Exploiting quirky ladder physics to get to the Mun and back in 2.022 tons

[tavert]

Not counting the Kerbal, anyway. I've sent Kerbals to the Mun and back hanging on to ladders in 0.19 before we had seats, but since 0.20 tripled the mass of EVA Kerbals (from 0.03125 tons to 0.09375), it looked like it was going to require more fuel to carry the Kerbal on this mission. A series of posts on the KSP subreddit converged at using ion engines to get down to 2.53 tons, and I did not think that would be beatable by much in 0.20 until I found an interesting quirk of ladders. If you mount a ladder perpendicular to the thrust direction, it appears any Kerbals hanging on to that ladder get accelerated for free, without requiring any more delta-V than the craft uses to accelerate itself.

So using a small probe, 1 solar panel, 1 small battery, 3 round-8 fuel tanks, and a single Rockomax 24-77 engine, which gives a vacuum delta-V of 2750 m/s for my upper stage, I was able to make the round trip with a Kerbal hanging on inside a ladder cage. Everything else on my upper stage, cubic and octo struts, aircraft landing gear (to withstand harder landings), and the ladders are all massless in 0.20.

A jet with 4 intakes and 15.74 units of fuel was plenty to get this into orbit. I burnt off the oxidizer from the jet-stage fuel tanks before lifting off (by disabling liquid fuel in those tanks, disabling both liquid fuel and oxidizer in the upper-stage tanks, and switching just the Rockomax engine on - this is what the fuel line is for), but the 2.022 ton mass number is with all tanks full.

This took an extremely long time since having Kerbals on a ladder limits you to 4x physical time acceleration, and you have to temporarily let go of the ladder to quicksave. After getting to an initial 58 x 100 km "orbit" on the jet engine, I spent 22 hours (letting MechJeb fly while I did other things) doing periapsis kicks in the atmosphere to burn off the last 2.5 units of fuel in the jet stage, bringing my orbit up to 62 x 228 km. Stabilizing this orbit after ditching the jet stage only took 6 m/s, then getting out to the Mun (with everything lined up so the Munar injection burn is close to periapsis) took only 744 m/s.

Rest of the mission was fairly typical, aside from taking forever at only 4x time warp. To avoid krakensbane tearing the ship apart, it's best to let go of the ladders while crossing SoI boundaries. Landing back on Kerbin was done via powered landing, since a parachute would cost more delta-V than it could save. The TWR requirement for powered landing was the main reason for choosing the 24-77 engine. It may be possible to land on one or two ant engines with the fuel tanks nearly empty, but I suspect it would cost substantially more delta-V to do so with a TWR not much more than 1.

Anyway, here are the pictures from the mission.

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Edited July 5, 2013 by tavert

[AustralianFries]

Wow. Sweet mission, mate! That's definitely an interesting exploit!

[DJEN]

Maximum exploit of the FTL bug, always awe-inspiring.

[Tw1]

That's cool. How do you control the ship, if the kerbal gets accelerated on its own? Is there a way to control that acceleration?

[tavert]

Yeah I was thinking this is the same glitchy physics that makes the FTL egg work. Here it's just a Kerbal mass remover, the probe by itself can do the mission properly

There's a probe, solar panel, and battery to control the ship. It's technically unmanned, the Kerbal is just hanging on for the ride. Every now and then I have to switch to the Kerbal and shift him up or down the ladder a bit to avoid colliding with the edges of his cage.