Tutorial:Basic Rocket Design

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Revision as of 11:17, 1 July 2013 by Senshi (talk | contribs) (Fixed language throughout the document: Clearer statements, better grammar&spelling, less filling words. Some more content (Gravity turn).)
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by Senshi

This tutorial describes important basics on how to use the VAB and how to get your first rocket into orbit without catastrophic failures. It will describe basic VAB functions, shortly explain available part groups and their uses, introduce you to basic staging and provide a simple example rocket as well as describe how to complete your first launch (and hopefully return) successfully.

  • Length: 30-45 minutes
  • Difficulty: Beginner
  • For version: Every version

Feedback is much appreciated on the Discussion page! If things are too complicated or confusing, the fault lies with the tutorial for not being elaborate and/or clear enough, so complain!

A tiny intro to space flight physics

The main rules for getting an object into space from Kerbin and keeping it there (which is called achieving a stable orbit) are fairly easy to explain: Your main adversaries to overcome are gravity and drag. Gravity decreases as the square of the distance you get away from the center of the planet. Drag decreases almost exponentially until you reach the top of atmosphere (which is at around 68,5km). The effect of gravity increases based on the total weight of your vehicle. The effect of drag increases based on the shape of your object. The slimmer and "smoother" it is, the less drag you encounter. If you build a brick, don't be surprised if it flies like a brick. Overall, mass of your vehicle is far more important than drag, but a good design still can make valuable contribution to your rocket's efficiency. To overcome both and achieve our goal, you need to generate vast quantities of thrust (about 4000m/s Delta-V).

So the rules for designing rockets are simple: Make them light, make them smooth, and give them plenty of power. Fairly straightforward.

It is good practice to always first construct your payload and afterwards the rocket that is supposed to bring it into space (the latter is also often referred to as "launch vehicle"). Later, this will allow you to employ standardized launch vehicles for similar payload weights and target orbits.


Placement helpers

Symmetry

On the bottom left portion of your screen, you see a button with a yellow dot in the middle. If you click on it, you will enable the symmetry option. This means that now, whenever you place a part of your vehicle that is not in the middle vertical axis (top-down), it will be placed symmetrically on both sides. This option is immensely helpful when constructing launch vehicles, as this assures that you do not accidentally misplace parts which would shift your center of mass from the middle of the rocket. There will be cases where it becomes unavoidable to have a center of mass that is offset from the middle of the vehicle, but every offset has to be continuously corrected during the ascent, as it causes your rocket to "lean" into a specific direction. And energy used for correction is not available to push your rocket upwards. You can click this button various times to cycle through different symmetry modes (1,2,3,4,6,8).


Angular Snap

Just next to the Symmetry button is another button with a yellow circle and a dot within. This is a simple on-off toggle. Enabling this will cause all your placed parts to be angularly snapped to sets of 22.5° . This also is very useful in assuring that your vessel will have a mostly symmetrical center of mass, even if you place singular parts opposite of each other. This option should be enabled throughout this tutorial (actually, there only rarely is a case where disabling this ever is useful).


Physics indicators

Below the yellow buttons are three more, green buttons. From left to right these allow you to get a visual marker for: Center of mass (CoM), center of lift (CoL) and center of thrust (CoT). All should be fairly self-explanatory, however "center of lift" is mainly useful for atmospheric vehicles (planes) and will be rather useless for rockets. The other two can be helpful to verify that both center of mass and center of thrust are well placed. Both should be centered horizontally. Center of mass should be in the lower third of your rocket (or rather: Close to your center of thrust). If it is far above, your rocket will be "top-heavy" and be more difficult to control during ascent. Center of mass will later change dynamically during flight as fuel is burnt up or stages are decoupled.


Rotating pieces

This is described in the Controls in detail. In short: Use the WASDQE buttons to rotate pieces in 90° steps before placing . When holding Shift while pressing WASDQE, rotation will be gradual.


Designing your payload

Upon creating a new vehicle, you are first required to choose a suitable command module. The decision here depends on if you plan a manned or unmanned missions. If you want to send your Kerbals into space, make sure you pick a crew pod. If you want to keep your Kerbals safe while you dabble with your first excursions into the void, you can pick any of the unmanned modules. Again, remember the golden rule to keep overall weight as low as possible while maintaining your mission objective.


