Wanderfound's Spaceplane Design Guide for FAR/NEAR

[Wanderfound]

Spaceplane Design for FAR/NEAR 102

1) Decide on a basic engine/fuselage/wing configuration. How many engines and what sort? Mk 1 or 2 fuselages? Full length for all engines, or nacelles for some? Swept or delta wings? Wings set high, low or centred? Mid-mount or rear? Canards? Tailplanes?

2) Build the core fuselage first, with engines but without detailing. Then make the sides: additional fuselage sections, engines, intakes, nacelles, wing pieces, control surfaces. Engage symmetry and slide this piece up and down the core until you're happy with the CoM/dCoM position and relationship. If you can be bothered, remove the intakes and engines and replace them in order to control the air-demand priority: http://forum.kerbalspaceprogram.com/threads/64362-Fuel-Flow-Rules-%280-23-5%29

3) Tweak the wing position, add or remove canards and stabilisers. Get the CoL placed right. Adjust the control surface tweakables.

3a) Check the aerodynamic analyses. Continue wing-fiddling until these are right.

4) Add accessories; batteries, solar panels, science gear etc.

5) Add landing gear, fuel lines and struts.

6) Place RCS thrusters. Balance.

7) Place Vernors.

8) Action groups.

9) Paint

If you need to go backwards through these stages (e.g. moving the wings after placing the RCS thrusters), it's safest to re-do from scratch all stages starting from the earliest returned-to stage. This will avoid a lot of the quirks of the SPH symmetry function.

Fuel Pumping

Get familiar with the fuel flow rules: http://forum.kerbalspaceprogram.com/threads/64362-Fuel-Flow-Rules-(0-24-2)

The TLDR version of that is that fuel comes from the farthest tank first, but you can manipulate which tank is "farthest" by use of fuel hoses and parallel tanks.

My normal arrangement on a fuselage + outriggers design is to run fuel hoses at the nose from the lateral tanks to the core. This makes the rear-lateral tanks the farthest (and therefore first-drained) from the core engines. I then add a pair of fuel lines from the rear of the core directly to the lateral engines; this forces them to draw via the core, making rear lateral the first tank to drain for them as well.

This arrangement ensures that any initial mass shift will be forwards, enhancing stability. After the lateral tanks drain, the core will burn from front to rear.

In some circumstances, particularly on single-fuselage planes, it can be worthwhile to lock off a forward tank as a "reserve". Just use the right-click tweakable to disable fuel flow, and remember to reenable it when appropriate.

All of these balancing tricks become superfluous if you have TAC Fuel Balancer (good) or Goodspeed (better) installed. Goodspeed in particular allows for all sorts of pumping/balancing possibilities.

dCoM Hunting

Get RCS Build Aid. Seriously. You need it.

Anyway, now that you can see your CoM and dCoM (dry, unfuelled centre of mass) markers at the same time, you want to start bringing them together. You want to get the CoM/dCoM offset down to 1m or less. Fortunately, it isn't too hard to do.

The markers update live. Build your tank/engine/intake/wing combo, engage symmetry, and slide them up and down the fuselage. Watch the CoM/dCoM relationship change as you do. Spread your fuel load laterally, not longitudinally. Mid-mounted engines provide more flexibility in fuel placement.

If you're carrying cargo, place the CoMs under the middle of the cargo bay. Think about the location of CoMs in relation to landing gear and takeoff rotation. Get your weight balance right, then fiddle about with CoL.

When adjusting more precisely, remember that parts without fuel move both CoMs but parts with fuel move CoM much more than dCoM. Structural fuselages are useful, as are partly-full tanks. Weight a long way from CoM has a big effect, weight close to CoM a small effect.

You can design perfectly well just by eyeball and flight testing; the flight data isn't absolutely necessary. But it does help if you can use it.

The first screen in Ferram is the static analysis screen. This gives you pretty

graphs.

It has two buttons down at the lower right: Sweep AoA and Sweep Mach. There are boxes to the left of that which say Lower, Upper and Mach/AoA. This page can produce two different graphs: if you press the Sweep AoA button, it shows the behaviour of your plane from Angle of Attack values between Lower and Upper, at the speed shown in the Mach/AoA box. If you press the Sweep Mach button, it shows behaviour at speeds between Lower and Upper at the AoA shown in the Mach/AoA box.

The blue line is the Coefficient of Lift. It's good when this is high.

