SSTO Air intakes

[psyper]

I'm refining my SSTO design and have noticed that there are some different air intakes and its confusing as to which ones to use - Everytime when I launch my SSTO the air intake bar always starts on about 50% no matter how many intakes i have (even though the actual value has increased). Even when I get going it always seems like it starts from about 50% and drops from there. Firstly, am I doing anything wrong in that respect or is this normal?

Also looking at the intakes there are various different versions now and I'm wondering which ones to use best. My SSTO (I can't post a pic yet as i'm at work - will post when I get home) has several stack mounted points where I can place 6 intakes. I used to use the Ram Air Intakes but I noticed the Shock Cone Intakes that are although heavier have slightly better intake - if I'm using 6 of these would it make my ssto more efficient or is any gain in altitude is lost due to its higher mass?

Lastly, Is there any difference between the Engine Nacelle and the Radial Engine Body apart from the cost? Would it be worth me putting some of those on my craft or are structural intakes good enough?

Edited November 17, 2014 by psyper

[Streetwind]

As far as I understood it, air intakes can basically be reduced down to intake area per engine.

Higher intake area per engine = higher atmospheric flight ceiling = less flight time spent on rockets = more dV left in orbit

Are you happy with your current atmospheric flight ceiling?

a.) Yes --> Do not change anything

b.) No --> Add more intakes

[juanml82]

There are some threads about which intakes are better. I think it works sort of this way:

The Shock intake is the best of all the inline intakes

2 of the ugly radial intakes (the short ones) are slightly better than the shock intake

4 of the longer structural (radial) intakes are sort of equal to one shock intake

The air intake bar at the resource panel measures how much intake air is accumulated in the intakes but it doesn't show the actual amount of intake air. Kerbal Engineer Redux and Mechjeb both have displays that show the amount of air your intakes are taking in and the amount required for the engines to work.

[LordFjord]

I'm refining my SSTO design and have noticed that there are some different air intakes and its confusing as to which ones to use - Everytime when I launch my SSTO the air intake bar always starts on about 50% no matter how many intakes i have (even though the actual value has increased). Even when I get going it always seems like it starts from about 50% and drops from there. Firstly, am I doing anything wrong in that respect or is this normal?

Just treat intake air as a resource like electricity. The amount shown in the resources display is what is stored (yes, the intakes store a certain amount). But as Streetwind already mentioned, the stored amount is not what is important, its the amount of the air resource that is generated by the intakes and used up by your jet engines.

TLDR:

If you have the shock cones available, use them as they are the most efficient ones at high altitudes.

Lastly, Is there any difference between the Engine Nacelle and the Radial Engine Body apart from the cost? Would it be worth me putting some of those on my craft or are structural intakes good enough?

Stat-wise those are almost identical, except the price. Go for the better fitting looks here

I use them often as its a nice combined jetfuel/intake combo and fits good to some designs.

[Red Iron Crown]

As far as I understood it, air intakes can basically be reduced down to intake area per engine.

I've come around to Jouni's line of thinking on this: It's not intakes per engine that is the important metric but intakes per ton of mass. When intake air limited at high altitude, the amount of thrust that can be delivered is proportional to the number of intakes, whether that thrust is delivered by one engine or four is borderline irrelevant as the single engine will just run at a higher throttle setting.

[Wanderfound]

AFAIK, the two nacelle types are functionally identical. I'm quite fond of them; as well as the extra intake, they also provide a useful little bit of pure LF storage so that you don't find yourself with surplus oxidiser in orbit. And they make for handy engine and shock cone mounts:

There are some threads about which intakes are better. I think it works sort of this way:

The Shock intake is the best of all the inline intakes

2 of the ugly radial intakes (the short ones) are slightly better than the shock intake

4 of the longer structural (radial) intakes are sort of equal to one shock intake

The other consideration is: FAR/NEAR or stock aero?

In stock, load up on intakes and get crazy high and fast (if you want, it's not compulsory; you can make perfectly good stock SSTOs with <1 intake per engine). In FAR/NEAR, don't bother: no matter how many intakes you put on, you're not going to get any faster than about Mach 5.5 until you light the rockets. More intakes than you need is just wasted weight and drag.

The air intake bar at the resource panel measures how much intake air is accumulated in the intakes but it doesn't show the actual amount of intake air. Kerbal Engineer Redux and Mechjeb both have displays that show the amount of air your intakes are taking in and the amount required for the engines to work.

As does Kerbal Flight Data:

Edited November 14, 2014 by Wanderfound

[Streetwind]

I've come around to Jouni's line of thinking on this: It's not intakes per engine that is the important metric but intakes per ton of mass. When intake air limited at high altitude, the amount of thrust that can be delivered is proportional to the number of intakes, whether that thrust is delivered by one engine or four is borderline irrelevant as the single engine will just run at a higher throttle setting.

That's actually a good point that I hadn't thought about. I can see the reasoning.

[psyper]

Many thanks for all the replies - its been a great help. Forgot to mention in the OP that i'm using stock aero.

with regards to weight vs flight ceiling is there a point where more intakes weight out weighs the higher altitude you get. I'm getting to about 20km with my current setup before mechjeb start to throttle down the speed due to flameout prevention. If my TWR on the ground is > 1 is it okay to keep adding more and more intakes. PS I'm trying not to intake spam so won't be overlapping intakes or putting them in stupid places just to get more air!

