Lunar free returns and reentry troubles (RSS)

[NFunky]

Hey all,

I'm trying to do a crewed lunar flyby in RSS, and I don't really have the tech yet to build a crewed lunar orbiter yet.  I was planning to use the science I got from the flyby to unlock better engines and allow a follow up orbital mission.  I was planning to do a nice free return trajectory, but I seem to be having serious problems plotting what I am after.

 

Ideally, I want a free return trajectory with the lowest possible perilune (for the science), that is less than 15 days from launch to reentry (life support), and especially a posigrade reentry.  I've learned the hard way that even a lunar rated heat shield is generally not enough protection for a retrograde lunar return reentry.  Delta-V is not really a barrier, as I have around 4 km/s in the transfer stage.

 

I keep either getting retrograde reentry trajectories, or trajectories that barely enter lunar SOI and take 13+ days to complete.  I guess I don't really understand how free returns actually work.  I am also curious about the Apollo 13 trajectory, and specifically how the "PC+2"  burn (that shortened flight time) worked.

 

Thanks!

[Streetwind]

11 hours ago, NFunky said:

I guess I don't really understand how free returns actually work.  I am also curious about the Apollo 13 trajectory, and specifically how the "PC+2"  burn (that shortened flight time) worked.

But did the Apollo missions have to work with the same constraints that you're working with, i.e. insufficient heat shields? I don't play Realism Overhaul myself, but this post is by far not the first I see along the lines of "my heatshield isn't saving my hide even though it seems like it should". It's one of the most common RO questions that I see, period.

So maybe there's actually something wrong there, and you should technically be able to reenter from your free return trajectory with a lunar rated heatshield regardless of direction.

As a stopgap measure, though: why does it need to be a set-and-forget trajectory? A small mid-flight course correction in the right place should easily let you swap reentry direction even if the initial trajectory didn't give you what you wanted.

[blakemw]

How a free return works is you start with a transfer orbit with an apogee higher than the moon's orbit (by maybe 20-30%) - this will get to the moon's orbit significantly faster than a hohmann transfer - then you do a gravity assist at the moon that lowers the orbit so that perigee intersects the Earth's atmosphere - having the right amount of surplus velocity when encountering the moon for the moon's gravity to bend is key, if you have too much surplus velocity it'll bend it into a retrograde orbit rather than simply lowering the orbit. Getting a low perilune is a matter of precise timing, aka tweaking the manoeuvre node for a while.

I can't comment on the heat shield thing except that in RSS/RO it's hard to make everything work as it should, I think I just ended up turning down reentry heating a bit in the difficulty settings.

Edited April 9, 2018 by blakemw

[Laie]

14 hours ago, NFunky said:

Ideally, I want a free return trajectory with the lowest possible perilune (for the science), that is less than 15 days from launch to reentry (life support), and especially a posigrade reentry.  I've learned the hard way that even a lunar rated heat shield is generally not enough protection for a retrograde lunar return reentry.  Delta-V is not really a barrier, as I have around 4 km/s in the transfer stage.

IIRC low lunar PE and free return are mutually exclusive. So the best suggestion I have for you is to try something different.

2 hours ago, Streetwind said:

It's one of the most common RO questions that I see, period.

If this even is an RO question... @NFunky only mentions RSS, after all.

So, standard disclaimer:

• RSS scales up the solar system, but stock parts are not really suited to that environment. It takes rather large rockets to get to space, and stock heatshields are taken to their limits if not beyond.

[Norcalplanner]

On 4/9/2018 at 5:28 AM, Laie said:

IIRC low lunar PE and free return are mutually exclusive. So the best suggestion I have for you is to try something different.

If this even is an RO question... @NFunky only mentions RSS, after all.

So, standard disclaimer:

• RSS scales up the solar system, but stock parts are not really suited to that environment. It takes rather large rockets to get to space, and stock heatshields are taken to their limits if not beyond.

RSS with SMURFF works well, though, because the mass of the capsule is reduced so much.

+1 on suggestions to make small corrections far away from Earth.

I'll also add two other suggestions - first, dump or burn any excess monoprop that you don't need to maintain orientation during reentry prior to entering the atmosphere to reduce the craft's mass. Second, burn every bit of remaining fuel in your service module or transfer stage prior to reentry to slow down. I've had good success with creating a node between 1000 and 3000 km altitude so that you can dial in the precise amount of radial out needed to maintain your desired Pe. I recall that the heating flux is a function of the cube of the velocity, so scrubbing off just a few hundred m/s is still a worthwhile exercise.

[Buisson]

Hello

Ok this thread is quite old, but for the people who are struggling getting a free return trajectory I cannot stress this enough :

On 4/9/2018 at 12:35 PM, blakemw said:

Getting a low perilune is a matter of precise timing, aka tweaking the manoeuvre node for a while.

[...] for a while [...]

I just passed the last hour to get it right. So there's always hope !