Space plane help

[RizzoTheRat]

I appreciate there's a couple of other spaceplane threads on the go at the moment, but don't want to send one off on a tangent hence another one.

I'm having my first go with space planes and having some trouble, so I've read a couple of threads on here and am still not too sure where I'm going wrong. My current design is a bit over powered and can climb at 70-80 degrees nose up at lower altitudes, I'm getting up to 20km or so and leveling off to pick up speed, with the intention of shutting off the turbojets and switching to the rocket once I've got plenty of speed on and before the turbojets run out of air. However it seems to lose control somewhere around 20-25km and 800-900m/s, without the engines running out of air and conking out, and go in to a slow spin which can't seem to recover until I get in to thicker air

Design philosophy so far

• Centre of mass ahead of centre of lift (COL high though which seems silly as tail gives down force not lift)
  
• Centre of mass aligned with the middle of the fuel tank so it doesn't vary much with fuel usage (moves back a bit as jet fuel depletes but stays ahead of CoL
  
• Biplane design gives lift/mass approx 0.5
  
• Ram air and radial intakes seem to give plenty of air air to 25+km
  
• Canards added as I was struggling to get enough pitch up before
  
• Winglets added to get more area behind the CoM to see if that helped with the loss of control, it didn't
  
• Way more rocket fuel than needed as my goal is to end up with some form of tanker so I wanted a bit of weight on board the prototype
  

Any suggestions what I'm doing wrong?

Edited October 7, 2014 by RizzoTheRat

[QuesoExplosivo]

It sounds like an asymmetric thrust problem due to one engine starving for air before the other. In what order were the intakes and engines placed?

[Pecan]

Not fast enough, if you're having problems. Try 1km/s at 20km altitude and 100m/s more for every additional km up - so 1,500m/s by the time you get to 25km. The higher you go the thinner the air so the faster you can go but the higher you go the thinner the air so the faster you have to go to get enough air for your engines. If that doesn't work - spam the intakes!

The other thing I'd say is; are you sure about "tail gives down force not lift"? KSP wings are symmetric so lift depends primarily on angle of attack.

Edited October 4, 2014 by Pecan

[Tex_NL]

My guess: as you use fuel your CoG actually DOES shifts backwards to behind the CoL. Or at least far enough backwards to make your plane extremely unstable.

Since you already use KER I'll assume you're not afraid to use mods. Install TAC Fuel Balancer and use fuel from the rear tanks first.

[RizzoTheRat]

Think I did both radial intakes, both ram intakes, then both engines. Couldn't quite get my head around the best order.

Re the tail plane, in the real world it would be providing downforce, and I assume it must in KSP as the COM is forward of the COL, so the tail has to push town to prevent it nose diving, the tail I'm using the all flying one (not that compressibility is issue in KSP presumably ) so you can see it change angle of attack

I've tried draining the fuel in the hanger and with the rocket tank full and the jet tanks empty the COL is about on the back edge of the COM ball

I'd assumed I wanted to keep the speed down below 10k in the same way i do with rockets, I'll try a shallower full power climb and see what happens.

Edited October 4, 2014 by RizzoTheRat

[Wanderfound]

As Pecan says: the efficiency of intakes is a function or angle of attack, air pressure and speed. You can fly higher if you're going faster when you get there. Lose the steep climb, set a 20° pitch after takeoff, and hold that until 20,000m. Then level off such that your VSI reduces to about 10m/s climb rate.

Your other issue is asymmetric flameouts. Consider switching to a design with a central air-breather and flanking rockets or RAPIERs. If not, carefully watch your air in the resources tab, and throttle back to maintain air supply and avoid flameouts until you shut the engines down.

If you just want piloting practice and something to compare your ship to, use https://www.dropbox.com/s/lbnz9s8k9h7gwgb/Kerbodyne%20Benchmark%20StockAir.craft?dl=0

[RizzoTheRat]

Not researched the Rapier yet. Both engines are still running at the point I lose control so I don't think it's asymmetric thrust, I'd put the intakes on before the engines and was keeping an eye on the intake figures.

I'll give it a go with a shallower climb, and compare with that other aircraft too, and report back, thanks.

