Help with a space plane design: how much air do i need? How to read the Air Intake

[Brainlord Mesomorph]

Help with a space plane design: how much air do i need? How to read the Air Intake Resource Meter?

I understand the fuel resource meter. 1440/750 means that I have 750 units of fuel (kg?) left of my capacity of 1440.

But when my air intake resource meters says: 9.61/24.30 what does that mean?

Does it mean that an excess capacity? (maybe I do, adding air intakes is awfully easy …and fun!)

Currently my space plane has four turbo jet engines and two RAPIER engines With 6 of the new “shock cone intakes†and 20 (I didn’t realize there were 20) of those new little square “structural intakesâ€Â

So:

How much your do I need?

And what the heck and does that air resource meter mean?

tia

brainlord

BTW: Love this game! Love this game! Love this game! Love this game!

Edited October 17, 2014 by Brainlord Mesomorph
answered

[Alshain]

Some intakes provide more air in a moving position than they do sitting on the runway. That is why your resource meter looks like it's half gone. For stock, a couple of intakes per engine should be enough, that's a generalization though, it depends on the plane really.

[Red Iron Crown]

Liquid fuel is shown in 5kg increments.

Intake air is more complicated. It shows how much is in the IntakeAir pool at the end of a frame. Due to the unusual nature of how intake air is calculated, the amount shown can be different for two craft with exactly the same parts in the same locations. The order in which the parts were placed determines how the gauge will read.

To maximize the usefulness of the gauge, place all intakes, then place all engines. The amount shown in the gauge will be the surplus air after all engines have taken their requirements.

Unfortunately, this placement strategy is not the best for performance. It is better to divide your intakes by the number of engines. Place that number of intakes, then an engine, repeat until all are placed. This strategy minimizes asymmetric engine flameout. Do not place airbreathing engines using symmetry.

A more thorough explanation of intake air flow can be found here, with a simpler practical example here.

[Brainlord Mesomorph]

I’m trying to understand what the numbers mean.

The jumbo jet engine says it consumes 5.3701/sec of intake air. 5.3 what? (I’ll guess liters because everything else is metric ) the new “shock cone intake†as in intake air of .4 (not per sec) so I would need 14 shock cones for every turbojet? (I’m clearly misunderstanding that)

I have read the jet engines work up to about 30 km, mine sputter out at 23. (I already have 26 air intakes)

So does somebody know what these numbers actually mean?

[Brainlord Mesomorph]

Thanks Red,

I was typing at last response, while you posted yours.

I'll go read that,

B

[juanml82]

Stock won't give you intake air readouts, they'll give you readouts of the intake air temporarily stored in the intakes. You need KER or Mechjeb (and in MJ the readout is hidden and has to be brought up by customizing a menu) to see the actual intake air and required intake air.

I think 3 of the shockwave intakes per engine is good enough to cruise at around 25-30 km of altitude. Apparently, it may be that one of those is roughly the same of four structural intakes.

[Wanderfound]

I’m trying to understand what the numbers mean.

The jumbo jet engine says it consumes 5.3701/sec of intake air. 5.3 what? (I’ll guess liters because everything else is metric ) the new “shock cone intake†as in intake air of .4 (not per sec) so I would need 14 shock cones for every turbojet? (I’m clearly misunderstanding that)

I have read the jet engines work up to about 30 km, mine sputter out at 23. (I already have 26 air intakes)

So does somebody know what these numbers actually mean?

Do you mean turbojet or basic jet? The basic jets are only good for subsonic low altitude stuff; they conk out at about Mach 1 / 20,000m. Turbojets and RAPIERs handle the high & fast stuff.

Unless you're running a dozen engines or more, you have more intakes than you need. One intake per engine will get you to orbit just fine if you do it right.

Intake effectiveness is a function of air density, AoA and speed. You can go a lot higher if you level off and crank up the speed as high as you can before trying to climb too high. You can go a lot higher if you keep your AoA minimised. You can go a lot higher if you shut down as many engines as possible to concentrate the airflow into a single jet. You can go a lot higher if you throttle back gradually as you climb, keeping the air requirement just below the flameout threshold.

