Spaceplane Ascent Profile?

[Nicholander]

I can't figure out how to get spaceplanes into orbit without running out of fuel. Before the aerodynamics update, I've used this profile for spaceplane ascents:

And that worked, but now I have large trouble getting up to 10 km, because my ship doesn't have a lot of thrust, and when I've finally reached 10 km I can barely sustain my altitude, let alone increase my speed. After that, when I switch to rocket engines, I always run out of fuel soon after my apoapsis is above the atmosphere.

Obviously this profile was designed for the souposphere, and doesn't work now. But I can't find any new updated ascent profile for the updated atmosphere.

Do any of you have any idea how to do it? Because now I'm just kind of lost.

Thanks in advance for helping!

Edited November 15, 2015 by Nicholander

[AeroGav]

I don't like working with Pitch because it's highly dependent on the thrust:weight ratio of your aircraft.

I prefer Angle of Attack, which is the difference between the orange -W- marker and the yellow prograde -o- .

According to this thread,

http://forum.kerbalspaceprogram.com/threads/138291-Wing-Lift-Wing-Lift-to-Drag-Ratio-Charts

Lift : Drag ratio peaks at about 2 deg alpha at lower altitudes, 5 degrees when you're getting up near orbital velocity.

So, that is what i try to maintain for most of the ascent. I do my subsonic climb at 2 degrees AoA, pitching the nose up and down to try keep AoA constant. Actual pitch angle will depend on thrust weight ratio, and airspeed will depend on wing loading ie. how much wing area you have versus the weight of your vessel.

I tend towards larger wings and moderate thrust weight, the climb generally starts at 120 m/s and gets up towards double that at 10km, with same AoA, because the air is thinning.

I will deviate from this optimal pitch angle theory however, when it comes to the sound barrier. Drag nearly doubles at mach 1, about 330m/s, before settling back to near-subsonic levels. It starts to rise above 240 m/s and starts to ease off again at 430. So, i'll often fly at up to 10 degrees AoA, to stay subsonic as high as possible. If my pitch control surfaces are maxing out though, this creates a lot of drag so i'll transition before reaching 10 alpha.

When I've decided it's time to go supersonic, I drop the nose by either setting prograde on autopilot, or just release the stick and let it fall back to the 2 deg AoA that's hopefully been trimmed for. Depending on wing area, this happens at altitudes from 8km-16km.

After that it all gets very hectic. You're probably using Whiplash or Rapier motors, so once you go supersonic thrust goes through the roof just as drag is falling away, and a lot of this theory becomes moot.

I generally level out from my sound barrier dive 10-13km and start pitching up to a STEEP climb, as the engines go ballistic. I'm still relatively low, so whilst I believe the lowest drag coefficient north of mach 1 is around mach 2, i don't want to get too fast yet, hence the pitch up. This is especially important after 1.05, if you don't want the cockpit to explode.

OTOH, you need to start levelling off in time to make your speedrun. I used to set autopilot to prograde at about 15km which would level me for an 19km speedrun. Going over 1000 below 20km is no longer safe however. It's tough to strike a balance between nosing over too hard, and blowing up, and allowing the ship to balloon to too great a height, and not having the thrust to hit top speed.

Looking at the engine config files, it might be safer to just aim for the optimum power speed of the engines, rather than absolute max speed. Below 22km at any rate. The Whiplash max out at 1050-1100, after that , the faster you go, the less thrust you get , whilst for the rapiers it's about 1200.

Above 22km i generally light up the nuke, if carried.

I switch over to rocket mode when the air breathers flame out or when the aircraft is no longer gaining altitude/airspeed.

Maintain 5 deg AoA during the rocket part of the climb until above 40km or i can see from the aerodynamic forces display that the wings are no longer making significant lift. Then i set autopilot to prograde again and switch to map mode, and start thinking about apoapse and periapse.

[GoSlash27]

Nicholander,

It really varies widely now with t/w, drag, and air breathing engine type.

I build my SSTOs to rely on aerodynamic efficiency rather than raw thrust, so I have never pitched up to 45° during my ascents.

Step 1 is to build enough speed to get the angle of attack (difference between pitch and prograde vector) down below 5°

Step 2, I climb as rapidly as I can while steadily increasing speed.

Step 3, get your speed above 360m/sec

Step 4, See step 2.

Step 5, reduce your climb rate as engine thrust diminishes until you have hit max speed and altitude for your engine. This might be 13km and 800m/sec for a Panther or 25km and 1600 m/sec for a RAPIER.

Step 6, engage rockets and *gently* increase pitch up to about 20° to climb out of the atmosphere, or as much as you can get while still accelerating.

Finally, gradually reduce pitch to prograde as the air thins.

HTHs,

-Slashy

[Nicholander]

Nicholander,

It really varies widely now with t/w, drag, and air breathing engine type.

I build my SSTOs to rely on aerodynamic efficiency rather than raw thrust, so I have never pitched up to 45° during my ascents.

Step 1 is to build enough speed to get the angle of attack (difference between pitch and prograde vector) down below 5°

Step 2, I climb as rapidly as I can while steadily increasing speed.

Step 3, get your speed above 360m/sec

Step 4, See step 2.

Step 5, reduce your climb rate as engine thrust diminishes until you have hit max speed and altitude for your engine. This might be 13km and 800m/sec for a Panther or 25km and 1600 m/sec for a RAPIER.

Step 6, engage rockets and *gently* increase pitch up to about 20° to climb out of the atmosphere, or as much as you can get while still accelerating.

Finally, gradually reduce pitch to prograde as the air thins.

HTHs,

-Slashy

Hmm... I'll try using your method and tell you what happens.

It's also worth noting that when I was using the outdated method I described in the first post, I was flying a ship I made which now that I think about it didn't have big enough wings or enough thrust,

[SpaceplaneAddict]

Here's a generic rule-of-thumb for acsents:

Pitch according to engine thrust. If thrust is declining below "speed run" altitude, pitch down. Pitch up if you're confident the engines thrust will still increase.

[Nicholander]

It worked! The plane I was using, the stock Dove spaceplane, it had a lot of thrust, and I kind of forgot to pitch down for the speed run until the RAPIER engine it had was loosing thrust. So I ended up switching to rockets at ~900 m/s at 31 km altitude. I got into an 81 - 83 km orbit just finely, but when I tried to reenter and land I had to use infinite fuel to deorbit.

(Speaking of landing, I successfully landed it on my first try. I over shot KSC by a lot, but I landed on the island runway with my remaining jet fuel.)

[Sanic]

It worked! The plane I was using, the stock Dove spaceplane, it had a lot of thrust, and I kind of forgot to pitch down for the speed run until the RAPIER engine it had was loosing thrust. So I ended up switching to rockets at ~900 m/s at 31 km altitude. I got into an 81 - 83 km orbit just finely, but when I tried to reenter and land I had to use infinite fuel to deorbit.

(Speaking of landing, I successfully landed it on my first try. I over shot KSC by a lot, but I landed on the island runway with my remaining jet fuel.)

If this ever happens in career, get out and push

[GoSlash27]

Nicholander,

Congratulations! Glad it worked out for you.

FWIW, you're probably not going to be able to max out a RAPIER without overheating and exploding parts on your plane, so you may want to trade some top speed for altitude as you did here.

Best,

-Slashy