How to Laythe?

[Lucius Sejanus]

I've recently mastered interplanetary navigation enough to able to do a manned Duna return. Having done that, I set my eye on a loftier goal, a manned Laythe return. Now, I like to meticulously plan everything in advance, but in this case, don't have all the info. And what better way to fix that than by starting a generic-named Howto thread, and dump a bunch of vaguely-related questions into it?

First question concerns those delta-v maps I see floating around. As I read them, I need a vessel in LKO to have circa 4000 m/s delta-v for the return trip, assuming aerobraking on Jool capture with the resultant apoapsis around Laythe's orbit. But, I am unsure (How much to kick that periapsis out of Jool atmosphere? How costly is the Laythe capture?). What are the rough requirements, assuming liberal use of aerobraking?

Secondly, my Duna mission vessel had abysmal TWR, which resulted in rather long burns. Not long enough to jeopardize the mission, but long enough to annoy me. How low do people commonly go, before low TWR becomes a serious problem?

And lastly, it looks to me that the best way to go about designing a Laythe lander is to build a hybrid jet and rocket powered VTOL SSTO. Which raises a whole host of questions, since I can test my designs on Kerbin, not Laythe, and I don't want to overbuild the lander to make it Kerbin-ascent capable. My current design has around 1500 m/s delta-v as reported by MechJeb and Kerbal Engineer, which I think accounts only for the rocket-powered ascent phase. Is there a good way to find out the delta-v of the jet phase? I guess I could build a purely rocket-powered ship with 2800 m/s delta-v, and try and match its ascent profile with my hybrid lander, but that wouldn't be entirely accurate, since I imagine jets perform slightly differently on Laythe. Or do they?

Anyway, thanks in advance for any insights, I'm sure as I keep designing I'll stumble into more questions.

[Jesrad]

Because of its atmosphere, Laythe is indeed the ideal destination for spaceplanes. The only difference to jets is that it's slightly denser so you get as much thrust as if you were higher in altitude - but it's fine since gravity is also less.

Note that if you aim directly at Laythe for aerocapture instead of stopping at Jool then intercepting Laythe, you can save yet some more fuel. You can basically do Kerbin orbit <-> Laythe orbit on just 2000 m/s of deltaV. It works both ways.

IIRC someone was able to make a round trip to Laythe with a single-stage spaceplane, using just jet engines and ion engines (and lots of solar panels). Low TWR too alas... The lowest TWR I allowed myself on a Laythe mission was 0.16 and I had to thrust for 36 minutes (24 minutes ingame, but there was some dilation because of the size and complexity of the mothership). It's fine as long as you only have to do it once.

Edited May 17, 2013 by Jesrad

[Fractal_UK]

I would definitely opt for a VTVL jet powered SSTO for Laythe landing and return to orbit rather a spaceplane. I have landed planes on Laythe but in general the terrain is poorly suited to extended landings so your plane needs to be very durable and capable of landing very very slowly. Jets do function a bit differently on Laythe, I think you get a little less intake air. My advice though would just be to build a VTVL jet rocket that can achieve Kerbin orbit, that way you have a more versatile craft than something that can only achieve orbit on Laythe. I find a lot about this game is reducing the challenges you face to problems you have previously solved, so while you may not need a small Kerbin ascent capable SSTO at the moment, you might find you'll need it later and then you won't have to do a complete redesign.

As for long burns to get to Laythe, I wouldn't worry about it. My typical burns to get there are between 15 minutes and an hour, time accelerate and read a book or something, it's more efficient that way. I would definitely suggest doing the initial burn to Laythe as a series of periapsis kicks and possibly, if the alignment suits, a munar gravity assist as well - every little bit of fuel saved is a bonus. On your way back, a Tylo gravity assist might be a good way to leave the Joolian system but I've not actually attempted a return flight from Laythe yet. My Kerbals go there and they colonise.

You can estimate the delta-v of your jets with the rocket equation. Turbojet ISP is 800s, so exhaust velocity is effectively 8000ms^-1. If you want, say, 2000 delta-v from your jets you need 31% of the starting mass of your rocket to be liquid fuel (note this is approximate because jet ISP changes with altitude).

[Rage097]

(Warning: Non-helpful joke reply)

This is how you Laythe ^^^

[tomf]

If you go straight to aerobraking on Laythe from you transfer orbit that bit will be almost free. For the return to Kerbin a simple plan involving leaving Laythe 70km orbit for a Joolian orbit and then burning again to go home will cost about 2200m/s. If you feel you can pull it of though you can get all the way to Kerbin with a single burn from a 70km Laythe orbit for an amazing 1062m/s. That Oberth effect is quite significant!

Attempting to explain this to myself - a 70km Laythe orbit whips around pretty quickly at 200m/s. When you add another 1000m/s you leave Layth'e soi so quickly that it's gravity hardly has time to slow you down at all, you leave Laythe at 1200m/s + laythe's speed of 3223 which is enough to escape jool and take you home.

Edited May 17, 2013 by tomf

[numerobis]

It is indeed about 2km/s to Laythe from LKO, and just over 1km/s for the return. Aero capture directly to Laythe at an altitude somewhere between 20 and 25 km. You will need either MechJeb or lots of quicksaving and reloading to pull that off.

MechJeb 2 will make running the jets easy; otherwise you need to make yourself a table to know what throttle you can maintain at each altitude.

I built an single-stage spaceplane with return capacity that weighed in about 45t for a big payload; it was centered on an LV-N. It took off at 60 m/s, and landed at less than 20 m/s since it had burned so much fuel. That allowed me to land on an uphill. There is a nice landing spot at the antimeridian, just north of the equator -- lots of flat ground. Other than that, Laythe is a steep place.

[bsalis]

First question concerns those delta-v maps I see floating around. As I read them, I need a vessel in LKO to have circa 4000 m/s delta-v for the return trip, assuming aerobraking on Jool capture with the resultant apoapsis around Laythe's orbit. But, I am unsure (How much to kick that periapsis out of Jool atmosphere? How costly is the Laythe capture?). What are the rough requirements, assuming liberal use of aerobraking?

It's about 2 km/s to get there --> http://www.skyrender.net/lp/ksp/system_map.png

Plane changes can be expensive since the SOI is small, so cheap very high Ap changes are not available.

Aerobraking means getting into low circular orbit is cheap, so is landing.

Back to orbit is 2.8 km/s, with transfer to Kerbin a bit over 1 km/s as pointed out by numerobis.

You can capture directly into Laythe, i'm not going to bother typing that yet again --> http://forum.kerbalspaceprogram.com/showthread.php/29169-delta-v-requirements-for-Laythe?p=362095&viewfull=1#post362095

Secondly, my Duna mission vessel had abysmal TWR

You will want some decent TWR for Laythe, but not as much as needed for Kerbin.

And lastly, it looks to me that the best way to go about designing a Laythe lander is to build a hybrid jet and rocket powered VTOL SSTO.

Oh yes, yes yes yes. Any SSTO Spaceplane that works well on Kerbin will work even better on Laythe. VTOL SSTO is better, since you can handle the rough terrain better, or splashdown if needed. Problem is they are not trivial to make (however, i have an example of one in my sig if you want to d/l it).

Anyhow, have fun on Lathe. I do!