Reasoning for design decisions in Advanced Construction Tutorial

[Zja]

I have been playing Kerbal Space Program for a few days now and all my designs are created experimentally (build and fly a rocket - if it doesn't go far enough, add more fuel, etc.).

I get the feeling that I am supposed to design ships in a more design oriented approach, so I decided to go through the Advanced Construction tutorial again to see what the game recommends for an orbital craft. After completing the tutorial, I still don't understand why most of the design decisions were made.

In the tutorial, von Kerman starts off by recommending setting the parachute min pressure to .75.  Why was that the recommendation? And then he recommends adding an RCS tank for fine maneuvers, but what fine maneuvers are needed to achieve orbit and why is an RCS tank the most efficient for these tasks (how can I tell that an RCS tank is the best for fine maneuvers by looking at the part selection screen?)? Then, von Kerman says that we only need 96 units in the tank.  How was that number determined and how can I determine how much I need for a different ship?  Then von Kerman says that we need to add another liquid tank to offset the weight of the RCS tank.  How could I determine if I needed to add more liquid fuel for a different craft and then determine how much I need to add?  He then says that the Terrier engine is highly efficient in space. Is there a way to see which engines are efficient in different conditions?  These type of questions continue throughout the tutorial.

So, I'm wondering if there is either a good resource that explains how to make these types of design decisions for general ships, something that explains the reasoning behind the different steps in the tutorial (to the point where I could determine the correct steps and adjustments for a new craft), or if you have any advice to offer on making these design decisions.

I looked around for 20 minutes or so for a resource but couldn't find anything. If you could guide me to the answer, I'd appreciate the help.

[Snark]

Hello, and welcome to the forums! 

1 hour ago, Zja said:

von Kerman starts off by recommending setting the parachute min pressure to .75.  Why was that the recommendation?

So, I've never done any of the tutorials myself, so I don't actually know.  However, that doesn't stop me from speculating.

• It might be that they did that just to make sure you're aware that "this setting is a thing" so you know that you can.
• Or, maybe the tutorial is an older one?  Once upon a time, you had to worry about your parachutes getting ripped off if you activated them too soon.  However, that's not a thing anymore, because they changed the default setting of parachutes so that you can trigger them whenever you want and they won't actually deploy until it's safe to do so.  So, maybe the tutorial was made before that change to the game, and then nobody bothered to circle back and update the tutorial accordingly.

In any case, I suspect that your real question is "should I be tinkering with the parachute settings?" and the answer is no, you really don't have to in most cases.  Certainly there's no reason I can think of to set the minimum-pressure setting that high.  Defaults work great, in my experience.

1 hour ago, Zja said:

And then he recommends adding an RCS tank for fine maneuvers, but what fine maneuvers are needed to achieve orbit and why is an RCS tank the most efficient for these tasks (how can I tell that an RCS tank is the best for fine maneuvers by looking at the part selection screen?)?

RCS thrusters are super convenient and useful if you ever need to dock with anything... but pretty much pointless otherwise.  So when you're designing your own ships, ask yourself:  "Does this ship need to dock?"

• If the answer is "yes", then it would be a good idea to make sure it has RCS thrusters and some monopropellant aboard.
• If the answer is "no", then you don't need any RCS thrusters or monopropellant.

Note that even if you do plan on docking, you may not actually need a monopropellant tank.  That's because most crew pods actually have a small amount of monopropellant storage built in, which is enough for a few docking maneuvers.  So you only need to add an actual tank if either, 1. you have a really big heavy ship that's gonna need a lot of RCS muscling around, and/or 2. you have a ship where you plan to dock / undock a lot and you're worried that the small built-in supply may run out.

1 hour ago, Zja said:

Then, von Kerman says that we only need 96 units in the tank.  How was that number determined and how can I determine how much I need for a different ship?

No clue.  I'm guessing they just want to make sure you're aware that you can set the tank contents... though why they bother, I don't know, because in practice you pretty much always want to have all of your tanks completely full on launch.

1 hour ago, Zja said:

Then von Kerman says that we need to add another liquid tank to offset the weight of the RCS tank.  How could I determine if I needed to add more liquid fuel for a different craft and then determine how much I need to add?

It's mostly an academic exercise, I'm guessing.  He's trying to show you how when you adjust your payload you may need to adjust fuel accordingly.

1 hour ago, Zja said:

He then says that the Terrier engine is highly efficient in space. Is there a way to see which engines are efficient in different conditions?

