1.0 rocket launching. Why does it tip over ?

[Baleine]

Hello,

I'm quite embarrassed now because I'm a KSP veteran. I used to play with FAR/DRE/etc... but with the 1.0, I have the impression of being suddenly dumb.

I managed to send one or two ships in orbit, but I struggled to do so. My last attempt was peculiary nightmarish as the rocket tipped over almost immediately after ignition.

I removed all the unneeded parts (batteries, antenna, science) and it seems the craft is a little more stable, but it tips over nonetheless, albeit later (when booster are almost empty).

I can't figure out what happens. I'm pretty sure I wouldn't have any problem with the same craft in 0.90+FAR, so I'm left asking you guys about what is wrong with my ship.

You can download my craft here

You can launch it vertically, and it will tip over after 20 to 30 seconds. Just be careful to adjust the staging (the boosters aren't set to fire when stability enhancer release the ship).

Here is a picture of the craft, with CoL, CoM and CoT shown

I'll be grateful if you could explain me what is the problem with that simple craft.

Edited April 30, 2015 by Baleine

[Starwhip]

Did you turn on the SAS?

Don't stray too far from your prograde vector under 25,000 meters.

If none of those are the case, then you might have a thrust discrepancy somewhere. I'll download it and see.

EDIT:

Flipping behavior confirmed.

It's your staging sequence. I'll fix it up for you and post a screenshot.

Your boosters and the LV-T45 should fire in tandem. The LV-T45 doesn't have to be at full thrust, it can be around 25%, and it will still have control authority over your rocket.

EDIT:

It helped for a while, disregard.

Investigation pending.

- - - Updated - - -

Making progress, though this thing does NOT want to get to space.

Few more things to do.

EDIT:

Success!

Booster separation confirmed.

Your problem was that the SRBs were mounted too high. Try moving them down a tad, so that the nozzle is level with AT LEAST the top of the "Swivel" near the bottom, or level with the Swivel's nozzle.

If you want I can send you the fixed .craft file. Ah, here, I'll put it up for you if you want to grab it.

https://www.dropbox.com/s/bpwig586vjf7yjh/Munar%20explorer%20Mk1.craft?dl=0

Edited April 29, 2015 by Starwhip

[cybersol]

It is definitely flipping. To get it flying well, I added three control AV-R8 winglets at the bottom between the boosters. I then moved the launch stability arms to the boosters to let the winglets clear the tower. Also, I needed to lower the booster thrust to 60%, which might still be a touch high. I finally removed the struts, as its too small to need them and they can sometimes lead to problems by themselves. Now it flies really nicely.

EDIT: Similarly to Starwhip, I also lowered the boosters to lower the CoM as well. That plus putting an even lower CoL behind it with the winglets gave very nice control. GLHF!

Edited April 29, 2015 by cybersol

[Starwhip]

It is definitely flipping. To get it flying well, I added three control AV-R8 winglets at the bottom between the boosters. I then moved the launch stability arms to the boosters to let the winglets clear the tower. Also, I needed to lower the booster thrust to 60%, which might still be a touch high. I finally removed the struts, as its too small to need them and they can sometimes lead to problems by themselves. Now it flies really nicely.

You did exactly what I did, except for the winglets, as I didn't want to add in something he might have not gotten yet.

[capi3101]

I think y'all might've figured this one out already - to be sure, can somebody post a screenie of the unmodified rocket in the VAB with the CoM highlight turned on? Could simply be a case of the CoM going too far aft during flight (the old "throw a dart backwards" problem).

[Wintersdark]

I think y'all might've figured this one out already - to be sure, can somebody post a screenie of the unmodified rocket in the VAB with the CoM highlight turned on? Could simply be a case of the CoM going too far aft during flight (the old "throw a dart backwards" problem).

Errr, for a rocket under powered ascent, don't you want the CoM aft?

[csiler2]

Errr, for a rocket under powered ascent, don't you want the CoM aft?

You want the CoM forward for stability. The heavier ends of objects like to lead and the lighter ends like to follow. That's why darts are heavy at the front and light at the tail.

[Starwhip]

You want the CoM forward for stability. The heavier ends of objects like to lead and the lighter ends like to follow. That's why darts are heavy at the front and light at the tail.

More correctly, you want the CoM ahead of the CoL (Center of Lift), so that drag keeps the engines pointing away from space. If your engines are pointing towards space, you are not going to space today.

[csiler2]

More correctly, you want the CoM ahead of the CoL (Center of Lift), so that drag keeps the engines pointing away from space. If your engines are pointing towards space, you are not going to space today.

Ah, true. Which is why the wingy-bits of the darts are at the back

[Baleine]

Wow, thanks for the answer. I'll edit the first post (if I can) to put a picture of the original craft with CoM and CoL shown.

[A_name]

Your problem was that the SRBs were mounted too high. Try moving them down a tad, so that the nozzle is level with AT LEAST the top of the "Swivel" near the bottom, or level with the Swivel's nozzle.

Why would the boosters being too high cause this behavior? What forces are in play here?

[moronwrocket]

Does having a high CoM have more to do with balancing than with the "dart effect"? In my head, I see a thrusting rocket as similar to a stick with a weight on it. If the stick is fairly long with the weight at the top, it's easier to balance because shifts of the bottom of the stick have a small effect and are easy to control, and if the weight starts tipping over, it's got plenty of time before it "falls". On the other hand, if the weight is moved towards the bottom of the stick, it becomes much harder to balance because even a small change is going to cause it to topple over, and when it does it does it fast. I know it's about angular momentum etc as well, but this in my head makes more sense than the dart analogy, because there aren't any thrusting forces acting on the dart.

[Starwhip]

Why would the boosters being too high cause this behavior? What forces are in play here?

The aerodynamic quality of the boosters, coupled with a higher CoM, led to instability in the rocket.

Remember, drag is actually correctly calculated based on your profile now. Before it was based on mass. Lift has also been redone. Lift can now occur as a result of drag.

[SRV Ron]

Ship on the right is similar design but note how low the SRBs are mounted. Also, tweaked thrust to control burnout speed to a reasonable level. Liquid engine doesn't need to be powered in SRB phase. An LV-45 gives more control during powered liquid flight then just an LV-30. Key is don't turn too fast in the lower atmosphere or you will flip over due to angle of attack.