Troubleshooting Spaceplanes

[Guest]

Level: Intermediate

If you've pored over Basic Aircraft Design, Explained Simply, With Pictures, and manage to get something into orbit and back, at least some of the time, but it doesn't quite work like you want to, and you're not sure why, this troubleshooting checklist is for you. It's compiled from my own experience as well as wisdom gathered from various places in this forum.

This checklist has been written for Stock 1.2-pre, but is mostly applicable to 1.1.x and Ferram Aerospace Research as well. Atmosphere Autopilot eliminates much of the need for aerodynamic tweaking, but stable planes do handle better with it enabled as well. 

Take-off

• Craft won't go straight on the runway
  1. Unsuitable landing gear configuration
     • -> Make sure landing gear is straight on both axes
     • -> Adjust position of landing gear - wider or narrower, nose gear forward or back
  2. Landing gear is too light
     • -> Tune up springs and damping on main gear
     • -> Use heavier main landing gear
• Craft won't pitch up for take-off
  1. Nose landing gear is lower than tail landing gear
     • -> Adjust relative landing gear height
  2. Tail landing gear is too far behind centre of mass
     • -> Move tail landing gear forward
  3. Wing doesn't generate enough lift
     • -> Add a bigger wing
     • -> Adjust wing's angle of incidence (tilt wing so the leading edge is higher)
     • -> Add canards to the nose of the plane
       • TIP: start extended and key them to the gear action group
• Craft wants to do a backflip after it unsticks from the runway
  1. Problem 1 or 2 above: you have to apply a big control movement to unstick the nose, and when it does you back-flip
     • -> Correct the problem
  2. Pitch control surfaces aren't far enough back: can happen with delta wing or canard design
     • -> Add a tail
     • -> Move the tail further back
     • -> Add bigger control surfaces to the tail
     • -> Move canard back
     • -> Reduce control authority on canard
     • -> Reduce angle of incidence of canard

Flight

• Craft is too roll-happy
  1. Lack of inherent stability
     • -> Adjust wing camber so the wingtips are above the wing root
     • -> Adjust wing placement so it's higher up the fuselage
  2. Roll control surfaces have too much movement
     • -> Disable roll control for the inner ailerons and tail, enable it only for the wingtip ailerons, and adjust control authority on them
• Craft won't stay pointed where I'm pointing it
  1. Lack of inherent stability
     • -> Add a bigger tail, another tail surface, or move tail further back
• Craft wants to nose down a lot when rolling
  1. Centre of mass too far forward of centre of lift
     • -> Adjust centre of mass and centre of lift
  2. Lack of inherent stability
     • -> Add a double tail, cambered outwards
• Craft won't accelerate like I expect it to
  1. Angle of attack is too high (check on the navball)
     • -> Add more wing, or adjust wing pitch so the craft stays pointed at prograde once it's going reasonably fast
  2. Construction problem: one or more parts didn't "snap" properly and register as surface attached, which generates obscene amounts of drag
     • -> Disassemble and reassemble the craft, making sure all parts "snap" together rather than having to be adjusted by hand
  3. Craft is too heavy
     • -> Reduce fuel load or payload
     • -> Redesign for better aerodynamics and reduce the number of engines, if you're using much more than roughly:
       • A typical Mk 1 spaceplane will need two Whiplashes and an Aerospike
       • A typical Mk 2 will use two Whiplashes and 1-2 Aerospikes
       • An entry-level Mk 3 (built around the shorter cargo bays) will use four Whiplashes and the equivalent of 3-4 Aerospikes (one Skipper, one Vector etc.)
       • A heavy Mk 3 (built around the long cargo bay) will will use 6-7 Whiplashes and a Vector or equivalent
     • TIP: Ramjets will get into a positive feedback loop once past 400 m/s or so. Consider firing your rocket engine for a few seconds to boost your craft past that point for the in-atmosphere acceleration to orbit

Deorbit and re-entry

• Craft burns up on re-entry
  1. Your re-entry corridor is too steep
     • -> Use a shallower corridor. Pe between 55 and 60k for an Ap up to 250k should be safe for most designs.
  2. You're not producing enough drag
     • -> Stay pointed up for longer (45 degree AoA or more, until you're at 1500 m/s or below)
     • -> Add more wing
  3. You're nosing down too soon
     • -> Move the CoM back by pumping fuel from the fore to the aft tanks. With SAS engaged, watch the Pitch control. Try to keep it at around halfway up until down to 40 k altitude or so.
• Craft flips out of control on re-entry
  1. CoM has moved back as fuel and payload have changed, and is now behind the CoL
     • -> Pump fuel from aft tanks to fore tanks. With SAS engaged, watch the Pitch control. If it's below the 0 line, you're in trouble.
  2. Hypersonic aerodynamics are different than regular ones: components in the forward part are producing too much drag
     • -> Redesign, moving the draggy parts further back, but still keeping the CoL behind the CoM
• Re-entry ends up halfway across the globe from KSC
  • -> Experiment to find the right corridor. Putting Pe between 55 and 60k right above KSC (v 1.2) will get you in the ballpark for most designs. Tweak Pe altitude for your particular design.

Landing

• It's more like crashing really
  1. You're going too fast
     • -> Glide down at about -10 degrees pitch until you're within a couple of hundred metres. Then pitch to 0 to +5 degrees (between -5 and -10 m/s vertical speed) until you're very close to the ground. Then pitch up so you're between 0 and -5 m/s vertical speed, ideally with your nose close to 0 degrees. You will float down to the runway. Then hit the brakes.
  2. You're not producing enough lift
     • -> If your plane is stalling at 100 m/s or so, consider adding more wing
• It's more like a trampoline really (big plane running empty, still flying at around 30 m/s?)
  1. Your landing gear springs/dampers are too hard
     • -> Tune down the springs and dampers, or use lighter landing gear
  2. Your nose gear is too high
     • -> Lower nose gear – note that this will make take-offs harder!
  3. You're producing too much lift
     • -> Add drogue chutes and pop them the instant before touching down
• I get a tail strike and blow up my engines
  • Practice. Ideally your wings are straight and level with your speed at 50 m/s or lower when you touch down. This should avoid both bouncing and tail strikes.
  • Use taller landing gear
  • Add a tailgear as far aft as feasible and high enough up that it'll catch the tail if it drops down too far
  • Redesign so the CoM is further forward so you can move the main gear further back
• I miss the runway all the time
  1. Well, it is hard!
     • -> Don't worry about it, land in the nice flat area next to it instead, or if you miss that, ditch in the sea
     • -> Practice!

Edited October 8, 2016 by Guest
Added description of tail strike problem when landing