Multiple problems with ship

[Steve C.]

After creating a ship to go to the mun virtually without any problem at all, my next, larger ship is giving me all kinds of grief.

First of all, in an effort to increase the thrust of my first stage, and lacking any decent SRB's (the Hammer seems so short-lived as to be practically useless) I attached a couple of Mainsail engines and liquid fuel tanks onto my Mammoth engines and their tanks.  The problem is that when I used a single radial coupler on each of the side structures, they would bow outwards similar to the picture below.  So I figured that could be remedied by putting one coupler on each tank segment such that there will be four couplers holding the side structures to the main rocket.  But even with the multiple couplers, I get the same problem.  In fact, it does't seem as though the multiple tank segments are using the couplers I provided. 

https://imgur.com/a/3zuCukf

https://imgur.com/a/XZMLMnB

Finally, I have found it virtually impossible to maneuver this ship.  I don't need translation so I thought that reaction wheels would be sufficient.  I put an advanced reaction wheel module in what is more or less the middle of the ship but once I am not using my thrusters, I have almost no control.  I tried adding four RCS thrusters near the center of mass but it still seems like that barely helped at all.

https://imgur.com/a/aYTNyxW

What am I missing?

Thanks,

Steve

 

[Space boy]

31 minutes ago, Steve C. said:

 The problem is that when I used a single radial coupler on each of the side structures, they would bow outwards similar to the picture below.

 

Always, always attach struts to your radial decouplers. 

Edited November 1, 2019 by Space boy

[Goody1981]

3 hours ago, Steve C. said:

After creating a ship to go to the mun virtually without any problem at all, my next, larger ship is giving me all kinds of grief.

First of all, in an effort to increase the thrust of my first stage, and lacking any decent SRB's (the Hammer seems so short-lived as to be practically useless) I attached a couple of Mainsail engines and liquid fuel tanks onto my Mammoth engines and their tanks.  The problem is that when I used a single radial coupler on each of the side structures, they would bow outwards similar to the picture below.  So I figured that could be remedied by putting one coupler on each tank segment such that there will be four couplers holding the side structures to the main rocket.  But even with the multiple couplers, I get the same problem.  In fact, it does't seem as though the multiple tank segments are using the couplers I provided. 

Hi Steve

Unfortunately, due to the way that the game engine works, you can’t make loops out of parts. If there’s a “parent” part, and you attach a “daughter” part to it, that daughter part can never come back to connect to the parent part, or any other part already on the rocket. Think like the branches of a tree - you can only branch out, it can never come back to join the tree in a loop (cue photos of just such a thing happening with trees probably :p)

There are two workarounds: 

1 - struts. They are a kind of cheat way of creating a loop. So you could place the radial decoupler, place your booster, then use struts to connect back to the main body.

2 - autostruts. Because struts look ugly, I prefer autostruts... which are just pretty much invisible struts. You have to turn them on via “advanced tweakables” in the settings. Once turned on, you can right click on a part (in the VAB or in flight) and cycle through a few options for where the autostrut is connected.

So to do big boosters like that, my go-to method:

- place radial decoupler and attach booster

- add a nose cone on the booster and set it to autostrut to “root part” (this creates a solid connection between the top of the booster and the top of the rocket)

- autostrut the main tank of the booster, and/or the engine on the bottom, to “grandparent part” (this creates a solid connection between the bottom of the booster and the bottom of the rocket)

In the VAB there are orange lines that show you where the autostruts are connecting.

3 hours ago, Steve C. said:

Finally, I have found it virtually impossible to maneuver this ship.  I don't need translation so I thought that reaction wheels would be sufficient.  I put an advanced reaction wheel module in what is more or less the middle of the ship but once I am not using my thrusters, I have almost no control.  I tried adding four RCS thrusters near the center of mass but it still seems like that barely helped at all.

https://imgur.com/a/aYTNyxW

What am I missing?

Thanks,

Steve

 

As for the second problem, it looks like you have no fins on the bottom of your rocket. Adding some steerable fins may help.

In order to steer you need to think of maximising torque - which you do by having the steering/turning force act as far from the centre of mass as possible. Imagine trying to use a spanner to turn a bolt - it works best if you grip the spanner further from the bolt, not really close to it. So the torque/ turning force needs to act at the ends of the rocket, not in the middle. Putting RCS right in the middle of your rocket makes it almost impossible for them to steer your rocket (actually, mathematically IMpossible if they are exactly on the centre of mass). Also, for a rocket of that size/mass, those RCS are pretty underpowered.

