delta v requirements for Laythe

[chemieglennie]

I've been trying to design a manned land and return mission to Laythe.

I think I need 10,000 dv in my lander to get from Laythe surface back to Kerbin. Plus another 7,000 dv to get from Kerbin orbit to Laythe. I calculated this from the delta v chart I've seen on this forum. Is this correct?

The chart doesn't account for aerobraking. How much can I save by aerobraking at Jool and Laythe?

Also what are the optimal heights for aerobraking?

I want to get my figures sorted before I redesign my lander.

Thanks

[Fractal_UK]

Getting from Kerbin to Laythe can be done fairly cheaply, I think 3,000dv should be more than adequate assuming you go for the optimal transfer window, are careful with your trajectory while en route to Jool and you aerobrake. You have two options for the aerobrake too, 1) you can aerobrake at Jool to bring your apoapsis in line with Laythe orbit (this is usually done with a periapsis around 116-119km) then set up a Laythe injection and aerobrake again at Laythe (usually about 22km) or 2) Head in to Laythe directly from an interplanetary trajectory and aerobrake there (only tried this once but I think the aerobrake was around 21km).

Obviously 2 is more convenient but it relies on getting that Laythe encounter conveniently, plus Laythe's sphere of influence is much smaller so you have less time (and thus it costs more dV) to set up your periapsis for aerobraking.

[SunJumper]

This depends on how long you are willing to wait, but, my d/v aims for such a mission:

KSC - LKO: 4.5km/s

LKO - HSO: 0.9km/s

HSO - Jool: 3km/s (can do the above as one burn to save 1.4km/s, requires better timing)

Correction: 0.2km/s

Capture: 0km/s (aerocapture). Aim for an apoapsis at Vall, not Laythe, this will make the process somewhat easier.

Periapsis raise: 0.1km/s

Correction: 0.3km/s

Laythe Capture: 0km/s (aerocapture)

Periapsis raise: 0.1km/s

De-orbit: 0.1km/s (must deorbit to land on, well, land)

Braking: 0.0km/s (use liberal parachutes and repack)

Re-orbit: 3km/s

Laythe escape: 0.6km/s

Jool escape: 1.4km/s (can reduce with gravity assists, takes much longer)

Burn to lower periapsis (while still at Jool): 1km/s (at most) (can save some d/v by doing all of the above as one burn)

Burn to "match" orbital period with Kerbin (You want to hit Kerbin at your next periapsis): 1km/s (Can be zero if you are in the perfect spot)

Correction: 0.2km/s

Kerbin landing: 0km/s (More if you specifically want to land on land, even more if you want to land at KSC)

Periapsis corrections are negligible.

D/v to Laythe: 9.2km/s

D/v to Kerbin: 6.6km/s

Total: 15.8km/s (50% requires high TWR, 1.5+).

So, although the d/v to Laythe is much bigger, the d/v to Kerbin is much smaller than you expected. By combining liberal flybys with the other d/v saving methods, I can imagine savings of at least 4km/s, bringing the total to about 11.8km/s, and the high TWR percentage to 65%.

[DMagic]

I think the 7km/s to Laythe was from LKO, not the surface. But still, that is way more than you need. If you get your timing right for the burn from LKO to Jool it should take no more than 2km/s, and then another 500 m/s or so for orbital corrections, and lining up aerobraking right.

[capi3101]

Let's play it conservative:

Kerbin - Jool Intercept: 1915

Jool Orbit: 2630

Jool - Laythe Intercept: 1600

Laythe Orbit: 780

Landing/Launch: 2800

Laythe Orbit: 780

Laythe-Jool Intercept: 1600

Jool Orbit: 2630

Jool-Kerbin Intercept: 1915

TOTAL DELTA-V: 16,650 m/s

There's a couple of fudge factors in there - you probably don't need to go back to Jool after you've lifted off of Laythe; that's 4230 you could apply for other purposes or ditch. You should take SunJumper's advice on the chutes; otherwise the launch/landing is going to take more than 2800 to pull off.

Shoot for 120 km to aerobrake at Jool (that'll put your apoapsis close to Laythe's orbit) and 50k for Kerbin. I assume you could aerobrake with Laythe itself but I don't know the altitude off the top of my head.

Caveat: I've never been to Laythe (as is obvious from my ribbons); take my advice with more than grain of salt. Good luck.

[SunJumper]

I think the 7km/s to Laythe was from LKO, not the surface. But still, that is way more than you need. If you get your timing right for the burn from LKO to Jool it should take no more than 2km/s, and then another 500 m/s or so for orbital corrections, and lining up aerobraking right.

Which I wrote in my posts. Honestly, a brachiostrone trajectory would be cool.

[chemieglennie]

Thanks for all the quick replies. Looks like I can get away with a smaller lander which should make things easier all the way down my ship.

I'll update my progress when I get chance. Will probably come down to a few good manoeuvres. Quick save/load should let me experiment a bit.

[RoboRay]

Which I wrote in my posts. Honestly, a brachiostrone trajectory would be cool.

KSP really doesn't support those, with the 4x physics limit during thrust. Even if you had enough propellent to sustain one, you'd get tired of watching the long, slow burn for days/weeks.

[bsalis]

Correction: 0.2km/s

Capture: 0km/s (aerocapture). Aim for an apoapsis at Vall, not Laythe, this will make the process somewhat easier.

