Lowest dV to Laythe?

[Aanker]

Hey there!

I'm right now busy building a spaceplane SST-Laythe (Single Stage to Laythe, in case the term is too novel), and while I have managed to get there and take many beautiful pix, I only had roughly 300 m/s of dV left and my approach velocity was close to 6 km/s. Suffice to say, the struggle was too hard on the spaceplane hull, and an explosion ensued.

So. For my second attempt, I'd like to at least be able to land on Laythe. My spaceplane has, after insertion into a 100x100 Kerbin orbit, roughly 1800-2000 m/s of dV (I think, if the maneuver node dV is anything to go by). I want to approach Laythe at a survivable velocity. Do I absolutely have to build a new spaceplane with more fuel, or can I a) make a more accurate/efficient Kerbin-Jool transfer which puts me directly on a lower speed encounter with Laythe, use an easy gravity assist to cut my fuel consumption and save some dV for a retro burn or c) a combination thereof?

I find it especially difficult to get the encounters accurate enough, perhaps because as part of weight saving measures I've not included RCS/monopropellant. But I don't really have an idea on how to use accurate gravity assists either.

Thanks!

[LostOblivion]

You could use one of Jool's other moons to slow down using a gravity assist, but you'd have to use the biggest one (Tylo?), because the others are too small to be of any use.

Also, for an efficient trajectory, use this fab tool to calculate your launch windows.

http://alexmoon.github.io/ksp/

Edited July 9, 2015 by LostOblivion

[Caelib]

Reverse Gravity Assist off Tylo is your best option.

[MadChris48]

I'll just leave this here. http://forum.kerbalspaceprogram.com/threads/80455-How-To-Go-From-Low-Kerbin-Orbit-to-Jool-Orbit-for-1051-m-s

[Aanker]

Ok, good answers so far, but my problem with trajectories is not so much timing (I use Kerbal Alarm Clock's transfer windows, which I assume are accurate enough?) as it is using the maneuvre node tools to get me onto a good encounter. If I am to use a reverse gravity assist off of Tylo, for instance, how do i set up my approach on the Jool system? Aim for Tylo?

I'll just leave this here. http://forum.kerbalspaceprogram.com/threads/80455-How-To-Go-From-Low-Kerbin-Orbit-to-Jool-Orbit-for-1051-m-s

That sounds really good, but one goal of the 'Project Laythe' spaceplane is for it to function as a shuttle service of sorts, so I would need more opportunities than just the Year 1 specific date, although maybe I missed something in the thread.

[Red Iron Crown]

I'll just leave this here. http://forum.kerbalspaceprogram.com/threads/80455-How-To-Go-From-Low-Kerbin-Orbit-to-Jool-Orbit-for-1051-m-s

I tried this transfer a couple of times while working on a single-stage-to-laythe-return mission, I couldn't make it work (I fly and make nodes manually). I'm not going to say that an autopilot is required but it takes a *lot* of precision, more than I seem to be capable of.

[winged]

I tried this transfer a couple of times while working on a single-stage-to-laythe-return mission, I couldn't make it work (I fly and make nodes manually). I'm not going to say that an autopilot is required but it takes a *lot* of precision, more than I seem to be capable of.

Maybe you will be interested in my tutorial:

http://forum.kerbalspaceprogram.com/threads/118470-1-0-2-Any-hope-left-for-SSTOs?p=1931674#post1931674

I can make it using 1220 m/s. 1051m/s must be very special transfer window + Mun gravity assist.

[cantab]

For a capture at Laythe you want your trajectory compared to Laythe's orbit round Jool to look like this.

On the "Good" trajectory you will hit Laythe's atmosphere at around 3 km/s. Fast, but possibly survivable. Bear in mind you only need to barely capture around Laythe, or even barely capture around Jool itself, you don't have to go straight to a low Laythe orbit or a landing.

On the "Very Bad" trajectory you would hit Laythe's atmosphere at around 8 km/s. Kaboom.

To arrange a "Good" Laythe encounter you want to set up a course correction. I usually do one around the orbit of Dres, then another just after entering Jool's SOI if needed. Then the secret is to trade off pro/retrograde and radial/antiradial components. By doing that you can change your arrival time at Jool periapsis while keeping the height of that periapsis the same. (Around 27,000 km, the radius of Laythe's orbit round Jool). Reach your periapsis when Laythe reaches the same spot and there's your encounter.

The same general geometry applies for encountering any planet or moon if you desire either an efficient powered capture or a gentle aerocapture.

