How to build a rocket ship for a mission

[Foxster]

Many people who are fairly new to KSP want to build a craft to satisfy a contract, to meet a Challenge or to just boldly go where no-one has gone before, but find it's not easy.

What engines to use? What shape should the craft be? How much fuel to take?

Well, this is a quick guide to building a ship capable of getting your payload to where it needs to go. It certainly does not cover everything but I want to give you some information to get you started building workable rockets. 

We need to understand a few concepts first... 

Payload

This is the bit with the mission's important stuff. This may include crew and passenger accommodation, return engines+fuel, science experiments, electrical power generation, mining equipment, etc.  

You must minimise your payload! Don't add crew if you don't need them. Don't add lots of landing struts you don't require. Below we will check how much dV is needed for a journey and pack only enough engines+fuel for this. 

dV

dV (delta V) is the measure of how much shifting ability or range that your rocket has. Let's say your rocket has 5000dv. Whether it is a tiny rocket or a huge one, if they have the same dV then they have the ability to travel the same distance (in theory, ignoring atmospheric drag). For instance, both these rockets have ~5000dV and could make Kerbin orbit:

They have about the same dV but the the difference is the amount of payload being shifted for the expenditure of that dV. The first craft can take it's payload of ~0.9t to orbit and the second can do the same with its payload of ~4.2t. 

The dV needed to achieve common things is given in this handy chart: http://i.imgur.com/8jGWLCg.png. 

We generally talk about the total vacuum dV of a whole craft when thinking about it's "range".

Isp

Isp is the miles-per-gallon of an engine. Some engines being more efficient than others. The same engine will have a range of Isp depending on the craft's altitude in an atmosphere.

Isp tends to be worse in the presence of a thick atmosphere. So, for instance, the Terrier engine has an Isp at sea level of just 85, but in vacuum it has an Isp of 345. This means it is mostly useless as a first stage on Kerbin but becomes excellent once the air gets thin beyond 10km and it is at its best in vacuum. 

You can find the Isp for engines by right clicking the part in the parts selection panel. 

So, for your first stage or two, if they are going to burn in the lower atmosphere, you want engines with a high sea-level Isp. Once out of the thick atmosphere, you want engines burning with a high vacuum Isp. 

Drag

This tends to be a more obvious concept. Skinny, pointy things will have less drag in the atmosphere and so will not waste so much dV. Fat, blunt craft or ones with lots of bits sticking out will waste fuel getting to orbit. 

Looking again at the two craft above, the first will have less drag than the second. So the second will waste dV fighting the air on the way to orbit.  

A poorly designed rocket, even though it has the needed dV in theory, may not make orbit at all due to drag. 

Drag can be reduced in various ways, including: 

• Using thinner and fewer stacks of engines and boosters
• Making the craft pointy
• Putting things in cargo bays, service bays and protective shells

Stability

I'll just mention this briefly as there is a lot to this. 

The most likely place your rocket will have problems is a few km up, on it's gravity turn towards orbit. The front end will be lighter than at launch and the craft will no longer be balancing on vertical engine thrust. It may also have more drag at the front than the rear. This will lead to the rocket tilting too far over and perhaps finally pointing down. The simplest and most often-used fix is to add some fins to the bottom of your craft. Don't use more than you need as they will add unnecessary drag and mass.

The first craft here will not make orbit but the second will...

TWR

Our final piece of useful information is the TWR (Thrust to Weight Ratio). 

If we have a heavy craft with low-power engines then our TWR will be low. If we have powerful engines but a light payload then we may have a high TWR. 

In space, the TWR is comparatively less important. Any TWR will make the rocket go. High thrust engines will mean burn times are less but they usually have the disadvantage of being heavier and that is mass you have to haul into space. A light craft powered by a Terrier can be efficient, as can a heavier craft powered by Nervs. Using a Rhino to push a small craft around in space is not so good. 

One place TWR is very important is in the atmosphere. Your craft must have a TWR of at least 1.0 or it won't lift off the launchpad. If you have too high a TWR then the craft will move quickly and this will increase drag and heating, leading to wasted dV and even heatplosion of the craft. 