As you may gather from the preceding paragraph, it's as much about piloting as design.

Kerbal Flight Data also gives a % of air requirement filled figure, BTW, as well as other useful stuff. Highly recommended.

[Brainlord Mesomorph]

THANKS Wanderfound

(of course I meant turbojet (speech recognition LOL)

and thanks for the flight instruction. I was starting to figure that out

umm... how do I mark this as a answer?

[Alshain]

umm... how do I mark this as a answer?

Sticky thread at the top of the forums.

http://forum.kerbalspaceprogram.com/threads/85063-How-to-change-Unanswered-to-Answered

[capi3101]

Edit your original post, go into advanced mode and you should be able to change the thread status from there.

EDIT: Ninja'd.

[Geschosskopf]

Currently my space plane has four turbo jet engines and two RAPIER engines With 6 of the new “shock cone intakes†and 20 (I didn’t realize there were 20) of those new little square “structural intakesâ€Â

So:

How much your do I need?

And what the heck and does that air resource meter mean?

To be honest, the intake air line on the resource list is pretty much meaningless, mainly because back in the update that introduced the RAPIER engine, Squad also introduced a mechanism that automatically throttles down jet engines as the amount of intake air decreases with altitude (you used to have to do this manually) to postpone flameout for as long as possible. This automatic system has such fine control that it can use tiny amounts of intake air too small to appear on the resource list. Thus, these days planes routinely show 0.00 IntakeAir remaining on the resource line and still keep running on jets while gaining another 8-10km of altitude.

So.... instead, it's generally best to just follow certain rules of thumb. If you desire to make a spaceplane, as in something that can get to orbit, then in general you'll need 2 of either the old ram intakes or the new shock cone intakes per turbojet or RAPIER.

As to the other inline intakes, don't even bother putting them on a spaceplane. They should be paired with the basic jet engine on things that fly fairly low and slow.

Then there are the old radial and new structural intakes, which are functionally about the same. Ignore any stats about quantity of IntakeAir. What matters for these intakes is their intake area in comparison to the intake area of the ram and shock cone intakes. The radial and structural intakes are roughly similar and both have about 1/4 the area of the ram and shock cone intakes, so you need 4 of them to take the place of 1 of the latter. IOW, 1 turbojet or RAPIER needs either 2 ram/shock intakes OR 1 ram/shock + 4 radial/structural OR 8 radial/structural to keep running long enough to be of use to a spaceplane.

Also note that when installing intakes and engines, it's a good idea to do it WITHOUT symmetry. Install the intakes that will be used by 1 engine, then install that engine, then the next batch of intakes, then the next engine, etc. This helps make all the engines flameout simultaneously so your plane won't go into a flat spin from 1 flaming out before the others.

Also, I'm curious as to why you're putting both turbojets and RAPIERs on the same plane. The turbojets are better jets than the RAPIERs and things like LV-909s or (even 48-7S engines if your spaceplane is small enough) are better rockets than RAPIERs. The only reason to use RAPIERs is if you don't have enough room to mount separate jet and rocket engines. If you've already got a bunch of turbojets, maybe change out the RAPIERs for pure rockets?

[Alshain]

Also, I'm curious as to why you're putting both turbojets and RAPIERs on the same plane. The turbojets are better jets than the RAPIERs and things like LV-909s or (even 48-7S engines if your spaceplane is small enough) are better rockets than RAPIERs. The only reason to use RAPIERs is if you don't have enough room to mount separate jet and rocket engines. If you've already got a bunch of turbojets, maybe change out the RAPIERs for pure rockets?

Well that's not that strange. Depending on the size of the plane, maybe 2 rapiers in air breathing mode wasn't enough, nor was 4 turbojets. 6 air breathers drops to 2 rockets @ 30k, I don't see why that can't work. His alternative would have been 6 turbojets and 2 aerospikes?

