Any More Tips for Rocket Building

[mackster]

Having played for a few hours, very impressed with this game. Gone through loads of video tutorials on building and have landed successfully on minmus so far - tho moon is still trouble in landing.

My main issue is I want to take two people so my ships start to be huge and unstable. Some things I still havent got a complete hold of is:
- where should the CoM be for each stage?
- dont really understand the aerodynamic overlay symbol in VAB, this is always way above CoM, is that right?
- having the 2 or 3 crew module, makes the demands for TWR to get close to 1.5 get huge. Always over building to make it work which inevitably makes the lift off a challenge, especially on stability
- there are several tips when to start the drift towards 45 degrees, starting from straight away or 100m/s or 10km above ground. What this probably depends on is wieght, is there a technique to understand when this should be?

Appreciate any tips you guys have

[GunsForBucks]

I wouldn't bother with the aerodynamic overlay in rocket building, I only use it when making planes.

In the command and control section is reaction wheels. They use electric charge to stabilize your craft. Also you have RCS engines that use monopropellent fuel. You can also add fins to the lower end to increase drag in the rear and control.

I never watched any youtubes and personally don't sweat trying to perfect turning into orbit at launch so I can't really advise much on that aspect of it as I just go up until I reach about 30k and turn to the horizon. I'm not a minimalist so I tend to build stuff with extra fuel anyway.

[Eekhoorn]

- where should the CoM be for each stage?

Generally speaking you want this to be as high as possible. Think about it as a dart arrow... heavy on the front makes for more stability. A rule of thumb for me is when looking at the entire rocket, the CoM should be no lower than 2/3 from the top. For landers you want the CoM to be as low as possible.

- dont really understand the aerodynamic overlay symbol in VAB, this is always way above CoM, is that right?

Don't worry about it

- having the 2 or 3 crew module, makes the demands for TWR to get close to 1.5 get huge. Always over building to make it work which inevitably makes the lift off a challenge, especially on stability

You really don't need a giant ship to go to the Mun. TWR is the thrust to weight ratio. You always want this to be higher than 1.3 (= a slow rocket), preferable a bit higher but don't overdo it. I aim for anywhere between 1.6 and 2.0 for an ascent from Kerbin.

- there are several tips when to start the drift towards 45 degrees, starting from straight away or 100m/s or 10km above ground. What this probably depends on is wieght, is there a technique to understand when this should be?

You want to start your gravity turn as soon as possible. There's no thing as escape height, it has to do with the escape velocity (3 431.03 m/s on Kerbin). As long as you keep the nose slightly above the horizon, you're good. The flatter the trajectory, the better. IRL it may look like a rocket is going straight up, but they roll pretty fast. Going straight is fighting gravity = more DeltaV needed to reach escape velocity.

[Manwith Noname]

Originally posted by mackster:

- where should the CoM be for each stage?

For your lander and reentry stages, ideally you want the CoM closest to the part of the craft you want facing the surface. For a complete rocket in launch configuration, you ideally want it as close to the nose as possible.

Originally posted by mackster:

- dont really understand the aerodynamic overlay symbol in VAB, this is always way above CoM, is that right?

As mentioned above, probably best to ignore it for the most part in the VAB. Just consider what creates drag (this includes fins / wings) and where those parts are in relation to your CoM and the craft as a whole. Generally not a great idea to be generating drag in the upper area of the rocket. A well designed rocket will not need fins but having some near the bottom of your launch stage can help with designs bordering on unstable.

Originally posted by mackster:

- having the 2 or 3 crew module, makes the demands for TWR to get close to 1.5 get huge. Always over building to make it work which inevitably makes the lift off a challenge, especially on stability

Hard to tell what exactly you are referring to here. Your entire rocket launching from Kerbin wants a TWR higher than one, other stages intended for flying in space can have lower TWR but this will of course impact burn times. Also, TWR is body dependent. So if you design a lander in the VAB with a TWR of 1.5 for Kerbin, on the Mun or Minmus, this will be significantly higher.

I can't recall off hand if the Stock dV calculator can be switched between bodies but snooping around in the options accessed by the toolbar icon in the VAB will probably reveal it if it's there. Failing that, KER allows you to select bodies for calculations when designing so you don't over engineer your Munar lander.

Originally posted by mackster:

- there are several tips when to start the drift towards 45 degrees, starting from straight away or 100m/s or 10km above ground. What this probably depends on is weight, is there a technique to understand when this should be?

