KerbalX

Description

This is a simple, light, completely stock, low tech passenger spaceplane with decent performance, built for ferrying up to three Kerbals to and from low Kerbin orbit, as well as to serve as an example of elements of good spaceplane design.

Built in the SPH in KSP version 1.2.2.

Features

• 28 parts, takeoff weight under 16 tons - requires no runway or hangar upgrades
• Requires only two tech tree nodes that cost 160 science (Supersonic Flight and Advanced Electrics)
• Has a probe core and can be flown remotely if you’re uncertain about your piloting skills
• Pleasant handling characteristics with well balanced trim and no tendencies to depart from controlled flight in any flight regime - center of mass does not shift as fuel is burned
• Design has upgrade potential, can easily be extended to carry more crew or more fuel or both
• Cheap, at around 17000 spesos
• Reaches LKO with around 350-400 m/s delta-V to spare, giving decent margin for orbital rendezvous maneuvers

Flight manual

The rudder has yaw authority disabled. That’s intentional, SAS and control surface yaw do not get along. To turn left and right while in atmosphere, just roll carefully in the direction you want to turn and the aircraft will go that way without sideslipping.

The aircraft is very easy to fly without SAS if you like that, but there is a very weak tendency to roll left that you may have to trim out. Probably caused by phantom forces in the physics engine.

There is no antenna other than the builtin one, so if flying remotely with extra ground stations disabled, make sure you have good relays. Or add an antenna (preferably hidden in the service bay), whichever you prefer.

Ascent

• Start engine at low throttle, align with runway heading carefully, and once stable, throttle up and turn SAS on. Rotate carefully and take off at around 80 m/s - the pitch authority is quite strong so beware tailstrikes (you may want to press caps lock to turn precision control on).
• Gear up, then nose up to just under 10 degrees over the horizon and start climbing. No need for the afterburner just yet.
• Turn on afterburner (action group 1) at any point between 1000 and 2500 meters. Keep nose pointed at 10 degrees above the horizon (the nose has a weak tendency to wander upwards during atmospheric flight - this is normal and you do not need to correct for it very often).
• Once above 2500 meters, lower nose slightly to around 5 degrees above horizon and accelerate to at least 350 m/s or Mach 1.25. Once reached, pitch up again and maintain 10 degree climb.
• Rough ascent profile points of reference: 550 m/s or Mach 1.85 at 10000 meters, 670 m/s or Mach 2.25 at 12500 meters, at least 700 m/s at 14000 meters and over 750 m/s or Mach 2.6 at 16000 meters. If you’re going slower, you climbed too steeply. Going faster is kinda okay, but the Panther’s thrust starts dropping dramatically above Mach 2.6 or so, so you don’t want to exceed that. Also, if the flight path marker on the navball is below the prograde marker, you’re going too fast (you have a negative angle of attack, which is draggy).
• Pitch down slightly to around 5 degrees above the horizon around 15000 meters.
• Ignite rockets around 17-18000 meters or when you start losing too much speed (either horizontal or vertical speed - try not going below 50m/s vertical speed). You should be going at least 750 m/s when you do this.
• Shut down jet engine (Abort action group) at 19000 meters or when thrust drops below 10 kN, whichever comes first. Or let it flame out by itself.
• Pitch up again to between 10 and 15 degrees above the horizon. Do not exceed 15 degrees above horizon - it causes a lot of drag.
• Above 30000 meters or so, start gradually pitching down towards the prograde marker. You should reach it at around 38000-40000 meters.
• Above 40000 meters, throttle down to 75%, make sure you’re following the prograde marker, and open the map screen. Cut engines once desired apoapsis is reached.
• Coast to apoapsis (stay pointing prograde while doing this, you can still cause a small amount of drag by pointing your nose away from it) and circularize. Circularization should require around 100 m/s for a 75 km circular orbit. You should have around 350-400 m/s delta-V left for orbital maneuvers. Save around 75-80 m/s for deorbiting.
• Open the service bay and extend solar panel hidden therein to recharge batteries. Keep in mind the Terrier does not have an alternator, so this is the only way to get electrical power while in orbit.

Descent

• Create a maneuver node 90° west (or before, on the orbit path) of the KSC. Drag retrograde until periapsis is at around 5000 meters, 90° east (or after) the KSC. If you did this well in advance, make sure to check that the maneuver node is still in the right place when you get close to it - the planet will have rotated below you.
• Atmosphere reentry checklist: rockets off, jet engine on but at zero throttle, afterburner off, solar panel retracted, service bay closed.
• Once you descend below 70000 meters, pitch aircraft up to 70-80° above the horizon and then just keep the nose as far up as the control surfaces will allow for the entire descent (you can even turn authority up a bit on the tailplanes if you want). This slows you down fast enough for your temperature-sensitive mk1 parts to survive reentry.
• Once below 30000 meters or so, adjust trajectory as necessary to reach KSC.
• If you over- or undershoot keep in mind you can cruise in atmosphere for a very, very long time on the Panther’s dry thrust and the liquid fuel you have left (there should be some left over from ascent).
• Landing should be fairly easy, the aircraft is quite docile at low speed. If you need to lose speed just pitch up or do S-turns.

