Do air intakes need to be closed at high altitude jet engine shutdown?

[Kelderek]

I can understand closing an air intake at low altitude if you have lots of intakes and don't need the air yet, opening it higher up when the air is thinner. After you've reached a really high altitude and your jet engines start to flame out, is there any need to close the intakes then? I would assume that if the air is thin enough to not help your engines, then it would be thin enough to not cause any drag either. Is this true? Or is the drag somehow still magically strong enough that it would help to close my intakes at high altitude?

Not a particularly big deal, but it would affect how I set up my action groups.

Edited February 6, 2015 by Kelderek

[tsotha]

I think it depends on where you switch over. If you switch over manually at 25km the air is probably still thick enough that the drag will have some effect on your fuel consumption.

If, on the other hand, you're milking your engines up to 55km then I doubt closing intakes will have any noticeable effect whatsoever. I think at one point if you left the intakes open your engine would use fuel even if it had flamed out, but I haven't noticed that to be the case.

[eddiew]

I do set my intakes to close in the same action that toggles rapiers, but I'm not entirely sure how much use it is. I've never really bothered closing intakes when I have more than enough open - will maybe try that later

[SaturnV]

Right click on it then you can check the real-time drag of air intakes. It won't make a difference under current stock aerodynamic since the true drag is an weighted average number not only related to drag but also the mass of each part. Air intakes are really lightweighted.

[LordFjord]

Intakes usually provide air up to roughly 35km, so as long as jets are running, keep them open.

Between those 35km up to space, they would just create unnecessary drag if left open. That saves you a bit fuel.

And I've also never thought about start intakes closed and open them when I really need them - which might be a good idea to save a bit more fuel in lower atmosphere.

Could be useful for monstrous SSTOs with lots of intakes.

Edited February 6, 2015 by LordFjord
typo/wrong wording

[GoSlash27]

I don't bother closing them on single engine spaceplanes. Kicking in the rockets at flameout is often enough to restart the turbojet, saving fuel.

Multiengine spaceplanes actually require the turbojets to be shut down, so I close the intakes as well.

Best,

-Slashy

[numerobis]

Intakes are light (10kg) but their drag coefficient is large when they're open (10x or 11x as much as normal parts), so treat them as a 100kg part when it comes to drag.

If you're above 40km, drag pretty much doesn't matter. If you're below 20km, it matters a lot.

A minor secondary thing: intakes hold air. When you close the intake, that air remains in the intake, and you'll be carrying it around with you. For that reason, when I want to switch to some other propulsion, I close the intakes but leave the jets running.

[Kelderek]

I guess it won't hurt to close the intakes high up, so I'll keep them as part of my action groups. Thanks all.

[RexKramer]

It is not necessary to close them- at stages where I do not yet have action groups available I generally don't bother with closing them, and get to orbit just fine.

However, if you have action groups available, it certainly doesn't hurt to close them, and depending on your altitude it may provide a great deal of benefit.

My spaceplanes generally reach 40km and ~2350m/s on jets alone, all stock, with no intake spam (one intake per engine..). Using two intakes per engine gives the same results, except I reach that speed and altitude faster, so less jet fuel is needed. If you can reach 35-40km on jets, the drag is minimal to begin with, but closing your intakes is like getting FREE energy, so why not?

[panzer1b]

Its a matter of how much you care about 100% efficiency.

Any open intake that isnt running a jet is wasting more or less dV. Personally, if its above 60km, i really dont care at all. The atmosphere there is so thin, its more or less negligible. And loosing 10dV isnt really important for me (its not like 10dV is going to save a mission that ran outta fuel).

below 30km (none of my SSTOs actually go that low), id keep them closed if you are using rockets that low. Ive never actually had such a situation, as 90% of my SSTOs have enough intakes to gun it to 70km minimum before they need to switch to rockets. The few that dont make it, can at least make 60km, which is well above what id call thick atmo. never made a SSTO that unable to go above 60km sofar (stock aero, realistic or not, i really prefer FAR to not screw over jets, SSTOs are already hard enough to get to laythe and back, let alone my next endevour, of landing a SSTO on eeloo and returning home without a single bloody refueling using 100% stock parts).