RO/FAR/RSS Tipping Rocket...

[Grimm Spector]

Trying to follow @NathanKells tutorials playing RP-0, building some sounding rockets. I've got the Aerodynamic point (blue ball) right down near the main engine, using four fins to stabilize. And whether I set them perfectly straight, or on a very slight angle in the hopes of spin stabilizing, my rocket will tip to the side shortly (before 700 m) after launch. I've only got about 8k delta-v on this thing, it's nothing crazy. Using an RD-107 on the main stage. It tips, and keeps tipping, this isn't a gravity turn built into it by design, it tips until it's going back down, and it does this very quickly. I can't seem to set up a rocket without avionic control stabilizing it. I would love some help in understanding what I'm doing wrong....

 

[Spricigo]

If I understood correct it's not a uncontrolled spin, rather a speed up gravity turn. 

Performing a gravity turn is in some sense just matching the time it takes to turn the rocket in the direction it need to be to complete an orbit and the time it takes to reach orbital velocity /height. 

If the rocket is turning to fast try to start the turn later,  reduce the initial angle or increasing the thrust.  In short  turn slower or accelerate/climb faster. 

[Grimm Spector]

14 minutes ago, Spricigo said:

If I understood correct it's not a uncontrolled spin, rather a speed up gravity turn. 

Performing a gravity turn is in some sense just matching the time it takes to turn the rocket in the direction it need to be to complete an orbit and the time it takes to reach orbital velocity /height. 

If the rocket is turning to fast try to start the turn later,  reduce the initial angle or increasing the thrust.  In short  turn slower or accelerate/climb faster. 

I am aware of what a gravity turn is. I'm not trying to do a gravity turn at all. I don't want my rocket to turn, I want to go more or less straight up, with the goal of achieving height to fulfill sounding rocket contracts.

 

• My rocket is NOT angled on the launch pad, so it shouldn't just drift in one direction.
• My rocket does not have any strangely balanced items, everything is symmetrical
• My rocket has no avionics, no control input is occuring

 

All I want is to stop the thing from rapidly tipping to one side, as though it was undergoing a gravity turn. It certainly even if setup for one shouldn't be turning at 700m and being at a lower angle than horizontal not much later.

[Spricigo]

Question: it's a slow(ish) but continuous and inexorably turn or the rocket just change heading at random? 

If what is happening it's the former you are doing a gravity turn.  Just happens to not be your intention. It happens because at some point point it incline just a bit and you don't have torque to turn it up again. 

You are trying to launch straight,  it means you already is trying to turn later (never)  and reduce the initial angle (0°), more thrust may help. 

Other than this it's the usual keep the CoM as high as possible and the CoD as low as possible. 

[EpicSpaceTroll139]

With FAR, making sure the COL ball is behind the COM marker is no longer sufficient. You need to make sure the center of drag (aka center of pressure, COP) is also behind the COM. The true COP unfortunately does not have a visual indicator, so you just kind of have to guesstimate. Try checking the data + stability derivatives in the FAR tab. I find things usually work best when there aren't any red numbers. 

Just looking at your rocket I'm fairly sure your COP is ahead of your COM, meaning you either need some form of guidance to keep it from tipping, or to redesign it with more mass toward the top and/or drag at the bottom.

[Spricigo]

22 minutes ago, EpicSpaceTroll139 said:

Just looking at your rocket I'm fairly sure your COP is ahead of your CoM. 

I'd like to know what are you seeing.  Personally I can see the CoM below the geometrical center,  and the fairly low drag difference between top and bottom portion of the rocket.  But,  if you forgive the pun,  where is the turning point ? 

Edited April 21, 2017 by Spricigo

[EpicSpaceTroll139]

4 hours ago, Spricigo said:

I'd like to know what are you seeing.  Personally I can see the CoM below the geometrical center,  and the fairly low drag difference between top and bottom portion of the rocket.  But,  if you forgive the pun,  where is the turning point ? 

Ok, so I spent a few minutes in Microsoft Paint, and I believe I've come up with a reasonable representation of the problem.

Not sure how reasonable the representation of the fins is, I was trying to get it like I was looking from one side, and not from this 45 degree angle or so.

So I painted the lateral area fore of the center of mass red, and the lateral are aft of the COM blue, and all the forward facing / leading area yellow.

In the second picture I demonstrated the balancing of area fore and aft of the COM. The rocket has an excess of lateral area at the back, which is good. This means that if flying sideways it will generally try to rotate its nose towards prograde. 

However, the forward facing / leading area is a different story. The yellow circles, rings, and rectangles show approximations of the location and amounts of surface area (and thus approx drag) of areas when looking down from the top of the rocket. I didn't bother actually do a similar demonstration of areas canceling out, but it was easy to find that there was significantly more area towards the top. This means that when pointing close to prograde, it ends up wanting to turn to the side.

I'm guessing the effects of lateral and forward facing area balance out at some angle between 0 and 90 degrees, so without control the rocket will end up tilting over to this angle, causing problems. If given active control surfaces, it could probably go straight though, as they would keep it from deviating enough from prograde for aerodynamic forces to cause this issue.

[NathanKell]

Pull up the FAR GUI and do an AoA sweep at Mach 0.5, Mach 1.0, and Mach 1.5. Make sure the yellow line slopes downward. Then empty the tanks and do it again (recall that CoM will likely shift downwards as the tanks empty).

[Grounder]

Probably already tried this, but did you try a slower ascent, keeping it under 500 m/s until you get out of the thicker atmosphere?