The red line is the Coefficient of Drag. It's good when this is low.

The yellow line is the Coefficient of Manoeuvrability/Instability. You don't want this to be above zero, and it's usually best when it's angling down like it is here.

The green line is lift divided by drag. It's good when this is high.

The picture above shows how the plane will act at Angles of Attack between 0° and 25° while travelling at Mach 2.

Some of the lines split into two lines. This shows how the plane responds after a stall: you get a sudden loss of lift and increase in drag that lasts until you return your AoA to where the line isn't split any more.

This is the same picture at Mach 0.8. See how the plane no longer stalls at that speed?

If you click the Sweep Mach button, you instead get a look at a bunch of different speeds with Angle of Attack held constant. This shows Mach 0-6 with a 3° AoA. The bumpiness on the left shows the effect of breaking through the sound barrier.

The second page of Ferram is data and stability derivatives. This produces scary looking numbers.

To get those numbers, you need to put in values for temperature, altitude (air density) and speed.

Don't worry much about temperature; just use 20 for sea level, 0 for low altitude and -20 for high up. For altitude, use these numbers:

0m = 1.225 kg per cubic metre.

5,000m = 0.45

10,000m = 0.16

15,000m = 0.06

20,000m = 0.02

25,000m = 0.008

30,000m = 0.002

All of the confusing letters next to the output numbers relate to this picture here:

x is forwards, y is sideways, z is down. P is roll, Q is pitch, R is yaw. Don't worry about the Greek for now.

If you hover your mouse over any of the numbers, it'll pop up a tooltip explaining what it refers to. Mostly, however, all you want to do is make as many as possible of the numbers green and as few as possible red.

The one other useful thing on this screen is the "level flight" stuff up top right. If you set the analysis for sea level pressure and temperature and the speed for whatever you think you can reach on the runway, you can find out how much AoA you need to take off (the "level flight" value). Try to keep that number below ten for easy takeoffs.

How to Apply FAR Aero Analyses

The pretty graphs: use these to check for excessive drag and regions of instability. AoA graph is the more useful of the two.

Numbers: use the Level Flight figure to work on your takeoff speed and flap settings. For the stability figures, check takeoff (1.125 pressure and .35 or so speed), low altitude (1 pressure and .8 speed), Mach 1 (speed obvious, pressure up to you) and edge of rocketry (0.01 pressure, Mach 5). You want green everywhere. You won't get it, at least to start with.

Hover your mouse over the red ones and use the picture above to work out what the tooltips mean. Once you decode the x's and y's, usually they just mean something like "has a tendency to roll when pitching up" or similar. Sometimes the solutions are obvious (e.g. too much yaw slippage, add a rudder), sometimes they take a great deal of trial and error to sort out.

Yes, these do affect the analyses.

FAR allows some options beyond stock here. You can activate or deactivate axes, adjust max deflection and set flaps and spoilers.

With the axes, think about what each control surface is intended to do. Control surfaces should be as far from their axis of rotation as possible, and as close as possible to the other axes. So, elevators at front and rear on the centreline, ailerons out to the side, flaps and spoilers at mid-centre, rudder right up the back, maybe some elevons mid-wing if it's a rear delta design. It's usually best if a control surface has one job only; you don't want inputs for one thing affecting something else.

Maximum control authority is basically the thrust limiter for your control surfaces. Too little, and the plane won't do what you tell it to; too much, and the plane becomes a hair-triggered bomb waiting for an aerodynamic failure. How much is largely a matter of personal taste.

Generally though, you want the values on rear surfaces to be higher than those up front (canards etc.). Manoeuvrability and stability are two sides of the same coin; improving one degrades the other and vice-versa. Forward-mounted control surfaces are better for manoeuvrability; rear-mounted control surfaces are better for stability. Mid-mounted surfaces should be ailerons on the wingtips and pure flaps/spoilers near the centreline.

Flaps add drag and increase lift. Use them to lower your stall speed at takeoff and landing. Spoilers add drag and reduce lift. Use them to lower your plane to the runway without needing to pitch down.

In either case, the control surfaces need to be right on CoM or balanced either side of it. If this isn't the case, activating the flaps/spoilers will also cause an alteration in pitch. Don't bother with flaps unless you need them for takeoff, and be sure to set a pair of action groups to raise and lower them. Do consider spoilers; they make landing much easier.

Edited October 1, 2014 by Wanderfound