Rocket flips out even though my rocket has a good design

[Space boy]

I have read a lot of forums and followed as they say. Put fins at the bottom, fuel cross feed bottom to top, not straying far from the prograde marker etc. But my rocket just keeps on doing this flip. Here's a screenshot.

https://imgur.com/a/juCbCEN

[RCgothic]

I would hazard that as you burn through fuel the CoM moves backwards as most of your fuel tanks are ahead of the CoM. Your margin of stability (difference between CoM and CoD) probably isn't high enough to compensate.

Empty all the tanks of fuel and see where the CoM goes.

Edited September 5, 2019 by RCgothic

[VoidSquid]

Hmm... I'd like to test that rocket myself, could you make the .craft file available?

[mk1980]

the spherical (soviet style) capsules are pretty bad in my experience. i've tried using them in my latest career game and had issues getting them to orbit.

try replacing the capsule with a cone shaped one (the basic "Mk1" Mercury capsule for example).

also, the gap between the capsule and the fuel tank is bad. causes a lot of drag. you could probably just put that reaction wheel below the nosecone to avoid the gap. or put it into a 1.25m service bay.

i can't identify the other gap between the upper and lower fuel tank, but whatever it is - you should probably avoid it.

[Guest]

I think it's the gaps. You're not using the same size decouplers as your stack. This creates flat surfaces which create a lot of drag near the nose. 

- Fix: use 1.25 m decouplers all through. If necessary, switch to a 1.25 m engine also.

[Space boy]

4 hours ago, VoidSquid said:

Hmm... I'd like to test that rocket myself, could you make the .craft file availabl

Can you tell me how to do that please?? 

[Space boy]

4 hours ago, mk1980 said:

the spherical (soviet style) capsules are pretty bad in my experience. i've tried using them in my latest career game and had issues getting them to orbit.

try replacing the capsule with a cone shaped one (the basic "Mk1" Mercury capsule for example).

also, the gap between the capsule and the fuel tank is bad. causes a lot of drag. you could probably just put that reaction wheel below the nosecone to avoid the gap. or put it into a 1.25m service bay.

i can't identify the other gap between the upper and lower fuel tank, but whatever it is - you should probably avoid it.

The other gap is because I am trying to test the ant engines. And that's the only way the game would let me place the decoupler

[Space boy]

4 hours ago, RCgothic said:

I would hazard that as you burn through fuel the CoM moves backwards as most of your fuel tanks are ahead of the CoM. Your margin of stability (difference between CoM and CoD) probably isn't high enough to compensate.

Empty all the tanks of fuel and see where the CoM goes.

https://imgur.com/hhNU2sm here you go

[Space boy]

4 hours ago, mk1980 said:

the spherical (soviet style) capsules are pretty bad in my experience. i've tried using them in my latest career game and had issues getting them to orbit.

try replacing the capsule with a cone shaped one (the basic "Mk1" Mercury capsule for example).

also, the gap between the capsule and the fuel tank is bad. causes a lot of drag. you could probably just put that reaction wheel below the nosecone to avoid the gap. or put it into a 1.25m service bay.

i can't identify the other gap between the upper and lower fuel tank, but whatever it is - you should probably avoid it.

thanks i got into orbit following your steps. I thought it had something to do with center of mass. https://imgur.com/undefined

[Space boy]

What should I do if I want to send two kerbals at the same time. All i have for a two seater pod is the pea pod, which unfortunately, leads to aerodynamic havoc.

[VoidSquid]

You could for example use two Mk1 pods on top of each other. Or combine a Mk1 pod with a Mk1 lander can. Or you could use the Mk2 pod from Making History. Or you could use a mod mod for two Kerbals.

Or you could use a Mk1 pod with the 2-Kerbal Mk1 crew cabin  below, which would be my choice.

Edited September 5, 2019 by VoidSquid

[sh1pman]

The only reason your rocket flipped out is because Vostok-style soviet capsules are incredibly draggy. It has to be this way in order for them to slow down quickly enough in the atmosphere. Solution: put the capsule inside a fairing.

