Difference between revisions of "Tutorial:Basic Rocket Design"

Revision as of 13:06, 19 October 2015

This tutorial describes all important basics on how to use the VAB and how to get your first rocket into orbit without a catastrophic failure. It will describe basic VAB functions, explain available part groups and their uses, and introduce you to basic staging while providing a multistage example rocket as well as describe how to complete your first launch and return successfully. It will also attempt to introduce various common science and engineering terminology that is frequently used in game without being too nerdy. Inter-Wiki-Links are used to allow you to gather more knowledge about specific aspects you may be curious about.

• Length: 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!

Intro to rocket science

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 craft must overcome gravity and drag.

Gravity decreases with the square of the distance as you move away from the center of Kerbin. But simply gaining altitude isn't enough. You also have to gain sufficient horizontal velocity to, as a respected author once defined flight, throw yourself at the ground and keep missing. Gravity will continually pull a craft straight down toward the ground; but by traveling sideways fast enough, the craft continually avoids falling downward straight enough to hit the atmosphere, much less the surface of Kerbin. This is called orbital speed and requires about 4000 m/s of Delta-V for Kerbin.

Drag decreases almost exponentially until you clear the top of Kerbin's atmosphere (which is at 69,078m). Overall, the mass of your vehicle is far more important than drag, but a good design still makes valuable contribution to your rocket's efficiency. A rocket is simply the attempt to push mass through the air into the void. Air acts much like a liquid, and as such your rocket will have to push it aside continuously on its way to orbit. This is way real life rockets and planes are streamlined: They are shaped in a manner that makes it easy to part the air in front and lead it around the rocket. A wide, short "pancake rocket" will experience much more drag than a tall slim one, because air can only move out of the way of the rocket at limited speeds as well.

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

It is good practice to first construct your payload and only later build the rocket that is supposed to bring it into space (the latter is also often referred to as a “launch vehicle”). Later, this will make it easier to better estimate what launch vehicle you'll need for your missions.

Placement helpers

This section discusses how to use placement assists. For how to use the VAB/SPH controls, see "controls" and "using the VAB/SPH controls.

Symmetry

On the bottom left portion of your screen, you can 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. 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 ( = wasted). 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

Next to the yellow buttons are three more, green buttons, vertically aligned. From top to bottom these allow you to get a visual marker for: Center of mass (CoM), center of thrust (CoT) and center of lift (CoL). "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 (look at the rocket from above to verify). 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 5° per rotation (useful for angling panels, ladders, thrusters, etc.).

Payload Design

First, you need to decide on what the objective of your mission is. Then you figure out how to achieve said objective with the least cost and effort. The main objective of this tutorial mission is to get a satellite into a stable orbit around Kerbin. Upon creating a new vehicle, you are first required to choose a suitable command module. The available modules fall into two major categories: Manned and unmanned. 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. Remember the golden rule to keep overall mass as low as possible while maintaining your mission objective.

Reaction Wheels

→ See also: Reaction wheel

To facilitate rotation of a vessel, reaction wheels can be used to apply torque to the vessel at its part location, so bear in mind that reaction wheels far off the center of mass can induce undesired rotation effects. They are mostly used to rotate your craft precisely without the need to use up fuel. Instead, reaction wheels require electric charge. All command pods (manned and unmanned) contain basic reaction wheels. The torque provided scales roughly with the pod's mass (the bigger, the more torque). Reaction wheel torque is grossly overpowered ingame compared to reality, making it much easier to control crafts.

If you have a very heavy and bulky payload, you might want to add additional reaction wheels to speed up rotation time or help with stabilization during ascent. There are several inline wheels available in the "Controls" panel.

SAS

→ See also: Stability Augmentation System

This toggleable assistance system included in all command modules (default: "T") supports rotation control (pitch, yaw, roll) of a vessel. Manual control input will be dampened while SAS is active and slow rotation rate to zero ("dampening mode"). This can be useful if you have very powerful rotation wheels that would cause too strong of a torque otherwise or to "brake" rotation.