SAS

A fairly special device in KSP is the SAS and the ASAS (both found in the "Control" tab). Both are designed to automatically stabilize the trajectory your rocket. This system can be toggled during any stage of the flight. Upon activation (default: "T"), your ability to influence the flight path is restricted while the SAS tries to keep the current direction. The strength of the SAS to correct flight paths is limited, do not expect it to be able to recover a totally out of control rocket in atmosphere. The lighter the vehicle, the more effective the SAS forces are. SAS efficiency can be increased by simply attaching multiple SAS modules. However, this usually indicates that your vehicle is very unstable by design and rather than trying to solve the issues with more SAS, your design should be revised. Also, more SAS modules mean more weight. SAS will employ all means of control that are available on your launch vehicle, e.g. thrust vectoring ("gimballing") or control surfaces (the latter only work in atmosphere, obviously). Many of the main liquid propulsion rockets allow for slight thrust vectoring, which in general suffices for steering and control of the rocket during ascent.

All command pods feature a small amount of SAS force as well, allowing you to magically rotate your craft without using any kind of fuel. This "base SAS" is considerably less than the additional modules offer.


Electric Energy

Some devices will require electrical energy to work. Especially unmanned probes need a constant energy supply to remain controllable.

The easiest way to provide electricity is by attaching battery packs to your payload. Placement of these is irrelevant, all parts of your vehicle will be considered "wired" and can be supplied by the batteries, no matter what's in-between.

However, they are not unlimited, and if you intend to keep a satellite in space for a prolonged period of time it is advisable to provide a way to generate energy. There are three ways to do so in the current version (all found in the Utility tab):

Long-term satellites use solar panels, which have a high output at no cost and are fairly light weight. However, they require space to unfold, need unobstructed sight of the sun and cannot take a lot of physical stress (impacts, bumps, etc.). Planetary probes that have to be compact or robust thus sometimes prefer the thermoelectric generators, which have an infinite running time at the cost of a bit more weight.

When relying on solar power to provide energy for a satellite on equatorial orbit, it is advisable to have sufficient amounts of batteries on board which can provide energy while passing the dark side of the planet. Once the sun rises again, battery packs will get recharged. 2-3 small battery packs or one single battery ring will comfortably suffice for this.

Attaching two OX-4 solar panels or four OX-STAT panels in a symmetric fashion will ensure sufficient energy for most uses. Symmetry will also ensure that at least one panel will always face the sun, disregarding the vessel's current orientation.

RCS

RCS is a simple thruster system that creates thrust by expelling compressed gas. This gas is stored in a super-compressed form called Monopropellant (often shortened to Monoprop or Mono) as it does not require a mix of two propellants like all other engines. Their main design purpose is to allow precise omni-directional trajectory changes in vacuum (where no drag or gravity have to be overcome, hence a low thrust is sufficient). The thrust generated by these thrusters is very low, but the thrusters do not apply force on nearby other objects (unlike solid or liquid thrusters which can propel another object away), making them the first choice for docking maneuvers in space. Another possible field of application is for probe propulsion on low-gravity, no-atmosphere planetoids or moons. A lightweight probe with lots of thrusters can be controlled there effectively. They have two major upsides: While they require special fuel to be carried along (meaning extra weight), the actual thrusters are very lightweight and omnidirectional. Also, it is not necessary to take care of fuel lines for the thrusters, as they can be placed virtually anywhere, and do not have to be directly connected to the monopropellant tank. Another upside is that fuel consumption is extremely low. Even a small tank of 25 units of monoprop can last for minutes of thrust. When mounted, RCS is not active by default. Like SAS, it can be toggled anytime (default: "R"). If RCS and SAS are active, RCS will be used for course corrections. This can be important if you have to speed up correction maneuvers, but take care to not leave it active for prolonged periods of time, as SAS likes to do "mini"-boosts of RCS for continuous correction. If you do not require fast course alterations or precise orbital corrections in space (as for docking maneuvers), you can simply skip the entire RCS and save some tons of weight.

If you decide to put them on, there are some things to consider: As the RCS thrusters are very low-power, you need many of them. A regular satellite module with ~3 tons weight that is intended to be docked at a space station should have 6-8 thrusters. As explained earlier, RCS is (almost) omnidirectional. Still, in order to complete orbital maneuvers like rotating in a timely fashion, you need to be able to thrust into various directions.


For thruster placement, you should enable the "center of mass" indicator. Using the Symmetry-3 or Symmetry-4 option, place one set of thrusters radially near the bottom of your payload. Place another one equidistant from the centre of mass near the top of your payload. For bigger crafts, you might want to add an additional ring of thrusters around the CoM to facilitate lateral movements.

Failing to properly space your RCS thrusters equally from the centre of mass will result in a failure to properly translate laterally. This is particularly important during docking when a steady focus on the target is necessary.