[VoidSquid]

2 hours ago, Space boy said:

Can you tell me how to do that please?? 

Look for the folder \Kerbal Space Program 1.7\saves\<your save game name>\Ships\VAB

There should be a file with the name of your vessel, having a .craft file extension. Upload to Dropbox or any other file sharing service and provide the link here, ok?

[mk1980]

in my experience the aerodynamic properties of the capsule become less relevant if it sits on top of a larger rocket. i successfully used a 2 seat "pea" capsule mounted on top of some mid-sized 1.875m stack for the first mun landing in that new career for example. and that thing even had a bunch of 1.25m droptanks near the top, so it actually was a lot more draggy then anything a sane rocket scientist would build.

 

fairings are also an option to make whatever capsule or payload you hide below them aerodynamic.

 

regarding the ant engines and the gap in your rocket: you can use an "engine plate" part. applications like what you did with those ant engines are basically the purpose of the engine plate parts. the plate has an attachment node at the bottom, so you can add the rest of the rocket below it and it automatically adds a fairing to close the open gap in between. don't remember if the game actually has a 1.25m engine plate though.

[Space boy]

40 minutes ago, VoidSquid said:

Look for the folder \Kerbal Space Program 1.7\saves\<your save game name>\Ships\VAB

There should be a file with the name of your vessel, having a .craft file extension. Upload to Dropbox or any other file sharing service and provide the link here, ok?

k,will send you the file

[Space boy]

1 hour ago, VoidSquid said:

Look for the folder \Kerbal Space Program 1.7\saves\<your save game name>\Ships\VAB

There should be a file with the name of your vessel, having a .craft file extension. Upload to Dropbox or any other file sharing service and provide the link here, ok?

https://kerbalx.com/spaceboy/Mun-test-for-ant-engines  I posted on kerbalx. FYI- i dont have any mods.

1 hour ago, VoidSquid said:

Look for the folder \Kerbal Space Program 1.7\saves\<your save game name>\Ships\VAB

There should be a file with the name of your vessel, having a .craft file extension. Upload to Dropbox or any other file sharing service and provide the link here, ok?

I dont know why but the photo on the page is completely unrelated. The rocket in the photo is not my craft.

[VoidSquid]

Thanks, now I'm curious to test  

I'll come back to you soon!

[Norcalplanner]

What's your initial TWR? I see a lot of thrust for not much mass, so you may be going too fast in the lower atmosphere. Try adding more fuel tanks to that center stack so the initial TWR is down around 1.4 or so.

And when during the launch is it doing the flip? It looks like you're only staging the outer LVT-30 engines initially, and they have no gimbal - which means that you're at the mercy of aerodynamics.

[Aegolius13]

13 hours ago, Space boy said:

What should I do if I want to send two kerbals at the same time. All i have for a two seater pod is the pea pod, which unfortunately, leads to aerodynamic havoc.

Been said above, but putting everything from the Ants in a fairing should solve your aero worries.  This will also let you lose the nosecone, and not have to worry about blowing up science instruments or anything.

I actually like the Soviet capsules for certain things.  Since they come with a built-in decoupler and heatshield, it's a decent tradeoff for the needed reaction wheel.  Plus, for some reason, the abator burns off incredibly slowly, so you can cut down the amount to save some more weight.  And, OP mentioned, they're nice for cramming many Kerbals into a small volume.  And I often use a fairing even with the conical capsules anyway, if I have a draggy spacecraft, low-temp-tolerance instruments, etc.

[Space boy]

5 hours ago, Norcalplanner said:

What's your initial TWR? I see a lot of thrust for not much mass, so you may be going too fast in the lower atmosphere. Try adding more fuel tanks to that center stack so the initial TWR is down around 1.4 or so.

And when during the launch is it doing the flip? It looks like you're only staging the outer LVT-30 engines initially, and they have no gimbal - which means that you're at the mercy of aerodynamics.

My rocket flips out around 10km. It's always gotta be around 10km to flip. 