If your vessel does not rotate, SAS switches to "Locking" mode. It will now try to hold the current orientation (which is not to be confused with heading).

SAS uses the same means of control available to you as user (reaction wheels, RCS, control surfaces, engines, etc.).

Electric Charge

→ See also: Electric charge

If your vessel runs out of electric charge, you will lose control over all devices that need it and all engines will be automatically cut. Unmanned probes require a continuous current for operation, and all reaction wheels require energy while active. 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 have finite reserves, 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 except the engines are in the Utility tab):

• Radioisotope thermoelectric generators (Currently there is only the PB-NUK Radioisotope Thermoelectric Generator available)
• Some liquid engines produce a small amount of electric charge while running as a byproduct (Low power output, try not to rely on it.)
• Solar panels

Long-term satellites often use solar panels, which have a high output at no cost and are very lightweight. However, they require space to generate meaningful amounts of energy, need unobstructed sight of the sun and cannot take a lot of physical stress (drag, heat, impacts, bumps, etc.). The amount of energy generated also decreases the further the probe moves away from the sun. The radioisotope generator is heavier than solar panels and provides little energy, but it has an infinite lifetime, is independent of solar radiation and is much more compact, making it popular for planetary probes that are active during night time. However, they are also very sensitive to physical stress and have to placed securely.

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 Z-100 Rechargable Battery Packs or a single Z-1k Rechargable Battery Bank will comfortably suffice for our mission. Batteries in KSP do not deteriorate in capacity or recharge ability as real life batteries do.

Attaching two OX-4W solar panels or three OX-STAT panels in a symmetric fashion will ensure sufficient energy for most uses. Symmetric placement ensures that at least one panel will (almost) always face the sun, disregarding the vessel's current orientation.

RCS

→ See also: Reaction Control System

The reaction control system is a special system that creates thrust by expelling compressed gas. This gas is stored in a compressed form called Monopropellant (often shortened to Monoprop or Mono) as it does not require a mix of two propellants like all other engines. The thrust generated by these thrusters is very low, but - unlike in reality - they do not apply force on nearby other objects (the exhaust of regular solid or liquid thrusters can push another object away), making them the first choice for all kind of high-precision rotations and translations in vacuum, where no significant resistance (drag, gravity) has to be overcome. The low thrust also allows a very fine control (even more so in precise control mode, default "Capslock"), perfectly suited for docking maneuvers. Another possible field of application is for ultra light probe propulsion on low-gravity, no-atmosphere planetoids or moons. A lightweight probe with multiple RCS thrusters can be controlled very effectively. RCS has two major upsides: While it requires special fuel to be carried along (meaning extra mass), the actual thrusters are very lightweight and omnidirectional. Also, RCS thrusters don't 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. 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 rotations or translations, but take care to not leave it active for prolonged periods of time, as SAS likes to do "mini"-boosts of RCS for continuous corrections. If you do not require fast course alterations or precise orbital corrections in space (as for docking maneuvers), you can simply choose not to install RCS at all and save valuable 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) omni-directional. 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 center 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 translating movements.

Failure to space your RCS thrusters equally from the center of mass will result in diverted torque application. This means rotational maneuvers will introduce slight movement, and movement thrusts will also introduce rotation. This is particularly important during docking when a steady focus on the target is necessary.

Science

→ See also: Science modules

These parts provide scientific data for research as well as measures to transmit said data back to Kerbin. They also mean extra drag and weight, so I'll forgo them in this tutorial.

Parachutes

→ See also: Parachute

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 best 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 ~100 m/s vertical speed), the parachute gets ripped off, leading to a rather unpleasant touchdown.

