Air intake dropping while in Kerbin atmosphere?

[skyfire322]

I've been noticing something for the past few SSTO builds lately. From take off to landing (note that the highest I'll go is ~8000 M) the air intake drops like a rock, which in turn burns my liquid fuel quicker even though I'll slap 4-6 radial air intakes on and use the one or two RAPIER engines (depending on the size of the plane) which if I recall have the auto-switch.

The thing that confuses me is that when I time accelerate, the air intake increases. Am I completely missing something?

EDIT

*Not radial intakes. I use the structural intakes. Not enough coffee.

Edited December 31, 2014 by skyfire322

[worir4]

Air intake will drop if you have a high angle of attack. Try keep your nose pointing towards the velocity vector to maximize the air intake.

[Anquietas314]

From take off to landing (note that the highest I'll go is ~8000 M) the air intake drops like a rock

I'm not sure if this is the problem or not, but even in stock air intakes care about your angle of attack - if it's too steep, you don't get intake air at all. If you have action groups I would recommend disabling autoswitch on the RAPIERs though, particularly if you have more than one of them. It's a tiny bit too sensitive, and if you have an asymmetric flameout having one engine switch to rocket mode and not the other one can be very bad.

The thing that confuses me is that when I time accelerate, the air intake increases. Am I completely missing something?

That's just how physics (and to some extent normal) timewarp works. Basically the simulation timesteps are increased, which means each chunk of intake air (and everything else) you collect is bigger.

[Hobbes Novakoff]

The thing that confuses me is that when I time accelerate, the air intake increases.

I think that's a bug. Physics warp in general is very buggy, especially on slower computers (At max physwarp you're basically quadrupling the load on the processor) and shouldn't be used with anything over 100 parts (Maybe more if you're on a gaming rig, but not much more). I think it's just a bug in the way that KSP handles intakes and engines.

EDIT: Someone already provided an answer to the physwarp thing and they obviously know more than me about KSP physics

Edited January 4, 2015 by Hobbes Novakoff

[skyfire322]

I'm not sure if this is the problem or not, but even in stock air intakes care about your angle of attack - if it's too steep, you don't get intake air at all. If you have action groups I would recommend disabling autoswitch on the RAPIERs though, particularly if you have more than one of them. It's a tiny bit too sensitive, and if you have an asymmetric flameout having one engine switch to rocket mode and not the other one can be very bad.

When it comes for the AoA: I usually only fly increasing my inclination by 5 or 10 degrees on the navball at most. I'll occasionally do a quick 30-45 degree increase to test the integrity and recovery of the plane, but other than that it's a pretty flat trajectory.

As for the RAPIER's, I'll disable the autoswitch to see what happens. Thanks!

[Anquietas314]

As for the RAPIER's, I'll disable the autoswitch to see what happens. Thanks!

The problem there has nothing to do with intake air btw. It's more the whole asymmetric thrust = crazy flat spin fun time thing

[lirg03]

I've been noticing something for the past few SSTO builds lately. From take off to landing (note that the highest I'll go is ~8000 M) the air intake drops like a rock, which in turn burns my liquid fuel quicker even though I'll slap 4-6 radial air intakes on and use the one or two RAPIER engines (depending on the size of the plane) which if I recall have the auto-switch.

The thing that confuses me is that when I time accelerate, the air intake increases. Am I completely missing something?

Radial air intakes are very inefficient for SSTOs. Ram air intakes and shock cone air intakes are much better choice. And in my experience, you need more than one of those for each engine. Use a couple of radial intakes to boost up initial intake air reserve.

Also like others said, if the angle of attack is too large, air intakes reduce their efficiency.

[Anquietas314]

Ram air intakes and shock cone air intakes are much better choice.

The structural ones are quite good for packing them into a small area, which is good for some of the smaller planes where you simply don't have anywhere handy to put the node-attached ones. They're not the most efficient, granted, but they do look pretty cool

[skyfire322]

The structural ones are quite good for packing them into a small area, which is good for some of the smaller planes where you simply don't have anywhere handy to put the node-attached ones. They're not the most efficient, granted, but they do look pretty cool

I use the structural ones all the time, and just attach them around the top part of the fuselage. I really have never tried the other ones because I get paranoid about mass, placement, and yes.... Looks.

I'll play around with some designs, as well as different engines. I just installed FAR for the first time this morning so it just make things even more interesting! lol.

[Anquietas314]

I really have never tried the other ones because I get paranoid about mass, placement, and yes.... Looks.

Yeah, don't worry too much about it. Most of my (smaller) designs just use the structural ones too. They fly just fine

On bigger planes the mass of the intakes is completely negligible.

I'm not entirely sure if FAR affects intakes much though. I don't use it.

[Srpadget]

I must confess I'm a bit surprised to hear that stock aero cares about angle of attack for intakes.

One of my more sinful abominations--err, less-than-completely-realistic designs--is a wingless SSTO "jet-powered booster" with a cluster of turbos around a NERVA core. Takes off vertically from the launchpad, flies to orbit using a spaceplane-like trajectory, lands vertically with chutes and landing legs somewhere near KSC (I'm nowhere near good enough to land it back on the pad, but someone else doubtless could)...and part of its standard landing profile is to fire up the jets while it's faced tail-first into the 200-300 m/s freestream flow, as near 180 degrees AoA as makes no difference. The intakes seem to function just fine at the "ultimate high angle of attack", so I guess I'm surprised to find that they apparently stop working somewhere in the vicinity of 90 degrees??

[Anquietas314]

Srpadget: do you have a screenshot of that design at all?

[GoSlash27]

Radial air intakes are very inefficient for SSTOs. Ram air intakes and shock cone air intakes are much better choice. And in my experience, you need more than one of those for each engine. Use a couple of radial intakes to boost up initial intake air reserve.

Also like others said, if the angle of attack is too large, air intakes reduce their efficiency.

I strongly disagree with everything said here (except for the last sentence).

The XM-G50 radial intake is all- around the most effective and efficient intake in the game.

Reference: http://forum.kerbalspaceprogram.com/threads/97615-0-25-KSP-air-intakes-compared

And you really don't *need* to air-hog if you've properly matched the number/ type of wings and turbos to your spaceplane. My workhorse SSTO uses 4 XM-G50 intakes and a single turbojet and nets 50% payload fraction.

A single ram or shock cone coupled with 2 XM-G50s is plenty sufficient to feed a turbo for an SSTO spaceplane.

Best,

-Slashy

[skyfire322]

Here are a few shots of a design I worked with.

First is with a RAPIER as the middle engine. Five structural intakes, plus intakes on each wing for good measure. Same results each time.

Second is all regular jet engines, same air intake set up. Same results while in flight.

I right clicked on each of the intakes to see what was happening and noticed that all of them were at about .10. Is that normal, with me being at a 5-10 degree inclination?

[Anquietas314]

skyfire322: Your screenshots look perfectly normal. Intake air isn't like most resources; it's updated every game tick based on your altitude, AoA and so on. You should normally be able to run the engines up to and sometimes beyond the point where the bar hits zero. Since you have KER already, I recommend adding its intake air readouts to the display; in particular the percentage one. I've found that for turbojets you can run them up until 1000% intake air usage. RAPIERs seem to be around 300%, and I never tested normal jets.