A TWR of about 1.4 for low atmosphere stages is good. After that it can be a little lower, but generally keep it above 1.0. In space it can be as low as you like.

Note though that TWR varies with the gravity. A rocket with a TWR of 1.14 at sea level on Kerbin will have a TWR of 7.34 on the surface of the Mun. So you can have much weaker engines to return from the Mun than were used to lift the craft from Kerbin. 

How to know the stats for a rocket

KSP is a bit light on performance stats to help design a ship. Luckily there are essential mods out there to help. The two favorite ones are Mechjeb and KER. Personally, I tend to use Mechjeb mostly but do have KER installed too. Neither of these mods are "cheating" - you think NASA doesn't use computers to help design rockets?

If you don't have it already then go install Mechjeb and open the "Delta-V Stats" and "Vessel Info" windows whilst in the VAB. 

Building a craft

That's pretty much all the background you need to know to get your mission started.

So, how do we actually go about designing a rocket to achieve an objective? Well, let's go to the Mun!

I won't be making the lightest, most efficient or cheapest craft but it will demonstrate the design process. It will be designed on-the-fly though, as this guide develops. 

For a trip to the Mun the chart linked above tells we need something like 5000dV in total. So let's build a craft to do that, with a margin for error. 

 

I always design from the end of the mission to the beginning. So we will begin with the payload and work back to the first stage lifting engines. I'm going to ignore cost but if we keep things light and simple it shouldn't be an issue. 

Stage 0, Mun payload

Let's make this a science probe that can collect a good lot of science and then radio it back. We will need experiments, electrical power, an antenna, a landing engine+fuel and something to cushion the landing...

 

Note that in the Delta-V Stats window I have selected "Mun". This will get the TWR figures accurate for the SOI (Sphere of Influence) this stage will be operating in. 

Let's look at the stats:

dV: 1338. We need about 600 or so for a Mun landing so we should be OK here. Best to play on the safe side. 

TWR: 2.77. Needs to be well over 1.0 to overcome the Mun's gravity during landing. We should be OK at 2.77.

Isp: 290. There are three Spider engines, one more or less could be used and it would probably be OK. Spider engines don't have the best vacuum Isp at 290 but they are very light. 

What I did was put the top half together first with everything that had to make it to the Mun; the experiments, a probe core, power, etc. Then I added a small fuel tank and tried various small engine combinations until I got the stats I wanted as above.

It is topped off with a nose cone, as this is going to be the pointy end of our rocket at launch. At the bottom we have the minimum number and size of landing struts.  

Hopefully you can see already the basic ideas behind this guide being used: Keep the mass low, keep the craft streamlined, then get the dV and TWR right to match the mission and payload needs. 

Stage 1, transfer stage

A stage to get us from Kerbin-orbit to Mun-orbit could probably be combined with another stage but let's make it a separate stage just to demonstrate the process. This stage will also operate in vacuum and it needs about 1200dV. Something like this...

 

dV: 1754. That's over our target so should be OK. 

TWR: 1.0. We are only operating in space here, so this doesn't matter too much, but 1.0 will mean the burns won't be too long. 

Isp: 290. I used two Twitch engines. Again, not a great Isp, but at least these are light engines. 

An inline engine might have had less drag but the only suitable one would have been the Terrier, which would have been over-powered for this use.

Stage 2, upper lifter stage

All we need now is get this top section into Kerbin orbit. The dV for that is something over 4000dV. It's not exact because it depends on the drag, the flight profile we use, etc. 

We know we have to get 3.25t to orbit, so I'll experiment with a couple of stages to do that. I'll try to use the smallest engines and tanks that will do the job. I won't start by throwing on some big tanks and engines. We will need the TWR to be about 1.4 or so at launch and a bit less in the upper atmosphere. We will also need to think about stability. 

Here's a simple stage 2...

dV: 2611. Not enough to get us to orbit, so we will need a stage 3.

