observation during the rocket phase when flying spaceplanes to orbit

[Cirocco]

So I noticed a bit of a weird thing this evening in my spaceplane's behaviour. Maybe others have seen it already (haven't been on the forums in a few weeks) but just in the off chance that you guys haven't, here's a thread about it.

When I fly my spaceplanes to orbit, I usually follow a flight path that has been posted many times here in the past: fly up to about 12km, level out and possibly shallow dive to break sound barrier, go up to 20k and about 1000m/s when you will likely run out of air. Activate rocket engines, pull up hard and boost into orbit.

Now, it's that "pull up hard and boost into orbit" bit that I noticed something strange about. Usually I just angle up to 45-60° and try to get the heck out of the atmosphere as soon as possible. Seems sensible, right?

Well.... not exactly.

in my latest spaceplane, I didn't pack enough oxidizer so I the RAPIERS ran out before I even got apoaps above 70km. I had 2 NERVA's active (was trying to go for a long range spaceplane), but they weren't giving nearly enough thrust to push me out of the atmosphere or give me enough lateral velocity to get orbital speed before I'd reach the apoaps. But I figured, hey, let's just burn em till they run dry. At least I'll see if they overheat or not. Sure enough, the NERVA's never were able to push me to space, and I soon began falling back into the atmosphere.

and then at about 30 km, I stopped falling.

It seems that, given a decent amount of speed (I was around 1600m/s at the time), at around 30 km altitude, spaceplanes will "float" on the atmosphere. This gave the NERVA's more than enough time to increase my velocity past orbital velocity and hey, presto, apoapsis at 120km, and almost all of it was done on NERVA power, meaning it was super efficient.

So yeah, I'm not sure if other people know or do this already, but the next couple of days I'm going to be experimenting with a flight path where my spaceplane levels out again at 30-33km in its boost phase, and goes to orbital velocity there on nothing but NERVA power.

just thought I'd share.

Cirocco

Edited June 4, 2015 by Cirocco

[KerikBalm]

Activate rocket engines, pull up hard and boost into orbit.

Now, it's that "pull up hard and boost into orbit" bit that I noticed something strange about. Usually I just angle up to 45-60° and try to get the heck out of the atmosphere as soon as possible. Seems sensible, right?

I never do that....

I don't level off, but maintain a climb on jets... the climb may be shallow (15 degrees), but I keep the climb (some of my heavier 1.0 designs did level off),

With jets I get my apoapsis over 30km, and then when jets cut out, I just follow prograde.

When I did have desings that would level off to try and get maximum speed, I would pitch up.... but not 45-60... more like 20... just long enough to raise my apoapsis above 30km... and then I'd again just follow prograde into orbit.

1,600 m/s surface velocity is about 1,800 orbital... which basically means you only need 40% as much lift to maintain altitude (due to "centrifugal" force)

It also means your craft is producing 4x the lift that it did at 800 m/s...

Its not hard to see how this will enable it to stay up.

[problemecium]

I did not observe this particular effect, but I have found that pitching up too far when using LV-Ns actually is counterproductive, as the LV-N lacks the TWR to "hover", so increasing speed is the only way to raise the apoapsis, and therefore you don't want to deviate more than about 30 degrees from prograde or you waste thrust pushing up and lose too much of the needed thrust pushing forward.

[Teilnehmer]

I did that. Or something similar.

My test spaceplane ‘Ape’ utilizes two Nervs and two turbojets. The only way to get to orbit after leaving the atmosphere is falling into the atmosphere with Nervs on till the plane gains enough speed to stop falling using the wing lift (?) and leave the atmosphere again.

I considered this way not efficient and time-wasting.

Edited June 5, 2015 by Teilnehmer

[cybersol]

After rocket ignition, I find pulling up really hard can be counter productive due to losing too much of your air breathing speed. However, I also feel that it is time to get out of dodge (32km) as quickly as possible. I have usually finished air breathing at around 10* upwards pitch to gain as much speed as possible at the edge of the air. When the rockets fire, I find a 20* pitch to usually be a good compromise between losing too much speed and getting out of dodge. After 32km, I follow prograde until I reach my apoapsis. No matter if you are flying a spaceplane or a rocket, its time to level off by 35-40 km altitude.