Yes.  The magic phrase here is specific impulse, which is usually abbreviated as Isp.  Higher Isp = more fuel efficient.  If you right-click on the engine in the VAB, you'll see info on the part tab.  Isp will generally have two numbers listed:  "ASL" (for "atmosphere sea level") and "vacuum".  ASL number is always lower than vacuum, but how much lower depends on the engine.

Some engines are good both in air and vacuum-- their ASL number isn't much lower than the vacuum number.  These are the engines that are good for launching off the launchpad with.  Other engines-- the so-called "vacuum engines" like the Terrier-- are super fuel efficient in vacuum, but absolutely terrible in atmosphere.  So you'd never want to launch off the pad with them, but they're great for upper stages.  (Note that you don't have to be all the way in space with these engines-- as long as you're above 10 km or so, they work fine because the air's pretty thin.)

Oh, by the way.  If you're new to KSP and trying to wrap your head around the various design considerations for spacecraft, you might find it worthwhile reading through this thread:

...I mention it only because it's ongoing now, and it's a person in a pretty similar situation to your own (i.e. fairly new to KSP, kinda scratching his head about some stuff), and he asks a bunch of questions, to which various people (myself included) have given detailed answers.  I suspect there may be a pretty big overlap between his questions, and the questions you're likely to be asking fairly soon. 

[4x4cheesecake]

Welcome to the forum

I want to add a few things to the already really detailed explanaition of Snark.

1 hour ago, Zja said:

In the tutorial, von Kerman starts off by recommending setting the parachute min pressure to .75.  Why was that the recommendation?

It can be still a valid option to use a lower pressure value then the default one, depending on your craft. If it is really heavy and doesn't slow down enough it can happen to you that the parachutes will open too late to slow down your craft before you reach the ground. But for the beginning, you should at least keep in mind that this option exists

2 hours ago, Zja said:

Then von Kerman says that we need to add another liquid tank to offset the weight of the RCS tank.  How could I determine if I needed to add more liquid fuel for a different craft and then determine how much I need to add?

If you play on the latest version of KSP, you got already the stock deltaV (dV) readout in the editor. As soon as you add parts to your craft which doesn't provide any fuel for the engines, your dV will be decreased. If you want to know more about calculating dV, I would recommend the ksp wiki article about this topic.
And if you want to know how much dV you need for your rocket: Well, after some time you will get a feeling for it but the deltaV map is always helpful
To read the map, start at the bottom and add each number you come across on the way to the planet/moon you want to go but keep in mind, that these are calculated values, so you should always add some more fuel to spare. And if oyu also want to return to Kerbin, just do the same thing but backwards. Since Kerbin got an atmosphere, you dont need to perform a powered brake, so you can also save the fuel for circularization and launch, but it depends on your ship and your mission

2 hours ago, Zja said:

Then, von Kerman says that we only need 96 units in the tank.  How was that number determined and how can I determine how much I need for a different ship?

It is probably pre-calculated for the tutorial ship and if you want to know it for your own ship, all you need is to watch the dV numbers (or you can calulate it via the rocket equation which is explained in the linked wiki article).
For the beginning it should be fine to keep in mind that adding non-fuel-mass will reduce the dV of your ship. (In this case, you will actually still get some more dV for your RCS thruster but the amount is usually tiny compared to your main engine(s))

Apropos RCS thruster: Try to place the close to the Center of Mass (CoM) which can be displayed in the editor via a button in the lower left corner, above the symmetry buttons. If you want to use more thruster, place them in equal distance to the CoM.

Also, as soon as you build a big ship, you may want to add RCS thruster just to assist the reation wheels, so it becomes easiert to change the orientation your ship in space Of course, this works for small ships as well but it is less necessary.

Regarding engine ISP: It's is already perfectly explained but

55 minutes ago, Snark said:

(Note that you don't have to be all the way in space with these engines-- as long as you're above 10 km or so, they work fine because the air's pretty thin.)

AFAIK it is 20km

[bewing]

2 hours ago, Zja said:

I have been playing Kerbal Space Program for a few days now and all my designs are created experimentally (build and fly a rocket - if it doesn't go far enough, add more fuel, etc.).

I get the feeling that I am supposed to design ships in a more design oriented approach, so I decided to go through the Advanced Construction tutorial again to see what the game recommends for an orbital craft. After completing the tutorial, I still don't understand why most of the design decisions were made.

Welcome to the forums.

As said above, the tutorials exist to demonstrate the user interface to you, and to show you what is possible -- it's not a guideline to show you what's clever when designing.

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In the tutorial, von Kerman starts off by recommending setting the parachute min pressure to .75.  Why was that the recommendation?