TWR: 1.42 (sea-level) to 1.66 (vacuum). Chances are this stage will operate in atmosphere and vacuum, so this should be OK. 

Isp: 290 (SL) to 340 (vac). The Dart is a very good all-round engine. I think it is going to be mostly operating at or close to its vacuum Isp, so the Isp looks good for this. 

Stage 3, 1st stage lifter

We now just need to find a good first stage with 2000 or so dV, with a good sea level Isp and a TWR of around 1.4. We could have used some cheap solid rocket boosters but I am not a fan, so a bit of experimentation gave this...

dV: 2375. That, plus stage 2, should give us more than enough umph to make orbit. 

TWR: 1.50 (SL) to 1.75 (vac). Pretty good. Should get us most of the way to orbit without too much drag or heat. 

Isp: 290 (SL) to 340 (vac). As before, a good engine just about anywhere. 

Note that all three Dart engines fire at launch and that the outer tanks feed the central tank i.e. we have asparagus staging (Google it). 

Now, I did take the craft as it is for a spin. It flopped a few km up and it all went badly. It needs some drag at the rear end and the lack of engine gimbal on the Dart engines is not helping, so some sort of attitude control seems in order too. Maybe there's a little more dV than we really need as well. 

So, how about this?...

dV: 1919. Plenty to get us to orbit with the next stage. 

TWR: 1.71 (SL) to 2.01 (vac). High but we can live with that, as long as the craft doesn't overheat on the way to orbit. 

Isp: 290 (SL) to 340 (vac). Fine. 

The complete craft has 7479 dV (vac), which is over-engineered for our needs but better too much than not enough. 

Let's take this craft for a spin to the Mun...

 

So, we had a successful mission! We are down on the Mun ready to do some  science and radio the results home. 

The craft was over-engineered. However, with some relatively minor modifications, it could be made into a return science mission so that we get more science next time or even become a manned return mission craft. It certainly didn't look like a massive Saturn5-like craft that I see some folks using for such as mission. 

Craft file: https://www.dropbox.com/s/ac3nmb4p0gewhiq/Guide%20ship%201.craft?dl=0

I hope you found this guide useful. If so then rep me. If not then give me feedback and I'll try to make it better. 

Happy New Year!

Fox

Edited December 29, 2015 by Foxster

[Pecan]

Nice start Foxster :-)

[Proof-reading:
Payload "This is the unpowered bit" conflicts with "  return engines+fuel ... electrical power generation".  Might want to re-word that a bit.
Isp typos " miles-per-gallon", "iof an engine", "range iof Isp"
TWR typo "Thrust to Weight Ration"]

[Foxster]

Thanks. Appreciate the input. Might be best to save it until the end though, I have additional content and proof-reading to do yet . 

[GoSlash27]

A good summary.

A couple minor points to throw in: DV is expressed in m/sec. We would not say "it takes 5000 DV", we would say "it takes 5km/sec DV". Isp is shorthand for "specific impulse" and represents the ratio of thrust to fuel consumption. It is expressed in seconds; the Terrier has a sea level Isp of 85s.

Best,

-Slashy

[GoSlash27]

Foxster,

 This is a good outline of the design process. I have a couple more rules of thumb that should prove helpful:

 I design my upper "transstage" to orbit to have at least 0.7 t/w and 1,700 m/sec DV. No aerodynamic considerations are necessary for this stage and it uses a vacuum engine. If I have less than 1,000 m/sec to get to the next step, I fold that into this stage and crash it into the surface at the destination. If not, I jettison it when barely suborbital so that it won't be left as clutter. I always design my stages so that they don't get left floating around. The main criteria for this stage is "light" followed by "cheap".

 The lower "boost" stage is 1,800 m/sec DV and at least 1.4 t/w. This stage incorporates atmospheric engines and any control fins. 1,800 m/sec is the magic number here because that gets you to 30km altitude in a gravity turn, which is where aerodynamic stability is no longer important. The main criteria for this stage is "cheap", with "light" only coming into play when operating under a pad mass restriction.

Best,

-Slashy

 

[Bandus]

This is a fantastic guide. Thank you for the effort!