Edited October 15, 2014 by Alshain

[Red Iron Crown]

To be honest, the intake air line on the resource list is pretty much meaningless, mainly because back in the update that introduced the RAPIER engine, Squad also introduced a mechanism that automatically throttles down jet engines as the amount of intake air decreases with altitude (you used to have to do this manually) to postpone flameout for as long as possible. This automatic system has such fine control that it can use tiny amounts of intake air too small to appear on the resource list. Thus, these days planes routinely show 0.00 IntakeAir remaining on the resource line and still keep running on jets while gaining another 8-10km of altitude.

In my experience manual throttle management is still necessary, especially for multi-engine designs. Even if you place the intakes and engines in the optimal order some thrust asymmetry will still occur and create undesirable torque that throttling back can eliminate, and in single engine designs flameout will occur sooner if you leave the throttle at 100%. The automatic thrust reduction you mention does happen, and can provide a useful warning that you're approaching flameout (much better than the old system where your warning was one engine flaming out while the other was still happily thrusting at 100%).

[Geschosskopf]

In my experience manual throttle management is still necessary, especially for multi-engine designs. Even if you place the intakes and engines in the optimal order some thrust asymmetry will still occur and create undesirable torque that throttling back can eliminate, and in single engine designs flameout will occur sooner if you leave the throttle at 100%. The automatic thrust reduction you mention does happen, and can provide a useful warning that you're approaching flameout (much better than the old system where your warning was one engine flaming out while the other was still happily thrusting at 100%).

Personal preference I guess. I haven't touched the throttle on my spaceplanes since the automatic system came out, except maybe to throttle back some to start with so as not to exceed terminal velocity at low altitude. But otherwise, I leave it full on the whole way up, even with multi-engine planes that for some unknown reason continue to have asymmetrical flameout despite my best efforts in the SPH. On the 1st flight, I let them go until flameout. If that's symmetrcail, then on subsequent flights I just continue to fly to flameout at full throttle. If it's asymmetrical and I can't solve this by tinkering with the part placement, then I note the altitude that happens at. This is almost always a bit over 37km for planes with 2x ram/shock intakes per engine. So then on subsequent flights, I hit the action group that switches from jets to rockets at 36km, still without touching the throttle. That way, the rockets kick in at full throttle immediately.

I have occasionally noted instances of asymmetrical thrust prior to flameout. However, when this happens, it usually starts about 25-26km, way below flameout height. This is a sure sign the part order of engines and intakes is buggered and I've always found that I can solve this problem by reattaching all the intakes and engines in the correct order (like I thought I'd done the 1st time). But even though I can cure the asymmetrical thrust, this doesn't always prevent asymmetrical flameout, even though you'd think both were caused by the same issue. So that's a mystery to me. Still, I've got a simple workaround for that. So these days, with the automatic throttle system, I find getting spaceplanes to orbit requires less attention and control intervention on my part than launching a rocket. It's a huge change from the old days of it being a very intense and stressful ordeal lasting 10-20 minutes. I don't think this is because I"m particularly good at flying or designing spaceplanes, but because I'm lazy so I focused my attention on those aspects that make the job easier .

[Geschosskopf]

Well that's not that strange. Depending on the size of the plane, maybe 2 rapiers in air breathing mode wasn't enough, nor was 4 turbojets. 6 air breathers drops to 2 rockets @ 30k, I don't see why that can't work. His alternative would have been 6 turbojets and 2 aerospikes?

Spaceplanes run a wide spectrum in their ratio of thrust to wing area. The less wing you have, the more the spaceplane is like a rocket so the more thrust it needs just to keep itself heading up. OTOH, the more wing it has, the less thrust it needs because even a little thrust, if allowed to act for a long enough time, will eventually get it fast enough while the wings do the work of keeping the thing in the air. And it gets to use this thrust more efficiently for increasing speed because the bigger the wing, the less angle of attack it needs to maintain and gain altitude, so the thrust is almost all applied in the desired direction.

I usually build my spaceplanes, especially the bigger ones, to have a lot of wing. Thus, they don't need much thrust (and they also have nice, easy landing speeds of 50m/s or less). Take this ugly, hulking abonination for example:

It's only got 2s turbojets and 2s 909s, the latter only having a TWR of 0.34. Actually, they have a bit more than that when they come online because by then about 1/2 the jet fuel is gone, but they're still around 0.4, way less than 1.0. Despite this, because the jets eventually (in about 15-18 minutes) get it to about 2200m/s at 36km, it can easily reach a 200km orbit with 1/2 its rocket fuel remaining for maneuvering and de-orbit even without docking, and upon reentry still has 1/2 its jet fuel for crossing continents and oceans to reach the runway if you badly misjudge your descent path. Plus, it can kill up to 18 Kerbals at once .

