Launch from Kerbal - Something special at 18-20k? Rocket keeps flipping backwards.

[WanderingKid]

Hey folks,

I'm relatively new here, so if I missed some information that is needed to assist me, please just let me know.

Is there something special at the 18-20k mark in atmo for rockets (not SSTO)? I'm playing around trying to get a massive lifter up with minimal science so I've got a few hundred parts on this rocket, but I've tested each stage uniquely. Finally having enough Delta-V to put the entire assembly into space, and with three rebuilds, I constantly run into the same problem.

Using a set of stable LV-T30's, at 18-20k when I drop the SRB assist (I think this actually starts around 16k) the rockets constantly start an 'upward' spin, trying to stabilize out to straight up again. And quickly. Once they hit that point they flip the thing over. I've tested each stage from rocket pad and the stage itself is stable. It's only once I have outer stages that it becomes a problem.

I've confirmed that the SRB's aren't hitting anything on their departure (to the point of putting Seperatrons on them to make sure they get away from me). All engines are burning well and all at 100% thrust. Nothing's significantly shifted (it's all built via symmetry), and I don't have fuel lines. All tanks are burning at maximum directly down to their engines.

I'd assumed rounding errors or clipping problems or something the first two times I did this. At three attempts, with individual stage tests confirming that each one is stable, I constantly get this 'flip' shortly after the orbital turn. If it started pointing down I'd assume I had a momentum problem and needed to stack on more command pods or something to counter the movement, but it's not. It's reversing and tossing me backwards.

Any ideas? Anyone ever seen anything like this?

On a side note, I should mention I've been to Jool, etc. I can get typical rockets up I'm just trying to get greedy with low tech to see what can be done. Most of my craft haven't done anything like this. I run Kerbal Alarm Clock as my only mod. This particular ship is 459 parts.

A quick shot of the ship is here: http://i35.photobucket.com/albums/d178/GUDare/KSP/AlbatrossMulti.jpg~original

Edited June 7, 2014 by WanderingKid
Marking as Answered

[Pecan]

So your rocket is not single-stage and the problem starts when you drop a stage and are in that part of your ascent where you've burnt a lot of fuel and are pitching-over through your gravity turn.

There isn't anything special about 18-20km apart from the thinning atmosphere but I suspect that your rocket's centre of mass is moving (as you burn fuel/drop heavy stages) to such an extent that it's just not stable any more. Your link just gets me 'not available' so I can't comment on the ship itself.

[Vanamonde]

It's hard to tell from that picture, but it looks like you're using a lot of non-vectoring engines, which do not help with attitude control. What does the ship look like at the time of the problem, without the ejected stages?

[Taki117]

Holy crap that's a big rocket. I think you have run into the classic rocket equation problem of it takes fuel to lift fuel. Where are you trying to go with your design? If it's the Mun I suggest building smaller and more efficient. If you are trying to go interplanetary with that, my best advice is don't. Here's how you should progress in career mode:

Do some pad science. Simply put a capsule on the pad, take a crew report, EVA and do an EVA report (if he's still on the ladder it counts as flying). Store that and get off the ladder. Do another EVA report and take a surface sample, store that. Repeat off the launch pad and recover kerbal and pod.

Sub orbital hop: take along 2 goo cans (4 is better, placed with symmetry) use one just after liftoff, one above 36km, one above 70km, and one above 200km, do EVA and crew reports above 70 and 200km. Grab your science from your too, hop back in your capsule and deploy chutes.

Orbit: get into a stable orbit and do EVA reports over all the biomes. (In addition to other science stuff you have). Make sure you can get back down.

[WanderingKid]

@Pecan: Thanks, I'm not sure why you're getting not available. I can follow the link directly and it looks like Taki was able to as well. I've burn maybe half my fuel off at that point, when I drop the stage with the SRBs. The problem is exactly opposite of pitching over as I understand the term. I'm pitching UP... and then momentum carries me over.

@Vanamonde: I'll record a flight of the Albatross here shortly and get it up on YouTube. Probably the easiest method to show you.

@Taki: It's actually trying to lift 5 separate ships for a low-tech multi-interplanetary run, 4 of them will go to distant worlds, one will run Mun/Minmus/Kerbal for biome EVAs. May attempt a few extreme evas on Minmus with Jeb too. It seems like a fun idea. I just have to get the core to stable orbit and separate them. They're fine from there. I've done the 'normal' science progression. I wanted to do something different... and fun.

[Taki117]

@Taki: It's actually trying to lift 5 separate ships for a low-tech multi-interplanetary run, 4 of them will go to distant worlds, one will run Mun/Minmus/Kerbal for biome EVAs. May attempt a few extreme evas on Minmus with Jeb too. It seems like a fun idea. I just have to get the core to stable orbit and separate them. They're fine from there. I've done the 'normal' science progression. I wanted to do something different... and fun.

Well, if you're dead set on doing that, I suggest (And this isn't the most efficient method) going straight up until the boosters run out, jettisoning them, and then doing your gravity turn.

