How to mount and set up AIRBRAKES?

[Jestersage]

While I know AIRBRAKES are required to make a reusable Falcon 9 clones, I have a few questions:

1. Which direction to mount them, assuming down is ground? As in: whether it's the concave side (where Hydralics are) or the convex (opposite side) facing the ground?
2. Should I enable pitch and/or yaw?

Edited November 17, 2016 by Jestersage

[EpicSpaceTroll139]

It doesn't matter how you put it relative to the ground really. You just stick em on your craft with and they deploy outward. The only situation you would need to worry about the ground is if the airbrakes are longer than your gear, in which case you would want to avoid placing them on the bottom of your craft.

Since you are making a F9, your airbrakes will be at the top of the rocket, so this is irrelevant. If you want more realistic deployment, you will want the hinged round end to be facing down.

As for pitch/yaw function, that is entirely up to you and what you want out of your design. With those on, the airbrakes can be used as control surfaces, deploying differentially to incur assymetric drag, thus steering your craft.

 

This flying wing demonstrates how airbrakes can be used for steering. The principle can be applied to a rocket too.

Edited November 17, 2016 by EpicSpaceTroll139
Details

[Jestersage]

Thanks. To be honest I am trying to make not only the Falcon 9, but the Dragon V2. However, at an entry speed of 2.1km/s (using in-build cheat to bring it to 400k altitude and then burn retro grade), my airbrake was killed before the ship start flipping.

The idea is to create a 100% reuse launcher + pod, minus the cargo/fairing section, that would allow direct re-entry from Minmus, without the use of the inflatable heatshield (since it cannot be deflated) and orbiting airbrake -- in short, a Dragon V2 Functional clone. Not sure if I should just give up on using 2.5m and use the Mk3/3.75m system for that.

Edited November 17, 2016 by Jestersage

[cicatrix]

Here's how you should do it properly:

As for the re-entry, you shouldn't deploy them until you're slowed down enough so that dynamic pressure won't exceed the limit stated in the part information (you can see the max pressure in the VAB or SPH).

[EpicSpaceTroll139]

11 hours ago, Jestersage said:

...2.1km/s (using in-build cheat to bring it to 400k altitude and then burn retro grade), my airbrake was killed...

What is your periapsis at?

Also: Managing your airbrake deployment during re-entry can help to a certain degree. While deployed, even in the thinnest part of the atmosphere, will help slow your rocket down earlier, and thus reduce peak heating. In the deployed state, the airbrakes have a somewhat lower heat tolerance, however the flat surface sticking out creates a bow shock, reducing this effect slightly. When stowed, the airbrake has a higher heat tolerance,  but will not provide the extra drag to slow you down, and thus won't reduce peak heating.

I would suggest keeping them open as long as you can, and then when you think they will overheat, use the authority limiter to clean them partially, so that they provide some drag while avoiding overheating.

I've used this technique for returning rockets from lower orbits (75-120km), and I think actually my airbrakes would survive if I kept them completely open due to the aforementioned bow shock effect (I mainly just take these things into effect while using them to target the KSC).

I'm not certain, but I think you might be able to keep them from overheating by using careful management of their deployment.

If you're willing to sacrifice a little payload capacity, using a little fuel to slow down just prior to reentry would likely help your heating problems a lot.

[Jestersage]

TBH it was a test entry, so it's an extreme steep re-entry; I placed it so KER state AP=400k, pe is at -380k. Drained tank to 1/4 full.

The actual designed usage would be return from Minmus, direct, with an expected pe of 25 to 30k. Would not do any airbraking. The AIRBRAKES is used to prevent flipping.

I can post the craft files tonight.

[Sharpy]

Airbrakes got a heavy nerf of heat tolerance when extended. Pe of 25 to 30 from Minmus will blow them up within seconds. You can open them above 48-50km, when heating isn't rapid enough (though watch them and retract if they go red) and once your speed is down to some 1200-1400m/s (when they will brake you to safe speeds before they can overheat). But a direct reentry from Minmus is practically a suicide.

Set your PE to 53km, extend airbrakes, and keep doing airbraking passes until your AP drops below some 150km. If your PE drops too much and airbrakes heat up too much with each pass, perform a minimal burn at AP to raise PE a little bit.

Once your AP is at some 150km, burn off the rest of your fuel at periapsis (it should be below 50km by then), and when airbrakes start overheating, retract them. Extend them again once you're down to safer speeds.

[Foxster]

Pro tip:

Air brakes produce their drag if occluded by other parts but don't get hot.