Heat shield

→ See also: Heat shield

You also have to care of excessive heat. Usually, it is generated during reentry, when you hit the dense atmosphere at insane orbital speeds of multiple times the speed of sound. The air being pushed out of the way is superheated, and transfers some of its heat to the payload. Very small and sturdy satellites can be risked to survive reentry a bit warmed up if they follow a shallow enough reentry trajectory, but heavier payloads (usually manned pods) will require a dedicated part, the heat shield. A heat shield is composed of ablative material, that gradually melts away during reentry, absorbing a lot of heat in the process. It should be placed right below the payload that is to return to Kerbin, and has to have a sufficient diameter to cover the payload fully. It also is directional, so you have to ensure to hit the atmosphere with the heat shield facing forward. If your payload tumbles just slightly during reentry, a non-protected area could face the heated atmosphere and suffer explosion by overheating.

The heat shield is fairly heavy. With experience, you can reduce its weight by reducing the thickness (right-click the part and reduce the amount of ablator), shaving off some more valuable kilograms. It also has an integrated decoupler, meant to be triggered after leaving the dense atmosphere, to reduce the weight the parachute has to slow down.

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 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 for an efficient ascent. Later you can refine your stages in advanced tutorials or experiments.

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 your propulsion system before reentry to reduce weight.

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. Usually the more powerful an engine is, the lower is its efficiency and the larger its mass. A light, low-thrust engine can also be used to allow orbital maneuevers. 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.

Using a three stage launch vehicle will give us a lot of (surplus) energy to ensure we can establish a stable orbit and even allow further orbital maneuvers. A long, slim rocket also is much more aerodynamically stable during ascent than a short stubby one, making it less likely to flip over and disintegrate.

The main launch stage should be two FL-T800 Fuel Tank which will power our escape of the atmosphere. An additional FL-T200 Fuel Tank and a light low-thrust LV-909 Liquid Fuel Engine will serve to finalize the orbit and allow us to deorbit again. Two radially attached boosters will give us the kick we need (and then some) for the thick lower atmosphere. Use TT-38K Radial Decoupler with a two-symmetry option and place them radially on the lower end of the main fuel tank. After that, attach three Rockomax BACC Solid Fuel Booster on these decouplers. You will notice that the symmetry function works even on these "child objects", placing them symmetrically 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 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 orbiter engine
• Stage 2: Decouple launch vehicle and enable orbiter engine
• Stage 3: Decouple booster rockets and enable main engine
• Stage 4: Enable booster rockets

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 (~31 tons), the boosters alone provide plenty of thrust. 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.

Checklist

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 working example. The point here is to see if you missed anything necessary (see Catastrophic failure). From the top to the bottom:

1: Mk16 Parachute
2: Command Pod Mk1

3: Z-100 Rechargable Battery Pack 2x, symmetrically attached to the command pod

4: Heat Shield (1.25m)
5: TR-18A Stack Decoupler
6: FL-R25 RCS Fuel Tank

7: RV-105 RCS Thruster Block 4x, symmetrically attached to the R25 fuel tank

8: FL-T200 Fuel Tank

9: RV-105 RCS Thruster Block 4x, symmetrically attached to the T-200 fuel tank

10: LV-909 Liquid Fuel Engine
11: TR-18A Stack Decoupler
12: FL-T800 Fuel Tank
13: FL-T800 Fuel Tank
14: LV-T45 Liquid Fuel Engine

15: TT-38K Radial Decoupler 2x, symmetrically attached to the FL-T800 Fuel Tank

16: Rockomax BACC Solid Fuel Booster 2x, symmetrically attached to the TT-38K Decouplers

17: Aerodynamic Nose Cone 2x, placed on top of the boosters to reduce drag

Quick guide to your first orbit

Launch

Engage SAS and increase throttle to 100% (default: "Shift" and "Ctrl") before launch. Setting thrust has no effect on our boosters, but will be applied to the second stage main engine. Now you can trigger the first stage to ignite the boosters (default: "Space"). While your boosters burn out, hold "D" to slowly lean the rocket on an eastward trajectory (up to about 80° pitch). When the boosters are empty, stop holding "D" and trigger the next stage which decouples the boosters and ignites your main engine. Now switch to the Map View (default: "M"), enable the navball there and monitor your apoapsis point on the trajectory you see.