[Northstar1989]

Hey folks,

I'm relatively new here, so if I missed some information that is needed to assist me, please just let me know.

Is there something special at the 18-20k mark in atmo for rockets (not SSTO)? I'm playing around trying to get a massive lifter up with minimal science so I've got a few hundred parts on this rocket, but I've tested each stage uniquely. Finally having enough Delta-V to put the entire assembly into space, and with three rebuilds, I constantly run into the same problem.

Using a set of stable LV-T30's, at 18-20k when I drop the SRB assist (I think this actually starts around 16k) the rockets constantly start an 'upward' spin, trying to stabilize out to straight up again. And quickly. Once they hit that point they flip the thing over. I've tested each stage from rocket pad and the stage itself is stable. It's only once I have outer stages that it becomes a problem.

I've confirmed that the SRB's aren't hitting anything on their departure (to the point of putting Seperatrons on them to make sure they get away from me). All engines are burning well and all at 100% thrust. Nothing's significantly shifted (it's all built via symmetry), and I don't have fuel lines. All tanks are burning at maximum directly down to their engines.

I'd assumed rounding errors or clipping problems or something the first two times I did this. At three attempts, with individual stage tests confirming that each one is stable, I constantly get this 'flip' shortly after the orbital turn. If it started pointing down I'd assume I had a momentum problem and needed to stack on more command pods or something to counter the movement, but it's not. It's reversing and tossing me backwards.

Any ideas? Anyone ever seen anything like this?

On a side note, I should mention I've been to Jool, etc. I can get typical rockets up I'm just trying to get greedy with low tech to see what can be done. Most of my craft haven't done anything like this. I run Kerbal Alarm Clock as my only mod. This particular ship is 459 parts.

A quick shot of the ship is here: http://i35.photobucket.com/albums/d178/GUDare/KSP/AlbatrossMulti.jpg~original

Are those SRB's in the TOP of the rocket?

I think the problem might be that your rocket is a little "top-heavy" at that point in your launch...

Just as you can have problems with a rocket being too bottom-heavy, you can also have a problem (although it is rare) with a rocket being too top-heavy.

I suspect what is happening is that the combination of non-vectoring thrust and an excessively top-heavy rocket is attempting to straighten the thing up in the atmosphere... Try building a rocket with less weight on the top (LFO instead of SRB's in the upper stages, for one- which will also get you better ISP). See how it works.

Regards,

Northstar

[WanderingKid]

So, as I mentioned, I made a video demonstrating what's going on here. I do three liftoffs. One is to show the general problem, a second to confirm the problem, and the last launch shows that it's perfectly stable going up, just can't turn it, and what the rest of the ship looks like afterwards.

http://youtu.be/VpLFxKwGKbI

Northstar, They are, but they're not part of the main design. That's a combination of 5 ships at once that will separate once in orbit all tightly packed into a nice little cube. If you check near the end of the video I show the deployment.

[Kasuha]

You can have a top-heavy lander but you cannot have a top-heavy rocket. Neither in KSP, nor in reality. Trying to suggest so is known as "pendulum fallacy".

On the other hand, those SRBs are the problem. SRBs have high drag and they are trying to stay behind other parts with lower drag.

It is possible to launch with a rocket that has high-drag parts on its top but in such case you must keep your rocket aiming prograde relative to the air most of the time in atmosphere - otherwise those high drag parts will turn your rocket around. Other options are leaving all high-drag parts on the tail of the rocket, compensate for them with some control surfaces, or compensate for them with low-drag parts such as nosecones.

[WanderingKid]

You can have a top-heavy lander but you cannot have a top-heavy rocket. Neither in KSP, nor in reality. Trying to suggest so is known as "pendulum fallacy".

Well, that was an interesting read. Not sure how that applies here though. The fallacy you mention is that gravity won't keep your butt towards the ground if the point of thrust is above the Center of Mass, at least according to the wiki and the followup article on Goddard.

On the other hand, those SRBs are the problem. SRBs have high drag and they are trying to stay behind other parts with lower drag.

It is possible to launch with a rocket that has high-drag parts on its top but in such case you must keep your rocket aiming prograde relative to the air most of the time in atmosphere - otherwise those high drag parts will turn your rocket around. Other options are leaving all high-drag parts on the tail of the rocket, compensate for them with some control surfaces, or compensate for them with low-drag parts such as nosecones.

Now, that's quite interesting, and good to know. I still would have assumed it would drag the nose down, however, not up. Down would have made sense to me. It's also odd that it only triggers at some point after I perform the gravity turn. For a while it's fine, then suddenly it's not.

Good information to keep in mind, however, thanks.

[SRV Ron]

Looks like a bunch of sub orbitals strapped together into one huge rocket. Would help to know what is the goal of the mission. That payload is a massive anchor of highly inefficient SRBs. With that many heavy capsules being needed to control that beast, something is extremely unstable in the design in the first place.

