TWR vs Drag

[Jestersage]

If I ignore aethestic, When should I consider TWR and when to consider drag, for a VAB rocket plane? Debating between using small hardpoint and decopulers

Edited March 30, 2017 by Jestersage

[Foxster]

For Kerbin you are probably fine using either, unless its a really small and marginal craft. For somewhere like Eve though you need to absolutely minimise drag. 

[Spricigo]

 

I'd say you should consider (T-D)WR.

 

[Cunjo Carl]

@Jestersage This depends a lot on your desired launch profile. Steep profiles care more about TWR (in conjunction with low drymass as in your case) and shallow ones care more about drag (specifically ^Lift/_Drag). I agree with @Foxster that on Kerbin, the difference shouldn't be too severe. As a rule of thumb, minimizing the number of stacks is typically best for space planes, but the best way to tell is probably just to try! (or send us a picture and we can hmm and hah.) Also, if you'd like me to foist the governing equations at you (and the thoughts that govern them) let me know.

Edited March 29, 2017 by Cunjo Carl

[Jestersage]

Related: does small hardpoint have less drag then tt38k decoupler?

[fourfa]

1 hour ago, Jestersage said:

Related: does small hardpoint have less drag then tt38k decoupler?

Yes - it has only 1/6 to 1/10 the drag in exchange for weighing twice as much (50kg instead of 25kg).  Also the small hardpoint is bizarrely cheap - only 60 to the TT30K's 600.

[Wanderfound]

8 minutes ago, fourfa said:

Yes - it has only 1/6 to 1/10 the drag in exchange for weighing twice as much (50kg instead of 25kg).  Also the small hardpoint is bizarrely cheap - only 60 to the TT30K's 600.

...which is why I use hardpoints and structural pylons in place of large radial decouplers wherever possible.

They look nicer and leave a clean surface after decoupling, too.

11 hours ago, Jestersage said:

If I ignore aethestic, When should I consider TWR and when to consider drag, for a VAB rocket plane? Debating between using small hardpoint and decopulers

Depends on the flight profile.

The flatter your ascent, the more important drag becomes. The steeper your ascent, the more important mass becomes.

If you're actually using your wings as you ascend, go with low drag. If you're just rocketing up on a ballistic trajectory and gliding back down, go with low mass.