Gravity turn

→ See also: Gravity turn

An efficient ascent does not go "straight up", but uses gravity to prepare an orbit trajectory by angling your rocket in-atmosphere and allowing gravity to help pull your craft to orbital speeds by slowly pitching eastwards (90° heading) during launch. As your main engine has a gimbaled engine, it allows thrust vectoring to control your rocket much better than with the boosters. Slowly keep pitching over, always aiming about ~5-10° below your current prograde marker. Maintain this until your map's apoapsis point is at about 80 km (edge of atmosphere is at ~69 km, but a buffer never hurts), then cut your engines (default: "X") and float into space! Make sure your rocket points mostly prograde ( you can use the SAS "prograde" help system, one of the buttons left of the navball) while you coast through the atmosphere to reduce drag.

Establishing a full orbit

While floating to your apoapsis, monitor your fuel reserves in the main tank; Once it is empty, trigger the next stage to use the last engine. Then 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). Now slowly continue to drag it out until you see the orange apoapsis and periapsis markers "flip" positions. Now both the apoapsis and periapsis should be around 80 km, meaning a stable, round orbit. Your navball will now show a blue marker (look for it near the prograde marker), so you can fly the maneuver either in map view or viewing your ship directly. Align your vessel so it faces this blue indicator perfectly either manually or by usiong the "maneuver node helper" of your SAS. 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 before reaching the maneuver point. Divide the estimated burn time by 2 and you know how soon you should start your maneuver (e.g. if estimated burn duration is 60sec, start when the timer hits T-30sec). Use full burn (unless your engine overheats) to complete this burn as quickly as possible. Always monitor the navball to ensure you still face the blue marker. If you notice you are far off the marker and want to correct, always cut your engine first (X key). Only then correct your course 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. You can delete the maneuver node now by clicking on it again and clicking the small red “X” symbol.

Congratulations, you safely put the first satellite into orbit which could circle Kerbin for centuries! If it's a manned mission, 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! Best to quicksave (F5) before taking that spacewalk!), perform RCS maneuvers or just enjoy the planet zipping past below you.

Reentry

When your Kerbal notices he forgot to bring snacks, you can initiate 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 maneuver and orbit predictions do not take atmospheric braking into account. Once you enter atmosphere, atmospheric drag and gravity will provide additional deceleration energy, so your actual touchdown point will be considerably short of the one the map trajectory shows!

Decreasing periapsis too sharply will mean your flight through the atmosphere will be shorter. This means that the upper atmosphere will also have lesser time to slow you down, and there willbe a much harsher heating and deceleration when you hit the thick lower atmosphere. By aiming for a periapsis of ~32km, a shallower, slower and safer reentry is guaranteed. This way the craft will fly almost horizontal in the upper atmosphere, bleeding speed. It also allows greater control in choosing a landing site, given sufficient aerodynamic control and thrust.

Once your deorbit burn is complete and you are confident to land near your target, decouple the orbiter's fuel tank. Reduced weight means a more efficient aerobraking and less stress on your vessel. Fuel tanks are much less resistant to heat than your heat shield. Ensure that your vessel faces retrograde, so the heat shield at the bottom of your pod faces forward. Once you reach the upper atmosphere (~50km), the airstream should stabilize your vessel and no more control input should be required, so best deactivate SAS. Now you just have to wait until your pod has slowed down enough: The parachute gives you an indicator when it'll be safe to deploy it: Right-click the parachute, and it'll tell you it's either "Unsafe", "Risky" or "Safe" to deploy. You can certainly wait until this is "Safe" (usually below the speed of sound, ~330m/s) before you deploy. Don't rush this, as a ripped off parachute will mean a very unpleasant touchdown. You can also just wait until your speed doesn't decrease anymore and the G forces on your pod are close to 1 (around 100-150m/s, probably at 3000-5000m altitude), this will certainly be a safe deploy.

~500m above ground the parachute will fully deploy, abruptly slowing your vessel down. After that you just enjoy the view while floating for a safe touchdown, which hopefully will not be on a steep mountain slope.

Conclusion

You mastered basic rocket science and engineering 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 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 efficiently.

References