Center of Pressure in Rocket Stability

[Apollo13]

When I was active in model rocketry some 45+ years ago, we designed our rockets such that the Center of Pressure (CoP) was behind the Center of Gravity (CoG). In KSP, we refer to CoG as Center of Mass (CoM).

In the VAB, we can see CoM and CoT, but not CoP when designing rockets. Why is CoP not needed here? Now, I just read on the NASA site that modern rockets depend on thruster vectoring rather than aerodynamics to maintain stability. So, I'll assume that's the answer to my question.

I'm looking at all this because at about the 30km altitude, my rockets tend to fall over, i.e., pitch drops to 0 or negative. Some spin about their CoM. I'm guessing my rockets are too tall (have three stages: booster, cruise, command module with tank and engine). Usually, the rockets falls over during the cruise stage.

Other than making the stages very wide by packing on tanks and boosters (which I've seen plenty of videos), what need I do to stabilize the taller rockets? I've been able to coax many into orbit. But the failure rate is about 75-80%.

Any thoughts?

[KasperVld]

Can you show us some screenshots of your rockets? Adding some stability enhancers should help your rockets maintain stability a little. A thrust vector capable main engine is also a must have

[Whackjob]

I suspect aerospike nozzles or aircraft engines. I see the same thing with those, from time to time. Their thrust is treated differently.

[Johnno]

Fins/canards, gimballed main engine(s), reaction wheels. That's all there is to it really.

[AmpsterMan]

What is Center Of Pressure? I haven't been taking it into acount at all

[Apollo13]

Sorry for the night shot. Also, I've tried moving those boosters up and down. I've used reaction wheels as well. The main center engine is thrust vectored. However, I usually lock it during boost phase and unlock/lock during cruise.

Edited September 1, 2013 by Apollo13

[sojourner]

1. remove the control surfaces you have high up on the rocket. KSP doesn't work well with control surfaces on rockets above the COM. Those control surfaces cause problems in low atmosphere and are useless in the thin high atmosphere. 2. leave the gimbal active on your first stage center engine. disable all others. (second stage central engine can also remain active gimbal) *yes, it's annoying that compared to real life KSP numbers stages in reverse order.

Edited September 1, 2013 by sojourner
Damn the forum for not preserving formatting. Hope this is fixed soon.

[lajoswinkler]

There is center of pressure. It's called center of lift. The blue marker. At least that should be it.

Magenta is center of thrust and it's always on the nozzle of the currently lowest rocket.

I think the blue marker doesn't work. Mine is constantly on the ground and doesn't move no matter what I strap to my rocket. Only the center of mass moves.

[Supernovy]

It sounds to me like it might be a Drag issue, they are hard to detect due to the lack of a stock centre of drag indicator (similar to centre of pressure, as I've heard). Since KSP drag is proportional to mass, the centre of drag is usually right on the centre of mass, resulting in no torque. When you add parts that don't have a drag value of 0.2 (which most parts have), this can move the centre of drag, making the rocket stable or unstable depending on where the part is. Even more so with high mass differing drag parts like the Aerospike and Cupola.

So, you can make it more stable by adding higher drag parts near the bottom or lower drag parts near the top. You could also try a gentler gravity turn (e.g. following prograde) or slower ascent to reduce the aerodynamic forces on the craft.

[Apollo13]

There is center of pressure. It's called center of lift. The blue marker. At least that should be it.

Center of Pressure is not Center of Lift. CoL applies to spaceplanes.

Thanks for the replies folks. I'm using your suggestions in-program right now. Wish me luck.

[CalculusWarrior]

Apollo13, I have had similar issues with early unplanned catastrophic pitchovers while designing my DreamChaser replica. For an unstable rocket like that, it is best to forget everything the forums have taught us in pitching over early to improve efficiency; take it veeerrrrryyyyy slowly. I don't even begin to turn until after 15 km, and often still lose control. I find that placing RCS thrusters on the topmost section of your rocket is useful as it gives you that extra little bit of control needed to prevent the rocket from flipping upside-down. Placing larger-than-normal control surfaces on the bottom of the rocket is handy as well. Lastly, (I don't know how well this would apply to you) making a two-stage lifter will help greatly in launching, as a small second stage with a high-gimbal engine is a massive blessing when in the 20-30 km range. Above 35 km, the air becomes thinner and it is the capsule torque and engine gimbal that matter more for control. Hope this helps!

[lajoswinkler]

It's analogous. Rockets don't use aerodynamic lift, but they're influenced by wind or simply drag from their lateral movement. Knowing where that point is is very important and we don't know.

You've said you were into model rockety. I'm sure you remember the easy method of determining the approximate position of this point. The cardboard cut out method.

Mos of the rockets we design in KSP don't follow that point position rule at all because they rely on SAS. Too bad we can't use that rule, too. It would make our rockets so much stable.

[Apollo13]

Calculus Warrior: I've used a two-stage lifter with some success. I'll keep at it. Also, sometimes, I won't make a turn until 20km+. Quite often, I'll turn to an 80 degree pitch, and it still just falls over.

lajoswinkler: when designing model rockets, I'd first calculate the CoP, then design fins so as to move the CoP behind CoG. I'd use the cardboard method as first test. Then, for "first flight" I'd tie a string at the CoG (with engine loaded) and twirl it around (about a 6 foot diameter) to verify the design was correct.

