Aerobraking with Sledgehammer-shaped crafts

[Commissioner Tadpole]

I have a craft made in 0.90(the souposphere era) which is currently en-route to Duna and is inserted into a prograde aerocapture trajectory, periapsis at 14,400-something. Unfortunately, due to it being made prior to the atmospheric upgrade, its shape is aerobrake-unfriendly, and I found that my aerobraking trajectory failed by a wide margin, the craft instead plummeting into Duna's surface.

I'm using the Trajectories mod to determine my aerocapture route, and before plunging into the atmosphere it says it'll sucessfully slow me down enough for the apoapsis to be above Duna's atmosphere but not too far. After doing so, though, it updates to say I'll crash. I am utterly confused... and just when I decided to get back into KSP.

The craft in question is a rover meant to transport fuel from storages into crafts which need them, attached to the top of a booster stage which is essentially a Rockomax X32 tank with a Poodle engine. I would upload a screenshot if my internet weren't trying to see if it can be slower than dial-up.

Edited August 5, 2015 by Commissioner Tadpole

[Eric S]

Duna's got a bit more atmosphere in 1.0 than it did in 0.9, try a higher altitude. I can think of a few reasons Trajectories might make the wrong projection. You've updated Trajectories, right? The other thing that's possible is that Trajectories assumes a certain angle of attack, and if you didn't aerobrake the way it expected you to, the atmospheric drag would be different.

[CalculusWarrior]

Make sure you've correctly set the angle of attack (prograde/retrograde, or more exotic angles) in Trajectories; that has a huge impact on the atmosphere's effect on you due to the game calculating lift now.

[cantab]

Yeah, trajectories needs to be told the direction you'll come in at. And even then I don't think it can handle everything.

What I usually do for caution when aerobraking is to put a manoeuvre node at my periapsis, find out how much delta-V it takes to just capture, how much it takes to capture into a low orbit, and then take the midpoint delta-V wise. The resulting orbit, which will have an apoapsis somewhat lower than half the height of the SOI, is what I try and capture into. I believe this gives me the maximum margin for error both ways. I can then make subsequent cautious aerobraking passes to lower my apoapsis.

If you have the delta-V spare you could instead make a powered capture into an initial just-captured orbit, then make as many aerobraking passes as you like. Watch out for Ike though, you might want an inclined orbit to help stay clear of it.

[Commissioner Tadpole]

Duna's got a bit more atmosphere in 1.0 than it did in 0.9, try a higher altitude. I can think of a few reasons Trajectories might make the wrong projection. You've updated Trajectories, right? The other thing that's possible is that Trajectories assumes a certain angle of attack, and if you didn't aerobrake the way it expected you to, the atmospheric drag would be different.

Make sure you've correctly set the angle of attack (prograde/retrograde, or more exotic angles) in Trajectories; that has a huge impact on the atmosphere's effect on you due to the game calculating lift now.

Ah, alright. I plan for the craft to come in retrograde(not that I have much of a choice; the craft's shape makes it want to fly butt-first), so I'll have to inform Trajectories of that.

How do I tell it which angle of attack I'll use, though?

[cantab]

Retrograde means a 180 degree AoA as far as Trajectories is concerned.

[CalculusWarrior]

Ah, alright. I plan for the craft to come in retrograde(not that I have much of a choice; the craft's shape makes it want to fly butt-first), so I'll have to inform Trajectories of that.

How do I tell it which angle of attack I'll use, though?

In map view, click the Trajectories button on the toolbar. This will bring up the settings menu for the mod, where you can set the entry options.

[Reactordrone]

The denser atmospheres introduced after 1.0.0 mean you want to aim for about 20-25km at Duna. The fairly low entry speeds shouldn't give you any heating problems, I've done an 18km aerocapture with a pre 1.0.0 craft that didn't even break off the non-retractable solar panels.

[Commissioner Tadpole]

Thanks for the help, everyone! I'll post the results of the aerocapture re-attempt once it's done.

EDIT: Nope, no dice. Trajectories still gets the predictions wrong. What's really egregious, however, is how, after doing the 20km suggestion above, I thought I was into a safe orbit around Duna, and quicksaved... then realized Trajectories was COMPLETELY wrong and I was actually in an escape trajectory out of Duna(with a small Ike flyby). I got so angry I just HyperEdited my craft into a Duna orbit as revenge against the game. (though I could've used an Ike gravity assist to stay wtihin the Duna system)

The only thing keeping me from uninstalling it is because it is still useful for landing. But enough of this, it's quickly becomming a Trajectories-related thread which doesn't belong in this forum.

Edited August 6, 2015 by Commissioner Tadpole