Space plane

[Sras]

Hello there fellow space explorers!

Ive set myself the task of making a spaceplane and I ran into a minor stability problem which I don't seem to be able to figure out so here I am.

First of, the stats given by Kerbal Ingineer Redux:

TWR: 1.50

Mass: 28.802kg

Thrust: 380kN

(This is with the aerospike disabled)

The plane should be balance since everything is doubled by the doubling tool. Yet still I am running into stability problems...

From takeoff its alitle difficult to control I keep having to pull up alitle to not make a nose down. Around 160 m/s it gets uncontrollable and basically makes a flat stall and fall out of the sky.

Any suggestions ?

Here is some pictures of my attempt of a spaceplane:

[capi3101]

What kind of control issues is it giving you?

I'm kind of new to spaceplanes my own self; that said I have made it to orbit and back in a spaceplane at this point. Taking a guess I'd say you don't have enough in terms of control surfaces for pitch. Your center of lift may also just a hair too far forward compared to your center of mass; ideally you want it behind the center of mass but still sticking through the ball. A pic of your three centers from the side would also be useful in diagnosing problems as well. Raising the center of lift slightly above the center of mass would should make your craft more stable.

Might suggest http://forum.kerbalspaceprogram.com/threads/52080-Basic-Aircraft-Design-Explained-Simply-With-Pictures for general principles, especially if this is your first foray.

[numerobis]

When you hit a flat spin, what altitude are you at? A flat spin usually indicates you're running out of air. When you do, one engine will get all the air it needs, and the other will lose thrust, so you spin. Throttle back.

Yank the landing gear off if you want an accurate read of the center of mass. You appear to have CoM and CoL right on top of each other in the editor, so when the flight starts and the landing gear go massless (KSP physics deviates a bit from ours) your CoM will move. From the pictures and your claim to need to pull up, I'm going to be the center of mass is ahead of where it shows, which is good. You can use SAS to do the pulling up for you.

[SuperBigD60]

From the look of your plane, I can't fathom why it would want to nose down. if anything it really should be barely stable once you're supersonic.

I imagine you don't run into too many flat spins with those rapiers. In fact you could probably ditch the aerospike, as the rapiers are rockets, too.

Personally, I like to move my center of lift well behind my center of mass, as that will make any aerodynamic forces restore the plane to a prograde direction (assuming you don't over steer and stall)

Also, if you like spaceplanes, FAR makes them at least twice as enjoyable, and coupled with B9 twice as useful (to laymen like me).

[Redshift OTF]

I'm not sure if you could get that into space with a low TWR of 1.5. Are you only using 2 ram intakes or are there multiple intakes clipped together? As others have said, the centre of lift is too close to the centre of mass so as you use fuel the centre of lift will end ahead of the centre of mass which can cause your plane to flip out, (although usually up and not down). Also you don't have a lot of tail and what tail you do have is angled, reducing it's effectiveness. Try angling them straight up to see if it helps. Ditch the aero spike and show the centre of thrust in a picture as if that is off it can cause you plane to pitch too high and too low at high altitudes where the air has less of a stabilising effect on the wings.

[numerobis]

TWR of 0.75 is plenty for a decently made plane to take off.

The visible intakes, two rams and two radials, will let you max out going about 1 km/s at 25 km (or higher if you can make it; you've got 1.5 TWR at that speed, so you should be able to trade a bit of speed for a good bit of altitude). From there it's 1.5 to 2 km/s to orbit. If you want the aerospike, switch the rapiers into turbojets to save a tonne of mass and improve rocket mode Isp. If you want the rapiers, ditch the aerospike to save a tonne and a half. Either way the rocket phase will be cheaper, and you'll get a bit higher/faster off the jets.

[Claw]

Since it's hard to see everything in your pictures, I spent a little time trying to replicate your ship. I was a bit thrown off because you said the mass is 28.802 and the picture shows 25.802 so I tried to match both of those weights with the CoM and CoL picture you put up.

I also noticed you said it "flat stall" and not "flat spin" so I will address the "stall."

By the way, I love the way the wings look.

So...

