designing for aero

[steuben]

Is designing rockets for aero worth it?

I know that some experiments with the nose cones have shown increases in m/s at burn-out. But, in the cases of production rockets do you save enough to shave off a tank or two, or a whole set of boosters?

[bewing]

No. The difference is only one or two units of fuel. A dozen meters per second of deltaV. If fuel is an issue, it may make the difference between getting to your destination or not.

[Pecan]

Streamlining obviously only has any use within an atmosphere so for space-only operations nosecones and the like just add unwanted mass.
If you're launching a rocket through an atmosphere and it's intended to do much once it's in space the added mass may waste more dv on those operations than it saves during launch.
On the other hand ... if you can't launch the thing in the first place because drag keeps making it flip, then it may well be worth adding streamlining/fins just to keep it pointing the right way.

[wumpus]

If you are dealing with career mode silliness [part limits], I'd ditch the aero caps first (but not the fins).  If you are trying to reduce gravity losses (by going fast), I'd recommend making the rocket more aerodynamic.  Note, aero effects should be much [less*] significant on more massive rockets.  Remember, aero drag is proportional to frontal area:

1.25m = 4.9m^2

2.5m = 19.6m^2

3.75 = 44.1m^2

Yet very rarely is a 3.75m rocket only ten times the mass and thrust of a 1.25 rocket.

* thanks, Red Iron Crown

Edited July 26, 2016 by wumpus
oops.

[Foxster]

Depends - like a lot of things in KSP. 

It's a good idea to keep the number of stacks small and to use the thinnest practical stacks. Streamlining does reduce the dV used to get to orbit but whether that difference bothers you or you'd prefer to just over-engineer is up to you. If your rocket is overheating without streamlining then you will likely have to add it though. You definitely will need to streamline for Eve launches as otherwise the drag losses make reaching orbit unlikely. 

Here's the difference between two simple rockets...

Using Mechjeb to get the two craft to orbit with the same flight profile left 619 dV at 100km orbit for the first one and 730 for the one with my favorite mk1 "nosecone". An 18% improvement, which is quite a lot and will be more if you have more stacks. 

Edited July 25, 2016 by Foxster

[Red Iron Crown]

12 hours ago, wumpus said:

Note, aero effects should be much more significant on more massive rockets.  Remember, aero drag is proportional to frontal area:

1.25m = 4.9m^2

2.5m = 19.6m^2

3.75 = 44.1m^2

Yet very rarely is a 3.75m rocket only ten times the mass and thrust of a 1.25 rocket.

Do you mean that aero effects should be less significant for larger rockets, rather than more? I.e. the frontal area has increased by a bit less than 10x, but mass and thrust have increased by much more (typically), so drag should be a less significant factor, no?

[mk1980]

17 hours ago, steuben said:

Is designing rockets for aero worth it?

I know that some experiments with the nose cones have shown increases in m/s at burn-out. But, in the cases of production rockets do you save enough to shave off a tank or two, or a whole set of boosters?

i like to use fairings for stuff that isn't naturally aerodynamic. like satellites or station parts or dedicated space ships. this way you get better aerodynamics in the atmosphere and you don't haul useless nosecones etc. around in space.

i usually put nosecones on radial booster rockets. not necessarily to save deltaV. it's more about making the whole thing easier to control. large planar faces perpendicular to the airstream contribute a lot to flipping issues. nosecones pretty much eliminate flipping issues.

to me, it's more convenient to launch an aerodynamic rocket that will be reasonably stable on its own (even with SAS off) rather than manually fine control some contraption with the aerodynamics of a barn door. i guess it also saves a bit of deltaV. on the flip side, it costs more. not a big deal either way economically (in my opinion)

[wumpus]

2 hours ago, Red Iron Crown said:

Do you mean that aero effects should be less significant for larger rockets, rather than more? I.e. the frontal area has increased by a bit less than 10x, but mass and thrust have increased by much more (typically), so drag should be a less significant factor, no?

Yep.  LESS.  I meant less.  Can't believe I typed more.

Luckily I have coffee now.

[Norcalplanner]

On 7/25/2016 at 3:19 PM, Pecan said:

Streamlining obviously only has any use within an atmosphere so for space-only operations nosecones and the like just add unwanted mass.
If you're launching a rocket through an atmosphere and it's intended to do much once it's in space the added mass may waste more dv on those operations than it saves during launch.
On the other hand ... if you can't launch the thing in the first place because drag keeps making it flip, then it may well be worth adding streamlining/fins just to keep it pointing the right way.

Or you can use one of my favorite tricks, which is attaching a nose cone to a docking port on the pointy end of the rocket. You can ditch the nose cone before your circularization burn to minimize unneeded orbital mass, but still enjoy the beneficial aero during the launch.

I'd also like to suggest using the C7 tapered fuel tank, which adds aerodynamic benefits for the same mass and price as the Rockomax 8.