Trying to make a Darkstar SR72

[AeroGav]

Well if M$ FlightSim can have one,  surely to goodness we can in KSP, right ?

Only thing is,  some of the asthetic elements conflict with the way I usually build.    The underslung engine tunnels are a case in point ! 

If I put engines here, it will pitch up under power.  If I put fuel here,  and leave my engines at the height of the main stack,  it will pitch down.   Are you supposed to compensate by thrusting slightly off axis ?  I've always struggled to make that work,  possibly because i like to keep my engines as close to CoM as possible.

The problem is if you have two NERV and a Whiplash (8 tons of engine) hanging out the back,   you can only really balance that by putting loads of fuel up front..  which causes a problem as the tanks start to empty.

I created an extra stack for the intakes and used offset tool to move them down,  and tried to disguise (not v successfully) the number and location of engines by putting a tunnel around them. These engine tunnels should be longer, but that's a lot of drag to add if you're just faking it with empty fuselage sections that are not storing fuel.

Or should i go with fairings ?

 

Or shouldI bit the bullet, mount engines and fuel low with only the cockpit above centreline ?  Like a sukhoi flanker ?

 

https://kerbalx.com/AeroGav/DARKSTAR

 

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[ColdJ]

5 hours ago, AeroGav said:

Well if M$ FlightSim can have one,  surely to goodness we can in KSP, right ?

If you hide elevons in the front fuselage you can adjust them to compensate for the thrust pushing the nose up.

[AeroGav]

Alright,  my second attempt,   "Big S Wing Strakes are the Meta".

The cockpit has type B nose cones fore and aft.     This section sits above the main aero surface plane of the vehicle.    There is another type B cone mounted low to the far end of the aft cockpit cone, to which the Whiplash engine is mounted.

 

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Then it was merely a matter of building up the planform out of Big S strakes.    I also built a pair of engine tunnels with these strakes hanging under the wing.  This is a feature of the SR72 design and the channel formed in the middle is probably for hypersonic compression lift. 

The NERVs ?  I use NCS adapters to attach radially, though the NCS face backward.  On their 1.25m attach face goes the rear node of the NERV, and on the front face of the NERV I put adjustable ramp intakes,. Abuse the offset tool so the nozzle is no longer blocked and we're good to go.  Only thing is, I didn't quite get the look I wanted because the strakes are so thick, but the upside is they hold a lot of fuel - more on that later. The NERVs are mounted near the front of the engine tunnels which helps with CG and to disguise the true nature of the powerplant..  you just see F-22 style rectangular "nozzles".

The fact that there is fuel in the "floor" of the engine ducts helps stop CoM shifting vertically as fuel burns off.   I did rotate and offset vertically the engines in an effort to remove the thrust torque, and think i more or less succeeded.   However, the type B nose cones do create aerodynamic effects not taken into account of by the stock Aero force indicator.  Also, when all the strakes are filled with fuel CoM shifts to the rear. I was not able to keep CoM from moving fore-aft except by violating conservation of volume (hiding a trim tank inside the cockpit!),  which is something i'm very loth to do.

So , instead I set one pair of rear "fuselage" strakes to higher priority and empty first.   Once these are gone,  there is no further CoM shift as fuel is used,  but that does make her very squirrelly just after takeoff.  Oh well,  the P-51 Mustang was like that too apparently, when the rear fuselage tank was full.

With all the wing area provided by all them strakes, the airplane really does not want to go supersonic below 10km.   For the 17km speedrun,  acceleration dies away above 1100m/s.  But once you light the NERVs and dial in enough pitch trim to get a steady 5 degree AoA, lift drag ratio can get to 3:1 or better (the previous version of the craft was nowhere near) and it soars upstairs.    Once you've got the right amount of pitch trim, it can more of less be left to its own devices , even with SAS off.  Just the odd roll correction, which let me pan out and take screenies.

All those strakes hold a lot of fuel - we still had 50% left by the time we'd made orbit.   Easily enough to reach Duna, though this craft lacks amenities.   I'm struggling to think where a docking adapter or service bay could go - attaching to cockpit makes the airplane look like it needs serious rhinoplasty.

Flight timeline in spoilers

 

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Will add a link shortly in KerbalX,  just need  a quick break from Kerbalizing..

 

[AeroGav]

Alright, fresh from getting my a55 kicked in Civ 6,  I made a link to the craft file if anyone wants to try it / dissect it in the name of science.

 

https://kerbalx.com/AeroGav/Dark-Star-2