Design:

The only way I could get that much weight was by using all fuel/oxidizer tanks. I don't know if that's what you did, but if you're going to use the RAPIER in air breather mode, you don't need excess oxidizer. Unless you have RCS/monopro hidden in there somewhere.

I gotta ask: Are you flying SAS on or SAS off? SAS off it sure wants to pitch down.

Takeoff:

I noticed this thing is a beast to takeoff. I had to wait till I ran off the end of the runway, but it flew.

Climbout:

If I get the nose way up then let it droop, the airplane caught itself and climbed out slowly. If I tried to fight and hold a specific pitch angle, or tried to climb too aggressively, the nose eventually fell and stalled out.

So if this is what you're running into, try climbing out very shallow until you can get high enough to get some speed. I found my pitch to be around 30 degrees with the prograde at 5 to 10 degrees. Around 20k (when the RAPIERS start to go asymmetric), you can throttle back and hit ignite the aerospike. Your pitch is already high enough for the aerospike to put you in the right direction, and throttling down allows the RAPIERS to air breath a bit longer. Soon after, switch RAPIER's mode and go full throttle. Now you just have to play out orbital mechanics as for a rocket. With a big HOWEVER...

Orbital insertion:

My airplane wanted very badly to pitch up. I'm guessing it's because you have the tail fins, ladder, and parachutes on and your CoM is so far back. Your plane is so neutrally stable that when you burn off fuel, the CoM shifts back and up (behind your CoL but that doesn't matter in space). When it's time to ignite the rockets, all that trust makes it buck like a bronco because your CoM is above the Center of Thrust.

80 km

So I tried a few tweaks if you want to give something similar a try. I ditched about 1/4 to 1/3 of the oxidizer the RAPIERS were carrying. I moved the rear landing gear forward and outboard from the engines to wing connector (closer to the CoM), and removed the ones from the wings. I also added a canard to the tail (so it's actually a horizontal slab) in between the wings, but just in front of the RAPIERs, to move the CoL back a little and give some more flight control authority (which the SAS made full use of.) I also placed it slightly low on the fuselage to offset the weight of the tails but it wasn't quite enough. If you're going to add a canard to the back, you might want to put a strut or two in between the sandwich of the wings because the extra force caused them to flex. If you put it inside, I doubt you'll be able to see it.

That being said, the thing still flew with about 20 degrees AoA but took off and climbed out a bit easier. I just parked the pitch at 30 degrees and let it ride till 18km and lit the aerospike. If you adjust pitch AT ALL at this point, it will fall due to lack of speed.

During orbital insertion, the plane still wanted to pitch up. So if you want all those chutes and ladders (no pun intended), then you'll have to figure out how you want to shift your CoM down some. You might also want to check your CoM location after you burned off your fuel. Right now it's neutrally stable but will probably be unstable when you want to reenter the atmosphere.

(By the way: Make sure you remove your gear while you play with the CoM. They mess up the CoM in the SPH and don't really contribute to it in flight.)

Good Luck!!

Edited January 13, 2014 by Claw

[Claw]

I think if you just pull off the aerospike, you might not have enough thrust to overcome the slow speed during climbout since you have to fight the pitch so much. Your aircraft is pretty heavy right now and the RAPIERs just aren't pushing it fast enough. You can swap out the RAPIERs for TurboJets, but you might end up with the same problem.

Can't hurt to try.

Good luck!

Edited January 13, 2014 by Claw

[SuperBigD60]

I think if you just pull off the aerospike, you might not have enough thrust to overcome the slow speed during climbout since you have to fight the pitch so much. Your aircraft is pretty heavy right now and the RAPIERs just aren't pushing it fast enough. You can swap out the RAPIERs for TurboJets, but you might end up with the same problem.

Can't hurt to try.

Good luck!

Yeah, you're probably right about that. I guess I'm just used to FAR where lift from your wings is somewhat more significant.

[Sras]

Since it's hard to see everything in your pictures, I spent a little time trying to replicate your ship. I was a bit thrown off because you said the mass is 28.802 and the picture shows 25.802 so I tried to match both of those weights with the CoM and CoL picture you put up.