This is why I consider the RAPIER an engine of last resort, for use only when available space precludes using separate engines. Sure, 1 RAPIER weighs a bit less than 1 turbojet + 1 909, but you more than make up that difference in fuel weight because the RAPIER is a gas-guzzler. Sure, the RAPIER has a lot more thrust in rocket mode than a 909, but if the jets have done their job properly and gotten you nearly to orbital velocity already, then you only need a tiny bit of thrust to get the rest of the way up. The RAPIER's extra thrust is total overkill useful only for pushing the mass of the fuel it needs to get that thrust, and that mass makes the whole spaceplane more of a pain to build and fly.

[Red Iron Crown]

Personal preference I guess. I haven't touched the throttle on my spaceplanes since the automatic system came out, except maybe to throttle back some to start with so as not to exceed terminal velocity at low altitude. But otherwise, I leave it full on the whole way up, even with multi-engine planes that for some unknown reason continue to have asymmetrical flameout despite my best efforts in the SPH. On the 1st flight, I let them go until flameout. If that's symmetrcail, then on subsequent flights I just continue to fly to flameout at full throttle. If it's asymmetrical and I can't solve this by tinkering with the part placement, then I note the altitude that happens at. This is almost always a bit over 37km for planes with 2x ram/shock intakes per engine. So then on subsequent flights, I hit the action group that switches from jets to rockets at 36km, still without touching the throttle. That way, the rockets kick in at full throttle immediately.

It definitely sounds like a play-style difference. I ride the airbreathers as high as I can, until their thrust is only a few newtons each at 50-55km and over 2000m/s and my ap is safely out of the atmosphere, then do a fairly small rocket burn much further downrange to circularize. They're both valid strategies (after all, we both get to orbit ) though yours seems to take less flight time.

I have occasionally noted instances of asymmetrical thrust prior to flameout. However, when this happens, it usually starts about 25-26km, way below flameout height. This is a sure sign the part order of engines and intakes is buggered and I've always found that I can solve this problem by reattaching all the intakes and engines in the correct order (like I thought I'd done the 1st time). But even though I can cure the asymmetrical thrust, this doesn't always prevent asymmetrical flameout, even though you'd think both were caused by the same issue. So that's a mystery to me. Still, I've got a simple workaround for that. So these days, with the automatic throttle system, I find getting spaceplanes to orbit requires less attention and control intervention on my part than launching a rocket. It's a huge change from the old days of it being a very intense and stressful ordeal lasting 10-20 minutes. I don't think this is because I"m particularly good at flying or designing spaceplanes, but because I'm lazy so I focused my attention on those aspects that make the job easier .

I don't think asymmetric flameout can be completely eliminated, just delayed as you say. If I understand it correctly, even if you evenly divide your intakes between identical engines one engine will always flameout before the other within a frame, meaning the engine immediately after it gets the air the first engine didn't use and has enough to continue operating. The second engine essentially has both sets of intakes feeding it until either that amount is not enough to keep it going or the throttle is dialed back enough to allow the first engine to restart.

Edit: That's an impressive plane, I'm only up to killing kerbals six at a time.

Edited October 16, 2014 by Red Iron Crown

[Wanderfound]

Edit: That's an impressive plane, I'm only up to killing kerbals six at a time.

Still plenty of room for more:

[Geschosskopf]

It definitely sounds like a play-style difference. I ride the airbreathers as high as I can, until their thrust is only a few newtons each at 50-55km and over 2000m/s and my ap is safely out of the atmosphere, then do a fairly small rocket burn much further downrange to circularize. They're both valid strategies (after all, we both get to orbit ) though yours seems to take less flight time.

