Which and how many intakes to use?

[seaces]

Hey there!

So finally made my mind to try and build some nice space plane. So far I am able to make some nice plane which is not able to get into orbit for several reasons. One of them is I run out of intake air around 20-26km. I am using turbojets and I cant figure it out which intake is best and how many of those are needed for a turbojet to make it work for longer time. Also I have heard about those RAPIERs. Should I use them instead of those turbojets?

[Bobe]

I'm still a bit of a spaceplane novice, but I can get an SSTO into LKO with a little bit of fuel left, nothing that can leave Kerbin orbit yet.

As far I know, the shock cone intakes are the best, but I tend to supplement them with one or two pairs of structural intakes.

I've tried a lot of different combinations of engines, but the bottom line is a RAPIER or two is more or less essential to boost you out of the atmosphere. Turbojets work great in atmosphere to get your speed up, but once you are forced to shut them down you need to have something running. RAPIERS run for longer with less intake air before they switch modes, but it is a matter of whether they will provide enough thrust by themselves.

I haven't tried toroidal engines yet, they seem like they would be ideal upper atmosphere/vacuum engines, but they wouldn't provide any thrust from the ground (unless you wanted to waste oxidizer).

I've been trying to optimize a configuration of two turbojets, two RAPIERS and one LV-N, but balancing the fuel and weight is troublesome.

[Jouni]

Shock cones provide most air per part, while ram intakes provide most air per intake mass. The differences between the two are small enough that it doesn't really matter which one you use.

Having 1 intake per 10 tonnes of vehicle mass is generally enough to reach orbit with a reasonable efficiency. Doubling that to 2 intakes/10 tonnes increases the efficiency significantly, while doubling it again to 4 intakes/10 tonnes provides only minor benefits. Increasing the number of intakes beyond that yields only negligible benefits, unless you actively manage the number of open intakes to reduce drag.

Having 1 intake/10 tonnes should get you to around 32 km before engine flameout, and even higher if you manually throttle down before that. Every time you double the number of intakes, that altitude increases by 3.5 km. If you run out of air sooner, you should reduce you climb rate above 20 km and try to accelerate more. The faster you fly, the more air you will get from intakes, and the higher you can climb.

RAPIERs and turbojets are more or less the same in KSP. Any differences between them only become apparent, if you try to maximize the efficiency. If you use RAPIERs, you should switch between airbreathing and closed cycle modes manually (preferably using an action group), as the automatic switching usually happens too soon.

[Bobe]

Having 1 intake per 10 tonnes of vehicle mass is generally enough to reach orbit with a reasonable efficiency. Doubling that to 2 intakes/10 tonnes increases the efficiency significantly, while doubling it again to 4 intakes/10 tonnes provides only minor benefits. Increasing the number of intakes beyond that yields only negligible benefits, unless you actively manage the number of open intakes to reduce drag.

This is confusing. What intake are you referring to? Shouldn't you instead be using intake volume/tonne?

[Jouni]

This is confusing. What intake are you referring to? Shouldn't you instead be using intake volume/tonne?

Either ram intakes or shock cone intakes. The difference between the two is usually too small to matter.

The amount of air stored in an intake is usually irrelevant. Intake effectiveness is essentially a product of two factors: airspeed and intake area. For shock cone intakes, the area is 0.012 units, while the area of ram air intakes is 0.010 units. There is also another intake parameter (intakeSpeed) that is slightly higher for shock cones than for other intakes (12 vs. 10), but I'm not sure what it does.

[seaces]

So by reading this I have also realized that I am not very good in flying space planes efficiently. could anybody explain me how do I fly them properly?

Oh and one more question. In stock game I struggle to build the wing that I have in my mind. I am just not good enough in using the parts. I see on forum a lot nice wing design but when I try something is just cant find a way on how to use the aviable parts to get the wing from my mind in KSP.

[Anquietas314]

So by reading this I have also realized that I am not very good in flying space planes efficiently. could anybody explain me how do I fly them properly?

I can't speak for FAR, but in stock aero you want to climb as steeply as you can while still gaining speed (or holding speed if you have TWR close to 1), until you reach about 10-12km, then start levelling out. You want to be at around 10-20 degrees ascent on the navball by the time you reach 20km depending on how high you can go before running out of intake air. From there, just try to keep the thing going on jet power as long as you can - if you can reach/exceed orbital velocity (can do this with turbojets) that's ideal.

[Jouni]

So by reading this I have also realized that I am not very good in flying space planes efficiently. could anybody explain me how do I fly them properly?

