Panther NERV Career Spaceplanes

[AeroGav]

Preamble

Most people building Space Planes use RAPIER engines,  however  you are unlikely to see those till the very end of career mode,  being Tier 9  (1000 Science points).   While not as popular as RAPIER designs,  Whiplash Space Planes aren't uncommon,  but as Tier 8 engines, they have a Science cost of 550,  which means you aren't getting them without a  max level R&D  building.  And that's a hugely expensive upgrade,  given that funds tend to be harder to acquire than science points.  

 

So,  for 90% of our career game, the only realistic engine option is the Panther.   It's a fine engine, but it's thrust output peaks at mach 2.5 and goes to zero by mach 3.    Realistically, your space plane is not getting much above mach 2.5 on jet power,   and that means your rockets need to add another 4.5 mach's worth of velocity to reach orbit.      On chemical engines,  this requires a very large fuel fraction that leaves little room for payload, and that payload fraction soon disappears altogether if you attempt to  go beyond low orbit.         

However,  you can get NERV engines with a Tier 2 science building,  and with over twice the ISP  you don't need to turn your craft into one big fuel tank just to reach orbit.   These engines do have a miserable TWR of course - the measly LV-909 Terrier has the same thrust, but weighs just 0.5 tons instead of 3.      Your vessel is never going to have a TWR > 1 or even close to it.      For a rocket,   this is a major problem.      However ,  you have to remember that your spaceplane is an airplane and it does not need a TWR over 1 to fly.     The engines will be thrusting horizontally to accelerate you to orbital velocity,   the wings take care of gravity, to stop you plummeting to the ground.      So long as your thrust exceeds drag, and the wings are making sufficient lift to counteract gravity,  you will go to space today.

 

Lift to Drag Ratio

 

Angle of attack is the difference between where the  nose is pointing and where the airplane is actually going.    

For wing and control surface parts,  lift and drag are both zero,  at zero angle of attack.    As AoA goes above zero, lift increases rapidly, but is subject to  diminishing returns.  After 30 degrees , lift starts decreasing with further increases of AoA because you've stalled.       Drag increases steadily with angle of attack, all the way to 90 degrees.      

Non wing parts also produce more drag with increasing AoA, however they still produce considerable drag even at zero AoA.    They also produce little or no lift.

The important things to remember from this -

• In supersonic flight,  best lift to drag ratio happens at about 5 degrees ApA.  
• Unlike real airplanes in KSP,  the wing generates less than 20% of the drag your craft is producing. 
• Some parts are very draggy.    Solar panels, even deployable ones, need to be in a service bay.  1.25m bicouplers are meant for use on rocket stacks,  and are very draggy on an airplane.   Mk2 fuselage parts should be avoided, they make  250% of the drag of a mk1 part of the same length and fuel capacity.
• Mismatched attach nodes create massive drag.   A 1.25m attach node  should only ever join to another 1.25m  node.  If your fuselage is tapering down to 0.625m,  use an NCS adapter to smooth the transition.  If it's flaring out to a mk2 cargo bay, use a mk1 to mk2 adapter short.

 That second point is counterintuitive - people often reduce wing area thinking it will reduce drag,  but you end up needing a higher angle of attack to get the same lift, or  flying at a lower altitude where the air is thicker.    When on jet power, a smaller wing can  improve performance, because at lower altitudes your engine can make more power.  

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TIP '#1 - When flying to orbit,   stay below 14km when on jet power.   Accelerate in level flight between 10km and 14km until you've reached 750m/s.  At that point,  jet thrust will fall away rapidly, time to start the nukes.  

Once you've staged in the nukes however,  you are burning fuel rapidly and it's all about getting the best exchange rate possible when converting your precious forward velocity into lift.

This means, angle the nose for 5 degrees above prograde.  It doesn't matter whether prograde is showing a climb or a dive -- just point  5 degrees above where it is pointing.

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NOOB ERROR 1 - Surely If i can get enough lift at 2 degrees,  i will have lower drag than if i pitch to 5.

ANSWER -   No.   Whilst it is correct you will have lower drag in that exact instant,  it will delay your airplane climbing to higher altitudes, where drag is less.  Over the course of the entire ascent, the lowest total drag is obtained by remaining pitched at optimal ;L/D - 5 degrees - even if that produces surplus lift at times, and insufficient at others.       The only time you pitch away from optimal L/D is when you are on jet power, because the Panther has ISP  5 times higher than the NERV even in afterburner, so the overriding consideration is to make sure you reach its top speed (750m/s) before staging.

