Rocket wobbling on the launch pad

[satorarepo]

I was trying this very tall rocket build and it became extremly unstable, to the point of wobbling on the launch pad even before launching. In fact, it moved so much I could only speed up the game up to 4x. In the picture you can see that the modules seem to get disconected from each other.

What causes this and how can I prevent it? 

Thanks!

 

[RainThorn]

Your have 140 T rocket and engine that can lift just 40 T.

Your stack separators are for 1.2m tanks and you use 2.5 m tanks

Launch clamp is at final stage and you need to release it as soon as you fire first engine.

All that mass connected with weak decouplers and suspended on launch clamp will go up and down like yo-yo

Use smaller tanks or stronger engines and correct size decouplers.

 

also, you need to move reaction wheel up. If it is at first stage, it is useless, you have fins for that stage, and if it is at second stage it is right on top of your second engine adnd whole thing will blow up.

Edited November 11, 2019 by RainThorn

[CBase]

Using small decouplers if the engine above is as well small is fine. Actually you need to add struts (in structural pane) between the tanks above and below the small sections anyway.

The smaller the diameter the less rigid it is.

3 hours ago, RainThorn said:

Your have 140 T rocket and engine that can lift just 40 T.

It looks to me like a Mainsail, which can lift 120 T with a small dV penality due to the low TWR at launch.

However check the most upper stage design: It looks extrem overpowered, since it is likely a LV-909 Terrier and therefore the SL 1400 m/s will be more like 6000 m/s in vacuum. Or if you really need that much, adding a smaller stage could result in the same dV with less mass.

 

 

[RainThorn]

13 minutes ago, CBase said:

Using small decouplers if the engine above is as well small is fine. Actually you need to add struts (in structural pane) between the tanks above and below the small sections anyway.

The smaller the diameter the less rigid it is.

It looks to me like a Mainsail, which can lift 120 T with a small dV penality due to the low TWR at launch.

However check the most upper stage design: It looks extrem overpowered, since it is likely a LV-909 Terrier and therefore the SL 1400 m/s will be more like 6000 m/s in vacuum. Or if you really need that much, adding a smaller stage could result in the same dV with less mass.

 

 

Mainsail has silver nozzle, and his engine has black nozzle, it must be Skipper. But even with Mainsail TWR is too low.

Rocket is unstable because it is hanging on launch clamp. Lower stages are pulling top stage down instead top stage pushing lower stages down. It will swing even with struts, just like pendulum.

 

[Fierce Wolf]

1) Decouplers are not made the same: Bigger decouplers hold tighter than smaller ones.

2) Use struts: Struts struts. Can be added, or the automatic type, but please use those!

3) Is it not too tall for a rocket? I don't think it will fly up in a good fashion once drag becomes like a punch

[Curveball Anders]

Moar Struts!

[satorarepo]

Thanks for all the answers. Let me clarify a bit about my weird design, though.

I know this rocket is way too tall and perhaps lacking some delta V. This screenshot was taken after stripping out the radial SRBs I was using to make the unstability more evident (it was also a kind of experiment with super tall rockets). I did not even try to fire the engines with that thing  

20 hours ago, RainThorn said:

Launch clamp is at final stage and you need to release it as soon as you fire first engine.

I know, this setup was just for experimenting (and desperately try for more stability).

20 hours ago, RainThorn said:

also, you need to move reaction wheel up. If it is at first stage, it is useless, you have fins for that stage, and if it is at second stage it is right on top of your second engine adnd whole thing will blow up.

I'm a bit ignorant about reaction wheels. So in principle they should be just below the command module?

16 hours ago, CBase said:

Using small decouplers if the engine above is as well small is fine. Actually you need to add struts (in structural pane) between the tanks above and below the small sections anyway.

The smaller the diameter the less rigid it is.

It looks to me like a Mainsail, which can lift 120 T with a small dV penality due to the low TWR at launch.

However check the most upper stage design: It looks extrem overpowered, since it is likely a LV-909 Terrier and therefore the SL 1400 m/s will be more like 6000 m/s in vacuum. Or if you really need that much, adding a smaller stage could result in the same dV with less mass.

It is a skipper.

Concerning struts, I was used to adding them to radial tanks and SRBs... where exactly should I put them in a ship like this?

The top stage is indeed oversize, but that's because this ship is supposed to refuel a stranded one. Maybe there's a more clever way to do that?

12 hours ago, Fierce Wolf said:

1) Decouplers are not made the same: Bigger decouplers hold tighter than smaller ones.

2) Use struts: Struts struts. Can be added, or the automatic type, but please use those!

3) Is it not too tall for a rocket? I don't think it will fly up in a good fashion once drag becomes like a punch

Yeah, I'm concerned with the size too. Same answers as to @CBase though  

[CBase]

6 hours ago, satorarepo said:

Concerning struts, I was used to adding them to radial tanks and SRBs... where exactly should I put them in a ship like this?

You need to connect the tanks above and below. Out of real world stability reasoning, I am normally using triple symmetry, however I am not sure it is really needed. You could place them as well on the outside, stock aerodynamic will be the same.

Even with a 2.5m decoupler your engine is the weak point, so you still need struts. But the larger decoupler adds weight. Note that I am still connecting the heavy parts with struts.

But maybe you want a bigger engine as well ? If your burntime is more than 5 minutes, the loss due to gravity and node execution tend to get bigger than adding more weight.

That is what actually a skipper is for: Intermediate stage. With all 2.5m parts you do not need to strut . At least not around this separation area.

6 hours ago, satorarepo said:

It is a skipper.

Then RainThorn is right, it will not lift, but only look pretty at launch site

Moar power !

6 hours ago, satorarepo said:

this ship is supposed to refuel a stranded one

Then it is fine to use a oversize tank if part of the fuel is meant as payload. No way to optimize.

Good luck on your rescue !

 

[RainThorn]

Same rocket with struts, Mainsail engine and Kickback SRB

 

don't use launch clamps, be careful when attaching SRB to rocket, connect SRB somewhere above middle of SRB, 2:3 length ratio works fine, fine adjust if rocket is not stable. If you connect SRB too high or too low, rocket will start to vibrate, and if you try to fix that with clamps, it will vibrate even more.

Use struts at bottom for clean separation

 

 

start gravity turn early and change angle slowly and you will go into orbit with more than enough fuel in second stage and that stage will start to pick up speed when burns more than half of fuel

Design is far from optimal and highly inefficient but it can go into orbit with more than enough fuel left.

 

Edited November 12, 2019 by RainThorn

[satorarepo]

Wow, you even replicated the rocket! Thanks a lot, I managed to make it launch very stably with struts and a larger decoupler (plus moar boosters, of course!).