Question about ship design for aerobraking

[M4ck]

I haven't really done any aerobraking in my KSP career, so I don't know how exactly how ships behave then. I will be sending this ship(the 3rd and the last picture) to Jool, then to Laythe orbit. The front RCS tanks will be replaced with a heatshield structure, and there will be two one-kerbal rovers on the "arms" on the back side. The rear rocket has already been detached.

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My question: if I want the ship to be properly aligned while aerobraking(heatshield to front), should I put something at the rear end to keep it straight? Like a small chute, little wings to make it something like an arrow? What would work best here?

[thereaverofdarkness]

I have not used a heat shield (or even 3/4ths of the parts you are using) but I have seen that ships push the lower density parts toward the back. You can definitely line it up with a drogue (MK-25) parachute at the back, and that will partially inflate as high as 25km IIRC. But the partially inflated drag is small and really just good for keeping it smooth. The most important thing will be to put the density forward. You want the craft to have its center of thrust furthest back, with the center of mass furthest forward, and center of lift anywhere in between, for launching or any time you are running the engines, especially in atmosphere. So if you are able to move the center of mass far enough forward, you should be able to both have a steady launch and a steady re-entry. The real trick is dealing with the high density of engines, fairly high density of fuel tanks, and low density of most every other in-line part.

Edited August 31, 2013 by thereaverofdarkness

[M4ck]

Thanks for the tip. It will be an interplanetary vessel only, so it won't be landing anywhere, only using Jool's atmosphere to slow down. The "spine" is made from fuel tanks, for the aerobraking I'll move all the fuel left to the main, front tank, maybe it'll help.

[NGTOne]

Unless you have Deadly Re-entry, you don't need heat shielding (unless you like to roleplay, anyways). But, as thereaverofdarkness said, differential drag forces along your vehicle's length may send it tumbling. Generally speaking, though, aerobraking takes place too high in the atmosphere to make parachutes useful. I recommend using lots of struts to tie everything together, so a tumble doesn't tear your ship apart. Other than that, you should be fine.

[Motokid600]

I would try to test it with a Kerbin areobrake if you can. Send the ship out as far as possible without it escaping. Then have it come back in for an areobrake. You won't be traveling at interplanetary speeds, but that should give you a good idea of how the ship behaves when it hits atmosphere. Id say no amount of struts will save a ship like that if it tumbled over. Either way I wouldn't chance it. Testing testing and more testing.

Also..what are those engines at the end of the arms your using?

Edited August 31, 2013 by Motokid600

[Amoun]

Exactly what NGTOne said. You only need to use a heat-shield worry about those effects if you have a Deadly Re-entry installed, otherwise it is not required at all

.....well, unless you want to go fancy.

[Specialist290]

Some resources you might wish to make use of:

• Alterbaron's Aerobraking Calculator, which outputs an estimate for your ideal target periapsis for a given set of initial orbit parameters and a target apoapsis.
  
• Since you mentioned Jool, alterbaron has created a reference chart for that planet specifically as well.
  
• I created a comparative atmospheric density table a while back, and a few other people have put that data in the form of some handy charts as well. (Kudos to ROFLCopter64Bit and Zeroignite.)
  

[Geschosskopf]

I've done aerobraking with some very clunky, non-aerodynamic things with no problem, but I'm not using FAR right now so I'm getting away with it.

The most important thing about aerobraking is to quicksave while you're still quite far from the planet, before the point where you'll be setting the maneuver to tweak your periapsis. This is because 1) there is zero in-game instrumentation to help you and 2) every single one you do is different, even with the same ship at the same planet aiming for the same final apoapsis, because you'll always at least have a different speed. Thus, it's always trial and error, meaning you'll have to go back to your quicksave, put the periapsis in a different place, and try again.

The charts and tables Specialist290 mentions are very helpful for giving you a rough idea of what sort of periapsis to start with, but you'll almost certainly have to tweak things from there to end up with what you want. Also, you really can't use these tables until you enter the target's SOI and know what your orbital stats are, and for everything except Jool itself with its huge SOI, this is also about the same place you want to quicksave before tweaking your periapsis. So that means save, exit to stop time, read the chart, then restart the game.

Also note: When you set your periapsis while you're out near the edge of the SOI, you have to consider that just an estimate. It will certainly change the closer you get to the planet. So, when you're fairly close to the planet, like say 2-3x the height of the atmosphere to give yourself some time, make a final tweak of the periapsis to put it exactly where you want it. But also understand that as soon as you hit the atmosphere, you'll start slowing down as intended, so the periapsis you set will get lower as you approach it. Don't freak out about that, just roll with it. It's the apoapsis you need to keep an eye on. If it ends up being to high or too low, remember you have a quicksave to go back to .

To me, the single greatest problem with aerobraking is that because it's trial-and-error, you have to spend most of the time in the map view watching what's happening to your apoapsis. Thus, you can't watch much of the pretty fireworks and will probably miss any spectacular fail that occurs . This is why I really wish there was in-game instrumentation to set it up perfectly ahead of time. Then you wouldn't have to look at the map view again until the show was over.

[M4ck]

Thanks a lot, people I'll take two small modules with me as well and see what configuration works best(F5/F9 a few times), then detach the not needed one(s). They will be a line of empty fuel tanks and a chute. I will try it in about 12 hours, as the ship is now fully assembled, refueled and is only waiting for a small kethane refinery to be added to its end. Thanks for the links, and for the "Aerobraking for noobs" tutorial, you all amaze me by how kind and helpful people can actually be online

By the way, the shields are only supposed to look pretty and provide some visual effects for nice screens, so no need to worry about stuff exploding from overheating.

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[Captain Sierra]

Heat shields (especially in stock game or with FAR) tend to work like chutes and flip you around. There is no real reason to use them at all unless you are using deadly re-entry. It really comes down to being a waste of weight.