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Hey there! I’m Cruzan, bringing you another episode from the KSP Quick Guide Series. This
video will cover everything you need to know about flying at high altitudes! High Altitude
flying is very useful and is the quickest and most efficient way to get around Kerbin
without leaving the atmosphere. This is the last step before going to space. Understanding
how your plane operates at high altitudes will make you a much better player and the
transition from planes to SSTOs from this point will be a breeze! So let’s get started
There is a fairly big difference between regular and high altitude designs. The most obvious
changes will be swapping out the Basic Jets for TurboJets. The Basic Jets are the more
efficient and powerful of the two jet engines initially, but once you start flying higher
and faster you’ll want to swap them out for Turbos. The TurboJet gets more efficient
the higher you climb meaning that its specific impulse or Isp is dependent on altitude. The
thrust however is dependent on surface speed. Here is a graph that better illustrates these
concepts. You can see that the Turbo has its max specific impulse at 0.3 atmospheres or
at around 6000m and its maximum thrust comes when it hits 1000m/s surface speed. Just stuff
to keep in mind, but the basic gist of the matter is if you want to fly high and fast,
slap on some Turbos!
For air intakes the go-to selection is the ram-air intake. It performs the best out of
the three options and is designed for exactly what we’re trying to do. The main thing
to think about is if you’ll need to angle your intakes at all. This is only really a
concern if your plane doesn’t have a lot of lift, which causes you to have to keep
your nose well above your prograde marker. The intakes work best when they are aimed
directly at the direction of travel. If you only need to pitch up less than 5 degrees
then I wouldn’t worry about it.
Mounting the air intakes can be tricky, but it mainly comes down to whatever you think
looks good. You can use extra tanks, tail connectors or structural parts like the cubic
strut. My personal preference is to use the cubic strut. You can place them anywhere which
is very handy and they can easily be angled if you them to be. On top of that you can
actually mount two intakes to it if you want which is the basis for airhogging. I’ll
cover air-hogging when we start talking about SSTOs! Whatever you do though, please, PLEASE
don’t mount your intakes like this. I think it looks terrible haha.
A good ratio to aim for with high altitude designs is about 1 to 3 intakes per engine.
The more you add, the higher you’ll be able to climb so feel free to add more if you feel
like it! I typically add extra parts like this adapter to make things look nicer, but
these parts add weight and drag, so keep that in mind.
Something you won’t have experienced when flying at low altitudes is the dreaded flameout.
Flaming out is when an engine runs out of the required amount of air to keep the itself
running. This causes it to shut down and in almost every case this is a bad thing. There
are some ways to design around this to make it less of an issue, but the easiest way to
counter flaming out is to know your performance ceiling and understand how throttling down
can actually prevent flaming out (up to a point.) The basic rule of thumb when it comes
to intake air for an engine is that it needs 0.1 units of intake air to continue running
at max throttle. It can usually run a little below this if you want to push your luck,
but typically once you hit 0.1 units you should begin thinking about throttling back slightly
to safely avoid flaming out. You can gauge what throttle you need to be at by thinking
of the intake air value as a percentage of your throttle. For a single engine this would
mean 0.1 for 100% throttle, 0.05 for 50% throttle and so forth. These give you a decent buffer
and should keep you from flaming out. Multiply these values by however many engines you have,
so 0.3 at 100% throttle with 3 engines for example.
Your plane gives some warning signs when it is about to flameout if you’re climbing
gradually. You can usually hear weird sounds start happening. Also if you right click on
your engines, you can actually see which engine is going to flameout by looking at which one
has the lowest thrust. This will usually cause your plane to pull to one side and want to
ayw even though you haven’t flame out. You can use this sign as good indicator that you
need to throttle back. If you do flameout you should instantly throttle down o hopefully
kickstart the dead engine right away. At low throttle you shouldn’t be put in to as much
of a death spin.
The other way to avoid major flameout issues is to design your plane using an odd number
of engines. The reason behind this is that you can control which engine flames out first!
