Kerbin aerobraking at 5 Km/s?

[NeverEnoughFuel!!]

So, I have a mother ship in Duna orbit waiting for the transfer window... almost year and a half of waiting... pilot is frozen and supplies are not a question... since I'm inpatient and have about 3900 delta V last night I tried to go earlier... I can get a rendezvous with Kerbin but the problem is I'm entering Kerbin SOI with 5 Km/s and with about 1500 delta V fuel left... no heat shields... a maneuver node at Kerbin 80 Km (out of atmosphere) Pe asks for about 3750 delta V for circular orbit... would assisted aerobraking be possible, to get a capture orbit... roundness of the orbit is not important...  BTW I have air brakes on the ship and the thing itself weights about 22 tonnes it has (retractable) radiators for interstellar reactor and the thermal rocket engine...

Edited October 1, 2016 by NeverEnoughFuel!!

[ForScience6686]

If give it a shot, but not too deep into the atmosphere.  That's a lot of speed which means a lot of heat.  But you should have enough dv to get into an orbit at least.  Which at that point you could just make multiple passes to slow down further.

[Aegolius13]

Not sure, but only way to know is try.  You could always make a copy of your save and skip forward. Aerocapture is a lot harder than regular aerobraking since you have to do it all in one pass.  But if you can engine brake into an elliptical orbit, then you can take it nice and slow with multiple passes.

You could also try getting gravity assists from the Mun and Ike to save some fuel for breaking.  But it's not gonna be a huge difference.

[Foxster]

Mun gravity braking?

[klesh]

36 minutes ago, Foxster said:

Mun gravity braking?

Bingo.  

[NeverEnoughFuel!!]

So basically if it survives up to let's say 40 Km altitude and with hard retro burn maybe I'll make an elliptical orbit... exactly the answer I was afraid of... up to recently  I was playing 1.0.4 and got really good "feeling" for that version of atmosphere... this 1.1.3 scares the s**t out of me... the roasting  starts higher... about 52 km (sometimes even 58) but doesn't really slows you down all the way to 30... only burns you to ash and heat shields are melting like butter ...  about the Mun... during the setup of maneuver nodes on few occasions ship passed right beside it... at 5 km/s the orbit didn't even flinch let alone bend a bit... like the Mun wasn't even there... It would be nice to have a mod who can predict atmosphere influence on the orbit... not like this "let's see what happens" with our 750 000 ship and 1500 points... maybe there is such a mod? I know about trajectories mod, and I have it, but in this case the white "predicted" orbits do not appear... maybe because its a third node long time in the future... they didn't appear on Duna also... only when I hit the atmosphere, only then... but that aerocapture was ridiculous compared this one...    

[foamyesque]

You don't need to hit a circular orbit -- once you get into an elliptical one, repeated aerobraking passes will eventually circularize it for you. Can you hit an elliptical orbit with an in-atmo periapsis on your fuel? Apoapsis doesn't matter.

[NeverEnoughFuel!!]

1 minute ago, foamyesque said:

Can you hit an elliptical orbit with an in-atmo periapsis on your fuel? Apoapsis doesn't matter.

Exactly what I would like to know also ... I was hoping for some math whiz kid to calculate it on a whim depending on more data about the ship... weight, speed, TWR, atmosphere density, periapsis atitude, etc. etc. ... how would that formula look like... I'm not a mathematician in fact I'm an anti talent for it... 

[Snark]

I would say that you have basically zero chance.  Would love to be wrong. 

The problem is the lack of heat shields.  As you start to dive deeper into atmosphere, you get scads of heat first, and actually-enough-aerobraking-to-make-much-difference second.  Let's run the numbers:

• You're arriving at 80 km with 3750 m/s of excess speed, relative to a circular orbit.
• You have 1500 m/s of dV, so that knocks it down to 2250 m/s.
• You don't actually need to get down to circular orbit.  Any elliptical orbit will do, as long as it's captured to Kerbin.  So you can be going faster than circular-at-80-km speed.  IIRC, the dV needed to escape Kerbin from an orbit like that is 950ish m/s.  I might be off by a few dozen m/s, but not by more than that.  So that takes it down to 1300 m/s.

So.  You need to kill, at the absolute minimum, 1300 m/s of dV by aerobraking, in a single pass.  When you're going well over 4000 m/s surface-relative, after using all your dV.