Yes pardon after you pointed it out I've been hack in checking that the 25.802 is the right one and I hadn't taken parts off on the picture. I must have misread or remembered wrong when writing my post here. I am happy to supply my craft file if interested

I also noticed you said it "flat stall" and not "flat spin" so I will address the "stall."

Yes it might be my terms that are wrong but, suddenly it looses its lift and start falling like a brik uncontrollable.

By the way, I love the way the wings look.

Thank you, I like it too its mainly made by stock parts apa

So...

Design:

The only way I could get that much weight was by using all fuel/oxidizer tanks. I don't know if that's what you did, but if you're going to use the RAPIER in air breather mode, you don't need excess oxidizer. Unless you have RCS/monopro hidden in there somewhere.

Inbetween the two wings which are "folded" together I have added several science modules such as goo tank, thermometer etc. 1 on each side. Plus there is a RCS controller aswell one on each side.

I gotta ask: Are you flying SAS on or SAS off? SAS off it sure wants to pitch down.

Generally I fly with SAS, but I have tried to fly it with MechJeb, which doesn't do any greater good

Takeoff:

I noticed this thing is a beast to takeoff. I had to wait till I ran off the end of the runway, but it flew.

Exactly, which I why I added the aerospike, just in case I need a extra punch to get off quickly on some moon or planet. Notice I've added parachutes as for a "resettable" quick stop if needed.

Climbout:

If I get the nose way up then let it droop, the airplane caught itself and climbed out slowly. If I tried to fight and hold a specific pitch angle, or tried to climb too aggressively, the nose eventually fell and stalled out.

So if this is what you're running into, try climbing out very shallow until you can get high enough to get some speed. I found my pitch to be around 30 degrees with the prograde at 5 to 10 degrees. Around 20k (when the RAPIERS start to go asymmetric), you can throttle back and hit ignite the aerospike. Your pitch is already high enough for the aerospike to put you in the right direction, and throttling down allows the RAPIERS to air breath a bit longer. Soon after, switch RAPIER's mode and go full throttle. Now you just have to play out orbital mechanics as for a rocket. With a big HOWEVER...

Orbital insertion:

My airplane wanted very badly to pitch up. I'm guessing it's because you have the tail fins, ladder, and parachutes on and your CoM is so far back. Your plane is so neutrally stable that when you burn off fuel, the CoM shifts back and up (behind your CoL but that doesn't matter in space). When it's time to ignite the rockets, all that trust makes it buck like a bronco because your CoM is above the Center of Thrust.

So I tried a few tweaks if you want to give something similar a try. I ditched about 1/4 to 1/3 of the oxidizer the RAPIERS were carrying. I moved the rear landing gear forward and outboard from the engines to wing connector (closer to the CoM), and removed the ones from the wings. I also added a canard to the tail (so it's actually a horizontal slab) in between the wings, but just in front of the RAPIERs, to move the CoL back a little and give some more flight control authority (which the SAS made full use of.) I also placed it slightly low on the fuselage to offset the weight of the tails but it wasn't quite enough. If you're going to add a canard to the back, you might want to put a strut or two in between the sandwich of the wings because the extra force caused them to flex. If you put it inside, I doubt you'll be able to see it.

That being said, the thing still flew with about 20 degrees AoA but took off and climbed out a bit easier. I just parked the pitch at 30 degrees and let it ride till 18km and lit the aerospike. If you adjust pitch AT ALL at this point, it will fall due to lack of speed.

Good ideas I am gonna try and play around with it again tonight deffinately!

During orbital insertion, the plane still wanted to pitch up. So if you want all those chutes and ladders (no pun intended), then you'll have to figure out how you want to shift your CoM down some. You might also want to check your CoM location after you burned off your fuel. Right now it's neutrally stable but will probably be unstable when you want to reenter the atmosphere.

Good point, Ive located the RCS thrusters centered to CoM, but of course I didn't think of I burned fuel to get up there.

The whole point of the plane is a reusable science plane which I can send back down to a planet or moon and then rendevouz with the orbiting lab.