Science

These parts are absolutely not mandatory for anything, however they do provide scientific data for research. They also are extra drag and weight, so I'll forgo them in this tutorial.


Parachutes

If you don't want to keep your satellite permanently in space or be able to deorbit your Kerbals safely, there's no better way than using parachutes. Placing one on top of your payload is the safest bet for a safe descent. If your payload is rather long in shape, it might be better to use a single radial parachute mounted at the center of mass to prevent toppling over on touch-down. Parachutes are very strong and can decelerate your payload safely onto Kerbin, thanks to its thick atmosphere. Don't be alarmed if parachutes are "floppy" after being deployed, they only fully deploy for the last 500m. Using the Mk25 heavy drag parachute is only necessary for heavy payloads to slow down enough for the final parachute to be able to do the rest. If your payload is descending too fast when the parachute deploys (aim for less than ~100m/s vertical speed), the parachute gets ripped off, leading to a rather unpleasant touchdown.

Launch vehicle

Propulsion

There are two types of main propellants used to generate the large amounts of thrust needed to push you into space: Solid and liquid fuel. The main difference is that liquids are burnt in a controlled fashion, allowing you to alter and adjust thrust levels during flight. Solid fuel on the other hand burns with full power as soon as it is ignited, and will continue to burn so until its entire fuel reserve is depleted. The upside of solids is that they have a somewhat better thrust to weight ratio (abbreviated TWR) than liquid systems and are cheaper to produce (which currently is of no concern in KSP) and recycle. As by far the most energy of every space flight is required for the effort to escape Kerbin's atmosphere and reach a stable orbit, solid engines are often attached to the main rocket and will be decoupled after they burn out (often called "booster rockets").

Even a basic rocket design such as this should use several stages to increase efficiency sufficiently to get into space. Later you can refine your stages in advanced tutorials.

At the bottom of your payload you should place a single TR-18A Stack Decoupler. This ring allows you to trigger a small explosion that separates the top from the bottom. This will be used to discard the chubby main launch vehicle once you have reached space.

Below that follows the launcher. This should be composed of a very strong first stage that can push you through the thick first kilometers of atmosphere where gravity and drag are the strongest. Once you pass the first kilometers, resistance (drag, gravity) is considerably less and subsequent thrust can be lower and still be effective. The other upside of burning much and hard at the start is that you immediately begin reducing your weight by burning fuel and soon after by discarding empty fuel containers, making the following ascent phase more and more easy.

As our rocket is an experimental vessel with a very low payload, there's no need for multiple stages. A simple, powerful main booster will suffice to carry us all the way to orbit after we pass the initial takeoff. For that, we will use powerful external solid booster rockets. These burn hard and fast and will be decoupled after the first ~12km when they burn out. The main launch stage should be a proper FL-T800 Fuel Tank to ensure escaping the atmosphere and establishing the required orbit. In addition, two or three booster rockets should give a healthy sling on launch. They should be attached like this: Use TT-38K Radial Decoupler with a two/three Symmetry option and place them radially on the lower end of the main fuel tank. After that, attach two/three Rockomax BACC Solid Fuel Booster on these decouplers. You will notice that the symmetry function works even on these "child objects", placing them perfectly on the other decouplers as well.

That should be enough total thrust.

Staging

Once you have finished adding parts to your vehicle (and really only then) you have to set up the stages of your launch. These are various groups of actions that can be triggered by toggling the next stage (default: Spacebar). Stages always count backwards from the highest to the lowest stage. For the example above the staging would look like this:

  • Stage 0: Deploy Parachute
  • Stage 1: Decouple launch vehicle (TR-18A)
  • Stage 2: Decouple booster rockets and enable main engine
  • Stage 3: Enable booster rockets

It's fairly straightforward. The only thing one could argue about is whether to start the main engine right from launch or not. As we have a relatively light vehicle overall, the thrust of the boosters is sufficient for quite a strong ascent. Enabling the main engine later will conserve its fuel which is needed for the upper atmosphere, but more importantly has to be used in a controlled fashion when circularizing the orbit (prograde boost at Apoapsis). The downside is that solid boosters have no thrust vectoring, so you have almost no means to correct your ascent trajectory while the main engine is shutdown. My example vessel's fuel ran out while establishing Low Kerbin Orbit (LKO, ~75km altitude), so I jettisoned the main launcher and enabled RCS to use its low-power thrusters in a long burn to finish circularizing. If you did not mount RCS at all, the weight saved will probably mean you will have no trouble reaching a full orbit stage.