Without heat shields.

I gotta say, I really think that is not gonna happen.  You're gonna go kaboom long before you shed anywhere near enough speed, would be my guess.

Yes, you can try for a reverse gravity assist from the Mun... but the Mun is just not that big, and you're going past it so fast that you're not going to shave a huge amount off your velocity.  Not even close.

I would love to be wrong about this   ... but I gotta say that I think you're pretty much doomed, here.

If I had to make a suggestion for pulling it out of the hole:  Use a rescue ship.

1. Set up an encounter that has your Duna ship going screaming past Kerbin just above the top of the atmosphere.
2. You'll use your 1500 m/s of dV to slow down at Kerbin Pe just as much as you can (every little bit helps), even though it won't be enough.
3. Shortly before it arrives at Kerbin, launch a rescue ship.  The rescue ship is a stripped-down, low-mass, high-dV hot rod.  It's just enough crew capacity to hold your returning crew, and some xenon tanks and ion engines, and not much else.  It accelerates so that it can match velocities with the Duna ship as it goes screaming past.
4. Rendezvous, then transfer the crew across from the Duna ship to the rescuer, bringing with them any science you've collected.
5. Let the Duna ship go hurtling off into the black void.  Rescue ship brakes to a halt and then heads back to Kerbin.

That's about the only hope I see to pull it off.

In any case, whether I'm right or wrong, would love to hear how it turns out-- be sure to let us know!

[NeverEnoughFuel!!]

Gentleman, DO NOT try this kind of exhibition ... no way...  hit the atmosphere at 5750 m/s Pe altitude set at 33 km equatorial... I think even 40 would be too much... solar panels evaporated instantly, air brakes two seconds later, at full throttle whole thing lasted about two seconds more... it would be highly elliptical orbit beyond the Mun ... if it worked... waiting for the transfer window...

Edited October 1, 2016 by NeverEnoughFuel!!

[NeverEnoughFuel!!]

1 hour ago, Snark said:

 

The problem is the lack of heat shields.  As you start to dive deeper into atmosphere, you get scads of heat first, and actually-enough-aerobraking-to-make-much-difference second.  Let's run the numbers:

• You're arriving at 80 km with 3750 m/s of excess speed, relative to a circular orbit.
• You have 1500 m/s of dV, so that knocks it down to 2250 m/s.
• You don't actually need to get down to circular orbit.  Any elliptical orbit will do, as long as it's captured to Kerbin.  So you can be going faster than circular-at-80-km speed.  IIRC, the dV needed to escape Kerbin from an orbit like that is 950ish m/s.  I might be off by a few dozen m/s, but not by more than that.  So that takes it down to 1300 m/s.

So.  You need to kill, at the absolute minimum, 1300 m/s of dV by aerobraking, in a single pass.  When you're going well over 4000 m/s surface-relative, after using all your dV.

Without heat shields.

 

I had some murky calculation like that somewhere in the back of my mind, but I think I wasn't clear enough in my first post... sorry English is not my language... this was only an optional theoretical aerocapture, I still have a save with that ship still in Duna orbit waiting for  launch window... "sigh"... a loooong wait... last night i just tried to setup nodes long before launch window... and this question came up... 1500 delta V spared for assisted aerocapture at 5 km/s... well I tried it and it didn't work...  but anyway thank you for your time and math... I really had high doubt's this could work but but it turns out the number I was really missing in my approximations was 950ish m/s escape Kerbin... this ship is built for massive delta V and precise launch windows with (optional) some mild aerobraking on duna like atmosphere... not some tomfoolery in Kerbin's invisible wall... 

[Jovus]

I know you said you were impatient, but if you want to try some tricksy flying you could use an Eve gravity assist and then hit Kerbin afterward.

Don't know how much you'd save, because you'd have to burn for Eve, but done right you can put Kerbin at your aphelion instead of perihelion.

Of course, this takes time...`

[foamyesque]

3 hours ago, NeverEnoughFuel!! said:

Exactly what I would like to know also ... I was hoping for some math whiz kid to calculate it on a whim depending on more data about the ship... weight, speed, TWR, atmosphere density, periapsis atitude, etc. etc. ... how would that formula look like... I'm not a mathematician in fact I'm an anti talent for it... 

