How to use engines and air intakes in a SSTO?

[Ferrius]

Hello there!

I've been playing KSP for a while, but I mostly use rockets, since I like them most.

I've built some planes but never a SSTO. At least a working one.

So I started to learn a lot about this things. I've seen in many videos that they close intakes at certain altitude, near atmosphere limit, and the engines keep running (RAPIER). Also, they angle the craft almost horizontally once in atmosphere, and they fire the NERV really late. 

I would like to get some guidelines about this matter. Thanks!

[Vanamonde]

This question about how to play the game has been moved to Gameplay Questions. 

Rapier engines will switch from air-breathing mode to rocket mode automatically, or by player command. In rocket mode, the intakes are not being used, and can be closed to reduce drag. 

Building up speed is more important than climbing quickly, which is why the planes are flying nearly horizontally (which I think is what you're asking about?). 

[bewing]

As I understand it, closing the air intakes was for a previous KSP version, and no longer makes any difference.

Some guidelines beside the standard stuff of minimizing weight and drag: you need to have enough wing that you don't need an excessive AoA when you are below 20km, you want to minimize the number of airbreathing engines because they become deadweight later, you want to run the airbreathing engines as long as you can because they use almost no fuel to produce thrust, you want to delay igniting the rockets as long as you can because they all use up your fuel really fast (and a spaceplane takes a long time to get to orbit), you want to get your speed as high as you can before igniting the rockets, and there is a danger zone between 20km and 40km where the speed you need to fly (or can fly) can also easily be enough to heatsplode most spaceplane noses -- so you always need to figure out a trick to get through the danger zone.

There are several workarounds for the heatsploding problem. Beyond that, there have been many threads created in this forum about tips for building spaceplanes. Or you will have to narrow down your question.

Edited July 17, 2016 by bewing

[AeroGav]

Part of the reason you see people flying nearly horizontal is that thrust falls off severely as jet engines get above 20km.  Like it halves with every additional 1.5km or so.  So, by the end you have very little excess power leading to a shallow climb angle.   Specific impulse is constant, so fuel consumption falls along with thrust, so it's just like the game is forcing you to throttle back.   However if you've got to the point where your jet engine is only making 101kn thrust and it takes 100kn just to sustain level flight,  it's time to bring a rocket engine into the play even if its ISP is worse.

Note that as well as power falling off with increasing altitude, going too fast has the same effect.   The R.A.P.I.E.R  peaks at mach 3.7, beyond that, the faster you go, the less power it produces.   So milking the jet engines for all they are worth involves a compromise between going fast and high.   In the case of the RAPIER,  it does hold on to 80% of its performance up to mach 4.5, after that the decline is steep -  zero thrust by mach 6.   

I generally look for mach 3 by 18 or 19km,   but try to stay below mach 4 under 20km to avoid excessive drag losses/heat.  After that i am for about 4.3 ish then steepen the climb as needed to keep me under mach 4.5.  By 23 or 24km I find I can no longer climb without loosing speed, so it's time to start up a rocket engine or two.

[bewing]

Oh, as far as general tips about air intakes goes: many of the air intakes cannot provide enough air at the top speed of the engines. Some of the air intakes also have a fair bit more drag than others. But weight is also a factor. Putting it all together, shock cones are the intakes that I'd recommend.

[dire]

What is Mach 1 at 20km? Like 300 m/s? So Mach 4 would be 1200m/s?

[Hodari]

10 hours ago, Ferrius said:

So I started to learn a lot about this things. I've seen in many videos that they close intakes at certain altitude, near atmosphere limit, and the engines keep running (RAPIER). Also, they angle the craft almost horizontally once in atmosphere, and they fire the NERV really late.

The exact ascent profile and best time to switch engines/mode will vary for each craft, so you may have to experiment a bit there.

For most of the ssto's I've built, I use only the RAPIER engines.  While still in the SPH, I set them all to manual switching and then set up an action group which toggles the mode on all of my engines and also toggles all of my air intakes.  I usually maintain a fairly constant angle of attack of around 15-20 degrees and then pitch up or down slightly as needed to control my speed.  The RAPIERS sometimes seem to have a hard time getting past 320-350m/s, so might need to level out for a little while there.  Once you're comfortably past Mach 1 though, the engine performance improves rapidly and you can climb again.  At around 23km altitude, you'll notice your acceleration starting to drop off as your engines are no longer getting enough air.  This is when you need to hit the action group to switch over to rocket mode.

[AeroGav]

5 hours ago, Hodari said:

The exact ascent profile and best time to switch engines/mode will vary for each craft, so you may have to experiment a bit there.

For most of the ssto's I've built, I use only the RAPIER engines.  While still in the SPH, I set them all to manual switching and then set up an action group which toggles the mode on all of my engines and also toggles all of my air intakes.  I usually maintain a fairly constant angle of attack of around 15-20 degrees and then pitch up or down slightly as needed to control my speed.  The RAPIERS sometimes seem to have a hard time getting past 320-350m/s, so might need to level out for a little while there.  Once you're comfortably past Mach 1 though, the engine performance improves rapidly and you can climb again.  At around 23km altitude, you'll notice your acceleration starting to drop off as your engines are no longer getting enough air.  This is when you need to hit the action group to switch over to rocket mode.