Tinkering

The aircraft as designed does not use all the fuel tank capacity. This is intentional, because I think what’s there is just right for the trips I usually want to make, but try experimenting with how much mass you can carry on one Panther. Adding more mass may require adding more wing panels or adjusting the angle of incidence on the wings, however. Careful with adding liquid fuel to the tank just in front of the Panther though; doing so shifts the CoM backwards in a dangerous way.

The cockpit can be replaced with another crew cabin if you want another crew spot. This does not move the CoG since the mk1 inline cockpit without monopropellant and the mk 1 crew cabin both weigh the same - one ton. If you do this though, keep in mind that you may need to add some more reaction wheel torque, and that the mk 1 crew cabin has no entrance hatch.

If you want to make slowing down easier, just add a pair of airbrakes just behind the center of gravity.

If you don’t have Advanced Electrics (needed for the extendable solar panel) yet, you can remove the extendable solar panel and just replace it with a small, fixed solar panel and rotate the plane so it gets exposed to the sun.

Design theory

This plane is indeed all I know about spaceplanes, or at least almost - it uses almost all the tricks I’ve learned. While this is not the place for a full design guide, a number of axiomatic design rules used in this design follow. I consider the mods RCS Build Aid and Correct CoL absolutely necessary for spaceplane design, and either KER or MechJeb are fairly essential as well.

• The aircraft center of mass shall not shift during flight. If the aircraft carries cargo, this rule must also be adhered to when deploying cargo. It follows that the center of mass must be very close to the center of the cargo bay.
• The center of mass shall not be located further from the longitudinal center of the aircraft than a bit less than ¼th of the aircraft length. It follows that if there is no heavy cockpit or equivalent in the front of the aircraft, engines must be placed either near the center of the aircraft or near both ends.
• There shall be no externally mounted, drag-causing equipment that is not absolutely necessary. RCS thrusters, solar panels and batteries must be placed inside cargo bays or equivalent.
• The wing shall be given an angle of incidence that is sufficient (but no more) for maintaining an angle of attack between 0.5 and 3.5 degrees in a slight climb throughout most of the air-breathing flight envelope, but especially near the sound barrier and near the air breathing top speed. Negative AoA shall be avoided.
• The center of lift shall be located slightly above (and behind, obviously) the center of mass. The main wings shall have 1-2 degrees of dihedral angle. This counteracts sideslip and stabilizes the plane in the roll axis.
• If canards are used, they shall have the same angle of incidence as the main wing, or slightly more, to prevent dynamic instability in certain conditions.
• The frontal surface area of the plane shall be minimized by using as few radially mounted stacks as possible.
• Angle of incidence shall be used on canards or tailplanes to ensure the need for pitch trim does not become excessive throughout most of the airbreathing flight envelope.

Thrust is a complicated thing, especially atmospheric thrust. Buying thrust costs weight, and weight is bad. You can counteract low thrust by adding more wings or adding angle of incidence to your wings to a certain degree, but those wings are dead weight in space, and if you accelerate too slow you use more fuel, which counteracts the weight saved by having less engines. Unlike on a rocket you don’t need a TWR greater than 1, not even on your rocket engines, but it shouldn’t be too far below 1 either or you’ll spend too much fighting gravity. Low drag is extremely important in any case.

The relation between engine TWR and ISP in vacuum is also something to think about. High ISP is good because it means less fuel used which means less weight, but low TWR is bad because it means you carry around a lot of dead weight. Nukes are right out - they’re for going interplanetary and if you want to go anywhere beyond medium Kerbin orbit, don’t use a spaceplane. Dead weight is the devil.

Things to avoid

This is all from experience, so learn from me and avoid the same mistakes :(

• All mk2 parts (godawful drag, no really usable cargo bay, if you want to ferry passengers just use mk1 parts)
• Control surfaces or wings mounted behind jet engines (that engine silently loses thrust leaving you scratching your head and wondering why you’re going so slow)
• Parts with middling temperature tolerance at the front of a stack, or low temperature tolerance in general (FAT-455 wings, I’m looking at you)
• Excessive torque caused by axis of thrust not being aligned with center of mass
• Autostruts on wing panels (can cause phantom forces that can create a constant roll moment in one direction, usually left)
• Wings or radial stacks mounted to anything other than the central stack - if you want underwing nacelles just attach them to the central stack and use the offset tool to get them where you want them, attaching them to the wings can cause the same phantom force problems as mentioned above
• Enabling yaw authority on the rudder, it just causes sideslip and drag and things you don’t want when SAS is on
• Steering enabled on any landing gear other than the nose gear
• Gimbaling enabled on airbreathing engines (ok well fine you can have like half a degree to counteract off-center thrust if you really want to but in general you want to avoid it)
• Struts
• External fuel lines
• Control surfaces that respond to more than one input axis (pitch, roll or yaw)

Other assorted advice and rules of thumb

• You almost always need less intakes than you think, but on the other hand they tend to make very low drag nosecones (but on the third hand they tend to weigh more than actual nosecones…)
• Total wing area - including fixed portions of control surfaces - needed for a spaceplane is usually somewhere around total mass in tons divided by three
• Closing intakes does nothing, these days