Ballistic coefficent

[Palaceviking]

Hey guys, looking for advice.

I have a 197 ton vessel coming from a Jool aerobrake(not orbit) into a Laythe aerobrake and I need an aerocapture.

It has the stock 10m inflatable at the front,fins at the back(attached to an Orion drive that wasn't originally invited to the party, but hey) and my COM is just slightly back from centre, I have about 4 large sas wheels in the mix but only 63 units of mono.

Due to the absurd amount of time it takes to turn this thing around I really need to aerocapture on first pass. Any recommendations on altitude?

https://imgur.com/KJqpsGW

Also if anyone knows the formulae for this kinda thing

 

Edited October 19, 2017 by Palaceviking

[OhioBob]

Altitude at Jool or at Laythe?

[Palaceviking]

29 minutes ago, OhioBob said:

Altitude at Jool or at Laythe?

Laythe, done Jool at a 196/7 km pe , didn't brake as much as wanted, going to adjust for Laythe approach.

https://imgur.com/a/xuJdd

https://imgur.com/hc9aRrk(Encounters)

https://imgur.com/Xl0OcgS   The ship(changed sinced the first iteration)

Edited October 19, 2017 by Palaceviking
Screenies

[OhioBob]

I did a bunch of simulations to estimate aerocapture periapsis, but that won't be much help unless we can obtain more data.  My numbers are a function of hyperbolic excess velocity and ballistic coefficient.  Right now we don't know either of those.

I would suggest initially setting your Laythe periapsis to about 35 km.  Right after you pass into Laythe's sphere of influence, pause and make a note of the vessel's altitude and velocity.  From that we can compute hyperbolic excess velocity.  That will give us an indication of how fast you're coming in.  The faster you're going, the lower you'll have to set the periapsis in order to achieve a capture.

The ballistic coefficient we may just have to take a guess at.  If you had known ahead of time, you could have opened the game's Aero GUI and read the BC as you passed through Jool's atmosphere.

 

Edited October 19, 2017 by OhioBob

[OhioBob]

I've just been playing around with the numbers.  Based on your vessel's current orbit (semimajor axis = -96,707.733 km), I compute that's its velocity when crossing Laythe's orbit will be 4,869 m/s.  And it looks like it's crossing the orbit at roughly 90 degrees.  Laythe's orbital velocity is 3,224 m/s.  If they cross at right angles, then the relative velocity is SQRT(4869²+3224²) = 5,840 m/s.  In this scenario the excess hyperbolic velocity is equal to the relative velocity.

I also guesstimated the ballistic coefficient by taking a inflatable heat shield and putting fuel tanks behind it until I got something with the same weight and roughly the same shape as your vessel.  I then hyperedited it to space and ran it through Kerbin's atmosphere at high Mach with the Aero GUI window open.  The BC varied, but it was around 1,700 kg/m² and the time of greatest aerodynamic deceleration.  Not a really accurate number but hopefully in the ballpark.

Based on those numbers, I'm guessing you need a periapsis of about 34-35 km.  Of course there's a lot of uncertainty in these numbers.  You should definitely quicksave first.  I'm also not clear on whether you're trying to aerocapture into Laythe orbit, or are you just using Laythe to aerocapture into Jool orbit.  You need to shed more velocity for the former than you do the latter.

I just hope your vessel can maintain stability.  The test vessel that I slapped together wanted to flip around when I got deep in the atmosphere.

[OhioBob]

2 hours ago, Palaceviking said:

done Jool at a 196/7 km pe , didn't brake as much as wanted

Yeah, that's way too high.  You're not going to get much aerobraking at all at that height.  I ran a bunch of Jool aerocapture tests and I found that the ideal periapsis altitude varied between about 145 km and 170 km, depending on the circumstances (velocity and BC) and desired outcome (apoapsis).
 

 

Edited October 19, 2017 by OhioBob