[Wanderfound]

P.S.: tailplanes absolutely do not provide downforce under normal circumstances. Lift vs downforce is a function of AoA (and shape a bit, but mostly AoA). Tailplane wings typically have a neutral or positive angle.

In particular, your tailplane is made of AV-R8's. Those are all-moving control surfaces that will provide the bulk of your pitch authority. Use right-click tweakables to set them to influence pitch only. Do this to the canards as well. Set the wing surfaces to roll only. You don't appear to have any active yaw control (unless there's a rudder hidden beneath the tailplane), but that's no big deal. One might help a touch, though.

Your other issue is CoM->dry CoM offset. As soon as you start burning fuel, your CoM is going to start shifting backwards. By the time the tanks are dry, it's going to be well behind CoL.

[Friend Bear]

If it is the spin out I am thinking about, it is because the engines are running at all before the intake dies. While it says there is some intake, you need to manual disengage those jets with an action group at the same time. I do this and engage the rocket at the same time.

Judging by the COM and weight of your jet, you would be completely fine to remove the back fuel and have only the middle and two sides. The jets won't get through the two sides in time and would even allow for a reentry/takeoff with enough fuel to enter orbit with the rocket and rendezvous.

But yeah, just set an action group that turns on the rocket, turns off the jets. and turn them off at the altitude you have been having problems.

[RizzoTheRat]

In the real world the wing provides more lift than the aircraft weight, the CoM is forward of the wing CoL, and the tail provides downforce to balance it. Surely the same has to happen in KSP to cope with the CoM/CoL offset?

No rudder but the cockpit torque seems to manage that bit ok so I didn't bother.

I turned the ailerons to roll only but didn't change the tail or canards, I'll give that a go too.

CoM>dryCoM doesn't seem to be be an issue, if I drain the fuel in the hanger the CoM is still ahead of the CoL

[Friend Bear]

In the real world the wing provides more lift than the aircraft weight, the CoM is forward of the wing CoL, and the tail provides downforce to balance it. Surely the same has to happen in KSP to cope with the CoM/CoL offset?

No rudder but the cockpit torque seems to manage that bit ok so I didn't bother.

I turned the ailerons to roll only but didn't change the tail or canards, I'll give that a go too.

CoM>dryCoM doesn't seem to be be an issue, if I drain the fuel in the hanger the CoM is still ahead of the CoL

That does become an issue with lots of planes, but with your COM/COL and how the fuel is used, it won't be a big one on yours. COULD be when you remove a fuel tank as suggested but just keep the COM and COL where they are.

[Wanderfound]

(Google Google)

Actually, it looks like you're right about the tailplane angles.

Cool. Build one like that and tell us how it goes?

[Wanderfound]

It's probably also worthwhile swapping the lateral Mk 1's for small LFO tanks and running fuel lines both directions between the main fuselage and the nacelles.

Also, check the CoM/CoL relationship when the side and front tanks are empty but the rear one is still full. That's the order in which you've got 'em draining.

Edited October 4, 2014 by Wanderfound

[Friend Bear]

It's probably also worthwhile swapping the lateral Mk 1's for small LFO tanks and running fuel lines both directions between the main fuselage and the nacelles.

Not a bad idea, but I noticed that planes built like yours will drain from the sides, then the middle, starting from the front of the front tank and working back, without lines. You could do lines to maybe change that and help with COM management?

[Wanderfound]

Not a bad idea, but I noticed that planes built like yours will drain from the sides, then the middle, starting from the front of the front tank and working back, without lines. You could do lines to maybe change that and help with COM management?

Yup. If you want it to drain back-to-front you need a pair of lines from front core to front lateral and a pair of lines from rear lateral direct to the rocket motor.

[RizzoTheRat]

LFO? I'm guessing that's somthgin I've not researched yet. At the moment I've just got jet fuel in the engine pods and rocket fuel/oxidiser in the centre tanks

[Friend Bear]

LFO? I'm guessing that's somthgin I've not researched yet. At the moment I've just got jet fuel in the engine pods and rocket fuel/oxidiser in the centre tanks

Unless its an atmo. only jet, I just make all tanks liquid/oxy. You don't use any liquid in the jets, really, so there is a little extra for the rocket. Plus, weight isn't hugely different.