Checklist

File:Basicrocket tutorial.png
A possible orbiter launcher configuration.
For the sake of this tutorial, I'll contribute a simple "This works"-list of a basic vessel you might want to check against the design you have come up with. If your vehicle is similar to this, don't bother altering it, I only provide a sloppy example. The point here is to see if you missed anything absolutely necessary (see Catastrophic failure).

From the top to the bottom:

I want to press the point that this is by far no perfect or particular useful vehicle. It's just a "thing" that will get your Kerbal into space and into a low Kerbin orbit (LKO), allow you to test your EVA, test RCS and finally return your Kerbal back home safely.

If you read carefully above you might wonder why there is no form of energy generator on board of this vessel. Simple: There's nothing that uses energy aboard this vessel in the orbit phase. The single battery pack is solely intended as a buffer and will assure that a necessary current is present to power all controls. Of course you could try and squeeze in some solar panels, if you want.

Quick guide to your first orbit

Launch

Reaching orbit is fairly easy. Just engage SAS before launch and then fire up your engines. Once you have decoupled your solid boosters and fired up your main engine to about 95% (to prevent overheating, default Shift and Ctrl), you should switch to the map (default: M) and monitor your apoapsis point on the trajectory you see.

Gravity turn

An efficient ascent does not go "straight up", but uses gravity to already start creating an orbit trajectory by angling your rocket in-atmosphere. For this tutorial,a simplified gravity turn is suggested: After decoupling the solid boosters (you should be at ~12km), disable SAS and angle your vessel towards 45° pitch, heading east (90°). Maintain this angle until your map's apoapsis point is above 70km (edge of atmosphere, better go for 75-80km), then cut your engines (default: X) and float into space!


Establishing a full orbit

Thanks to the new maneuver function you can establish a full orbit rather easy: While floating to your apoapsis, click on the apoapsis point of your orbit. This will open a small maneuver control gizmo. Use the green lever without the central cross (prograde) and drag it out until you see your planned orbit's periapsis appear on the other side of Kerbin (some fiddling with the map camera is required, yes). Hover over the periapsis point to see what altitude this point will be at. Once the periapsis is above 75km as well (it does not have to be circular, just out of Kerbin's atmosphere!) let go of it all, you can even leave map mode. Your navball will now display a new, blue marker (look for it near the prograde marker). Align your vessel so it faces this point perfectly, then enable SAS again to keep this orientation. At the right of the navball there's a bar showing how much thrust will be required to complete your planned maneuver, as well as when your maneuver point will be reached. As your burn will take some time, it's best to start 10-15 sec before reaching the maneuver point/apoapsis. Use full burn (unless you overheat) to complete this burn as quickly as possible. Always monitor the navball to ensure you still face the blue marker as well as possible. If you notice you are completely off the marker and want to correct, always cut your engine first (X key), then disable SAS. Only then correct your course, re-engage SAS and restart your engine. If you have completed the burn and check the map screen again, you'll see you now have an orbit that's very close to the maneuver orbit.

Congratulations, you safely put a metal pile into orbit that could circle Kerbin for centuries! You now can test out the EVA suit and take a space walk (Don't get lost! There's no safety line to keep you near your ship!), perform RCS maneuvers or just enjoy the planet zipping past below you.

Reentry

When your Kerbal notices he forgot to bring food, you can initialize reentry. To aim for a specific landmark along the way, use the maneuver tool again, this time you'll need the other green lever (retrograde) as you'll have to retroburn to lower your orbit. When targeting for a ground point, bear in mind that once you enter atmosphere, atmospheric drag and gravity will provide additional deceleration energy, so your actual impact point will be considerably short of the one the map trajectory shows (maneuver and orbit prediction do not take atmospheric braking into account).

Once you enter atmosphere and approach the ground, just remain cool, make sure you have decoupled the main fuel tank should it still be attached and wait. Once you are down to 1-2km altitude, toggle the last stage, which will deploy your parachute. During descent, it will just flop and provide minimal drag, but at 500m altitude it will fully deploy, slowing your vessel down rather abruptly (you can alleviate some of the G-force impact of this deployment by using upwards (RCS) thrust shortly before (default H), though Kerbals are too cool to care about G-shock). After that you just enjoy the view while floating for touchdown.


Conclusion

You mastered basic rocketry and can now shoot things and Kerbals into the void as you please (and even get them back home safely)! However, if you want to tackle targets further away or want to bring more stuff into space in a single launch, it is strongly recommended to take a look at the Intermediate Rocket Design tutorial and the Advanced Rocket Design tutorial to master the art of propelling even bigger piles of metal (and Kerbals) into space.

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