[metl]

I commend your efforts to try to put some of these concepts into more layman's terms. While "miles per gallon" may not technically be correct, both MPG and Isp symbolize the efficiency of the engine. Good read.

[egoego]

A nice guide. 5 stars and a like from me.

But  :

On 29.12.2015 at 4:44 PM, Foxster said:

If you don't have it already then go install Mechjeb

I would rephrase that to something like: "If you don't want to do the math yourself you could install MechJeb..."

There are ways to find out the DV of your rocket without any mod. KSP tells you all the numbers you need (total mass, fuel mass, engine Isp) and the required math can be found in the wiki or in some good places in the forum (can't search for it myself right now, because I'm on the phone).

Please don't tell the people not to try to learn something by doing it on their own and install a mod instead.

Edited January 22, 2016 by egoego

[GoSlash27]

On 1/22/2016 at 9:17 AM, egoego said:

A nice guide. 5 stars and a like from me.

But  :

I would rephrase that to something like: "If you don't want to do the math yourself you could install MechJeb..."

There are ways to find out the DV of your rocket without any mod. KSP tells you all the numbers you need (total mass, fuel mass, engine Isp) and the required math can be found in the wiki or in some good places in the forum (can't search for it myself right now, because I'm on the phone).

Please don't tell the people not to try to learn something by doing it on their own and install a mod instead.

^ I'll second that.

 You do need math to properly design stages, but most people prefer to not do the math themselves. For those people, mods like KER and MechJeb are mandatory. If you're not interested in working the math yourself, you really *should* have a mod for this.

If, OTOH, you are willing to get your hands dirty... the math opens up a whole array of possibilities that aren't available to people who use mods. Here's my tutorial on the mathematical design process for KSP:

Also, I'd like to cross-link NorCalPlanner's excellent tutorial and challenge for cheap and cheerful boosters. They contain lots of great tips and examples for how to design economical and effective mission packages for career use.

 

Best,
-Slashy

Edited March 6, 2016 by GoSlash27

[coguar]

Hello Great guide,

 

I have the following question, where do you get the value of the ISP, I don't see it on the screenshots, I've also downloaded the MechJeb mod but don't see a Column like "ISP" just get the same you have on your images, probabbly I miss something.

 

 

Album https://imgur.com/iRiyXou will appear when post is submitted

[Foxster]

2 hours ago, coguar said:

Hello Great guide,

 

I have the following question, where do you get the value of the ISP, I don't see it on the screenshots, I've also downloaded the MechJeb mod but don't see a Column like "ISP" just get the same you have on your images, probabbly I miss something.

 

 

Album https://imgur.com/iRiyXou will appear when post is submitted

 

It is on the extended information screen for each engine on the right side of the part's information display...

The first number is the Isp at Kerbin sea level and the second is in space (vacuum). The bigger the number the better (more "MPG"). 

For the Terrier engine above, you can tell that it makes an excellent space engine but is lousy in an atmosphere. 

Edited February 19, 2018 by Foxster

[michaelbezos1]

hi guys i stacked now at building my ship, this tutorial seems to be good but still not the easiest. This tuition is bad as i do not know how to build my spacecraft or even spacestation so i look for more i could not find at youtube as well step by step as the youtubes tuition are so quick and i do not know how to begin because i want to do awesome spacecraft. 

Edited December 17, 2019 by michaelbezos1

[έķ νίĻĻάίή]

2 hours ago, michaelbezos1 said:

hi guys i stacked now at building my ship, this tutorial seems to be good but still not the easiest. This tuition is bad as i do not know how to build my spacecraft or even spacestation so i look for more i could not find at youtube as well step by step as the youtubes tuition are so quick and i do not know how to begin because i want to do awesome spacecraft. 

Hi!

First of: if you are new to the game, I suggest to not go that ambitious and build a rocket that is that complicated 

Secondly: Start easy. The Ksp learning curve is rather steep

Thirdly: Ksp has built in tutorials in the missions(?) section, below the resume game (if I remember correctly)