[capi3101]

The old rule for stock aero was three Ram Air Intakes per engine - that'd get your atmo flight ceiling up in the 25-30k range going somewhere between 1700-1800 m/s or so. Not sure what the new rules for stock are...and I'm not sold on the notion of the Shock Cone Intake being a better intake just yet. BTW, I fly stock aero my own self.

If you're only getting to 20k before you're throttling back, I have to ask a few questions about the characteristics of your planes - General rules for engines are maximum take-off weight of 10 tonnes for Basic Jets, 13 tonnes for RAPIERs and 15 tonnes per Turbojet, and the total lift of your craft should be close to one lift rating per tonne (1.15 tonnes:1 unit lift is the actual ratio but 1:1 is easier to remember). I've mentioned the 3:1 intake-to-engine ratio.

I personally would not recommend Mechjeb for a plane flight, though that's based on stories I've heard about how well Mechjeb handles it and not from first-hand experience, so take that particular piece of advice with a grain of salt.

[Claw]

There is lots of good info/discussion in here. One extra thing to throw out, spcifically in this regard...

Everytime when I launch my SSTO the air intake bar always starts on about 50% no matter how many intakes i have (even though the actual value has increased). Even when I get going it always seems like it starts from about 50% and drops from there.

There are two intakes that appear to act differently than the others. One is the structural intake and the other is another small one whose name escapes me at the moment.

The thing with these two is that the air pool is set to a relatively higher value than the other intakes. I say they appear to act differently but in reality they function exactly the same. Intake air consists of the maximum pool available (how much the craft is capable of holding), and how much air you actually have for that frame (which is generated as a function of air density, speed, intake area, and probably a few other things). As an intake generates air, it can only ever store that air in its own pool. So it sort of acts like electricity, but is also different.

For the structural intake, it's not possible for the intake to fill its maximum pool. It simply doesn't generate enough air (under nearly all conditions conditions) to fill the pool all the way. The structural intake has the smallest area but is among the largest pool size. In this case, you're just seeing the fact that it can't fill all the way. And no other intake can fill the remaining space.

So if you are using a lot of structural intakes (and I forget the other), then it will look like you can never fill the air pool when really the pool is simply bigger than it needs to be. Or really to look at if from the other perspective, all of the other intake pools are too small to hold the maximum air each intake is capable of generating (so it's being "lost").

Cheers,

-Claw

[numerobis]

The old rule for stock aero was three Ram Air Intakes per engine - that'd get your atmo flight ceiling up in the 25-30k range going somewhere between 1700-1800 m/s or so. Not sure what the new rules for stock are...and I'm not sold on the notion of the Shock Cone Intake being a better intake just yet. BTW, I fly stock aero my own self.

Nothing has changed, so it's the same rules.

A shock cone has 20% more area than a ram, but 2.5x the mass. So to improve performance, replace your shock cones by rams, and every third shock cone you remove, add a radial intake. You'll have the same total area (0.36 either way) but less mass (0.4 versus 0.75). It's not a perfect replacement: when determining your speed for the purpose of airflow, the shock cone adds 72 m/s rather than 60 m/s, but if that bothers you, add another radial and you've overwhelmed the difference while still being lower mass.

The shock cone looks cool though, so if you aren't out to maximize performance, knock yourself out!

[capi3101]

Nothing has changed, so it's the same rules.

A shock cone has 20% more area than a ram, but 2.5x the mass. So to improve performance, replace your shock cones by rams, and every third shock cone you remove, add a radial intake. You'll have the same total area (0.36 either way) but less mass (0.4 versus 0.75). It's not a perfect replacement: when determining your speed for the purpose of airflow, the shock cone adds 72 m/s rather than 60 m/s, but if that bothers you, add another radial and you've overwhelmed the difference while still being lower mass.

The shock cone looks cool though, so if you aren't out to maximize performance, knock yourself out!

I'm definitely out to maximize performance, so I'll give your advice a try.

Hell...that looks like what my problem's been with the transport spaceplane I've been working on lately. By my calculations I'm about a half-dozen Ram Intakes shy.

Edited November 14, 2014 by capi3101

[panzer1b]

About shocks vs rams, ive found that shocks are better but they do not stack anywhere near as well as rams. A single shock will get my 3 engine ssto into 20km without running a singl eengine dry, whereas id need about 2-3 rams to accomplish the same thing, which = more drag and weight in total. Although i have found that stacking shocks doesnt help much, so most of my sstos use 1-2 shocks, and then a array of rams, with the shocks always on, and the rams are enabled only above 20km where i actually need them.

Also, rams are much lighter, and do not offer that much less surface area. Overall ive found 12-16ish rams and 1-2 shocks to be the best at least for a particular deisgn i have (3 engine cluster in back of jets). This can attain a 200km AP 50km PE using 150 jet fuel (just a single tank), the whole craft is a little above 15t mass.

[psyper]

yeah changing all 6 intakes from Ram to shock really really changes the center of mass and pretty much turned my sleek ssto into something that like to fly backwards. The design doesn't lend too much in the way of a design tweak to counteract that so I might just turn two rams in to shock intakes and make up for the loss with radial intakes. Thanks everyone for your help in this