Best way to launch a spaceplane?

[Xyphos]

I've done lots of SSTO space plane launches and I've never really given this much thought.
what's the best / optimal way to launch a spaceplane?

most of my flights usually take off from KSC then skim the surface of the ocean until I get to 900+ m/s surface speed then carefully pitch up to about 25 degrees, by the time I switch to rockets my AP is already above 40Km just from the upward momentum.

some of my flights have held a specific angle in which to increase both altitude and horizontal speed at the same time, but usually consume more jet fuel.

and fewer of my flights have slowly climbed to about 18Km then level-out until heating becomes an issue, but by the time I pitch up to use rockets, I've lost all that dV to drag forces.

Edited July 16, 2016 by Xyphos

[Alshain]

The second option is your best bet.  You need to find the right angle to take off so that you can continue to climb while evenly gaining speed.  Too much speed too low, and you overheat, too much altitude gain means your engines lose air too quickly and your orbital speed does not increase as much.  Every time you alter your velocity vector away from prograde you gain drag and take a cosign loss, so you want to avoid it once you get it where it should be.

Edited July 16, 2016 by Alshain

[Snark]

Moving to Gameplay Questions.

[bewing]

Basically, Alshain is exactly right. But with spaceplanes, it's always more a question of "whatever works." During the rocket phase you burn 8 million times more fuel than during the airbreathing phase. So if one method of climbing uses 11 more fuel than another during the airbreathing part -- it's basically irrelevant.

[Van Disaster]

If you're worried about losing small amounts of jet fuel to drag then the answer is most definitely not doing 900m/s at sea level...
 
A gentle acceleration climb is the thing, just try and keep a fixed AoA if you want a reference.

[FyunchClick]

Definitely agree with the steady climb, usually at < 20 degrees. Setting wings to a 5 degree incidence as the pros advised was a revelation for me and makes it much easier to steadily climb at a minimal body drag/off-prograde engine burn.

[Xyphos]

39 minutes ago, FyunchClick said:

usually at < 20 degrees. Setting wings to a 5 degree incidence

I usually hold 12 degree pitch, but what's this about wings 5 degrees?

Edited July 17, 2016 by Xyphos

[bewing]

18 minutes ago, Xyphos said:

I usually hold 12 degree pitch, but what's this about wings 5 degrees?

In the SPH, you can use the rotate tool to make the wings have a built-in AoA. An extra 5 degree tilt on the wings means you only need 7 degrees of AoA on the fuselage, which reduces your drag a bit -- and aims the engine along your prograde better.

[Xyphos]

That's GENIUS!
why didn't I think of that?

[bewing]

It actually changes the feel of the plane quite a bit. You may not be happy.

[Xyphos]

14 minutes ago, bewing said:

It actually changes the feel of the plane quite a bit. You may not be happy.

yeah, the ksp areo model isn't all that perfect, either.

[AeroGav]

Your goals are threefold

1. reach the highest speed and velocity your airbreathing engines can give you before switching on the much less efficient rockets.

2. fly at an angle of attack (angle of wing surface relative to direction of travel ) that minimises drag losses throughout the ascent.  That going to be about 5 degrees at high speeds and about half that when subsonic.    If you have a draggy fuselage then  the optimum angle may be a slightly more , since flying at a higher AoA creates more lift and will mean you are higher up and in thinner air for a given airspeed compared to being at a lower AoA.

3. be mindful of the effect of your airspeed on jet engine output so as to maintain a healthy thrust weight ratio

Let me expand on the third point and how it conflicts with the second -

At 10km, altitude,  my Spaceplane can fly subsonic with an angle of attack of 4 degrees, getting a L/D ratio of 6 to one.  Or we could go supersonic and fly at 1.4 mach at  2 degrees, and only have a 3:1 L/D ratio.

Most of the time, your goal is to maximise L/D, so you should be subsonic right?

Well, in this case, no.

At this altitude, our Rapier engines are feeble in subsonic flight, our total thrust is only 80kn  vs drag of 60kn.      So, three quarters of our thrust being used "treading water", gravity losses IOW.

Supersonic, we have double the amount of drag - 120kn.   But our engine quadruples in power, giving us 240kn thrust - only half of our power is being wasted "treading water", so to speak.   In practical terms, we're using fuel four times as fast but will gain energy at six times the rate , with 120kn excess thrust intead of 20kn excess thrust, so this phase of the climb will be over six times faster.

Note, the above all assumes your rocket and jet engines are running off the same fuel source eg.   a whiplash/nerv powered spaceplane.   If the rocket engines have their own, separate tanks it is in fact better to be almost at the point of running out of jet fuel when the jets shut down for lack of air, so your rocket engines have less dead weight to carry to orbit.     And even given all that, however you fly up to 20km is still small beer.   Making sure you hit max velocity on your jet engines before leaving the atmosphere is the most important consideration by far.

