Periapsis height for interplanetary capture

[Kepler68]

So recently I've started going to other planets in ksp but I haven't been able to get a good periapsis for aerobreaking and always fly off into solar orbit or crash. I was hoping someone could provide a cheat sheet for a good place to put the periapsis for a capture. 

Ps: I know aerodynamics affect it so just give a range like between 10-20km

[herbal space program]

What Pe you should shoot for depends entirely on the body you're trying to get captured by and how fast you're coming at it. Moreover the aerodynamic model just got changed again in a manner that significantly affects upper-air drag, so I'm not sure even the hardcore folks have got that all worked out yet for every situation. In the last model, there were plenty of aerobraking scenarios that used to work pre-1.00 that became instantly deadly. Perhaps if you could describe your specific scenario a little more, somebody could give you a more helpful answer.

[Gaarst]

I'd say 20km should be fine. I use 25-30km when coming back from Minmus. You're going a bit faster when coming back from interplanetary so a little lower peri should to the job.

[OhioBob]

There are many factors to come into play.  Most importantly, which planet are you talking about?  Also your incoming speed and the aerodynamic characteristics of your vehicle make a big difference.  I performed a  bunch of tests on this a while back.  I used three different test vehicles, all with a 2.5m heat shield but with different masses.  The first had a mass of 6410 kg (or about 1306 kg/m² of heat shield), the second had a mass of 19,910 kg (4056 kg/m²), and the third had a mass of 37,910 (7723 kg/m²).  I also tested approaching at two different hyperbolic excess velocities, which represented the approximate minimum and maximum that would be expected for a interplanetary transfer between Kerbin and the destination planet.  For Eve I used hyperbolic excess velocities of 800 m/s and 2000 m/s, for Duna 740 m/s and 960 m/s., and for Jool 1600 m/s and 3000 m/s.

Generally speaking, If you are coming in slow with a low mass, you want a high periapsis.  If you are coming in fast with a high mass, you want a low periapsis.  And for each set of entry conditions, there is a range of periapsis altitudes that will work - from just barely capturing inside the planet's sphere of influence, to just barely avoiding getting swallowed up by the atmosphere and not being able to pull out again.  There are also other limiting factors, such as overheating to the point that the ship explodes, or losing aerodynamic control.  In one case at Duna I found that I couldn't set my periapsis low enough to aerocapture without crashing into the terrain.

In rough numbers I found that the minimum and maximum periapsis altitudes at Eve ranged from about 53-75 km, with a median of about 62 km.  Of course that doesn't mean that you can pick a number anywhere in that range.  If you have a lightweight ship you'll probably want to be somewhere around 66 km or so.  For a heavyweight ship you'll need to be closer to 56 km.  It is best to experiment to find the periapsis altitude that works best for your particular circumstances.

At Duna the periapsis altitudes ranged from about 6-22 km, with a median of about 13.5 km.  At Jool the altitudes ranged from about 144-171 km, with a median of about 155 km.
 

Edited May 15, 2016 by OhioBob

[Kepler68]

8 hours ago, OhioBob said:

There are many factors to come into play.  Most importantly, which planet are you talking about?  Also you're incoming speed and the aerodynamic characteristics of your vehicle make a big difference.  I performed a  bunch of tests on this a while back.  I used three different test vehicles, all with a 2.5m heat shield but with different masses.  The first had a mass of 6410 kg (or about 1306 kg/m² of heat shield), the second had a mas of 19,910 kg (4056 kg/m²), and the third had a mass of 37,910 (7723 kg/m²).  I also tested approaching at two different hyperbolic excess velocities, which represented the approximate minimum and maximum that would be expected for a interplanetary transfer between Kerbin and the destination planet.  For Eve I used hyperbolic excess velocities of 800 m/s and 2000 m/s, for Duna 740 m/s and 960 m/s., and for Jool 1600 m/s and 3000 m/s.