 

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NOOB ERROR 2 - Oh no I'm descending , pitch up !

See above.  Just stay at 5 degrees above prograde, eventually as speed increases, so will lift and you'll start climbing again.

 

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NOOB ERROR 3 - Won't the wings stop working above 20km/25km/Whatever?   I need to pitch up to radial out to keep us climbing !

ANSWER - As you get closer to orbital velocity, centrifugal force counteracts an increasingly large portion of your airplane's weight,  which will cause it to drift higher as it needs less lift.   There is no need to yank the nose up, stall,  and start producing vast amounts of drag.  Most of the time you'll be in a shallow climb, prograde just a couple of degrees above the horizon.  You'll start the nukes iat about 14km and find yourself throttling back somewhere between 35 and 40 km because AP is going above 70km.  If your design is particularly slick and well flown,  you'll notice AP continues to go up despite the engines being cut, due to residual lift.

 

 

NERV in practice - Building an Airplane

The basics aren't all that demanding

• One NERV for every 12 tons or so of airplane
• One Panther for every 2 NERVs
• About 30% of the craft's weight should be fuel.  2 MK1 liquid fuel tanks are enough to get a basic 2 nerv design into orbit.
• A lift rating of 7 to 10 per 20 tons is about right.  There is a lot of wiggle room here, but in general more wing makes the NERV powered part of the climb easier, at the expense of the jet powered part. 

Avoid using mk2 parts due to their high drag, only if you must have a cargo bay.   Mk1 parts are not as heat tolerant,  so use an inline cockpit as it will be protected by the nose cone, intake or service bay in front.

The majority of your effort should be  in making sure the airplane is stable and easy to fly with precision.  This will make the flight more enjoyable and allow you to maintain the angle of attack values where lift to drag is good.

The major issue is balancing the heavy engines with non fuel parts,  and keeping the fuel amidships, so there is no centre of mass shift between full and empty.   For example, this works -

Cockpit, Passenger Cabin,  Engine Nacelle intake, 2 fuel tanks, then our Panther.  The nukes are either side.  RCS build aid is on ,  but you can't see the red ball because the dry CoM is in the exact same place as the wet one.

We have 6T liquid fuel for a total mass of 17T.  About 30% fuel so that's good,  the engine to mass ratio is good too, leaning slightly on the overpowered side. 

Now we just add wings.  Canard or Tailplane design is totally up to you, just make sure your wings don't get in the way of the exhaust plume.  Since the first pic is of a canard version of this airplane, let's go with a tailplane this time.

And there we go .  As you can see,  Kerbal Wind tunnel shows if your design has enough power to go supersonic, and what its top speed will be at every altitude.  Note, it takes account of drag from landing gear, so you need to raise the gear before running this analysis, or just do it before fitting any like i did here.  Note also that you can  bind your nukes to an action group so you can turn them on between 300-400m/s if your design has trouble busting mach 1 on jet power alone.

Note how the blue arrow is towards the back of the yellow ball.  Centre of lft must be behind centre of mass, but excessively aft CoL will suffer excessive drag, because you need to generate a lot of force with the control surfaces to get the nose up to make lift.   Large deflection angles on your control surfaces are draggy.   That is why it is better to fit generously sized ones and use the "limit authority" slider than use tiny ones that are maxed out.    Because we used RCS Build Aid to ensure that our centre of mass does not change as fuel burns off, we can get away with having CoL fairly close behind our Centre of Mass.

Test Flight

A complaint I often hear about spaceplanes is that you waste  20 minutes getting to altitude on every failed attempt.

However, there is a lot you can learn without getting above 500m.      Take SAS off,  and attempt to stall the plane,  how controllable is it?  Throttle up and down, does the nose rise and fall, have you got a centre of thrust problem?

Does it break mach 1 with one panther per 20 tons?  If it can do so, without being ridiculously overpowered, then its not got any drag problems that will stop it from flying under NERV power.  Of course , Kerbal Wind Tunnel will tell you this without even needing to leave the hangar.

If all of the above is true, and you are flying a sensible profile, and have stuck to my recommendations regarding engines per ton and fuel mass percentage,  it should make orbit ! 

 

 

 

 

 

Edited October 12, 2018 by AeroGav