Best way to areobrake to save fuel when changing orbit?

[THX1138]

In this screenshot is a craft I wanted to use to transfer fuel from the Mun to LKO. To save fuel when returning to LKO, I aerobrake and I thought I could facilitate the process by using the A.I.R.B.R.A.K.E. part but it tends to heat up much before the engines which I face towards the front/heat. It's a hassle to have to keep deploying and retracting them twice every orbit and I'm not sure they really make that much difference. Does anyone have any techniques I can use? I saw what I presumed to be airbrakes on video of the SpaceX rocket landing and they had holes in, presumably preventing them from getting as hot as they would otherwise, even if that makes them less effective per unit area. Perhaps a girder attached to the outside of an airbrake would behave similarly if the airbrakes were located in a more recessed position?

 

[archenemy_6]

I think if you want a more aggressive aerobrake you'll need to switch to a heat shield. Seeing as the airbrakes aren't as heat tolerant it would probably be best just to use those for a slow braking maneuver at high altitudes although it would probably take a few orbits. There's always a trade off especially with mass and heat tolerance

[bewing]

The "fin trick" always works better than airbrakes, except at Eve where you have to be super careful about staying behind a heatshield.

It's easiest to see the fin trick in action, rather than trying to describe it. If you want to see it, download this: http://www.virtualrealitytoursllc.com/pix/suborb1.craft

Launch it straight up. It'll get a little over 200km as I recall. Once above the atmosphere, there are two pairs of tailfins. Click on each one, and select "Deploy". One pair is already set "inverted" and must be that way. Then just watch it come back down. Doing this adds a tremendous amount of drag to the ship for aerobraking.

[Snark]

Try the inflatable heat shield.  It's amazingly effective, and you can use it for as many Kerbin-Mun round trips as you like, since it has no ablator to consume.  That thing's great.

 

[magnemoe]

8 hours ago, Snark said:

Try the inflatable heat shield.  It's amazingly effective, and you can use it for as many Kerbin-Mun round trips as you like, since it has no ablator to consume.  That thing's great.

 

This, my heavy tanker with 3 long MK3 tanks aerobrakes from Minmus to 500 km Ap.
has to go down to 28 km as its so heavy. 
Your main problem is danger of flipping over so you need very good symmetry,  plenty of reaction wheels, vernier engines also helps. 

[THX1138]

17 hours ago, Snark said:

Try the inflatable heat shield.  It's amazingly effective, and you can use it for as many Kerbin-Mun round trips as you like, since it has no ablator to consume.  That thing's great.

 

Is that a somewhat silly part? I suppose they're all silly parts but I thought heatshielding was introduced for the purpose of realism and an inflatable heat shield that is never consumed seems to undermine that. The one time I used it, my vehicle flipped, perhaps predictably (it came down fine engines first instead). How do you ensure that such a non-aerodynamic part stays front-first?

[Snark]

26 minutes ago, THX1138 said:

Is that a somewhat silly part? I suppose they're all silly parts but I thought heatshielding was introduced for the purpose of realism and an inflatable heat shield that is never consumed seems to undermine that. The one time I used it, my vehicle flipped, perhaps predictably (it came down fine engines first instead). How do you ensure that such a non-aerodynamic part stays front-first?

It's perfectly fine.  It simply has a high temperature tolerance.  The fact that it doesn't have consumable ablator means that it can't take heat quite as intense as the smaller ones can, that's all.

Keeping it pointed in front can be a challenge, yes.  There are a few options.

First, you can put fins or airbrakes on the back end.  As long as they're inside the ten-meter "shadow" of the heat shield, then they're protected from heat ... but they do still produce aerodynamic drag and thus serve their function.  (Yes, that's physically unrealistic, so you may consider it an exploit.)

Second, you can design a very squat "pancake" of a ship that's as close to 10 meters in diameter as you can make it, but is short on its longitudinal axis.  A pain to launch, yes, but fairly stable on reentry.

Third, depending on your ship design, you can actually just let it flip around behind your ship, as long as the thing in the rear is fairly temperature-resistant (e.g. an engine).  This won't work for a really massive ship.  But as long as the ship's not too huge, and if you keep your center core down to 2.5 meters, then that means that well over 90% of the total heat load will still be going to the heat shield, and it will slow you down enough that your engine will get hot but not go kaboom.

 

[N_Danger]

 I have had some recent experience with the inflatable heat shield on Laythe.  The faster you go the bigger your chances are for the shield to overcome control (SAS for me) and once it flips it will keep tumbling. And then you won't get control back until you get some drag (in my case open parachutes) behind it.

 The benefit was that it induced a lot of drag and I bleed off speed really fast while tumbling.

Edited June 10, 2016 by N_Danger

[bewing]

Control is overrated. Tumbling is a good thing. Tumbling also gives you an opportunity to safely detach the heatshield (if you want to), whenever the shield is "above" you.

[magnemoe]

8 hours ago, bewing said:

Control is overrated. Tumbling is a good thing. Tumbling also gives you an opportunity to safely detach the heatshield (if you want to), whenever the shield is "above" you.

Tumbling at more then 2km/s is bad, later on you might want to turn around to drop shield, you can always turn again.

[Montag]

Or you can just set your periapsis at 49k or higher and keep doing multiple passes to pull your apoapsis down.  Just keep boosting the periapsis above 49k for chump change untill you get your apopapsis down to where you want it.  No heat shield should be required.  It may take many passes for the desired altitude and your periapsis may need to be higher or could be lower depending on your craft shape.

[blorgon]

I know this isn't ideal (I'm generally not a fan of having to install mods for every little thing), but you could always install Trajectories. Lower your periapsis into the atmosphere, and the trajectories mod should* draw a new trajectory, and you'll be able to see ahead of time how low your apo will get after the aerobraking. And, as others have mentioned, the inflatable heat shield is the way to go for this application.

*I haven't used Trajectories in a long time, so I don't actually know if it would work exactly like this, but I don't see why not.

[THX1138]

The inflatable (disappointed it can't be deflated - it looks like a silly hat - but it makes sense since constantly folding it up would wear it out) heat shield combined with some airbrakes at the back have reduced the number of passes needed to three or four instead of about eight. In the screenshot below you can see that some of the airbrakes are only partially-covered. Is this properly simulated in the game to determine how hot the parts are? Would radiator panels allow parts to take more re-entry heat, provided the panels were in the shadow of the shield?

[Snark]

Just for grins, here's a completely reusable fuel tanker that uses the inflatable heat shield.  382 tons mass when it hits Kerbin's atmosphere, can aerobrake from the Mun into LKO with a single pass, no problem.  (Lowest altitude during the pass is a bit under 28 km).

Yes, there are some mod parts in there, but you could just as easily build pretty much the equivalent with stock parts.  I've built it around a single giant 5m tank from SpaceY; if you put three of the big stock Kerbodyne tanks end-to-end, you'd get pretty close to the same fuel load.

It's rock-solid stable during aerobraking, since its center of mass is slightly forward of center, and the drag from the inflatable on the back balances out the drag from the inflatable in the front.

Plowing almost 400 tons behind a 10-meter heat shield at lunar-reentry speed through air at 28 km gave me the longest aero-overlay drag arrow I've ever seen.