 

Well, you said you placed a manouver node at 80km, that's how you found out how much dV you'd need for a circular orbit. But you don't need a circular orbit to aerobrake. Shift your periapsis down to about 55km, and see how much the manouver planner says you need to close your orbit.

[NeverEnoughFuel!!]

Ahhh... I guess this all was a consequence of me getting used to warp drive... I got used to fly to Duna in 5 min... ... not all of these waiting for window transfers... year and a half waiting for window transfer... half a year there... one more year and a half for another window transfer... half a year back... with these new ratios of data/science conversion for mobile labs I had almost punched trough whole tech tree... yes it's a bit harder to feed (four of them) with enough data for conversion... but nevertheless only one interplanetary trip so far... I think it was second alignment with Duna when I got enough tech to go there in the classical way... but without gargantuan ships who need  assembling in orbit... something smaller, lighter and much more powerful ... but these are the gameplay questions, balances and philosophies... but I can bet that my Kerbal will feel stupid when he wakes up and realizes he had wasted years of life and the warp drive was invented while he was still on his way home... this game really good portraits the vastness of a solar system...  

[Aegolius13]

On 10/1/2016 at 11:30 AM, NeverEnoughFuel!! said:

Exactly what I would like to know also ... I was hoping for some math whiz kid to calculate it on a whim depending on more data about the ship... weight, speed, TWR, atmosphere density, periapsis atitude, etc. etc. ... how would that formula look like... I'm not a mathematician in fact I'm an anti talent for it... 

I don't think you can really math this without simulating pretty much the whole game.  Aerobraking performance depends greatly on the ballistic coefficient (is the ship long and skinny or short and fat?), mass distribution, available attitude control, heat tolerance of your parts, etc.

Only rarely would I expect to survive pure aerocapture in a ship not designed for it, except probably at Duna.  Since the atmosphere is so thin there, you can get low and spread the braking over a long time.

[StahnAileron]

On ‎10‎/‎1‎/‎2016 at 2:06 PM, NeverEnoughFuel!! said:

It would be nice to have a mod who can predict atmosphere influence on the orbit... not like this "let's see what happens" with our 750 000 ship and 1500 points... maybe there is such a mod? I know about trajectories mod, and I have it, but in this case the white "predicted" orbits do not appear... maybe because its a third node long time in the future... they didn't appear on Duna also... only when I hit the atmosphere, only then... but that aerocapture was ridiculous compared this one...    

I think you need to mess with the settings in both Trajectories and the KSP options config file. KSP limits patch conic predictions to 3 (per maneuver node) and Trajectories limits the aero influence calculations as well (I forget how many dips into the atmosphere it'll handle by default.) I played 1.0.5 mostly (until I heard about the 1.2pre, which I touch now and then) and never had issues with Trajectories displaying aerobraking effects. However, I manually edit KSP patch limit to 5 and Trajectories equivalent setting for its calculations to max (which I think is 10). I also don't usually have it working beyond a single maneuver node, so that may actually be the problem. Still, once you get into an atmo grazing intercept with Kerbin, the aero calcs from Trajectories should kick in. (Granted, the fastest I've done is about 3.3km/sec from the Mun or Minmus in 1.0.5 with RealHeat installed, so my experience may not help much here.)

Too bad you can't use a burn at a low Mun intercept PE to take advantage of the Oberth effect to get more efficient dV use out of your engine. I imagine any attempt like that would screw up your Kerbin intercept.

[Rath]

Mun gravity-Graze atmo-Repeat.  Might work.  Perhaps you could launch a dockable heatsheild?  10m one connected with a claw could even work.

[foamyesque]

Honestly you should be pretty close to being able to simply close your orbit with a burn. That's why I asked for the manouver node numbers; they'll give you an idea of how much, if any, you need to use the atmosphere to provide.

 

EDIT:
 

On re-read, you're entering the SoI with 5km/s, not hitting LKO with 5km/s. You're hooped.

Edited October 3, 2016 by foamyesque

[Plusck]

I think Snark has covered virtually all the options here. If it takes a 3750 m/s burn to circularise at 80km, that gives a V_inf of 5095 m/s.