Yeah, it kinda depends on each rocket, its aerodynamics, mass, TWR. You get a feel for it over time. If you have a sleek rocket with good TWR at launch, you can typically tilt sooner and power your way to space. If it's a monstrosity with huge draggy areas everywhere and not great in the TWR department, you'll want a different launch profile to prevent the rocket just falling out of the sky.

[GunsForBucks]

Originally posted by Manwith Noname:

Originally posted by mackster:

- where should the CoM be for each stage?

For your lander and reentry stages, ideally you want the CoM closest to the part of the craft you want facing the surface. For a complete rocket in launch configuration, you ideally want it as close to the nose as possible.

Originally posted by mackster:

- dont really understand the aerodynamic overlay symbol in VAB, this is always way above CoM, is that right?

As mentioned above, probably best to ignore it for the most part in the VAB. Just consider what creates drag (this includes fins / wings) and where those parts are in relation to your CoM and the craft as a whole. Generally not a great idea to be generating drag in the upper area of the rocket. A well designed rocket will not need fins but having some near the bottom of your launch stage can help with designs bordering on unstable.

Originally posted by mackster:

- having the 2 or 3 crew module, makes the demands for TWR to get close to 1.5 get huge. Always over building to make it work which inevitably makes the lift off a challenge, especially on stability

Hard to tell what exactly you are referring to here. Your entire rocket launching from Kerbin wants a TWR higher than one, other stages intended for flying in space can have lower TWR but this will of course impact burn times. Also, TWR is body dependent. So if you design a lander in the VAB with a TWR of 1.5 for Kerbin, on the Mun or Minmus, this will be significantly higher.

I can't recall off hand if the Stock dV calculator can be switched between bodies but snooping around in the options accessed by the toolbar icon in the VAB will probably reveal it if it's there. Failing that, KER allows you to select bodies for calculations when designing so you don't over engineer your Munar lander.

Originally posted by mackster:

- there are several tips when to start the drift towards 45 degrees, starting from straight away or 100m/s or 10km above ground. What this probably depends on is weight, is there a technique to understand when this should be?

Yeah, it kinda depends on each rocket, it's aerodynamics, mass, TWR. You get a feel for it over time. If you have a sleek rocket with good TWR at launch, you can typically tilt sooner and power your way to space. If it's a monstrosity with huge draggy areas everywhere and not great in the TWR department, you'll want a different launch profile to prevent the rocket just falling out of the sky.

There is a DV tab at the bottom right of the VAB window. With that up you can select what body and even select sea level or set it for any given altitude.

[Manwith Noname]

Cool. I can remember it has settings for atmospheric pressure but couldn't recall if it had a body selector.

[mackster]

great stuff. I think one of my pitfalls is the centre of mass being too low then on the whole rocket. It starts OK, with good TWR. The dv though isn't enough. I guess i just need more tests to get these rockets designed right to allow enough dv to get where i want while keeping that centre of mass as high as possible.

Once i get further in the science tree i can get the octo 2 which would mean i wouldn't need the pilot on these missions, as i like the SAS functions being there.

[Manwith Noname]

To be fair, keeping the CoM near the nose is kinda tricky. The heavier your payload, the more fuel and engines you need below it, for a traditional rocket. Something that helps keep traditional designs more stable is to make use of fuel flow priority. Empty the bottom tank first and the top tank last, well, assuming that the stage is made of multiple smaller tanks and not just one large, which tends to be the case in early career. This will drive the CoM closer to the nose as you go through your fuel.

[GunsForBucks]

Logically speaking though your Center of Mass is dynamic.

So when you start at launch your center of mass starts to shift up as you burn fuel in the lower stage. It is when it becomes top heavy that the stability becomes worse.

If anything I would think making the higher tanks in the stage drain first would tend to keep your nose up better. At least in the lift-off / climb stage.

[GunsForBucks]

https://steamcommunity.com/sharedfiles/filedetails/?id=2116710777

This rocket has really bad aerodynamics and forced me to keep the speed so slow it wasted too much fuel for the retro burn back.

I added fins and some structure to attach some side engines and I could launch at full throttle without worries and much better fuel usage.

https://steamcommunity.com/sharedfiles/filedetails/?id=2116713008

[Manwith Noname]

Originally posted by GunsForBucks:

Logically speaking though your Center of Mass is dynamic.

Yeah, exactly that is what I am getting at.

Originally posted by GunsForBucks:

So when you start at launch your center of mass starts to shift up as you burn fuel in the lower stage. It is when it becomes top heavy that the stability becomes worse.