Perhaps, my velocity at altitude is too low or too high. Might that be it?

Sidebar: I flew Estes rockets. I was just on their site. Engines are 4 for 10.39. Ouch. I thought 4 for $2 was expensive back in 1965.

Edited September 2, 2013 by Apollo13

[alioth]

I have a rover with a skycrane which must be launched 90 degrees from its landing attitude, which leads to a slightly off centre centre-of-mass. This lead to almost uncontrollable rockets when trying to launch. I found putting one of the small orange Rockomax engines on the "heavy side" of the rover stage near the top of the stack made for an almost always successful launch. (It feeds off the rover's fuel tank - I've got to remember to transfer fuel before detaching and landing the rover or bad stuff happens!)

I've stopped using the skycrane for these rovers, since munar gravity is sufficiently low (the intended operating environment for this rover) I can land the rover on its tail and use reaction wheels to tip it onto its wheels after landing.

Edited September 2, 2013 by alioth

[magnemoe]

It sounds to me like it might be a Drag issue, they are hard to detect due to the lack of a stock centre of drag indicator (similar to centre of pressure, as I've heard). Since KSP drag is proportional to mass, the centre of drag is usually right on the centre of mass, resulting in no torque. When you add parts that don't have a drag value of 0.2 (which most parts have), this can move the centre of drag, making the rocket stable or unstable depending on where the part is. Even more so with high mass differing drag parts like the Aerospike and Cupola.

So, you can make it more stable by adding higher drag parts near the bottom or lower drag parts near the top. You could also try a gentler gravity turn (e.g. following prograde) or slower ascent to reduce the aerodynamic forces on the craft.

Think part count also give more drag, I had an Eve lander who wanted to flip all the time as it had many struts on top. not much weight but an decent part count. Think drag is mostly an issue during landing and aerobraking as here 4-5 g with drag is not uncommon and you can run into fun like landers who insist of turning nose down during decent.

[AmpsterMan]

So I have a question. It has been said that KSP doesn't do well with control surfaces above the center of mass-because-reasons. Now, does this mean that since my command pod is practically a control surface because of it's reaction wheels that disabling them should give me more control?

[lajoswinkler]

lajoswinkler: when designing model rockets, I'd first calculate the CoP, then design fins so as to move the CoP behind CoG. I'd use the cardboard method as first test. Then, for "first flight" I'd tie a string at the CoG (with engine loaded) and twirl it around (about a 6 foot diameter) to verify the design was correct.

Perhaps, my velocity at altitude is too low or too high. Might that be it?

Sidebar: I flew Estes rockets. I was just on their site. Engines are 4 for 10.39. Ouch. I thought 4 for $2 was expensive back in 1965.

That's what I used, too. Cardboard and twirling, then small weights in the nosecone for the fine tuning. It worked fine. Never had any problems, except faulty engines.

It could be that your velocity is too high. I remember having lots of problems with that when launching larger rockets. The whole vehicle, supplied with ASAS+RCS, would start correcting the resonant motion and that would often end up terribly. Use the accelerometer and try staying below 2 G below 50 km. That's what I do...

$10.39? If you include the inflation, it doesn't seem like much has changed. I'd say they're even cheaper. I've never bought an engine. My rockets were all built from the scratch, totally homemade.

[Traches]

You can get an idea where your center of pressure is by rotating your rocket around in the VAB with the blue marker turned on-- the blue marker assumes air is always coming from the same direction. I'm pretty sure it doesn't take drag into effect-- just your control surfaces, so not insanely useful.

[Genius Evil]

So I have a question. It has been said that KSP doesn't do well with control surfaces above the center of mass-because-reasons. Now, does this mean that since my command pod is practically a control surface because of it's reaction wheels that disabling them should give me more control?

No. Command pods have reaction wheels, which work anywhere. Engine gimbals and fins, however, steer the wrong way when above the CoM.

[loknar]

<shameless plug of mod>

I hate mods in every game. But F.A.R aerodynamic consideration mod is essential to me since KSP alpha does not do a good job at all to account for drag and stall.

</shameless plug of mod>

With FAR:

1) you have to build tall, not wide. The OP's rocket would do great.

2) Remember not to exceed terminal velocity

3) use low AoA (also to avoid wasting fuel)

4) use your instincts.

[Conte_Vincero]

My instincts tell me that I should strap SRBs to every part with available space. Will that be OK?

[Apollo13]

<shameless plug of mod>

I hate mods in every game. But F.A.R aerodynamic consideration mod is essential to me since KSP alpha does not do a good job at all to account for drag and stall.

</shameless plug of mod>

I'll take a look at FAR. I've seen many references to it. I just didn't want to add another mod. (Full disclosure: I use many, such as MechJeb(for planning, but manually execute), NovaPunch,KAS,Alarm Clock). Thanks

[Traches]

I hate mods in every game.

Clearly you've been playing the wrong games/mods.

[JiWint]

umm I usually add inline reaction wheel and more struts,if my rocket starts wobbling or spinning.If your rocket begins to spin in atmosphere,you can add wings with control surfaces.

Respect for all the work you did before!

[numerobis]

If you lose control at 30km, it's unlikely to be because of aerodynamics -- there's little air left there (0.2% of sea level air density). Also, most KSP parts suffer the same amount of drag, which means there's no net moment for a symmetric ship.