There are basically four phases of ascent for stock spaceplanes:

1. You want to get above 10 km as soon as possible. The plane can only waste time and fuel there due to high drag.
   
2. You also want to get quickly above 20 km, but you need to be careful. If you climb too fast, you don't have enough time to accelerate above 20 km. As a rule of thumb, you should start leveling off above 10 km, and get the climb rate down to 100-150 m/s at 20 km. Once you know the basics, you can start experimenting how fast you can complete this phase of the ascent.
   
3. Jet engines do the real work between 20 km and 32 km, as the terminal velocity increases from around 700 m/s to around 2300 m/s. Keep the ascent rate below 100 m/s for this phase, until you know what your planes can handle. Most planes should be flying at least 1700 m/s by the time they reach 32 km.
   
4. You can keep climbing and accelerating for a long time above 32 km, especially if you throttle the engines gradually down. Eventually the engines either flame out or you're about to die of boredom, so you switch to rockets and finish the ascent.
   

Mods such as MechJeb that tell the precise ascent rate are useful for learning to fly spaceplanes.

[Anquietas314]

You can keep climbing and accelerating for a long time above 32 km, especially if you throttle the engines gradually down. Eventually the engines either flame out or you're about to die of boredom, so you switch to rockets and finish the ascent.

I actually built a plane yesterday that could keep running the engines (at low throttle) all the way out past 69km - at which point there's no atmosphere at all lol. It didn't take very long to get there either, so it can be done (this was also quite a small plane though).

I just noticed OP asked about RAPIERs vs turbojets: RAPIERs can switch to rocket mode late on to get you into space, but can only get up to 2km/s relative to the surface in jet mode - not enough to get out of the atmosphere on jet power alone. Turbojets have a top speed of about 2400m/s, which is enough

[Bobe]

Jet engines do the real work between 20 km and 32 km

I keep reading about people running their engines up to ~30 km, but mine (the turbojets at least) always flameout just before 20 km. I can often get the speed up to about 1,000-1,200 m/s during that phase, but even if I gradually throttle down to keep them running longer, that loss of thrust causes my speed to start decreasing, so I'm forced to shut them down and switch the RAPIERs to closed cycle. As a result I have to burn more oxidizer to get my speed back up and lift up out of the atmosphere. It doesn't help that my TWR is usually less than 1.5 at that point.

[Anquietas314]

I keep reading about people running their engines up to ~30 km, but mine (the turbojets at least) always flameout just before 20 km.

You need more intakes. At least double.

[seaces]

Thnx guys got some good all Mk2 parts space plane up and running then I went on Mk3 parts and yeah I have some questions about it. Now I don't know ho much problems is visible on this pic but I will try to explain it. The very tip of the wings and part of the structure it bends and it kind od seems that it detaches from the rest of the wing but not completely. Sometimes you can see when the tip flexes like that much you can see air through. Do I use these parts in wrong way not properly connected or the wing is too heavy and it needs some support if the later is case what I can do about it?

[Anquietas314]

seaces: that's because adjacent wings that you didn't directly attach together are, well... not attached. You can add struts between them (at the wing tips) to make them stick together, but unless it's making your plane uncontrollable, it's not really worth caring about. Plus it makes the plane a little ugly when there are struts all over holding wing pieces together If you were using FAR it might be a bigger problem though.

As for, erm... "other problems", you probably shouldn't use so many elevon pieces - there are bigger ones for a reason

[seaces]

seaces: that's because adjacent wings that you didn't directly attach together are, well... not attached. You can add struts between them (at the wing tips) to make them stick together, but unless it's making your plane uncontrollable, it's not really worth caring about. Plus it makes the plane a little ugly when there are struts all over holding wing pieces together If you were using FAR it might be a bigger problem though.

As for, erm... "other problems", you probably shouldn't use so many elevon pieces - there are bigger ones for a reason

Well how to make sure they are attached? there are not contact points...the parts do not turn green despite I put the really close together so I don't see any kind of indication are they connected or not.

[Anquietas314]

Well how to make sure they are attached? there are not contact points...the parts do not turn green despite I put the really close together so I don't see any kind of indication are they connected or not.

The parts connect if you place part A onto part B. If it placed properly, it's connected. Otherwise, struts are really the only way to connect them.

[numerobis]

There is also another intake parameter (intakeSpeed) that is slightly higher for shock cones than for other intakes (12 vs. 10), but I'm not sure what it does.

The speed your intake is going is the air speed plus 6x the intakeSpeed. That's +72 m/s for the shock cone, +60 m/s for the ram intake. It makes a bit of a difference when you're at a standstill, but you aren't at a standstill for long; at 1km/s it's already just a 1% difference.