KSP chooses the last engine placed on to the plane as the first one to flame out. What
this means is that if you place one engine on to either wing first using symmetry, then
place another engine on the tail of your plane, that tail engine will flameout first which
won’t cause your plane to go in to a flatspin! If you have an even number of engines, like
2, one of the two engines will flameout which will instantly put you in to a flatspin. This
is a trick that I’m not sure a lot of people know about so use it wisely. Having the middle
engine flameout lets you push the plane a little harder instead of trying to preemptively
throttle down and avoid flaming out. With this system you just go until you flameout
and then throttle down without risking any real major problems!
Another way to avoid flatspinning is to just put a ton of reaction wheels on your plane.
It’s pretty cheesy, but it works. If you’re okay with that then go for it.
The last tricky thing about flying at high altitudes is the lower amount of atmosphere
present. This means that your control surfaces will have less of an effect the higher you
climb. You’ll quickly find making small adjustment much harder. The ways to counter
this are to either add more control surfaces or to add reaction wheels. I recommend a good
blend of the two. Avoid going all out on control surfaces or all out on reaction wheels if
you can. Although the infiniglider characteristics of having tons of control surfaces can be
kind of fun to play around with sometimes. I see some designs that use RCS, but I don’t
support it for flying around in atmosphere. RCS is primarily used in space and even then
you don’t need very often. I prefer using reaction wheels when I can as they are lighter
overall and it’s one less resource to worry about in the end.
Alright that covers about all the high altitude design aspects. I recommend starting out small
with a single engine and two to four intakes, similar to this design. The single engine
means we won’t have flatspin problems if we flameout which is great for learning how
to control the throttle. The extra intakes will let us climb fairly high as well.
Flying is fairly straightforward and if you’ve watched my previous plane tutorials you should
be a pro by now. The only issues you’ll run in to is the lack of control that I mentioned
earlier. Depending on how many reaction wheels you have on your plane your experience may
vary.
When you’re first learning I recommend climbing at a fairly shallow angle, like 45 degrees.
Open up your resources tab and keep an eye on your intake air. At about 10km you’ll
see it start to drop. I try to make sure that I’m flying within 5 to 10 degrees of horizontal
by the time I hit 0.2 units of intake air. This gives me a little time to flatten out
and also makes sure I don’t flameout too quickly. Ideally you want to fly as long as
you can at max throttle. Whatever altitude you hit that leaves you at 0.1 units of air
at max throttle is what I call the acceleration ceiling. I have no clue what an actual term
for this might be haha. You want to make sure you are flying perfectly horizontal at this
point if you can with as little vertical speed as you can. Aim for somewhere between 10 and
-10m/s on the vertical peed gauge. Based on how much lift your plane has with
respect to the weight of your plane you might not be able to be pitched directly at the
horizon. As I said earlier, if you find that you need to be pitched up more than 5 degrees
you might consider designing your intakes with a slight downward angle. The more direct
contact they have while flying means you’ll have more air to work with!
After a while you’ll notice your acceleration decreasing. Once it gets slow enough you can
consider actually raising your altitude. This will require pitching up and watching out
for flameouts. Throttle back accordingly as you climb, keeping an eye on your intake air
in the resources tab. Play around with leveling off as you climb to gain extra speed. As your
acceleration slows down again feel free to climb some more. The more you climb and the
less throttle you have to use, the more efficient your flight becomes as you’re burning less
fuel, but flying as fast or faster than you were at a lower altitude. Typically you can
push this all the way to where you only have 1 to 2 ticks of throttle left. I call this
the cruising ceiling as this will be where you spend most of your time flying.
If your design is good enough you might even be able to poke yourself in to space. It will
be a purely suborbital hop as it is impossible to go fully in to orbit without some form
of rocket engine. If you find your cruising speed goes over 2100 m/s surface velocity
or 2350m/s orbital your apoapsis will slowly climb, even though you are flying horizontal.
Fly like this long enough and you’ll eventually see your apoapsis climb above 70km. Congratulations!
This is the basic principle behind flying SSTOs even though you won’t be able to get
to orbit just yet. The next video will be the SSTO tutorial and I’ll help you get
that extra kick you need to get in to orbit! It takes some tinkering, but it if you have
gotten this far with your plane design then it will actually be pretty easy! As always,
thanks for watching. If this video helped you out please press the Like button down
below! Take it easy!