So to maximise the torque/turning force you can move the RCS to the top and bottom, as well as adding steering fins to the bottom, and well as using engines at the bottom that can steer (have a gimbal range). Hope that’s helpful!

Edited November 1, 2019 by Goody1981

[Foxster]

Attach the boosters with the decouplers about 2/3 of the way up (only one per booster will do anything as explained above) and put struts near the bottom of the tank. This will help the boosters peel away when staged. 

If your maneuvering issue is in the atmosphere then add some steerable fins. If it's in space then chances are your craft is just too big or if it is carrying some huge payload for whatever reason then you'll need more reaction wheels AND the power generation they need. It makes zero difference where on the craft you place the reaction wheels cos of physics innit. 

[Streetwind]

1 hour ago, Goody1981 said:

In order to steer you need to think of maximising torque - which you do by having the steering/turning force act as far from the centre of mass as possible. Imagine trying to use a spanner to turn a bolt - it works best if you grip the spanner further from the bolt, not really close to it. So the torque/ turning force needs to act at the ends of the rocket, not in the middle. Putting RCS right in the middle of your rocket makes it almost impossible for them to steer your rocket (actually, mathematically IMpossible if they are exactly on the centre of mass). Also, for a rocket of that size/mass, those RCS are pretty underpowered.

Small addendum: while this is true for most things, reaction wheels actually ignore that rule. They work the same way regardless of where on the ship you place them.

[Goody1981]

2 hours ago, Streetwind said:

Small addendum: while this is true for most things, reaction wheels actually ignore that rule. They work the same way regardless of where on the ship you place them.

Cheers - I didn’t know that

[mk1980]

7 hours ago, Steve C. said:

First of all, in an effort to increase the thrust of my first stage, and lacking any decent SRB's (the Hammer seems so short-lived as to be practically useless) I attached a couple of Mainsail engines [...]

if you unlocked the mammoth and the mainsail, you should also have the kickback and thumper SRBs. and probably also the recently added 2.5m monsters (haven't memorized their names yet)

if you don't have those, you should probably check if your installation is broken.

[Streetwind]

The 2.5m SRBs are actually located in the very final rocketry node, the one that has Making History's high-end parts. And if he doesn't have Making History, then he won't have the 1.875m Pollux for the midgame. But yes, Kickback should definitely be unlocked, and it does have a decent "kick".

Alternatives:
- Orbital assembly of spacecraft with multiple launches and docking ports, if you're wanting a large mothership.
- Downsizing. Just because you have 3.75m rock parts doesn't mean that you need them to go places! People have demonstrated crewed Duna land-and-return missions on 1.25m hardware. Barebones, sure, but it's doable... and you can probably get a "full package" mission launched on a single Mainsail.

...In fact, that sounds like a fun project. Just how much capability can I cram onto a single Mainsail for a Duna return mission? Something to do for when I get home.  

[Steve C.]

Thank you all!!  I so appreciate your help.  I'll take a look at the struts.  As for "unlocking", in the interest of time (I don't have a lot to play this game), I am in sandbox mode... and I don't have the Making History add on.  But I have been thinking about getting it so this has pushed me over the edge.

As for the SRB's, I am kicking myself!  I do have thumper and kickback.  Because I am getting old and don't see as well as I used to, I have the font size cranked up and therefore all the engines don't fit on one page.  I didn't notice that I could scroll down to see kickback and I have no explanation for how I missed thumper.  Let's just say that one of my attempts to create multiple stages of clusters of hammer engines was an unsightly detour down a dead-end path.

Finally, the power of suggestion: I am a wannabe physic guy, or so I thought.  I figured that the physics of torque would make RCS thrusters most effective if they were farther away from the center of gravity, but I swear that the tutorial had you place them near the center of mass.  And thus I let that suggestion override my common sense.

Steve

Edited November 1, 2019 by Steve C.

[Streetwind]

32 minutes ago, Steve C. said:

I figured that the physics of torque would make RCS thrusters most effective if they were farther away from the center of gravity, but I swear that the tutorial had you place them near the center of mass.

The tutorial might have been teaching you to place RCS thrusters for translation control while in orbit - for rendezvous and docking. In that situation, you want to avoid inducing torque when translating, so you would distribute your RCS thrusters equidistant from your center of mass to allow them to cancel each other's torque out. Or, for very small vessels, a single ring of thrusters placed directly on top of the center of mass will do the trick.

Obviously, that single ring will not work very well for rotation! Which is kind of the point. Different setups for different use cases  

[Steve C.]

RIGHT!  That makes total sense.