I did consider that there is way too much thrust for a small payload. But if I put a single engine underneath, there isn't enough thrust to get me out of the atmosphere,  and if I put two: hence, my rocket. 

16 minutes ago, Aegolius13 said:

Been said above, but putting everything from the Ants in a fairing should solve your aero worries.  This will also let you lose the nosecone, and not have to worry about blowing up science instruments or anything.

I actually like the Soviet capsules for certain things.  Since they come with a built-in decoupler and heatshield, it's a decent tradeoff for the needed reaction wheel.  Plus, for some reason, the abator burns off incredibly slowly, so you can cut down the amount to save some more weight.  And, OP mentioned, they're nice for cramming many Kerbals into a small volume.  And I often use a fairing even with the conical capsules anyway, if I have a draggy spacecraft, low-temp-tolerance instruments, etc.

I would definitely use the fairings, buuut, I haven't unlocked them yet. 

[Space boy]

12 hours ago, mk1980 said:

in my experience the aerodynamic properties of the capsule become less relevant if it sits on top of a larger rocket. i successfully used a 2 seat "pea" capsule mounted on top of some mid-sized 1.875m stack for the first mun landing in that new career for example. and that thing even had a bunch of 1.25m droptanks near the top, so it actually was a lot more draggy then anything a sane rocket scientist would build.

 

fairings are also an option to make whatever capsule or payload you hide below them aerodynamic.

 

regarding the ant engines and the gap in your rocket: you can use an "engine plate" part. applications like what you did with those ant engines are basically the purpose of the engine plate parts. the plate has an attachment node at the bottom, so you can add the rest of the rocket below it and it automatically adds a fairing to close the open gap in between. don't remember if the game actually has a 1.25m engine plate though.

Your design did get me into an orbit, but during launch, The rocket didn't want to listen to me I had to like, force it to stay in one place. And after like 5,6 seconds from launch,  it changes it's direction from the ksc and I have to just deal with it. Of course, all the bad stuff happens around 10km.

[Space boy]

6 hours ago, Norcalplanner said:

What's your initial TWR? I see a lot of thrust for not much mass, so you may be going too fast in the lower atmosphere. Try adding more fuel tanks to that center stack so the initial TWR is down around 1.4 or so.

And when during the launch is it doing the flip? It looks like you're only staging the outer LVT-30 engines initially, and they have no gimbal - which means that you're at the mercy of aerodynamics.

My initial TWR is 2.42.

[Norcalplanner]

1 hour ago, Space boy said:

My initial TWR is 2.42.

That's very high for any rocket, let alone one with less-than-ideal aerodynamics.  If you can do so within your current pad limitations, I'd recommend stacking more fuel tanks to make the rocket taller and reduce the initial TWR down to 1.4 or so.  You may also want to consider removing the Reliants and replacing them with Swivels, as you'll have much better control authority.  And make the turn pretty gently - without more information, I'd crank it over 5 degrees to the east when you hit around 40 or 50 m/s, then engage SAS and lock it on prograde hold.  If you're pointed 45 degrees above the horizon somewhere between 8 and 12 km altitude, then you're in the ballpark.

[Space boy]

3 minutes ago, Norcalplanner said:

That's very high for any rocket, let alone one with less-than-ideal aerodynamics.  If you can do so within your current pad limitations, I'd recommend stacking more fuel tanks to make the rocket taller and reduce the initial TWR down to 1.4 or so.  You may also want to consider removing the Reliants and replacing them with Swivels, as you'll have much better control authority.  And make the turn pretty gently - without more information, I'd crank it over 5 degrees to the east when you hit around 40 or 50 m/s, then engage SAS and lock it on prograde hold.  If you're pointed 45 degrees above the horizon somewhere between 8 and 12 km altitude, then you're in the ballpark.

I have replaced the pea pod with mk1 pod. So aerodynamics isnt much of a problem now. I will try replacing the relliants with the swivels though. Is 1.42  TWR okay??

Edited September 6, 2019 by Space boy

[Guest]

Yep 1.42 sounds about right.