I'm guessing you must use more intakes per engine than I do . I can't get above 37km on jets with 2 intakes per engine, at least in multi-jet planes. For some reason, a single-jet plane with 2 intakes will get to 40km or a bit higher. Yet another mystery of IntakeAir.... But anyway, even with the lower flameout altitude, I can still get to 2200-2300m/s on jets. The rockets still have to get the Ap out of the atmosphere (usually) but that's not hard given the thing's going so fast already.

Because I skimp on thrust, my bigger spaceplanes typically need 15 minutes or more to reach orbit. But my small ones usually make it in half that time.

I don't think asymmetric flameout can be completely eliminated, just delayed as you say. If I understand it correctly, even if you evenly divide your intakes between identical engines one engine will always flameout before the other within a frame, meaning the engine immediately after it gets the air the first engine didn't use and has enough to continue operating. The second engine essentially has both sets of intakes feeding it until either that amount is not enough to keep it going or the throttle is dialed back enough to allow the first engine to restart.

I've found that if you make nacelle assemblies of 1 engine, its fuel tanks, and all its intakes all in 1 inline stack, you'll get symmetrical flameouts even with multiple nacelles. But if any intakes aren't on the same inline stack, then you'll probably have asymmetrical flameout no matter what you do, although you can at least minimize its effects.

For example, this blasphemous abortion from the bad old days before SP+ parts always flamed out perfectly symmetrically.

As you can see, it had all its intakes, fuel, and engines in linear nacelles. And it used RAPIERs, so it's not like I totally hate them. They were the best choice for this particular..... not "design", no, I was just drunkenly sticking random parts together . This was the BUS (Butt-Ugly Spaceplane).

Edit: That's an impressive plane, I'm only up to killing kerbals six at a time.

Thanks, but as you can see, I don't make pretty spaceplanes. They work well enough but I'm embarassed to show pics of them. That 18-seater one reminds me of a flattened bug so I call it the "Swatted Fly" .

[Geschosskopf]

Still plenty of room for more:

You've found a way to bottle genocide. I love it .

[Wanderfound]

You've found a way to bottle genocide. I love it .

Carries more passengers than a De Havilland Comet, BTW. Not a lot of range, however; it pretty much empties its tanks getting to orbit, even with a high-efficiency shallow ascent profile. The long-haul version replaces the nacelle economy cabins with extra tanks, and the aerospikes and rearmost cabin with a pair of nukes.

The pictured version will do interplanetary if you refuel at each end, though.

[Brainlord Mesomorph]

Well that's not that strange. Depending on the size of the plane, maybe 2 rapiers in air breathing mode wasn't enough, nor was 4 turbojets. 6 air breathers drops to 2 rockets @ 30k, I don't see why that can't work. His alternative would have been 6 turbojets and 2 aerospikes?

Precisely this.

It's an odd vehicle. I call it the Hawk Command Module. Going to be the CM for the flagship of my Joolian Explorer Fleet. SSTO spaceplane+CM+Laythe Lander with a payload bay full of science.

To function as CM it has to be built around a Clamp-o-tron Sr. (not a typical spaceplane part) and I figure there aren't going to be any runways on Laythe, so the thing has parachutes, landing struts, and is a tail-lander.

And how did that other guy make his plane RED?

[Padishar]

And how did that other guy make his plane RED?

Probably the KerbPaint mod...

[Red Iron Crown]

I'm guessing you must use more intakes per engine than I do . I can't get above 37km on jets with 2 intakes per engine, at least in multi-jet planes. For some reason, a single-jet plane with 2 intakes will get to 40km or a bit higher. Yet another mystery of IntakeAir.... But anyway, even with the lower flameout altitude, I can still get to 2200-2300m/s on jets. The rockets still have to get the Ap out of the atmosphere (usually) but that's not hard given the thing's going so fast already.

Because I skimp on thrust, my bigger spaceplanes typically need 15 minutes or more to reach orbit. But my small ones usually make it in half that time.

I use three ram intakes or more commonly six of the new structural intakes per turbojet, not sure of how the structural ones compare. I'd classify it as light airhogging though my intakes always have a clear path for air. 10-15mins to orbit is pretty typical for me in small planes (all I do so far).

