Help with rocket design for atmospheric flight performance

[Pahimarus]

Hi all,

I feel I've started to get the hang of my rocket designs from reading some good posts on the forums as well as from lots and lots of trial and error (feels like the scene from October Sky). I still have some challenges with the in-atmosphere performance of some of my rockets that have been pernicious and I was hoping some of you experienced designers could enlighten me on some more theory. 

For reference, I am playing vanilla KSP current release career mode and have all the 90 science parts unlocked except the aircraft tree with a few 180 science parts.

With some help from the forums I've basically solved my "flippy rocket" problem. What has helped me as far as design principles are:

Rockets fly in atmo like a badminton shuttlecock. You want mass in the front, drag in the back and for stability you want some distance between them. So for solving flippy rockets you can:

1.) Move your center of mass forward. This seems to be difficult as generally your payload is your payload and the basic layout of the booster rocket is often driven by the TWR of the engines you have access to. The main helpful tip here for some rockets has been turning off fuel consumption in my most anterior second stage fuel tank and releasing the fuel only after the after tanks have been nearly exhausted.

2.) Don't fly too fast. Throttling my LF engines to keep TWR around 1.5 has worked well for me. Knowing that I can throttle down SRBs in the vehicle assembly has also been helpful when working with lighter payloads.

3.) Add drag to the back. For me this has been the most helpful (though still feels like the least elegant solution). Sometimes you just need MOAR fins at the back of the rocket. 

I would love any other tips to help with this problem that are more elegant than MOAR fins, but in general, my problem with rockets taking a nosedive mid gravity turn have disappeared.

Unfortunately another trickier problem has cropped up. I've managed to solve it eventually for my various rockets, but I haven't been able to figure out the principles behind why it is happening and how to prevent/fix it.

The basic probably is that mid gravity turn, the rocket suddenly swings like a pendulum and points straight up. I feel like this may be related to the "flippy rocket" problem, the two solutions I've found on specific rockets have been adding MOAR fins on one rocket and using a fairing to cover a very draggy payload on another rocket (to much drag on the anterior part of the craft??). Like I said, I would love some help understanding the "why" this happens so I can avoid it in my designs and fix it without so much trial and error (I admit, I play hardcore once I get my flights into space, but I do use the revert flight option if I have problems in atmo and consider it the roleplay equivalent of the simulations and wind tunnel testing one would be able to do in real life).

Here is an example of the rocket where I solved the problem by adding the fairing.

(Can't figure out how to upload images, hopefully can access through the links).

Original design without fairing

Original Design penduluming during gravity turn

Successful design with fairing

For reference if needed, my typical flight profile is launching 90 degrees eastward with SAS enabled. I make a 5-10 degree gravity turn once velocity exceeds 50m/s and turn off SAS once it stabilizes (usually by 100 m/s if I fly well). I then adjust my throttle to maintain TWR ~1.5 until I reach 35km and reengage SAS, adjust course as needed to circularize as quickly as possible w/ apoapsis between 80-100 km

 

 

Edited January 12, 2020 by Pahimarus
Photo links reversed

[OHara]

You list of principles makes sense to me.  
If this wording makes a difference to you, fins and wing-like parts keep the rocket straight mostly by lift in the back, rather than just drag.  That is, they give a force to the side when they are turned relative to the rocket's direction of motion.  That side force has a nice long lever arm to straighten the rocket. 
And fins don't have much drag in the sense of holding the rocket back, when they are aligned to the airflow --- as they are when they succeed keeping the rocket going straight.

Another approach, when you have a pilot or probe core that can hold prograde  , is to let SAS hold prograde at the stage when you currently turn SAS off.  Steerable fins are helpful for this because their forces go up and down with the relative strength of aerodynamic forces.  But, SAS seems a more brute-force approach, and I would say that "more fins" is the more elegant solution.

The blue ball in the VAB shows center of lift; that is, where the wing-like parts will apply a side-force if the rocket tips relative to is motion through the air.  Great for airplanes, where that is the bulk of the force that turns the craft, but not the full story for rocket stability.  Drag forces are not counted in placing that blue ball, but drag does tend to flip the rocket.

The side mounted parts, and the exposed flat portions when different-sized parts meet along the main stack, get more drag in KSP than one would expect.  (I started a thread (link) collecting facts players have noticed about drag.) KSP treats these as if exposed to the full airflow, ignoring any protection from being tucked into the sides of the rocket.  That is why the fairing in your images was needed. 

.. and welcome to the forum. 
Your image links worked fine; many users take the URL that resolves to the image file, like "https://www.dropbox.com/s/k15dmv4ysbaruzq/screenshot8.png" ending in .png, and give that to the "insert image from URL" button, so that the image appears directly in the post.

