Aerodynamics for large rockets

[DeadMG]

Here's my rocket.

I've created this rocket in my career for exploration of Duna- specifically, I want to land on both Duna and Ike and return. I've got four main stages- liftoff, orbit, interplanetary, and lander. Liftoff is just a huge mass of SRBs. Orbit is a Mainsail with some fuel (hidden by all the SRBs). After that, the interplanetary stage is an atomic engine (the top fuel tank above the orange one also contains oxidiser for refueling the lander stage). Then underneath the fairing I have a 3-man lander powered by LV909 with a lot of parachutes, etc.

I haven't really played KSP much since about 1.1 or so, and it seems like some of the aerodynamics have changed and my rockets are no longer stable. I tried asking MechJeb to fly them instead but even Mr. Jeb can't fly my rockets stably. I've tried a few variants on this like adding a bunch more wings in various places, but it doesn't seem to have made much difference.

Any suggestions on how I can improve the aerodynamics of this rocket?

[Streetwind]

The first rule of rocketry is: "heavy stuff at the front, draggy stuff at the back". Like the rocket equation itself, this rule is true regardless of size.

That big fairing is a big drag producer, and if the payload inside is bulky but lightweight, you're in for a bad time.
- If possible, make your payload more compact. This not only reduces the amount of drag the fairing will create, but also the distance between the center of mass and the tip of the rocket (which you want to be as small as possible).
- Move your miscellaneous greebling (solars, batteries etc) to the bottom of your rocket stage. It too creates drag, which you do not want at the top.
- Using advanced tweakables (activated in the game settings IIRC), you can modify your rocket's fuel flow rules to drain tanks bottom-first. This makes your CoM move slightly forward in flight.
- Launch in a steeper trajectory. It will cost more dV to orbit, but it will make you leave the dense parts of the amosphere faster, while having less absolute velocity (since you're going less sideways).
- Make sure your payload doesn't wobble, especially if your control point is part of the payload.

Edited February 20, 2018 by Streetwind

[Kryxal]

This craft has a pretty high TWR, especially for matched SRBs.  I think it practically requires a pretty sharp turn off the pad, then gluing yourself to prograde.  If you deviate from prograde with any speed to speak of while the atmosphere still matters, you're going to have a bad time of it.  I'd probably go with 4+4 boosters, with the second set turned down to maybe 60-70%, and the Mainsail is more engine than you should need as well.

[Gargamel]

You really shouldn't need batteries and solar panels on the launch stage anyways.  Your payload (assumedely) has such things already.  Why add more weight?  You can stage the fairings at alt's higher than 50km, the air is thin enough the fairing just adds weight and doesn't do anything more for aerodynamics.    As you're already using MJ, it has the option in the ascent guidance to stage them for you, and then open your solar panels once you reach space.   The engines you're using, I'm assuming again, also have alternators on them, generating EC.  Your batteries should stay fully charged the whole time the engines are firing.   If your ship can't survive without juice for the few minutes it's launching, it's not going to make it to Duna and back. 

[bjerrang]

your fist stage is only boosters , you will need an engine to fire aswell for controls. its way to difficult for control. Does the rocked woble during launch ? , add more struts. 

[Starman4308]

Lots of dense SRBs and a Mainsail at the bottom are dragging CoM down. Fairly blunt fairing tip with a bulge lower down are helping drag center of pressure up. I'd remove the miscellaneous equipment like solar panels from the launch stage, attempt to make the payload a bit more compact, and use bigger fins on the SRBs.

It doesn't help that SRBs do not have any gimbal. With only reaction wheels and small fins, you don't have much control authority to fight dynamic instability, so I would advise ensuring you have enough fin area for full dynamic stability.

[CSE]

I think bjerrang has it right - using the gimbal of the engine you're already carrying is the most efficient way to go.

Reduce your SRBs from eight to six, add a little fuel for the Mainsail, and put the Mainsail in the first stage along with the SRBs. If the ascent still gives you trouble, you can reduce throttle to about half once you've cleared the pad and go a little more slowly through the thick lower atmosphere, then throttle up after; MechJeb may be able to do this for you. However, you might find that a running Mainsail gives you enough control to fly full throttle all the way, and then you can replace the expensive and draggy steering wings with cheap basic fins.

Edited February 22, 2018 by CSE

[GrouchyDevotee]

Could ditch the SRBs entirely, add a couple radial mount fuel tanks with Mainsail engines on them, attach some fuel lines from the new external tanks to the centre tank and use all three engines to launch.

[overkill13]

Unless something has changed since KSP 1.2.2, which I am using, Kickback SRBs have 2 degrees of gimbal.  Otherwise I agree with several people.  Fire the Mainsail with the SRBs, replace some of them with Mainsails or Skippers and set the fuel to burn bottom to top.

[Spricigo]

1 hour ago, overkill13 said:

Unless something has changed since KSP 1.2.2, which I am using, Kickback SRBs have 2 degrees of gimbal.  

Your kickback is modded. 

[Starman4308]

1 hour ago, overkill13 said:

Unless something has changed since KSP 1.2.2, which I am using, Kickback SRBs have 2 degrees of gimbal.  Otherwise I agree with several people.  Fire the Mainsail with the SRBs, replace some of them with Mainsails or Skippers and set the fuel to burn bottom to top.

 

3 minutes ago, Spricigo said:

Your kickback is modded. 

I can confirm that the Kickback is not supposed to have gimbal in stock. The part.cfg in my 1.2.2 install has no ModuleGimbal, and there isn't one on the wiki.

Granted, the Space Shuttle SRB it's based on had gimbal (even if it frankly would've been simpler and lighter to leave all the gimbaling to the RS-25 liquid engines).

[overkill13]

13 minutes ago, Spricigo said:

Your kickback is modded. 

Interesting... Now to find which mod did that...

[Starman4308]

1 minute ago, overkill13 said:

Interesting... Now to find which mod did that...

Do you have a Unix terminal? If you don't know what I'm talking about, don't worry about it.

You could navigate to the relevant directory, and run this command:

find . -name '*.cfg' -print0 | while IFS= read -rd '' file; do if egrep -q -e '@.*PART.+MassiveBooster' "$file"; then echo "$file"; fi; done

After running that on my own GPP install, the most likely candidate is VenStockRevamp/Squad/Parts/Engines.cfg, which adds a 1.5 degree gimbal range.

[Reactordrone]

Even something simple like sharpening up the top of the fairing  will reduce drag at the top of the rocket and improve controllability.

[overkill13]

1 minute ago, Starman4308 said:

Do you have a Unix terminal? If you don't know what I'm talking about, don't worry about it.

I just played around with one, but it didn't like the "while" command.  The easiest option for me might be just to write a final MM patch to remove the gimbal module.

[DeadMG]

Actually, the problem was the complete inverse of what I (and apparently everybody else) expected. There were too many control surfaces, not too few- since the rocket is so tall, a little goes a long way. Once I finally thought to replace the Winglet/Tail Fin with something less effective (the delta-deluxe winglet), it flew fairly easily.

Just for notes, the reason why I have that stuff on the interplanetary stage is because it's actually probe-controlled (there's a probe core hidden under the docking port) so that i can control it whilst the lander is detached.