[Wanderfound]

LFO? I'm guessing that's somthgin I've not researched yet. At the moment I've just got jet fuel in the engine pods and rocket fuel/oxidiser in the centre tanks

Liquid fuel (LF) as used by the jets is the same as liquid fuel used by rockets. It's interchangable. However, rockets also require oxidiser (O). A normal rocket tank is LFO; an aircraft fuselage is usually LF only.

At least until Porkjet's parts hit stock in a couple of days, anyway. Lovely lovely LFO lifting-body fuselages.

[RizzoTheRat]

Ah, hadn't twigged the fuel was the same, that would explain why I'd seen some posts saying about reducing the level of oxidant in the tanks before takeoff.

[LethalDose]

A simple solution may be to place fuel lines going from the center out and lock the fuel in the port and starboard tanks on take off. That should pull fuel from the aft tank first and prevent the CoM from shifting back. The lateral tanks can be unlocked when needed or on return.

[Redshift OTF]

My guess: as you use fuel your CoG actually DOES shifts backwards to behind the CoL. Or at least far enough backwards to make your plane extremely unstable.

Since you already use KER I'll assume you're not afraid to use mods. Install TAC Fuel Balancer and use fuel from the rear tanks first.

I would agree with this. OP, provide a craft file and I'll fly it for you.

[RizzoTheRat]

I hadn't realised the radial intake bodies also contain fuel, so a bigger impact on my CoM than I anticipated, but even with the intake bodies and Mk1 fuselages drained of fuel and the central FL-T800's full (not used the rocket by the point of loss of control, and no fuel pipes to cross feed so they're still full) the CoM is still ahead of the CoL

I've tried a 10 degree climb which got me 20km at about 700-800 m/s and had the same control difficulties. As an experiment I've tried dropping the rocket motor off the back and putting on a tail boom to shift the rudder and tailplane further back, this seemed a lot more controllable and I got to well over 1000m/s at about 22km before I lost control, and I'm pretty sure that time it was because I wasn't watching the air intakes and started getting thrust issues. I'll have a go later with a single engine on the centreline to check if that is my problem.

I would agree with this. OP, provide a craft file and I'll fly it for you.

I appreciate the offer thanks. I'm currently reading Eric "Winkle" Brown's autobiography and that fits nicely with the test pilot ethos of if you can't figure out what's wrong, find a more experienced pilot to get a second opinion

https://dl.dropboxusercontent.com/u/12134283/KSP/SSTO1.craft

Edited October 5, 2014 by RizzoTheRat

[Taki117]

I've skimmed the thread a bit, and while you keep saying that your engines aren't flaming out it still might be an asymetrical thrust issue, mostly because, unless I am mistaken, you dont have any YAW authority meanin as soon as one of your engines begins to lose thrust (even if it doesnt flame out) the other is going to push you around beecause you have no way to compensate for that. i willl say this, move your hoeizontal tail planes to your fuel tank just infront of your rocket, and then add the delta-delux winglet as a tail.

[Yakuzi]

Not researched the Rapier yet. Both engines are still running at the point I lose control so I don't think it's asymmetric thrust, I'd put the intakes on before the engines and was keeping an eye on the intake figures.

When you spin out, is the pitch or the yaw uncontrollable? Pitch issues are typically due to changes in your CoM as you burn fuel. Since you have an multi jet-engine setup in the horizontal plane, yaw issues are most likely caused by asymmetric thrust due to low oxygen (as Taki117 pointed out above, this does not directly result in a flame-out). Keep an eye on SAS yaw/pitch/roll correction as you ascend and throttle down (or switch engines) as soon as you see incremental changes in yaw. This should solve your problems.

Edited October 5, 2014 by Yakuzi

[RizzoTheRat]

I had the horizontal planes on the back of the central tank originally and the thing wouldn't move on the runway at full thrust, I think the model reckoned the drag on the tailplanes being in the jet exhaust was as much as the jets were providing in thrust!

Just tried it with a single engine on the centreline and having only just managed to get it off the runway, nursed it to 26km and over 1200m/s, so it looks like it is asymmetric thrust, despite the engines not flaming out completely