 

There is no "one size fits all" however.   The best profile depends on 

a) your engine selection

b) TWR - this particularly effects climbing angle, which is why it's so meaningless when people talk about climb at this or that angle etc.    AoA is independent of TWR  or how fast your climbing, it's the universal truth.

c) wing area vs craft weight

d) heat tolerance

e) parasite (ie not lift-induced) drag from fuselage etc.

Here's an annotated video of a flight i did, showing the thought processes behind launching this particular aircraft, hope it helps

 

 

[FyunchClick]

19 hours ago, Xyphos said:

That's GENIUS!
why didn't I think of that?

Thank @Val (or else @GoSlash27, I don't remember exactly) who was kind enough to bring it up in one of the many excellent posts both write on the subject.

18 hours ago, bewing said:

It actually changes the feel of the plane quite a bit. You may not be happy.

I actually find it's a lot easier not only to fly and land, but also to take off as you're generating lift just barreling down the runway, and it definitely makes the difference between a SSTO plane that only just doesn't make it to orbit, and one that does (or one that needs to dip around 6-7km to break the sound barrier vs one that can climb at a steady pace).

[GoSlash27]

1 hour ago, FyunchClick said:

Thank @Val (or else @GoSlash27, I don't remember exactly) who was kind enough to bring it up in one of the many excellent posts both write on the subject.

I actually find it's a lot easier not only to fly and land, but also to take off as you're generating lift just barreling down the runway, and it definitely makes the difference between a SSTO plane that only just doesn't make it to orbit, and one that does (or one that needs to dip around 6-7km to break the sound barrier vs one that can climb at a steady pace).

FyunchClick,
 It could've been either of us... or both.

 I like to tailor my wing incidence so that I am flying at zero angle of attack (forward velocity vector aligned perfectly with my nose) at Mach 1.
 This gives me the absolute minimum drag in the transsonic region, which allows me to use the least engine for the job.

Best,
-Slashy

[foamyesque]

It also depends a fair bit on what engines you're using. Trying to spaceplane with Panthers is an interesting exercise; they have very different flight characteristics than Rapiers.

It's very easy to go overboard with wing incidence. Even five degrees is, I find, too much. I tend to prefer basically the smallest increments I can put on them with the rotate tool in non-snap mode, or even just completely flat ones for better performance at hypersonic speeds.

[FyunchClick]

3 hours ago, GoSlash27 said:

I like to tailor my wing incidence so that I am flying at zero angle of attack (forward velocity vector aligned perfectly with my nose) at Mach 1.
 This gives me the absolute minimum drag in the transsonic region, which allows me to use the least engine for the job.

Interesting notion, taking the mach 1 hump is always the hardest I feel, so anything to alleviate that is helpful. I'll give that a go with my next one.

[Van Disaster]

If there really is a proper M1 hump alá FAR, then just diving through it should be enough: the drag profile is something like ___/\--- as you go through the transonic region. I don't use any incidence for FAR spaceplanes, things can get a little wierd at extreme speeds.

[AeroGav]

On 18/07/2016 at 8:15 PM, GoSlash27 said:

FyunchClick,
 It could've been either of us... or both.

 I like to tailor my wing incidence so that I am flying at zero angle of attack (forward velocity vector aligned perfectly with my nose) at Mach 1.
 This gives me the absolute minimum drag in the transsonic region, which allows me to use the least engine for the job.

Best,
-Slashy

I use 5 degrees of wing incidence on some of my designs. 

At hypersonic speeds, eg.  when you are accelerating slowly from air breathing speedrun velocity to orbital velocity on NERV power,  having the lifting surfaces at 5 degrees gives the best lift drag ratio possible.   If the wings are mounted to the fuselage at 5 degrees,  then you can keep the fuselage at zero degrees, ie. minimum drag, flying on prograde autopilot, whilst the wings are at the optimum 5 for lift:drag ratio.  If your craft has generous wing area it will never need to exceed that amount of nose up to get sufficient lift.

This approach gives the lowest possible drag for sustained flight (ie, not arcing downwards following a zoom climb) in the hypersonic regime. 

However, the downside is that it is less efficient lower in the atmosphere.

Ie. where the air is thicker,  5 degrees angle might give too much lift, so you end up angling the nose down with the fuselage at -2 degrees AoA to get the wing AoA down to something reasonable.   The negative AoA on the fuselage generates drag just as positive AoA does on a plane with flat wings, with the additional downside of generating negative body lift.

 

Also, when it comes to crossing the sound barrier, I'd say that having wings with incidence is a slight disadvantage.   This is because at this point, you don't want the lowest possible drag for sustained flight

ie  wing at 5 degrees, body at 0, whilst generating lift equal to the plane's weight.

You simply want the lowest possible drag, period

ie. wing at 0 degrees, body at 0 degrees

no lift will be made, and your flight path will arc downwards into a dive, you establish an upward path again once over 400m/s with your engines feeling the full Honda VTEC goodness.

Not a great example as i don't go to zero AoA, but at 8 minutes and 50 seconds, 13km up, you see me shove the nose down and light up the NERVs to get through the sound barrier.