Because the tutorial knows you're going to be landing over the ocean, and it knows the altitude of the ocean, and most people do not want to descend on a parachute for ages -- so you set the number so the parachutes will open quite low.

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And then he recommends adding an RCS tank for fine maneuvers, but what fine maneuvers are needed to achieve orbit and why is an RCS tank the most efficient for these tasks (how can I tell that an RCS tank is the best for fine maneuvers by looking at the part selection screen?)?

Fine maneuvers are for docking and rendezvous, as said above. And RCS thrusters are about the only things you can use for fine maneuvers, so that means they are "best" by default. So the thrusters need fuel, and the fuel comes in the tanks (or a limited amount in the crew pods). Once you are an expert at rendezvous maneuvering, you may well decide that RCS thrusters are worthless and you never want to put them on another ship again -- because they add mass, drag, and useless complexity.

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Then, von Kerman says that we only need 96 units in the tank.  How was that number determined and how can I determine how much I need for a different ship?

Tanks are adjusted in increments of 10% of the maximum, and the max of that tank is 120. So the number can't be just anything. But the amount you need depends on your maneuvering skill. Since Wehrner is educating newbies, he is suggesting an entire tank of monoprop (RCS fuel), with 96 units in it! Once you have some skill, you'll only need a couple of units of fuel for a complete docking.

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 Then von Kerman says that we need to add another liquid tank to offset the weight of the RCS tank.  How could I determine if I needed to add more liquid fuel for a different craft and then determine how much I need to add?

If you add something and then your rocket runs out of fuel before you get to your destination, then you needed to add more fuel. Or you can judge by the DeltaV readouts. Or you can just guess. Many players here are adamant about following the math. Others are just as adamant about playing by "feel".

 

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 He then says that the Terrier engine is highly efficient in space. Is there a way to see which engines are efficient in different conditions?

Isp is the rocket definition of efficiency and it's listed in the part's info window, as said above.

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 These type of questions continue throughout the tutorial.

Have you perused KSPedia? It goes into more detail about explaining all this. That's basically what it's there for. It really sounds like that's the next source of answers for the questions you are having.

So yes, in order of level of detail for answers to rocketry design questions, you have: tutorials, KSPedia, the Wiki, Wikipedia, trial and error, these forums, then engineers at nasa and spacex.

(BTW, it is useful to keep in mind that this is rocket science -- so this stuff is not simple and can't be explained in a soundbite.)

Edited January 4, 2019 by bewing

[Snark]

1 hour ago, 4x4cheesecake said:

AFAIK it is 20km

No, they work fantastic even at 10 km.

Bear in mind that there's no sharp cutoff.  Even at 69 km, there's some atmosphere and their Isp won't quite be at vac levels... but the difference is so microscopic at that point that nobody cares.

10 km is a pretty good rule of thumb.  At that altitude, you're in about 90% vacuum, which means you're 90% of the way to vacuum Isp, which is, 1. pretty good and 2. generally better than the Isp of the non-vacuum engines anyway.  So, at 10 km or above-- go ahead and use that Terrier or Poodle or NERV, it's fine.

[4x4cheesecake]

16 minutes ago, Snark said:

No, they work fantastic even at 10 km.

Bear in mind that there's no sharp cutoff.  Even at 69 km, there's some atmosphere and their Isp won't quite be at vac levels... but the difference is so microscopic at that point that nobody cares.

10 km is a pretty good rule of thumb.  At that altitude, you're in about 90% vacuum, which means you're 90% of the way to vacuum Isp, which is, 1. pretty good and 2. generally better than the Isp of the non-vacuum engines anyway.  So, at 10 km or above-- go ahead and use that Terrier or Poodle or NERV, it's fine.

Ok, I got curious now and checked the numbers for the Terrier in KER.
The result:

At 10.1km altitude, the ISP equals 297.3s which is ~86% of the max ISP.
At 20.0km altitude, the ISP equals 338.4s which is ~98% of the max ISP.

So yeah, you're absolut right there.
(It's not like I wouldn't trust your knowlegde, but I had to proof me wrong on my own because I was pretty sure it is 20km....now I wonder why I thought this...probably mixed up a number/topic here )

[Snark]

1 hour ago, 4x4cheesecake said:

now I wonder why I thought this...probably mixed up a number/topic here

Well, there is a gameplay-significant transition that occurs slightly over 20 km (I think it's about 23 km), which is the point at which Kerbin's atmosphere is 1% remaining.  That's relevant because any on-rails object that falls below that altitude is deleted from the game.