What does closing my air intakes do for me?

[Zipmafia]

I have heard more than a few people mention closing their air intakes at some point during their ascent with a SSTO, but I can not figure out why. Do they create less drag when they are shut off?

Edited October 12, 2014 by Zipmafia

[Boomerang]

Yup. IIRC, they have additional drag given by the intake area and when you close them, that extra drag goes away.

If you want to test it, get a plane flying at a moderate altitude at a decent clip with symmetrical intakes. Close them on one side only and you should notice that your plane wants to yaw towards the side with the open intakes.

[tsotha]

Yes, drag. You can save fuel by closing the ones you don't need.

[Zipmafia]

Ahhhh, so I should probably shut those when I get to Duna or Eve huh?

[technion]

Ahhhh, so I should probably shut those when I get to Duna or Eve huh?

The air on Duna is so thin it'll barely make a noticeable difference.

The air on Eve is.. I doubt anything still carrying air intakes is going to get back into orbit

[PikachuNL]

I know it works, but if you close an intake, it covers more space and thus increases the drag, right? :S Cause when it's open it flows right through, oh well, Magic Kerbin Airâ„¢

[Red Iron Crown]

Air doesn't "flow right through" intakes, it is slowed and fed to an engine. It is the slowing of the air that causes the drag IRL, and this is simulated by greater drag when open in KSP.

[PikachuNL]

Ah okay, thanks

[Clockwork_werewolf]

It is worth noting that at very low speeds it is better to have intakes open. At about 20m/s the drag will be about 0.10 (I can't remember), where as the closed drag is 0.3 .

Once you get above 100 m/s it starts to go above 0.3 while open so closing reduces the drag.

Drag maxes at 2 so 2 -> 0.3 is a great reduction.

When you start to get near to running out of air you will need to open them again.

It's also worth noting that you will always need some open or you will run out of air but in a intake heavy craft you can go up to 50% faster with them closed (50% is an estimate and it all depends on the number of intakes).

[Yakky]

BTW the other thing a closed air intake does is trap a tiny amount of air inside. This will allow you to fire up your jet engines in space for a brief moment. I believe this has been exploited to build jet-only orbital craft, which are only possible if you have some method of putting the finishing touches on your circularization burn after you've completely exited the atmosphere.

[Coga19000]

BTW the other thing a closed air intake does is trap a tiny amount of air inside. This will allow you to fire up your jet engines in space for a brief moment. I believe this has been exploited to build jet-only orbital craft, which are only possible if you have some method of putting the finishing touches on your circularization burn after you've completely exited the atmosphere.

I do doubt that the air trapped into the intake is really enough to combust the fuel long enough for the engine to provide sufficient thrust for raising the apoapsis abovethe atmosphere.

[LethalDose]

I do doubt that the air trapped into the intake is really enough to combust the fuel long enough for the engine to provide sufficient thrust for raising the apoapsis abovethe atmosphere.

I think it's theoretically possible. If you're traveling at orbital speeds in the atmosphere with an AP above the atmo, the burn needed at AP to circularize is pretty small.

However, setting this up probably requires some pretty, well, 'exploity' designs and flight (air hogging, part clipping, etc).