I've found that if you make nacelle assemblies of 1 engine, its fuel tanks, and all its intakes all in 1 inline stack, you'll get symmetrical flameouts even with multiple nacelles. But if any intakes aren't on the same inline stack, then you'll probably have asymmetrical flameout no matter what you do, although you can at least minimize its effects.

I haven't had luck with this approach so far, though I haven't used traditional nacelles but more along the lines of the side fuselages as in Wanderfound's example. I'll have to try being more careful in its assembly (those side pods get placed and removed so often as the design is refined).

Thanks, but as you can see, I don't make pretty spaceplanes. They work well enough but I'm embarassed to show pics of them. That 18-seater one reminds me of a flattened bug so I call it the "Swatted Fly" .

They certainly are illuminating though, it is easy to tell exactly how many of what parts are where. Some of the more aesthetic "sculpted" designs clip or hide parts which can make it difficult to tell exactly what goes into making it work.

[Alshain]

Spaceplanes run a wide spectrum in their ratio of thrust to wing area. The less wing you have, the more the spaceplane is like a rocket so the more thrust it needs just to keep itself heading up. OTOH, the more wing it has, the less thrust it needs because even a little thrust, if allowed to act for a long enough time, will eventually get it fast enough while the wings do the work of keeping the thing in the air. And it gets to use this thrust more efficiently for increasing speed because the bigger the wing, the less angle of attack it needs to maintain and gain altitude, so the thrust is almost all applied in the desired direction.

Snipped for brevity. I just wanted to say your plane there is a fraction of the size of my largest plane. At the time, you didn't know what he was building. More wing surface won't help on a big enough plane.

[Geschosskopf]

It's an odd vehicle.

Somewhat, yes. OK, I can see what you'd have both RAPIERs and turbos on the same ship now. You do have a space issue and need lots of thrust for a vertical launch. I was just asking because for conventional planes, it's usually not the best option.

I haven't had luck with this approach so far, though I haven't used traditional nacelles but more along the lines of the side fuselages as in Wanderfound's example. I'll have to try being more careful in its assembly (those side pods get placed and removed so often as the design is refined).

Yeah, any time you take something off even just to slide it forwards or backwards a bit to tweak the CoM/CoL, you screw up the magic part order. So once I have the airframe built and balanced, I have to remember next to take all the engines and intakes off and put the back on again in the correct order. Often this requires temporarily enabling part clipping because by then the rest of the airframe often interferes with putting the intakes and engines back on, even though they weren't put on with clipping to begin with.

Snipped for brevity. I just wanted to say your plane there is a fraction of the size of my largest plane. At the time, you didn't know what he was building. More wing surface won't help on a big enough plane.

Sure, that wasn't a huge plane, although fairly large for being 100% stock. Of course you can make runway-crushing monsters with the enormous B9 parts.

But it seems to me that it's often the case that most of the size and weight of spaceplanes is unnecessary for the payload you're trying to lift. And the cause of this seems to be that many folks, probably due to habit formed from building rockets beforehand, greatly overestimate the amount of thrust a spaceplane really needs. Sure, if you're trying to deliver full orange tanks to your fuel station, then you'll need a pretty huge spaceplane. But most things spaceplanes carry are low-density things like probes, crew cabins/station modules, etc. They might take up a fair amount of volume but don't weigh a lot. Because these might require a fairly large airframe to enclose them, though, folks assume it will need a lot of engine, and thus a lot of fuel, and then this snowballs. When they try to launch, they don't quite make it, so then they add more thrust and more fuel, etc., and pretty soon the runway is collapsing under the plane's weight. And because the plane is so heavy with all the fuel and big engines, you can't really put enough wing on it to get enough lift that way. The net result is they end up building a plane-shaped rocket that would go straight up just fine but they insist on trying to fly it horizontally.

So, if you realize thrust isn't all that important for a spaceplane, you can eliminate the majority of its weight by using fewer, smaller engines and a lot less fuel. Then for any given size of wing, you have a much smaller wing loading so the wing does you more good, meaning you need even less thrust. And thus you avoid building runway-crushers entirely, unless you have a legitimate need for them such as lifting full orange tanks.