(By the way: Make sure you remove your gear while you play with the CoM. They mess up the CoM in the SPH and don't really contribute to it in flight.)

Good Luck!!

Thank you so much your post has been very helpful and definitely given me things to think about:)

[numerobis]

I think if you just pull off the aerospike, you might not have enough thrust to overcome the slow speed during climbout since you have to fight the pitch so much. Your aircraft is pretty heavy right now and the RAPIERs just aren't pushing it fast enough. You can swap out the RAPIERs for TurboJets, but you might end up with the same problem.

I did the calculations in the post before your test. You save mass by changing the engines in those two ways, which doesn't really help the jet stage much, but it helps the rocket stage by reducing dry mass. You mention that you run out of air at 20km. How fast are you going then, and at what pitch? I calculate you should be able to go quite a bit higher.

By the way, does Kerbal Engineer know that landing gear is massless? If not, its mass estimate is 2.5t high.

[Redshift OTF]

Ah so the nose was dropping because the plane was too heavy and there was not enough speed for the wings to give enough lift.

[Claw]

I did the calculations in the post before your test. You save mass by changing the engines in those two ways, which doesn't really help the jet stage much, but it helps the rocket stage by reducing dry mass. You mention that you run out of air at 20km. How fast are you going then, and at what pitch? I calculate you should be able to go quite a bit higher.

To be honest, I don't remember what my speed was, but I know it was slow. It was slow because of how the plane flies. I guess I should have also said I wasn't using MechJeb to throttle modulate for me. I was flying by hand since I assumed SRAS was also and I didn't fully milk the engines since that's not what I was worried about.

That being said, I went back and flew the profile again to see if I could match your numbers. The plane is already unstable because of the angled wings so I figured I wouldn't get to 25 km and 1000 m/s. So I first ran a profile with the same craft using straightened wings. I was able to reach about 1050 m/s at 25,000m altitude.

However, when I ran the same test with the original craft, I maxed out at about 23,500 m and 920 m/s. I could give it a little boost with the aerospike, but the speed/altitude bled off. This was with a vertical climb rate of about 10 m/s through 21 km (and lower as I climbed). I'm not sure if your calculations account for the forward, angled wing. Plus it's a little less stable.

[Claw]

Thank you so much your post has been very helpful and definitely given me things to think about:)

Sure, I hope I was able to help.

By the way, here's the likely reason your plane is so hard to control in pitch.

Here is a test flight in level flight at about 8,000m and 160 m/s (where I assumed you were having problems). The red line is the airflow (represented by the prograde marker). If you look at your forward wings (the "structural wing" element), it is still at negative AoA. Your forward-top section is angled down to far and is providing a downward lift force, while your lower half is probably getting close to max lift (and will soon start to decrease lift). Essentially your forward angled sections are canceling each others lift out (sort of).

Also, a side effect of using an elevon is that you put a lot more force into your wings, causing them to flex. I went back to my first design and put a single strut inside the wings, on the main delta part. Also, I went and locked out the roll and yaw function of the lower elevon, leaving it for pitch only. This helped minimize pitch losses when rolling and pitching the craft.

Plus, activating your elevon (to pitch up) causes your wing to lose lift, making your heavily laden/low lift design slow down further. That's why minimal pitch changes (till you're higher and faster) are important for flying this design. Don't hesitate to use the aerospike in little bursts if you get too slow in that 8,000 to 14,000 ish altitude range.

In my (limited) experience, MechJeb has a hard time controlling these kind of characteristics. It worked well for dealing with throttling the air breathing RAPIERS though.

Exactly, which I why I added the aerospike, just in case I need a extra punch to get off quickly on some moon or planet. Notice I've added parachutes as for a "resettable" quick stop if needed.

If you move your gear a little forward, you won't need to use the aerospike. EDIT: Ahh, I misread what you said. I mean to say if you move the gear forward, takeoffs shouldn't be a problem at KSC.

Ah so the nose was dropping because the plane was too heavy and there was not enough speed for the wings to give enough lift.

Yes, essentially. Because of what I said above.

Edited January 14, 2014 by Claw