Periapsis raise: 0.1km/s

Correction: 0.3km/s

Laythe Capture: 0km/s (aerocapture)

Why not capture directly into Laythe?

[3_bit]

I'm planning on using no more than 2.1 k/s dV by using a tiny ion engine (lower thrust than RCS) to change my heading so I can aerobrake at laythe. If all goes wrong, I'll be on course for an ocean. BTW, the 2.1 k should be a bit more than enough to get from LKO to laythe I believe. Probably less if I were to use a slingshot.

[DMagic]

I think it would be really hard to go directly to a Laythe capture, I've had chance encounters with Laythe after enter Jool's SOI, but planning that ahead would be tough. And really it takes almost nothing to aerobrake at Jool first and then wait for a Laythe encounter (assuming you get a good orbit on the first attempt). All it takes to prevent another dip into Jool's atmosphere is maybe 5 or 10 m/s. And, at least for me, it never takes long to get an encounter with Laythe after that, usually 3 or 4 orbits at most. Depending on how close your subsequent Laythe encounter is, you may need another 100 or 200 m/s to get into it's atmosphere, but that's not too bad.

That said, planning ahead to get a Laythe encounter before getting to Jool would be really awesome.

[Fractal_UK]

I agree. The other problem with a direct Laythe aerobrake capture compared to a Jool aerobrake capture is that it's harder to set up your aerobraking a long way in advance. If you arrive in the Jool system many days out then you can correct your orbit for aerobraking very cheaply but when you arrive in Laythe SOI, you're only a few hours or minutes away from the planet so if you have to alter your trajectory substantially the delta-v required can be reasonably high.

Aerobraking at Jool first means you approach Laythe at lower speed and can adjust your orbit for a second round of aerobraking much more cheaply. You can plan ahead to minimise some of these effects but it's very fiddly and not worth the effort, in my opinion.

[magnemoe]

I think it would be really hard to go directly to a Laythe capture, I've had chance encounters with Laythe after enter Jool's SOI, but planning that ahead would be tough. And really it takes almost nothing to aerobrake at Jool first and then wait for a Laythe encounter (assuming you get a good orbit on the first attempt). All it takes to prevent another dip into Jool's atmosphere is maybe 5 or 10 m/s. And, at least for me, it never takes long to get an encounter with Laythe after that, usually 3 or 4 orbits at most. Depending on how close your subsequent Laythe encounter is, you may need another 100 or 200 m/s to get into it's atmosphere, but that's not too bad.

That said, planning ahead to get a Laythe encounter before getting to Jool would be really awesome.

You can either hit it on the way inn, would start plan outside Bob orbit. or you can slingshot around Jool, One warning an Laythe direct aerobrake will be an high g braking and could damage an fragile craft.

You might also not be able to do an complete brake. Last time I did this I had to use 500m/s extra as mechjeb landing autopilot did not give me an orbit either pass or land, but I lost 6000 m/s.

Mechjeb 2 might be more accurate here.

[bsalis]

I think it would be really hard to go directly to a Laythe capture -snip-

I seem to keep going over this like a broken record, but it's dead easy to get a Laythe encounter, with only tens of dV, if that.

As soon as you enter the Jool SOI, end time warp. Make small Normal or AntiNormal burns to make your trajectory intersect with Laythe orbit. Now (with our without checking with a maneuver node) make a slow pro or retrograde burn and wait for an intercept. Since you are so far away from Jool, only a few dV makes a big difference to your arrival time. I have not bothered with Jool aerocapture since like the first .18

Note that there are fine tolerances to direct capture to Laythe. I set my Pe to 20Km, then use the the MJ calculator to work out the Ap once in the SOI. To play safe you could probably try 21-22 Km and with more braking later as needed.

[Jesrad]

I second (third actually ?) that: doing the capture directly at Laythe is easier and cheaper. Here's how I do it:

- adjust your Kerbin-Jool injection to aim for a 20000 km Pe at Jool, preferably going behind the giant planet,

- correct the normal slowly and carefully so that your ejection trajectory (dashed line) is in the same plane as Jool's ecliptic: this will guarantee you run into Laythe,

- adjust your Pe some more until you get the intercept.

When you later get into Jool's SoI, adjust your Laythe Pe for very cheap. One problem that may arise, is that other Jool Moons get in the way, but that's never been too big a problem so far. If you really need to adjust the Pe from within Laythe's SoI, keep in mind that RCS is more than enough to do that.

When going prograde along with the blue moon, you need about 20 km aerocapture target, but when going head-first into it like I did, you'll need to very finely tune that around 11 or 12 km (11650 m precisely in my case) because you'll be plowing through its atmopshere at speeds around 8 km/s and that vastly amplifies trajectory differences. Doing it right you will have an exit Ap of ~100 km and can circularize with mere fumes.

Edited May 16, 2013 by Jesrad

[chemieglennie]

Finally managed to get Laythe and, more importantly, back again.

Did a Imgur mission log here,

http://imgur.com/a/rhEUS

Built a lander with 10,000dv for the return journey from Laythe surface. Plus IP stage with 6,000dv. Managed it with some serious aerobraking

[bsalis]

Well done!

I liked the use of asparagus when ascending from Laythe.

[chemieglennie]

Thanks. Other moons I've visited I just used a nuclear engine for max dv. Laythe gravity is quite high though so you need some extra thrust for liftoff.

Plus the wide arrangement let me land on water. My first lander was tall and narrow and fell over.