Generally speaking, If you are coming in slow with a low mass, you want a high periapsis.  If you are coming in fast with a high mass, you want a low periapsis.  And for each set of entry conditions, there is a range of periapsis altitudes that will work - from just barely capturing inside the planet's sphere of influence, to just barely avoiding getting swallowed up by the atmosphere and not being able to pull out again.  There are also other limiting factors, such as overheating to the point that the ship explodes, or losing aerodynamic control.  In one case at Duna I found that I couldn't set my periapsis low enough to aerocapture without crashing into the terrain.

In rough numbers I found that the minimum and maximum periapsis altitudes at Eve ranged from about 53-75 km, with a median of about 62 km.  Of course that doesn't mean that you can pick a number anywhere in that range.  If you have a lightweight ship you'll probably want to be somewhere around 66 km or so.  For a heavyweight ship you'll need to be closer to 56 km.  It is best to experiment to find the periapsis altitude that works best for your particular circumstances.

At Duna the periapsis altitudes ranged from about 6-22 km, with a median of about 13.5 km.  At Jool the altitudes ranged from about 144-171 km, with a median of about 155 km.

 

thanks! i almost went to low for eve and now i have a reference sheet! thank you for the help 

[Geschosskopf]

On 5/14/2016 at 9:35 AM, herbal space program said:

What Pe you should shoot for depends entirely on the body you're trying to get captured by and how fast you're coming at it. Moreover the aerodynamic model just got changed again in a manner that significantly affects upper-air drag, so I'm not sure even the hardcore folks have got that all worked out yet for every situation. In the last model, there were plenty of aerobraking scenarios that used to work pre-1.00 that became instantly deadly. Perhaps if you could describe your specific scenario a little more, somebody could give you a more helpful answer.

There is no answer to this question.  Aerocapture Pe depends on many factors that are different every time you do it.

• The planet you're doing it at.  All their atmospheres are different.
• The desired Ap altitude after the aerocapture pass.  IOW, how much braking effect do you need?  The lower your desired Ap, the more braking you need, so the deeper you have to go.
• The relative velocity (both magnitude and direction) of your ship compared to the target planet when you enter it's SOI.  The steeper the angle at which your ship's trajectory crosses the target planet's orbit, the greater the difference in their velocities,k so the greater braking effect you need to end up with the desired post-aerocapture Ap, so the deeper you have to dive.  But the greater the relative velocity, the faster you hit the atmosphere, so the greater the heat.  At some point, the heat will prevent you from going as deep as you need to get your desired Ap, so it pays to make very efficient transfers that have you arrive with minimal velocity relative to the target.
• The design of the ship.  Draggy ships don't need to go as deep as sleek ships to get the same braking effect.  Also, because sleek ships don't slow down very quickly, the spend more time going fast in the atmosphere, so experience more heat for a longer time, which can also limit how deep they can go.  Similarly, dense ships are harder to slow down than less-dense ships.

All of these variables make it pretty much impossible to predict in advance exactly what Pe you need for any given aerocapture attempt.  There's a mod called Trajectories that will give you a good estimate in advance, but it's only an estimate.

Therefore, the ONLY way to successfully aerocapture is to set your Pe where you think it might be close, hit F5 to quicksave, and then give it a try.  If it doesn't give you what you want, hold F9 to reload the quicksave, adjust your Pe up or down as needed, and try again.  

[Andem]

Unfortunately, like a lot of things in KSP, you just have to learn it, just always have a backup plan.

Edited May 15, 2016 by Andem
Damn forum software...

[herbal space program]

2 hours ago, Geschosskopf said:

Therefore, the ONLY way to successfully aerocapture is to set your Pe where you think it might be close, hit F5 to quicksave, and then give it a try.  If it doesn't give you what you want, hold F9 to reload the quicksave, adjust your Pe up or down as needed, and try again.  

In my practice I'd have to say that's pretty much true, but with some experience you do learn what will never work and also what the starting points are to try for different scenarios.

[Geschosskopf]

4 hours ago, herbal space program said:

In my practice I'd have to say that's pretty much true, but with some experience you do learn what will never work and also what the starting points are to try for different scenarios.

Quite true.  Still, even with experience, your initial Pe placement is just an estimate and will need to be adjusted via trial-and-error.