For a gravitational capture into the Joolian system you want to use either Laythe or Tylo for the gravity assist. Vall is a maybe, I've never really tried, Bop and Pol are definitely too small and too slowly orbiting. I find the exact trajectory hard to visualise myself and often end up trying stuff until I see a promising capture. The same technique as above is needed to set up your gravity assist though - a course correction using pro/retrograde and radial/antiradial components to tune the trajectory through the Joolian system and the timing of that trajectory independently from each other. You may also need to add normal/antinormal components to avoid being kicked into a highly inclined orbit.

However your gravity assist works out, you still want to finally encounter Laythe in a similar fashion to above, with your orbit round Jool just touching Laythe's orbit or close to that, not sharply cutting across Laythe's orbit and definitely not meeting it when it's going the other way to you.

For a gravity assist out to Jool itself you want to use Eve first of all. If you need more than one assist then Kerbin itself is good too. Duna is usable with repeated flybys but not really worthwhile. Then there's a crucial trick. You cannot use a gravity assist at peri to raise apo, or at apo to lower peri for that matter. So what you need to do is get on a trajectory with a Sun periapsis lower than Eve's orbit. This takes a little more delta-V than the most efficient Eve transfer, but that small extra delta-V gets repaid many times over by the gravity assist.

Whether you encounter Eve before or after your solar periapsis doesn't matter, and yet again the same course correction techniques will come in handy, but now with the additional variable of your Kerbin departure time. Once on course to the Joolian system you're then looking at either an aerocapture or a gravitational capture as covered in the two sections above.

On a final note, for any sort of gravity assist work I strongly recommend the mod Precise Node. In particular it makes changing the timing of a burn, which can be very important, far far easier than with the stock system.

[PLAD]

Aanker, you have a point that just a one-time low-dV path isn't too useful, but there are a lot more ways to get from Kerbin to Jool cheaply. The trick is that 3 Kerbin-Jool synodic periods =350 (Earth)days, and 5 Eve-Jool synodic periods = 346 days, so any path involving Kerbin, Eve, and Jool will often repeat (roughly) in about 350 days. For example, the best way I've ever found to get to Jool is that K146-E195.65-K296.2-K509.25-J821 method I used for the 1011m/s mission (and a few others). It costs 1068m/s (or 990 if you use Mun). There is another one 352 days later: K498-E546.9-K639-K852.05-J1062 (but the cost is 1163m/s). This path is very finicky so there are only a few repeats before it fades away, though I'm sure it reappears someday. A much better path for repeatability is K-E-K-J. I have found there is always one of these at least every 350 (Earth) days. A given 'cycle' of paths will run for a while and then fade out, but another cycle will arise before it does. Here are examples for the first 1200 (Earth) days of the game, which is about 11 Kerbin years:

K1-E46.4-K263-J454.5 : start boost (from a 75x75km orbit) is 1288m/s.

K344.5-E391.6-K608.95-J834 : 1336m/s.

that's the last in that particular chain but another has started:

K132-E197.4-K406.5-J1046 : 1515m/s.

K505.5-E544-K757.5-J1376.6 : 1493m/s.

K843-E892.4-K1108-J1742.4 : 1228m/s.

K1186-E1235.1-K1452.2-J2038 : 1256m/s.

and so on. These windows are more forgiving than the KEKKJ routes, but you still should have your Eve periapsis be within about 12 hours of the time given.

I do not think it is possible to do these multiple flybys without a node editor. You have to adjust a, say, 1300ms burn by +/- 0.1 m/s and +/- 1 second until you get the desired flyby time and I go crazy almost immediately when trying that by dragging the little node arrows. Red Iron Crown is way better than me at flying by hand, and it sounds like he couldn't do it either.

I found these flybys by using my Flyby Finder for the KEKJ routes and my Lambert spreadsheet (available in the same post with FF) for the KEKKJ routes. Another user here (winged, look a couple of posts up) came up with a way to find KEKKJ routes with just Flyby Finder, though sometimes that might give a route that has to do too low a 2nd flyby of Kerbin.

Note all my dates are given in Earth time (24h/365d) and not Kerbin time (6h/426d). I've been too lazy to update FF to do both time formats, arrgh.

[falloutaddict]

Whatever you do, don't accidentally come in the wrong way and be orbiting Jool opposite of all the moons' orbits. I made that mistake once...

[Red Iron Crown]

I do not think it is possible to do these multiple flybys without a node editor. You have to adjust a, say, 1300ms burn by +/- 0.1 m/s and +/- 1 second until you get the desired flyby time and I go crazy almost immediately when trying that by dragging the little node arrows. Red Iron Crown is way better than me at flying by hand, and it sounds like he couldn't do it either.

I'm not that good, I couldn't make it work using PreciseNode either. Though I was unaware of your Flyby Finder tool, that looks fantastic. Downloading and adding to my toolbox, rep for you!