Deorbiting and landing on specific islands on Laythe

[Lyra]

I'm flying a Jool 5 mission with a big old SSTO and just circularized at Laythe, but have yet to perform the landing. I know how to control roughly where I land on Kerbin after deorbiting, but Laythe is a very different beast and I have trouble predicting where I'm going to land. I have some experience with Laythe landings but almost every time my craft would skip off the atmosphere and mess up my trajectory.

My craft is in a 60x60 km low orbit and my target is an equatorial island at about 109º longitude. I have plenty of fuel left to run the rapiers in air breathing mode but would prefer to land as close as possible to the island, ideally just east of it. Any tips?

[Hotel26]

[Caveat: I am no expert on Laythe as I have never deorbited there. ^[1]]

In a similar situation, I would run this craft in orbit above Laythe in my 'Lab'^[2] world and devise and tune a 'clockwork' landing.  You simply deorbit one at a known longitude and see where it comes down.

Then cheat & repeat, adjusting the deorbit location until you can 'follow the numbers', which is a 'clockwork' landing.

In addition, I often use an approach in which the deorbit simply lowers the PE above a certain location to a certain altitude.  For example on Kerbin, targeting a landing at KSC, 50km above 110W (retro performed opposite at 70E) because 50km is the lowest altitude one wants to fire the final retro (PE:-280 km) and then revert to prograde direction for flight.

If I were approaching the skip problem, I'd certainly look at descending into the upper atmosphere for controlled aero-braking being tuned with a little thrust to allow the suborbit to decay, again: "by the numbers".

You find out what that whole profile looks like in your Lab world view repeated experiments; then document it with the craft so that you can play it from the can at any time thereafter.

[1] Yes, folks: Laythe is still in my future...
[2] I run 'Lab' for development, test and experimentation and 'Orbit' for 'production'.

Edited October 14, 2023 by Hotel26

[miklkit]

Laythe is similar to Duna in my experience in that it has less atmosphere to slow you down so you have to start a lot earlier than one is used to on Kerbin.   As for skipping, when they start to do that I rotate it to a nose down position to get it into thicker air, then level off. 

[king of nowhere]

no real way to determine where you have to place periapsis, depends a lot on the aerodinamic profile of your craft. if it's a plane, also on how you fly it. so you just have to go by trial and error. if you are running a no save game, you have to bring extra fuel to compensate for off target landings

[Lyra]

Sorta figured it out - 45km periapsis and deorbiting about 270º behind the target seems to work fairly well. In general you just have to anticipate atmospheric skipping and account for it when planning a deorbit burn. Laythe's atmosphere is weird compared to Kerbin's lol

[king of nowhere]

16 minutes ago, Lyra said:

Sorta figured it out - 45km periapsis and deorbiting about 270º behind the target seems to work fairly well. In general you just have to anticipate atmospheric skipping and account for it when planning a deorbit burn. Laythe's atmosphere is weird compared to Kerbin's lol

atmospheric skipping makes the prediction harder; the lander bounces on the atmosphere, goes upward again, and could turn around the whole planet. with a lower periapsis and the lander reentering hard it is easier to predict the landing spot - but it's also more expensive.

[Hotel26]

2 hours ago, Lyra said:

45km periapsis and deorbiting about 270º behind the target seems to work fairly well. In general you just have to anticipate atmospheric skipping and account for it when planning a deorbit burn.

well that sounds like "clockwork" to me...

Well done, Aviator!

[Lyra]

6 hours ago, king of nowhere said:

atmospheric skipping makes the prediction harder; the lander bounces on the atmosphere, goes upward again, and could turn around the whole planet. with a lower periapsis and the lander reentering hard it is easier to predict the landing spot - but it's also more expensive.

I did a few test runs with the craft to figure it out. It's got plenty of fuel left and I'm very familiar with flying it, so that helped with predictions. I've noticed that with a consistent periapsis and angle of attack, you can roughly predict where the skip will occur (usually quite high in the atmosphere before heating effects even show up) and account for it when planning the burn.

I think after finishing this mission and a few more test flights with different craft I'll try to write a basic guide to Laythe entry, to save others the pain of finding out the hard way