Gotta go fast! Career speedrun

[king of nowhere]

Part 13: The straight way back

La coscienza di xenon returns on Kerbin in day 383, establishing a new one-of-its-kind record.

Spoiler

When leaving Kerbin, the planets were in a bad alignment, I had to turn around the sun and take over 200 days to reach Jool. This time the alingment is favorable.

Planetary alingment while leaving Jool

Kerbin will be at the perfect place for an intercept in less than half an year, letting me complete this Jool 5 in one year. No need to turn around the sun or anything.

As I said before, I need a gravity assist to get out of the Jool system, else the ion engine is too weak and would require multiple orbits of apoapsis raising, unacceptably slow.

The gravity assist to escape Jool

I get away from Vall immediately, La coscienza di xenon is still close enough to periapsis that it can burn prograde for a few tens of m/s and escape. With low thrust, it's just more practical to adjust for an assist from a Jool orbit.

Tylo is right now in the perfect position for an assist, so by the time I can reach it it won't be anymore, and I'll have to wait 9 days for it to return in the same place. Hence I have to use Laythe. It takes 4 more days, I can't shorten that.

Jool periapsis lowering for Laythe encounter

At this point I'm using two engines and 13 RTGs (I started with 6 on the main ship and 4 on each of the long range landers, for a total of 18. I could not recover one from the Pol lander because it was the root part, and I lost all four on the Bop lander). But 12 RTGs can power a single ion engine, so I'm carrying a lot of extra mass just for a 10% extra thrust. As soon as I finish this maneuver - which is still in a relatively fast orbit, where higher thrust helps - I'm going to ditch the second ion engine and an RTG.

After the gravity assist

Ok, I have 21 km/s. A tad more, since I'll be dropping spent tanks. I must save some xenon to brake, I'll reach Kerbin so fast not even the thermal shield will save me. Ideally I want to meet Kerbin on the part of the orbit that's on the right side in the picture, this way I can touch it lightly and reduce intercept speed. Let's see how much it would cost.

That's how much it would cost

14 km/s, for a 162-day trip, ending in day 406. Maybe a couple days more, since those maneuvers are long and must be started in advance. And it would leave 8-10 km/s to brake. A good target.

Commencing the burn. It's almost prograde

I've done the 14 km/s burn, and I still have over 10 km/s. That's more than I need to brake. I can spare an additional... let's say, 2 km/s. How much time could I spare that way?

With an additional 2 km/s I can save 25 days. Let's do it

Now that I set my course, fast forward to the arrival. Of course, I can't just enter the atmosphere. I tried as an experiment, the whole ship got vaporized before even reaching 55 km of altitude.

What happens if I try to reenter atmosphere at 15 km/s

So I start braking one day before encounter. I'm still well out of Kerbin's SoI

Dropping the last tank. Kerbin is a small disc in the distance

I have 4 km/s left, and I reduced my speed by 5 km/s. I burn 2 more km/s, then decide to try and reentering atmosphere just like that.

Entering atmosphere at 8.5 km/s. A Dolphin survived a higher speed on the DREAM BIG mission, but they were sturdier

So far so good, but I'm running out of ablator - I only loaded a smidgen to reduce mass

As soon as the ablator runs out, the ship overheats and disintegrates

Whenever somebody asks how to salvage a high speed reentry, he gets told that a lower periapsis may do the trick, because the ship will slow down faster and spend less time overheating. It's normally a bad advice, especially for spaceplanes. Certainly not something I'd tell an inexperienced player. But there is exactly one case where it may work: when you need the ablator and you're short on it. I tried a 20 km periapsis, and La coscienza di xenon slowed to a lower speed before running out of ablator and burning. I tried a 0 km periapsis, and it burned despite the ablator. I'm still too fast. I'll have to use all the xenon.

Use all the remaining xenon, speed reduced to 6300 m/s

Dropping the engine. The time and speed do not match because this pic is from a previous attempt at reentry. It was the best image of the discarded plate

Entering atmosphere at 7 km/s; of course, La coscienza di xenon picked up some more speed falling towards Kerbin. The explosion on the left is the engine block

Getting past 15 g of deceleration. Good thing I left g limits off for this mission

Ablator is gone, but by now I am at a safe speed

Landed!

And so I completed the Jool 5 in day 383, 1 hour and 9 minutes of a new career. As far as I know it's a record, because I'm not aware of anyon else trying a speedrun. It could certainly be optimized a lot; spend one more week around Kerbin, get more money, get a lot more xenon, use 60 km/s for the outbound trip instead of 40, reduce travel time. I doubt anyone will want to try, though, unless they manage to make persistent thrust work.

Edited May 4, 2023 by king of nowhere