Simulating a direct re-entry to Kerbin from Duna?

[Jestersage]

If I am using the in-built hyperedit, what's the best way to simulate a direct-reentry to Kerbin from Duna? Somewhere around Eve?

[Snark]

44 minutes ago, Jestersage said:

If I am using the in-built hyperedit, what's the best way to simulate a direct-reentry to Kerbin from Duna? Somewhere around Eve?

Not sure what you mean.  I'm having trouble understanding what your actual question is.

What do you mean by "direct-reentry"?  As opposed to... what?

And why would you do anything involving Eve if you wanted to simulate something involving Duna and Kerbin?

I mean... if you want to simulate returning to Kerbin from Duna, why not just use the game's built-in "set orbit" function to put your ship in orbit around Duna, and then, well, just do the burn, timewarp, then do the Kerbin reentry and see what happens?  Takes just a couple of minutes, right?

Are you looking to do something else?  If so, need a little more description about what you're trying to achieve.

[AeroGav]

I don't think the heating is that severe,  i managed to aerocapture kerbin in my mk1 inline cockpit spaceplane from Duna.

To simulate it accurately, why not do the real thing?

https://alexmoon.github.io/ksp/

Warp to year 1, day 155 (kerbal alarm clock is good for this)

Then use f12 menu to put you in orbit of duna.  Burn prograde to eject from the planet at daytime, and you're guaranteed to intercept kerbin.   Wait for the fireworks 

[Zeiss Ikon]

The only difference that matters between one reentry and the next is the velocity you need to get rid of.  If you make a swing out past Mun, let your vessel fall all the way in, and then make a prograde burn (depressed vector to prevent raising periapsis) you can add velocity and test in the same gravity and atmosphere you'll be encountering in the actual mission.  You can even do this without hyperediting, though that will save time building launchers and waiting for orbits to go up as well as down.

What you need to know is mostly how deep in to the atmosphere you need to dive at a given encounter velocity to avoid overheating stuff, squashing your Kerbals flat from the G forces, and so forth.  I found out today that my usual choice from LKO and orbits up to about 400 km apoKerb, 37 km, apparently lets a vessel overheat if applied on a Munar return with 3000+ m/s entry velocity.  The vessel wasn't destroyed, but it melted off one RCS block, which I take as a warning.  Now I'm going to have to find out if shallower is better (scrub off speed in the upper atmosphere, and risk skipping out to make another, shorter orbit and do it again) or deeper (higher G forces, but dipping down into the thicker air will slow the vessel faster and reduce the heating -- I think).  You'll be going faster coming back from Duna than a normal return from Mun (though I accidentally set up a retrograde reentry, which added about 600 m/s compared to prograde due to Kerbin's rotation -- in theory, that's more than the dV difference for Duna vs. Mun).

 

[Jestersage]

25 minutes ago, Snark said:

What do you mean by "direct-reentry"?  As opposed to... what?

22 minutes ago, AeroGav said:

I don't think the heating is that severe,  i managed to aerocapture kerbin in my mk1 inline cockpit spaceplane from Duna.

And that's the thing (and the main reason why the NASA want to go with Orion capsules): A direct return. That means just a straight direct rendezvous from Mars, no Orbiting, no aerocapture -- Plunge direct right into Earth (or in this case, Kerbin) and Safely land the astronauts.

Of course, I could do a new deciated save to test it out... I just figure a plunge directly into Eve from a certain height would do the trick too.

 

 

Edited January 30, 2017 by Jestersage

[Snark]

45 minutes ago, Jestersage said:

That means just a straight direct rendezvous from Mars, no Orbiting, no aerocapture -- Plunge direct right into Earth (or in this case, Kerbin) and Safely land the astronauts.

Yes, that's what's puzzling me.  Does anyone ever do anything else?  I mean, if you want to go home to Kerbin, why would you try to orbit Kerbin first?  That would just make things considerably harder (both from the standpoint of engineering, and from piloting).

Even IRL, has anyone ever done anything else?  i.e. returning something to Earth that wasn't already in LEO-- has anyone ever brought back something to Earth in a way that involved going to LEO first?  Doesn't everyone just "plunge right in"?

Aerocapture to orbit only makes sense if your actual goal is to end up in orbit.  If you're heading to the surface anyway, it makes sense to just plow right in; there's not really any point to aerocapture first, as far as I can tell.

That's why the term "direct reentry" was confusing me.  To me, that's just "reentry". 

[FancyMouse]

I usually hyperedit the reentry vehicle for testing to a high orbit (several Mm), put on infinite fuel, burn until I reach a desired periapsis with KER showing a desired periapsis speed. The desired periapsis speed can be computed from data in TWP where it shows injection delta V into Kerbin. This delta V plus LKO speed (and plus a hundred or so as safety margin) is my desired periapsis speed for reentry testing. Once it can survive that, that reentry vehicle is ready to go.

You could very well just try a periapsis speed of something like 3500m/s for Duna in most scenarios, but for me I have different requirements (e.g. I may want to come back in non-optimal trajectory in order to arrive early), so even for the same planet I may need different delta V and I need to consult TWP for the exact value I want for each scenario.

[Jestersage]

40 minutes ago, Snark said:

Yes, that's what's puzzling me.  Does anyone ever do anything else?  I mean, if you want to go home to Kerbin, why would you try to orbit Kerbin first?  That would just make things considerably harder (both from the standpoint of engineering, and from piloting).

Even IRL, has anyone ever done anything else?  i.e. returning something to Earth that wasn't already in LEO-- has anyone ever brought back something to Earth in a way that involved going to LEO first?  Doesn't everyone just "plunge right in"?

Aerocapture to orbit only makes sense if your actual goal is to end up in orbit.  If you're heading to the surface anyway, it makes sense to just plow right in; there's not really any point to aerocapture first, as far as I can tell.

That's why the term "direct reentry" was confusing me.  To me, that's just "reentry". 

Based on the numerous posting (including the one by AeroGav here), it seems most if not every player in KSP  (but especially the SSTO crowd) do at least an aero-capture first, and by its nature will always result in some kind of eclipse orbit (otherwise you will be shooting back to a solar orbit). 

And as you point out, IRL, none of the re-entry plans, be it from Mars or Venus flyby involve an aero-brake before re-entry.

Edited January 30, 2017 by Jestersage

[bewing]

The SSTO crowd don't have any choice, because you can't put heatshields on a SSTO.

If you're not playing an SSTO, and you have heatshields, then it just comes down to an engineering question of how close can you come to the heatshields temp limits with a comfy safety margin. If that limit allows you to reenter directly, then you're good to go. If the heatshield would blow on a direct reentry, then you need to be a little tamer and do it in 2 stages -- an aerocapture followed by a reentry.

But yeah, you can SetOrbit your ship into high Kerbin orbit with a few vectors attached as an extra stage. Turn on infinite fuel, do a deorbit burn, turn basically prograde, burn until you are at whatever speed and Pe you want to test, decouple the vectors, and see how the ship performs when it hits the atmosphere.

[dci]

I tested Jool direct reentry in similar way as NASA tested Orion reentry in EFT-1 misison. Just get into highly eccentric Kerbin orbit (Ap around Mun orbit, Pe at 70km) and when my vessel passed apoapsis and went back to Kerbin's gravity well, I set and performed maneuver node to speed up the vessel towards Kerbin with periapsis about 20km. In map view I look at predicted orbit around Kerbol - it must (at least) reach Jool orbit. But, surely, predicted orbit went never into real, as I aerobrake. Of course, you'll need right argument of periapsis of your eccentric orbit to correctly determine your entry speed.