If anything I would think making the higher tanks in the stage drain first would tend to keep your nose up better. At least in the lift-off / climb stage.

It really depends on the rocket. I'm thinking more along the lines of early career where your rocket will most likely be a pencil made of multiple t-200 or t-400 tanks and you might not have engines with gimbal. Though as far as stability goes for launching, if you could have the CoM at the the tip of the nose that would be ideal as it is the pivot point all forces apply to. Here's a quickly thrown together example of how emptying tanks bottom to top can improve mass shift during atmospheric phases of the launch where CoM is important.

https://steamcommunity.com/sharedfiles/filedetails/?id=2116951299

CoM near the nose means more airflow along the length of the rocket keeps the rocket going straight. It also means that any engine with gimbal at the bottom has more torque when thrust vectoring to counter any pushing against the nose from drag if the craft points away from prograde.

Realistically, getting the CoM at the the nose is likely impossible. I'm not trying to say you can't build around this in other ways though, just that this is a way you can manage things like this.

Edit: It just occurred to me that by default, all the tanks probably drain evenly rather than top to bottom but the point still remains. Emptying the bottom tanks first moves the CoM up the craft quicker, which can be helpful during the transonic phase.

[1.10e+25 Worms]

the center of mass should ALWAYS be in front of the center of lift. Not all the way inside, But not super far back. the closer the col is to the com the more manuverable it is, and the farther the more stable. a good place for the col is right behind the com, touching or slightly inside.

the aerodynamic symbol just shows where the center of lift is, the arrow is what direction the force of lift is going, no arrow means that it is pushing straight up. the col should always be behind the com as stated before.

Not too sure what you mean here but, The TWR (thrust to weight ratio) should be over 1. a twr of 2 is the best for most standard engines though 1.5 is fine for getting off the ground. Vector engines are great for getting a higher twr. To decrease weight try reducing the fuel in your craft: you only need about 2000 - 3000 delta V to get to orbit if i remember (i have not played for a long while). If you still can get over 1 twr try adding some solid rocket boosters, or look into asparugus staging for more power with lower delta v costs. to increase stabillity while in flight try to make sure that the com is infront of the col as stated before.

What your talking about is gravity turning and the only thing that really affects it is delta v and drag but not too (the higher the . the 'optimal' gravity turn is to start turning 10 degrees after 10,000 m, then after 10 degrees every 10,000m until you hit 30 degrees (60,000 m) then just cruise to 70,000m

Sorry if this doesn't help. if you have any questions just ask me.

[Wobbly Av8r]

Hope I'm not too late to add exactly $0.02, nothing more nothing less...

First and foremost, when escaping a gravity well, less is more! The frustration (and learning curve) comes from finding out how to balance what you want/need into orbit/escape with the exponentially increasing resources required to do so.

Part of the solution is to coax more performance for less weight, which means using parts with the highest Isp and thrust-to-weight ratio (TWR). In an atmosphere it also means reducing aerodynamic drag. (That's $0.01)

All of these CoM / L / T relationships can be confusing but it all comes down to a playground teeter-totter: the forces of thrust and lift/pressure [the kids on each end of the teeter-totter] translate and/or rotate the rocket about the CoM [the fulcrum between them]. Unfortunately all three of these are also constantly changing which is why this game is challenging...

Of all three of these, the CoL changes are not / cannot be "accurately" displayed in the VAB - the CoL not only *RAPIDLY* changes in atmospheric flight, but as surfaces move they can become greater or lesser contributors to the overall CoL, which is why so many Kerbals suffer violent motion sickness (or worse!) after their rockets start spinning shortly after liftoff! (I think that's the whole $0.02)

[GunsForBucks]

After reading Manwith Nonames informative post about the forces involved I understand the forces at work and honestly it is pretty simple.

It only matters in atmosphere since that is the point you teeter against. The air going past you will help to keep you lined up and having less weight in the back gives it less mass and momentum and makes it easier for the airflow/thrust vectoring to control.

If you fly bricks or tend to go up far enough before you turn for orbit then it won't matter at all keeping the Com up high or not.

But I don't build for efficiency personally so I should probably bow out of this discussion ha ha.
Always great to learn new things though!

[Chibbity]

To simplify it to a "rule of thumb":

A proper rocket looks like, and flies like an arrow.

Mass is at the tip, you have a long sturdy shaft, and fins at the rear for stability. COM should be as high as reasonable, and COL should be lower than COM, the lower the better generally speaking. The simplest and most straight forward way to achieve this is by adding fins to the very bottom of the rocket to drag the COL down.