[Captain Sierra]

As I run out of air at ~24-26km, there's still that 6-8km barrier before drag falls away. I tend to be in around a 5 degree climb when i kick rockets and I pitch up to 20-30 degrees. Anything more than that and I end up losing speed from drag. The 20 degree AoA is sufficient to deflect airflow and boost me higher, only bleeding a little speed that the rockets easily counter.

[Cirocco]

As I run out of air at ~24-26km, there's still that 6-8km barrier before drag falls away.

That's exactly what I'm talking about: at around 30 km, drag drops to almost 0, but if you have a decent speed, you can still use lift to maintain altitude at almost no drag. Theoretically speaking it makes perfect sense that a plane could "float" in/on an atmosphere, but I just found it intriguing that it's actually possible in KSP.

[Acet]

In my experience it's not as much the pitching up to 20 degrees that looses speed in the air breather->rocket transition but rather how fast you do it.

In 1.0 deviating significantly from prograde increases drag a lot, which is why the ideal gravity turn for traditional rockets starts early and is pretty much just following prograde all the way to space.

Thus with SSTOs, even though when one goes into rocket mode one wants to get out of the lower altitudes asap to reduce drag, it is still a more efficient profile to just increase the pitch of the plane slowly up, allowing prograde to follow never more that 1 or 2 degrees behind. Just activate the aerodynamic forces overlay (F11, if I'm not mistaken) and you'll see what I mean.

[Mic_n]

I noticed that when I inadvertently took an 'easy' contract for some observations on Kerbin.. which were basically down at the south pole.. one above 18km, one below, and one landed. Had only the basic turbojet unlocked at this point so nothing other than rockets that could get up that high.. which meant I could either cruise to the pole at ~mach 1 (erk), get the two low ones then kick in a rocket and climb to the last, or (the option I went with) make a kind of sub-orbital lifter.. Was in space, out of oxidiser & having dumped the boost stages to be in pure 'plane' mode but still a little short of my destination... But I was very pleasantly surprised by just how much lift I could still get in the upper atmosphere, even with a relatively 'underwinged' craft.. more than enough to keep me above that 18km limit with plenty to spare.

So yeah, there's still a lot of lift to be had in the upper atmosphere. Just because you can't harvest oxygen from it any more doesn't mean it doesn't have something to offer.

[Red Iron Crown]

It's not lift that's keeping you up, it's the fact that you have enough velocity and your Ap is far enough away that your momentum keeps carrying you upward. Once you get into that situation a low-TWR stage can continue to raise Ap, you just need enough to overcome the small amount of drag. I've used a similar ascent before (see album below, not a spaceplane but the same idea applies) and found it better to point just a bit above prograde rather than pitching up significantly; the thrust component perpendicular to prograde is energy lost (effectively just changing the eccentricity of the orbit).

Javascript is disabled. View full album

[Mic_n]

Err... yes and no.

As I said... my little space hopper I invented had NO thrust. no rocket engines left on it at the time and still too high for the jets to even fire up, yet I was still able to glide (and glide quite well) at ~30km. The new lift model (using v^2 properly) really emphasizes that.. You most certainly can use high-altitude lift to your advantage.

I'm thinking you could probably test it out by getting an AP up to 30km, hitting that AP, locking to a zero degree AoA and burning. Any increased vertical velocity at that point is purely due to lift. You're only going to be raising your AP through velocity when it's at the other side of your orbit.

To be clear, absolutely yes - thrusting prograde while "on the rise" is the most efficient way to raise your Ap around.. But lift lift can still play a significant part.

Times like these I wish I could get the mechjeb ascent guidance to work right for spaceplanes to get some repeatable test methodology, if only to satisfy my own curiosity about more and less efficient methods.

Edited June 5, 2015 by Mic_n

[Red Iron Crown]

In stock aero wings only generate lift when there is an AoA, if you lock to surface prograde and your wings aren't angled there won't be any lift.

But yes, high altitude lift can be somewhat useful in keeping the AP ahead of you. You can also raise it with lift, at the expense of reducing your Pe.