Dropping Pe to 55km (which might be survivable), and plugging in that V_inf of 5095m/s gives a total burn to circularise of 3740m/s ... a saving of exactly 10 m/s. My calculations (which may be erroneous somewhere, but look right) tell me that to capture (Ap 84,000 km) at a Pe of 55km needs a burn (or braking pass) of 2790 m/s.

So yes, as @Snark said, deduct the available 1500 m/s from that and you're left with 1300 m/s (well, 1290m/s, that could be important  ). There is simply no way that you'll shed 1300 m/s going through the atmosphere at 55km in one pass. The Mun will hardly deflect the ship at those speeds, so a gravity assist is pointless and would probably cost more to set up than it gains in dv. 

Assuming that it's too late to try an Eve gravity assist, the only option is a rescue, either with a ≈10km/s crew carrier or a ≈10km/s plus additional fuel clawed/docked refueller (3.4km/s to orbit, 1km/s to SOI edge, 5.1km/s for a perfectly arranged RDV shortly after the ship arrrives in the SOI).

[Palaceviking]

On 01/10/2016 at 4:25 PM, ForScience6686 said:

If give it a shot, but not too deep into the atmosphere.  That's a lot of speed which means a lot of heat.  But you should have enough dv to get into an orbit at least.  Which at that point you could just make multiple passes to slow down further.

This 

[John FX]

I would suggest doing the smallest capture burn you can to remain in Kerbin SOI then at Ap reduce your Pe to just inside the atmosphere and do multiple gentle aerobrakes until your orbit is almost circular then raise your Pe to maintain a low orbit and send a rescue craft to get the kerbals back to KSC.

EDIT : reading the thread I see you have too much speed entering Kerbin SOI, maybe try a few kerbin gravity braking manoeuvrers to reduce your velocity before trying to capture?

Edited October 3, 2016 by John FX

[Karol van Kermin]

If none of these works, then the only thing I could recommend is:

Try to launch a vessel, which will rendezvous with the spacecraft and try to refuel it. But this is very expensive and it would cost you a lot of dV. What I would recommend is some kind of Mun/aero-breaking combined.

[Mjp1050]

On 10/1/2016 at 0:02 PM, Snark said:

Yes, you can try for a reverse gravity assist from the Mun... but the Mun is just not that big, and you're going past it so fast that you're not going to shave a huge amount off your velocity.  Not even close.

What if you not only used the Mun's gravity assist, but also burned at Mun periapsis to reduce velocity and adjust your orbit?

I mean, we know that burning from a Minmus orbit to interplanetary uses less delta-v than burning from LKO to interplanetary. Could we use this theorem in reverse?

[TheGuyNamedAlan]

On 02/10/2016 at 3:07 AM, Foxster said:

Mun gravity braking?

minmus or mud. double the price

 

[Snark]

10 hours ago, Mjp1050 said:

What if you not only used the Mun's gravity assist, but also burned at Mun periapsis to reduce velocity and adjust your orbit?

Not gonna help you much.

The OP's scenario requires shedding thousands of m/s of dV.  Just as a guess, I'd say that a Mun gravity assist at those speeds is going to give you dozens, not thousands, of m/s -- i.e. practically nothing.

Burning at Mun periapsis is basically an attempt to use Oberth effect to help you.  Sure, you can do that... but it's the wrong place.  The Mun is only a tiny fraction of Kerbin's mass.  By far the best use of Oberth effect there is to get the lowest Kerbin Ap you can get away with, and dump all of your ship's dV right at Kerbin periapsis.

It won't be enough, or even close, as discussed above.  But it's the best you can do.

10 hours ago, Mjp1050 said:

mean, we know that burning from a Minmus orbit to interplanetary uses less delta-v than burning from LKO to interplanetary. Could we use this theorem in reverse?

Nope.  Apples and oranges.

Burning to a low velocity interplanetary escape from Minmus orbit is less dV than burning to a low velocity interplanetary escape from LKO.

However... if you want to eject from Kerbin SOI with several thousand m/s of excess velocity, it's actually cheaper to burn from LKO than Minmus orbit, due to the Oberth benefit.

If you're coming in fast and what you want is to capture to a celestial body, then what you need to do is dive into the deepest gravity well possible, and dump all your dV right at the bottom of that well.  And Kerbin > Minmus.  By a lot.

(Plus Kerbin would allow aerobraking, if you had a heat shield, which the OP doesn't, so that's not really relevant here.)