Rescue from Retrosolar

[Laie]

*Youtube Voice* Hello my dear friends. Today I am going to rescue Val from a retrosolar orbit.

There has been a challenge about that a while ago, but somehow I never came around to submitting an entry. My interest in that kind of mission has been rekindled when @Mars-Bound Hokie had questions about a similar mission he had in a career game. So I put Val into a cockpit and used the Set Orbit cheat to place her in a retrograde solar obit based on parameter gracefully provided by Mar-Bound Hokie.

Seriously: if you're looking for interesting storytelling, this is the wrong place. I'm going through the motions of rescuing a Kerbal from a retrograde solar orbit. I'm writing it down mostly so it can be referred to later when the question comes up again in Gameplay Questions. Anyway, Val was stranded in space:

(only SMA, eccentricity, and inclination are known. I should probably have added an value for Arg Pe: as it is, AN/DN coincide with the Apsides. But too late. As i write this, the mission is already underway.)

And a rescue mission was launched to save her:

You may laugh at a vessel named "Something IIb" but this is how it goes. At least I didn't have to go up to Rescue IVf2 or something.

Once in space, I started to draw up a preliminary mission plan: From Kerbin down to a low PE around the sun, from there up to the target orbit. Still prograde, by the way: I'll have to reverse directions once I get there.

Note the two encounters: In the time I'm loitering at high AP, Val will swoosh right past me. Looks like a hit, doesn't it? But seen from the side, there's still quite some distance between us:

And besides, I have no hope of executing all maneuvers until them with pin-point precision. The takeaway from that plan is that I'll try to establish a high AP in that general area, and then make things up as I go along.

And, just by the way, this is the vessel as it looks in LKO:

"Making things up as I go along" starts immediately: the double Wolfhounds already had to provide the last 100m/s or so of making orbit. Whatever remains is used to send me to a low PE around the Sun. I dialed my remaining dV into the maneuver, but after execution was some 20m/s short.

Edited January 18, 2020 by Laie

[Laie]

After the departure burn, I know my precise PE around the Sun. And while I have several long burns of 20 and 40 minutes to perform, I will do so in a solar orbit. Even at low solar PE, my maneuvers will be "instantaneous" relative to the trajectory I'm on, and I can rely on them being performed with pretty good accuracy. Time to draw up a real flight plan, that will get me close to my target:

I got that plan through numerous iterations of slightly increasing the dV of the (prograde-only) burn at solar PE, followed by re-seating the other node at solar AP and tweaking that. I'm not sure if I could have done that with the in-game node editor. Mods remain indespensable to me. I'm using the Mechjeb node editor, those who abhor autopilots may use PreciseNode instead, but either way, a "Snap to AP" function saves a lot of time and hassle.

The time was not wasted, though: I could follow that plan almost to the latter. After the maneuver at solar PE, the next maneuver only needed 0.2m/s worth of adjustment.

 

 

[Laie]

Let's talk about heat management:

Here I'm running the nuke for 20 minutes. The next maneuver will even take 40min. The nuke reaches a temperature of about 1000K, and 20 minutes is long enough that a lot of the heat will seep through the entire vessel. Which at first glance doesn't seem too bad, nothing will blow up from 1000K.... however, Ablator starts to sizzle at 450K. It also slowly vaporizes when close to the sun.

I believe that the Ablator will be needed when I make my return to Kerbin, and would prefer to keep it, thankyouverymuch.

The only solution I could find was to use rigid radiator panels, the ones that "only cool nearby parts", and attach them to the command pod (their exact placement is thanks to the offset tool). The large folding radiators will try to shed heat from all over the vessel: the heat shield is only one part among many that desires cooling. And it is simply not practical to attach so many radiators that the vessel will remain below 450K.

So, on order to provide cooling to the heat shield, one has to employ radiators that are dedicated to the task, and not overwhelmed with trying to cool all and sundry. As it turns out, not much cooling power is need. But the mission would fail without it.

[Laie]

Back to the mission. After reversing the orbital direction at high AP, I was left with this:

Which is as good as it gets, from three years out. I started closing the gap when I was one hour from closest engounter, and eventually Val could escape her prison and move over:

 

[Laie]

Heading home: reverse orbit, leaving a low Solar PE again. From there, make encounter with Kerbin.

Reversing the orbit again is important: no heatshield will survive entering Kerbin's atmosphere at 20km/s. The safe limit remains to be determined: some say 8500m/s, personally I could enter with a 1.87m shield at 9200m/s. It probably depends on how deep and hard you hit the atmosphere, and how much exposed surface area there is on the shield to radiate with: if the shield is wider than the capsule on top, it can radiate a little better.

Still, coming at Kerbin from a retrograde solar orbit will result in velocities that are altogether too high. Can't do that.

I'm hitting a snag in that I'm running out of LiquidFuel. That was not supposed to happen quite that early: I still have more than enough dV from an Ion Engine, but I'm far from the sun, with only tiny solar panels. Still, I manage to bring the solar PE up to 2.5Mm, which I hope will be high enough for my radiators to cope (ablator also sizzels from solar heat, remember? If it wasn't for that, I could risk a much lower solar orbit).

Also note that the fuel tank is glowing red hot. All the parts between the tank and the pod have served to insulate me rather well, though, the radiators could cope easily.
As a final note, this is also where I leave the probe core and antenna behind. As I've picked up a pilot, those parts are no longer needed.

Yup, 2.5Mm above the sun is rather hot. Some ablator has vanished. The radiators went red-hot as I dropped out of timewarp, and recovered while I was in normal time / physics warp. Ablator loss only happens at high warp (>100).

Edited January 19, 2020 by Laie

[Laie]

The get-home maneuver was a bit too precise, I blow my remaining dV on reducing velocity and setting the PE. Ion thrusters need to be tweaked down to ~40% as that's all the electrical power I have. Another 20-minute burn, but by now I'm used to it.

And a little later: Terra firma.

Turns out that I never needed any ablator, as the base plate would withstand the entry. And even more surprising, the ion engine survived as well. Given the price tag, that may be good to know.

 

----  Mission Summary  ----

~2700 m/s to get from Kerbin to a low PE around the Sun
1922m/s  - high AP between Dres and Jool
5871m/s -  to reverse directions and set up a RV with the target
295m/s - matching velocities
7500m/s - reversing directions again, prograde low PE around the sun
1660m/s - lowering AP to Kerbin

Gross Delta-V on vessel: 24.000 (approx. 20.500 in LKO)

Launch Cost: 164.026
Recovered assets: ~11.600 (though at the far end of the world, at perhaps 50% recovery value)