From the video, something has broken early causing the pitch to go hard over. All the SAS in the heavy capsules are able to hold attitude for a while but once fuel gets used to where there is less mass in the liquid stages, the SAS in the capsules are no longer able to hold the rocket on course.

A much simpler career style Minmus lander;

Edited June 7, 2014 by SRV Ron

[Kasuha]

Well, that was an interesting read. Not sure how that applies here though. The fallacy you mention is that gravity won't keep your butt towards the ground if the point of thrust is above the Center of Mass, at least according to the wiki and the followup article on Goddard.

Pendulum fallacy is false belief that position of center of mass relative to center of thrust plays role in stability of the rocket. The only thing that in reality plays role is whether thrust vector goes through CoM or not.

In fact, it is better controllable than rockets with low CoM because engine gimbal has wider relative range.

On the other hand...

It is not unstable because it's top-heavy (it's not top-heavy, its CoM is safely at the bottom) but because it has high drag parts (cupola modules) at the top. Trying to do sharp turn in atmosphere (e.g. standard newbie-level gravity turn 45 degrees at 10 km) lead to uncontrollable turns.

Edited June 7, 2014 by Kasuha

[PrivateFlip]

[snip]

Now, that's quite interesting, and good to know. I still would have assumed it would drag the nose down, however, not up. Down would have made sense to me. It's also odd that it only triggers at some point after I perform the gravity turn. For a while it's fine, then suddenly it's not.

[snip]

When you look at the 5:30 point in the video. You can see on the navball your direction your vessel is facing upwards in relation to your prograde vector. So if indeed the top is taking more drag as Kasuha suggest, you would expect it to pitch upwards as this point. This is simply because at this point its already tilted upward in relation to it's direction of travel. At some moment this unbalanced drag force, due to increasing speed, rises above the strenght of the control and the ship starts to turn.

Some other tips: your ascent speed is pretty low after dropping the boosters. Due to this your spend a lot of time fighting gravity which costs a lot of fuel. I think you'll find if you leave out one fuel tank from the the first stage (so four fuel cans instead of five) you maintain a better acceleration, and you'll actually end up with more fuel remaining in orbit; it's more efficient to ascent with a speed closer to terminal velocity. Link

[Kasuha]

Here, schematic description why your ship pitches upwards. It's not perfectly accurate but doing it accurately would make it too complex.

In KSP, aerodynamics is not realistic. Drag depends on mass of the part and its drag coefficient. A rocket made of all parts with the same drag coefficient can fly through atmosphere in any orientation without any forces trying to rotate it. Put a part with lower drag on it and it will rotate the ship to stay in the front. Put a part with higher drag on it and it will rotate the ship to stay in the back.

Edited June 7, 2014 by Kasuha

[WanderingKid]

Kasuha, thank you. Sometimes a picture is worth a thousand words, and apparently I didn't quite grasp the basic issue of the fallacy, just the details of the particular explanation.

Also thank you Flip, that also helped. Just a different phrasing but between that and a good night's sleep it makes perfect sense.

So, to make sure I grok this, the problem is two fold. First, my choice of SRBs as injection boosters for the interplanetary ships is causing me to have significant drag across the top when I get into a gravity fight while still in atmosphere and I'm cruising more sideways than upwards. Secondly, when the mass in the lower portion lowers to some critical point, it no longer has the weight on one end to counterbalance the drag on the other. This sideways drag is what is causing the issue, where a straight up/down doesn't tackle the parts in the same manner, allowing for SAS to keep control of it for the minor differences.

By George, I think he's got it!

Thank you all.

[Kasuha]

Secondly, when the mass in the lower portion lowers to some critical point, it no longer has the weight on one end to counterbalance the drag on the other.

No, mass of that part plays no role in that. Here I prepared a little example for you. Three almost identical ships, each with a 41 ton fuel tank on one end and three items (total mass about 12 tons) on the other end. The fuel tank is more than three times heavier in all three cases.

The "item" is another fuel tank in one case (drag 0.2, i.e. equal to the rest), SRB in another case (drag 0.3 i.e. higher), and four inline docking ports in the third case (drag 0.08, i.e. lower; I used four of them as each weighs 1 ton).

I sent them on ballistic trajectory above atmosphere, lined them up horizontally, then let them fall through atmosphere. Notice that the ship with SRBs rotates with these SRBs up (retrograde) and falls slowest, ship with docking ports rotates with docking ports down and falls fastest, while the ship with fuel tanks keeps falling oriented horizontally, however unbalanced it is.

Note that of course they would not fall this way in Earth atmosphere and they would not fall this way if I used FAR. This is result of stock aerodynamics implementation

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[WanderingKid]

Okay, that all makes sense, Kasuha, and thanks for the very descriptive explanation. So the mass of the lower parts don't matter, it's that I've gained enough speed for the drag to overtake the SAS, which is why I get variable critical failure points for control? That makes sense in retrospect.

[Kasuha]

Oit's that I've gained enough speed for the drag to overtake the SAS, which is why I get variable critical failure points for control? That makes sense in retrospect.

Yes, exactly.