather than in Laythe's). My thinking was that when using an atmosphere to come screeching into orbit, it would be less violent to slow down over a distance of thousands of kilometers in Jool's atmosphere rather than slow down over a much shorter distance of hundreds of kilometers in Laythe's atmosphere. But this turns out not to be the case, for reasons given below.

First, you pick up more speed dropping down to Jool than just dropping down to Laythe's orbit. Second, Laythe is moving in its orbit, which increases the distance over which you are slowing down...and you only have to slow down to Laythe's orbital speed. Add to this the fact that you don't need to use additional fuel to intercept Laythe and brake into orbit (as I did after a Jool aerocapture), and aerocapture directly at Laythe becomes even more desirable.

But, to ensure minimal aerocapture heating, you need to do a couple other things. One: You want to make sure your ship's intercept trajectory with Laythe is as close to tangential to Laythe's orbit as you can make it; this minimizes the relative velocities (by quite a bit in some cases). You can do this by tweaking the prograde and radial components of your distant targeting burn (while focused on the Jool system to see the results) so that your ship will arrive at Jool when Laythe is in the correct part of its orbit. Two: Adjust your aerocapture altitude as needed to put your ship into a highly eccentric orbit around Laythe; this also minimizes to aerodynamic heating by minimizing the aerocapture delta-V. You can then use multiple aerobrakings to lower the orbit.

All of this depends on having a good aerobraking calculator. I use the one built into MechJeb's Landing Guidence feature, but I gather that there are aerobraking calculators online.

Below: The Comsat Refurbishment ship drops in toward Laythe. I apparently hadn't handled my plane-matching burn outside of Jool's SOI too well, since the ship is coming in from below the plane of Laythe's orbit. I not sure how I messed this up...but I apparently hadn't done the burn to match Jool's orbital plane right at the node (I don't recall if I used Kerbal Alarm Clock's opinion of where the node was, or using the marker in Map view...I should have double-checked as that burn approached).

The ship got as low as 26,160 km during the initial aerocapture, and the entry flames were unimpressive (as desired).

Below: A burn of 279 m/s was needed to align the ship's initial post-aerocapture orbit with Laythe's equator. Unfortunately, the nodes were both close to Laythe, so the burn was relatively expensive. This could have been avoided had I done a better job out in deep space matching the orbital plane of Jool.

Note: You might notice that the Map View above doesn't show all the orbital and surface assets that I have at Laythe. This is because, for reasons of lag reduction, I do my orbital assembles in Kerbin orbit and interplanetary transfers is a separate copy of my savegame file with fewer ships (the main savegame file has over 200 active ships, landers, bases, etc., which can bog down KSP). After the Comsat Refurbishment ship got into a stable orbit at Laythem I copied and pasted it into the main savegame file.

At apoapsis, a small burn of 1.4 m/s was made to lift the periapsis a little for an aerobraking pass that would drop the apoapsis near 1,000 km where the stricken comsats are located.

After the aerobraking pass, a burn of 293.2 m/s was needed at the new apoapsis to almost circularize the orbit. "Almost," because a slightly elliptical orbit was used to create an intercept with Laythe Comsat 1 upon the ship reaching its next apoapsis.

Below, the ship rendezvous with Comsat 1 in a 1,000 km circular orbit above Laythe.

For extra fun and excitement, the first thing I did to Comsat 1 was give it a bump to get it slowly tumbling. Then I backed off and changed position and then tried to dock. First I gave a few little bumps to try to minimize the tumbling...then docked. The magnetic docking force is strong with this one, being rather small in mass.

After docking, I saw that the RCS fuel was down to 79%...which seemed to indicate that I had used a lot more monopropellant than expected...though this does include monopropellant used during the interplanetary transfer targeting. But still...this seemed like a lot to have used. Maybe I forgot to top off the RCS tanks during the refurbishment of the Tug.

Anyway... The central spire on the nose of the ship is a stack of: Jr. Docking Port - Small Reaction Wheel - Jr. Docking Port - Jr. Docking Port - Small Reaction Wheel - Jr. Docking Port...etc. The reaction wheels in that stack had been deactivated previously to keep the stack from wobbling around during maneuvers...so before the comsat was released, its new reation wheel had to be activated, and then the comsat was separated with its shiny new reaction wheel in place (and a docking port still on the end for future refurbishment, if needed).

Then it was off to Comsat 2 by kicking the Comsat Refurb ship in a slightly higher orbit (just its apoapsis) to wait for the next comsat to move into position for a rendezvous.

The remaining three comsats were still aligned Normal, and I didn't bother to bump them off kilter to force docking with a tumbling object (I'd already had enough of THAT fun). Below, the docking with Laythe Comsat 2, and the subsequent separation of the comsat with its spiffy new reaction wheel attached.

Lather, rinse, repeat. Below, the releasing of Comsats 3 and 4, and a final picture of a comsat on station, 1000 km over Laythe's equator. You can see the central stack of docking ports/reaction wheel units shrinking after each refurbishment.

After refurbishing the four equatorial comsats, the RCS was down to 55%. The liquid fuel and oxidizer levels in the main X200-32 tank was just below 13%...but the two FL-T400 tanks on the nuclear engine nacelles are full, which is enough fuel to return the Tug to Kerbin for re-use.

The eight replacement comsats around the centra Comsat Bus core can be used later to replace the disabled comsats around Vall, Tylo, Bop, and/or Pol (as needed by future missions). The comsats orbiting those other moons were NOT equipped with docking ports, so they could not be refurbished like the Laythe comsats.

Two of the replacement comsats were deployed for placement into polar orbits around Laythe (since Laythe is where most of the exploration traffic is located). The two comsats were separated and their high gain and omni antennas and solar panels were deployed.

Shifting the orbital planes of these two comsats 90 degrees took most of the fuel they contained, but both ended up in polar orbits with their planes oriented about 90 degrees apart.

The final Laythe Comsta constellation is shown below.

Finally, the Tug was separated from the Comsat Bus. The Comsat Bus has a full X200-800 tank and four Rockomax 24-77 engines, which makes it capable of reaching any other Joolian moon (or multiple moons with careful planning). The Comsat Bus (which also functions as a comsat) will remain in Laythe orbit for now.


So... Back to my Eve and Duna missions (write-ups coming Real Soon Now).