Edited January 12, 2020 by OHara

[Confutus]

The CM (center of mass) changes in flight (moves backward) as fuel is burned, and the CL (center of lift, or aerodyanmic forces) also changes, as aerodynamic forces increase with speed.  When the CM and the CL exactly coincide, the rocket is highly sensitive to any changes,  When the CL is ahead of the CM, aerodynamic forces have a greater effect on the attitude of the rocket than the controls (such as engine gimbal and reaction wheel) do.  The rocket swings around the CM, to put the CL behind it. Also in flight close to horizontal, gravity acting on the CM but not on the CL may have a turning effect.  Since the drag increases with speed, horizontal flight at low speed also tends to be unstable.     What I've found works best is to pay strict attention to the CM and CL indicators while I am building the rocket in the VAB for both filled and empty tanks and make sure the CL stays behind the CM in all cases. Placing the fins as close to the rear as possible helps.  Then, in flight, aerodynamic forces help maintain my attitude instead of changing it.

[Cavscout74]

2 hours ago, Pahimarus said:

The main helpful tip here for some rockets has been turning off fuel consumption in my most anterior second stage fuel tank and releasing the fuel only after the after tanks have been nearly exhausted

Rather than turning off tanks, you can adjust the fuel flow priority on the part window of each fuel tank so the lower tanks drain first.  You may need advanced tweakables turned on in settings.  So for example if you have 3 fuel tanks in the first stage, the lowest should have +1 priority, the middle 0 and the top -1 priority so the lowest drains first, moving your CoM higher as you accelerate. 

[Pahimarus]

1 hour ago, OHara said:

If this wording makes a difference to you, fins and wing-like parts keep the rocket straight mostly by lift in the back, rather than just drag.  That is, they give a force to the side when they are turned relative to the rocket's direction of motion.  That side force has a nice long lever arm to straighten the rocket. 
And fins don't have much drag in the sense of holding the rocket back, when they are aligned to the airflow --- as they are when they succeed keeping the rocket going straight.

You phrased it very politely, but this is more than semantics, this is knowledge. Thank you, this was exactly the sort of information I was looking for.

Quote

Another approach, when you have a pilot or probe core that can hold prograde  , is to let SAS hold prograde at the stage when you currently turn SAS off.  Steerable fins are helpful for this because their forces go up and down with the relative strength of aerodynamic forces.  But, SAS seems a more brute-force approach, and I would say that "more fins" is the more elegant solution.

That makes sense. Sometimes I have some trouble with roll and yaw during the unassisted gravity turn. This comment made me wonder if I could use the advanced tweakables to disable pitch control from the SAS when I start my gravity turn to let gravity take over but still allow it to maintain yaw and roll constant. Nonetheless, there is something beautiful about a rocket than can ride a smooth gravity turn all the way to orbit without any controls inputs required (other than throttle). Makes you feel like you got the design just right.

Quote

The blue ball in the VAB shows center of lift; that is, where the wing-like parts will apply a side-force if the rocket tips relative to is motion through the air.  Great for airplanes, where that is the bulk of the force that turns the craft, but not the full story for rocket stability.  Drag forces are not counted in placing that blue ball, but drag does tend to flip the rocket.

The side mounted parts, and the exposed flat portions when different-sized parts meet along the main stack, get more drag in KSP than one would expect.  (I started a thread (link) collecting facts players have noticed about drag.) KSP treats these as if exposed to the full airflow, ignoring any protection from being tucked into the sides of the rocket.  That is why the fairing in your images was needed. 

I read through your drag post. Very thorough and interesting. Some of it is a bit beyond my ken but I did notice one interesting component of your analysis that may be relevant to this problem. If I understood your post correctly, it looks like the drag forces on trailing parts decline significantly at Mach 1. Looking at my screenshot, it looks like my "flip out" happens right as I approach the sound barrier. The fairing solving the problem does suggest that the underlying problem was drag at the front of the spacecraft. Perhaps at subsonic speeds the trailing part drag is sufficiently balanced with the leading part drag that the lift from the fins and the force of gravity on the COM are able to maintain straight flight, but when it passes through Mach 1, the relative increase in drag at the front of the craft as the rear drag decreases is enough to throw the forces out of balance and push it out of straight flight.

Quote

.. and welcome to the forum. 
Your image links worked fine; many users take the URL that resolves to the image file, like "https://www.dropbox.com/s/k15dmv4ysbaruzq/screenshot8.png" ending in .png, and give that to the "insert image from URL" button, so that the image appears directly in the post.

Thanks! I'll give that a try with my next post. I have few more nagging challenges I may pick your collective rocket scientist brains on.