If it's a three or four engine ship,  you don't necessarily want to switch them all at the same time.

For example, you might be unable to climb beyond 23.5km in airbreathing mode, but if you switch the centreline engine over to closed cycle (on a three engine ship) or switch the inboard pair (on a quad), you now have a comfortable margin of power over drag again, but you can still milk the atmosphere for whatever airbreathing thrust is left on the outboards, all the way up to flameout at 29km when they'll automatically switch.

[RuBisCO]

I have always wondered if opening and closing intakes actually has any effect on drag, I just close the intakes just in case once all the engines have switched to rocket mode.

With my most recent spaceplane I can put ~60 tons of cargo in it, run off the the runway with a set pitch of 20-25° all the way to a 80 km apoapsis. I just manually switch most of the engines to rocket mode once velocity begins to decrease and close the intakes once all the engines automatically switch to rocket mode. I can squeeze a little more performance out of it by reducing my pitch as velocity increases at the cost of increase heating. I use a shielded docking port as a nose cone because it is basically a cheat in how much heat it can take: I can get all the way to mach 5 at 20 km before a temperature bar even appears. 

 

[FyunchClick]

5 hours ago, AeroGav said:

For example, you might be unable to climb beyond 23.5km in airbreathing mode, but if you switch the centreline engine over to closed cycle (on a three engine ship) or switch the inboard pair (on a quad), you now have a comfortable margin of power over drag again, but you can still milk the atmosphere for whatever airbreathing thrust is left on the outboards, all the way up to flameout at 29km when they'll automatically switch.

I think you're thinking pre 1.x release. Nowadays, I find intakes don't matter much, except at takeoff, where I found that having few intakes at little to no speed makes my engines flame out as they're ramping up thrust after ignitoin. Just a few will do nicely until just over 20 km, at which point even rapiers will rapidly run out of power regardless.

The reason for a shallow climb (pitch for me usually <20 degrees, and with wings set at a 5 degree incidence) is to run up enough speed before you get to 20 km, where jets dramatically start to lose thrust (rapiers can hold out the longest, but are quite atrocious until you can ramp them up to mach 1.3/1.4 or so). Specifically, the window between 7 km and 20 km is where rapiers run best for me, and the TWR is terrific until it is very suddenly not.

Running this bit shallow makes you come out of it at higher speeds, but of course running it too shallow just keeps you in the atmosphere that much longer which is draggy and comes with a serious overheating risk).

So, tips I took away from the pro's:

* Wings at incidence so you can climb pointing prograde (dramatic drag reduction).
* With this version of KSP aero, go for steady shallow climb.

21 hours ago, Ferrius said:

(...) I would like to get some guidelines about this matter. Thanks!

I'm sure there's better examples and tutorialsout there, but this shows me running a quite heavy, very inefficient and relatively low TWR into orbit.

For one, if you want an efficient ascent, mix rapiers with other types of jets to make the run up to mach 1.4-ish more efficient; they'll sputter out before the rapiers do but the rapiers will have enough thrust left to compensate and then some when they do.

Oh, and set the rapiers to switch mode manually, then bind the mode switching to a hotkey that also toggles the intakes.

[AeroGav]

53 minutes ago, FyunchClick said:

I think you're thinking pre 1.x release. Nowadays, I find intakes don't matter much, except at takeoff, where I found that having few intakes at little to no speed makes my engines flame out as they're ramping up thrust after ignitoin. Just a few will do nicely until just over 20 km, at which point even rapiers will rapidly run out of power regardless.

 I never took part in the Beta,  my statement had nothing to do with intakes.  To be more specific, imagine this situation as you approach your airbreathing cieling at mach 4.3, 23500m up -

You have three RAPIERs in Airbreathing mode, each is only making 30kn in these conditions and of course the higher and faster you go the worse this gets.

Total thrust = 90kn

Total drag = 80kn,  which is why you can barely climb without loosing speed

Switch over the centre engine to Closed cycle, it makes 180 KN.   The left and right motors still only do 30 each, but that gives a total thrust of 240kN which is comfortably more than your drag, still 80kn.   You can climb again, whilst still milking the atmosphere for its remaining jet power up to the point the RAPIER flame out at 29km.

 

[FyunchClick]

2 hours ago, AeroGav said:

(...)AirSwitch over the centre engine to Closed cycle, it makes 180 KN.   The left and right motors still only do 30 each, but that gives a total thrust of 240kN which is comfortably more than your drag, still 80kn.   You can climb again, whilst still milking the atmosphere for its remaining jet power up to the point the RAPIER flame out at 29km.

Ah I get it now. It's about optimizing your fuel consumption even as your thrust dwindles. With the rate of climb you get at that point (several hundreds of m/s vertical I guess), do they last long enough before flaming out to be worth it?

I tend to design with a comfortable (wasteful?) margin of oxidizer as I usually include some rocket powered VTOL configuration for airless world landings, so that's an optimization that's never really been on my radar.